development of stage-discharge ratings...

Development of Stage-Discharge Ratings for Site 2240 – Bear Creek at Cold Spring

May 19, 2006 (Rev May 2007) Prepared for: Urban Drainage and Flood Control District 2480 W. 26th Avenue Suite 156-B Denver, CO 80211

1225 Red Cedar Circle, Suite A Fort Collins, CO 80524

(970) 225-6080

UDFCD Site 2240 Rating Development 2 Water and Earth Technologies, Inc. May 2006 (Rev May 2007)

Table of Contents

1.0 Certification ............................................................................................................................... 3 2.0 Introduction ............................................................................................................................... 4 3.0 Field Survey Methods................................................................................................................ 4 4.0 Data Reduction, Modeling And Analysis .................................................................................. 5 5.0 Rating Development .................................................................................................................. 6

5.1 Site 2240 Channel Survey ..................................................................................................... 7 5.2 Site 2240 HEC-RAS Modeling ........................................................................................... 17 5.3 Site 2240 Depth Above PT Discharge Rating ..................................................................... 23 5.4 Site 2240 Staff Gage Depth Discharge Rating .................................................................... 25 5.5 Site 330 Discussion of Rating Results ................................................................................ 27

6.0 References ............................................................................................................................... 28 7.0 Survey Field Notes .................................................................................................................. 29

Table of Figures

Figure 1. Site 2240 Instrument Standpipe and Upstream Diversion Structure. .............................. 6Figure 2. Site 2240 Location Map. ................................................................................................. 7Figure 3. Site 2240 Approximate Locations of Surveyed Cross Sections. ..................................... 8Figure 4. Site 2240 Cross-Section 1.0, Looking Downstream. ...................................................... 9Figure 5. Site 2240 Cross-Section 2.0, Looking Downstream. .................................................... 10Figure 6. Site 2240 Cross-Section 3.0, Looking Downstream. .................................................... 11Figure 7. Site 2240 Cross-Section 4.0, Looking Downstream. .................................................... 12Figure 8. Site 2240 Measured Landmark Elevations. .................................................................. 13Figure 9. Site 2240 Landmark Locations. .................................................................................... 14Figure 10. Staff Gage Installed at Site 2240. ................................................................................ 15Figure 11. Exposed PT Intake Pipe Observed at Site 2240. ......................................................... 15Figure 12. View Upstream at Floodplain Valley at Site 2240. ..................................................... 16Figure 13. Site 2240 Approximate Floodplain Boundaries at High Flow. ................................... 17Figure 14. Site 2240 Reduced Survey Notes and Estimated Cross Sections. ............................... 18Figure 15. Site 2240 Distances Between Surveyed and Estimated Cross Sections. ..................... 18Figure 16. Site 2240 Cross Sections Used in the HEC-RAS Model. ........................................... 19Figure 17. Site 2240 HEC-RAS Model Output. ........................................................................... 20Figure 18. Site 2240 Modeled Cross Sections at Maximum Flow. .............................................. 21Figure 19. Site 2240 Discharge Measurement Notes. .................................................................. 22Figure 20. Site 2240 Model-Predicted Stage-Discharge Relationship. ........................................ 23Figure 21. Site 2240 Stage-Discharge Relationship Comparison. ............................................... 25Figure 22. Site 2240 Stage-Discharge Relationship Log-Log Plot. ............................................. 25Figure 23. Site 2240 Staff Gage Stage-Discharge Relationship. .................................................. 26

Table of Tables

Table 1. Site 2240 Predicted Stage-Discharge Relationship Tabular Data. ................................. 24 Table 2. Current UDFCD Rating for Site 2240. ........................................................................... 24 Table 3. Stage-Discharge Relationship for The Stage Gage at Site 2240. ................................... 26


1.0 CERTIFICATION I, Richard Spotts, state that the information presented in this report entitled, “Development of Stage-Discharge Ratings for Site 2240 – Bear Creek at Cold Spring,” prepared for The Urban Drainage and Flood Control District, Denver, Colorado was prepared by me or by persons under my supervision and is correct to the best of my knowledge and information. _____________________________ Richard Spotts, P.E. Registration No. 26155


