Morris & Fan. Reservoir Sedimentation Handbook
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CONCEPTS OF RESERVOIR LIMNOLOGY 4.17
least abundant of the major nutritional and structural components of biota, is considered
to be the primary factor limiting primary production in many freshwater ecosystems,
and has been the focus of many control efforts.
Phosphorus is removed from aquatic systems rather rapidly and becomes stored in
sediments, where its concentration may be 2 orders of magnitude greater than the con-
centration in the overlying water. Continued inputs of phosphorus are necessary to
maintain enhanced productivity in many oligotrophic and mesotrophic lakes. Manipula-
tion of water levels and sediments will potentially impact phosphorus levels and
productivity in the reservoir and downstream ecosystems, making it important to
have a conceptual understanding of the phosphorus cycle and its relationship to lake
sediments. The phosphorus cycle in lakes is described in basic limnology texts.
The ratios of major elements found in aquatic organic matter, including both algae
and aquatic macrophytes are:
1P : 7 N : 40 C per 100 units dry weight
1P : 7 N : 40 C per 500 units wet weight
Thus, if phosphorus is limiting, the addition of phosphorus in assimilable form can
theoretically generate 500 times its weight in living cell mass. Phosphorus is naturally
supplied to lakes from the atmosphere and soils in the watershed, plus human
activity including the discharge of detergents containing phosphorus. A generalized
relationship between nutrient concentration and trophic status is summarized in Table
4.5.
TABLE 4.5 Epliminic Phosphorus and Nitrogen Concentrations in
Nautral Lakes, Related to Trophic Status
Trophic Condition Total P, μg/L Inorganic N, μg/L
Ultraoligotrophic <5 <2000
Oligomesotrophic 5 - 10 200-400
Mesoeutrophic 10 - 30 300 – 650
Eutrophic 30 - 100 500 – 1500
Hypereutrophic >100 >1500
4.7.2 Phosphorus
Most phosphorus in natural aquatic environments is contained in particulate form, pri-
marily in seston, which is both the living and nonliving organic particulate matter sus-
pended in water. The dissolved phosphorus fraction consists of the material passing a
0.5-µm filter and includes dissolved inorganic orthophosphate (PO
4
-3
), the only inor-
ganic form available for uptake by algae, and organic phosphate in a colloidal state. The
orthophosphate fraction usually constitutes about 5 percent or less of the total phosphorus
in natural water. When orthophosphate is added to water, its rate of assimilation by algae
is extremely rapid, on the order of minutes, with uptake tending to be faster in
oligotrophic systems having the most severe phosphorus deficiency. In systems with
higher concentrations of phosphorus, algae can assimilate phosphate in quantities
significantly exceeding their actual needs, a process termed luxury consumption.
The particulate fraction includes phosphorus contained in or adsorbed onto seston
and/or inorganic complexes such as clays, carbonates, and ferric hydroxides. Phosphorus
data from aquatic systems usually refers to either the total or the soluble inorganic
phosphorus (orthophosphate) without further differentiation of its various forms. While
CONCEPTS OF RESERVOIR LIMNOLOGY 4.18
most phosphorus in natural lakes is associated with seston, in reservoirs
experiencing significant sediment loads much phosphorus may be associated with
sediment, primarily the fine fraction that has a large surface area in relation to mass.
The total concentration and fractionation of phosphorus will vary seasonally.
The phosphorus contained in organic material is recycled through both algal and
bacterial waste products into forms that can be assimilated. However, sedimentation of the
particulate fraction and its incorporation into sediments effectively removes phosphorus
from the water column, and sedimentation is the primary sink for phosphorus. The rate of
phosphorus sedimentation in reservoirs is significantly higher than in natural lakes,
attributed by Canfield and Bachmann (1981) to the association of phosphorus with the
higher settleable sediment load delivered to reservoirs. Littoral vegetation also plays an
important role in both the uptake and release of phosphorus in natural lakes. It may also be
important in smaller reservoirs with stable water levels, significant vegetated shallows,
and dendritic geometry that produces an elongated shoreline. However, littoral vegetation
may be insignificant or absent in larger impoundments, in arid or alpine zones, and where
steep slopes or variation in pool elevation precludes significant vegetative growth.
