detailed project report section 205 silver creek dam early warning system october 2004

8/6/2019 Detailed Project Report Section 205 Silver Creek Dam Early Warning System October 2004

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Detailed Proj ect Reporf

Section 205 Silver Creek Dam Early Warning System

October 2A04



Table of Contents

1.0 INTRODUCTION.... 4

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1.2 Authority ...................1.3 Study Sponsorship ..............

1.4 Project Location1.5 Previous Studies.

2.0 STUDY AREA CHARACTERISTICS

2.1 Socioeconomic Characteristics ........

2.2 Basin Conditions ....................

2.2.1 Climatology and Hydrology ..........

2.2.2Basin2.3 Current Dam.....

2.4 Existing Condition ...............

2.5 Review of Existing Dam Safety Monitoring Program.2.5.7 Piezometers.

252Flow Measurements from Drains .

2.5.3 Survey Points

2.5.4 Reservoir Level.

2.5.5 Visual Observations............

2. 5 .6 D ata Evaluation/Management

3.0 PLAN FORMULATION.........

3.1 Problems and Opportunities...........

3.2 Planning Objectives ...

3.3 Evaluation of Conditions before Project Improvements

3.3. 1 Overtopping Scenario..

3.3.2 Piping Failure Scenario....

3.3.3 Seepage Failure Scenario...

3.4 Seepage Failure Scenario before Project Improvements

3.4. I Overview............

3.4.2 Property Damagos..............

Estimate of Residential Content Damages .

Estimate of Non-Residential Content Damages

3.5 Alternative Analysis.......

3.5.1 Flood Warning System...

3.5.2 Flood'Warning System Costs

3.6 Potential Reduction to Property Damages with Flood Warning Time...

3.7 Comparison of Benefits to Costs

4.0 RECOMMENDED PLAN.......

4.1 Overview.............

4.2Detection System

4.2. I Reservoir Monitoring

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4.2.2 Upgrading Piezometers

4.2.3 W eir Box Instrumentation ..

4.2.4 Monitoring Station.......

4.2.5 Reservoir Gauge

4.2.6 Measurement Control Units.........

t7t7t718

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4.2.7 Additional Detection System Detai1s.............

4.3 Notifrcation System.....

4.3. I Alternative Notification Options

4.3.2 Recommended Notification System

4.3.3 Specific Notification System Components .

4.4 Evacuation Plan ......

4.4. 1 Recommended Evacuation Zones.

4 .4 .2 Ev acuations of Special Facilities/Structure s

4.4.3 Coordination of Emergency Response ........

4.4.3.1 Police ........

4.4.3.2 Cify4.4.3.3 Fire............

4 .4.3 .4 Interagency Coordination ...........

4.5 Environmental Impacts....

4.6 Cultural Resources ...................

4.7 Geotechnical

4.9 Cost Estimate .....

5.0 COMPLIANCE AND COORDINATION

5.1 Regulatory Compliance and Environmental Statutes...

5.2 Public and Agency Coordination6.0 IMPLEMENTATION PLAN AND SCHEDULE..

6. 1 Finalizing Remaining Implementation Details

6.2 Tasks and Responsibilities ....

6.3 Acceptance Criteria Plan

6.4 Schedule.....

7.0 CONCLUSIONS AND RECOMMENDATION ..............

7.1 Conclusion ..........

7.2 Recommendation

Appendix A- Geotechnical lnvestigation of Silver Creek Dam

Appendix B- Baseline Cost Estimate for Silver Creek Dam Early Warning System

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l.O INTRODUCTION

1.1 PurposePotential flood problems associated with the Silver Creek Dam, and opportunities to minimize the

consequences with the implementation of a flood warning system are explored in this Detailed

Project Report (DPR). This study is a feasibility level decision document, prepared using the

current guidance contained in Engineering Regulation (ER) I105-2-100, Planning GuidanceNotebook and current cost sharing requirements cited in the Water Resources Development Act

of 1986, as amended.

The specific purpose of this study is to identifr a project that will reduce the risk of loss of lifeand flood damage in the City of Silverton while minimizing or avoiding environmental and

cultural impacts.

1.2 AuthorityThis report was prepared under authority of Section 205 of the Flood Control Act of 1948, as

amended. The Northwestern Division, Corps of Engineers granted specific authority to conduct

this analysis through correspondence dated 8 August 2003.

1.3 Study SponsorshipIn a letter dated March 6,2002, the City of Silverton requested a study under the Section 205

Continuing Authority to evaluate the viability of a flood warning system for Silver Creek, and

acknowledged their financial obligations after the first $100,000 study cost and requirements forimplementation of a solution.

1.4 Project LocationSilver Creek Dam is located on Silver Creek abotfi2 miles upstream of the City of Silverton, in

Marion County, Oregon (figure 1). Silver Creek meanders through the City of Silverton and the

potential flood zone encompasses the majority of the city.

1.5 Previous StudiesA substantial amount of information is available in work previously completed by others for the

Cþ of Silverton, including geotechnical, hydrologic, and system cost studies. Following is a list

of the most relevant studies used in this evaluation.

o Supplemental Information, Silver Creek Dam (Silverton, Oregon), Prepared by U.S.

Army Corps of Engineers, July 2003.

o Silver Creek Dam Early Warning System Preliminary Design Report, Prepared for Cityof Silverton, Oregon by Squier Associates, April2002.

¡ Silver Creek Dam Break Analysis Final Report, Prepared for City of Silverton, Oregon

by Philip Williams & Associates, Ltd, January 18,2000.

¡ Phase I Inspection Report, Prepared by Oregon Water Resources Department in

cooperation with the U.S. Army Corps of Engineers, June 1981.

o Seismic Stability Analysis, Silver Creek Dam, Prepared for City of Silverton by

Cornforth Consultants, July 21, 1999.

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Insert Figure I

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2.0 STUDY AREA CHARACTERISTICS

2.1 Socioeconomic CharacteristicsThe City of Silverton home page indicates that the history of Silverton began with the first settlers

who came to the banks of Silver Creek in the 1800s. In 1846, a sawmill and small settlementcalled Milford was established. By 1854, Milford was abandoned and the businesses moved

downstream to the current site of the city of Silverton. Silverton was incorporated in 1885. By

1894, the population was about 900. By 1921, Silverton industries were producing goods. The

Fischer Flour Mills on South Water Street was flourishing. Power for the mill was obtained by

damming Silver Creek at a point near the present pool, diverting water into a millrace that ran

along the creek to the mill and then dumped back into the creek. A short way downstream from

the Fischer mill, the creek was dammed again to furnish power for a sash and door plant. Timber

drove local industry, and the Silver Falls Timber Co. was once the largest sawmill of its kind in

the world. Metal piping was also part of the economy. Today, those kinds of industries have

been replaced with others. The newest major industry for the area is Champion Homes, a

manufactured home plant. The Oregon Garden attracts tourism. Silverton's major employers

include the Silver Falls School District (more than 400), Silverton Hospital (over 400), ChampionHomes (more than 200), Brucepac, a meat packing plant (more than 100), and Mallorie's Dairy

(e0).

The Oregon Employment Department's"2}l2 Regional Economic Profile" shows 2000 Census

data for population: Marion County had a population just under 285,000; Salem, the largest city

in Marion County, had a population just under 137,000; and, Silverton had a population of 7,414.

Thereportalsoshowed a7999 percapitaincomefigureof $23,828forMarionCounty. The

principal industries in Marion County are government, agriculture, food processing, wood

products, retail trade, education and tourism.

2.2 Basin Conditions

2.2.1 Climatology and HydrologyThe study area has a temperate maritime climate dominated by airflow from the Pacific Ocean.

Warm, dry summers and mild, wet winters are typical. Daily average temperature ranges from 40

degree Fahrenheit. Average temperature in January is 40 degrees Fahrenheit, and in July is 66

degrees Fahrenheit. Annual average precipitation is about 50 inches over the basin with almost

70 percent of that falling between November and March, inclusive. Most major floods in

Silverton occur as a result of general winter storms during December, January and February.

These are caused by heavy rainfall from storms off the North Pacific falling on sno\ry in the Silver

Creek Basin. Recent large flood events have occurred in 1964, 1972,1976 and 1996.

2.2.2B,asir'Silver Creek is located in the mid-Willamette Valley. It runs generally SE to NW, draining offthe lower western slope of the Cascade Mountains. It is a tributary to the Pudding River about 3

miles downstream of Silverton, Oregon. Drainage area at the Silver Creek Dam is about 45.6

square miles. Silver Creek has a drainage basin made up of forested foothills of the Cascade

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Mountains, which has maximum elevations of about 4000 feet in the basin. Silver Creek has cut

a relatively deep channel through most of the community of Silver Creek. The basin is fairly

naffow with channel slopes upstream of Silver Lake Dame in excess of 100 feet per mile.

The upper drainage basin is forested and includes portions of Silver Lake State Park and Santiam

State Forest. Some logging still occurs. Debris in the form of logs and felled timber enters the

lake during large winter storms and some partial blockage of the spillway has occured. Theseare typically a single large tree and have not caused major problems in the past. Based on

historical operation and maintenance records, debris has not and will not likely become a

significant emergency problem at the project.

2.3 Current DamThe Silver Creek Dam is located in Silverton, Oregon. Silverton is located about 10 miles

northeast of Salem, Oregon, along Silver Creek. The dam was constructed in 1974by the City of

Silverton for municipal water supply and recreation. It is owned and maintained by the City of

Silverton.

The dam is a zoned earth and rockfill embankment with a maximum height above the original

ground surface of 65 feet. Embankment slopes are 2H:lV downstream and 3H:1V upstream.Crest width is 20 feet, and crest length 680 feet, including the spillway. The spillway is located

on the right abutment and consists of a converging concrete chute with an entrance width of 120

feet. The dam stores approximately 1300 acre-feet of water and is approximately 2 miles

upstream from the Silverton downtown area.

2.4 Existing ConditionIn June 1981, the U.S. Army Corps of Engineers and the Oregon Water Resources Depaftment

completed a Phase 1 Inspection Report. It identified the Silver Creek Dam as a high hazatd dam

because of the potential loss of life risk and the level of potential property damage. The

inspection evaluated abutment and foundation conditions, embankment stability, hydraulic and

hydrologic conditions, and structural/mechanical features. The inspection found the dam to be in

satisfaetory condition for continued operation.

Following a 1993 earthquake, preliminary inquiries were initiated concerning the seismic stabilþ

of the dam, and the potential for loss of life and property in the event of a failure of the 65-foot

structure. The City of Silverton contracted with Cornforth Consultants to prepare a Seismic

Stability Analysis for the Silver Creek Dam. The 1999 report concluded that it is unlikely that an

earthquake would result in failure of the dam.

Then, in 2000, the City of Silverton contracted with Philip Williams & Associates (PWA) to

perform a Dam Break Analysis, based on two dam failure scenarios: piping and overtopping.

