LAKES AND RESERVOIRS - Water Quality Engineering in Natural Systems - page 281

an area of vegetation, although the benefits are
only temporary. Rates of plant regrowth can be
rapid (within weeks), but can be slowed if the
cutter blade is lowered into the upper sediment
layer. Cut plants are removed from the lake, elimi-
nating an internal source of nutrients and organics
with potential long-term benefits. However, some
plant species, such as milfoil, may be fragmented
and dispersed and actually increase in abundance
after harvesting operations. Also, small fish can be
caught and killed by mechanical harvesters. Har-
vesting operations should precede spawning
periods and avoid important spawning and nursery
areas.
Herbicides. Although herbicide treatments can
reduce macrophyte growths rapidly, the benefits
are short term and the potential for negative side
effects is high. Plants are left in the lake to die
and decompose, releasing plant nutrients, and, in
some cases, causing oxygen depletion and algal
blooms. Plants generally regrow after several
weeks or months, or may be replaced by other,
more tolerant macrophyte species. Generally,
because herbicides do not remove nutrients or
organics from the lake or address the cause of the
aquatic plant problem, herbicides should be used
only where other techniques are unacceptable or
ineffective.

concentration 10 cm above the bottom of the lake
is 100 mg/L, and a typical particle size is 10 µ m. If
the density of the individual particles is estimated
as 2650 kg/m 3 , estimate the suspended sediment
concentration near the surface of the lake.

7.5. Estimate the depth of the euphotic zone in a lake
where the suspended solids concentration is
3 mg/L.

7.6. The orthophosphate concentration in a lake is
measured as 30 µ g/L and the available nitrogen is
measured at 0.2 mg/L. Determine the limiting
nutrient for algal growth. If the biomass concen-
tration in the lake is too high, suggest a method to
limit the growth of biomass.

7.7. The TP concentration in a lake is measured as
15 µ g/L, and the concentration of TN is estimated
as 0.17 mg/L. Estimate the biomass concentration
and trophic state of the lake.

7.8. Measurements in a lake indicate a Secchi depth of
2.0 m, a chlorophyll a concentration of 10 µ g/L,
and a TP concentration of 20 µ g/L. Estimate the
trophic state of the lake.

7.9. Estimate the maximum depth of the euphotic
zone in an eutrophic lake.

7.10. The measured temperatures are 15 and 12°C on
the top and bottom of an estuary, respectively,
and the corresponding salinity concentrations
are 2 and 20 ppt. Estimate the density difference
between the water on the top and bottom of
the estuary and determine which is more respon-
sible for the density difference, temperature, or
salinity.

PROBLEMS

7.1. A rectangular flood-control lake is 100 × 70 m and
has an average depth of 5 m.

(a) If the average inflow and outflow rate is
0.05 m 3 /s, estimate the detention time in the
lake.
(b) Compare this detention time with that of a
larger lake that is 200 × 140 m and 10 m deep,
with an average inflow/outflow rate of 0.1 m 3 /s.

7.11. A 20.0-m-deep lake has an average temperature
of 12°C, and the temperature difference between
the top and bottom of the lake indicates a
density difference of 0.400 kg/m 3 . If wind induces
a surface current of 10.0 cm/s and currents on
the bottom of the lake are negligible, assess
whether significant wind-induced mixing is likely
to occur.

7.2. A large natural lake has a surface area of
2.5 × 10 6 m 2 and the catchment for the lake has an
area of 2.0 × 10 7 m 2 . Estimate the hydraulic deten-
tion time of the lake.

7.12. The density difference between the top and
bottom of a 7-m-deep lake is measured as 3.5 kg/
m 3 , and the mean density of the lake water is
998 kg/m 3 . If the velocity in the lake is typically on
the order of 2% of the wind speed, estimate the
maximum wind speed for the lake to remain
strongly stratified.

7.3. What is the shear stress on the surface of a lake
corresponding to a wind speed of 35 mph? Esti-
mate the order of magnitude of the turbulent
velocity fluctuations induced in the surface layer
of the lake.

7.4. A 4.0-m deep lake under wind-induced circulation
has a turbulent diffusion coefficient of 4 cm 2 /s and
a temperature of 20°C. The suspended sediment

7.13. The currents in a 7-m-deep lake are on the order
of 5 cm/s. If the mean density of the water is