FLOOD01888

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

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U.S. Department of Transporation 1989). Commonly
used in the Corps of Engineers is RMA-2 (King 1988)
which is the hydraulics module of the T ABS-2 modeling
system (Thomas and McAnally 1985). Synopses of these
and other programs are presented in HEC (U.S. Anny
Corps of Engineers 1982b). RMA-2 solves the vertically
(i.e., depth) averaged version of equations 4-1 to 4-4;
written as shown below.

Momentum equations:

hi + ~ + v~ + g~ + g~

(4-5)

hE.xx <flu
P ax2

hE.xy <flu
- -- + Sft + tx
P ay2

~ 0

~ + ~ + hdv + hila + ~ah
at ax v/iV g= g
y ay

hEyx <flv hE.yy <flv S ~ 0
P ax2 -Pily2 + 1'Y +ty

(4-6)

Continuity equation:

ah + iJ(hu) + a(hv) ~ 0
dtdXT

(4-7)

where

x ,y = the horizontal coordinate directions.
u,v = velocity components in the x and y
directions, respeclively.
t = time.
g = the acceleration due to gravity.
a = the bottom elevation.
h = the depth.
P = fluid density.
Ex:<' Exy, etc. = the turbulent exchange coefficients which
describe the diffusion of momentum in
the direction of the first subscript to that
of the second subscript.
Slx'SIY = terms for the nonlinear Manning or
Chezy representation of bottom friction.
'U, 't)' = terms representing boundary shear stresses
other than botlom friction (e.g., wind),
these terms also include the Coriolis
effect.

EM 1110-2-1416
1S Oct 93

4-8. Data Requirements

It is useful to think of "data" in three categories:
analysis input data, calibration data, and validation or
confirmation data, These categories are useful when
identifying data requirements for both physical and
numerical models.

a. Analysis input data. Analysis input data are those
items required 10 operate the model. They consist of a
geometric description of the study area (e.g., cross sec-
tions in one-dimension, contour maps, or a digital terrain
model for twlHlimensions), flow 10 be analyzed (a single
discharge for steady flow, or a hydrograph for unsteady
flow), other boundary conditions such as stages or rating
curves, and various coefficients that approximate the
effects of friction and turbulence. Of these, the geo-
metric description of the study area is usually the most
time consuming 10 develop and schematize; it is, how-
ever, not necessarily the most important data in terms of
simulation accuracy (U.S. Anny Corps of Engineers
1986). The density (i.e. resolution) and accuracy
required of the flow and geometric data are governed,
fundamentally, by the study purpose, not the analysis
technique (Cunge et al. 1980).

b. Calibration data. Calibration data consist of field
observations that are used to evaluate the performance of
a model and adjust the coefficients 10 improve its perfor-
mance, if necessary. "Performance" is a qualitative, or
subjective, measure of the degree to which the model
faithfully reproduces the field observations. This mea-
sure is applied by the engineer performing the study and
documented by means of the reporting process. The
complexities of river hydraulics do not allow the selling
of objective criteria to measure the accuracy of calibra-
tinn. Whether the model's performance is acceptable
depends on study objectives, sensitivity of study out-
comes 10 model results, and reliability of field data.

(I) The weight given 10 the performance of a model
with regard to different hydraulic variables, such as water
surface elevation or velocity, will vary with study objec-
tives, data availability and reliability, and the judgment of
the engineer. For example, floodway studies focus on
accurate computation of the water surface elevation while
constituent transport studies require accurate reproduction
of velocity, water discharge, and mixing. Surrogate data
should be used with caution. For example, if the study
objectives require the prediction of discharge, prototype
discharge should be measured for calibration rather than
derived from a rating curve.

4-5