2.0 INTRODUCTION

Water and Earth Technologies, Inc. (WET) was contracted by The Urban Drainage and Flood Control District (UDFCD) to develop a hydraulic rating at Site 2240, Bear Creek at Cold Spring. This station, located downstream of a drinking water treatment plant intake structure is already instrumented to measure river stage. Stage information for this site is telemetered in real time to local base stations that assess the flooding potential during large runoff events. In addition to stage, discharge information is valuable for decision-making. Stage-discharge rating relationships (ratings) are used to convert the stage, represented as a water depth in feet above a reference elevation monitored by a pressure transducer (PT) in the stream to values of discharge in cubic feet per second (cfs). The discharge rating developed in this report is based on precise measurement of the river channel and physical structures controlling flow and mathematical approximation of the hydraulics at this site. This report includes the following sections: • an introduction, • a discussion of field survey methods, • a discussion of office procedures for data reduction and analysis, • a description of each site and a discussion and presentation of survey data and model results

(including model output, a stage-discharge rating table and a plot of the rating curve, and recommendations relevant to the results),

• a compilation of references, and • cross section field survey notes

3.0 FIELD SURVEY METHODS A theoretical step-backwater technique using the U.S. Army Corps of Engineers HEC-RAS computer model (USACOE 2002) was used to develop the stage-discharge ratings. The modeling typically requires data for five cross sections at a site. Typically, the cross section in which stage is observed bracketed by one or more cross sections both upstream and downstream Cross sections were surveyed from left to right looking downstream. Cross sections were numbered from downstream to upstream. Bench marks and end points of each cross section were marked as appropriate. A mapping grade GPS unit was used to determine the latitude and longitude of monument bench marks at each site, the stage measurement sensor housing (with the cap removed) and each cross-section end point so that cross sections and bench marks can be easily located in the future. Additionally, the coordinates were used to establish cross-section orientation in the HEC-RAS model. A self-leveling level, tape and survey rod were used to measure each point in the cross section and to relate streambed and water-surface cross section elevations to the bench mark elevation. Variations in channel roughness (Manning’s n value) were determined for each cross section. The main channel and overbank areas within each cross section were subdivided into n-value break points (locations where n-values change), and n-values specific to each subdivision were estimated. A current meter measurement was made and was referenced to the water-surface elevation in the gaged cross section. Section velocity measurements and discharge determinations were made using the midsection method. This information was used to determine the Manning’s n-value associated with the measured stage, low-flow discharge, and gradient of


the water surface between cross sections. The information was used to check the calibration of the HEC-RAS model run associated with the measured discharge. Photos of the site and each surveyed cross section were taken. Cross-section location selection, spacing, and orientation; surveying techniques; roughness parameter selection (Manning’s n values); current meter/velocity measurement techniques; and photographic and methods documentation followed standard protocols (Arcement and Schneider 1990; Barnes 1987; Benson and Dalrymple 1984; Dalrymple and Benson 1984; Harrelson et. al. 1994; Schulz 1974; U.S Army COE 2002; U.S. Geological Survey 1977).

4.0 DATA REDUCTION, MODELING AND ANALYSIS As previously mentioned, the U.S. Army COE HEC-RAS computer model was used to analyze the field data and develop the stage-discharge rating relationships. HEC-RAS is an integrated system of software, designed for interactive use in a multi-tasking environment. The steady flow water-surface profile component of the modeling system was used to calculate water surface profiles and elevations for a wide range in flows, from very low flow to flood flows, or flow that would occur at the highest stage contained within the channel and surveyed overbank. The basic computational procedure is based on the solution of a one-dimensional energy equation describing gradually varied uniform flow through the channel. Energy losses are evaluated by friction (Manning’s equation) and contraction/expansion (coefficient multiplied by the change in velocity head). WET’s HEC-RAS modeling input applied the initial assumption that the downstream modeling boundary condition was controlled by normal depth as defined by measured channel conditions and slope. At higher flows however, other cross sections (such as the location of the bridge contraction) may control the flow. These controlling locations and conditions are additional valuable output from the HEC–RAS model. Output values from the HEC-RAS model include the predicted water surface elevations at each cross section for a range of known discharges. The water surface predictions at the pressure transducer cross section were used to develop the stage–discharge rating for the site. Where possible, the current rating defined by UDFCD are compared to the HEC-RAS modeling results based on the WET channel survey.


5.0 RATING DEVELOPMENT A general site investigation was performed by WET staff on January 10, 2006. The site was surveyed by WET staff on February 28, 2006. River cross sections were surveyed to describe the channel and used as input data to the HEC-RAS hydraulic model in order to develop a stage–discharge relationship at the PT cross section. Site 2240, Bear Creek at Cold Spring (Figure 1) is located between Morrison and Evergreen, CO in Bear Creek Canyon (circled in red in Figure 2). The PT is located to measure the depth in a pool below a small sheet pile dam, used to back up water for intake to a water treatment plant. Cold Spring Gulch enters the Bear Creek from the north just downstream of the PT cross section.

