FLOOD08565

• County: Statewide
• Basin: Statewide
• Title: HEC-2 Water Surface Profiles
• Date: 8/1/1979
• Prepared For: US
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

6. ENCROACHMENT OPTIONS

Six methods of specifying encroachments for floodway studies can
be used. Stations and elevations of the left and/or riqht encroachment
(method 1) can be specified for individual cross sections as desired.
A floodway with a fixed top width (method 2) can be specified which will
be used for all cross sections until changed. The left and right encroach-
ment station5'are made equidistant from the centerline of the channel,
which is half-way between the left and right banks stations. Encroachments
can be specified by percentages (method 3) which indicate the desired
proportional reduction in the natural discharge carrying capacity of each
cross section.

Encroachments can be determined so that each modified cross section
will have the same discharge carrying capacity (at some higher elevation)
as the natural cross section (method 4). This higher elevation is specified
as a fixed amount above the natural (e.g., 100-year) profile. The encroach-
ments are determined so that an equal loss of conveyance (at higher eleva-
tion) occurs on each side of the channel, if possible.

Encroachment method 5 is an optimization solution of encroachment
method 4. It determines water surfaces elevation differences between the
natural and encroached conditions such that the target difference is obtained
as near as possible.

Encroachment method 6 is an optimization solution similar to method 5;
however, method 6 optimizes on differences in the energy grade line eleva-
tions. Refer to Appendix II for a detailed explanation of encroachment
methods 1 through 6.

7. OPTIONAL FRICTION LOSS EQUATIONS

The friction loss between adjacent cross sections is computed as
the product of the representative rate of friction loss (friction slope)
and the weighted reach length. The program allows the user to select from
the following previously defined friction loss equations:

(1) Average Conveyance

(2) Average Friction Slope

(3) Geometric Mean Friction Slope

(4) Harmonic Mean Friction Slope

Any of the above friction loss equations will produce satisfactory esti-
mates provided. that reach lengths are not too long. The advantage sought
in alternative friction loss formulations is to be able to maximize reach
lengths without sacrificing profile accuracy.

Equation (1), the Average Conveyance equation, is the friction loss
formulation that has been standard in all HEC-2 source decks since 1971.
Previous HEC-2 source decks utilized equation (2), the Average Friction
Slope equation. Research by Reed and WoHkill (reference 5), indicates
that equation (2) is the most suitable for Ml prOfiles. (Suitability as
indicated by Reed and Wolfldll is the most accurate determination of a

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