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Floodwater Storage Design Information Sheet

Author: Robert P. Stone, P. Eng. - Engineer, Soil/OMAFRA
Creation Date: 13 June 2007
Last Reviewed: 4 April 2008

This form will be used to calculate the quantity of water exiting from a pond. It will indicate the effectiveness of a particular size of pond in reducing the peak flow associated with an upstream watershed. Follow all of the instructions on this form and on all of the associated figures.

  1. Watershed area
________ ha
________ ac
  1. Average grade of watershed
________ %
  1. Runoff curve number from Tables 2.2 — 2.4
________________
  1. Peak flow from watershed for a 25 year storm from
    Table 2.5-M — 2.11-M (2.5-I — 2.11-M)
________m3/s
________ft3/s
  1. Obtain one-day rainfall for the watershed location from Table G.1
________ mm
________ in.
  1. Obtain the depth of runoff (Vr) from Table G.2-M (G.2-I)
________mm
________in.
  1. Calculate the ponding volume available
________m3
________ft3
  1. Calculate the equivalent depth of storage over the entire watershed

Vs = (pond volume x 1000) ÷ (hectares x 10,000)
= ________ mm

Vs = (pond volume x 12) ÷ (acres x 43,560)
= (________ x 12) ÷ (________ x 43,560)
= ________ in.
________mm
________in.
  1. Refer to Figure G.3 to decide which chart to use

Table G.3 or Table G.4-M(G.4-I)

    
Table G.3

If Table G.3 is used, divide Vs by Vr (i.e. divide answer in Step 8 above by the answer in Step 6)

Vs ÷ Vr = _______ = _______

Using Table G.3, read the first decimal place of Vs/Vr on the left side and the second decimal place across the top. Obtain the answer where the two lines intersect:

Answer: _______

Multiply this answer by the peak flow in step 4 (above) to obtain the peak pond outflow.

_______ x _______ = _______ m3/s (ft3/s)

 

Table G.4

If Table G.4-M (G.4-I) is used, read Vs along the top of the chart and Vr along the left side ot obtain discharge:

Answer: _______ m3/s/ha (ft3/s/ac)

Multiply the answer (above) by the number of hectares (acres) in the watershed to obtain the peak pond outflow

_______ x _______ = _______ m3/s (ft3/s)

 

For more information:
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Local: (519) 826-4047
E-mail: ag.info.omafra@ontario.ca