Cassandra Deedee Fagan, PE Robin Lee, PE 1 What is HMP? - - PowerPoint PPT Presentation

Cassandra “Deedee” Fagan, PE Robin Lee, PE

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What is HMP?

 Hydromodification is the change in natural hydrologic processes caused by urbanization.  Hydromodification plans (HMP) are implemented to prevent erosion from increases in runoff peak flow, volumes, and durations from development.  HMP = detention basin

 BAHM compares pre-, and post- development (mitigated) runoff and has tools to design HMP control structures  This includes: ponds, detention basins, tanks, and vaults.

*San Mateo County Public Works - Streambank Erosion *SCVURPPP HYDROMODIFICATION MANAGEMENT PLAN FINAL REPORT

HMP Pond in Livermore

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Outlet Structure

What Projects Require HMP

 NPDES MRP 2.0 requirement (Permit Section C.3.g)

 Add/replace 1 acre impervious

AND

 Located in HMP applicable area

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BAHM Background

 BAHM2013 developed by Clear Creek Solutions for:

 Alameda County  Santa Clara County  San Mateo County

 BAHM is a tool to size HMP controls to meet permit requirements  Available for free from Clear Creek Solutions, Inc.

 http://clearcreeksolutions.info/sample-page/

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BAHM Overview

1.

BAHM uses continuous simulation hydrology that models the entire hydrologic cycle for multiple years

2.

Flow duration is based on erosive flows

 Flow Range: 10% of 2-year to 10-year

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Flow (Q)

Example 1

 2 acre development in Alameda County  Currently site has 0.5 acres impervious  Site wants to increase to 1.5 acres impervious  Mitigated scenario will use a trapezoidal pond with outlet control structure for HMP

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Scenario Impervious Area (acres) Pervious Area (acres) Grand Total Mitigated 1.5 0.5 2 C/D, Urban, Flat (0-5%) 0.5 0.5 Driveways, Flat (0-5%) 0.25 0.25 Roads, Flat (0-5%) 0.5 0.5 Roof Area 0.75 0.75 Scenario Impervious Area (acres) Pervious Area (acres) Grand Total Pre-Project 0.5 1.5 2 C/D, Grass, Flat (0-5%) 1.5 1.5 Roof Area 0.5 0.5

Pre-Project Mitigated

BAHM Files

• Project file: _______.WHM
• Backup project file: _______.WH2
• Database file: _______.WDM
• Report file: _______.RTF or
• Report file: _______.PDF

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BAHM Example 1 – Trapezoidal Pond

1.

Open BAHM Example Problem 1. In this screen you will see the project location is in Alameda.

2.

Confirm the precipitation factor is 0.8 using the Berkeley gage

3.

Select the (A) General Project Information tab from the ribbon to see the Pre-Project land use basin:

4.

(B) Select the land use basin as the point of compliance (POC) 5.

(C) Run the Pre-Project Scenario

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A B C

BAHM Example 1 – Trapezoidal Pond

1.

Switch to the Mitigated project scenario to view the post-development landuse basin.

2.

To create a trapezoidal pond, (A) drag the element to the gridded screen shown below:

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3.

The landuse basin has (B) been connected to a trapezoidal pond and,

4.

The trapezoidal pond has (C) been selected as the point of compliance

A B C

Example 1 –BAHM User Resources

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 Helpful tools in BAHM:

1.

BAHM User Manual

2.

Trapezoidal Pond Help

3.

Facility Dimensions Diagram

1 2 3

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Example 1- AutoPond

 AutoPond – analyzes the pre- and mitigated scenario flows to:

(1) Create pond design (will take a few minutes to run) (2) Optimize the Pond outflow structures (will take a few minutes to run) (3) Accept optimized pond design

12 Adjust optimization time (1) Select Create Pond (2) Select Optimize Pond (3) Select Optimize Pond

Example 1 – Pond Dimensions

(1) Confirm that pond dimensions make sense AND (2) Run the Mitigated Scenario with updated pond dimensions

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Example 1 – Analyze Pond Results



View the Duration Analysis – compares the pre- and mitigated flows for 10% of 2- year – 100% of 10-year



Often a Pond will not pass the first run, and this window will show you where the pond fails to meet the HMP requirements

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Example 1 – Analyze Pond Results

 View the flow frequency

(2-year to 25-year events)

 BAHM computes the pond

drawdown/retention depth (stage) for 1-5 days plus max. stage and drawdown time

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Example 1 – BAHM Report



(A) Open the Report tab from the Ribbon



BAHM Report summarizes the model



Draft report is the default - used for preliminary submittals



Uncheck “Draft Report” to remove Draft watermark

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A

Things to Check

 Volume check, i.e. 1-ft freeboard above riser structure

for open ponds

 Pond area required is the pond top surface area, not

bottom

 Max stage is reasonable  Drawdown time of vault/pond  Create Report in Model

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Example 2 - Vault

 4 acre industrial site  Replacing 1 acre impervious and adding 0.5 acres

imperious

 Developer wants underground storage vault

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Set 1: Located Project

 Alameda Island  Precip Factor should be 0.8  Gage = Berkeley

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Set 2: Set Up Pre- Project Basin

 Pre-Project Basin

1.

Drag Landuse basin onto grid

2.

Enter landuse data into basin

20 Scenario I mpervious Area (acres) Pervious Area (acres) Grand Total Predevelopment (Total) 1.5 1.5 3

C/D, Grass, Flat (0-5% ) 1.5 1.5 Roof Area 1.5 1.5

Set 2: Set Up Mitigated Basin

 Mitigated Basin

1.

