## Levee Overtopping Floodplain Interactions

Flows that overtop levees located along either or both sides of a main-stem river and/or its tributaries can be treated as lateral flow (q) in Eqs. (29) and (30) where the lateral flow diverted over the levee is computed as broad-crested weir flow. This overtopping flow is corrected for submergence effects if the floodplain water-surface elevation sufficiently exceeds the levee crest elevation. After the flood peak passes, the overtopping flow may reverse its direction when the floodplain water-surface elevation exceeds the river water-surface elevation, thus allowing flow to return to the river. The overtopping broad-crested weir flow is computed according to the following:

where ks, the submergence correction factor, is computed as in Eqs. (67) to (69) except h* = (h{p — hc)/(h — hc), in which C/ is the weir discharge coefficient, hc is the levee-crest elevation, h is the water-surface elevation of the river, and hfp is the water-surface elevation of the floodplain. Flow in the floodplain can affect overtopping flows via the submergence correction factor. Flow may also pass from the waterway to the floodplain through a time-dependent crevasse (breach) in the levee via a breach-flow equation similar to Eq. (66). The floodplain, which is separated from the principal routing channel (river) by the levee, may be treated as (a) a dead-storage area (A0) in the Saint-Venant equations, in which case Eq. (75) is not relevant, (b) a tributary that receives its inflow as lateral flows (the flows from the river that overtop the levee crest), which are simultaneously dynamically routed along the floodplain, and (c) the flows and water-surface elevations can be computed by using a level-pool routing method particularly if the floodplain is divided into compartments by levees (dikes) or elevated roadways located somewhat perpendicular to the river levee(s).

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