Section 10
Methodology Development for the Big Muddy River Watershed
v 10-7
FINAL REPORT
(Metcalf and Eddy, Inc. 1991). For analysis, a ratio of 1.3 was used to calculate BOD5
for each sample date.
Literature provides equations to calculate both the BOD5 decay rate coefficient (kd)
and reaeration rate coefficient (ka). The decay rate coefficient is dependent on stream
depth, and the reaeration coefficient is dependent on depth and velocity. Due to the
limits of the data set shown in Table 5-10, the decay rate coefficient was calculated
from either known depths or rating curves allowing the reaeration coefficient to be
calculated from the Streeter-Phelps equation presented above as the only unknown
variable. The rating curves used to determine depths are available in Appendix K.
The BOD5 decay rate coefficient (kd) at 20°C was calculated based on the following
equation (USEPA 1997b):
The BOD5 decay rate coefficient was corrected for temperature with the following
equation (Novotny and Olem 1994):
where kdT = BOD5 decay rate coefficient at temperature T; T in °C
= Thermal factor
The thermal factor () in the above equation has an accepted value of 1.047 for the
BOD5 decay rate coefficient (Novotny and Olem 1994). The decay rate coefficient
typically falls between 0.02 and 3.4 day-1. The reaeration rate coefficient typically
ranges between 0 and 100 day-1 (USEPA 1997b).
For comparison purposes, the reaeration coefficient (ka) was calculated based on the
following equation (USEPA 1997b):
where: v = Stream velocity (feet/s)
H = Stream depth (feet)
Like the BOD5 decay rate coefficient, the reaeration coefficient is corrected for
temperature with the following equation (Novotny and Olem 1994):
at 20 C
H
12.9 v
k 1.5
0.5
a
0.3 for H 8
for 0 H 8
8
H
k 0.3
0.434
d20
(T20) dT d20 k k
