Morris & Fan. Reservoir Sedimentation Handbook

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SEDIMENT PROPERTIES 5.5

FIGURE 5.4 Grain size distribution curve.

Sediment size distributions usually only approximate a normal distribution, and may be

skewed rather than centered on the median size. The gradation coefficient (Gr) defines

the departure from the sample median value as follows:

(5.4)

These distribution parameters should be used only if the grain size distribution curve is S

shaped.

The slope of the grain size distribution curve reflects the uniformity of the grain sizes in

the sample. Steep size-distribution curves are produced by samples which are more

uniform in size (well-sorted); flatter size-distribution curves are produced by samples

having a wide range of grain sizes (poorly sorted). The concept of sediment sorting is

visually presented in Fig. 5.3.

5.1.4. Classification by Mode of Transport

The movable bed consists of the sediment found on the streambed that is transported by

water, especially during high flows. Not all the material found in streambeds is consid-

ered movable, such as large-diameter avalanche debris or deposits of cohesive clays.

Stream sediments may be classified by relative grain size and abundance in the movable

streambed and by mode of transport (Einstein, 1964). The basic concepts of transport

classification based on relative size and mode of transport are summarized in Figs. 5.5

and 5.6. Suspended load refers to (1) the material moving in suspension and sustained

in the water column by turbulence or in colloidal suspension or (2) the material

collected in or computed from a suspended-load sampler. Wash load is the portion of

SEDIMENT PROPERTIES 5.6

FIGURE 5.6 Schematic representation of sediment transport and sampling zones in a stream.

the sediment load composed of grains smaller than those found in appreciable

quantities in the movable bed, often taken as the smallest 10 percent of the bed

material by weight. The wash load is normally transported in suspension. These small

sediments are washed through the fluvial system without significant interaction with

the bed. Bed material consists of the grain sizes which are predominantly represented in

the movable bed of a stream. Bed load may be used to designate either (1) coarse

material moving in continuous or intermittent contact with the bed or (2) material

collected in or computed from samples collected in a bed-load sampler or trap. The bed

load includes particles rolling or sliding along the bed plus the saltation load, which

consists of grains bouncing along the bed or particles moved by the impact of the

bouncing particles. The bed material load is that portion of the total sediment load

composed of bed material, whether transported in suspension or as bed load.

Classification by mode of transport depends on both grain size and turbulence within

the stream. Clays and fine silts are probably always transported in suspension, and most

gravel and cobbles as bed load. However, depending on the turbulent energy, sand may

be stable on the bed, may roll or bounce along the bottom, or may be carried in suspen-

sion. Thus, the suspended load in a stream may consist entirely of silts and clays at low

SEDIMENT PROPERTIES 5.7

discharges, but include sands at high discharge, causing the grain size distribution of the

suspended load to coarsen as discharge increases. In many streams the bed material load

constitutes a small fraction (less than 15 percent) of the total load.

5.2. CHARACTERISTICS OF SEDIMENT GRAINS

Sand, silt, and clay particles behave distinctly and have characteristics that are readily

identifiable in the field.

5.2.1 Sand

The term sand refers to a specific size particle, not a particular mineral. Thus, sand-size

particles in lowland rivers may consist exclusively of quartz particles, dark basaltic par-

ticles predominate in the streams of many oceanic islands, and beach sands may consist

of quartz crystals, volcanic or basaltic fragments, or carbonate fragments of shells,

urchins, calcareous algae, reef rubble, or carbonate precipitates (oolites). Sand grains are

too small to measure individually without use of magnification, yet are large enough to

be counted with the unaided eye and can visibly be observed to settle rapidly in water.