Figure 1. Site 2240 Instrument Standpipe and Upstream Diversion Structure.


Figure 2. Site 2240 Location Map.

5.1 Site 2240 Channel Survey On February 28, 2006 WET surveyed four channel cross sections at this site. An aerial photo with the approximate locations of the surveyed cross sections and landmarks is presented in Figure 3. Photos of the surveyed cross sections from downstream to upstream are presented in Figure 4 - Figure 7.


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Xsec 2.0

Xsec 3.0Xsec 4.0

StandpipeBM1

BM2

PT TOP

0 10050Feet

4

Figure 3. Site 2240 Approximate Locations of Surveyed Cross Sections.

Flow


Figure 4. Site 2240 Cross-Section 1.0, Looking Downstream.


Above Cross-Section 4.0, at the sheet pile dam, a pool exists to back up water for an intake structure for a water treatment plant. Approximately 500 feet upstream of the sheet pile dam, a concrete bridge over the river provides access to the Lair O’ the Bear open space. Just downstream of Cross-Section 3.0, the Cold Spring Gulch enters from the north. The PT stage measurement will not accurately account for flow entering the Bear Creek from Cold Spring Gulch. A survey grade GPS unit was used to measure the elevation of three landmarks at the site (Figure 9). Differential correction was used to remove errors from the logged data to provide more accurate elevations. The elevation of landmark BM 1, measured at 6,663.05 ft amsl, was used as the vertical control elevation for the survey. Elevations of the landmarks are presented in Figure 8.

Figure 8. Site 2240 Measured Landmark Elevations. A staff gage upstream of the sheet pile dam was observed. This gage is used by water treatment plant operators as a depth of water in the intake structure for pumping uphill to the plant. A photo of this staff gage is presented in Figure 10. The elevation of the channel bottom at the base of the staff gage was measured at 6657.21 ft amsl. At the time of survey the PT riser intake pipe was almost one foot out of the water (Figure 11). There was water approximately 1-foot deep in the PT cross section. Unless the PT riser has another hydraulic connectivity, it is unlikely that the PT as currently installed can correctly measure low stage.


Figure 9. Site 2240 Landmark Locations.

BM 1

BM 2


Figure 10. Staff Gage Installed at Site 2240.

Figure 11. Exposed PT Intake Pipe Observed at Site 2240.


Based on observations at the site, at very high flow conditions a split flow, or valley-wide flow may occur. In this extreme event, the PT installed at its present location may not be able to measure the flow properly. A view upstream of the floodplain valley adjacent to the Bear Creek is shown in Figure 12. Note the woody debris in the center left of the image. The PT is installed out of the picture to the right.

Figure 12. View Upstream at Floodplain Valley at Site 2240. An aerial photo of the possible inundation and split flow that could occur is presented in Figure 13. Note a bridge across the Bear Creek in the lower left corner of Figure 13. This bridge is very low and consists of concrete deck and concrete sides. At high flows this bridge is likely to become flooded and act as a constriction causing flow to leave the main river channel and flow overland short-cutting the river bend downstream. Extrapolation of surveyed cross sections was performed to expand the HEC-RAS model prediction for this type of high flow event. The approximate extent of the expanded surveyed Cross-Sections 1.0 – 4.0 as well as two estimated Cross-Sections 5.0 and 6.0 are presented in Figure 13.


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Xsec 1.0

Xsec 2.0

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Figure 13. Site 2240 Approximate Floodplain Boundaries at High Flow.

5.2 Site 2240 HEC-RAS Modeling The four surveyed cross sections were input to the HEC-RAS modeling system and a low-flow rating developed. As requested by UDFCD, surveyed cross sections were extended across the valley using distances measured from the available GIS data and end point elevations measured during the survey. To facilitate modeling of the valley-wide flows and for the prediction of water surface elevation at the staff gage location, two additional cross sections were estimated, numbered 5.0 and 6.0. The reduced survey notes with both the measured and estimated cross sections are presented in Figure 14.


Figure 14. Site 2240 Reduced Survey Notes and Estimated Cross Sections. The measured and estimated distances between the cross sections are presented in Figure 15.

Figure 15. Site 2240 Distances Between Surveyed and Estimated Cross Sections. Graphical descriptions of the surveyed and extrapolated cross sections are presented in Figure 16. HEC-RAS model output is presented in Figure 17. HEC-RAS water surface elevations at all cross sections for an extreme flow event are presented in Figure 18.


Figure 16. Site 2240 Cross Sections Used in the HEC-RAS Model.


Figure 17. Site 2240 HEC-RAS Model Output.