Under aerobic conditions, phosphate combines with iron to form ferric phosphate and
can also be absorbed onto ferric hydroxide and calcium carbonate. These chemical
equilibria produce highly insoluble forms of phosphorus causing oxygenated sediment to
act as a phosphorus sink. However, sediments with a significant organic content will
become anaerobic below the surface, and reduction reactions involving iron cause
dissolved iron and phosphate to be released simultaneously into the interstitial water. A
layer of oxidized surface sediment less than 1 cm thick can be very effective in
preventing the release of soluble phosphorus from the interstitial waters into the water
column, but when anoxic or reduced conditions extend to the sediment surface (e.g.
because of anoxia in the hypolimnion), the dissolved phosphorus is no longer trapped and
over a period of several months phosphorus can migrate into the water column from a depth
of at least 0.1 m within eutrophic muds. The rate of phosphorus release can be further
accelerated (e.g., doubled) by agitation from turbulence, but biological activity by bottom-
feeding fish and burrowing fauna seem to have little impact (Wetzel, 1983) The potential
for biological activity that disturbs P-enriched muds is reduced or eliminated by anaerobic
conditions in the bottom waters.
As a result of these interactions between phosphorus, sediment, and the water column,
oligotrophic lakes with a well-oxygenated hypolimnion show little variation in
phosphorus content with depth. There is a limited supply of phosphorus in the system and
the oxygenated sediments serve as a phosphorus sink. By contrast, in eutrophic lakes with
oxygen-depleted profundal water and anaerobic sediments, soluble phosphorus is
continuously released from sediments into the water column, and the phosphorus
concentration increases significantly with depth. Thus, water released from the
hypolimnion of productive reservoirs tends to be nutrient-enriched, and circulation
patterns within reservoirs that draw deeper anaerobic water into the epilimnion can
increase nutrient availability, productivity, and the loading of organic sediments (Fig 4.9).
Because of the longitudinal gradients characteristic of reservoirs, the significance of internal
nutrient cycling can vary from one location to another. These dynamics may be summarized
by stating that surface discharge reservoirs tend to trap nutrients and export heat, whereas
reservoirs with deep discharges tend to export nutrients and trap heat (Wright, 1967).
Lijklema et al. (1994) examined nutrient dynamics and light-shading associated will the
resuspension of sediment by wave action in shallow lakes in the Netherlands. They
pointed out that turbulent resuspension of sediments can increase productivity in the
overlying water because of the recirculation of phosphorus, but it can also reduce primary
CONCEPTS OF RESERVOIR LIMNOLOGY 4.19
FIGURE4.9 Seasonal variation in dissolved oxygen distribution in Lake DeGray,
Arkansas, showing seasonal anoxia in the upstream zone of the reservoir receiving the
highest rate of organic loading, and the transport of nutrient-enriched anoxic water due to
runoff events entering the reservoir as interflow. (Kennedy and Nix. 1987)
productivity as a result of turbidity shading. They modeled sediment erosion, transport, and
deposition in a variable-depth system consisting of a generally shallow lake containing deep
pits or channels into which suspended sediments would collect, but which were too deep for
wave action to resuspend the sediments. It was found that the pits would act as a trap for
fine sediments and their associated nutrients, which could be subsequently dredged, thereby
removing nutrients from the lake. This modeling simulated the size-dependent settling rate
of the sediment, and the preference of phosphate for the fine-sized fractions
4.7.3 Nitrogen
Elemental nitrogen (N
2
), the most abundant element in the atmosphere and the most
abundant form of nitrogen in aquatic systems, is biologically nonreactive except for
nitrogen-fixing organisms. Nitrate (NO
3
-
) is commonly the predominant form of reactive
nitrogen in aquatic systems. Other forms include ammonia(NH
4
+
), nitrite (NO
2
-
), and
CONCEPTS OF RESERVOIR LIMNOLOGY 4.20
both particulate and dissolved organics. Unlike phosphorus, nitrogen is very mobile
and is not highly absorbed by soil, and only ammonia (NH
4
+
) tends to be adsorbed
by clay or organic particles.
