ADW 2: Downstream Analysis...downstream analysis basin. • Onsite basin is 8.95 acres so 10% basin is ~89.5 acres • Delineation can be done by hand or using USGS StreamStats website

ADW 2:Downstream Analysis

Source: NCDOT

WHY ARE WE HERE?

…to discuss the reasoning and methodology for downstream analysis

POINTS OF DISCUSSION

What is a downstream analysis?Why is a downstream analysis important?How is a downstream analysis performed?

When is a downstream analysis not required?

DOWNSTREAM ANALYSISWhy do we do it?

– to ensure no adverse impacts to downstream structures or properties arising from project related post-developed discharges

When is it required?– all projects with a “significant” post-

developed flow increase (not all increases require detention)

– to evaluate effects of water quantity control structures (detention)

CPv, Qp25, and Qf can be waived where discharge locations drain directly to channels that have a drainage area larger than 5 square miles.

DOWNSTREAM ANALYSIS

What is it?Comparison of hydrographs at:• Each project outfall• End of project’s zone of influence in the

downstream watershed, either:– point where drainage area is 10 times the project

site drainage area, or– large receiving water (lake, river, estuary)

• Intermediate locations of concern (downstream confluences, structures, conveyances)

Project site’s drainage area

Downstream 10% point

Typical analysis points

Importance & Advantages• Reduce high velocity runoff that causes

stream erosion• Analysis can reveal potential downstream

flood impacts• Prevent harmful and unnecessary

detention

DOWNSTREAM ANALYSIS

Tributary to Snake Creek, near Whitesburg, Georgia, showing a washed out bridge. Heavy rain caused flooding in and around the Atlanta area. (Photo from USGS)

DOWNSTREAM ANALYSISImportance & Advantages• Identifies necessary downstream improvements

- Bridges/culverts- Increase existing pipe capacity

DOWNSTREAM ANALYSIS

Importance & Advantages• Avoid potential legal issues

- Protect both owner and engineer

• Document and Record Analysis- Record Downstream Analysis in MS4 Post-

Construction Stormwater Report

DOWNSTREAM ANALYSIS PROCESS FLOWCHART



• Determine pre- and post-developed basin characteristics (area, CN, Tc)

• Obtain precipitation depth from NOAA website

• Compute pre- and post-developed peak discharges

Qp25 = Overbank Flood Protection = 25 year storm

Qf = Extreme Flood Protection = 100 year storm








NOAA’s Website Link: https://hdsc.nws.noaa.gov/hdsc/pfds/index.html

https://hdsc.nws.noaa.gov/hdsc/pfds/index.html








• If Qp25POST increased, stormwater management facility likely required

• Use software for efficient iterative facility design process (Hydraflow Hydrographs, PondPack, HydroCAD, etc.)



• Determine downstream 10% point

• Note downstream confluences, structures and conveyances and include analysis point at these locations


• Compute Qp25PRE and QfPRE at these locations

• Determine hydrographs of downstream basins using software

• Use reach routing to accurately model hydrograph timing


• Develop post-developed conditions model (with detention)

• Compare to pre-developed model at confluences, structures and conveyances


• Check hydrographs for downstream adverse impacts

• Pond designed to meet outflow requirements at site…

At project outfallPre-developed

Post-developed(with detention)


At project outfall

At downstream culvert



Pre-developedPost-developed

(with detention)

At project outfall


At downstream culvert

At 10% analysis point



Pre-developedPost-developed

(with detention)


• If adverse impacts exist, consider alteration of pond outlet control structure to mimic pre-development conditions


Consider the following to mitigate impacts of detention:

• Upgrade of downstream infrastructure

• Additional drainage easements from affected property owners

Evaluate for adverse effects on FEMA-regulated floodplain

EXAMPLE1.4 MILE ROADWAY WIDENING AND RECONSTRUCTION PROJECT IN CHEROKEE COUNTY

Project includes:

• intersection improvements,

• addition of turn lanes,

• urban roadway section, and

• sidewalks.