The study indicates that if a failure due to piping or overtopping were to occur, the failure would

be catastrophic to the City of Silverton. The report notes, "Piping failure occurs if water migrates

through the dam material and develops a passage. This could be due to inadequate compactionduring construction of the dam, or to changes to dam integrity caused by seismic activity, slope

failure or vegetation. As water flows (pipes) through the dam material, it continues to carry away

more material and the passage grows in size. Eventually the size of the passage compromises the

structural integrity of the dam and causes collapse of the structure itself." An overtopping failure

is also described generically in the PIWA report. "Over-topping failure occurs when sustained

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reservoir inflow is greater than the combined spillway discharge and reseryoir storage capacity

Eventually the water surface elevation in the reservoir rises above the dam crest, causing flow

down the face. Flow over the downstream face of the dam causes erosion. Eventually, the

erosion compromises the structural integrity and a breach develops." An overtopping failure

scenario was not used as justification for commencement of this study

2.5 Review of Existing Dam Safety Monitoring ProgramThe existing dam safety monitoring program consists of collecting and evaluating water level

readings on an annual basis from piezometers installed in the embankment and the right

abutment. Flow measurements are also collected from the horizontal drains and the 4-inch

diameter perforated pipe drains along the toe of the drainage berm. During this annual inspection

visual observations of the total flow and presence of any turbidity in the water from the

downstream contact along the left abutment are also made. The results of the measurements and

observations are documented in a memorandum that is sent to the Oregon Water Resources

Department (OWRD), Dam Safety Division.

2.5.1 Piezometers

The water levels in piezometers are read on an annual basis. Typically, readings are made duringthe month of June after the reservoir has refilled and held full for several months which allows for

seepage and ground water conditions to equilibrate with full pool condition. Generally, this is

when highest ground water condition occurs. Water level readings are obtained by manually

sounding the standpipe using an electronic probe. The probe is lowered into the standpipe until an

audible signal indicates that the probe has been submerged in water. The depth to the water from

the top of the standpipe is then recorded. The elevation of the water level is calculated by

subtracting the depth to water from the elevation of the top of the standpipe.

2.5.2 Flow Measurements from Drains

Flow measurements are collected from horizontal drains, and from the 4-inch diameter perforated

drain pipes that run along the toe of the drainage berm. Measurements are taken using the timed

bucket method. The amount of time that it takes to fill up a bucket of known volume is recordedand used to calculate the flow rate.

2.5.3 Survey Points

Four settlement monuments are located on the crest of the dam. The settlement monuments

consist of a l-inch diameter steel rod set in concrete. Survey monuments consisting of a bronze

disc set in concrete are located on the left and right abutments. This network of survey points is

used to monitor for settlement and horizontal offset of the dam crest. Both concrete and survey

momuments are set a minimum of 18-inches below the ground surface to provide stability against

frost heave and effects of wetting and drying of the soils.

2.5.4 Reservoir Level

The reservoir level is recorded during the inspection for comparison with the instrumentation data

and seepage observations. This is accomplished using a staff gauge that is located on the spillway

training wall, Inspection typically is made during the month of June each year after the reservoir

has refilled to full pool and held for several months.

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2.5.5 Visual Observations

Visual observations are made of the general condition of the dam during the inspections. A visual

inspection checkoff list is used to maintain consistency over time. The list includes observations

of erosion on both the upstream and downstream side of the embankment, the presence of woody

vegetation, burrowing animals and wet areas that may indicate seepage, and the flow at weirs and

presences of turbidity in the flow.

Specific observations have also been documented regarding seepage that is occurring along the

downstream contact of the left abutment. These observations have included estimates of the flow

rate, the extent of the seepage area, and the clarity of the flow.

2.5 .6 D ata Evaluation/Management

The instrument readings are recorded daily and presented as time history plots. The results are

compared to reservoir level changes and evaluated regarding increasing or decreasing trends in

the seepage performance of the dam. The instrument plots and visual observations are then

documented in the memorandum that is sent to the OWRD, Dam Safety Division.

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3.0 PLAN FORMULATION

3.1 Problems and OpportunitiesThe "silver Creek Dam Break Analysis" report prepared by Philip Williams & Associates in

2000 concluded that an overtopping failure or piping failure would be catastrophic to the City of

Silverton. If either were to occur, there is a potential for both significant loss of life and propertydamage.

Providing a flood warning system would increase the amount of time residents have to evacuate,

thereby reducing the risk of loss of life and reducing some portion of damage to property and

vehicles.

3.2 Planning ObjectivesThe planning objective is to contribute to the National Economic Development (NED) in a way

consistent with protecting the Nation's environment. NED features are those that increase the net

value of goods and services provided to the economy of the United States as a whole. Only

benefits contributing to the NED may be claimed for economic justification of the project. The

specific objective is to reduce the risk of flood damage to Silverton while minimizingor avoidingenvironmental and cultural impacts to the area.

While reducing the risk of life is not a NED planning objective it is an important component. The

City of Silverton is situated along Silver Creek two miles downstream of the dam. The results ofdam failure would be catastrophic for the City of Silvefion. In the event the dam failed

significant portions of the valley would be inundated putting thousands of people in harms way.

3.3 Evaluation of Conditions before Project ImprovementsThree potential without project scenarios were considered.

3.3. I Overtopping Scenario

The "silver Creek Dam Break Analysis" prepared by Philip Williams & Associates in 2000defined the overtopping scenario. "Over-topping failure occurs when sustained reservoir inflow

is greater than the combined spillway discharge and reservoir storage capacity. Eventually the

water surface elevation in the reservoir rises above the dam crest, causing flow down the face.

Flow over the downstream face of the dam causes erosion. Eventually, the erosion compromises

the structural integrity and a breach develops."

The report indicates that the water surface elevation (WSE) in the reservoir associated with the

overtopping scenario is taken just below the crest of the dam (at 439.3 ft). In order for the WSE

to reach this elevation it assumed there is a debris blockage of the spillway.

Conceptual flaws in the logic of the debris blockage scenario became apparent with further

evaluation. The capacity of the spillway is about 22,400 cubic feet per second (cfs). At one-footbelow the top of the dam the capacþ of the spillway is on the order of 18,500 cfs. This is a

capacity of 2-ll2 times the 0.2 percent annual probability peak discharge so even very infrequent

flood flows would safely pass through the dam spillway. Dam failure due to overtopping would

require a blockage of 60 to 75 percent of the spillway capacity. Based on historical operation and

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maintenance records, debris has not and will not likely become a problem at this project.

Therefore, improvements to the existing structure are not required based on the capacity to pass a

large flood event.

3 .3 .2 Piping Failure Scenario

The Philip Williams and Associates 2000 Dam Break analysis identified potential consequences

of a piping failure, but there was no analysis indicating that such a failure is likely to occur. Theirpiping failure analysis assumed high water. Such high water conditions would require a blockage

of the spillway and this has been determined to be highly unlikely.

3.3.3 Seepage Failure Scenario

Silver Creek Dam was designed and constructed in accordance with the state of the industry at the

time, and is considered a safe dam, but all dams have some risk of failure.

The dam was designed and constructed in the early 1970s in accordance with the state of the

industry atthattime. The structure failed to perform in an acceptable manner due to seepage

problems affecting the right side of the dam and the right abutment. Mitigative actions were

implemented which appear to have successfully corrected the problem. The structure has

performed in a satisfactory manner since, and has been deemed to be safe for continued operationwith no restrictions.

Historically 2.0 percent of embankment dams experience failure or incidents of unsatisfactory

behavior due to internal erosion or piping (Fell et aL.2003). Seepage occurs in all dams, and is

not a problem unless the seepage is able to move material in the structure causing it to be less

able to resist additional seepage, normally referred to as internal erosion or piping. Seepage in

dams can over time develop conditions that will result in higher volumes of flow and significant

internal erosion of the structure, abutments, or foundation. This erosion, if not detected, willnormally result in failure of the structure. In most earthen dams seepage is controlled by the

interaction of an impervious zone, upstream and downstream filters, and the material making up

the mass of the dam. Seepage problems result when there is an incompatibility of embankment

materials or an incompatibility of the embankment materials and the foundation or abutments.There is usually some attempt made to ensure the abutments and foundation of the structure are

compatible with the structure and are themselves not prone to seepage failure. In some cases

embankment material can be eroded into a more porous foundation or abutment, resulting in

eventual failure of the structure, but with no obvious downstream seepage. An example being the

failure of the right abutment of Canyon Meadows Dam, Grant County, Oregon, where

embankment material'was over time eroded into a more pervious right abutment landslide

deposit. This condition eventually lead to the inability of the structure to store water and would

have resulted in a catastrophic failure of the dam had it not been for the nature of the downstream

shell of the dam.

In the case of Silver Creek Dam, the right abutment is composed of landslide material. The

formation is relatively porous in nature, and probably prone to seepage problems, based uponobservation made in the two borings made during foundation investigations. In an attempt to

prevent excessive abutment seepage an upstream impervious blanket was placed on the abutment

from the dam for some distance upstream. The purpose of the blanket was to increase the length

ofthe seepage path so that seepage pressures would be reduced to an acceptable level.

Conditions in the abutment were not as assumed or the upstream blanket was compromised in

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some manner resulting in excessive seepage during the f,rst filling of the dam. The seepage was

believed to be coming through the abutment and into the downstream shell of the dam,

threatening the stability of the structure. The pool was drawn down, horizontal drains were

installed into the abutment, and additional material was added to the downstream slope of the

dam at the abutment. These actions appear to have been successful in controlling the seepage at

the present time. However, some potential for unsatisfactory performance in the future remains.

Determination of the Probability of Unsatisfoctory Perþrmance: The probability ofunsatisfactory performance due to seepage was evaluated using a procedure developed by MA

Foster, R. Fell, and M. Spannagle (1998) "Analysis of Embankment Dam Incidents," The

University of New South Wales, UNIGIV Report No. R-374. This procedure evaluates the

structure against the performance of other similar embankment dams, is intended only to

determine the probability of unsatisfactory performance, and does not determine a factor of safety

for seepage related problems. A spreadsheet was developed based upon this procedure. The

formulas and selected weighting factors and the procedure and weighting factor selection criteria

are shown in Appendix A. The computed annual probability of failure due to seepage and piping

is 0.0032.

Determination of Rate of Failure: The time required for failure of the structure is that amount oftime from initiation of piping to the loss of pool. The value was determined using the procedure

presented by Fell, Wan, Cyganiewicz, and Foster (April 2003) "Time for Development of Internal

Erosion and Piping in Embankment Dams" Journal of Geotechnical and Geoenvironmental

Engineering,Yol. 729,Issue 4. As with the above process to estimate probability ofunsatisfactory performance this procedure generates a rate offailure based upon the historical

performance of similar structures. Based upon this procedure it is probable that the time between

initiation of a piping failure and actual failure will be between 12 and 24 hours.

3.4 Seepage Failure Scenario before Project Improvements

3.4.1 Overview

Although all three failure scenarios were found to be highly unlikely, the seepage failure scenariowas determined to be the most likely and therefore carried forward in this analysis. The Silver

Creek Dam Break Analysis report (PWA 2000) indicates that a flood wave would travel down the

Silver Creek channel reaching the downtown area within 15 minutes of the failure with flood

wave heights in excess of 10 feet in some areas.

If failure were to occur, there is a potential for both significant loss of life and property damage.