Nitrogen is commonly a limiting nutrient in the ocean and lakes in warm climates.
Inputs of biologically reactive forms of nitrogen into reservoirs and other aquatic systems
are derived from atmospheric sources (precipitation and dry fallout), from runoff
(including pollutants), and by fixation of molecular nitrogen by blue-green algae and
photosynthetic bacteria. The bacterial reduction of nitrate to nitrite and then to elemental
nitrogen, a process termed denitrification, occurs in areas of oxygen deficit including
the hypolimnion and sediments. It is an important process for the removal of biologically
reactive nitrogen from aquatic ecosystems. Many aquatic systems experience regular
seasonal variations in nitrogen levels (Home and Goldman, 1994).
The nitrogen cycle is complex, and nitrogen may exist simultaneously in many
forms in the water column. Nitrogen transformations are controlled largely by microbes.
The rate of nitrogen exchange between sediment and the water column varies greatly as a
function of various factors including redox potential and sediment composition, and
Wetzel (1983) characterized the nitrogen dynamics of sediments as "poorly understood.”
4.8 GRADIENTS, SEDIMENTATION, AND
BIOLOGICAL PROCESSES
A large fraction of the organic matter and nutrients delivered to reservoirs is in particulate
form and subject to the same sedimentation processes that affect suspended mineral
particles. Allochthonous inputs of both detrital organics and nutrients are delivered to
the impoundment by the inflowing river. Both living and dead suspended organic
particulate material, the seston, will settle out by physical processes, and a sedimenta-
tion gradient would normally be expected from the headwater to the dam. Dissolved
nutrients in the water will be incorporated into the algal biomass, or floating
macrophytes (e.g., water hyacinth) in some tropical reservoirs, which will also
contribute to the sedimentation of organic material as these organisms die and sink.
Thus, seston from both allochthonous and autochthonous origin contributes to the total
organic sediment load, and in hydrologically large reservoirs this organic load may be
expected to be higher in the headwater region and lower nearer the dam.
Gradients in the nutrient content in the water and the resulting deposition rate of
organic sediments can have important ecological consequences from the standpoint of
oxygen distribution, nutrient cycling, and reservoir metabolism. In reservoirs that are
longitudinally short, hydrologically very small, or subject to periodic emptying, these
gradients may not be pronounced. However, in larger reservoirs these gradients can be a
primary factor controlling ecological conditions. The existence of a sedimentation
gradient and several ecological consequences were documented at Lake DeGray by
James and Kennedy (1987).
The 808-Mm
3
Lake DeGray impoundment on the Caddo River in Arkansas is
hydrologically large lake with C:I = 1.2. The mean depth is 9 m, maximum depth is 60
m, and the 1162 km
2
watershed is mostly forested. The reservoir configuration is
shown in Fig. 4.4. Sediment deposition was monitored by triplicate traps suspended in
the water at depths of 5 and 15 m, with an additional 45-m trap near the dam.
Trap deployment periods ranged from 20 to 48 days. Upon retrieval the samples were
recovered, the traps were cleaned and redeployed, and the triplicate samples were
mixed and analyzed.
CONCEPTS OF RESERVOIR LIMNOLOGY 4.21
Because of sedimentation, the sediment loading rates measured in the traps declined
in the direction of the dam (Table 4.6). Although deposition at the extreme upstream end
of the reservoir was limited by high flow velocity that can entrain the low-density organic
material and transport it farther downstream, considerable deposition occurred in the
vicinity of the plunge point. As nutrient-rich river water moved through the reservoir in
early spring, uptake by algae and removal by sedimentation progressively decreased
nutrient concentrations down-reservoir, thereby limiting productivity near the dam. Storm
events were found to account for about 95 percent of the annual suspended solids
loading. The generalized pattern of production and sedimentation of organic matter from
both allochthonous and autochthonous sources is summarized in Fig. 4.10.
