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Sizing Retention & Detention. UP 503 Fall 2011. Step 1:. Determine areas draining into basin Determine area occupied by: Low-density residential Medium-density residential Multi-family residential Industrial office Commercial. Step 2:. Multiply area by inches for retention/detention

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step 1
Step 1:
  • Determine areas draining into basin
  • Determine area occupied by:
    • Low-density residential
    • Medium-density residential
    • Multi-family residential
    • Industrial office
    • Commercial
step 2
Step 2:
  • Multiply area by inches for retention/detention
    • You now have watershed inches
steps 3 4
Steps 3 & 4:
  • Determine Volume
    • Volume = Site Area x Depth [in inches, from Table 1-5]
  • Determine Detention Basin Depth
    • Volume = Basin Area x Depth
    • So, Depth = Volume / Basin Area
example
Example:
  • Site Area:
    • 12 acres (your “watershed”) = 522,720 sq ft
  • Site Depth:
    • 0.2 inches uniformly over the watershed
      • Given, from Table 1-5, you have chosen to use a detention basin for a site that is 100% medium density residential
example1
Example:
  • Volume = Area x Depth = 12 ac x 0.2 in

= 2.4 ac inches

    • The Volume is 1 ac x 2.4 in, or 2.4 ac in
    • To convert the Volume to cubic feet:
      • 2.4 ac in = 2.4 in x 1 ac = 0.2 ft x 43,560 sq ft

= 8,712 cubic ft

Which means: for your site, if you have a one acre basin, it would have to be 2.4 inches (or 0.2 feet) deep--with vertical sides--to handle the water.

slide7
Area

3 x 208.71 ft

1 ac

4 x 208.71 ft

Area = 522,720 sq ft = 12 acres

0 2 watershed inches for a medium density residential detention basin
0.2 watershed inches for a medium density residential detention basin

3 x 208.71 ft

1 ac

0.2 inches

4 x 208.71 ft

Area = 522,720 sq ft = 12 acres

volume area x depth
Volume = Area x Depth

12 acres x 0.2 in = 2.4 ac in

2.4 in

(0.2 ft)

3 x 208.71 ft

1 ac

4 x 208.71 ft

Area = 522,720 sq ft = 12 acres

convert volume to cubic feet
Convert Volume to Cubic Feet

2.4 ac in = 2.4 in x 1 ac = 0.2 ft x 43,560 sq ft = 8,712 ft^3

3 x 208.71 ft

1 ac

4 x 208.71 ft

Area = 522,720 sq ft = 12 acres

example continued
Example continued:
  • A 1 acre Detention Basin was used in this example, but you can scale your Basin Size and Depth as necessary to fit your site
    • If you halve the area of the Basin (0.5 acre instead of a 1 acre), the Depth will double (4.8 in instead of 2.4 in)
      • Depth = Volume / Area
      • Depth = 2.4 ac in / 0.5 ac
      • Depth = 4.8 in
suggestions for assignment
Suggestions for Assignment
  • Using the acreages you determined for each of your different land use types, calculate the size and number of retention ponds/detention basins that your site will require.
  • Calculate the amount of retention/detention that will be required for your preferred scenario as well as the amount that would be required for a “conventional” scenario, and compare the results.
  • Create maps of where you plan to locate the ponds in both scenarios based on existing drainage and water flows on site.