Sands are normally size-classified by sieves, and the smallest sands (0.062 mm) are the

smallest grains that, as a practical matter, can be mechanically sieved. Terrigeneous sand-

size particles consist of small rock fragments or crystals, but especially silicates, quartz

being the most common form. Feldspar follows silicates in abundance. Silicates are

abundant crustal minerals and also the most solution-resistant, and remain intact after the

surrounding rock matrix has eroded or dissolved away. In mountainous areas where the

parent material is being actively decomposed, the sand-size particles will consist of

freshly eroded rock fragments, whereas in areas distant from the parent material sand-size

particles may consist exclusively of quartz grains. The composition of the sands in a

streambed reflects not only the composition of the source material, but also the physical,

chemical, and biological processes acting on the sediment. A generalized idea of the

effects of climate and topography on the erosional and weathering processes affecting the

composition of sands is presented in Fig. 5.7.

5.2.2 Silt

Silts consist of particles in the range of 0.004 to 0.062 mm. All but the largest particles

are too small to be individually discerned with the naked eye, but a small sample placed

between the teeth will feel gritty. Individual silt grains are small mineral fragments,

essentially smaller versions of sand grains, but silt-size particles suspended in water may

also consist of flocs formed from clay-size particles. Silts settle much more slowly than

sand, and the settling rate of a silt suspension in a graduated cylinder may be barely

discernible. When silts are deposited, water can become trapped between grains, and if

saturated silt deposits are agitated they will liquefy, causing phenomena such as quick-

sand. Loess soils are deposits of windblown silt.

5.2.3 Clay and Clay Flocculation

Clay is too fine for its structure to be detected between the teeth. However, a moist sample

containing a significant percentage of clay can be rolled into a ribbon by hand, its

SEDIMENT PROPERTIES 5.8

FIG

URE 5.7 Generalized effects of climate and topography on the processes of chemical

weathering and mechanical erosion and its influence on quartz/feldspar ratio in residual

sands (after Siever,1988).

"stickiness" caused by interparticle cohesion. Unlike the larger grains, clays are not

simply minutely abraded rocks but are minerals typically consisting of sheets of silica

tetrahedra cross-linked with alumina octahedra. Individual clay particles can be con-

ceptualized as sheet-like or disk-like and are termed platelets. Nonclay minerals are

generally not smaller than 0.002 mm, and this size is frequently used to split

between the clay and nonclay fraction.

A cohesive sediment is one which contains a concentration of fines and colloids suf-

ficient to impart plastic properties at some water content, and the ability to resist shear

stress at some other water content, a property termed cohesion. The plastic and cohesive

properties of clay are due to colloids, particles with a specific surface area (area per unit

weight) so large that behavior is controlled by surface rather than gravitational forces. To

illustrate the large surface associated with small particles, assume all particles are

spherical and compute the surface area for 1 cm

of solid material as a function of grain

diameter. The undivided diameter is 1.24 cm and the surface area is 4.84 cm

, but, if

subdivided into 0.001-mm-diameter spheres, the same mass of sediment will have a sur-

face area of 60,000 cm

. The electrochemical surficial forces may be 106 times stronger

than gravitational forces in 0.001-mm clay platelets (Partheniades, 1962). Deposited

clays can gain strength over periods of time ranging from minutes to years as the indi-

vidual particles become rearranged into more stable positions.

Clays have extremely low settling velocities, and without turbulence or Brownian

(thermal) motion a 0.001-mm clay particle will require about 2 weeks to settle 1 m.

SEDIMENT PROPERTIES 5.9

However, as a result of Brownian motion, individual clay particles may be very stable

even in quiescent water, and remain dispersed for months. Short-distance van der Waals

forces cause all particles to be attracted to one another and are responsible for the aggre-

gation of clays and other minute particles into flocs large enough to settle at appreciable

velocities. At near-neutral pH values, most particles suspended in water, including clays

and organics, are characterized by negative surface charges. In a neutral suspension, each

particle with a negative surface charge must be surrounded by a cloud of positively

charged ions to achieve electroneutrality. The concentration of the positively charged

particles is greatest at the particle surface and becomes more diffuse as a function of

distance from the particle, giving rise to an electrical double layer around each particle.

