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					WWF Water Balance Applications
Steve van Haren, P.Eng. Senior Project Engineer
MMM Group
Toronto’s Fast-Paced Development




                         STORMWATER MANAGEMENT TECHNIQUES | October 19, 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.

                                  Collection by Gutters, Curbs,
          RAIN                             Catchbasins




                                        Sheet or Pipe Flow to
                                 Infiltration, Cistern, Green Roof



 Treatment by OGS / Vegetative                                              Spill to On-site Storage with
   or Other for TSS Removal                                                    Orifice / Weir Control

                                       Discharge to Outlet
                                        (Sewer / Creek)




                                                         STORMWATER MANAGEMENT TECHNIQUES | October 19, 2007
Water Balance: Project Planning Stage
Strategy:



        • Fill Water Balance Measures First!

        • Spillover to Flood Control Structures

        • Treatment by Water Quality Structures

        • 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:                 62%
   Soil Type:                                       BC
   Water Balance Target:                            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)


                                              100
   % of Total Average Annual Rainfall Depth




                                              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


                                        Target                 By Design
Green Roof Storage:                     23.1 m3                46.7 m3
Bioretention Storage:                   28.0 m3                49.4 m3
Bioswale Storage:                       21.6 m3                36.3 m3
Total:                                  72.7 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      – Extremely Low Percolation Rate – can be modified
Silt      – Moderate Percolation Rates
Sand      – Excellent Percolation Rates
Gravel    – Excessive Percolation Rates – Too High


2006 Building Code Supplement – Supplementary   2003 MOE SWM Planning & Design Manual
                 Standard SB-6                  Minimum Infiltration BMP Percolation Times
     Percolation Time and Soil Descriptions
    Gravelly Sands through to Clayey Sands
            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.




                                                  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
                                             Coefficient of
      (Unified Soil Classification)                           Percolation Time,
                                             Permeability,                                 Comment
             Coarse Grained                                     T – mins/cm
                                              K – cm/sec
     More than 50% Larger than #200

   G.W. – Well graded gravels, gravel-sand        10-1               <1           Very permeable
   mixtures, little or not fines                                                  unacceptable

   G.P. – Poorly graded gravels, gravel-          10-1               <1           Very permeable
   sand mixtures, little or no fines                                              unacceptable

   G.W. – Silty gravels, gravel-sand-silt      10-2- - 10-4        4 – 12         Permeable to medium
   mixtures                                                                       permeable depending on
                                                                                  amount of silt

   G.C. – Clayey gravels, gravel-sand-clay     10-4 - 10-6         12 – 50        Important to estimate
   mixtures                                                                       amount of silt and clay

   S.W. – Well graded sands, gravelly          10-1 - 10-4         2 – 12         Medium permeability
   sands little or no fines

   S.P. – Poorly graded sands, gravelly        10-1 - 10-3          2–8           Medium permeability
   sand, little or no fines

   S.M. – Silty sands, sand-silt mixtures      10-3 - 10-5         8 - 20         Medium to low permeability

   S.C. – Clayey sands, sand-clay mixtures     10-4 - 10-6         12 - 50        Medium to low permeability
                                                                                  depending on amount of
                                                                                  clay

                   Column 1                         2                3                          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
   • 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