FLOOD06898

• County: Statewide
• Basin: Statewide
• Title: Managing Floodplain Development in Approximate Zone A Areas
• Date: 4/1/1995
• Prepared By: FEMA
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

Guide For Approximate Zone A Areas

Developinq BFBs

WP = wetted perimeter (ft)
R = A/WP

step 7 - Compute the channel bottom slope, S, from the
topographic map or from field survey.

step 8 - Compute the discharge, Q, for each segment of the
cross section at each elevation by multiplying K by
SO.5.

step 9 - Add the discharges from each segment at the same
elevation to obtain the total discharge.

step 10 - Plot the total discharges and the corresponding
elevations on graph paper.

step 11 - The BFE can be determined from this graph for the
IDO-year flood discharge computed in step 2.

An example of a normal depth hand calculation is included in
Appendix 8.

Critical Depth

After computing normal depth, the type of flow should be
checked. If the velocity head from the normal depth
computation is equal to or more than half the hydraulic depth,
the flow is supercritical and the critical depth should be
used to establish the BFE. The velocity head, HV, for an
irregular cross section is computed using the following
equation:

HV = .V"
2g

velocity coefficient

mean velocity = Or (fps)
Ar .
total discharge (cfs)
total flow area (ft~)
acceleration due to gravity = 32.2 ft/sec~

The velocity coefficient, a, is determined using the following
equation:

where:

a =
V =
Q.r =
AT =
g =

J <Ki/ At> + <X:/A;> + <K.:/ A;l]
~/A~)

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