The potential for loss of life is high because of the proximity of the Silver Creek Dam to the

population in the city of Silverton. While loss of life is not considered part of the economic

justification for a flood warning system, it is a key issue. The Silver Creek Dam sets just 2 miles

upstream of a highly populated area in the city. The Dam Break Analysis indicates that in the

existing condition, water could travel to the downtown area within 15 minutes and that the

floodwave would progress through the study area in approximately I hour. The average wavespeed through the 4.12-mile study area is approximately 6 feet per second (fps). The depth ofwater would vary, but would range between 6.0 feet and 15.8 feet within the area impacted by the

1 percent annual flood event. With the depth, velocity, and short timeframe, several lives could

be lost.

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3.4.2Property Damages

A significant number of properties lie within the flood inundation boundary. Therefore, there is a

corresponding potential for significant property losses ifthe dam failure event occurs. Structural

damages to residential and commercial properties cannot be prevented by a flood warning system.

However, some portion of damages to contents and vehicles can be prevented. Consequently,

only those categories are evaluated.

Residential Properties;Average home cost in Silverton is $203,956 (source: Coldwell Banker)'

Assuming thatTl3 of the value is for land, then remaining average structure value is about

$135,000. EGM 0l-03 for generic depth-damage relationships shows content damages as a

percentage of structure value. Housing stock is split between 1-story no basement and 2-story no

basement structures. Percentage damages are38.4o/o and32.0Yo, respectively. As an average, use

35%ofor the average l0-foot floodwater depth. There are over 800 residential properties in the

zone of dam failure inundation.

Based on the seepage evaluation, an event has a frequency of I in3l2 years. It is estimated such

an event would result in over $37 million dollars in residential content damages, and equates to

$173,900 in average annual damages.

Non-Residential Properfies.' Assume a conservative structure value of $200,000 for non-

residential properties. There are a few fairly large properties, as well as many small commercial

properties. There are two schools and a care home in the inundation zone. There are over 100

non-residential properties in the zone of dam failure inundation. For this estimate, the 1995 FIAcommercial contents depth-damage relationship will be used. At a depth of 10 feet, the

percentage of content damages is 59.98% (round to 60%). For this estimate, contents are

assumed to be half the value of the structure or $100,000.

Based on the seepage evaluation, an event has a frequency of I in3l2 years. It is estimated such

an event would result in $6 million dollars in non-residential content damages, and equates to

$27,600 in average annual damages.

$13s,000 135,000tructure Value

3s% 0.3sontent Damase%o

s47,250ubtotal:80000umber of properties

$37.800,000otal Content Damage:

$100,000 $100,000ontents Value0.60%ontent DamageYo

$60,000ubtotal

100 100umber of properties

$6.000.000otal

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Vehicles: It is assumed there are L5 vehicles per residence. The Institute for Water Resources

(IWR) funded a post flood survey in Salem, Oregon following the 1996 flood, with over 800

residences in the sample, and it showed that there were 1.5 vehicles per home in the flooded area

(IWR 1997). With over 800 residences in the inundation footprint, it is assumed there are 1200

vehicles in the footprint. While more vehicles may happen to be at non-residential properties

during an event, they were not accounted for so as to avoid double-counting. Also, some

residents may be gone to work outside the footprint during an event. To be conservative, it willbe assumed that 80 percent of the vehicles will be in the footprint during an event in the night and

evening hours, while 50 percent will be in the footprint during work hours. Weighting the two

equally, a conservative approach, results in about 65 percent ofthe 1200 vehicles having a

probability of being in the footprint during an event. Sixty-five percent of 1200 vehicles is 780

vehicles. With an average water depth of l0 feet, assume the loss of the vehicles. Assume an

average value of$7,500 per vehicle, for 780 vehicles, or $5,850,000.

Based on the seepage evaluation, an event has a frequency of I in3I2 years. It is estimated such

an event would result in almost $6 million dollars in vehicle damages, and equates to $26,900 in

average annual damages.

3.5 Alternative AnalysisTypically, in addition to the without project condition (no action plan), both structural and non-

structural alternatives would be considered. However, there are no reasonable structural

improvements to consider and the Sponsor is not interested in pursuing structural alternatives.

One non-structural alternative to be considered is a flood warning system. Given the short

warning time the only feasible approach is to establish a flood warning system that will: 1. Detect

a developing condition and2. Provide time to initiate the notification/evacuation of people based

on a "failure is imminent" condition.

3.5.1 Flood Waming SystemA flood warning system that provides sufficient time for residents to evacuate in the event of a

dam failure will reduce or eliminate the loss of life and a portion of the property damages(typically contents and vehicles).

The proposed early warning system consists of the detection system and the notificationsystem/evacuation plan. The detection system is to identif, a developing condition of concern in

advance of failure. The notification system is used to communicate to all inhabitants in the

inundation area of Silverton. The evacuation plan outlines how people are to exit the inundation

zone.

The detection component includes improvements to the current monitoring equipment and

installation of new equipment. Design components include reservoir level monitoring

instruments, piezometers with vibrating wire pressure transducers to detect changes in seepage

performance, weir box instruments to collect and measure seepage, and installing an onsitemonitoring station.

The notification system/evacuation plan will be a combination of methods to insure evacuation ofthe inundation zone. The proposed system consists of a siren network and a personal notification

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component. In addition, certain procedures and policies such as notification flow charts, on-

going testing, maintenance and public education will be required.

3.5.2 Flood Warning System Costs

The cost estimate for installation of a flood warning system at Silverton is $616,100 plus $5,000

peryear in operation and maintenance expense. Average annual costs are $37,000 based on the

discount rate of 5.625 percent and 50 year project life plus $5,000 peryear in operation andmaintenance expense. Refer to the Recommended Plan Section and Baseline Cost Estimate

Appendix for additional details.

3.6 Potential Reduction to Property l)amages with Flood'Warning TimeThe Day Curve estimates potential reductions in content damages, given additional flood warning

time. (See the "National Economic Development Procedures Manual-Urban Flood Damage,"

IWR Report 88-R-2, dated March 19S8). Results are summarized in the following two tables,

based on the potential additional time provided by a flood warning system. The estimates based

on engineering judgment indicate potential flood warning time ranging from 6 to 18 hours.

Potential Reduction in Residential and Non-Residential Content

EXPECTED REDUCTION IN

AVERAGE ANNUAL CONTENTDAMAGES

ADDITIONALTIME

(HOURS)

PERCENT

REDUCTIONIN DAMAGES

13.5 s27,200

t8 26 $52,400

Potential Reduction in Vehicle

The following table is a summary of benefits due to the implementation of a Flood Warning

System depending on the additional time to evacuate the inundation zone.

ADDITIONALTIME

(HOURS)

PERCENT

REDUCTION INDAMAGES

EXPECTED REDUCTION INAVERAGE ANNUAL VEHICLE

DAMAGES

$21,s0080

s24,2008 90

15



6

l8

of Benefits Due to Added Flood



4.0 RECOMMENDED PLAN

4.1 OverviewThe early warning system itself consists of two subsystems that have individual performance

objectives but are fully integrated into the overall system. These include the detection system and

the notification system. A third and critical component of a flood-warning program is the

evacuation plan. The evacuation plan identifres specific evacuation zones within the inundation

area andthe routes for the movement of the evacuees out of the inundation areas.

The primary objective of the detection system is to identifu a developing condition of concern tn

advance of failure to allow time for making a decision regarding evacuation. The detection

system has a secondary purpose of providing information to assist in the decision making process

during a developing condition of concern. The notification system is used to communicate the

need to evacuate once the decision that failure is imminent has been made. The objective of the

Silver Creek Dam notification system design is to provide notification to all inhabitants in the

downstream inundation area within the city limits of Silverton, so that they may be evacuated.

The following sections describe the recommended design for the detection and notificationsystem. The described system is similar to what was presented in the preliminary design report

prepared for the City of Silverton by Squier Associates (April 2003).

4.2Detection System

The recommended detection system consists of improving the monitoring capability by

enhancing existing instruments and adding new instruments installed at various locations on the

dam. The improvements will include:

4.2. 1 Reservoir MonitoringInstall a reservoir level monitoring instrument that includes the use of a vibrating wire pressure

transducer to monitor the reservoir water level, and detect a rapidly rising/dropping reservoir

level condition. Redundancy will be provided by a reservoir staff gauge that can be read manually

or by remote video camera. Consideration will be made of using a second pressure transducer to

provide redundancy.

4.2.2 Upgrading Piezometers

Outfit the existing piezometers with vibrating wire pressure transducers to detect changes in the

seepage performance of the dam and abutments.

4.2.3 Wefu Box Instrumentation

Install new weir box instruments to collect and measure seepage at the toe of the dam, the contact

with the left abutment, and from the horizontal drains. The weirs will be used to monitor changes

in the seepage performance of the dam.

4.2.4 Monitoring Station

Install a monitoring station at the dam to provide a base station for on-site monitoring during an

alarm condition.

1,7



4.2.5 Reservoir GaugeA new Reservoir Level Site Gauge will be installed to provide a back-up point of reference forvisual monitoring during high flow condition. A remote controlled camera will be installed to

visually observe the reservoir staffgauge and to observe the critical outlet controls for securitypurposes.

4.2.6 Measurement Control UnitsAll of the automated sensors would be connected to Measurement Control Units (MCU) that

would collect the data from the sensors and compare the readings to predetermined threshold

values. If a threshold value is exceeded and verified by redundant instrument values, then the

MCU network will initiate a phone call to the assigned city personnel to alert of a developingcondition of concern.

4.2.7 Additional Detection System DetailsA more detailed discussion of the individual system components follows

Automated Data Acquisition and Alarm Notffication: The MCU located at the dam will collectthe data from the instruments and compare the data to threshold values. This MCU will also

collect and store readings on a daily basis from the other MCU's for use in long termperformance and trending evaluations. Each MCU will be programmed with logic such that it willindependently poll its sensor's and compare the readings to predetermined threshold levels. This

architecture strengthens the integrity of the system by reducing the risk of the entire system going

down due to an equipment failure of one component.

Resemoir Level Monitoring: An automated sensor will be installed to monitor the reservoir level.

This instrument will be located on the upstream face of the dam and will be installed within a

PVC pipe buried on the face. The sensor will be located at approximately elevation 400 feet. The

instrument will be monitored hourly and compared to the threshold levels. If the reservoir levelrises to within 8 feet of the crest, then the system will activate the callout procedure to Citypersonnel to indicate a condition of concern. The reservoir level monitoring will also be used

during an alarm condition to keep track of the reservoir level and rate of rise. During normaloperation, daily readings will be stored for use in evaluating the historical performance of the

piezometers and weirs. A manually read site gauge will also be installed to provide a visual

confirmation of the reservoir level during a flooding condition. This gauge will be constructed in

the vicinity of the right abutment at an elevation of 430 feet so it can be easily observed during a

developing unstable condition.

Piezometer Level Monitoring: Ín order to improve the ability to detect changes in the seepage

performance of the dam and right abutment, automated sensors will be added to the existingpiezometers. These sensors will be monitored hourly and compared to predetermined threshold

levels. If a threshold level is exceeded, the system will activate the callout procedure to Citypersonnel to indicate a developing condition of concern. In addition to the hourly readings, daily

readings will be stored for historical evaluation purposes.

