Zones and Shapes in Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 326

TABLE 11.1
Comparison of Three Corps of Engineers Reservoirs

Characteristics DeGray Lake Red Rock Lake West Point Lake

Impoundment type Tributary Main stem Main stem

Major tributary Caddo River Des Moines River Chattahoochee River

Volume (10 6 m 3 ) 808 78 746

Surface area (km 2 ) 54 26 105

Length (km) 32 12 53

Mean depth (m) 14.9 3.1 7.1

Maximum depth (m) 60 11 31

Annual hydraulic residence time (year) 2.06 0.05 0.13

High-low hydraulic residence time (year) 1.11 0.02 0.09

Low-low hydraulic residence time (year) 3.39 0.12 0.18

Source: Modiied from Kennedy, R.H., Lake and Reservoir Management , U.S. Environmental Protection
Agency, Washington, DC, 1984.

The residence time of reservoirs is variable. Main-stem, run-of-the-river reservoirs have little
storage and the residence time may be days to weeks. Main-stem storage reservoirs have residence
times that are often weeks to months, while larger reservoirs may have a residence time from weeks
to months to years. For example, Kennedy (1984) compared three Corps' reservoirs, as shown in
Table 11.1. The residence time of the tributary reservoir (DeGray Lake) averaged 2.1 years (range of
1.1-3.4 years), while the main-stem reservoirs were typically much less than 1 year.
The residence or retention times are not temporally constant, but vary with volumes and lows.
A common practice is to use mathematical models of hydrodynamics and transport in lakes or res-
ervoirs to estimate the residence time (Martin and McCutcheon 1999). Two common methods are
simulating a dye tracer and simulating the water age. For the dye tracer, commonly a concentration
is speciied initially everywhere within the reservoir with all inlows having concentrations of zero.
The residence time is computed by how long it takes for the dye to be lushed out, or reduced to
some fraction of its initial concentration. For the water age, a nonconservative tracer is simulated,
which accumulates at a positive (zeroth-order) rate of 1.0 day -1 . In essence, the water ages at that rate
unless it is lushed out. The water age allows the visualization of areas of a lake with little lushing,
density inlows, and other features that may then impact the reservoir's quality and management.
Loga-Karpinska et al. (2003) and Camacho and Martin (2013) provide discussions of the methods
used to estimate residence times.

11.3.11 d raInaGe a reaS
The characteristics of lakes and reservoirs are also largely impacted by their watersheds. Thornton
(1984) compared the Corps' reservoirs with natural lakes that were surveyed as part of the U.S.
EPA's national eutrophication survey (1972-1975). This comparison indicated that, in general, res-
ervoirs have greater drainage and surface areas, drainage area/surface area ratios, mean and maxi-
mum depths, shoreline development ratios, and aerial water loads (Table 11.2).
The characteristics of the watershed also impact the characteristics of the lake or reservoir.
One example is the differences in lakes in urban as opposed to rural environments. Schueler and
Simpson (2002) used the following criteria to distinguish urban lakes:

• They tend to be rather small, and generally have a surface area of 10 mi 2 or less (this

excludes larger lakes).

• They tend to be shallow, with an average depth of 20 ft. or less.