Drag Landuse basin onto grid

2.

Enter landuse data into basin

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Scenario I mpervious Area (acres) Pervious Area (acres) Grand Total Mitigated (Total) 2.75 0.25 3

C/D, Urban, Flat (0-5% ) 0.25 0.25 Roads, Flat (0-5% ) 0.25 0.25 Roof Area 2.5 2.5

Add Vault to Mitigated Scenario

 Drag vault into grid  Connect basin to vault  Connect vault to POC

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Size Vault using Auto Vault

 Run pre basin scenario  Switch to the (1) Mitigated Scenario select the (2) vault, and (3) select Auto Vault

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1 2 1 2 3

Size Vault using Auto Vault

 Select Create Vault

(This will take a few minutes)

 Select Optimize Vault

(This will take a few minutes)

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1 2 3 4

Example 2 – Vault Dimensions

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(1) Confirm that vault dimensions make sense AND (2) re-run the Mitigated Scenario with updated pond dimensions

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Example 2 – Analyze Vault Results



View the Duration Analysis – compares the pre- and mitigated flows for 10% of 2- year – 100% of 10-year



Often a Vault will not pass the first run, and this window will show you where the pond fails to meet the HMP requirements

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Example 2 – Analyze Vault Results

 View the flow frequency

(2-year to 25-year events)

 BAHM computes the pond

drawdown/retention depth (stage) for 1-5 days plus max. stage and drawdown time

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Things to Check

 Volume storage required at max stage not at top of

riser

 Max stage is not higher than depth of vault  Drawdown time of vault/pond

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Quick Tips for Successful BAHM Results

 Run model from Desktop  If model has been changed over many times, copy and

save .whm file to new folder and start modeling again

 When sending model, only need to send the .whm file  Run model from multiple machines to test result

consistency

 Close and open model when issues arise  Start Over with .whm file when all else fails

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BAHM Checklist (1/2)

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BAHM2013 Reviewer Checklist: Yes No

• 1. Received BAHM project (WHM and WH2) files?
• 2. Received BAHM WDM (WDM) file?
• 3. Received BAHM report (PDF) file?
• 4. Project (WHM) file loads okay?
• 5. Project location matches location on BAHM screen?
• 6. Pre-project scenario runs okay?
• 7. Mitigated scenario runs okay?
• 8. Compare BAHM Report screen with report file:
• a. Project location descriptions match?
• b. Precipitation gages match?
• c. Precipitation scales match?
• d. Flow frequency results match?
• e. All flow duration values PASS?
• f. Any pervious (PERLND) land use changes?
• g. Any impervious (IMPLND) land use changes?
• h. Any scaling factor changes?
• i. Any duration criteria changes?
• j. Hydromodification facility (pond, tank, vault)

dimensions match?

• k. Hydromodification outlet structure info matches?
• 9. BAHM hydromodification facility dimensions match

drawings?

BAHM Reviewer Checklist (2/2)

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BAHM2013 Reviewer Checklist: Yes No

• 10. Compare outlet structure(s) in BAHM Report to

drawings:

• a. Do configuration and dimensions match,

including low flow orifice?

• b. If low flow orifice diameter is enlarged, is the

larger diameter mitigated according to User Manual Appendix D?

• 11. Are Precipitation Applied and Evaporation Applied
• ptions selected appropriately for each element?
• 12. Infiltration set to YES for infiltration facilities?
• 13. Infiltration Reduction Factor set to appropriate value

(consistent with User Manual Appendix D)?

• 14. Total BAHM drainage area matches drainage

maps/drawings?

• 15. Mitigated drainage area(s) match Pre-project?
• 16. Vegetation correct? (lawn as Urban, not Grass)?
• 17. Post-project land use areas correct?
• 18. Routing and land area conductivity correct?
• 19. Check facility drawdown (if included):
• a. Used POC Mitigated stage?
• b. Drawdown times okay?
• 20. Options set to default values?
• 21. Other issues?

BAHM submittal APPROVED?

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Optimization

1.

Trapezoidal Pond in example 1 has precip applied to the basin AND the area of the basin in the drainage area, therefore runoff is double counted. You can go back and remove the top surface area of the trapezoidal pond from the basin area

2.

Industrial site developer determines the underlying soils have infiltration capacity of 1.0 in/hr, how much can we reduce the vault size by?

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Optimization Example 1

 Remove the top surface area from drainage area in

Example 1

 Show Pond Table  Top Surface is 0.094 acres  Remove area from Pervious  Reduce pond dimensions  Revise basin area  Reduced top surface area from 0.094 to 0.08 acres

 15% surface area reduction

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Optimization – Example 2

 Currently vault is around 0.5 acre-feet of storage, can

this be reduced with infiltration?

 Use factor of safety of 2, which is entered as 0.5

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Extra Volume

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Optimization - Infiltration

 Leaving the outlet structure the same

 Reduces volume from 0.5 acre-feet to 0.38  24% reduction!

 Use AutoPond and Optimize new Outlet Structure

 Reduce volume down to 0.35  30% reduction

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Other Uses of BAHM

 Can revise storms to include up to 100-year  Determine flow rates for storm events (however, differs

from event based modeling)

 Determine ponding over wetland over 30 years to

determine appropriate plant selection

 Determine Stage-storage-discharge (SSD) tables that

can be entered into HMS

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