Dam Tailwaters - Hydro-Environmental Analysis: Freshwater Environments - page 491

18 Dam Tailwaters

18.1 INTRODUCTION AND ISSUES
Tailwaters (Figure 18.1) begin immediately downstream of some hydraulic structure, such as a dam.
As such, the spatial extent of the tailwater is easy to deine, at least at its beginning. The conveyance
areas (channel and structures) below the dam are often called the tailrace. The downstream extent
of the tailwaters is more problematic, and is taken as that zone inluenced by the structure and the
outlows from that structure. That zone can reach far downstream of the dam.
Since tailwaters are directly impacted by the upstream structure, they may be considered a
unique type of environmental system (WES 1998) with physical, chemical, and biological char-
acteristics directly impacted by the operation of the dam. The characteristics of the tailwater are
largely inluenced by the dam, the associated hydraulic structures, and how much, when, and from
where water is released from. By their nature and operation, dams often modify or eliminate the
natural low hydrograph in the tailwater and downstream river reach.
The tailwater inlow hydrograph (the outlow hydrograph from the dam) varies with the opera-
tion and design purpose of the dam. Operational purposes include hydropower, water supply, lood
storage, and other uses, and, most commonly, large reservoirs are operated for multiple uses.
The purpose of a lood storage facility is to attenuate the lood inlow to the reservoir, so that
the inlow hydrograph to the tailwater would have a lower peak but a longer duration than the res-
ervoir inlow hydrograph. An example of releases from a lood storage reservoir is illustrated in
Figure 18.2. The timing and lows of the releases would depend on the timing and the magnitude of
the lood events, which would vary between locations. The impact for the tailwater and the down-
stream river is to (Richter and Thomas 2007):

• Eliminate small loods, that is, 2- to 10-year recurrence interval loods.

• Eliminate all but the most extreme large loods (>50 year).

• Introduce artiicially long, high-low pulses following lood peaks.

For a storage reservoir, the tailwater inlow hydrograph may be relatively constant. One impact
of these facilities, particularly those with a large storage capacity, is to rearrange the seasonal pat-
terns of water lows. In storage facilities, wet-season lows are stored for release in the dry season
to support water supply users. When demands are low, lows may be limited or eliminated. When
demands are high, release lows may be unnaturally high (Richter and Thomas 2007).
Run-of-the-river hydropower facilities may have only a limited impact on natural variations in
river discharge. For a peaking hydropower facility, the inlow may be limited to seepage during
nongeneration and then rapidly increased to some peak during periods of generation, as illustrated
by Figure 18.2.
The inlows to the tailwaters may also be from and over spillways, from mid-level or bottom
release structures, and through turbines or a variety of types of conduits (see Chapter 10). The loca-
tion and the magnitude of the lows, and the characteristics of the reservoir at the level(s) from which
the water was withdrawn, impact not only the hydraulic characteristics of the low but also the
low water quality. For example, as previously discussed, the bottom waters of many reservoirs are
hypoxic during much of the year, so that bottom releases are low in dissolved oxygen (DO), often