Sizing Retention & Detention

1 / 12

# Sizing Retention & Detention - PowerPoint PPT Presentation

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

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

## PowerPoint Slideshow about 'Sizing Retention & Detention' - jolene

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

### 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
• You now have watershed inches
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:
• 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
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.

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

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

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

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:
• 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
• 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.