Morris & Fan. Reservoir Sedimentation Handbook
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CONCEPTS OF RESERVOIR LIMNOLOGY 4.7
T
= 999.8395 + 6.7914 × 10
-2
T -9.0894 × 10
-3
T
2
+ 1.0171 × 10
-4
T
3
-1.2846
× 10
-6
T
4
+ 1.1592 × 10
-8
T
5
(4.2)
The density increment due to dissolved solids may be estimated from:
TDS
= C
TDS
( 8.221 × 10
-4
- 3.87 × 10
-6
T + 4.99 × 10
-8
T
2
) (4.3)
where C
TDS
= total dissolved solids (mg/L) and T = temperature (
°
C). The density
increment due to suspended solids is:
SS
= C
SS
(1-1/G) × 10
-3
(4.4)
where G = specific gravity of the solids and C
SS
= suspended solids in g/m
3
or mg/L.
When G = 2.65, the equation is reduced to:
SS
= 0.00062 C
SS
(4.5)
The modification of density as a function of pressure is given by:
P
= /(1 - P/K) (4.6)
where P = pressure in bars (1 bar 10.2 m of water depth) and K is given by:
K = 19652.17 + 148.376 T - 2.329T
2
+ 1.3963 × 10
-2
T
3
-5.90 × 10
-5
T
4
+ (3.2918 - 1.719 × 10
-3
T + 1.684 x 10
-4
T
2
) P + (-0.8985 + 2.428 × 10
-2
T + 1.114
× 10
-2
P ) C
TDS
× 10
-3
(4.7)
Temperature differences will be the predominant mechanism creating density differences
in reservoirs when inflowing turbidity is low. At 25°C the density difference caused by a
1 °C difference in temperature is equivalent to the effect of approximately 420 mg/L of
suspended solids with a specific gravity of 2.65.
4.3 HYDRAULIC SHORT-CIRCUITING
If reservoirs were plug-flow-type reactors, the hydraulic retention time for any parcel of
inflowing water could be computed by dividing the total volume by the inflow rate.
However, the flow entering a reservoir may pass directly from inlet to the outlet as a discrete
current flowing along a path that is restricted or focused both horizontally and vertically,
thus limiting the mixing with water in the impoundment. This process is termed hydraulic
short-circuiting and is a common phenomenon in reservoirs. Hydraulic short-circuiting is
influenced by three factors: (1) density differences between the impounded and inflowing
water which cause stratification and vertical focusing of inflow, (2) reservoir geometry
that horizontally focuses flow, and (3) the opening of an outlet in the dam at a location and
discharge rate appropriate to release the focused flow.
4.3.1 Horizontal Focusing
When flood flow enters a reservoir, the flow will tend to bypass dendritic branches, side
storages, or other hydraulically "dead" zones as it flows toward the outlet. This phe-
nomenon of horizontally focused flow has been documented along the length of Lake
DeGray (Fig. 4.4). Because of horizontal focusing, storage areas off the main channel will
receive much less sediment loading than the main reservoir. For example, Lowe and Fox
CONCEPTS OF RESERVOIR LIMNOLOGY 4.8
FIGURE 4.4 Lake DeGray, Arkansas, illustrating horizontal focusing of flow (afte
r
Gunkel et al., 1984), the meandering thalweg, and the sampling locations used by James and Kenned
y
(1987).
(1995) noted that insignificant sedimentation has occurred in the portion of
Pakistan
's Tarbela reservoir that is off channel, although the main reservoir body itself
faces severe sedimentation problems.
In reservoirs having a deep, well-defined submerged river channel, bottom
density currents will be horizontally constrained by the channel geometry. If the
reservoir submerges an uncut forest, the "channel" constraining density current
movement may be defined by the stream corridor running through the still-standing but
submerged forest. However, as sediment deposition from turbid density currents
infill the original river channel, this horizontal focusing effect will be reduced or
eliminated and the density current will tend to spread out laterally and dissipate.
