OCEANS AND ESTUARIES - Water Quality Engineering in Natural Systems - page 338

outfall with a three-port diffuser having
1-m-diameter ports spaced 5 m apart. This
astute engineer asserts that it would be best
to space the ports such that the plumes do not
merge. Compare the outfall dilution when the
plumes do not merge with the dilution for
merging plumes.
(c) Assuming a near-field dilution of 30, a
three-port diffuser with merging plumes, and
an effluent coliform concentration of
100,000 CFU per 100 ml, estimate the coli-
form count at the beach closest to the diffuser.
If the coliform criteria for the beach is 50 CFU
per 100 ml, should the beach be closed to the
public?

9.19. The tidal river has a mean velocity of 1 cm/s, a
reaeration constant of 0.75 d −1 , a longitudinal dis-
persion coefficient of 50 m 2 /s, and a temperature
of 20°C. After initial mixing with a wastewater
effluent, the river has an ultimate BOD of 12 mg/l
and a BOD decay constant of 0.55 d −1 . Determine
the location and magnitude of the maximum
oxygen deficit.

9.20. An estuary has a surface area of 20 km 2 , an average
depth of 7.5 m, an average salinity of 15 ppt, a
tidal range of 10 cm, and a tidal period of 12.42
hours. The average freshwater flow rate into the
estuary is 15 m 3 is and the surrounding coastal
waters have an average salinity of 35 ppt. Esti-
mate the flushing time of the estuary.

9.18. A wastewater is discharged into a large tidal river
that has a mean velocity of 5 cm/s, a reaeration
rate constant of 0.6 d −1 , and a longitudinal disper-
sion coefficient of 30 m 2 /s. After initial mixing of
the wastewater, the river has an ultimate BOD of
15 mg/l and a BOD decay constant of 0.3 d −1 .
Account for longitudinal dispersion in determin-
ing the oxygen deficit 1 km downstream of the
outfall. Assess whether it is important to consider
longitudinal dispersion in this case.

9.21. The peak tidal inflow rate to an estuary during the
flood cycle is estimated as 2.0 × 10 4 m 3 /s, and the
peak outflow rate during the ebb cycle is esti-
mated as 1.5 × 10 4 m 3 /s. If the flood and ebb cycles
have durations of approximately 12 hours each,
estimate the net outflow from the estuary over
each tidal cycle.