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Sizing Retention & Detention

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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Sizing Retention & Detention

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  1. Sizing Retention & Detention UP 503 Fall 2011

  2. Step 1: • Determine areas draining into basin • Determine area occupied by: • Low-density residential • Medium-density residential • Multi-family residential • Industrial office • Commercial

  3. Step 2: • Multiply area by inches for retention/detention • You now have watershed inches

  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

  5. 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

  6. 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.

  7. Area 3 x 208.71 ft 1 ac 4 x 208.71 ft Area = 522,720 sq ft = 12 acres

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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.

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