Because the charges in the diffuse cloud surrounding the suspended particles are of like

sign, when particles approach one another the like-sign charges in the diffuse double

layer tend to repel each other, thereby impeding the particles from approaching to within

the short distances required for attractive van der Waals forces to dominate.

The facility with which clay particles can combine to form flocs depends on the bal-

ance between the attractive van der Waals and the repulsive electrostatic forces, and one

of the principal mechanisms for destabilization of clays and other colloids is to compress

the double layer thickness by increasing the ionic strength of the solution. The greater the

concentration of ions in the solution, the smaller the thickness of the diffuse layer around

the charged particle necessary to maintain electroneutrality (Fig. 5.8). The use of trivalent

aluminum or iron salts (A1

, Fe

) as coagulants in water filtration plants, and the

destabilization of riverine colloids when discharged into seawater, are two examples of

destabilization by double-layer compression. The rate of destabilization is also a function

of the rate of interparticle collisions resulting from Brownian motion, turbulence, or

differential settling velocities. In quiescent water, floc particles may agglomerate by

differential settling rates, with larger particles sweeping downward, overtaking, and

enmeshing smaller slower-settling particles, causing settling velocity to increase over

time as a result of floc growth. Ultimate floc size is limited by the turbulent shear stress

that tears the larger floc particles apart. Destabilization of clays and other colloids can

also occur as a result of adsorption to produce charge neutralization, enmeshment in a

precipitate, and interparticle bridging caused by certain organic polymers (Amirtharajah

and O'Melia,1990).

Most clays do not exist in aquatic systems as individual platelets, but instead are

flocculated by naturally occurring salts. Coagulation refers to the process of destabilizing

the particles in a solution, and flocculation refers to the process of continued mechanical

contact between particles which enables them to adhere to one another to create larger

particles. Flocculated clay particles can settle several orders of magnitude faster than

individual clay platelets and may exhibit sedimentation velocities characteristic of silts.

Clay flocculation is a common phenomenon and is responsible for the rapid clarification

of most natural waters. The grain size distributions in Fig. 5.4 show two curves for a

single sample corresponding to settling analysis with and without a dispersant to

deflocculate the clay fraction. For sedimentation studies, the grain size distribution for

conditions of natural flocculation would normally be used.

5.3 BULK PROPERTIES OF SEDIMENT

5.3.1 Sediment Density and Weight

The mass density of a solid particle 

is the mass per unit volume and is expressed in

units of g/cm

or kg/m

(slugs/ft

). The specific weight of solid particles 

is the weight

SEDIMENT PROPERTIES 5.10

FIGU

RE 5.8 Double-layer and clay flocculation mechanisms.

per unit of volume. Weight is the product of mass and gravitational acceleration and the

specific weight is the mass density multiplied by gravitational acceleration: 

= 

g. It is

expressed in units of kg/(m

•s

) or N/m

. For sediment with a specific gravity of 2.65,



= 2650 kg/m

and 

= 25988 kg/(m

•s

). The submerged specific weight of a parti-

cle 

 is expressed as the difference between the specific weights of the solid 

and the

surrounding fluid :

(5.5)

SEDIMENT PROPERTIES 5.11

The specific gravity G

of a solid is expressed as the dimensionless ratio of the

specific weights or of the density of the solid and of distilled water at 4°C, the

temperature of maximum water density.

(5.6)

5.3.2 Unit Weight or Bulk Density of Sediment

Unit weight, specific weight, and bulk density are all used to express the dry weight per

unit volume of a bulk sediment sample, including both solid grains and voids, after

drying to a constant weight at 105°C. Drying time may vary from a few hours for sands

to several days for hygroscopic clays. Accurate bulk density values depend on obtaining

representative, undisturbed in situ samples of the sediment. Under submerged conditions,

samples of predominantly fine-grain reservoir deposits will normally be collected by a

piston-type sampler. Some submerged sediments will be so weak that a core can be

pushed through several meters of material by hand, and in poorly compacted sediment a

piston core or a large diameter gravity core (e.g., 10 cm or 4 in) may give more accurate

results than smaller diameter gravity cores. Representative sampling of submerged

deposits of fines can be difficult, where alternating layers of coarse and fine sediments

occur, since denser material in the core barrel may push through finer sediment layers

without capture. Carefully compare the length of core penetration versus length of the

core captured to check for problems such as compaction and core loss.