4.3.2 Vertical Focusing Focusing
Stratification can cause inflowing water to pass through a reservoir as overflow,
interflow, or underflow, depending on the relative density of the inflow and the vertical
density structure of the impounded water (Fig. 4.5). Warm water will flow across cooler
water as overflow, water of intermediate temperature will flow across the surface of the
thermocline, and cool or sediment-laden inflow will flow beneath warmer water as a
bottom current. The combination of stratification plus inflow can produce complex
vertical profiles in reservoirs, such as that shown in Fig. 4.6, created by the plunging of
turbid, oxygenated storm runoff within a tropical reservoir. The depth at which
water is released from a dam will also influence the flow patterns through the reservoir
upstream of the outlet.
Stratification can cause inflowing water to pass through a reservoir as overflow,
interflow, or underflow, depending on the relative density of the inflow and the vertical
density structure of the impounded water (Fig. 4.5). Warm water will flow across cooler
water as overflow, water of intermediate temperature will flow across the surface of the
thermocline, and cool or sediment-laden inflow will flow beneath warmer water as a
bottom current. The combination of stratification plus inflow can produce complex
vertical profiles in reservoirs, such as that shown in Fig. 4.6, created by the plunging of
turbid, oxygenated storm runoff within a tropical reservoir. The depth at which
water is released from a dam will also influence the flow patterns through the reservoir
upstream of the outlet.
The presence of inflow focusing does not guarantee that hydraulic short-circuiting
will occur. A high degree of short-circuiting will occur only if outlets are opened at the
appropriate depth, time, and discharge rate to intercept and release the current passing
through the reservoir. Consider the case of cold or turbid water entering a warm
reservoir, resulting in a strongly stratified system and a bottom density current that
reaches the dam. Hydraulic short-circuiting will occur if the inflow is released through a
bottom outlet. However, if a surface discharge is used there will be no short-
circuiting in the outflow because only the warm surface water will be discharged while
the inflow accumulates in the bottom of the impoundment.
The presence of inflow focusing does not guarantee that hydraulic short-circuiting
will occur. A high degree of short-circuiting will occur only if outlets are opened at the
appropriate depth, time, and discharge rate to intercept and release the current passing
through the reservoir. Consider the case of cold or turbid water entering a warm
reservoir, resulting in a strongly stratified system and a bottom density current that
reaches the dam. Hydraulic short-circuiting will occur if the inflow is released through a
bottom outlet. However, if a surface discharge is used there will be no short-
circuiting in the outflow because only the warm surface water will be discharged while
the inflow accumulates in the bottom of the impoundment.
CONCEPTS OF RESERVOIR LIMNOLOGY 4.9
FIGURE 4.5 Vertical focusing of flow in a stratified reservoir. The flowthrough pattern may be
characterized as overflow, interflow, or underflow depending on the density of the inflowing wate
r
relative to the vertical stratification structure of the impoundment (Kennedy and Nix, 1987).
FIGURE 4.6 Vertical profile of the eutrophic and oxygen-depleted La Plata Reservoir, Puerto Rico,
showing a bottom density current of turbid, oxygenated river inflow.
4.4 SELECTIVE WITHDRAWAL
Water quality within an impoundment varies with depth, and an intake structure which
allows water to be withdrawn from any of several levels can be used to modify the
quality of water discharged from the dam or delivered to users. The highest level of
control is achieved with an intake that allows water from different levels to be mixed in
variable proportions. A review of selective withdrawal techniques is provided by Cassidy
(1989).
CONCEPTS OF RESERVOIR LIMNOLOGY 4.10
Selective withdrawal can improve water quality for a variety of uses. Anaerobic
profundal water quality is generally not desirable for municipal water use, potentially
creating taste and odor problems and objectionable levels of dissolved iron and
manganese that will form staining precipitates when oxidized. Cold irrigation water can
affect temperature-sensitive crops such as rice. Anaerobic decomposition of organic
matter on the bottom of a reservoir can produce hydrogen sulfide gas which can corrode
hydropower equipment and is toxic to aquatic life. The quality of water discharged to
the river downstream of a dam heavily influences the biological conditions in the
regulated river reach, with the two principal parameters being temperature and
dissolved oxygen. Without selective withdrawal capability, the temperature of water
released from a reservoir cannot be controlled and thermal shock can be created
downstream. Selective withdrawal allows the temperature of releases to be regulated
by the controlled mixing of colder hypolimnic water with warm epilimnic water. A
vertical array of temperature sensors in the reservoir can help operators select gate
openings to achieve the desired temperature.
