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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Figure 5-4. Computed versus observed flow et Hickman

both reasonable. Because of the variable friction slope,
Manning's equation does display the looped rating curve;
but the boundary condition still should be placed well
downstream of the area of interest. Any model which
uses Manning's equation as the downstream buundary
condition should be tested for sensitivity to confirm that
its use at the boundary bas no affect on the area of
interest.

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c. Lateral inflow. Lateral inflow (or outflow for a
diversion) data also constitute a boundary condition.
Unlike upstream and downstream boundary conditions
which translate into an independent equation, lateral
inflow (qL) augments the equations of continuity and
momentum (see Equations 5-2 and 5-3). Lateral inflow

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can come from gaged and ungaged areas, and can be
located at a point and/or uniformly distributed along the
length of the valley.

(1) In any river system a part of the drainage will
not be gaged. To provide an accurate and consistent
simulation, the modeler must estimate the inflow from
those ungaged areas. Along the Illinois River, for exam-
ple, there is 2,579 square miles of ungaged drainage
between the Marseilles and Kingston Mines gages, which
is about 52 percent of the total gaged area. Figure 5-6
shows a simulation result at Kingslon Mines without the
ungaged drainage. The omission of the ungaged drainage
produced a uniform error of about I foot in the simulated

5-7