In sampling from nonsubmerged sediments, a metal ring or box can be driven into

sediments, the surrounding sediments cleared away, and the sample of known volume

then carefully removed. This method can be used on both horizontal and vertical

sediment surfaces. In the sand replacement method (Bureau of Reclamation, 1987) a 0.5-

m square is leveled, the sample is excavated from within this horizontal surface, and then

the in-situ volume of the sample is determined by refilling the hole with calibrated sand.

In noncohesive soils, the sample hole will necessarily taper toward the bottom. The

calibrated sand should be air-dried, passing No. 16 and retained on No. 30 sieve, and of

known unit weight. Sample volume is determined as the difference in weight of the

calibrated sand in the container before and after filling the hole.

Bulk densities for surface soils classified as clay, clay loam, and silt loam typically

fall in the range of 1.0 to 1.6 g/cm

, whereas sandy soils typically range from 1.2 to 1.8

g/cm

, with the higher value corresponding to uniformly graded silica sand. Bulk

densities for compact subsoils may exceed 2.0 g/cm

(Brady, 1974). There is a wide

difference in bulk densities in reservoir sediments, both between sites and within the

same reservoir. Highest densities typically occur in coarse delta deposits and lowest

densities occur in fine sediment near the dam. Lara and Pemberton (1963) published data

from 1129 samples in 101 reservoirs, mostly in the United States. Bulk densities ranged

from 0.30 to 1.88 g/cm

. The average deposit bulk density for 800 U.S. reservoirs was

0.96 g/cm

(60 lb/ft

), as reported by Dendy and Bolton (1976).

5.3.3 Angle of Repose

The angle of repose is the slope angle formed with the horizontal by granular material at

the critical condition of incipient sliding. The angle of repose for both dry and submerged

noncohesive material, as a function of grain size, may be determined from Fig. 5.9. When

sediment has accumulated against a low-level outlet, and the outlet is opened, the angle

of repose can be used to describe the side slope of the cone which is excavated within the

SEDIMENT PROPERTIES 5.12

FIGURE 5.9 Angle of repose for uniform noncohesive sediment (Lane, 1953).

sediment, if cohesive materials are absent. When cohesive sediments are present, slopes

can be steeper than the angle of repose for cohesionless material. Also, sediment can

liquefy and flow under certain conditions, such as earthquake shaking, producing slopes

much lower than the angle of repose.

5.4 SEDIMENT WATER MIXTURES

5.4.1 Void Space

A fluid-sediment mixture consists of solid particles (subscript s) and void space

(subscript v), where the voids between the sediment grains are understood to be occupied

by the fluid. The volumetric ratio of voids (Vol

) to sediment grains (Vol

) in a sample is

termed the porosity p and is defined as

(5.7)

SEDIMENT PROPERTIES 5.13

Porosity should not be confused with permeability, which is the rate at which a

medium will transmit a fluid when subject to a pressure gradient. A clay sample may

have a porosity value equal to that of a gravel sample, but whereas the gravel will readily

transmit water, and is thus said to be highly permeable, the clay will be very impermeable

because the pores between grains are too small to transmit water at an appreciable rate.

The void ratio e is the ratio of voids to solids and is given by

(5.8)

5.4.2 Sediment Concentration

Three different methods may be used to compute the concentration of sediment in water

(Julien, 1995). Concentration by mass C

refers to the sediment mass per unit volume of

water-sediment mixture and, by definition:

(5.9)

Val

ues of Cm are usually expressed in units of mg/L, but units of g/L (or kg/m

) are more

convenient for high concentrations (1 g/L = 1 kg/m

= 1000 mg/L).