It may be desired to vent turbid density currents from the reservoir, to withdraw
water from sediment-free zones in the reservoir, or both. The selective withdrawal struc-
ture described in the case study of glacial-fed Gebidem dam was added to allow
water to be withdrawn near the surface where both suspended solids concentration and
grain size will be reduced compared to that in the existing fixed deep inlet, reducing
wear on turbine runners. The inlet structure at Lost Creek Lake (Fig. 4.7) combines
both selective withdrawal with a conduit for venting turbidity currents. The turbidity
conduit is intended to allow the rapid release of turbid water during and after large
flood events, with the idea of rapidly clarifying surface waters and thereby allowing
clear water releases to be reinitiated as soon as possible. This measure was
implemented to reduce impacts on downstream andromadous fisheries (Cassidy, 1989)
FIGURE 4.7 Outlet structures at Lost Creek Lake, Oregon. The Low-level outlet is on the left of
the photo, and the selective withdrawal tower and the turbidity conduit connected to its base are
visible on the right. (Courtesy U.S. Army Corps of Engineers)
CONCEPTS OF RESERVOIR LIMNOLOGY 4.11
4.5 LIGHT AND TRANSPARENCY
Lake transparency has traditionally been measured by recording the disappearance depth
of a Secchi disk, a white disk 20 cm in diameter, which is lowered into the water between
10 A.M. and 2 P.M. on the shaded side of the boat. This is one of the most basic of all
limnologic parameters measured in the field; the method is simple, and a Secchi disk is
low-cost, portable, and virtually indestructible. Extensive Secchi disk data have been
collected at numerous sites over many years. In an oligotrophic alpine lake, such as Lake
Tahoe on the California-Nevada border, Secchi disk transparencies can attain 40 m,
whereas in highly turbid water the disk may disappear at a depth of only a few
centimeters. Secchi disk transparency will vary seasonally and spatially as a result of
turbid inflows and variations in algal populations.
Turbidity can be measured by submersible sondes instrumented with turbidi-
meters and data loggers. These instruments, which have become widely available at
relatively low cost, can provide rapid data collection and reduction, can be used under
any ambient light conditions, and can measure the variation in turbidity with depth.
This last capability is quite important since the vertical distribution of turbidity is not
uniform, being affected by turbid density currents, settling of suspended solids, and
planktonic organisms which can be concentrated at specific depths and migrate
vertically. If turbidity is correlated to suspended solids concentration based on water
sampling at different depths and locations, turbidity data can be used to quickly map the
suspended solids structure within the impoundment and to monitor its changes, including
the movement of turbid density currents. A sonde used for turbidity profiling would also
normally include additional parameters such as temperature, dissolved oxygen, depth,
and pH, allowing several important profile parameters to be measured and recorded
simultaneously. When turbidimeters are used it is a good idea to use a Secchi disk for
comparative purposes. Also, different types of turbidimeters can give different readings
for the same natural water sample despite proper calibration against a formazin standard
(see Sec. 8.4.3).
Incident solar radiation is polychromatic, and different wavelengths are absorbed at
different rates. Red and infrared wavelengths are absorbed most rapidly, and blue light
has the greatest penetration. Over half the incident light is usually transformed into heat
within the upper meter of the water column. Light intensity is attenuated in both ocean
and lake waters according to the Beer-Bouguer law having the form:
I
z
= I
o
e
–kz
(4.8)
where I
o
= incident light intensity at the water surface
I
z
= light intensity at depth
z = depth, m
k = extinction coefficient
The relative rate of extinction can be determined by assigning I
0
= 1.0, or the absolute
value of irradiance at any depth can be determined by expressing I
0
in units such as of
W/m
2
or kcal/m
2
. The adsorption coefficient for polychromatic solar radiation can be
estimated from the Secchi disk transparency Z
D
by the relationship k = 1.7/Z
D
.
