CIVE 530 - OPEN-CHANNEL HYDRAULICS LECTURE 17A: SEDIMENTATION ENGINEERING I

17.0  INTRODUCTION

The study of sediments can be divided into: (1) production, (2) transport, and (3) deposition. Sediment production is primarily the result of raindrop impact at the watershed level. Surface runoff acts to transport sediment downslope. Deposition occurs where the kinetic energy of the water is insufficient to continue to entrain sediment in the flowing water.

17.1  SEDIMENT PROPERTIES

SEDIMENT FORMATION Sediments are the products of disintegration and decomposition of rocks. The disintegration is caused either by large temperature changes or by alternate cycles of freezing and thawing. Decomposition refers to the breaking down of mineral components of rocks by chemical reaction. Decomposition includes the processes of: (1) carbonation, (2) hydration, (3) oxidation, and (4) solution. Carbon dioxide in the atmosphere readily combines with water to form carbonic acid. Carbonic acid reacts with feldspars to produce clay minerals, silica, calcite, and other relatively soluble carbonates containing potassium, sodium, iron, and magnesium. The addition of water to many of the minerals present in igneous rocks results in the formation of clay minerals such as alluminum silicates. Oxidation is accelerated by the presence of moisture in the air. Oxygen combines with other elements to form sulfates, carbonates, and nitrates, most of which are relatively soluble. The amount of dissolved solids carried by streams in the contiguous United States has been estimated at more than 50% of the amount of suspended sediment. PARTICLE CHARACTERISTICS The characteristics are: (1) size, (2) shape, (3) specific weight and specific gravity, and (4) fall velocity. Size A classification of sediments according to size is shown in Table 15-1. Five groups are included: (1) boulders and cobbles, (2) gravel, (3) sand, (4) silt, and (5) clay. A No. 200 sieve is used to separate sand particles from the finer particles such as silt and clay. Shape Particle shape is numerically defined in terms of its sphericity and roundness. True sphericity is the ratio of the surface area of a sphere having the same volume as the particle, to the surface area of the particle. Alternate definition: sphericity is the ratio of the diameter of the sphere having the same volume as the particle, to the diameter of a sphere circumscribing the particle. A sphere has a sphericity of 1, whereas all other shapes have a sphericity of less than 1. Roundness is defined as the ratio of the average radius of curvature of the particles edges to the radius of the largest inscribed circle. It refers to the sharpness of the edges of sediment particles and is commonly used as an indication of particle wear. The shape factor is often used as an indicator of particle shape: SF = c / (ab)1/2 The quantities a, b, and c are three orthogonal particle length dimensions. According to Corey, a is the longest, b is the intermediate, and c is the shortest length dimension. According to McNown and Malaika, c is measured in the direction of motion, and a and b are perpendicular to c. Specific weight and specific gravity Specific weight of a sediment particle is the weight per unit volume. Specific gravity of a sediment particle is the ratio of its weight to the weight of an equal volume of water. Most sediment particls consists of quartz or feldspar, which are about 2.65 times heavier than water. The specific gravity of sediments is generally considered to be about 2.65. Exceptions are heavy minerals (for instance, magnetite, with specific gravity of 5.18), but these occur infrequently. Fall velocity The fall velocity of a sediment particle is its terminal rate of settling in still water. Fall velocity is a function of size, shape, and specific gravity of the particle, and the specific weight and viscosity of the surrounding water. For spherical particles, the fall velocity (derived from a balance of submerged weight and drag) can be expressed as follows: w = [(4/3) (gds/CD) (γs - γ)/γ ] 1/2 in which w = fall velocity, g = gravitational acceleration, ds = particle diameter, CD = drag coefficient (dimensionless), γs = specific gravity of sediment particles, and γ = specific weight of water. The drag coefficient CD is a function of the particle Reynolds number R, defined as: R = wds/ν in which ν = kinematic viscosity of the fluid. For particle Reynolds numbers less than 0.1, the drag coefficient