TABLE 4.6
Composition of Trapped Sediment as a Function of Location in lake
DeGray, Arkansas (mg/m
2
) during the Study Period
Parameter Headwater Middle Dam
Dry weight 8550 2980 1520
Carbon 456 202 123
Nitrogen 45 22 15
Phosphorus 18 5 2
Iron 269 103 50
Chlorophyll a 2.93 0.92 0.23
Source: James and Kennedy (1987)
FIGURE 4.10
Generalized longitudinal pattern of organic matter production and
sedimentation.
Kennedy and Nix (1987) studied oxygen dynamics in Lake DeGray, which were found to
be related to the longitudinal pattern of sediment deposition. When the reservoir was
impounded it submerged a mixed pine-hardwood forest. Oxygen demand from
submerged vegetation and soils resulted in severe oxygen depletion within the
hypolimnion during the first years of impounding. However, as the readily oxidizable
materials were consumed, oxygen dynamics in the lake came to be controlled by
allochthonous and autochthonous organic materials. Anoxia is initiated near the head-
water region of the reservoir, the zone where organic inputs are greatest from riverborne
detritus plus increased photosynthetic activity in the lake due to higher nutrient levels.
CONCEPTS OF RESERVOIR LIMNOLOGY 4.22
Dissolved oxygen in the headwater region is depleted rapidly in the spring because of
the limited volume in the hypolimnion plus the high rate of organic loading in that zone.
Organic sediments in the headwater zone release nutrients when subject to anoxic con-
ditions. Subsequent streamflow entering the reservoir as interfow transports anoxic water
and its associated nutrients deeper into the reservoir, thereby extending longitudinally
the zone of higher productivity in the epilimnion (Fig. 4.9).
4.9 CLOSURE
Little justice can be done to the field of limnology in a single condensed chapter. Readers
are encouraged to acquire a basic college text in limnology, and pursue a basic
understanding of the fundamental physical and biological processes occurring in
reservoirs.
CHAPTER 5
SEDIMENT PROPERTIES
5.1. SIZE CLASSIFICATION OF SEDIMENT
5.1.1. Size of Sediment Grains
The size of sediment particles transported by water ranges over 7 orders of magnitude
from individual clay platelets to boulders. Grain size is the most important
parameter describing sediment behavior in water, and a variety of terms may be used to
describe the size characteristics of individual grains and composite samples. The term
coarse normally implies sand and larger grains; fine refers to silts and clays. We will
use the size classification system recommended by the American Geophysical Union,
which is a geometric scale based on a ratio of 2 between successive sizes (Lane, 1947).
This classification method is presented in Table 5.1 along with sieve sizes. The phi ()
classification scale used by many researchers is a geometric grade scale in which particle
diameter in millimeters is expressed as the base 2 exponent (phi sizes for grains 0.5, 1, 2,
and 4 mm in diameter are -1, 0, 1, and 2 respectively). The phi size for a particle is
computed as
Φ= -log
2
d = 3.3219 log
10
d (5.1)
where d = diameter in mm. The phi scale was adopted to facilitate application of con-
ventional statistical methods to sediment-size particles (Krumbein,1934). Differences
occur between the size scales used internationally, especially for coarse sediment (Fig.
5.1). Some size-dependent characteristics of sediments are summarized in Fig. 5.2, along
with size ranges for several other types of particles.
Sediment particles are never exactly spherical and the term "diameter" only
approximates their size. Several methods are used to determine and express grain
diameter. Grains may be measured along three mutually perpendicular axis of which there
will be a longest, an intermediate, and a shortest axis. The triaxial diameter is the
arithmetic average of the three axes. Sieve diameter is the length of the side of a square sieve
opening through which a particle will just pass and closely approximates the intermediate
axis. Sands and fine gravels are normally sized with sieves, and the reported diameter is
characteristically the sieve diameter. Individual larger particles, such as gravel or
cobbles, can be directly measured by using a scale and the intermediate axis length is
reported as the nominal diameter. Sedimentation diameter is the diameter of a sphere
which, in the same fluid, has the same terminal settling velocity as the particle. The
reported diameter values for silts and clays are actually sedimentation diameters, since
the size of these minute particles is determined by their fall velocity in water. When
analyzing the settling properties of small particles, including clays which are more plate-
like than spherical and whose settling velocity is often strongly influenced by
flocculation, the sedimentation diameter is the most appropriate measure. Nominal
diameter is the diameter of a sphere having the same volume as the particle.
