Stormwater Infrastructurefor Water Quality

Management

Dr. Larry A. Roesner, P.E.

CE 394K.2 Surface Water Hydrology

University of Texas, Austin

April 8, 1999

Urban Runoff Quality Management Practices Urban Runoff Quality Management Practices

The Next Generation of UrbanThe Next Generation of UrbanStorm Water ManagementStorm Water Management

Larry Roesner, Ph.D., P.E.

Camp Dresser & McKee

Larry Roesner, Ph.D., P.E.

Camp Dresser & McKee

The Urban Stormwater Problem

Flow Quality

ReceivingWater

Regulated Principallyby EPA & TNRCC

Regulated byLocal Agencies

Increasing Imperviousness Increases Runoff

Return Period, yrs

FlowRate

Stormflow Impacts

• 100 year peak flow increases 2 X

• 15 year peak flow increases 3 X

• 2 year peak flow increases 57 X (Denver)

• 2-yr peak flow occurs – 3 X per year (residential development)– 6 X per year (commercial development)

The STORM Model(Storage Treatment Overflow Runoff Model)

Treatment

Wet-WeatherStorage

Q=CIARunoff

Overflow

Precipitation

Typical Capture Curves

50% Impervious

100% Impervious

Treatment Volume Required, inches

0.5

1.0

Per

cen

t C

aptu

re o

f A

nn

ual

Ru

no

ff

200

50

100

90

The Design Storm

So What’s the Problem

• BMPs target the control of the quality of runoff

• Conventional drainage facilities control downstream flooding

• Neither of these activities has as its objective protection of the aquatic environment - If it occurs, it is incidental

Stormwater Mgmt Must Address the Entire Flow

Frequency Curve

Return Period, yrs

FlowRate

Frequency Curvewith Flood & WaterQuality Controls

The Fact Is:

Simply reducing pollutants in the runoff to theMaximum Extent Practicable will probablynot result in significant improvement to theecological condition of the receiving waters

Flow management is also required

Urban Runoff Hydrology

Small storms account for most of the runoff and are affected most by urbanization.

85 percent of the storms in east Texas are less than 1 inch of rainfall

85 percent of the storms in west Texas are less than 0.65 inches

The Design Storm

Maximized Water Quality Capture Volume

Po= a C PA

where:

Po = Maximized Water Quality Capture Volume (in.)

a = Capture Volume Coefficient

C = Watershed Runoff Coefficient

PA= Mean Storm Precipitation Volume (in.)

The Design Storm

Maximized Volume for TexasPo= a C PA

a = 1.3 - 1.6 for 85% capture of annual runoff

PA = 0.6 inches in east Texas (wet) = 0.4 inches in west Texas (dry)

Po (wet) = 0.12 - 0.14 inches (residential, C=0.15) = 0.70 - 0.86 inches (commercial, C=0.9)

Po (dry) = 0.08 - 0.10 inches (residential, C=0.15) = 0.50 - 0.58 inches (commercial, C=0.9)

The Design Storm

The Stormwater Treatment Train

• Public Education• Spill Prevention• Used Oil Recycling• Lawn Chemical Mgmt

• Filter Strips• Swales• Modular Pavement• Infiltration Trenches

Pollution Prevention

Treatment Controls

Source Controls

• Extended Detention• Retention Ponds• Wetlands

Design of Source Controls

Minimize Directly Connected Impervious Area

• Drain Hard Lot Surfaces onto Pervious Areas

•Use Modular Pavement where Feasible

•Drains Streets to Swales

Lot/Site Drainage

Modular Pavement

Grassed Parking Area Reinforced with Geotextile Fabric

Depressed

Grassed Area

Lot/Site Drainage

Percolation Trench

Basic Design CriteriaPercolation Trench

• Seasonal High groundwater or bedrock more than 4 ft below trench bottom

• Do not locate in fill material or recompacted soils

• Soil should be type A or B with minimum hydraulic conductivity of 6.5 x 105 ft/sec

• Po based on lot size and %I

Use Swales for Road and Parking Lot Drainage

Design Criteria Swales

• Provide 1-2% slope• Max V < 1 ft/sec• Max bottom width, 8 ft• Min bottom width, 2 ft

• Minimum length 100 ft• Maximum water height for maximized storm than 1/2 the height of standing vegetation• Po sized for road runoff plus the portion of maximized storm not captured on building site

Infiltration Basins

Design CriteriaInfiltration Basins

• Seasonal groundwater or bedrock > 4 ft below basin bottom

• Do not locate on fill or compacted soils

• Soil must be type A or B with saturated surface infiltration rate > 0.3 in/hr

• Size to drain Po in 12 hour

• Use point system in book for rating suitability of a site

Design of Treatment Controls

Extended Detention

Extended DetentionDesign Criteria

• Size to detain Po for 12

to 24 hours, then add 20%

for sediment storage• Use two stage design (empty less than 50% of

volume in first 1/3 of detention time• Sediment forebay recommended• Clogging outlets are most common failure• Emergency spillway• Sideslopes 1:4

Extended Detention (cont)

Detention with Filtration

Detention with Filtration Classic Application

Detention with FiltrationDesign Criteria

• Capture Po or 1/2 inch of runoff from impervious area• 24 - 40 hour drawdown time• Minimum sand bed = 18 inches• Seal bottom of filter chamber •Underdrain the sand filter

• Provide smooth flow transition from presedi- mentation chamber to filter chamber

Retention

Retention - Design Criteria

• Design by one of two methods - Solids-settling theory - Lake eutrophication theory• Both facilities are larger than an extended detention basin for the same drainage area• For biochemical design, size to hold runoff from wettest two weeks for 14 days• Design as regional facilities as landscape amenity

Constructed Wetland

Constructed WetlandDesign Criteria

• Use same guidelines as biological retention, but detention time is 14 days during wettest month• Open water is less that 50% of total facility surface area• Use a wetland biologist for developing planting program

Where Can I Learn More?

North Central Texas Council of Governments

(1995)

The Joint ASCE/WEF Manual of Practice

Pragmatic

BroadlyBased

(1998)

Targeting Highway Runoff

(1997)

The Internetwww.txnpsbook.org

(1999)

Summary

• Design for the small storm

• Minimize Directly Connected Impervious Area

• Use the treatment train concept

• Design outlet controls as multi-stage to reproduce natural flow frequency curve