8. detailed hydraulics analysis

Presentation #8Detailed Hydraulic Analysis

An Introductory Guide to Hydrology Using

WinTR-55

Prepared By:Stormwater Solutions Engineering, LLC

100 East Sumner Street | Hartford, Wisconsin 53027www.stormwater-solutions-engineering.com |262-673-9697

http://www.stormwater-solutions-engineering.com/

Overview• How are runoff flows generated?• How WinTR-55 can be used to calculate

runoff/storm water flows

Overview• Section 1: Hydrology• Section 2: WinTR-55

Sub-area/Reach Concepts• Watershed - system of sub-

areas and reaches• Sub-areas – areas within a

watershed that generates runoff

• Reaches - represent watershed flow paths (stream channels) or structures

04/27/2023TR-55 Tutorial

5

A

DCC

B

EE

Reac

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Rea

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http://www.wsi.nrcs.usda.gov/products/W2Q/H&H/docs/WinTR55/WinTR55_exp_users.ppt

Section 1: Hydrology

Reading Topographic Maps for Tc

Contour “V” points upstream

Possible sheet flow location – long, shallow

slope

Possible shallow concentrated flow:2 or 3 distinct flow

regimes (slope)1 calculation for each

regime

Channel FlowMay be able to disregard culverts under roadways.

These contour lobes identify divides where

flow splits left and right


Boundary indicates limit of watershed – ridge of high points that define

flow boundary

Closed area indicate a localized depression or a high point (peak)

04/27/2023 TR-55 Tutorial 7

Sub-area/Reach Concepts• Watershed - system of sub-areas and reaches• Sub-areas – areas of a watershed that generate runoff

- Flows/discharges into the upstream end of reaches • Reaches or Routing Elements- represent watershed stream flow

paths or structures– Channel Routing elements - Stream Reaches– Structure Routing elements - Reservoir/Structure Reaches

• Watershed Outlet - downstream end of the watershed (required for all watersheds)

» Typically discharges to a reach



Sub-area/Hydrology Concepts

• Hydrology is based on several factors:– The size of the drainage area.– The ability of the native soil to infiltrate water at the start of a

rainfall event, or capture otherwise (infiltration + vegetation capture + evapotranspiration = initial abstraction). This is approximated in the TR-55 method by utilizing CN (aka RCN), or Runoff Curve Numbers.

– The time it takes for the entire area (or subarea) to contribute flow to the downstream point of interest (Time of Concentration).


Sub Area Concepts - Schematics

04/27/2023TR-55 Tutorial

9

Reach 2c

(Reach Routing)

Outlet

Reach 1a(storage routing)

Sub-area B

Sub-area AS

ub-a

rea

C

Legend:Storage AreaSub-Area Inflow Points



Determining CN - Soil Types• Based on “Hydrologic Soil Group” classification

– In the U.S., can be found at:websoilsurvey.nrcs.usda.gov

• A = sand/gravel sub soilsTo

• D = characteristics of a wetland– Silt or clay soil, poorly drained

• Outside the US, need soil engineer or local info on soil types


Hydrologic Soil GroupsSection 1: Hydrology

Hydrologic Soil GroupsFrom USDA:• Hydrologic soil groups are based on estimates of runoff

potential. Soils are assigned to one of four groups according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly wet, and receive precipitation from long-duration storms.

• The soils in the United States are assigned to four groups (A, B, C, and D) and three dual classes (A/D, B/D, and C/D).


Hydrologic Soil Group - A• Group A. Soils having a high infiltration rate (low

runoff potential) when thoroughly wet. These consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission.


Hydrologic Soil Group - B• Group B. Soils having a moderate infiltration rate

when thoroughly wet. These consist chiefly of moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately coarse texture. These soils have a moderate rate of water transmission.

• Consists of sandy silts, silty sand…


Hydrologic Soil Group - C• Group C. Soils having a slow infiltration rate when

thoroughly wet. These consist chiefly of soils having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture. These soils have a slow rate of water transmission.

• Consist of silty clays, sandy clays…


Hydrologic Soil Group - D• Group D. Soils having a very slow infiltration rate

(high runoff potential) when thoroughly wet. These consist chiefly of clays that have a high shrink-swell potential, soils that have a high water table, soils that have a claypan or clay layer at or near the surface, and soils that are shallow over nearly impervious material. These soils have a very slow rate of water transmission.

• Consists of predominantly clayey soils.