Coefficient values for monochromatic wavelengths, summarized by Wetzel (1983), range
from 2.45 at 760 nm (infrared) to 0.0181 at 473 nm (blue). As a precautionary note, the
terminology for the extinction coefficient used here follows the nomenclature of Cole
(1994), who points out that the term absorption coefficient also applies to the Beer-
Bouguer law, but formulated for base 10 logarithms. Thus, the value of the absorption
coefficient is 2.3 times the value of the extinction coefficient. Not all authors conform to
this nomenclature and there is Potential for confusion.
CONCEPTS OF RESERVOIR LIMNOLOGY 4.12
In most systems, photosynthesis is negligible when light intensity declines to about 1
percent of the intensity at the surface. Light reflected to the observer from a submerged
Secchi disk has to travel both directions through the water column, and as a rule of thumb
the depth of the euphotic zone, the level of 1 percent irradiance, is about 3 times the
Secchi disk depth (2.7 times for the Beer-Bouguer law and the relationship k = 1.7/Z
D
).
However, in some lakes and in highly turbid waters, the euphoric zone can be as much as
5 times the Secchi disk depth.
4.6 PRODUCTIVITY AND EUTROPHICATION
4.6.1 Primary Production
The primary producers or autotrophs create organic material from inorganic substances
using sunlight as an energy source. The process of primary production, the
conversion of radiant solar energy into chemical energy, can be represented by the
simplified photosynthetic equation:
Photosynthesis
6CO
2
+ 6 H
2
O
C
6
H
12
O
6
+ 60
2
(4.9)
Respiration
This equation represents the continual cycling of carbon between inorganic and organic
forms. The organic matter in turn feeds heterotrophic and decomposer organisms.
The linkages of energy transfer pathways, from primary producers to herbivores, and
ultimately to carnivores, is termed the food chain. Each level in a food chain is termed a
trophic level, with the primary producers occupying the first level. Only about 10 to 20
percent of the energy available at each trophic level is passed to the next, which usually
limits ecosystems to about four trophic levels. All food chains may be conceptualized as
pyramids with a few carnivores such as largemouth bass at the top, supported by lower
trophic levels containing more numerous populations and greater total biomass. An
example of a simple grazing food chain is given in Fig. 4.8, which shows energy
pathways through the system. Food chains are of two basic types: grazing food chains are
based on the direct consumption of green plants by herbivores, and detrital food chains
are based on the decomposition of nonliving organic matter by microbes, with the
microbe-enriched material subsequently being processed by detritivores, which also
shred the detritus into smaller particles.
The organic material that forms the base of the food chain in aquatic ecosystems may
be autochthonous material produced within the lake by littoral or floating vegetation,
aquatic plants, and microflora including diatoms and algae. The microflora may be either
attached to submerged surfaces (periphytonic) or floating within the water column
(planktonic). Allochthonous organic inputs may be delivered by inflowing streams or
from adjacent terrestrial ecosystems and include colloidal or dissolved material, organic
detritus, and terrestrial animals that fall prey to lake-dwelling organisms.
4.6.2 Diurnal Variations in Dissolved Oxygen
As indicated by the photosynthetic equation, there is a net release of oxygen (and con-
sumption of CO
2
) during photosynthesis, and the opposite occurs during respiration.
Algae and other primary producers release oxygen into the water column during the day,
and the respiration by the algae and heterotrophs at night depletes oxygen, resulting in
CONCEPTS OF RESERVOIR LIMNOLOGY 4.13
FIGURE 4.8 Principal energy flow pathways through a simplified food chain in a
Georgia (United States) pond managed for sport fishing. Numbers are energy flow between each
compartment in kcal/m
2
/yr. (Modified from Odum, 1971.)
a diurnal variation in oxygen concentration in the epilimnion. The diurnal range of
oxygen variation increases as a function of increasing primary productivity, and in
hypereutrophic water bodies characterized by high algal biomass the dissolved oxygen
levels may be supersaturated by day and sufficiently depleted by night to cause fish kills.
