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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with surveys of various degrees of accuracy can be esti-
mated from "Accuracy of Computed Water Surface Pro-
files" (U.S. Army Corps of Engineers 1986, 1989).

b. Changes. Visual inspection of a reach must be
done 10 identify the nature of the boundary material,
vegetation, and human activities. Alluvium is subject 10
scour and deposition with possibly major changes in
cross section shape accompanying a major flood event.
As gross changes in cross section occur within alluvial
streams, roughness also changes as dune patterns change
during a flood event. Estimated changes in roughness
can be applied 10 a rigid bed model to evaluate the
importance of their effect. Prediction of boundary move-
ment lies outside the scope of this Chapter; refer to
Chapter 7 and EM 1110-2-4000.

c. Micro-geometry. Visual inspection should be
used 10. identify the boundary roughness and other char-
acteristics, such as potentia! infiltration, of a reach.
Infiltration is usually of concern for overland flow;
occasionally however, significant water loss (or gain)
from a channel will occur in sand, karst, or volcanic
geology. Boundary roughness affects the passage of a
flood wave. Inspection of the study reach will indicate
the nature of the roughness elements: cobbles, boulders,
trees, houses, their density and distribution, and variance
of roughness with stage and distance down the reach.
First approximation values for roughness parameters can
be gleaned from past experience with similar roughness
elements (Chow 1959, Chapter 5); the drag of trees, and
small structures can be estimated from expected veloci-
ties, areas of projection normal to the expected flow, and
an estimated drag coefficient. Improved values of rough-
ness are obtained by comparing computed and observed
flows and stages for events of record.

5-5. Controls

a. Hydraulic controls. Hydraulic control sections
should be sought out because these are natural reach
delimiters. At such a section, there is a unique stage-
discharge relation (except for the hysteresis induced by
unsteady flow), unaffected by flow conditions down-
stream; hence, it is ideal for a gaging station. It is possi-
ble that a control is weak; that is, a rising downstream
water level can drown the control section and force its
effect upon the subject reach. In that case the reach
cannot be studied independently of downstream reaches.
This possibility can be investigated with steady flow
analyses based on projected flood discharges.

EM 1110-2.1416
15 Oct 93

(1) The issue of downstream control is signifICant to
the choice of flood routing method. Influences on water
levels within a reach stemming from conditions down-
stream (tidal levels, or increased levels due 10 small
slope, high roughness, or flow constrictions downstream,
for example) preclude application of hydrologic methods.
Known water levels (say, tidal) at the downstream end of
a reach allow use of hydraulic methods. Otherwise the
downstream boundary must be extended until a control
(or known level) is encountered.

(2) Downstream from a critical depth control is
supercritical flow. If the channel downstream is hydrau-
lically steep and suffICiently long to encompass the reach
of interest, supercritical flow will persist all the way
down the reach. No independent downstream boundary
condition is possible, since downstream depth and dis-
charge are dictated by the flow in the reach. The correct
way of modeling such a flow is with an unsteady flow
model. If available models cannot deal with supercritical
flow, a diffusion model will yield a reasonable result if
water surface elevations are not needed and the stream is
not extremely flat.

(3) In most cases, the zone of supercritical flow is
relatively short, ending either in a plunge inlo a pool of
subcritical flow or joining subcritical flow downstream in
a hydraulic jump. In unsteady flow, this jump (called a
hydraulic bore) can move about.

b. Friction control. A so-called friction control
pertains to a section in a nearly uniform reach, suffi-
ciently long 10 insulate the section from downstream
backwater. Then, the stage-discharge relation is gov-
erned by a condition of normal depth (near normal in the
case of unsteady flow). This type of downstream bound-
ary condition is well suited for all flood routing tech-
niques that recognize downstream boundary influences.

5-6. Boundary Conditions

"Boundary conditions" is a mathematical term which
specifies the loading for a particular solution to a set of
partial differential equations. In more practical terms,
boundary conditions for an unsteady flow model are the
combination of flow and stage time series, which when
applied to the exterior of the model either duplicates an
observed event or generates a hypothetical event such as
a design flood, or dam break. For an observed event, the
accuracy of the boundary conditions affects the quality of
the reproduction. In a similar but less detectable manner

5-5