Hydrologic Soil Group – Dual Groups

• If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is for drained areas and the second is for undrained areas. Only the soils that in their natural condition are in group D are assigned to dual classes.

• Can consist of peat, wetland soils, clayey soils, or even undrained sands/gravels.


Hydrologic Soil Group – Web Soil Survey

In the U.S:1. Find Location2. Define “Area of Interest”

From tool bar, draw polygon around project watershed

3. “Soil Data Explorer” tab4. “Soil Properties and Features”5. “Hydrologic Soil Group”6. “View Rating”

43

5

1,2

6


GroupGroup

Hydrologic Soil Group – Web Soil Survey Output

Soil Name Group Area, %


CN – Soil Type• 210-VI-TR-55, Second Ed., June 1986...

– Actual Technical Release (TR in TR-55)– Not a manual for the program, but the back ground

literature for the program– http://www.hydrocad.net/pdf/TR-55%20Manual.pdf

• Defines CNs for general areas (impervious, pervious, etc.) as well as select land uses.


http://www.hydrocad.net/pdf/TR-55%20Manual.pdf

CNFrom 210-VI-TR-55

Impervious Area:

CN=98

General Land Use(based on imp. area


CNFrom 210-VI-TR-55

Not all cropland is created equal…


CNFrom 210-VI-TR-55


CNFrom 210-VI-TR-55

• Some regulatory agencies require comparison to pre-developed conditions.– Pre-development– Or pre-European settlement

• Some restrict the CNs in pre-developed models to pasture or meadow– This can artificially lower the CN (less runoff) compared to

agricultural CNs


Area• Either by CAD, or from Web Soil Survey, or

estimated.• Area of individual sub-watersheds, composite

areas, or by similar CN• Commonly, areas are divided by sub watershed, or

areas with same Time of Concentration


Composite CNWinTR-55 does this for you!

• For use when adding areas (A) with different CNs.• Composite CN=(∑Ai*CNi)/(∑Ai)• Example:


Storm Data From 210-VI-TR-55

• “The highest peak discharges from small watersheds in the United States are usually caused by intense, brief rainfalls that may occur as distinct events or as part of a longer storm. These intense rainstorms do not usually extended over a large area and intensities vary greatly. One common practice in rainfall-runoff analysis is to develop a synthetic rainfall distribution to use in lieu of actual storm events” (B-1).


• Generally, there are 4 standard rainfall distributions within the US.


Storm Data From 210-VI-TR-55

Time of Concentration (Tc)• Based on longest hydraulic path of watershed or

sub-watershed.• 3 (actually 4) different types of water flow:

1. Sheet Flow2. Shallow Concentrated3. Ditch4. Pipe (not modeled in some programs, such as WinTR-55)

• Add all individual flow segments for Tc


Longest Path• Once the rain that falls on the furthest (time) point

from the discharge, the entire watershed is assumed to be reaching the outfall. This can help define the peak of the hydrograph.

• The longest hydraulic path used when analyzing times of concentration typically start at either the high point in the sub-watershed or a long, flat area.

• Sheet flow will typically control – maximize length at shallow slope.


Longest Path – Minimum Time From 210-VI-TR-55

• Regardless of Longest Path Tc calculations, The minimum Tc used in TR-55 is 0.1 hour (page 3-4).

• In large watersheds this constraint will probably not be an issue.

• This constraint may be an issue when modeling smaller or highly impervious areas.

• This minimum Tc is also typically applied to Rational Method calculations for storm sewer sizing


Sheet FlowFrom 210-VI-TR-55

• Sheet flow is flow over plane surfaces• Typically short (100 feet, maximum), and can be restricted to

lower lengths by regulatory agencies (or by WinTR-55)

• Professional judgment needed for end conditions


Sheet FlowFrom WinTR-55 FAQs

• Is there any way to increase the sheet flow length beyond 100 feet [in WinTR-55]?

• “No. After much discussion and research, the development team felt that sheet flow greater than 100’ was very unusual in natural watersheds. For more information on the subject read W.H. Merkel’s “References on Time of Concentration with Respect to Sheet Flow” as posted in Technical References and H&H Papers on Various Topics, http://www.wsi.nrcs.usda.gov/products/W2Q/H&H/Tools_Models/WinTR55.html in the USDA-NRCS West National Technology Support Center website.”


Sheet FlowFrom 210-VI-TR-55

• “The range of mean depth is 0.002' for paved areas to 0.02' for vegetated areas.”

