Channelization and Channel Modification Activities and Impacts
The design elevation of levees is based on containing a design discharge, generally for a short
period of time. The levee cross section is generally designed as a trapezoid, with an access road running
along the levee crown. To control seepage, a long, tapering berm may be extended on the landside of the
levee. Fill material for levees is generally obtained locally from borrow areas adjacent to the riverside of
the embankment. Although the local materials may not be ideally suitable for construction, economic
necessity dictates its use. Less than ideal materials can be compensated for by constructing larger levee
sections.
4.2.5.1 Hydraulic Effects
Levees can confine river flows to a narrower cross section, thus higher stages and discharge result
during flood flows. If levees are not set back from the main channel, the hydraulic connectivity of the river
is lost with the floodplain, thus confining flows and putting more energy into flow. A study reported by
Schumm (1977) estimated that levees and dikes on the middle Mississippi River had increased the stage
for a discharge of 800,000-900,000 cfs by approximately 10 ft at St. Louis, Missouri.
On un-leveed streams, flood flows spread out over the floodplain. The floodplain acts as storage
for the additional flows. The construction of levees decreases the floodplain storage, thus increasing the
peak discharge.
Channel instabilities may arise from leveed streams because degradation of the bed and banks may
occur. Debate continues on the effect of levees on the Mississippi River. Aggradation may occur due to
the increased sediment load in the main channel and the lack of available floodplain sediment storage. The
precise response is complex and is a function of the width of levees, the effects on duration of flows, and
other factors.
The Midwest flood of 1993 initiated efforts to define a long term, nationwide approach to
floodplain management. The results of this effort are summarized in a document commonly referred to as
the Galloway report (IFMRC, 1994). It presents an overview of floodplain management, current risks,
and the application of structural measures such as levees to minimize flood impacts.
Seepage is a major problem with levees during high water. When water is contained on one side,
a head differential exists across the levee. This tends to force water through the porous soil, eventually
seeping out to the landward side of the levee. This seepage carries both fine and coarse particles through
the levee. This internal erosion of the levees can lead to piping through the levee and catastrophic failure.
To prevent excessive seepage, impervious barrier materials such as clay can be built into the levee. Flows
from tributaries that are cut off from the river system due to levees must be addressed to prevent flooding
on the landward side of the levee. Pumping stations can be applied to divert tributary flows.
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