WWF Water Balance Applications Steve van Haren, P.Eng. Senior Project Engineer MMM Group
Toronto’s Fast-Paced Development
STORMWATER STORM WATER MANAGE MANAGEMENT MENT TECHNIQ TECHNIQUES UES | Octo October ber 19, 19, 2007 2007
Stormwater Management Traditional SWM Procedure
Flood 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 Guidelines Heavier Focus on Water Quality Measures Must Address Water Balance Requirements
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance Rain
Infiltration, Evapotranspiration, Runoff
Split based on Vegetation, Imperviousness, Soil Type. Goal of Water Balance: Inside Toronto: ensure no increase in overland runoff. Development: Addressing Water Balance in sync with Flood Control and Water Quality Improvement
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance •
Inside Toronto •
•
•
Mostly Redevelopment, Isolated pockets of New Development Existing High Levels of Imperviousness No change or better runoff coefficient
Still Doesn’t Address Water Balance •
Water Balance – Focus on dealing with weekly RAINFALL rather than stormwater (intense, low frequency events)
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance Attempt to get site to the meaningful water balance needed to meet the goals of the WWFMMP. •
Grab FIRST 5 mm of rainfall – Surrogate for Water Balance Goals – Find another use instead of discharge to sewer.
RAIN
Collection by Gutters, Curbs, Catchbasins
Sheet or Pipe Flow to Infiltration, Cistern, Green Roof
Treatment by OGS / Vegetative or Other for TSS Removal
Spill to On-site Storage with Orifice / Weir Control Discharge to Outlet (Sewer / Creek)
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance: Project Planning Stage Strategy:
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Fill Water Balance Measures First!
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Spillover to Flood Control Structures
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Treatment by Water Quality Structures
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Discharge to Outlet
Sheet or Pipe Flow to Infiltration, Cistern, Green Roof STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance - Example
South Beach Condominiums Post-development Imperviousness: Soil Type: Water Balance Target:
62% BC 9 mm
Total Event Volume to be diverted from runoff:
93 m3
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
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)
h t p e D l l a f n i a R l a u n n A e g a r e v A l a t o T f o %
100 90 80 70 60 50 40 30 20 10 0 0
5
10
15
20
25
30
35
40
Daily Rainfall Depth (mm)
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance - Example
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance - Example
Green Roof Storage: Bioretention Storage: Bioswale Storage: Total:
Target 23.1 m3 28.0 m3 21.6 m3 72.7 m3
By Design 46.7 m3 49.4 m3 36.3 m3 132.4 m3
Note: Difference between 93 and 72.7 is direct rainfall on pervious, landscaped site areas.
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance - Example
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance – Infiltration - Concept Natural Filtration - Percolation Rate
•
Clay Silt Sand Gravel
– Extremely Low Percolation Rate – can be modified – Moderate Percolation Rates – Excellent Percolation Rates – Excessive Percolation Rates – Too High
2006 Building Code Supplement – Supplementary Standard SB-6 Percolation Time and Soil Descriptions Gravelly Sands through to Clayey Sands
2003 MOE SWM Planning & Design Manual Minimum Infiltration BMP Percolation Times
K, cm/sec
T, min/cm
BMP
T, mm/hr
10-1 – 10-3
2-8
Infiltration Basin
>= 60 mm/hr
10-2 – 10-4
4-12
Soakaway Pit
>= 15 mm/hr
10-3 – 10-5
8-20
Pervious Pipes
>= 15 mm/hr
10-4 – 10-6
12-50
Landscaped Areas
>= 15 mm/hrz
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
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 acceptable for 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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STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance Concept – Percolation Time Table 2 Approximate Relationship of Coarse Graned Soil Types to Permeability and Percolation Time Soil Type (Unified Soil Classification) Coarse Grained More than 50% Larger than #200
Coefficient of Permeability, K – cm/sec
Percolation Time, T – mins/cm
Comment
G.W. – Well graded gravels, gravel-sand mixtures, little or not fines
10
-1
<1
Very permeable unacceptable
G.P. – Poorly graded gravels, gravelsand mixtures, little or no fines
10
-1
<1
Very permeable unacceptable
-2-
-4
4 – 12
Permeable to medium permeable depending on amount of silt
-4
-6
12 – 50
Important to estimate amount of silt and clay
-1
-4
2 – 12
Medium permeability
10 - 10
-1
-3
2–8
Medium permeability
-3 -
-5
8 - 20
Medium to low permeability
10 - 10
-6
12 - 50
Medium to low permeability depending on amount of clay
2
3
4
G.W. – Silty gravels, gravel-sand-silt mixtures
10
- 10
G.C. – Clayey gravels, gravel-sand-clay mixtures
10 - 10
S.W. – Well graded sands, gravelly sands little or no fines
10 - 10
S.P. – Poorly graded sands, gr avelly sand, little or no fines S.M. – Silty sands, sand-silt mixtures
10
S.C. – Clayey sands, sand-clay mixtures
Column 1
10
-4
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance – Infiltration - Technique Porous Pavement
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance – Infiltration – Technique
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance – Infiltration - Technique Underpavement Disposal
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance – Infiltration - Technique Infiltration Basins
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance – Evapotranspiration - Principle Parking Lot Perimeter Bioretention
.
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance – Evapotranspiration - Technique Green Roofs
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance - Evapotranspiration Green Roofs
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance - Runoff Amount of stormwater captured for water balance measures is directly subtracted from Flood Control storage requirements. Offset of $$ for water balance measure installation Cold Weather Performance – Green roofs: Dormant vegetation in winter – evapotranspiration reduced some evapotranspiration remains, similar to surrounding vegetation. Snow melts slower than black roof Insulates from heated areas below No additional roof Loads, just longer melt times.
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance Key Points •
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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 water balance requirements!
STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