• Sheet flow Manning's “n” values are for very shallow flow depths.

• “It is important to note that, particularly for unpaved surfaces, these friction factors are different than those traditionally used for channel flow.”

USDA, “TECHNICAL NOTE N0. N4.”


Sheet Flow Manning’sFrom 210-VI-TR-55


Shallow concentrated flowFrom 210-VI-TR-55

• After a maximum of 300 feet, sheet flow usually becomes shallow concentrated flow.

• Determine Velocity• Determine segment Travel Time

(Tt)• By equation:

– V=20.3252*(s0.5) [paved]– V=16.1345*(s0.5) [unpaved]

• Tt=Length/V


Channel and Pipe FlowFrom 210-VI-TR-55

• Typically non-pressure flow is assumed for pipes and culverts (WinTR-55 does not calculate pressure flow)

• Use standard equations for open channel flow

• Determine Velocity• Determine segment Travel

Time (Tt)• Tt=Length/V


Reading Topographic Maps for Tc

Contour “V” points upstream

Possible sheet flow location – long, shallow

slope

Possible shallow concentrated flow:2 or 3 distinct flow

regimes (slope)1 calculation for each

regime

Channel FlowMay be able to disregard culverts under roadways.

These contour lobes identify divides where

flow splits left and right


Boundary indicates limit of watershed – ridge of high

points that define flow boundaryClosed area indicate a

localized depression or a high point (peak)

What is a Reach/Influence on Tc

• Reaches - represent watershed flow paths (stream channels) or structures

• Generally utilized to route one sub basin through another

• “Reach C1” as shown can also be a part of the Tc for Sub Area C.


What is a Reach/Influence on Tc

• In this scenario, the Tc of sub area B is sum of its times for sheet flow, shallow concentrated, and any channel or pipe flow.

• The Tc of sub area B at the downstream end of Reach C1 is Tc(B) + Tt(Reach C1).

• Reach routing is a crucial step in runoff hydrograph development


Summary of CN, and Travel DataInput Data


Runoff QuantificationFrom 210-VI-TR-55

• The Technical Report offers 3 ways of determining runoff quantities:


Applicability to Professional Use

• With limited input, computer programs can automatically calculate travel times, composite CNs, and times of concentration.

• Some programs allow for fewer roughness variables than provided in reference documents.

• It is useful to manually calculate travel times, composite CNs, and times of concentration for easier review/verification purposes.

• Most computer programs also allow a “direct” input of these manually calculated values.


Hydrograph Development From 210-VI-TR-55

• Hydrographs can be developed using several methods.

• Commonly used is the procedure listed in Chapter 5 of 210-VI-TR-55

• While each runoff hydrograph is different, typically the distribution is similar, only differ in time scale and quantity of runoff– Dependant on Tc and CN


http://isddc.dot.gov/OLPFiles/FHWA/010593.pdf

Model Outputs – Technical Basis

• HEC-22, Chapter 8: Retention and Detention Facilities, provides discussion on hydrograph routing through detention facilities.


http://isddc.dot.gov/OLPFiles/FHWA/010593.pdf

Model Outputs – Technical Basis

• Stage-Discharge Ratings for an Outlet is shown• Stage-Discharge-Storage Relationships that some programs can

present, just depicts another known variable

HEC-22, p 8-40


Model Outputs – Technical Basis• Storage-Indicator

tables, and graphical representation

HEC-22, p 8-40 & 8-46


Applicability to Professional Use

• Computer programs are commonly used to bypass time consuming development of hydrographs and routing calculations.

• A common benefit for computer models is interaction of ‘interconnected ponds’ and development of hydrographs with data points every minute.

• Some professionals still prefer manual calculation though – it is wise to be familiar with both.


About WinTR-55 (general)• WinTR–55 is a single-event rainfall-runoff, small

watershed hydrologic model. • The model generates hydrographs from both urban

and agricultural areas and at selected points along the stream system.

• Hydrographs are routed downstream through channels and/or reservoirs.

• Multiple sub-areas can be modeled within the watershed.

From Win TR-55 User Guide, Page 1

Section 2: WinTR-55

How to Download• WinTR-55 (Computer Program)

– http://www.wsi.nrcs.usda.gov/products/W2Q/H&H/tools_models/wintr55.html

Optional Additional Reference Materials• TR-55 (Technical Release 55)

– http://www.hydrocad.net/pdf/TR-55%20Manual.pdf• HEC-22 (Hydraulic Engineering Circular No. 22, Second Edition,

Chapter 8: Detention And Retention Facilities)– http://www.fhwa.dot.gov/engineering/hydraulics/library_listing.cfm?

archived=true – http://isddc.dot.gov/OLPFiles/FHWA/010593.pdf

FREE!