EXAMPLEPre-Developed Basin

Basin Outfall

Drainage Basin

EXAMPLEPost-Developed Basin

Basin Outfall

Drainage Basin

EXAMPLE1. Determine pre- and post-developed basin characteristics2. Compute pre- and post-developed peak discharges3. Determine if detention is needed4. Design detention as required5. Determine the basins of the 10% analysis point and all intermediate locations of

concern6. Determine basin physical parameters7. Determine pre-developed peak flows at desired study point8. Determine post-developed peak flows at desired study point without detention9. Determine post-developed peak flows at desired study point with detention10. Compare hydrographs11. Redesign pond Outlet Control Structure (OCS) as necessary

Repeat steps 6-11 for each study point

EXAMPLE

Step #1 – Determine pre- and post-developed basin characteristics

Basin Characteristics:

• Drainage Area: LIDAR, USGS topo maps, DEM

• Land Use: Aerial Photography, GIS datasets, StreamStats

• NRCS Soil Type: Web Soil Survey: http://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx

http://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx

EXAMPLEStep #1 – Determine pre- and post-developed basin

characteristics

EXAMPLEStep #2 – Compute pre- and post-developed peak discharges

EXAMPLE

Step #3 – Determine if detention is needed

• Compare pre and post: Drainage areas CNs Flows Tc

EXAMPLE

Detention pond design and the downstream analysis can be facilitated using software programs such as Hydraflow Hydrographs and PondPack

Step #4 – Design detention

EXAMPLE

• Detention pond design and the downstream analysis can be facilitated using software programs such as Hydraflow Hydrographs and PondPack

• Spreadsheet tools are available for computation of required volumes (WQv and CPv)


EXAMPLE

Create Pre- and Post-Developed basin inputsStep #4 – Design detention

EXAMPLE


EXAMPLE


EXAMPLE


EXAMPLE


EXAMPLEStep #4 – Design detention

EXAMPLE

Using L=2W plan layout coupled with volume estimation, you can design your detention pond.


EXAMPLEStep #4 – Design detention

Hydrograph output below shows that the 25-year pond routed outflows are less than or equal to pre-developed rates.

EXAMPLE

100-yr peak discharge from pond is much higher than pre-developed


EXAMPLE

Redesign the OCS for 100-yr Pre vs Post


EXAMPLE

• Point downstream where project basin is ~10% of the downstream analysis basin.

• Onsite basin is 8.95 acres so 10% basin is ~89.5 acres

• Delineation can be done by hand or using USGS StreamStats website tool.

Step #5 – Determine the basins of the 10% analysis point & intermediate locations of concern

https://streamstats.usgs.gov/ss/

https://streamstats.usgs.gov/ss/

EXAMPLEStep #5 – Determine the basins of the 10% analysis point &

intermediate locations of concern





Project Outfall



Project Outfall



EXAMPLE

• The downstream analysis point should be taken downstream

of the project site just upstream of the next confluence point if

moving the downstream analysis point to the confluence point

increases the size of the downstream analysis basin too

much.

• This may result in the downstream analysis basin to be less

than 10 times the size of the onsite area


EXAMPLE

Note: 10% basin is comprised of 2 stream basins


EXAMPLE

1: Southern segment of the confluence that the proposed project outfalls into directly


EXAMPLE

2: Northern stream segment of the confluence.


EXAMPLE

Sub-basin 1

Sub-basin 2

Project Outfall

10% analysis point @

confluence

Study point just upstream of confluence


EXAMPLE

StreamStats allows for a shapefile of the drainage basin and its longest flow path (Tc computation) to be downloaded

Step #6 – Determine basin characteristics of desired study point

EXAMPLE

To retrieve basin characteristics, select “Continue”.


EXAMPLE


EXAMPLE


EXAMPLE

EXAMPLE

• Tc – Lag Method• Used for drainage basins where a large segment of

the area is rural in character and has a long hydraulic length

• Can be adjusted to smaller urban watersheds


EXAMPLE

Tc = 𝑳𝑳𝟎𝟎.𝟔𝟔

, and 𝑳𝑳 = {𝒍𝒍0.8(S+1)0.7}/1900Y0.5

where:• L = Lag, hr • Tc = Time of Concentration, hr• 𝑙𝑙 = flow length, ft• Y = average watershed land slope, %• S = maximum soil retention, in• S = 1000

𝐶𝐶𝐶𝐶- 10

• CN = NRCS curve number


EXAMPLE


EXAMPLEModeling Scenarios:

• Determine the pre-developed peak flows at the desired study point

• Determine post-developed peak flows at desired study point without detention

• Determine post-developed peak flows at desired study point with detention

EXAMPLE

Use the same process for on-site basin to input the physical parameters of the downstream basin

Step #7 – Determine the pre-developed peak flows at the desired study point

EXAMPLE

Perform channel reach routing to the downstream study point.


