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 Por Approximate Zone A Areas

Developing BPEs

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Detailed Methods

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Three essential factors must be determined either by hand
calculations or by computer model to determine a BFE by'
detailed methods. These factors are: 1) floodplain geometry
(topography); 2) flood discharge and/or volume (hydrology);
and 3) flood height (hydraulics).

Topography involves the measurement of the geometry of a cross
section(s) of the floodplain, which includes horizontal and
vertical coordinates. The vertical coordinate, or elevation,
is related to a vertical datum, such as the National Geodetic
Vertical Datum of 1929 or North American Vertical Datum of
1988. The horizontal coordinate, or station, is measured from
a reference point along the cross section to establish actual
ground points.

Hydrology for the particular location along a stream involves
the determination of the peak rate of stream flow [usually
measured in cubic feet per second (cfs)] that will occur
during a flood (for purposes of determining the BFE, the 100-
year flood). When determining lake or pond elevations, a 100-
year flood hydrograph is required to determine the BFE.

Hydraulics involves the determination of the water-surface
elevation that will occur during a flood (for purposes of
determining the BFE, the 100-year flood), the selection of a
method to relate the flood discharge to a flood depth, and the
selection of Manning's roughness coefficients or "n" values.
These "n" values vary depending on the type of materials;
degree of irregularity; variation of shape, obstructions, and
vegetation; and degree of meandering related to the channel
and the floodplain of a stream.

The following sections discuss various methods for determining
the topography, hydrology, and hydraulics for a particular
location in order to determine a BFE.

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Existing Topographic Maps

Before initiating field surveys, determine if there is
existing detailed topographic mapping that can be used to
generate cross-section data. To adequately describe a
floodplain and for use with a hydraulic method to calculate a
BFE, topographic map scales and contour intervals must be the
same as, or more detailed than, those used to prepare the

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