Stormwater Management Techniques

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WWF Water Balance ApplicationsSteve van Haren, P.Eng. Senior Project Engineer MMM Group


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STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007

Torontos Fast-Paced Development


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Stormwater Management

Traditional SWM ProcedureFlood Control

Control Peak Flows to Pre-Development Rates Maintain Pre-Development Runoff Coefficients

2, 5, 10, 25, 50, 100 Year Storms If Post-Dev Coefficient < Pre-Development, Nothing to Do!

Water Quality Control New Development, If direct discharge to Creek, 80% TSS

Removal Stormceptor (or other OGS / Structural)

If discharge to downstream SWM Pond Pre-treatment or nothing

City of Toronto: Wet Weather Flow Management GuidelinesHeavier Focus on Water Quality MeasuresMust Address Water Balance Requirements


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Water Balance

Rain Infiltration, Evapotranspiration, Runoff

Split based on Vegetation, Imperviousness, Soil Type.

Goal of Water Balance: Inside Toronto: ensure no increase inoverland runoff.

Development: Addressing Water Balance in sync with Flood Control and Water

Quality Improvement


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Water Balance

Inside Toronto

Mostly Redevelopment, Isolated pockets of New Development Existing High Levels of Imperviousness No change or better runoff coefficient

Still Doesnt Address Water Balance

Water Balance Focus on dealing with weekly RAINFALL rather than stormwater (intense, low frequency events)


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Water Balance

Attempt to get site to the meaningful water balance needed tomeet the goals of the WWFMMP.

Grab FIRST 5 mm of rainfall Surrogate for Water BalanceGoals Find another use instead of discharge to sewer.

RAINCollection by Gutters, Curbs,

Catchbasins

Sheet or Pipe Flow toInfiltration, Cistern, Green Roof

Spill to On-site Storage withOrifice / Weir Control

Treatment by OGS / Vegetativeor Other for TSS Removal

Discharge to Outlet(Sewer / Creek)


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Water Balance: Project Planning Stage

Strategy:

Sheet or Pipe Flow to

Infiltration, Cistern, Green Roof

Spillover to Flood Control Structures

Treatment by Water Quality Structures

Discharge to Outlet

Fill Water Balance Measures First!


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Water Balance - Example

South Beach Condominiums

Post-development Imperviousness: 62%Soil Type: BCWater Balance Target: 9 mm

Total Event Volume to be divertedfrom runoff: 93 m 3


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Water Balance


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Water Balance

Figure 1a - % of Total Annual Average Rainfall Depth Vs. Daily Rainfall Amounts(based on 1991 Toronto Rainfall Data from 16 Rain Gauge Stations)

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30 35 40

Daily Rainfall Depth (mm)

% o

f T o

t a l A v e r a g e

A n n u a l

R a i n

f a l l D e p

t h


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Target By DesignGreen Roof Storage: 23.1 m 3 46.7 m 3

Bioretention Storage: 28.0 m 3 49.4 m 3

Bioswale Storage: 21.6 m 3 36.3 m 3

Total: 72.7 m 3 132.4 m 3

Note: Difference between 93 and 72.7 is direct rainfall on pervious, landscapedsite areas.


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Water Balance Infiltration - Concept Natural Filtration - Percolation Rate

Clay Extremely Low Percolation Rate can be modifiedSilt Moderate Percolation RatesSand Excellent Percolation RatesGravel Excessive Percolation Rates Too High

2006 Building Code Supplement SupplementaryStandard SB-6

Percolation Time and Soil DescriptionsGravelly Sands through to Clayey Sands

K, cm/sec T, min/cm10 -1 10 -3 2-8

10 -2 10 -4 4-12

10 -3 10 -5 8-20

10 -4 10 -6 12-50

2003 MOE SWM Planning & Design ManualMinimum Infiltration BMP Percolation Times

BMP T, mm/hr Infiltration Basin >= 60 mm/hr

Soakaway Pit >= 15 mm/hr

Pervious Pipes >= 15 mm/hr

Landscaped Areas >= 15 mm/hrz


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Water Balance Infiltration - Concept

15 mm/hr (MOE Units) = 1.5cm / 60 min = 40 min/cm (SB-6 Units)

Interpolating into SB-6 table K >= 2.71x10 -5 cm/s acceptablefor minor infiltration purposes

(i.e. infiltration trench, landscaping areas, soakaway pits, pervious pipes, etc.)

Lower K soils good for larger, lower head water balance BMPs Porous pavement, landscaping application

Higher K soils good for smaller, higher head water balance BMPs Infiltration trench, bioretention.


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Water Balance Concept Percolation TimeTable 2

Approximate Relationship of Coarse Graned Soil Types to Permeability and Percolation Time

Soil Type(Unified Soil Classification)

Coarse GrainedMore than 50% Larger than #200

Coefficient ofPermeability,K cm/sec

Percolation Time,T mins/cm Comment

G.W. Well graded gravels, gravel-sandmixtures, little or not fines

10 -1


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Water Balance Infiltration - Technique

Porous Pavement


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Water Balance Infiltration Technique


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Underpavement Disposal


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Infiltration Basins


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Water Balance Evapotranspiration - Principle

Parking Lot Perimeter Bioretention

.


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Water Balance Evapotranspiration - Technique

Green Roofs


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Water Balance - Evapotranspiration

Green Roofs


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Water Balance - Runoff

Amount of stormwater captured for water balance measures is directlysubtracted from Flood Control storage requirements.

Offset of $$ for water balance measure installation

Cold Weather Performance Green roofs:

Dormant vegetation in winter evapotranspiration reducedsome evapotranspiration remains, similar to surroundingvegetation.

Snow melts slower than black roof

Insulates from heated areas belowNo additional roof Loads, just longer melt times.


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Water Balance

Key Points Utilize Water Balance measures first Spill to flood control and

water quality controls Vegetative measures may be cheaper to install and maintain

when compared to below ground, structural measures Soils with clay content may still support infiltration Use landscaping, grading and sheet flow to maximum benefit

- Preserve hydraulic head 5 mm rain volume diversion is minimum water balance target

Your site may require more!

Above, at and below grade areas all have potential to address waterbalance requirements!

Stormwater Management Techniques

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