EP 1165-2-1
30 Jul 99
a. Reservoirs. Reservoirs regulate floods downstream from the
dam by temporarily storing some part of the flood volume and releasing
it later. The impact downstream is to lower flood stages, increase
the duration of flooding, and shift the flood to a later time. It is
normal for dam and reservoir projects to effect some control on, and
lower flood stages for, all magnitudes of floods. This is especially
true of dams with ungated spillways. The amount of control and
effectiveness will, however, decrease when flood volumes exceed the
storage reserved for flood control. For the large flood, dams with
gated spillways may exert lesser control on downstream flood stages
than comparable ungated dams. Reservoir releases downstream can raise
groundwater levels in fields adjacent (and even more distant) to the
river and rapid change in stages can exacerbate bank caving.
Downstream of dams, uncontrolled tributaries will continue to
contribute to flooding, causing stage reductions to become less and
less farther downstream. (Tributary flooding may then assume
increased significance.) Channel capacities downstream of dams may
increase over time; however, farther downstream, especially below a
tributary carrying heavy sediment loads, channel capacity may be
reduced. (Reservoir regulation tends to shift channel rating curves
upward--less flow at a given stage--especially upstream of
tributaries.) Upstream of a dam, sediment deposition can be expected
to occur mostly in upper pool areas, decreasing the flood control
effectiveness over time and raising flood stages and ground water
levels around the pool.
b. Channel Enlargements. Channel enlargement will act like a
negative reservoir, raising flood stages downstream, shortening flood
durations and shifting the flood to an earlier time. Flood stages
will be lower in the enlarged channel reach for all floods including
those exceeding the channel capacity, if the channel is not
excessively long. (Long, oversize channels may have increased flood
stages in the lower part of the channel.) With main stem flooding
reduced, direct overbank flooding from tributaries may assume
increased significance. How flows from upstream and from tributaries
are collected, controlled, and transitioned into the enlarged channel
can greatly influence the project's beneficial impacts. Some control
is generally required to direct overbank flow into the channel.
Erosion and considerable attendant damage may occur upstream of the
enlarged channel unless there is appropriate hydraulic control; the
same applies where tributaries enter. All artificially enlarged
channels will tend toward a new equilibrium state where sediment
inflow and carrying capacity are in balance; the trend may be to a
smaller or larger channel than the one constructed. Whatever the
trend, it may be so slow as to be hardly noticeable, may occur at some
intermediate rate, or may take place suddenly with one dramatic large
flood.
c. Levees and Floodwalls. Levees and floodwalls are
constructed to exclude flood waters from the protected area, up to a
certain magnitude of flood. Unlike reservoirs and channel
enlargements, the flood control effectiveness of a levee or floodwall
will cease abrubtly if a flood should overtop it. Interior runoff
impeded by the structure may cause interior flooding if there are not
proper provisions for interim storage behind it or discharge past the
barrier. Potential effects outside a levee, upstream and downstream,
are too complex and too site dependent to generalize otherwise, but
generally the constriction of flow area caused by the structure will
raise flood stages upstream. Within the levee reach, flood stages may
be increased or decreased depending on whether the structure forms a
hydraulically long or short constriction. A levee may reduce valley
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