EXAMPLE

Combine previous hydrographs• downstream basin (sub-basin 1)• pre-developed basin reach routing


EXAMPLE

The resulting hydrograph reflects pre-developed conditions at the downstream study point


EXAMPLE

Combine previous hydrographs

• downstream basin (sub-basin 1)

• post-developed basin reach routing

Step #8 – Determine post-developed peak flows at desired study point without detention

EXAMPLEStep #8 – Determine post-developed peak flows at desired study

point without detention

EXAMPLE

Combine previous hydrographs

• routed pond• post-developed basin

reach routing

Step #9 – Determine post-developed peak flows at desired study point with detention

EXAMPLEStep #9 – Determine post-developed peak flows at desired

study point with detention

EXAMPLE

Below are the comparison hydrographs for the study point upstream of the confluence (25-year)

Step #10 – Compare hydrographs

EXAMPLE

Below are the comparison hydrographs for the study point upstream of the confluence (100-year)

Step #10 – Compare hydrographs

EXAMPLE

Over-detention may be required to mitigate adverse effects at the study point.

This step is not applicable for our example.

Step #11 – Redesign pond OCS as necessary

EXAMPLETo review:

1. Analyzed pre- and post-developed onsite basins and determined basin needs detention.

2. Designed a detention pond that meets the 25- and 100-yr requirements at the site and at the downstream study point upstream of the confluence.

3. Now, we need to check the same requirements at the 10% study point. To accomplish this, repeat steps 6-11 for the secondary downstream basin that drains to the 10% point.

EXAMPLEStep #6 – Determine basin characteristics of desired study point

EXAMPLE

• Modeling the 10% basin, including the northern stream segment of the confluence

• Model with physical parameters from StreamStats

Step #7 – Determine the pre-developed peak flows at desired study point

EXAMPLE

Combine the hydrographs

• Northern downstream basin hydrograph (sub-basin 2)

• Pre-developed conditions upstream of the confluence

Step #7 – Determine the pre-developed peak flows at desired study point

EXAMPLEStep #7 – Determine the pre-developed peak flows at desired study

point

EXAMPLE

Combine the hydrographs • Northern downstream basin hydrograph (sub-basin 2)• Post-developed conditions without detention at the

study point upstream of the confluence

Step #8 – Determine post-developed peak flows at the desired study point without detention

EXAMPLEStep #8 – Determine post-developed peak flows at the desired study

point without detention

EXAMPLE

Combine the hydrographs

• Northern downstream basin (sub-basin 2)

• Post-developed conditions with detention at the study

point upstream of the confluence

Step #9 – Determine post-developed peak flows at desired study point with detention

EXAMPLEStep #10 – Compare hydrographs

EXAMPLEStep #10 – Compare hydrographs

EXAMPLE

Re-design pond to over-detain further to mitigate adverse effects at the 10% pointFirst trial reduces weir length in half

Step #11 – Redesign pond OCS as necessary

EXAMPLE

EXAMPLE

EXAMPLE

Need to reduce the weir length even further…Step #11 – Redesign pond OCS as necessary

…until a good design is achieved.

EXAMPLE

EXAMPLE

Detention Outfall

Study Point above confluence

Study Point at confluence

EXAMPLE

SUMMARY

1. Determine pre- and post-developed basin characteristics

2. Compute pre- and post-developed peak discharges

3. Determine if detention is needed

4. Design detention as required

5. Determine the basins of the 10% analysis point and all intermediate locations of concern

6. Determine basin physical parameters

SUMMARY7. Determine pre-developed peak flows at desired study point

8. Determine post-developed peak flows at desired study point without detention

9. Determine post-developed peak flows at desired study point with detention

10.Compare hydrographs

11.Redesign pond OCS as necessary

Repeat steps 6-11 for each study point

QUESTIONS

Brad McManus, PE MS4 Program ManagerOffice of Design Policy and Support [email protected]

mailto:[email protected]

Documents

ADW 2: Downstream Analysis...downstream analysis basin. • Onsite basin is 8.95 acres so 10% basin is ~89.5 acres • Delineation can be done by hand or using USGS StreamStats website