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 5-14. Hydrogrephs for the Illinois River at Havena with overbank storage
increased by 40 percent

(I) This section seeks 10 relate the so-called hydro-
logic and hydraulic approaches to flood routing. The
hydrologic approaches, which are simpler 10 use but
harder to defend theoretically, are viewed from the point
of view of the hydraulic approaches, which are better
grounded in basic theory but relatively difficult to apply.

(2) The aim of both approaches is the same: 10
determine the response in a given reach of a watercourse
10 a given inflow sequence (usually a flood hydrograph),
and, both recognize the physical principle of conservation
of mass. They both seek to account, at all times, for all
of the volume of water initially in the stream and that of
the inflow(s) and outflow(s). The volume of water
stored in a reach varies with time as a flood wave passes
through.

(3) Mathematically, with 1ft) representing an inflow
sequence (hydrograph), T(t) the net lateral inflow along
the length of the reach (tributary inflow minus infiltra-
tinn, etc.), 0(1) the outflow hydrograph, and S(t) the
volume of water (storage) between the inflow and out-
flow sections, the principle of conservation of mass can
be written:

I(t) + T(t) - OCt) = dS(t)
dt

(5-1)

(4) The argument, t, is explicitly stated to under-
score the premise that the equation holds true at each
instant of time. With the inflow hydrograph given, and
with the tributary hydrograph given, estimated, or

5-19