Figure 18. Site 2240 Modeled Cross Sections at Maximum Flow. A comparison was performed between model simulations using the estimated and expanded cross sections and a simulation using only the surveyed Cross-Sections 1.0 – 4.0. Based on the


modeling results, water is predicted to be maintained in the main surveyed channel for all surveyed and estimated cross section for flows only below about 500 cfs. At higher flows, water is predicted to flow over the right bank near Cross Section 1.0 and a split flow is likely to occur at the bridge at estimated Cross-Section 6.0. Model simulations demonstrate that for flows between 1,000 and 1,500 cfs, water is predicted to be overbank at the PT cross section. Based on these results, above 1,500 cfs, a split flow or valley-wide flow is likely to occur and may not be properly measured by the PT. On February 28, 2006 a discharge measurement was taken at the 2240 site. A flowmeter was used to measure the water depth and velocity along a cross section just downstream of Cross-Section 2.0. The measured data and calculated discharge are presented in Figure 19. The flow measured on February 28, 2006 was 11.1 cfs.

Figure 19. Site 2240 Discharge Measurement Notes. The HEC-RAS water surface elevation at the PT cross section for a range of discharges is presented in Figure 20. The low-flow flowmeter measurement matches the rating quite well.


0

2000

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6652 6654 6656 6658 6660 6662 6664 6666

Water Surface Elevation at the PT (ft amsl)

Disc

harg

e (c

fs)

Figure 20. Site 2240 Model-Predicted Stage-Discharge Relationship.

5.3 Site 2240 Depth Above PT Discharge Rating The rating desired by UDFCD is expressed as stream discharge as a function of water depth at the PT. The reference elevation for zero depth at the PT has been assigned to the minimum measured elevation at the PT cross section. (6,653.29 ft amsl). The minimum channel elevation at the PT control Cross-Section 2.0 was measured as 6,653.40 ft amsl, 0.11 ft higher than the channel bottom upstream at the PT cross section. This elevation is considered the zero flow elevation, where water could pool to a depth of 0.11 ft before water begins to flow down the channel. These values are used to calculate depth of water above the PT from the HEC-RAS water surface elevation predictions for a range of discharge values (Table 1).


Table 1. Site 2240 Predicted Stage-Discharge Relationship Tabular Data.

Predicted Water Surface Elevation (ft

amsl) Depth above PT (ft)1 Discharge (cfs)

Condition

6,653.29 0.11 0 6,653.52 0.23 0.1 6,653.75 0.46 1 6,654.24 0.95 10 6,655.06 1.77 50 6,655.59 2.30 100 6,656.35 3.06 200 6,656.95 3.66 300 6,657.44 4.15 400 6,657.87 4.58 500 6,658.69 5.40 750 6,659.33 6.04 1,000 6,660.43 7.14 1,500 Split flow likely to begin

6,660.70 7.41 2,000 6,662.00 8.71 5,000 6,663.29 10.00 10,000 6,664.27 10.98 15,000

1 Depth is calculated as predicted water surface elevation minus minimum surveyed channel elevation at Cross-Section 3.0 (6,653.29 ft amsl) 2 The zero flow elevation is the minimum measured channel depth at Cross-Section 2.0 (6,653.40 ft amsl)

The current UDFCD rating for this site is presented in Table 2 and a graphical comparison between these data and the HEC-RAS rating are presented in Figure 21. At flows between 500 and 10,000 cfs, the HEC-RAS rating predicts higher depths for discharges, compared to the UDFCD rating. At the highest modeled discharge of 15,000 cfs, the ratings are similar. A Log-Log plot of the HEC-RAS rating is presented in Figure 22.

Table 2. Current UDFCD Rating for Site 2240. Depth (ft)1 Discharge (cfs)

0 0 1 30 2 150 3 450 4 850 5 1,500

5.2 1,750 8 5,850

9.5 9,500 14 25,000


0

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10000

15000

20000

25000

30000

0 2 4 6 8 10 12 14 16

Depth at PT (ft)

Disc

harg

e (c

fs)

HEC-RAS ratingUDFCD rating

Figure 21. Site 2240 Stage-Discharge Relationship Comparison.

0.01

0.1

1

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100000

0.01 0.1 1 10 100

Depth at PT (ft)

Disc

harg

e (c

fs)

Figure 22. Site 2240 Stage-Discharge Relationship Log-Log Plot.

