Stormwater Infrastructure for Water Quality Management

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Stormwater Infrastructurefor Water Quality
Management

• Dr. Larry A. Roesner, P.E.
• CE 394K.2 Surface Water Hydrology
• University of Texas, Austin
• April 8, 1999

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The Next Generation of UrbanStorm Water
Management
Urban Runoff Quality Management Practices
Larry Roesner, Ph.D., P.E. Camp Dresser McKee
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The Urban Stormwater Problem
Flow
Quality
Regulated by Local Agencies
Regulated Principally by EPA TNRCC
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Increasing Imperviousness Increases Runoff
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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)

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The STORM Model(Storage Treatment Overflow
Runoff Model)
Precipitation
Wet-Weather Storage
Runoff
QCIA
Treatment
Overflow
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Typical Capture Curves
The Design Storm
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So Whats 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

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Stormwater Mgmt Must Address the Entire Flow
Frequency Curve
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The Fact Is
Simply reducing pollutants in the runoff to
the Maximum Extent Practicable will probably not
result in significant improvement to
the ecological condition of the receiving
waters Flow management is also required
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Urban Runoff Hydrology
The Design Storm
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
Small storms account for most of the runoff and
are affected most by urbanization.
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Maximized Water Quality Capture Volume
The Design Storm
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.)
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Maximized Volume for Texas
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The Stormwater Treatment Train
Pollution Prevention

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

Source Controls

• Filter Strips
• Swales
• Modular Pavement
• Infiltration Trenches

Treatment Controls

• Extended Detention
• Retention Ponds
• Wetlands

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Design of Source Controls
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Minimize Directly Connected Impervious Area

• Drain Hard Lot Surfaces onto Pervious Areas
• Use Modular Pavement where Feasible
• Drains Streets to Swales

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Lot/Site Drainage
Depressed Grassed Area
Modular Pavement
Grassed Parking Area Reinforced with Geotextile
Fabric
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Lot/Site Drainage
Percolation Trench
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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

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Use Swales for Road and Parking Lot Drainage
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Design Criteria Swales

• Provide 1-2 slope
• Max V lt 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

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Infiltration Basins
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Design CriteriaInfiltration Basins

• Seasonal groundwater or bedrock gt 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 gt 0.3 in/hr
• Size to drain Po in 12 hour
• Use point system in book for rating suitability
  of a site

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Design of Treatment Controls
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Extended Detention
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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 14

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Extended Detention (cont)
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Detention with Filtration
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Detention with Filtration Classic Application
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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

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Retention
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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

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Constructed Wetland
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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

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Where Can I Learn More?
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North Central Texas Council of Governments
(1995)
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The Joint ASCE/WEF Manual of Practice
Pragmatic
Broadly Based
(1998)
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Targeting Highway Runoff
(1997)
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The Internetwww.txnpsbook.org
(1999)
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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