SEDIMENT PROPERTIES 5.2
TABLE 5.1 Grain Size Classes
Grain diameter, mm
Sieve size retained on
Size class Min. Max. Tyler U.S. Standard
Boulders:
Very large 2048 4096
Large 1024 2048
Medium 512 1024
Small 256 512
Cobbles:
Large 128 256
Small 64 128
Gravel:
Very coarse 32 64
Coarse 16 32
Medium 8 16
Fine 4 8 5 5
Very fine 2 4 9 10
Sand:
Very coarse 1 2 16 18
Coarse 0.5 1 32 35
Medium 0.25 0.5 60 60
Fine 0.125 0.25 115 120
Very fine 0.062 0.125 250 230
Silt:
Coarse 0.031 0.062
Medium 0.016 0.031
Fine 0.008 0.016
Very fine 0.004 0.008
Clay:
Coarse 0.002 0.004
Medium 0.001 0.002
Fine 0.0005 0.001
Very fine 0.00024 0.0005
5.1.2. Particle Shape and Roundness
The shape factor (SF) is an expression of the "sphericity" of a particle and may be
computed from the relative lengths of the longest (a), intermediate (b), and shortest (c)
particle
axes:
(5.2)
The s
hape factor for a sphere is 1.0 and for natural sands is usually about 0.7
(Interagency Committee, 1957).
Roundness is defined as the ratio of the average radius of the corners and edges of
a particle to the radius of a circle inscribed in the maximum projected area of the particle.
SEDIMENT PROPERTIES 5.3
It
is geometrically independent of sphericity and has a relatively small effect on the
hydraulic behavior of the particle, but it is of primary importance in determining the
abrasiveness of a particle to hydraulic equipment. Freshly weathered particles have
highly angular structures and become more rounded over time as a result of interparticle
abrasion. Thus, sands in streambeds tend to be more angular in headwaters and less
angular farther downstream. Aeolian (wind-transported) sands are typically more rounded
than fluvial sands because the impact and abrasive forces between grains transported by
air is greater than between sands transported in water. A visual representation of
roundness and sphericity is shown in Fig. 5.3.
SEDIMENT PROPERTIES 5.4
FIGURE 5.3 Sphericity, roundness, and sorting of sediment gains.
5.1.3. Grain Size Distribution
Sediment deposits have a range of grain sizes, and both field and laboratory techniques
can be used to separate sediment grains according to their size. The granulometric char-
acteristics of the sample may be described by a grain size distribution curve (Fig. 5.4)
which represents the cumulative dry weight of the sample in each size fraction. The par-
ticle size is displayed horizontally and the cumulative weight percent of grains smaller
than the stated size is displayed along the vertical axis. In a sieve analysis the vertical
axis corresponds to the weight percent passing each screen size.
For both computational and descriptive purposes, it is frequently desirable to use a
single diameter which describes the behavior of a sediment sample containing a range
of grain sizes. The median (d
50
) diameter is most frequently used to describe a sediment
sample. Fifty percent of the sample weight is composed of grains smaller than the
median diameter. However, different researchers have proposed using "representative"
sizes ranging from d
3S
to d
max
to describe the transport and related properties of sediment
mixtures. Stelczer (1981) states that the movement of bed load of mixed sediments
is best predicted by the particle size around d
80
, with both the coarser and finer
fractions of the bed being set into motion almost simultaneously, at the same critical
velocity as the d
80
fraction. The d
16
,
d
84
,
and d
90
diameters are also frequently used to
describe sediment mixtures.
Assuming the grain size curve is a cumulative distribution function (CDF) following a
normal (Gaussian) distribution, which is often approximately the case, the S-shaped CDF
can be converted into a straight line on normal probability paper. For a normal distribution,
the d
16
and d
84
diameters may be used to compute the standard deviation of the grain
size of the sample:
(5.3)