Seepage Flow Monitoring: Excessive seepage through the dam or at the contact with the

abutments could lead to a developing unstable condition. To monitor for this condition, weirboxes will be installed. The weir boxes will measure the flow rates from the horizontal drains and

4-inch diameter toe drains that are currently installed in the dam. One of the weir boxes will be

18



used to collect and measure seepage from the left abutment contact area. Flow measurement will

be collected hourly and compared to predetermined threshold values. If a threshold level is

exceeded, then the system will activate the callout procedure to City personnel to indicate a

developing condition of concern. In addition to the hourly readings, daily readings will be stored

for historical evaluation purposes.

Data Evaluation/Management: Data will be recorded daily for historical evaluation purposes. Thepurpose of collecting and evaluating this performance data on a regular basis is to identiff

developing conditions ofconcern through observation of increasing or decreasing trends in the

dam's long-term performance. This regular monitoring is a key part of developing an

understanding of how the dam normally performs so that changes in performance can be detected

and properly evaluated regarding the on-going safety of the dam. Because of the quantity of data

being collected and the long term monitoring objective of the system, a database application

needs to be integrated with the system. The database application will be installed on a personal

computer in the City's ofhce and will be used to manage and evaluate the data. The database willalso aid in dam-safety reporting tasks.

On-Site Monitoring Station: In addition to the automated instruments, an On-site Monitoring

Station will be constructed on the right abutment above the fish ladder. The On-site MonitoringStation will be used during alarm conditions to monitor the on-going instrument readings and to

make visual observations of the dam structure. The structure will be a prefabricated building that

will house the MCU station. In addition, the building will be equipped with floodlights for use in

observing the condition ofthe dam at night and a telephone that can be used to call out as part ofthe notification callout procedures. A permanently installed generator within the station building

will power the floodlights in the event of a power failure. The building will also have a laddèr

installed to provide access to the roof of the building to provide a better vantage point for

observing the dam.

4.3 Notification System

The Notification System is designed to provide evacuation notification to the people inhabiting

the flood inundation area (Figure I shows inundation area). This includes inhabitants withdisabilities such as the hearing-impaired and the blind. In addition to the people inhabiting the

flood inundation area, certain emergency response personnel must be notified in a timely fashion

to assure proper and orderly execution of the emergency response plan. The boundaries of the

flood inundation area are based on the failure scenario as presented in the Silver Creek Dam

Break Analysis Report, (PWA, January 18, 2000). The evacuation notice would be issued as a

result of an "imminent dam failure condition". In order to properly discern a dam failure

notification from other types of disasters, the Notification System would also include instructions

to the public as to the nature of the evacuation.

4.3. I Alternative Notihcation Options

Various notification altematives with different strengths and weaknesses were evaluated

TV or Radio Broadcast Notification: This type of notification is based on using television or radio

station broadcasts to provide flood notification to the public. In an emergency, special messages

would be broadcasted to the public.

Strengths:

t9



. Instant communication to all affected people.

. Gives detailed information and can keep people up-to-date.

. Generally available (most people have TV's).

Weaknesses:. Emergency management control is limited (involvement by broadcast stations is

voluntary).. Only applicable to local cable and antenna broadcast stations (satellite or non-localprogramming would not carry the broadcast).. Limited usefulness during most times of day or when TV/radio is turned off.. Limited usefulness for people outside of the house at time of emergency.. Not a selective broadcast audience so emergency message will carry to all people

receiving TV/radio signal.. Not available during po\À/er outages.. Not able to reach sight or hearing-impaired citizens unless specially equipped withTDD/TDY equipment.

This type of notification is typically used for school closures and other types of warning or news

information affecting alarge population area with notifications that do not require immediateaction. Therefore, using this type of notification for the early warning system is not feasible.

Audible Sirens: Audible sirens provide immediate notification by broadcasting a tone consistingof a siren wail and a voice message. Audible sirens typically consist of equipment mounted on a

utility pole and can include options to broadcast prerecorded voice messages to provideinstructions during an evacuation. Most sirens are powered by AC povver with a battery backupsystem in case of power outages and can be controlled via a direct telephone type connection or

individually via radio signal from a central Siren Control Unit. Audible sirens are available inboth a directional signal of varying degrees and an omni- directional signal broadcasting in a 360o

circle. Both types of sirens come in models that can broadcast varying distances up toapproximately 5,000 feet. In order to assure appropriate sound quality, the audible sirens are

driven by speaker amplifiers similar to those used in public address systems or large musicconcerts.

Strengths:. Instant communication to all affected people.. Can give detailed informatión as to what action to take.. Easily maintained and City owned.. Flexible and expandable for future emergency action plans.. Available during both phone and electric outages.

Weaknesses:. Limited usefulness for people inside sound dampening facilities/buildings at time of

emergency, or for the hearing-impaired.. Not a selective broadcast audience so emergency message will carry to all people withinsound range.

An additional method of siren notification is via sirens or warning lights installed within special

structures or facilities such as police departments, fire stations, hospitals, etc. Buzzers, sirens,

20



and/or waming lights can be installed within the structures or buildings housing these facilities to

provide notification to the occupants or special personnel within these facilities.

Notification Via Automated Telephone Service: The use of automated telephone services is

another type of notification system available. Automated telephone notification is a service

provided by an outside (non-City) service provider that uses a computer to phone the affected

population, delivering a warning message or evacuation notice. If the line is busy or does notanswer the computer keeps dialing the number. If an answering machine answers the phone then

the notification message is recorded. To initiate this type of notification system, a phone call is

placed to the telephone service provider. Once security authentication is completed, the service

provider starts a call-out operation to notifu the population of the need to evacuate. Most service

providers operate with the ability to place 200 calls simultaneously, with the average callout

capacity of approximately 3000 calls in one hour (based on a 2O-second notification message).

Typically, these service providers charge an initial "setup fee", an annual "subscriber fee", and a

per call charge for each number called during the emergency.

Strengths:. Targeted, rapid notification to only those affected or "on the phone call lisf'

. Can give detailed information as to what action to take.

. Relatively low cost.

Weaknesses:. Only effective if people have a phone, and it is turned on, it is not in use, and they

answer the call.. Cannot reach people if they are not within reach of the phone (outdoors).

. Not available during telephone outages.

. Not effective for hearing-impaired people unless TDD/TDY equipment is used.

Mobile Loudspeakers.' Another type of notification method is the use of loudspeakers mounted in

vehicles. Police or fire crews typically carry out this notification method by driving a vehicle witha loudspeaker through the effected area broadcasting a live or recorded message to the public.

Strengths:. Targeted notification to only those affected.. High degree of credibility to audience.

Weaknesses:. Limited information can be delivered.. Only a limited area can be notified quickly.. Cannot reach hearing-impaired people.. Messages can be hard to decipher if people are indoors.

. Risks exposure to personnel delivering message.. Ijses equipment and personnel that could be used more efficiently elsewhere.

Personal Notification: Waming each and every citizen personally in the aflected area is also a

method of notification. With this method, police officers, ftre crew, or City personnel would

2l



spread out and blanket the entire inundation area, delivering the evacuation notice personally to

all occupants. While this method does insure notification to everyone within the inundation zone,

it is not feasible to use it as the only notification system because of the limited amount of time

available to get the notice to the evacuees. Similar to the use of mobile loudspeakers, this method

of notification is only effective for small areas and taking the crews away from other more

essential tasks and risking the personnel and equipment going door-to-door.

Strengths:. Targeted notification to only those affected.. High degree of credibility to audience.. Very strategic notification.. Can reach all people, including those with disabilities.

Weaknesses:. Extremely time consuming. Risks exposure to personnel delivering message.

. Uses equipment and personnel that could be used more efficiently elsewhere

4. 3.2 Recommended Notification System

After examining the strengths and weakness of each of the systems, it is recommend that a

combination of methods should be used to provide evacuation notification for the early warning

system. The recommended system improvements consist of a siren network and a personal

notification component. In addition to notification sirens and personal notification, certain

procedures and polices such as notification flow charts, on-going testing, maintenance, and public

education, are recommended to assure proper operation of the notification system. When the

evacuation notice is executed, the audible siren network will be used to notiff the majority of the

population. The notification flow chart will be used as a guide to notif, certain City, police, and

fire department personnel as part of the emergency response plan. For the population within the

inundation zone with disabilities, the personal notification component will be used to assure they

receive the evacuation notification and assistance as necessary.

4.3.3 Specific Notification System Components

Audibte Sirens: To provide audible notification to the general public, four sirens will be installed.

Locations are shown on Figure l. Each of the audible sirens is controlled by a centrally located

Siren Control Unit which uses radio frequency broadcasts to control the sirens located throughout

the evacuation zones. When the evacuation notice is executed, the sirens will broadcast a wail

tone followed by a prerecorded voice message such as: "Silver Creek Dam emergency. Evacuate

the area immediately". In addition to the warning tone and message, an all clear message such as:

"It is safe to return; Silver Creek Dam is secure" will be used to indicate when the flood risk has

subsided.

Notification Flow Charts: Details of who will be notified should be evaluated and incorporatedinto the City's Emergency Action Plan as Notification Flow Charts. The Notification Flow charts

will be used as a guide to execute a call out to certain City staff and crew that are expected to

respond to the evacuation emergency action plan. Any inter-agency coordination or notification

(i.e., interagency coordinators, Marion County, downstream communities, etc.) will also be

warned via the Notification Flow Charts. The specific details will be developed as part of the

22



design and construction phases

Personal Notificationfor the People l{ith Disabilities: Another vital component of the

Notification System is a system or procedure to provide notification to the population within the

flood inundation area that have disabilities. The police department should maintain a list ofaddresses of households for people with disabilities that will be affected by the flood inundation.

This list of households will then be used during an evacuation event so that police and firepersonnel will notifr those households personally and can provide assistance as necessary. The

individual who will be responsible for assuring the households have been notified and evacuated

should be identified in the Notification Flow Charts.

Policies and Procedures: Policies and procedures should be incorporated into the City's

Emergency Action Plan to assure proper execution of an evacuation. These policies and

procedures should include:. A Notification Flow Chart.. A comprehensive public education program.. An on-going public awareness program.. An on-going interagency and interdepartmental coordination program.

. Regular scheduled maintenance of the notification equipment.

. Regular testing of the notification system and evacuation procedures.

4.4 Evacuation PlanA preliminary evacuation plan for the Silver Creek Dam Early Warning System has been

formulated. Essentially the evacuation plan shows how the inundation area is subdivided into

specific evacuation zones and identifies general routes for the movement of the evacuees out ofthe inundation areas. For this plan, it is assumed that most evacuees will use automobiles as the

method of evacuation. Some areas located in remote locations adjacent to

Silver Creek might be required to evacuate on foot if floods or other conditions have damaged

roads or private driveways to the point where they are impassible by automobile. Evacuation of

the facilities located within the inundation zone requiring special attention, such as schools, arealso discussed. Coordination ofemergency personnel is vital to the success ofan evacuation and a

brief description of that effort is provided.

