**4.3.4.2 Theory**

The theory of operation of a submerged skimming weir is based on the modification of the

withdrawal limits by the weir crest. Using the epilimnetic water quality as the desired release quality,

operation of a weir can be simulated, and the resulting quality predicted for a given weir crest elevation,

thermal stratification, and discharge. A numerical model can be used to simulate various stratification

patterns and discharges and to determine the optimum crest elevation for the weir (Figure 4.3.4). The

effects of various meteorological and hydrological events on the reservoir operating with a submerged

weir can then also be simulated.

Figure 4.3.4 Schematic of submerged skimming weir

**4.3.4.3 Design Methodology**

Design of a submerged skimming weir should begin on ly after clearly defining the existing

release water quality problems. The design methodology is based on installation of submerged

skimming weirs at existing Corps of Engineers' (CE) projects.

To determine this, a numerical model simulation can be used to predict the reservoir quality and

stratification under a variety of meteorological, hydrological, and operational conditions. If the project

is already operational, simple examination of release quality and in-reservoir quality profiles will indicate

if a problem exists. If the problem involves release of hypolimnetic water, such as low DO during

stratified periods, a submerged skimming weir may be a viable alternative to minimize hypolimnetic

releases.

The numerical model simulations described above, conducted to determine the viability of a

submerged weir, can also be used to determine the crest elevation of the weir. The crest is usually set

at an elevation equal to or higher than the highest thermocline elevation observed during the summer. If

the thermocline elevation is above the weir crest elevation, withdrawal of hypolimnetic water can occur,

thereby diminishing weir effectiveness.

4.3-12