Reaeration By Turbulent Flow Over A Dam

Reareation will occur when water falls over a dam, weir, or other structure in the stream. To simulate this form of reaeration, a “structure” command line is added in the watershed configuration file (.fig) at every point along the stream where flow over a structure occurs.

The amount of reaeration that occurs is a function of the oxygen deficit above the structure and a reaeration coefficient:

$\Delta Ox_{str}=D_a-D_b=D_a(1-\frac{1}{rea})$ 7:3.5.10

where $\Delta Ox_{str}$ is the change in dissolved oxygen concentration (mg O$_2$/L), $D_a$ is the oxygen deficit above the structure (mg O$_2$/L), $D_b$ is the oxygen deficit below the structure (mg O$_2$/L), and $rea$ is the reaeration coefficient.

The oxygen deficit above the structure, , is calculated:

7:3.5.11

where is the equilibrium saturation oxygen concentration (mg O/L), and is the dissolved oxygen concentration in the stream (mg O/L).

Butts and Evans (1983) documents the following relationship that can be used to estimate the reaeration coefficient:

7:3.5.12

where is the reaeration coefficient, is an empirical water quality factor, is an empirical dam aeration coefficient, is the height through which water falls (m), and is the average water temperature (C).

The empirical water quality factor is assigned a value based on the condition of the stream:

= 1.80 in clean water

= 1.60 in slightly polluted water

= 1.00 in moderately polluted water

= 0.65 in grossly polluted water

The empirical dam aeration coefficient is assigned a value based on the type of structure:

= 0.70 to 0.90 for flat broad crested weir

= 1.05 for sharp crested weir with straight slope face

= 0.80 for sharp crested weir with vertical face

= 0.05 for sluice gates with submerged discharge

Table 7:3-5: SWAT+ input variables used in in-stream oxygen calculations.