is equal to CD = 24/R (laminar flow). Substituting the value of CD into the equation of fall velocity leads to Stokes' law: w = [(gds2/(18ν)](γs - γ)/γ For particle Reynolds numbers greater than 0.1, the drag coefficient is still a function of Reynolds number, but the relationship cannot be expressed in analytical form. The relationship of CD versus R for a wide range of particle Reynolds numbers is shown in Fig. 15-1. Since fall velocities vary with fluid temperature and viscosity, two particles of the same size, shape, and specific gravity, falling on two different fluids of different viscosity, or in the same fluid at different temperatures, will have different fall velocities. To provide a measure of comparison, the concept of standard fall velocity was developed. The standard fall velocity of a particle is the average rate of fall that it would attain if falling alone in quiescent water of infinite extent at the temperature of 24oC. The standard fall diameter of a particle is the diameter of an equivalent sphere having the same standard fall velocity and specific gravity. Example 5-1. Calculate the fall velocity of a spherical quartz particle of diameter ds = 0.1 mm and drag coefficient CD = 4. Solution: Use the equation for fall velocity: w = [(4/3) (gds/CD) (γs - γ)/γ ] 1/2 with γs = 2.65 g/cm3, γ = 1 g/cm3, g = 9.81 m/s2, ds = 0.0001 m, to find w = 0.023 m/s. Example of calculation of fall velocity online. Calculate the fall velocity of a spherical sediment particle, given ds = 0.15 mm, and temperature T = 24oC. Solution: Proceed by iteration. Assume R, calculate CD from Fig. 15-1, calculate w, verify assumed R or iterate. Use online fall velocity. Answer: 0.01619 m/s. SIZE DISTRIBUTION OF SEDIMENT DEPOSITS Particle size distribution is the key to predicting the behavoir of a sediment deposit and estimating its specific weight. A sediment sample containing a wide range of particle sizes is well graded, or poorly sorted. A sediment sample containing a narrow range of particle sizes is poorly graded, or well sorted. The size distribution can be measured in many ways. The coarsest fraction can be separated by direct measurement for boulders and cobbles, and by sieving for sands and gravels. The Visual Accumulation tube (VA tube) is a fast, economical, and accurate method of determining the size distribution of sediment samples. In the VA tube method, the particles start falling from a common source and become stratified according to their relative settling velocities. At a given instant, the particles coming to rest at the bottom of the tube are of a certain sedimentation size, finer than particles than have already settled, and coarser that those still remaining in suspension. VA tube assembly. SPECIFIC WEIGHT OF SEDIMENT DEPOSITS The specific weight of a sediment deposit is the dry weight of sedimentary material per unit volume. The specific weight of a sediment deposit is always less than the specific weight of the individual particles. A knowledge of the specific weight of a sediment deposit allows the conversion of weight to volume. Factors influencing the specific weight of a sediment deposit are: its mechanical composition, the environment in which the deposits are formed, time. The specific weight of coarse materials such as boulders, gravel and coarse sand change very little with time. The specific weight of silt and clay deposits may reduce considerably in time. The Lane and Koelzer relation for specific weight as a function of time: W = W1 + B log T in which: W = specific weight of a deposit after T years, W1 = initial specific weight, measured after 1 year of consolidation, and B = a constant. Table 15-2 shows values of W1 and B. Drying or aeration of a sediment deposit helps to accelerate consolidation through removal of the water from the pore spaces. Table 15-3 shows the effect of aeration on the specific weight of sediment deposits for several types of soil mixtures. Example 5-2. Calculate the specific weight of a sediment deposit in a reservoir after an elpased time of 50 years, with the sediment always submerged or nearly submerged. Assume the following size distribution: sand, 30%; silt, 45%; and clay, 25%. Solution: Using Table 15-3, the specific weights for the various sizes are:   sand, 93 lb/ft3; silt, 75 lb/ft3; and clay, 57 lb/ft3. Therefore, the weighted average is:   75.9 lb/ft3. Example 5-2 Online. Use online Lane-Koelzer. The specific weight is:   75.8 lb/ft3.