4.4. I Recommended Evacuation Zones

To facilitate movement of inhabitants within the inundation zone, evacuation zones will be

identified. It has been determined that for each zone there will be more than one evacuation route

with a road or highway to be used to carry the flow of vehicular traffic. [n order to facilitate

vehicular traffic flow during the evacuation, an Evacuation Traffic Control Plan will be created

by the City and made part of the evacuation plan. Recommended road closures, vehicle traffic

counts, one-way traffic flow, and traffic direction by emergency personnel, barricades, etc. willbe considered and planned for.

4 .4 .2 Ev acuations of Spe cial Facilities/Structure s

In addition to evacuation zones, special facilities such as schools or other buildings that have a

large number of inhabitants require special coordination to assure that the occupants are

evacuated. Both Eugene Field Elementary School and Silverton Union High School are located

within the inundation zone. Each of these schools should receive the notification of the

23



"Developing Alarm" condition and should initiate procedures to prepare for evacuation before the

evacuation order is given. Occupants from the Silverton Union High School may be able to

evacuate on foot to higher ground outside of the inundation zone to the west. However, the

location of Eugene Field Elementary School precludes this alternative, and busing of the students

will most likely be required.

4.4.3 Coordination of Emergency ResponseDuring an evacuation, the coordination of City, police, fire, and medical personnel is essential.

To achieve an orchestrated response and to assure the proper personnel are notified, twonotification flow charts should be developed. The notification flow charts should be based on the

following alarm conditions :

. Unstable Condition has Developed

. Failure is Imminent or Has Occurred

The flow charts should provide a detailed process of notification for the individual alarm

conditions. The notification flow chart should detail the names, titles, and phone numbers ofthose who are responsible for notification, from the individual observer to the responsible agency

representatives. The flow chart should denote the priority or order in which each person on the

chart is notified. The Notification Flow Chart should be distributed to all key supervisory and

operational employees. The chart itself should be posted at key locations such as, but not limitedto:

. On-site Monitoring Station at the dam;

. City Hall (administrative offices);

. Police Station (dispatch);

. City Shops (maintenance facilities); and

. Fire Stations.

In addition to the notification flow charts, all personnel must be hained on the proper response

including where and how they report in and what their responsibilities are during the alarm

conditions. Each assignment must be fully understood and coordinated. The agency that willcoordinate the evacuation must be identified and an emergency chain of command must be

established in advance. The following is a brief description of some of the responsibilities thateach agency should be planning for. The final evacuation plan should include the names of the

individuals and what they will be responsible for under each alarm condition.

4.4.3.1PoliceGenerally, the police department should direct the evacuation operations. They should establish

and maintain an outer perimeter to maintain the outward flow of traffic from the inundation area.

They should provide for traffic and crowd control. They should be prepared to provide security

for any emergency housing or shelter facilities established during the evacuation.

4.4.3.2 City StaffDuring an evacuation order, City Staffshould be prepared to provide resources such as vehicles

equipment, and personnel to assist with traffic movement and crowd control. They should also beprepared to keep evacuation routes open and free ofdebris and to provide signs and barricades fortraffic control.

4.4.3.3 FireFire personnel should establish and maintain an inner perimeter of the inundation zones. They

24



should be prepared to rescue trapped victims within the inundation zones or provide assistance to

evacuees with special needs that cannot otherwise leave the area on their own. They should

provide fire stations for use as reception points for the evacuees. They should assist in the

evacuation process as requested by the Police department or incident command.

4.4.3 .4 Interagency Coordination

In addition to the coordination of the City services, interagency coordination must be establishedto aid in the evacuation process and to notifr other populated areas downstream of the City of the

Silverton. The City's Emergency Management Coordinator should compile a list of all agencies

that should be notifred when the City's evacuation order is given.

4.5 Environmental ImpactsThe early warning system will include a detection system and notification system. The detection

system will require minor ground disturbance to install sensors within the dam structure itself,

improvements and upgrades to current monitoring equipment, and the installation of a small, pre-

fabricated shed to house the monitoring station. The notification system will require the

installation of a siren network that will include four, pole-mounted sirens on existing right-of-

ways within the city of Silverton. No in-water work is planned for the construction activities.The scale of construction activities for this project is considered small'

Federally listed species that may occur in the proposed project area include bald eagle

(Haliaeetus leucocephalzs), Fender's blue butterfly (Icaricia icarioidesfenderi), golden Indian

paintbrush (Castilleja levisectc), Willamette daisy (Erigeron decumbens var. decumbens),

Howellia (Howellia aquatilis), Bradshaw's lomatium (Lomatium bradshav,ii), Kincaid's lupine

(Lupinus sulphureus var. kincaidil), Nelson's checker-mallow (sidalcea nelsoniana), steelhead

(Onc or hync hus my ki s s), and Chinook s almon (Onc ho rync hu s t s høwyt s c ha).

Based upon site surveys, literature searches, discussions with natural resource agency personnel,

and considering the scale of disturbance associated with this project, it was determined that there

will no effect to any threatened or endangered species or critical habitat.

4.6 Cultural Resources

There will be minor earth disturbance to install and upgrade sensor and monitoring equipment at

the Silver Creek Dam. The siren network will include installing four, pole-mounted sirens on

existing road right-of-ways at locations within the city of Silverton. All construction activity will

occur on or adjacent to the existing dam structure or on previously disturbed sites. The scale of

construction activities for this proposal is considered small. It is unlikely that any cultural

resources would be affected by construction activities.

4.1 GeotechnicalSilver Creek Dam is located in the foothills of the Cascade Range Physiographic Province of

Oregon near the margin of the Willamette Valley. The area is characterizedby eroded terrain ofaccordant peaks and ridges, with moderate to steep sided dendritic stream valleys. At the dam

site elevations range from 350 feet in the valley floor to over 1000 feet on the surrounding ridges.

Landforms along the creek include alluvial terraces, benches, bluffs formed by basalt flows, and

hummocky surfaces of landslide deposits. Extensive landslides have been mapped along the

north side of the valley at, and near the dam site. These landslides have developed at locations

25



where weak marine sedimentary rocks underlie thick flows of Columbia River Basalt, and at

deeply incised portions of the stream valley. The slide debris at the site is generally composed offragmented angular basalt with a fine-grained plastic silt matrix. The material is dense to very

dense. No major geologic structures have been identified at the dam site. Columbia River Basalt

is exposed in the left abutment, and alluvial deposits of sandy gravel cover the valley floor.

There are no known regional faults at the site.

4.8 Real EstateThe lands, easements and rights of way in possession of the city are adequate to fulfill project

requirements. The lands for Silver Creek Dam, where the dam monitoring instruments will be

sited, were acquired by the City of Silverton through a condemnation action in June 1974. Itislocated in the NE %, NE % of Section 12, Township 7 South, Range 1 West, Willamette

Meridian, Marion County, Oregon. Access to the dam is via boat. A boat ramp currently exists

on Silverton Reservoir only a few hundred yards away from the dam. In addition to monitoring

instrumentation, four sirens are proposed throughout the city of Silverton. Although the exact

footprint of each siren location will be determined during plans and specifications, the towers for

each siren will be on city owned property or right-of-way. The total acreage for the dam safety

monitoring structures, associated warning systems, and appurtenant equipment is estimated to be0.1 acres. A slightly modified utility easement estate has been used for LERRD crediting

purposes. The total LERRD credit for the project is considered to have a value of $4,000 ($500 x

4 tower sites * $2,000 for improvements on or near the dam). The city understands and has

agreed to the requirements established by P.L. 97-646, although no relocations are expected.

4.9 Cost EstimateEstimated costs for the implementation phase are presented in the complete Baseline Cost

Estimate shown in Appendix B. The cost estimate is based on the design as described in the

previous sections, equipment and material quotes, standard labor rates, anticipated time required

to complete the work, and on typical unit costs. Consideration was given to constructability. To

allow for uncertainties and unknowns that will remain until the design and installation details are

finalized in the next phase of work, the cost estimate includes a 15 percent contingency.

26



5.0 COMPLIANCE AND COORDINATION

5.1 Regulatory Compliance and Environmental Statutes

The proposed action does not have significant effects on the qualþ of the human environment.

Under EF.-200-2-2 (Procedures for Implementing NEPA), the proposed action is categorically

excluded. The Categorical Exclusion satisfies the requirement for meeting the National

Environmental Policy Act (NEPA) compliance procedures. An environmental assessment is not

required.

Endangered Species Act: It was determined that there would be no effect to any endangered or

threatened species or their critical habitat.

Clean Water Act: There will be no fill in any waters of the United States.

Cultural Resources: Due to the scale of proposed activities, it is unlikely that any cultural

resources would be affected by construction, operation, and maintenance activities.

5.2 Public and Agency CoordinationThe proposed action has been coordinated with resource agencies including the U.S. Fish and

Wildlife Service, NOAA-Fisheries, and the Oregon Department of Fish and Wildlife.

27



6.0 IMPLEMENTATION PLAN AND SCHEDULE

Implementation of the system improvements includes new instrumentation, the Automated Data

Acquisition System (ADAS), the data evaluation/management tool, and the notification system

and would need to be programmed and installed.

6.1 Finalizing Remaining Implementation Details

The remaining design and installation details that need to be determined in the implementation

phase consist of 1) developing project database and data management tools using Damsmart,2)

interconnecting the system into the City's Scada system, 3) developing threshold levels for the

instrumentation, 4) final siting of the siren locations, and 5) frnalizing the details of the

notification and evacuation procedures. In addition, the City is concerned about vandalism at the

dam site. The final installation details will need to consider methods for reducing the exposure of

the equipment. A detailed implementation schedule should be prepared that defines when the

components will need to be installed and operational.

6.2 Tasks and ResponsibilitiesIn general, the implementation phase will include six main tasks:

1) System construction,

2) System installation,

3) System calibration and testing,

4) Preparing Operations and Maintenance Documentation,

5) Training on the use and required maintenance of the monitoring and early warning

system, and

6) Educating the public about the system and how they should respond during an

incident.

The system construction task will consist of procuring the instrumentation, ADAS, and

notification equipment; programming the ADAS; development and programming of the database

tool; and performing bench tests of the detection and notification system components. The bench

testing is performed in a controlled environment to assure that the system components are

working properly and communicating appropriately before they are installed. Installation of the

systems would then proceed with 1 ) installing the new instrumentation, 2) installing the MCU' s,

3) installing the data evaluation database tool, 4) installing the notification sirens and 5)

performing a complete test of the system operation to demonstratethat it is functioning properly.

Operations and maintenance documentation will be prepared to provide guidance for the

operation and maintenance of the detection and notification systems. This would also include

documentation on the system configuration. The operations and maintenance documents will

serve as the main references for the training task. After the system has been successfully installed

and tested, the training task will be performed. The training will involve a discussion of the

response procedures for alarm conditions, activation ofthe notification system and how to operate

and maintain the system. The task will also include "hands on" experience for the users in

performing the typical operations that will be required for successful operation of the system,

including the education of the public. This task will be considered complete when the system has

been accepted in accordance with the plan as described in the following section.

28



6.3 Acceptance Criteria PlanThe Acceptance Criteria Plan (ACP) outlines necessary acceptance criteria for successful

completion of the implementation phase. The ACP is initially developed by the system

developers and then reviewed and commented on by the City of Silverton personnel. The

acceptance criteria plan will be used during the system training session for final acceptance of the

system. The system will be deemed successfully complete when the following items are

completed to the satisfaction of the City or otherwise resolved with the development team.