5.4 Site 2240 Staff Gage Depth Discharge Rating A cross section was estimated to describe the pool with bottom elevation the same as the staff gage bottom elevation, located 20 feet upstream of the sheet pile dam. This cross section (numbered 5.0) was added to the model to provide water surface elevation predictions for the

Flow likely to leave main channel and form split flow


staff gage location at the site. For better hydraulic connectivity in low-flow conditions, the pool at the staff gage location may provide a better PT intake location. If the PT intake were to be relocated to the staff gage site and the PT reference elevation set to 6657.21 ft amsl, the measured bottom of the staff gage, the stage-discharge relationship presented in Figure 23 and Table 3 would be valid, based on model predictions.

0

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0 1 2 3 4 5 6 7 8 9

Staff Gage Depth (ft)

Disc

harg

e (c

fs)

Figure 23. Site 2240 Staff Gage Stage-Discharge Relationship. Table 3. Stage-Discharge Relationship for The Stage Gage at Site 2240. Staff Gage Depth (ft) Discharge (cfs)

0.79 0 0.83 0.1 0.89 1 1.07 10 1.45 50 1.78 100 2.27 200 2.67 300 3.04 400 3.31 500 3.6 750 3.81 1,000 4.16 1,500 4.44 2,000 5.65 5,000 7.12 10,000 8.34 15,000


5.5 Site 330 Discussion of Rating Results The rating of the depth of water above the PT is based on the reference elevation of the channel bed at the PT cross section 6,653.29 ft amsl or 5.82 ft below the top of the PT riser pipe with the cap removed. At the time of the survey the PT riser intake pipe was measured at an elevation of 6,654.92 ft amsl, 1.63 ft higher than the channel bottom elevation used as the PT reference level. Before using this rating with confidence, the intake pipe should be lowered and the elevation of the installed PT should be lowered below the reference elevation and verified in the field. A field verification of water depth above the reference level should be performed using the installed PT reading and a field measurement of water surface elevation. Based on modeling results, some overbank flows are predicted to occur downstream from the PT cross section above about 500 cfs. Above an estimated 1,500 cfs, overbank flows are predicted to occur throughout the surveyed reach and a split flow is likely to occur beginning at the open space bridge upstream of the PT. Flows higher than 1,500 cfs have been modeled using estimated valley-wide cross sections. Water depths at the PT for discharge values above 1,500 cfs have been provided, but the complex hydraulic conditions of split or valley wide flows for these discharges may not be properly measured by the PT. Therefore, discharge predictions above stage values of about 7 feet of depth at the PT should be considered estimates.

An additional rating for the channel cross section at the staff gage location was also provided in case the PT intake were to be located there to provide better hydraulic connectivity in low-flow conditions. This rating relationship has been calculated for the Bear Creek above the Cold Spring Gulch and assumes that inflow from the Cold Spring Gulch does not occur between PT and PT control downstream.


6.0 REFERENCES

Arcement, G.J., and Schneider, V.R. (1990). “Guide for Selecting Manning’s Roughness Coefficients for Natural Channels and Flood Plains.” United States Geological Survey Water-Supply Paper 2339. Barnes, H.H. (1987). “Roughness Characteristics of Natural Channels.” U.S. Geological Survey Water-Supply Paper 1849. United States Government Printing Office, Washington. D.C. Benson, M.A., and Dalrymple, T. (1984). “General Field and Office Procedures for Indirect Discharge Measurements.” Techniques for Water-Resources Investigations of the United States Geological Survey, Book 3 Applications of Hydraulics, Chapter A1. United States Government Printing Office, Washington, D.C. Dalrymple, T., and Benson, M.A. (1984). “Measurement of Peak Discharge by the Slope-Area Method.” Techniques for Water-Resources Investigations of the United States Geological Survey, Book 3 Applications of Hydraulics, Chapter A2. United States Government Printing Office, Washington, D.C. Harrelson, C.C., Rawlins, C.L., and Potyondy, J.P. (1994). “Stream Channel Reference Sites: An Illustrated Guide to Field Technique.” General Technical Report RM-245. U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. Fort Collins, Colorado Schulz, E.F. (1974). “Problems in Applied Hydrology.” Water Resources Publications, Fort Collins, Colorado. U.S. Army Corps of Engineers, Hydrologic Engineering Center. (2002). “HEC-RAS River Analysis System.” Version 3.1 User’s Manual, Hydraulic Reference Manual, and Applications Guide. Institute for Water Resources, Davis, California. U.S. Geological Survey. (1977). “National Handbook of Recommended Methods for Water-Data Acquisition.” U.S. Department of the Interior, Prepared under the sponsorship of the Office of Water Data Coordination, Geological Survey, Chapter 1, Surface Water. Reston, Virginia. U.S. Geological Survey. (2005). Aerial Photography via Terraserver.


7.0 SURVEY FIELD NOTES

development of stage-discharge ratings...

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