Zones and Shapes in Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 319

The “true lake” (or reservoir) zone is the lacustrine zone. This zone is wider and deeper and
has a much longer residence time. The velocities are small and impacted by the dam operation,
and wind mixing dominates the surface layers. The reservoir often becomes thermally stratiied
due to the density differences between the warm upper layer (epilimnion) and the colder bottom
waters (hypolimnion). Productivity is usually limited to the surface photic zone (or well-mixed
epilimnion) and is dominated by planktonic algae. Internal productivity (autochthonous) domi-
nates and nutrient concentrations in the photic zone become depleted. In the deeper, profundal
zone, oxygen is often seasonally depleted and there are high concentrations of reduced materials.
As a result, the characteristics of the lacustrine zone become more similar to natural lake ecosys-
tems (Wetzel 2001).
11.3 LAKE BASIN AND CHARACTERISTICS
Lake basin morphology inluences lake hydrodynamics and lake responses to pollution. Some of
the more commonly used metrics to assess lake basin morphological metrics are discussed next.
11.3.1 d eptH and e LeVatIonS
Depth impacts the lake or reservoir in terms of mixing, light penetration, and the propensity to
stratify. Shallow lakes and reservoirs are also commonly more productive. In deep lakes, it is less
likely that the bottom sediments would be impacted by mixing due to waves.
Some of the common depth metrics include the mean depth, the maximum depth, and the relative
depth. The mean depth is typically estimated by the lake volume divided by the lake surface area,
or by just averaging the depth measurements. The relative depth is the maximum depth divided by
the mean diameter (Hutchinson 1957). All of these spatial averages are also commonly taken as
time-averaged values, such as annual averaged mean depth.
Depths vary spatially and temporally. For a natural lake the maximum depth may be located near
the lake's center, while for reservoirs the greatest depth is near the dam (Figure 11.2), or in “borrow
pits” or areas near the dam from which earth was “borrowed” in the dam's construction.
Rather than the depth, the water-surface elevation, referenced from some datum, is most com-
monly measured, such as near the dam or reservoir operations. This measurement is also commonly
referred to as the “gage height” (obtained from a gage) or the “stage height.” The stage height is usu-
ally measured with a water-stage recorder, encased in a stilling well to reduce the impact of waves
and other local disturbances on the measured stage or water-surface elevation.
In rivers and streams, there is typically some slope to the water surface. For some special systems
of a constant shape (prismatic), the water-surface slope is the same as the bottom slope (steady-
uniform low).
In reservoirs, while the bottom (such as an old inundated river channel or “thalweg”) slopes
toward the dam (Figure 11.2), the water-surface elevation (or stage) becomes nearly constant. That
is, the water surface becomes relatively lat. This is referred to as a backwater impact, and in hydrau-
lics, the curve of the water surface is referred to as an “M1” curve (Martin and McCutcheon 1999;
Sturm 2001). The surface of lakes is also relatively lat. Flat is, of course, a relative term, since the
water surface varies as a function of waves, seiches, and other variations, as will be discussed in
Chapter 14.
11.3.2 f LowS and V eLocItIeS
Rivers are advective systems and their lows are commonly estimated using a rating curve, which is
the relationship between water-surface elevations (and/or depths) and lows (see Chapter 4). Rating
curves are developed at points in a river called control points where there is a unique relationship