1. Demonstrated ability to detect and notiff City personnel of an exceedance of threshold

levels representing a developing condition of concern at the Dam.

2. Demonstrated ability to transfer historical instrument data from MCU to the designated

City workstation PC using a telephone modem communication link.

3. Demonstrated the abilþ to initiate a reading of the instruments from the remote

workstation PC and the on-site monitoring station using a laptop PC.

4. Demonstrated the ability to activate the evacuation notification system.

5. Demonstrated the ability to test (silent and/or audible) the evacuation notihcation

system.

6. Demonstrated the ability to load the dam monitoring instrumentation data into the

database application and generate the required time history plots for on going dam safetyevaluation.

7.The Operations and Maintenance provides the information needed to operate and

maintain the system.

8. The user training session objectives have been completed.

9. Public has been notified and educated.

6.4 SchedulePlans and Specifications Phase:

Initiate December 2004

Construction Phase:

Initiate April2005

29



7.0 CONCLUSIONS AND RECOMMENDATION

7.1 ConclusionInstallation of a flood warning system in Silverton is economically justified based on the analysis

of the benefits and costs. Assuming the more conservative estimate of 6 additional hours for

evacuation, the benefit to cost ratio is 1.2 to l 0.

Most importantly, providing a flood warning system would increase the amount of time residents

have to evacuate, thereby reducing the risk ofloss oflife.

7.2 RecommendationThe results of this study provide a design for an improved monitoring and early warning system

for the Silver Creek Dam in Silverton, Oregon. The system design is based on the need to

improve the ability to detect a developing condition of concern regarding the safety of the dam,

and to provide a system for notif,ing the downstream inhabitants of the need to evacuate under

and imminent failure condition.

It is recommended the next phase of work include finalizing the remaining design and installationdetails, and implementing the design improvements as recommended in this report.

30



LIST OF REFERENCES

Cornforth Consultants , July 21,1999, Seismic Stability Analysis, Silver Creek Dam, Prepared for

City of Silverton.

Foster, M.4., R. Fell, and M. Spannagle, 1998, "Analysis of Embankment Dam Incidents," TheUniversity of New South Wales, UNIGIV Report No. R-374.

Fell, R., Wan, Cyganiewicz, and Foster, April2003, "Time for Development of Internal Erosion

and Piping in Embankment Dams" Journal of Geotechnical and Geo-Environmental Engineering,

Yol.729,Issue 4.

Oregon Water Resources Department in cooperation with the U.S. Army Corps of Engineers,

June 1981, Phase 1 Inspection Report.

Philip Williams & Associates, Ltd, January 18, 2000, Silver Creek Dam Break Analysis Final

Report, Prepared for City of Silverton, Oregon.

Squier Associates, April2002, Silver Creek Dam Early Warning System Preliminary Design

Report, Prepared for City of Silverton, Oregon.

U.S. Army Corps of Engineers, July 2003, Supplemental Information, Silver Creek Dam

(Silverton, Oregon).

U.S. Army Corps of Engineers, Institute for Water Resources, July 1997, Unpublished

Residential, Commercial, and Vehicle Damage Data due to Flooding in Salem in 1996.

U.S. Army Corps of Engineers,Institute for rWater Resources, March 1988, "National Economic

Development Procedures Manual-Urban Flood Damage," IWR Report 88-R-2.

U.S. Army Corps of Engineers, Engineering Regulation (ER) 1105-2-100, Planning Guidance

Notebook and current cost sharing requirements cited in the Water Resources Development Act

of 1986, as amended.

U.S. Army Corps of Engineers,Institute for Water Resources, March 1988, "National Economic

Development Procedures Manual-Urban Flood Damage," IWR Report 88-R-2.

31



Appendix A

Geotechnical Investigation of Silver Creek Dam

32



Geotechnical Investigation of Silver Creek Dam

Silver Creek Dam Silverton Or.

Probability of X'ailure due to

Probability of Failure by Piping

Weighing Factors

Ws

Embankment Mode

(frlt) 0.2

(ego) 1.25

(cst) L2

(cc) 1.2

(con) 1

(ft) 1.2

(obs) 2

(mon) 2

Seepage

(From "Analysis of Embankment Dam Incidents"

Foster, Fell, and Spannagle; University ofNew South Wales,

UNIGIV ReportNo. R-374, Sep 1998.)

W¡

Foundation Mode

(filt) 1.2

(fnd) s

(crt) 0.9

(rg) 3

(obs) 2

(mon) 2

wnr'

Emb into Found

(filt) I

(cot) I

(fnd) l.s(ecm) 5

(er) 1.3

(se) s

(cog) 1.25

(cst) L2(cc) 1

(ft) 1.1

(obs) 3

(mon) 2

r.728 64.8

Average Probability of Failure for Zoned EarthfrllEmbankment Foundation

P": 0.000025 Pr: 0'000019

WE w¡n 482.625

Emb into Found

P"r: 0.000004

Probabilíty of Failure byPiping

Po: w6P" * wpP¡* \ilsFPer

Pp: 0'0032049

Inverse Po: 312.022216

ESTIMATEDFAILURE TIME - (From table 8 "Time for Development of lnternal Erosion and Piping in

Embankment Dams" Fell, Wan, Cyganiewicz, and Foster; Journal of Geotechnical

Engineering, Apr 2003)

Time from initiating pipe to development of Breach - 12 to 24Hrs.

JJ



Justiflrcation for Selection of Weighting Factors

Weighting Factors for Piping through the Embankment (Factors from Table 11.2)

Embankment Filters Ws (slt) - A factor of 0.2 was selected because the embankment was

constructed with a filter but there are no indications that a specific gradation was specified northat any test were performed on the material. It must therefore be assumed that the filter may be

poor quality.

Core Geologic Origin WE (ceot)- The core material is of colluvial origin therefore the selected

factor is 1.25

Core Soil Type W¡ i.,9 - The core material is described as plastic silts and clays therefore a factor

or 1.2 was selected.

Compaction We (".) - Available information indicates that at least a modest amount of control was

maintained during compaction, which would lead to the selection of a factor of 1.2.

Conduits Vy'e ("on) - The outlet conduit is located at the embankment foundation interface,

construction drawing indicate that the conduit \ilas constructed using practices similar to USBR

practices of the time, which allowed a factor of L0 to be selected.

Foundation Treatment Ws(n) - Drawings indicate that the foundation contained some

irregularities and steep areas, but was treated using standard practices of the time allowing a

factor of 1.2 to be selected.

Observations of Seepage WE(or.) - Seepage was initially observed on the right side of the

downstream face during initial filling. Horizontal drains were installed into the right abutment,

which to date appears to be controlling the seepage. Seepage continues to flow from the left

abutment contract but does not appear to be increasing at a detectable rate. Based upon theseconditions a factor or 2 was selected.

Monitoring and Surveillance W¡ 1.on¡ - The embankment in monitored annually, therefore a factor

of 2 was selected.

Weighting Factors for Piping through the Foundation (Factors from Tabte 11.3)

Filters W¡ (rr,) - A factor of 1.2 was selected because the design drawing show no foundation

filter.

Foundation Type Wp 1¡,q - A factor of 5 was selected because about half of the embankment is

founded on slide debris.

Cutoff Type Soil Foundation W¡1"1.¡- Construction drawing show a cutoff trench with an

upstream blanket on the right side and a cut of to rock (well constructed cut off trench on the left.

The left abutment is the more critical in this case therefore a weighting factor of 0.9 was selected,

34



Soil/Rock Geology Type W¡ 1.r¡¡- The left abutment is basalt rock, the right abutment is derived

from landslide debris. A factor of 3 was selected due to the basalt bedrock. The factor could

possibly have been set at a higher number but soils of landslide origin are not addressed.

Observations of Seepage Wp (ou,) - Seepage was initially observed on the right side of the

downstream face during initial filling. Horizontal drains were installed into the right abutment,

which to date appears to be controlling the seepage. Seepage continues to flow from the leftabutment contract but does not appear to be increasing at a detectable rate. Based upon these

conditions a factor or 2 was selected.

Monitoring and Surveillance W¡1.on¡ - The embankment in monitored annually, therefore a factor

of 2 was selected.

Weighting Factors for Piping from the embankment into the Foundation @actors from

Table 11.4)

Filters War (nr,) - From table 1 1.4, all cases have a factor of l.

Foundation Cutoff Trench Ws¡i"og - A factor of 1 was selected. The cutoff trench is considered tobe of average depth and width.

Foundation Type W¡¡ 16,a¡ - The embankment is partly founded on rock and partly on slide debris.

The partly founded on rock condition appears to control therefore a weighting factor of l 5 was

selected.

Erosion Control Measures of Core Foundation Ws¡(""r)- A factor of 5 was selected due to the

lack of erosion control measures in the landslide debris. The landslide debris were considered

equivalent to open jointed bedrock or open work gravels.

Grouting of Foundation Wsp6¡- Records indicate that grouting of the rock did not occur,

therefore a weighting factor of 1.3 was selected.

Soil Geology Type War (rct) - The left abutment is basalt rock, the right abutment is derived from

landslide debris and considered similar to colluvial material. A factor of 5 was selected due to the

nature ofthe soil.

Soil Geology Type W¡¡ 1"go¡ - The core material is believed to have been borrowed form colluvial

deposits at the site, therefore a weighting factor of 1.25 was selected.

Core Soil Type Wsp p.9 - The description of core material best representing the material used is

Clayey and Silty sands with a weighting factor of 1.2.

Foundation Treatment WEr (r) - Drawings indicate that the foundation contained someinegularities and steep areas, but was treated using standard practices of the time allowing a

factor of 1.1 to be selected.

Observations of Seepage W¡r(ouÐ - Seepage was initially observed on the right side of the

downstream face during initial hlling. Horizontal drains were installed into the right abutment,

35



which to date appears to be controlling the seepage. Seepage continues to flow from the left

abutment contract but does not appear to be increasing at a detectable rate. Because of the way

the seepage initially manifested itself it was felt that a factor of 2.0 was probably inappropriate,

but since seepage flows have not increased significantly a factor of 10 seemed to high, therefore a

weighting factor of 3 was selected.

Monitoring and Surveillance Ws¡1-oo; - The embankment in monitored annually, therefore a

factor of 2 was selected.

Document from UNICIV Report - Analysis of Embankment I)am Incidents. September

1998. Page 123.

To assess the annual probability of failure of an embankment dam by piping:

1. Determine the average annual probabilities of failure from Table 11.1 for each of the

three modes of piping failure:

- piping through the embankment

- piping through the foundation, and- piping from the embankment into the foundation,

allowing for the age of the dam, i.e. whether less than or older than 5 years (about 213 ofpiping

failures occur on first filling or in the first 5 years of operation).

2. Calculate the weighting factors WE, WF mad WEF from Tables 77.2,11.3 and 11.4 to

take account of the characteristics of the dam, such as core properties, compaction and

foundation geology, and to take account of the past performance of the dam. The

weighting factors are obtained by multiplying the individual weighting factors from the

relevant table. So, for example,

WE : WE(filt) X WE(cgo) X WE(cst) X WE(cc) X WE(con) X WE(ft) X WE(obs) X

WE(mon).

