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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15 Oct 93

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FIgure 5-15. Hydrogrephs for the Illinois River et Hevana with adjusted flow-
Manning's n relationship

neglected, the outflow hydrograph can be computed if the
relation of the slorage to the hydrographs is also known.
It is on this issue, the relationship between the geometri-
cal quantity, slorage, and the kinematic quantities, dis-
charge hydrographs, that the hydrologic and hydraulic
approaches differ.

of record for the reach. or theoretically on the basis of
some simplifying physical assumption. In the most
empirical of the hydrologic techniques, the storage is not
even considered; inflow hydrographs are manipulated by
an averaging technique flexible enough 10 allow matching
of computed and measured outflow hydrographs.

(5) The hydrologic techniques focus attention on
discharge hydrographs. The outflow discharge hydro-
graph constituting the response of the reach 10 the inflow
hydrograph is computed directly, and after that is done,
the water levels in the reach are somehow related 10 the
discharges. To achieve such a direct solution for the
outflow hydrograph, a storage versus flow relation is
assumed, either empirically on the basis of flood events

(a) Furthermore, in hydrologic methods, the study
reach is treated as a whole. Even if the reach is broken
inlo subreaches, as some of the techniques propose, it is
assumed that the outflow hydrographs can be determined
sequentially, from upstream 10 downstream. The outflow
hydrograph of one subreach serves as the inflow hydro-
graph for the neighboring downstream subreach.

5-20

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