below.A field investigation is the most effective way of defining the gas transfer characteristics of a
hydraulic structure and the potential for concentrating flow. In a preliminary assessment, water
temperature and oxygen concentration should be measured upstream and downstream of the project.
If the upstream DO is near saturation, measurements of oxygen uptake across the structure will reveal
very little about the gas transfer characteristics and nothing will be gained by implementing this
technique. If, however, the upstream oxygen is low (more than 2.5 mg/l below saturation), then a field
investigation will reveal the applicability of concentrating flow.
To determine the oxygen uptake characteristics, upstream and downstream DO concentrations
should be measured for a range of spillway discharges. We recommend that recording instruments be
deployed in an array that covers the immediate tailrace of the spillbay. These data will specifically show
the effects of increasing discharge through a single spillbay. The specific operational characteristics of
the structure should also be determined during the field investigation. This particularly includes the
stilling basin performance to note changes in the submergence of the discharge jet or in air entrainment
in the stilling basin. This experimental method was developed by Wilhelms (1988) at several lock and
dams on the Ouachita and Red Rivers in Louisiana and has been used at many projects (Hettiarachchi,
et al 1997; Portland and Walla Walla Districts, Corps of Engineers).
If the field investigation shows an improvement in downstream DO with increasing discharge
per spillbay, then hydraulic concerns should be addressed prior to full implementation. Since most
stilling basins and energy dissipators were designed for equal gate operations, unequal gate operation
may create scour or erosion problems downstream. The operating limits for this technique must be
The reaeration that is occurring with the existing gate operation can be compared to generalized
relationships developed for some types of hydraulic structures given by Wilhelms (1988), ASCE
(1990), and Gulliver, Wilhelms, and Parkhill (1998). Some research suggests that as the unit discharge
increases, a decrease in reaeration may occur (Thene, Daniil, and Stefan 1989).
Several of the lock and dams on the Ohio River are operated to maximize the unit discharge
through a few gates and thereby increase DO downstream (Price 1990). Field investigations at
Montgomery Lock and Dam on the Ohio River indicated that a 2-ft gate opening on one gate achieved
1.5 times the DO downstream as two gates open 1 ft (US Army Engineer District, Pittsburgh 1975).