3. Obtain the overall annual probability of failure by piping (Pp) by summing the weighted

probabilities:

SO Pp: WEPe + WFPf + WEFPef.

If the probabilities are high, allowance must be made for the union of events in this

calculation.

36



I)ocument from UNICIV Report - AnalysÍs of Embankment I)am Incidents. September1998.

Tabte 11.1: Average probability of failure of embankment dams by mode of piping and dam

zoning.

FOUNDATION EMBANKMENT INTOFOUNDATION

EMBANKMENT

AVERAGEANNUALPef(x 10-6)

AVERAGE ANNUALpe

(x 10-6)

AVERAGE ANNUALPf

(x 10-6)

First 5

Yea¡sC)neration

After 5

YearsOoeration

AVER-AGEPte(xl0-3 First 5

Years

Ooeration

After 5

Years

Ooeration

AVER-AGEPTf

(x l0-3)

First 5

Years

Operation

After 5

YearsOperation

AVER-AGEPtef

(xl0-3)

ZONINGCATEGORY

4

l6

1.5

8.9

t.2

1.2

(<1.1)

5.3

(<l)

93

(<l)

(<l)

(<l)

2080

190

I 160

160

150

(<140)

690

(<130)

t200

(<130)

(<130)

190

3t

160

25

24

(<34)

75

(<17)

38

(<8)

(<13)

(<s)

t.7 255 19 0.18 19

Homogeneous

earthfill

Earthfill with

filter

Earthfill with

rock toe

Zoned earthfill

Zoned earth

and rockfill

Central core

earth and

rockfill

Concrete face

earthfill

Concrete face

rockfill

Puddle core

earthfill

Earthfill with

corewall

Rockfrll with

corewall

Hydraulic fill

450 56 1,7 255 19 0.t8 t9 4LL DAMS 3.5

Notes: (1)(2)

PTe, Ptg and Ptef ars the average probabilities of failure over the life of the dam.

Pe, Pq and Pef are the average annual probabilities of failure.

37



I)ocument from UMCfV Report - Analysis of Embankment Dam Incidents. September

1998. Page 124

If a factor has two or more possible weighting factors that can be selected for aparticular

damcharacteristic, such as different zoning types or different foundation geology types, then the

weighting factor with the greatest value should be used. This is consistent with the method of

analysis that was used to determine the weighting factors, as only the characteristics relevant

to the piping incident were included in the analysis.

The method is intended for preliminary assessments only. It is ideally suited as a risk ranking

method for portfolio type risk assessments to identiff which dams to prioritize for more

detailed studies. Since the method is based on a dam performance database approach, it tends

to lump together these factors which influence the initiation and progression of piping, and it

is not possible to assess what influence each of the factors is having. It is recommended that

more rigorous event tree based methods be used for detailed studies so as to gain a greater

understanding of how each of the factors influenceseither the initiation or progression of

piping, or the formation of a breach.

The user of the method is cautioned against varying the weighting factors significantly when

applying the method to actual dams as they have been calibrated to the population of dams so

that the net effect on the population is neutral.

It is recommended that the effect of the length of the dam is not included in the assessment of

the probability of failure using this method. Vanmarke (1977) demonstrates the length of the

dam might be expected to influence the probability of failure of sliding as long dams are

more likely to have some defect or other feature in the dam or foundation that could

potentially cause failure of the dam. However, for piping this is considered not to be a

significantfactor, as the piping failures often occurred at locations such as conduits passing

through the dam or steep abutments, which are independent of the length of the dam.

0

38



I)ocument from UNICIV Report - Analysis of Embankment Dam Incidents. September1998, Page 125

Table 11.2: Summary of the weighting factors for piping through the embankment mode offailure

GENERAL FACTORS INFLUENCING LIKELIHOOD OF FAILURENEUTRAL LESS LIKELY MUCH LESS

LIKELY

FACTORMUCHMORELIKELY

MORE LIKELY

ZONING Refer to Table I 1. I for the average annual probabilities of failure by piping through the embankment

depending on zoninq typeOther dam types

t-l I

Embankment filterpresent-poorquality

t02'l

present-welldesigned and

constructed

EMBANKMENTFTLTERS WB(filr)

No embankmentfilter (for damswhich usually havefilters (refer totext) 121

Residual,Lacustrine, Marine,Volcanic [1.0ì

Glacial [0.5]

CORE

GEOLOGICALORIGIN we(CGO) Alluvial U.5l Acolian, Colluvialu 2sl

Clayey andsiþ gravels(GC, GM)

t0.81

Low plasticityclays [0.8]

High plasticityclays (CH)

t0.31

Dispersive clays

tslLow plasticity silts(ML) t2.51Poorly and wellgraded sands (SP,

sw) t2l

Clayey and siltysands (SC, SM)Í1.21

Well graded andpoorly gradedgravels (GW. GP)

t1.01High Plasticity silts(MH) tl0ì

CORE SOILTYPE WB(cst)

Puddle, Hydraulicfill n.Ot

Rolled, goodcontrol [0 5l

COMPACTIONWElcc)

No formalcompaction [5ì

Rolled, modestcontrol [1.21

Conduit throughembankmenl-including

downstream filterst0.8.|

No conduit throughthe embankment

t0.51

Conduit throughthe embankment-many poor details

t5l

Conduit throughthe embankment-some poor details

l2l

Conduit throughembankment-typical USBR

practice [.0]

CONDUITSWE(con)

Careful slope modification by cutting,filling with concrete [0 9]

FOLTNDATIONTREATMENT

Untreated verticalfaces or overhangsin core foundationt21

Irregularities infoundation orabutment, Steep

abutments [1.21

Leakage steady,

clear or notobsewed [ 0]

Minor leakage

t0.71

Leakage measured

none or very small

t0.sl

OBSERVATIONSOF SEEPAGE WE(obs)

Muddy leakageSudden increasesin leakage [up to101

Leakage graduallyincreasing, clear,

Sinkholes, Seepage

emerglng ondownstream slope

t21Irregular seepage

observations,inspections weekly

tl.01

Weekly-monthlyseepagemonitoring, weeklyinspections t0 8l

Dailymonitoring ofseepage, daily

inspectionst0.51

MONITORINGANDSURVEILLANCEWE(mon)

Inspectionsannually

t2t

Inspectionsmonthly

Ir.2l

39



Document from IIÌ\IICIV Report - Analysis of Embankment Dam Incidents. September1998,Page126.

Table 11.3: S of factors for the foundation mode of failureOF FAILUREENERAL F

NEUTRAL LESS LIKELY MUCH LESSLIKELY

FACTORMUCHMORELIKELY

MORE LIKELY

of failure theONING Refer to table I l. I for the

u.0l

No foundationfilter

Foundationfilter(s) present

I0.8t

FILTERS WF(frlt) No foundationfilter present whenrequired Í1.21

Rock-closedfractures and non-erodible substancet0.05t

Rock-clay infilledor open fracfuresand/or erodiblerock substance[1.01

Better rock qualityOUNDATIONTYPE (belowcutoff) WF(fnd)

Soil foundation

tsl

Partiallypenetratingdeep cutoff

trencht0.71

Shallow or nocutofftrench

lr.2lWell constructeddiaphragm wall

tl.sl

Partiallypenetratingsheetpile wall orpoorly constructedslurry trench wallt1.01Average cutofftrench [.0]

Upstreamblanket,Partially

penetratingwell

constructed

slurry trench\4iall [0.8]

Well constructedcutofftrench [0.9]

CUTOFF TYPE(Soil foundation)WF(cts) OR

CUTOFF TYPE(Rock foundation)WF(ctr)

Sheetpile wallPoorly constructeddiaphragm wall [3]

Sandstone, Shale,

Siltstone,Claystone,Mudstone,Homfels [0.7]Agglomerate,Volc. Breccia [0.8|

Alluvial t0.91

Conglomerate [0 5]Andesite, Gabbro

[0.5]Granite, Gneiss

t0.21Schist, Phyllite,Slate t0.51

Glacial t0.51esidual U.2l

TuffRhyoliteMa¡bleOuafzits

l.sl12)I2lt21

Aeolian, Colluvial,Lacustrine, Marine

[l 0]

SOIL GEOLOGYTYPES (belowcutoff) WF(sg),ORROCK GEOLOGYTYPES (belowcutoff) WF(rg)

Dispersive soils [5]Volcanic Ash t51

Saline(gypsum)

tslt3lt5lt3lasalt

LimestoneDolomite

Minor leakage

t0.71

Leakagemeasure none

or very smalll0.sl

Low pore pressures

in foundation [0.8]

Leakage graduallyincreasing, clear,

Sinkholes,Sandboils A)Graduallymcreasrngpressufes lnfoundation 121

Leakage steady,

clea¡ or not

obsewed tl.0lHigh pressures

measured infoundation U.0l

OBSERVATIONSOF SEEPAGE

wF(obs) OROBSERVATIONSOF POREPRESSURESWF(obp)

Muddy leakageSudden increases

in leakage fup tol0lSudden increasesin pressures [up to101

Weekly-monthlyseepagemonitoring, weeklyinspections t0.81

Dailymonitoring ofseepage, daily

inspectionst0.sl

Ir.2l

Inspectionsmonthly

Irregular seepageobservations,inspections weekly

tl 0l

MONITORINGANDSURVEILLANCEWF(mon) t2l

Inspectionsannually

40



I)ocument from UNICIV Report - Analysis of Embankment Dam Incidents. September

1998, Page 127,128.

