Hydrograph Modification Management

Using Simplified Low Impact Development Design

Christie Beeman, P.E.Philip Williams & Associates

runoff

infiltration

precipitation

Hydrograph Modification

Hydrograph Modification

Run

off

Time

Pre-Development

Urbanization tends to increase stormwater runoff:

volume

peak flows

frequency

Post-Development

Hydrograph Modification: Flow Peak

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 1 2 3 4 5 6 7 8 9 10

Recurrence Interval (years)

Peak

Flo

w (c

fs)

Impervious Drainage Area Runoff

Group D Soil Runoff

Group A Soil Runoff

Hydrograph Modification: Flow Duration

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0.00% 0.02% 0.04% 0.06% 0.08% 0.10% 0.12% 0.14% 0.16% 0.18% 0.20%

% Time Exceeded

Flow

(cfs

)

Impervious Drainage Area Runoff

Group D Soil Runoff

Group A Soil Runoff

Hydrograph Modification Management

SFRWQCB adding C.3 to Bay Area NPDES permits

C.3.f requires Hydrograph Modification Management Plan (HMP)– Control runoff peak flows &

durations to pre-project levels

– Prevent erosion impacts to receiving streams

Contra Costa HMP - approach

Encourage LID for Hydrograph Modification Management

Guidelines already in place for water quality treatment– Typical details

– Sizing spreadsheet

Expand for hydrograph modification management

Contra Costa HMP: Compliance Options Compare Pre- and Post-project Site Conditions

Implement LID using IMP GalleryImplement LID using IMP Gallery– Design details, specifications and sizing factorsDesign details, specifications and sizing factors

Demonstrate flow peak and duration mitigation – Continuous hydrologic model

– Flow peak and duration control

Assess risk of erosion impacts (receiving stream) – Demonstrate low risk of erosion

– Propose alternative mitigation

LID Integrated Management Practices

runoff

precipitation

infiltration

– Mimic natural hydrologic conditions• Retention, infiltration

– Can fit in site landscaping– Practical for small projects

Contra Costa HMP

Challenge:Challenge:Hydrograph Modification Management

IMP design guidance – Make IMP design and sizing easy

– Demonstrate peak and duration control

Solution:Solution:Develop IMP sizing factors using

continuous simulation hydrologic model

IMP Design Guidance – sizing factors Develop Pre-Project and Post-Project Runoff

Time Series using HPSF– Flow peak and duration plots

Build IMPs into the HSPF model

Adjust IMP size until peak and duration plots below pre-project– Sizing factor = (facility size)/(area managed)– Most sizing factors between 0.05 and 0.13

LID Integrated Management Practices

Planter box

Infiltration trench/basin

Vegetated swale

Bioretention basin

Dry well

Flow-Through Planter Example

Flow-Through Planter: Flow Peak Control

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0 1 2 3 4 5 6 7 8 9 10

Recurrence Interval (years)

Peak

Flo

w (c

fs)

IMP OutflowPervious Area Runoff0.5Q2

Flow-Through Planter: Flow Duration Control

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.00% 0.02% 0.04% 0.06% 0.08% 0.10% 0.12% 0.14% 0.16% 0.18% 0.20%

% Time Exceeded

Flow

(cfs

)

IMP OutflowPervious Area Runoff0.5Q2Q10

IMP Sizing Worksheet

Define Project

DMA Input

IMP Sizing

Total Drainage Area (SF):Number of Post-project DMA*:Select Rain Gauge:

Total: SF

Bottom Width: ftSwale Depth: ft

NoneNone

NoneNone

Dimension: UnitsDimension: Units

Dimension: UnitsDimension: Units

Dimension: UnitsDimension: Units

Dimension: UnitsDimension: Units

Dimension: UnitsDimension: Units

Dimension: UnitsDimension: Units

Dimension: UnitsDimension: Units

Units

Units

Units

Units

Units

Units

Select IMP Type IMP Dimensions IMP Size

Units

Surrounding Soil Group

CFS

Underdrain Discharge Limit

CFS

CFS

Post-project

Select IMP ID to Manage Runoff

SFTotal:

Units

LF

SF

SF

Units

Units

Units

Units

Units

Units

IMP ID

Pre-project

Project Name:

DMA ID

Hydrologic Soil Group

Land CoverArea of DMA (SF)

No. of Identical

Areas

Total Area (SF)

Example

145,000

6

1 40,0001 NoneScrub Scrub

1 50,0002 AScrub Scrub

3 15,0003 BScrub Impervious

2 10,0004 CScrub Impervious

1 15,0005 AScrub Impervious

1 15,0006 AScrub Grass

A

D

A

D

D

D

A 6

B

C

287

400

200

1

145,000

40,000

50,000

5,000

5,000

10,000

15,000

5,618

*DMA = Drainage Management Area

Vegetated/Grassy Swale

In-Ground Planter

In-Ground Planter

D

A

D

Martinez 2S (Mean Annual Precip = 20.2 inches)

N/A

0.027

N/A

IMP Sizing Worksheet - example

    

A soilsD soils

IMP Sizing Worksheet – Delineate DMAs

Total Drainage Area (SF):Number of Post-project DMA*:Select Rain Gauge:

Project Name: Example

145,000

6

*DMA = Drainage Management AreaMartinez 2S (Mean Annual Precip = 20.2 inches)

66

55

3c3c

3a3a

3b3b

4a4a

4b4b

2211

66

55

3c3c

3a3a

3b3b

4a4a

4b4b

2211

IMP Sizing Worksheet – DMA inputs

DMA ID

Area of DMA (SF)

1

2

3

4

5

6

40,000

50,000

5,000

5,000

10,000

15,000

Total: SF

No. of Identical

Areas

Total Area (SF)

1 40,000

1 50,000

3 15,000

2 10,000

1 15,000

1 15,000

145,000

IMP Sizing Worksheet – DMA inputs

66

55

3c3c

3a3a

3b3b

4a4a

4b4b

2211

DMA ID

1

2

3

4

5

6

SF

Hydrologic Soil Group

A

D

A

D

D

D

IMP Sizing Worksheet – DMA inputsSelect IMP ID

to Manage Runoff

None

A

B

C

A

A

DMA ID

1

2

3

4

5

6

66

55

3c3c

3a3a

3b3b

4a4a

4b4b

2211

B

B

B

C

C

A

IMP Sizing Worksheet – Select IMPs

1 2

Select IMP Type Surrounding Soil Group

IMP ID

A

B

C

Vegetated/Grassy Swale

In-Ground Planter

In-Ground Planter

D

A

D 66

55

3c3c

3a3a

3b3b

4a4a

4b4b

BB

BB

BB

CC

CC

AA

2211

IMP Sizing Worksheet –IMP dimensions

1 2

IMP ID

A

B

C

Bottom Width: ftSwale Depth: ft

NoneNone

NoneNone

Dimension: UnitsDimension: Units

Dimension: UnitsDimension: Units

IMP Dimensions

6

1

66

55

3c3c

3a3a

3b3b

4a4a

4b4b

BB

BB

BB

CC

CC

AA

2211

IMP Sizing Worksheet –IMP size

1 2

IMP ID

A

B

C

IMP Size Underdrain Discharge Limit

LF

SF

SF

Units

Units

287

400

200

Units

Units

CFS

Underdrain Discharge Limit

CFS

CFS

N/A

0.027

N/A 66

55

3c3c

3a3a

3b3b

4a4a

4b4b

BB

BB

BB

CC

CC

AA

2211