RIVERS AND STREAMS - Water Quality Engineering in Natural Systems - page 133

PROBLEMS

4.1. A natural river has a top width of 18 m, a flow area
of 75 m 2 , a wetted perimeter of 25 m, and the
roughness elements on the wetted perimeter have
a characteristic height of 7 mm. If the flow rate in
the river is 100 m 3 is and the temperature of the
water is 20°C, estimate the friction factor and the
turbulent diffusion coefficients in the vertical and
transverse directions. (Hint: Use the hydraulic
depth [= flow area/top width] as the characteristic
depth of the channel.)

4.2. (a) If the river described in Problem 3.32 has a
characteristic roughness of 8 mm, estimate the
vertical and transverse mixing coefficients.
(b) If the outfall discharges the effluent across the
entire bottom width of 6 m, estimate the dis-
tance downstream to where the effluent is
completely mixed across the channel cross
section.

Figure 4.25. Removal of the Brownsville dam (oregon).
Source : national oceanic and Atmospheric Administration
(2012).

4.3. A single-port outfall is located on the side of a
stream that is 15 m wide and 3 m deep, and the
flow velocity in the stream is 2 m/s.

must be taken when the sediment is dredged
because of resuspension of the contaminants and
the possibility of contaminated sediment move-
ment downstream. In extreme cases, underwater
dredging is not feasible and the river must be
diverted from the contaminated site for the site to
be dredged, sediment removed to suitable landfill
sites, and the channel restored. Sediment capping
is used in situations where the channel is not
aggrading and there is a steady supply of clean
sediment from upstream. The contaminated sedi-
ment is encapsulated by a clean cover with or
without a geomembrane encapsulation.

(a) If the friction factor is estimated to be 0.035
(calculated using the Colebrook equation),
how far downstream from the discharge loca-
tion can the effluent be considered well mixed
across the stream?
(b) How is this mixing distance affected if the
single-port outfall is replaced by a 5-m-long
diffuser located in the center of the stream?

4.4. A regulatory mixing zone in a river extends 200 m
downstream of a 5-m-long industrial diffuser. The
river has an average depth of 3 m, an average
width of 30 m, and an average velocity of 0.8 m/s;
the diffuser discharges 10 m 3 /s of wastewater con-
taining 5 mg/L of a toxic contaminant. It is esti-
mated that the plume width on the downstream
boundary of the mixing zone is 15 m. Estimate the
plume dilution on the (downstream) boundary of
the mixing zone.

A significant concern in many streams is accelerated
streambank erosion caused by human-induced changes
in the watershed. Such erosion increases sediment load
and inhibits aquatic life. Achieving natural stream sta-
bility is essential to minimizing stream bank erosion and
the resulting sediment pollution. natural stream stabil-
ity is achieved by allowing the stream to develop a
stable profile and pattern and stable dimensions, so that
the stream system neither aggrades nor degrades. Pres-
ervation of stream geomorphology is an important goal
in the identification of stream reaches that require res-
toration, and aids in the development of sustainable
stormwater management programs. The specialty area
dealing with the natural formation and equilibrium
shape of rivers is fluvial geomorphology .

4.5. A relatively clear river containing 5 mg/L of sus-
pended solids has a temperature of 15°C and
intersects a turbid river with a suspended solids
concentration of 35 mg/L and a temperature of
20°C. If the discharge in the clear river is 100 m 3 /s
and the discharge in the turbid river is 20 m 3 /s,
estimate the suspended solids concentration and
temperature downstream of the confluence of the
two rivers.