FLOOD03190

• County: Morgan
• Community: Weldona
• Basin: South Platte
• Title: Flood Mitigation Project - Schaefer Draw/South Platte Basin
• Date: 8/1/1996
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

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The required detention volume is the maximum difference between the
cumulative inf~ow and the cumulative outflow volumes or

v = max (Vin - Voud

(Equation 11.3)

As per Urban Drainage Design Guidelines, if the procedure results
in an increasing storage volume at the end of two hours, use the 2-
hour storage volume.

This procedure assumes a constant outflow rate which is the rate of
discharge when the detention pond is full. Discharge varies,
however, with the depth of water. This fact is partially
compensated for by the outflow adjustment factor k.

~1~\_.UlJ11~lI!f:,1

All detention facilities are to be designed to release not greater
than the 5-year historical (prior to any development) peak runoff
during the 100-year storm event.

11.4.3

~draulic Desiqn

Hydraulic design procedures for sizing of detention facilities
outlet works are described below.

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1. Weir Flow

The general form of the equation for horizontal crested weirs

is:

Q = CL(H)3/2

(Equation 11.4)

Where Q = discharge (cfs)
C = weir coefficient (see Table 11-1)
L = horizontal length (feet)
H = total energy head (feet)

Another common weir is the v-notCh, whose equation is as
follows:

Q = 2.5 tan (S/2)Hs/2

(Equation 11. 5)

Where e = angle of the notch at the apex (degrees)

When desiqning or evaluating weir flow, the effects of
submergence must be considered. A single check on submergence
can be made by comparing the tailwater to the headwater depth.
The example calculation for a weir design on Fiqure 11-2
illustrates the submergence check.

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