Section 2: WinTR-55

WinTR-55 INPUTSGeneral• User Name• Project Name• State, County (if

applicable)• Sub title• Area Characteristics• Reach• Structure

Area Characteristics• Sub-Area Names• Sub-Area Description• Sub-area Flows to Reach

Outlet– Weighted CN– Tc (hours) – Area (units)

Section 2: WinTR-55

Composite CN – WinTR-55 Click this…

…To get this window

Then enter acreage next to appropriate CN

Section 2: WinTR-55

Storm Data – WinTR-55 • Program requires storm data entry prior to Tc• Custom unit hydrographs and rainfall distributions

are also accepted inputs• USDA rainfall information provided for all counties

in U.S.– Will auto populate rainfall depths– Will auto populate rainfall distribution type

Section 2: WinTR-55

Storm Data – WinTR-55

Click this…


Section 2: WinTR-55

Storm Data – WinTR-55 Select this button to auto populate

storm data based on State and County indicated on home screen.

Or enter specific data (as required by regulatory agency)

accept

Select rainfall type

Section 2: WinTR-55

Tc Inputs – WinTR-55 • How to enter Time of Concentration Data

Click this…


Section 2: WinTR-55

Reach Data - WinTR-55 • Can model stream, channels, etc. downstream of a

subarea or watershedClick this…


Section 2: WinTR-55

Putting It All TogetherWinTR-55 Data Screen

Section 2: WinTR-55

Putting It All TogetherFlow Path

Section 2: WinTR-55

Putting It All TogetherResults

Section 2: WinTR-55

Additional Tools

Reservoir Routing

Section 2: WinTR-55

Outlet Structure- WinTR-55 • A “Structure” in WinTR-55 refers to structure routing

elements– A reservoir/pond– A reach with significant storage and/or a constricted

outlet

Section 2: WinTR-55

Outlet Structure- WinTR-55 • Unlike HEC-RAS, WinTR-55

does not have dynamic routing capabilities…– As a reach discharges, the

structure fills and begins to discharge.

– Eventually the reach may stop flowing following a rain event – a dry ditch, for example – while the structure may have only a portion of the total inflow.

– This may be problematic.

Modeled Reach

ModeledStructure

Downstream Reach

Ground

HGL

This condition NOT modeled

Section 2: WinTR-55

Modeled Reach

ModeledStructure

Downstream Reach

Outlet Structure- WinTR-55 • Note culverts and

riser/pipe combinations are modeled the same

• Also, pipe invert to spillway is measured at the pipe exit, not entrance

Section 2: WinTR-55

Outlet Structure- WinTR-55 …or Double click this…


Click this…

Pipe or weir?

Section 2: WinTR-55

Outlet Structure- WinTR-55

Double click this…


Section 2: WinTR-55

Reach Routing - WinTR-55

Assign the reach to the sub area

Section 2: WinTR-55

Reach Flow - WinTR-55

Click this…

…To get this window…or click this…

Section 2: WinTR-55

Run Model - WinTR-55 • The model can be run for numerical or graphical

output• The model can be run for multiple storm events

(numerical output)• Errors may be the result of “structure” overtopping –

i.e., the structure outlet is undersized, or not enough storage is available

Section 2: WinTR-55

Run Model - WinTR-55 Click this…


Select rainfall events, then Run



Section 2: WinTR-55

Run Model (output) – WinTR-55

Subarea peak

Reach (upstream) peak

Reach (upstream) peakReach (upstream) peakPeak Flow Lag (in to out)Structure outflow

is same as downstream reach

outflow

Structure outflow is same as

downstream reach outflow

Structure outflow is same as

downstream reach outflow

Section 2: WinTR-55

Run Model (Graphical) – WinTR-55

Click this…

…Select Areas, storms, and structure

trials

…Then Plot…


trials


trials


trials…

…to get graphical representation

Section 2: WinTR-55

Model Outputs• Use the numerical outputs to analyze peak flow,

time to peak, structure peak lag times, etc.• The graphical outputs can be used to estimate time

to drain a structure– (some communities require drain times of 24 to 48

hours)

Section 2: WinTR-55

8. detailed hydraulics analysis

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