Oxygen levels required for the survival of different species varies significantly, and
oxygen depletion will eliminate the most oxygen-sensitive species (often desirable game
species) from an impoundment.
4.6.3 Productivity
The gross primary productivity, or simply productivity, is the total rate of organic matter
formation, adjusted to include losses such as respiration, grazing, and excretion. The net
primary productivity refers to the rate of change in organic material, or net growth, which
equals the gross primary productivity less the respiration or metabolic consumption by
the primary producers.
The productivity, expressed in terms of gross carbon fixation, is measured in aquatic
systems using the light-dark bottle technique. In this technique, a water sample is
withdrawn from a desired water level and placed into three glass-stoppered biological
CONCEPTS OF RESERVOIR LIMNOLOGY 4.14
oxygen demand (BOD) bottles. One of these is transparent [the light bottle (LB)], a
second is entirely wrapped with black electrical tape [the dark bottle (DB)], and the third [the
is entirely wrapped with black electrical tape [the dark bottle (DB)], and the third [the
initial bottle (IB)] is immediately analyzed to determine initial dissolved oxygen (DO)
concentration. The LB and DB containing the original water sample and its planktonic
population are placed back into the water column at the sampled depth to incubate until a
significant oxygen change occurs, but without reaching oxygen depletion in the dark
bottle. Incubation times can be less than 1 h in eutrophic waters, whereas 8 h of
incubation may not significantly change oxygen levels in oligotrophic waters. After the
light and dark bottles are recovered, their oxygen content is measured. The net oxygen
depletion in the dark bottle represents the respiration in the aquatic community, while the
net oxygen increase in the light bottle represents net production. Gross production (GP),
or simply production, equals net production plus respiration and can be computed from
the oxygen levels in the bottles as follows: GP = LB - DB. Oxygen is then converted to
carbon equivalent to express production over the measurement period, which is then time
adjusted to (g C)/m
2
/day. Carbon uptake may be measured directly in the light-dark bottle
method by adding isotope-labeled
14
CO
2
to the sample, then filtering and measuring the
radioactivity contributed by
14
carbon absorbed in the filtered phytoplankton. This method
is more sensitive than oxygen measurement and can be used for short incubation periods
or in oligotrophic waters where productivity is too low for the oxygen measurements.
Rates of carbon production and selected other parameters as a function of trophic status
are compared in Table 4.2. From a comparison of productivity data from 64 reservoirs
and 102 natural lakes, Kimmel et al. (1990) concluded that reservoirs tend to be more
productive (more eutrophic) than natural lakes.
Productivity may also be expressed in terms of the conversion of solar energy. The
productivity of several ecosystem types is compared in Table 4.3 in energetic terms, and
several useful energy conversions relating to primary productivity are summarized in
Table 4.4.
4.6.4 Factors Limiting Primary Production
The rate of primary production in aquatic ecosystems may be limited by a variety of
factors. Limiting factors may include the rate of light extinction in turbid water, concen-
tration of available forms of necessary nutrients, temperature, and toxins. The primary
factors limiting the rate of primary production in aquatic ecosystems may change con-
tinuously over time scales ranging from hours to years.
