FLOOD01888

• County: Statewide
• Title: River Hydraulics
• Date: 10/15/1993
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

EM 1110-2.1416
15 Oct 93

(I) In the headwater regions the terrain is mountain-
ous, but in the lower reaches, the valleys are wide and
the hills gentle and rounded. Through most of the area,
the river flows in deep, narrow, sinuous valleys between
steep side ridges. Williamson is located in the lower
third of the Tug River Basin, where the valley is 800 10
900 feet wide.

(2) The original water surface profile study reach
extended from Kermit, West Virginia, to the central
business district of Williamson, a distance of 20 miles.
The general slope of this reach is about 2 feet per mile.

(3) The channel is alluvial with a botlom width of
about 150 feet and stable banks with heights ranging up
to 25 feet above low water. Bed sediments are sand and
gravel. Vegetation, predominately conifer, lines both
banks and covers the floodplain except where cleared for
agricultural or industrial use.

c. Summary of water suiface prof/Ie model and
parameter evaluations. Refmements to the original
HEC-2 data fIle included substituting field data at
bridges, developing reach lengths, and assigning
Manning's roughness coeffICients by vegetation and land
use. Channel bank limits were reestablished 10 better
approximate the limits of bank vegetation.

(I) Sensitivity of calculated profIles was evaluated to
determine the significant hydraulic parameters. Super-
elevation, bed scour during floods, local inflows, over-
bank flows, relative roughness, and seasonal vegetation
roughness were analyzed. Key sources of field data for
these evaluations were high-water maIks from 1984 and
1977 floods and USGS gage records at Williamson.

(2) Some of the results from these evaluations were
bed scour doring these events was found 10 be negligible,
superelevation did not impact except to indicate that the
calibration Iolerance should be relaxed from 0.5 foot 10
I foot, and local inflow changes improved agreement
between calculated and observed profiles between gages.

(3) The three most significant hydraulic parameters
were the identification of significant overbank flow
through the town of Williamson, changes in the values of
roughness as rare flood events overtopped all trees, and
seasonal changes in vegetative roughness.

(4) The maximum discharge during the 1977 event
was so signifICant that two extrapolations were made, one
for a 94,000 cfs event and one for a 117,000 cfs event.

6-4

The procedure for extrapolating the rating curves
followed EM 1110-2-1601 which utilizes "relative rough-
ness" and uses observed data to calculate roughness
height. The details of the extrapolation procedure and
other details of the stody are presented in Williams
(1988a, 1988c). Calibration of the HEC-2 model 10 the
two flood events is discussed in a later section under the
heading "Model Calibration and Verifu:ation" (6-11).

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Section II
Data Requirements

6-6. Introduction to Data Requirements

The time and effort required for completion of water
surface profile studies depend uprm the detail of the
analysis required to secure the results desired. In some
cases the character of available basic data and the time
. available impose practical limitations on the scope of the
study. In preliminary investigations a rapid approximate
method may give results fully as satisfactory for the
purpose involved as a more accurate but time consuming
computational procedure. In other cases, the utmost
degree of accuracy possible by a detailed and thorough
analysis may be profitable and essential for reliable engi-
neering. Accordingly, profile computations should be
initiated with a careful appraisal of the degree of
accuracy necessary for satisfaclory results, considering
the purpose and character of the investigations involved,
the detail and probable accuracy of basic data available,
the complexity of flow conditions in the stream, and the
budget and time limit for completion of the studies.

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a. Theory. Hydraulic theory is well established for
channels with rigid boundaries, and computer simulation
models based on this theory produce consistent and accu-
rate results if properly applied. Major sources of error
are inaccuracies in data and improper modeling of flow
conditions.

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b. Categories of data. Basic data are grouped into
five categories: cross sections, reach lengths, loss coeffi-
cients, flow regime, and starting condition. The accuracy
required for this data depends upon the accuracy needed
in the final results. At times, it seems most economical
to compensate for inadequacy of data by using safety
factors such as providing liberal amounts of freeboard.
In rural areas such procedures may be acceptable, but in
urban areas both property damage and loss of life can
result from designs based on inadequate and inaccurate
data. Cross-sectional data and loss coefficients are dis-
cussed in Appendix D.

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