FLOOD01659

• County: Statewide
• Title: Floodplain Management Handbook
• Date: 9/1/1981
• Prepared By: Flood Loss Reduction Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

Channel Alterations

Channel alterations reduce flood-
ing by increasing the flow carrying
capacity of a stream's channel. The
various types of alterations include:
straightening, deepening or widening
the channel; removing debris; paving
the channel; raising or enlarging
bridges and culverts that restrict
flow; and removing dams that interfere
with flow. Underground conduits can
also be installed to carry part or all
of a small stream's flow.

Shortening a stream improves its
flow carrying capacity and reduces
flooding. Cut-offs must be carefully
designed so that increased water
velocities do not cause erosion and
greater flooding in downstream
areas. Channel alterations often
have undesirable effects on fish and
wildlife.

All of these channel al tera tions
contribute toward reducing the height
of a flood. It is sometimes possible,
by extensively reconstructing a stream
channel, to contain major floods within
its banks. Unfortunately, such al tera-
tions sometimes increase flooding
downstream by accelerating the flow of
flood waters.

Channel deepening is not very
well suited to major streams because

I
t

Highway fills, low bridges and
obstacles to flow can Increase
heights. In the flood shown,
highway fill raised upstream
depths by several feet.

other
flood
this
flood

sediments can quickly fill in the
excavated area. Frequent redredging
often is necessary to maintain the
deeper channel even on smaller streams.
This can become a significant expense
to local governments. Care must also be
taken to avoid causing erosion when
changing a channel.

Channel . alterations are like
levees and floodwalls in that they can
be used to protect a specific site or
region. However, they differ in .that
they're not subject to sudden or dis-
asterous failure. Channel alterations
for flood control can sometimes be used
for other purposes such as navigation
and recreation. For example, features
such as boat launching facilities can
be included in projects to deepen a
channel.

i
,

The environmental impact of
altering a stream channel depends on
the specific techniques used. Some,
such as reconstructing bridges and
culverts, usually have only a temporary
effect during construction. However,
widening, deepening or paving of
channels may destroy fish and wildlife
habitat and other natural values for
several years, decades or perhaps, even
permanently.

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