Sizing Stormwater Control Facilities to Sizing Stormwater Control Facilities to

Address Stream-Bank Erosion Control Address Stream-Bank Erosion Control

Sizing Stormwater Control Facilities to Sizing Stormwater Control Facilities to

Address Stream-Bank Erosion Control Address Stream-Bank Erosion Control

Anthony M. Dubin, PEBrown and Caldwelltdubin@brwncald.com

Anthony M. Dubin, PEBrown and Caldwelltdubin@brwncald.com

Major Topics to AddressMajor Topics to AddressMajor Topics to AddressMajor Topics to Address

1. Why Hydrograph Modification Management is important

2. How Low Impact Development controls work

3. Describing the technical analysis that generated the set of pre-sized IMPs

Effects of UrbanizationEffects of UrbanizationEffects of UrbanizationEffects of Urbanization

• Impervious surfaces produce higher runoff rates, volume and duration of large flows

Effects within the WatershedEffects within the WatershedEffects within the WatershedEffects within the Watershed

• Urbanization alters the watershed• Channels respond with incision and/or

armoring

Continuous Hydrologic Modeling Continuous Hydrologic Modeling Examines Full Range of Local Examines Full Range of Local ConditionsConditions

Continuous Hydrologic Modeling Continuous Hydrologic Modeling Examines Full Range of Local Examines Full Range of Local ConditionsConditions

Sizing to one ‘design storm’ is not enough

Peak Flow FrequencyPeak Flow Frequency(Partial Duration Statistics)(Partial Duration Statistics) Peak Flow FrequencyPeak Flow Frequency(Partial Duration Statistics)(Partial Duration Statistics)

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 1 2 3 4 5 6 7 8 9 10

Recurrence Interval (years)

Pea

k F

low

(cf

s)

Impervious

Pre-Project Site

0.5Q2

Identify all HSPF storms in record and rank

Flow DurationsFlow DurationsFlow DurationsFlow Durations

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

% Time Exceeded

Flo

w (

cfs

)

Impervious

Pre-Project SiteQ10

0.1Q2

Rank hourly outputs from HSPF model

Example IMP: In-Ground PlanterExample IMP: In-Ground PlanterExample IMP: In-Ground PlanterExample IMP: In-Ground Planter

18-in sandy loam

Peak Flow Matching ExamplePeak Flow Matching ExamplePeak Flow Matching ExamplePeak Flow Matching Example

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 1 2 3 4 5 6 7 8 9 10

Recurrence Interval (years)

Pea

k F

low

(cf

s)

ImperviousMitigated Post-Project SitePre-Project Site0.5Q2

IMP Reduces Impervious Runoff to Less Than Pre-Project Levels

Duration Matching ExampleDuration Matching ExampleDuration Matching ExampleDuration Matching Example

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

% Time Exceeded

Flo

w (

cfs

)

ImperviousMitigated Post-Project SitePre-Project SiteQ100.1Q2

IMP Reduces Impervious Runoff to Less Than Pre-Project Levels

BMP Sizing Factor SummaryBMP Sizing Factor SummaryBMP Sizing Factor SummaryBMP Sizing Factor Summary

IMP Sizing Factors

In-Ground Planter

Group A: 0.08Group B: 0.11Group C: 0.06Group D: 0.05

Flow-Through Planter

Group C: 0.06Group D: 0.05

Vegetated/Grassy Swale

Group A: 0.10 to 0.14

Group B: 0.14 to 0.21

Group C: 0.10 to 0.15

Group D: 0.07 to 0.12

Bioretention Basin

Group A: 0.13Group B: 0.15Group C: 0.08Group D: 0.06

IMP Sizing Factors

Dry Well Group A: 0.05 to 0.06Group B: 0.06 to 0.09

Infiltration Trench

Group A: 0.05 to 0.06Group B: 0.07 to 0.10

Infiltration Basin

Group A: 0.05 to 0.10Group B: 0.06 to 0.16

Infiltration Only:Under-Drain or Infiltration:

Adjusting IMP Sizing to Account Adjusting IMP Sizing to Account for Rainfall Variability for Rainfall Variability Adjusting IMP Sizing to Account Adjusting IMP Sizing to Account for Rainfall Variability for Rainfall Variability

Group A, y = 0.0020x + 0.08Group B, y = -0.0005x + 0.11Group C, y = -0.0022x + 0.06Group D, y = -0.0022x + 0.05

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

-14 -12 -10 -8 -6 -4 -2 0 2 4 6

Mean Annual Rainfall (MAP) Relative to Martinez Gauge (in)

Siz

ing

Fac

tor

Group A soils

Group B soils

Group C soils

Group D soils

Sizing Conclusions for Sizing Conclusions for ImplementationImplementationSizing Conclusions for Sizing Conclusions for ImplementationImplementation

• IMPs in Group D soil sites are generally smaller than Group A soil BMPs

• Steep side walls produce smaller sizing factors

• Sizing factor may be particularly important for on-site BMPs

• Swales and Bioretention basin footprint may be less important if BMPs fit into otherwise undeveloped space

Questions?Questions?Questions?Questions?

Contra Costa Approach to Contra Costa Approach to Hydrograph ModificationHydrograph ModificationContra Costa Approach to Contra Costa Approach to Hydrograph ModificationHydrograph Modification

• Encourage LID to control stormwater flows

• HMP is technically rigorous and easy to apply

• Assumes need to match pre-project condition

BMP GalleryBMP GalleryBMP GalleryBMP Gallery

BMP GalleryBMP GalleryBMP GalleryBMP Gallery

Instructions for Computing Local Instructions for Computing Local Sizing Factors Sizing Factors Instructions for Computing Local Instructions for Computing Local Sizing Factors Sizing Factors

1. Describe each DMA on the project site, including area, soil type, post-project surface type

2. For DMA's draining to IMPs, select an IMP and configuration (e.g. swale width, dry well depth)

3. Pick the appropriate sizing factor from the summary sizing factor table (see handout)

4. Compute the rainfall adjustment using the regression equations (see handout)

5. Local sizing factor = Sizing Factor x RainAdj