FLOOD04629

• County: Statewide
• Community: Nationwide
• Basin: Statewide
• Title: Managing Floodplain Development in Approximate Zone A Areas
• Date: 4/1/1995
• Prepared By: FEMA
• Floodplain - Doc Type: Educational/Technical/Reference Information

Guide For Approximate Zone A Areas

Developing BFEs

step 2 - Compute the lOO-year discharge by applying one of
the methods described in the hydrology section of
this guide.

step 3 - Plot the cross section on graph paper with the
stations and the corresponding elevations. (The
stations and elevations are obtained from the
topographic map and/or from field survey).

step 4 - Select the left and right channel bank stations.
The channel bank stations are those stations where
the ground slope becomes flatter moving away from
the channel bottom as shown in Figure 22, "Channel
Bank stations." Photographs taken in the field and
the contours on the topographic maps are also
helpful when defining the channel bank stations.
Do not place the channel bank stations at the
bottom of the channel.

I CHANr-EL BANKS I
. .

Figure 22 - Channel Bank stations

step 5 - Select appropriate Manning's roughness coefficients
for the left overbank, channel, and right overbank
from the "n" values given in Appendix 5. These
values should be determined by reviewing the field
photographs and visiting the site.

step 6 - Compute the cross section area, wetted perimeter,
hydraulic radius, and conveyance for each segment
(i.e., left overbank, channel, and right overbank)
for at least three elevations. The conveyance, K,
of a segment is given as:

K = (1.486/n) x A x RO.667

where:

A =
R =

cross section area (ft2)
hydraulic radius (ft)

V-25