Table 11.4: Summary of weighting factors for piping from the embankment into the foundation-

accidents and failures.GENERAL FACTORS INFLUENCING LIKELIHOOD OF INITIATION OF PIPING-ACCIDENTS AND

FAILURES

NETJTRAL LESS LIKELY MUCH LESS

I,IKELY

FACTOR

MUCH MORELIKELY

MORE LIKELY

from embankment into foundationONING Refer to Table I I .l for the

FILTERS

WEF(filt)

Appears to be independent ofpresence/absence ofembankment or foundation filters [1.0]

Shallow or no

cutofftrench [0.8]

Deep and narrow

cutofftrench [1.5]

Average cutofftrench width and

depth tl.Ol

FOUNDATION

CUTOFFTRENCHrWEF(cot)

Founding on or

partly on soil

foundations [0.51

Founding on orpartly on rockfoundations [l.51

FOTINDATION

TYPE WEF(fridO

Good to very good

erosion control

measures present

and good

foundationt0.3-0.1-l

No erosion control

moasufes, average

foundation

conditions

tt.2\

No erosion control

measures, good

foundation

conditions

t1.01

Erosion control

measures presgnt,

poor foundations

t0.sl

ERIOSION

CONTROLMEASURES OF

CORE

FOUNDATIONWEF(ecm)

No erosion control

measures, open

jointed bedrock or

open work gravels

[up to 5]

Soil foundation

only-not applicable

tl.01

Rock foundations

grouted t0.81

GROUTINGS OF

FOUNDATIONS

No grouting on

rock foundations

tt.3l

Agglomerate,

Volcanic breccia

Granite, Andesite,

Gabb¡o, Gneiss

tl.0l

Residual t0.81

Sandstone,

Conglomerate [0 8]

Schist, Phyllite,

Slate, Homfels

t0.61

Alluvial, Aeolian,

Lacustrine, Marine,

Volcanic [0.5]

Shale, Siltstone,

Mudstone,

Claystone

t0.21

SOIL GEOLOGYTYPES wEF(sg),

OR

ROCKGEOLOGYTYPES wEF(rg)

Sandstone

interbedded withshale or limestone

t3lLimestone,

gypsum 12.51

Colluvial t5l Glacial tzl

Dolomite, Tuff,

Quartzite U.5l

Rhyolite, Basalt,

Marble ll.2l

Glacial [0.5]lluvial [l 5l Acolian,

Colluvial[1.25]

Residual,

Lacustrine, Marine,

Volcanic tl.01

CORE

GEOLOGICAL

ORIGINWEF(ceo)

High plasticity

clays (CH)

t0.31

Clayey and silty

sands (SC, SM)

[1.2]

Well graded and

poorly graded

gravels (GW, GP)

tl.0lHigh plasticity silts

(MH)t1.0.1

Clayey and siltygravels (GC, GM)

[0.8]Low plasticity

clays (CL)

t0 8l

CORE SOILTYPE

WEF(cst)

Dispersive clays[5]

Low plasticity silts

(lvfr-) I2.slPoorly and wellgraded sands (SP,

sw) t2l

CORE

COMPACTIONWEF(est)

Appears to of compaction-all compaction types [ .0]

Irregularities in

foundation or

abutmenl, Steep

abutments tl.ll

Careful slope modification by cutting,

filling with concrete

t0.el

FOUNDATION

TREATMENT

wEF(ft)

Ilntreated vertical

faces or overhangs

in core foundation'

I l.sl

Leakage measurednone or very small

t0.5ì

Leakage graduallyincreæing, clear,

Sinkholes

tz'l

Leakage steady,clear, or not

monitored

n.0l

Minor leakage

t0.71

OBSERVATIONSOF SEEPAGE

WEF(obs)

Muddy leakage,Sudden increases

in leakage

lup to I 0lDaily monitoring

ofseepage, daily

inspections

t0 5l

Inspections

monthly

u.2l

Irregular seepage

observations,

inspections weekly

t1.01

Weekly-monthlyseepage

monitoring, weekly

inspections I0.81

MONITORINGANDSURVEILLANCE

Inspections

annually

t21

4t



Appendix B

Baseline Cost Estimate for Silver Creek Dam Early Warning System

Section 205 Project

42



Insert Total Project Cost Summary sheet 1.

43



Insert Overall Project Summary Page

44



Narrative

1. Proiect Description: Silver Creek Dam is located just south of the city of Silverton,

OR. It is a 65 foot high embankment dam with a concrete spillway. The early waming

system is intended to reduce potential hazards posed by dam. The system will measure

piezometers and weirs, route the information through control units and to cþ offices. Asiren notification system is also included.

2. Basis of Desisn and Estimate:

a. Basis of Design . The basis of the design is the Feasibility Report for subject

project dated I|lIay 2004.

b. Basis of Estimate. The estimate for this project was developed using

information provided by the designers, including plans and quantities. The construction

cost estimate is a detailed MII estimate using labor and equipment crews, quantities,

production rates, and equipmentlmaterial price quotes. The Total Project Cost Summarysheet includes costs for construction; real estate; planning, engineering and design;

construction management; contingencies; and infl ation.

3. Construction Schedule: Construction will be accomplished during the summer of 2005,

taking about one month to complete.

a. Overtime. Assume that no overtime will be required.

b. Construction Windows. In-water work periods are not applicable to this

project.

4. Acquisition Plan. It is anticipated that this job will be accomplished through either an

A-E contract or a sole source construction contract.

5. Subcontracting Plan. It is envisioned that all work will be done by the prime

contractor except earthwork, electrical and telephone work.

6. Project Construction.

a. Site Access. Access to the right abutment and spillway will be via highway.

Access to the main embankment will be by a boat crossing the reservoir.

b. Borrow Areas. Not applicable.

c. Construction Description. A description of required work is given below:

45



1) Mob & Demob. Assume mob/demob will be from the Portland area.

Equipment mobilized will include an excavator /loader, a small crane, a small

boat and a small barge for transporting equipment to the south side of the dam. Adirect cost of $5,000 was allowed.

2) Miscellaneous Prime Contractor Work. The prime contactor will likely

be a geotechnical engineering AE hrm that specializes in this type of work. Atwo-man crew is likely to perform the installation of instrumentation equipment.

Labor ratçs are assumed to be similar to typical for geotechnical engineers.

a) Contractor Manaqement of Subcontractors. This is assumed at

40 hours.

b) Field Inspection of Subcontractors. Allow 4 weeks, or 160

hours.

c) Order Equipment from Suppliers. Allow 40 hours.

3) Measurement and Control Units. The units would be Geomation MCUs

or equal. Three MCUs will be procured and installed. Each MCU will be configured

somewhat differently, see report and backup for details. MCU equipment costs were

estimated using costs for similar MCUs installed for a recent job at John Day Dam. It is

anticipated that each MCU will require 2 crew-hours to install, using a2-man crew.

MCUs are to be installed at locations shown in the report. One will be installed on the

north side of the dam, while the other two will be installed on the other side of the

reservoir, and will have to be transported by boat.

4) Prefab Building. A 10' by 12' f,rberglass building was assumed for this

building. A quote was received from TRACOM, Inc. for the building, see backup.

Assume the site will be prepared and a 4" thick concrete slab will be placed. The

building will be erected (20 crew hours), then wired for power and phone service

(covered in Electrical Work and Phone Lines items). Costs for an access road and

floodlights were computed using Cost Book items. The cost for a backup generator was

obtained from the Grainger catalog.

5) Electrical Work. This item covers all power needed for MCU-I,transducers, weir sensors, and the prefab building. Power would be brought over to the

building from a nearby restroom in the park. MCUs on the south side of the spillway

would transmit data to MCU-1 via radio signals, and these MCU's would have solar

power. So no power or phone lines would need to be installed to these MCUs.

Quantities for required trenching and cable installation were provided by the designer.

Appropriate Cost Book items were used to price these items. Production rates were

reduced due to the small quantities on this job.

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6) Phone Lines. This item covers installation of a phone line from a

nearby neighborhood across the highway from the dam, to the prefab building.

Again, quantities for required trenching and cable installation were provided by

the designer. Appropriate Cost Book items were used to price these items.

Production rates were reduced due to the small quantities on this

7) Pressure Transducers for Piezometers. The transducers would be

installed in 10 existing piezometers at the site. Quotes were obtained from Geokon for

the transducers. The cost ofjunction boxes, transient protection, grounding rods,

modifications to the tops of the piezometer casings, and miscellaneous hardware were

estimated at $1,000 per transducer. Grounding rods must be installed 10 to 20 feet deep.

An estimated 3 crew-hours would be required for installation of each transducer.

8) Pressure Transducer for Reservoir Level. This would be another

Geokon transducer installed near the upstream toe of the dam, underwater. The

transducer would be installed via a boat. Cables would be placed as necessary to carry

signals to MCU3. Again, the cost ofjunction boxes, transient protection, grounding

rods, and miscellaneous hardware was estimated at $1,000 for this transducer. An

estimated 8 crew-hours would be required for installation of this transducer in the water.

9) Reservoir Level Staff Gage. This item covers a staff gage to be

installed on the south side of the spillway. It would be a2x6 section of lumber, about l0feet long, with a white piece of plastic fastened to its surface. Elevations would be

marked on the plastic. The gage would be installed on the spillway wall. An elevation

survey would be required to assure installation at the proper level. A rough cost of$1,000 was obtained from Bruce Duffe for materials and installation, complete.

10) Pressure Transducer for Barometric Correction. This would be

another Geokon transducer. It would be installed inside of MCU1, to measure barometric

pressure. About 1 crew hour would be required for installation.

11) Weirs. Four weir boxes are to be installed. Anticipate using

fiberglass weir boxes. The weirs will be placed on concrete pads. A quote was obtained

from TRACOM, Inc. for the weirs boxes, see backup. A Geokon weir-measuring sensor

will be installed in each weir box. As above, the cost ofjunction boxes, transient

protection, grounding rods, and miscellaneous hardware was estimated at $1,000 for each

weir. Installation will require about 4 crew-hours for each weir.

12) Testine and Troubleshooting System. Estimated to require 16 crew-

hours

13) Emergency Operations Center (EOCI. This item covers computers

and software required to process instrument data collected at the dam. Pricing and crew-

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hours were estimated by David Scofield of the NV/P Geotechnical Design Section. Items

include:a) A standard PC and a laptop PC at the EOC in a City of Silverton

building.b) A PC at the dam site, installed in the prefab building.

c) Damsmart software(single users).

d) Geonet Module for Damsmart.

e) Programming. Estimated to require 40 crew-hours.

Ð MS Office Suite (Excel and Access).

g) Data backup.

h) Installation at EOC. Estimated to require 16 crew-hours.

i) Miscellaneous Hardware. Estimated at $1,000.

14) Notifrcation System. This covers all work related to installation of the

siren system. Four sirens are to be installed. This system may be constructed by the City

of Silverton. Design and cost data were obtained from Barry Myers. Included are:

a) Siren Activation Panel.

b) Radio Transmitter.

c) Siren Control Station.

1. Sirens (Federal Signal MOD6024 or equal)

2. Utility Poles (decorative style)

3. Wiring4. Power to each siren

d) Testing System.

15) Operations Manual. Estimated at 160 man-hours at $100/hr:$16,000.

16) Evacuation Plan. These items would be developed during design and

implemented by the City of Silverton. An Evacuation Plan and Brochures would be

developed.

17) Training and Svstem Support. Training materials would be developed

and training would be provided after the system was installed. Then system support

would be provided for a short period after system installation was completed. Costs and

hours were estimated during discussions with Dave Scofield.

d. Ouantities. Quantities were provided by a designer in Geotechnical Section.

e. Government Furnished Property. There is no government furnished property

on this job.

f. Unusual Conditionq (Soil, Vy'ater, V/eather). There are no unusual conditions

expected on thisjob.

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g. Unique Construction Techniques . N/A

h. Equipmenllabor Availability and Distance Traveled. Equipment and labor

will be provided by the Contractor. Assume labor and equipment originates in Portland.

i. Overhead. Proht and Bond. A low JOOH percentage (5%) was used because

several overhead type items were detailed in the estimate. Standard percentages were

used for HOOH and bond. Profit was computed using weighted guidelines.

7. Environmental Concerns. The contractor must assure that no hazardous construction

materials enter the reservoir or creek.

8. Contingencies. A contingency of l5o/o is used for all activities to cover uncertainties

in design and quantities.

9. Effective Dates for Labor. Equipment. Material Pricing. Effective date for all pricing

is March 2004. The most recent Davis-Bacon labor rates were used. The 2001 Cost

Book database was employed, covering labor, equipment and unit price items.

Costs: Costs for Real Estate, Engineering and Design, and Construction0.

Management were provided by the project manager

detailed project report section 205 silver creek dam early warning system october 2004

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