The concentration by volume C

is the dimensionless ratio of the sediment volume to

the mixture volume Vol

mix

(5.10)

The

volume concentration can be computed knowing the mass concentration and

sediment density. For C

= 10,000 mg/L and 

= 2.65 g/cm

, adjust all units to kg and

to solve for Vol

/Vol

mix

= (10 kg/m

)/(2650 kg/m

) = 3.77X 10

-3

. Volume

concentration may be converted into mass concentration using

(5.11)

(5.12)

ncentration by relative weight C

is the sediment weight divided by the total weight of

the water-sediment mixture:

(5.13)

Concentration by relative weight is usually reported in units of parts per million (ppm)

where C

ppm

= C

× 106. For low concentrations there is little difference between mg/L

and ppm; the difference exceeds 1 percent when concentration exceeds about 16,000 ppm

for sediment having a specific gravity of G

= 2.65. The following equation may be used

to convert ppm into mg/L:

(5.14)

The water: sediment ratio is frequently used to describe the volume of water required

to remove a unit volume of sediments during flushing operations, in which case the

eroded deposit volume includes both sediment grains and voids. To compute the average

SEDIMENT PROPERTIES 5.14

mass concentration in the sediment-water mixture resulting from these operations, first

compute concentration by volume [C

from Eq. (5.10)], then compute mass concentration

from Eq. (5.11) using the dry bulk weight of the deposit instead of the specific weight

of the solid sediment particles. Thus, for a deposit composed of sediment particles with a

specific gravity of 2.65 and 30 percent voids, use a bulk weight of 1.86 g/cm

rather than

2.65 g/cm

. For a 25:1 water: sediment ratio with the water volume measured prior to

mixing with sediment, compute C

= 1/(25 + 1) = 0.038 and C

= 71,500 mg/L.

5.4.3 Density of Water-Sediment Mixtures

The density of a sediment-water suspension is greater than the density of water alone and

may be computed by the following equation:

(5.15)

whe

re 

mix

= density of mixture, C

= sediment concentration by mass in the mixture, 

= sediment density, and  = liquid density. Express all quantities in consistent units of

mass per unit volume (e.g., g/cm

, kg/m

). Example: for a suspended solids concentration

of 32,100 mg/L ( = 0.0321 g/cm

), with 

= 2.65 g/cm

, and  = 1 g/cm

, the density of

the mixture is 1.020 g/cm

5.5 SETTLING VELOCITY OF SEDIMENT GRAINS

Settling velocity is a primary determinant of sediment behavior in a fluid. A sediment

particle can be transported in suspension only if its settling velocity is less than the ver-

tical component of hydraulic turbulence. Settling velocity is also a primary determinant

of the percentage and grain size of the inflowing load that becomes trapped in a reservoir,

and the pattern of sediment distribution along the length of a reservoir. The fall velocity

of sediment particles depends on characteristics such as size, shape, and density, and also

on fluid characteristics such as temperature, salinity, and sediment concentration which

affect both the fluid density and viscosity. To make comparisons between the intrinsic

sedimentation properties of particles, without differences imposed by the fluid, the

standard fall velocity is computed as the average rate of descent for a particle falling

alone in quiescent distilled water at 24°C. The standard fall diameter of a particle is the

diameter of a sphere having the same specific weight and the same standard fall velocity

as the given particle (Interagency Committee, 1957).

5.5.1 Fluid Viscosity

Viscosity is a measure of fluid resistance to deformation. Higher-viscosity fluids have

Greater shear strength and are more resistant to deformation, causing particles to fall

more slowly in media such as oil, seawater, and cold water, as compared to lower-vis-

cosity warm fresh water. Shear  in a fluid is proportional to the velocity gradient du/dy

at a given point:

(5.16)

Morris &amp; Fan. Reservoir Sedimentation Handbook

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Morris & Fan. Reservoir Sedimentation Handbook