4.6.5 Trophic Status
Lakes and reservoirs may be classified in accordance with their trophic status, which
reflects the nutrient concentration and rate of primary productivity within a body of
water. An oligotrophic lake is characterized by low levels of nutrients and biological
activity per unit of volume, resulting in high water clarity and high levels of dissolved
oxygen throughout the entire water column. Alpine lakes and reservoirs with small
watersheds in areas of erosion-resistant granitic parent material lack significant inputs of
nutrients and sediments and are typically oligotrophic. A eutrophic lake has high levels of
nutrient inputs and high rates of primary productivity, including algal blooms and in
some cases prolific growths of submerged or floating vegetation. Water clarity is low, the
substrate may become smothered in organic mud as a result of increased primary
production, anaerobic conditions occur in deeper water, and temporary oxygen depletion
can occur even in shallow water, causing fish kills. In highly eutrophic lakes, sustained
anoxia can occur at depths less than 2 m. Eutrophic conditions are normally found in both
TABLE 4.2 General Ranges of Primary Productivity of Phytoplankton and Related Characteristics of Lakes of Different Trophic Categories
Phyto- Phyto- Light Total
Mean primary plankton, plankton, Chloro- extinction Total inorganic
productivity,* density biomass, phyll a, Dominant coefficients, organic Total P, Total N, solids.
Trophic type (mg C)/m
2
/ day) cm
3
/m
3
mg• C/m
3
mg/m
3
phytoplankton k/m carbon, mg/L µg/L µg/L mg/L
Ultraoligotrophic <50 <1 <50 0.01-0.5 0.03-0.8 <1-5 <1-250 2-15
Oligotrophic 50300 20-100 0.3-3 Chrysophyceae, 0.05-1.0 <1-3
cryptophyceae,
Oligomesotrophic 1 - 3 dinophyceae, 5-10 250-600 10-200
bacillariophyceae
Mesotrophic 250-1000 100-300 2-15 0.1-2.0 < 1-5
Mesoeutrophic 3-5 10-30 500-1100 100-500
Eutrophic >1000 >300 10-500 Bacillariophyceae, 0.5-4.0 5-30
cyanophyceae,
Hypereutrophic >1 0 chlorophyceae, 30->5000 500->15,000 400-60,000
euglenophyceae
Dystrophic <50-500 <50-200 0.1-10 1.0-4.0 3-30 <I-10 <1-500 5-200
* Referring to approximately net primary productivity, such as measured by the
14
carbon method. Source: After Wetzel (1983)
CONCEPTS OF RESERVOIR LIMNOLOGY 4.16
TABLE 4.3 Productivity of Aquatic and Terrestrial Ecosystems. *
Site
Gross Primary
Production,
kcal/m
2
/yr
Total for the World,
10
11
kcal/yr
Estuaries and reefs 20,000 4
Tropical forest 20,000 29
Fertilized farmland 12,000 4.8
Eutrophic lakes 10,000 -
Eutrophic seasonal wetlands 10,000 -
Unfertilized farmland 8 8,000 3.9
Coastal upwellings 6,000 0.2
Grassland 2,500 10.5
Oligotrophic lakes 1,000 -
Open oceans 1,000 32.6
Desert and tundra 200 0.8
* Net carbon production converted to gross energy values, where necessary, by multiplying by 20.
Source: After Horne and Goldman (1994)
TABLE 4.4 Biological Productivity Energy Conversions
To convert To Multiply by Reciprocal
g O2 released kcal 3.510 0.285
g CO2 absorbed kcal 2.553 0.392
g C fixed kcal 9.361 0.107
Kcal kJ 4.187 0.239
kcal/s kW 0.239 4.187
Source: After Cole (1994)
natural and artificial lakes that receive runoff or waste discharges from agricultural or
urban areas, or shallow water bodies in which nutrients in bottom sediments can
be continually brought to the surface water by processes such as wave action. A
mesotrophic lake has characteristics intermediate between oligotrophic and
eutrophic. Eutrophication refers to the process by which biological productivity
increases within an aquatic ecosystem, as compared to natural conditions, typically
as a result of human disturbances which increase the availability of limiting
nutrients and results in greatly increased populations of algae or macrophytes. The
eutrophication process can affect lakes at all trophic levels.
4.7 NUTRIENTS
4.7.1 The Concept of Limiting Nutrients
It is now widely recognized that phosphorus and nitrogen are the primary nutrients
limiting productivity in most lakes and reservoirs. Of these, phosphorus is the