Introduction to Design & Calculation Tools for LID
Transcript of Introduction to Design & Calculation Tools for LID
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Introduction to Design & Calculation Tools for LID
Frank W. Hahne, PE
NC Certified LID Professional #002
803-520-6731
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Various BMPs for Low Impact Design:
Bioretention (Rain Garden)
Infiltration Trench
Grassed Swale
Infiltration Swale (Bio-swale)
Vegetative Filter Strip
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Low Impact Design Tools:
TR-55 “Urban Hydrology for Small Watersheds”
Rain Garden (Bioretention) Spreadsheet
Infiltration Trench Spreadsheet
Vegetated Water Quality Swale and Enhanced WQ Swale (Bio Swale)
Vegetative Filter Strip Spreadsheet
Hydraulic Software (Pond Pack, HydroCad, Hydroflow, ICPR etc.)
“Site Evaluation Tool” S.E.T. (Developed by Tetra Tech )
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Question:
What storm do we use for water quality control?
1 inch rainfall
1 year storm
2 year storm
10 year storm
All storms
Answer:1 inch rainfall, sometimes called “First Flush”
Represents about 90% of annual storms and contains over 90% of pollutants in the runoff.
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1” Rainfall or 1” Runoff ?
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Water Quality Runoff:
WQv = ( 1.0RvA ) / 12 ( Schueler’s “Simple Method”)
Where WQv = water quality volume (acre-feet)
Rv = volumetric runoff coefficient
= 0.05 + 0.009 (I) , I = % Impervious
A = total drainage area (acres)
Example:
1 acre of parking lot with 1-inch rainfall
WQv = ((1.0 x (0.05 + 0.009 (100)) x 1 Ac) /12
= 3448 cubic feet of runoff
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Question:
What duration storm do we use for our design?
1 inch instantaneously
1 inch in one hour
1 inch in 24 hours
1 inch in 6 hours
Since the mean storm duration for this area (Charlotte, NC) is about 6 hours, we chose that storm event.
This storm yields a peak of 1.71 cfs per acre of impervious surface for Charlotte, NC.
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Bioretention ( Rain Gardens )
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Infiltration Trench
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Infiltration Pit/Trench
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Grassed Swale
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Design Considerations for Grassed Swale:
1. Keep velocities at or below 1 fps to allow settlement of solids.
2. Keep depth of flow below tip of grass for contact with suspended solids.
3. Use trapezoidal or parabolic shape for maximum capacity and low flow rate.
4. Keep side slopes at 3:1 or flatter for ease of mowing purposes.
5. Use trench drain at driveway crossing to minimize depth and ease of cleaning.
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Infiltration Swale ( Enhanced, or Bio Swale )
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Vegetated Filter Strip
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Site Evaluation ToolS.E.T.
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General Site Information
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Land Uses
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Drainage Area Breakdown
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Proposed BMPs
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BMP Storage & Storm Data
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User Defined BMPs
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Model Output
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Any Questions ?
Site Evaluation Tool http://www.charmeck.org/Departments/StormWater/Contractors/Huntersville+LID.htm