stormwater infrastructure for water quality management
DESCRIPTION
Stormwater Infrastructure for Water Quality Management. Dr. Larry A. Roesner, P.E. CE 394K.2 Surface Water Hydrology University of Texas, Austin April 8, 1999. The Next Generation of Urban Storm Water Management. Urban Runoff Quality Management Practices. Larry Roesner, Ph.D., P.E. - PowerPoint PPT PresentationTRANSCRIPT
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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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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
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The Urban Stormwater Problem
Flow Quality
ReceivingWater
Regulated Principallyby EPA & TNRCC
Regulated byLocal Agencies
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Increasing Imperviousness Increases Runoff
Return Period, yrs
FlowRate
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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)
Treatment
Wet-WeatherStorage
Q=CIARunoff
Overflow
Precipitation
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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
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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
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Stormwater Mgmt Must Address the Entire Flow
Frequency Curve
Return Period, yrs
FlowRate
Frequency Curvewith Flood & WaterQuality Controls
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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
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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
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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
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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
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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
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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
Modular Pavement
Grassed Parking Area Reinforced with Geotextile Fabric
Depressed
Grassed Area
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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 < 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 > 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
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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 1:4
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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
BroadlyBased
(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