17.2  SEDIMENT PRODUCTION

The presence of sediment in rivers has its origin in soil erosion. Erosion encompasses a series of complex and interrelated natural processes that have the effect of loosening and moving away soil and rock materials under the action of water, wind, and other geologic factors. In the long term, the effect of erosion is the denudation of the land surface. The rate of landscape denudation can be quantified from a geologic perspective. The number of cm per 100 years can be used as a measure of the erosive activity of a region. Geologic measures of landscape denudation appear insignificant whe compared to the typical timespan of human activity. However, the quantities of sediment removed may concentrate downstream and negatively impact the operation of hydraulic structures. SEDIMENT PRODUCTION AND SEDIMENT YIELD A distinction should be made between the quantity of sediment removed at the source(s) and the quantity of sediment delivered to a downstream point. Usually the sediment delivery is a fraction of the sediment produced. Gross sediment production refers to the amount of sediment eroded and removed from the source. Sediment yield refers to the actual delivery of eroded soil particles to a given downstream point. The ratio of sediment yield to gross sediment production is the sediment delivery ratio (SDR). Gross sediment production is measured in metric tons per hectare per year. Sediment yield is measured in metric tons per day at the catchment outlet or other point of interest. NORMAL AND ACCELERATED EROSION Erosion can be classified as: Normal erosion has been occurring at variable rates since the first solid materials formed on the surface of the Earth. Normal erosion is extremely slow in most places. It is a function of climate, parent rocks, precipitation, topography, and vegetative cover. Accelerated erosion occurs at a much faster rate than normal, usually through reduction of vegetative cover. Deforestation, overgrazing, overcultivation, forest fires, and urban sprawl result in accelerated erosion. SEDIMENT SOURCES Erosion can be classified as: sheet erosion, rill erosion, gully erosion, and channel erosion. Sheet erosion is the wearing away of a thin layer on the land surface, primarily by overland flow. Rill erosion is the removal of soil by small concentrations of flowing water (rills). Gully erosion is the removal of soil from incipient channels that are large enough so they cannot be removed by normal cultivation. Channel erosion refers to erosion occurring in stream channels in the form of streambank erosion or channel degradation. Upland erosion is made of sheet and rill erosion. Channel erosion excludes sheet and rill erosion. UPLAND EROSION AND THE UNIVERSAL SOIL LOSS EQUATION The prediction of upland erosion is made by the Universal Soil Loss Equation (USLE): A = R K L S C P in which A = annual soil loss due to sheet and rill erosion in tons per acre per year, R = rainfall factor, K = soil eodibility factor, L = slope-length factor, S = slope gradient factor, C = crop management factor, and P = erosion control practice factor. Rainfall factor When factors other than rainfall are held constant, soil losses from cultivated fields are shown to be directly proportional to the product of the storm's total kinetic energy E and its maximum 30-minute intensity I. The product EI reflects the combined potential of raindrop impact and runoff turbulence to transport dislodged soil particles. The sum of EI products for a given year is an index of the erosivity of all rainfall for that year. The rainfall factor R is the average value of the series of annual sums of EI products. Values of R applicable to the contiguous United States are shown in Fig. 15-2. Soil erodibility factor The soil erodibility factor is a measure of the resistance of a soil surface to erosion. Its is defined as the amount of soil loss, in tons per acre per year, per unit of rainfall factor R for a unit plot. A unit plot is 72.6 ft long, with a uniform lengthwise gradient of 9%, in continuous fallow, tilled up and down the slope. Values of K for 23 major soils are shown in Table 15-4. K factors for other soils are estimated by comparison with those values in Table 15-4. Slope-length and slope-gradient factors The rate of soil erosion by flowing water is a function of slope length (L) and gradient (S). For practical purposes, these two topographic characteristics are combined into a single topographic factor (LS). The factor LS is defined as the ratio of soil loss from a slope of given length and gradient to the soil loss from a unit plot of 72.6 ft length and 9% gradient. Values of LS are shown in Fig. 15-3. Crop management factor The crop management factor C is defined as the rate of soil loss from a certain combination of vegetative cover and management practice to the soil loss resulting from tilled, continuous fallow. Values of C range from as little as 0.0001 for undisturbed forest land to a maximum of 1 for disturbed areas with no vegetation. Values of C for cropland are estimated on a local basis. Values of LS are shown in Tables 15-5 and 15-6. Erosion control practice factor The erosion control practice factor is defined as the ratio of soil loss under a certain erosion-control practice to the soil loss resulting from straight row farming. Values of P have been established for contouring and contour strip cropping. In contour strip cropping, strips of sod or meadow are alternated with strips of row crops or small grains. Values of P used for contour strip cropping are also used for contour-irrigated furrows. Values of P are shown in Table 15-7. Use of the Universal Soil Loss Equation The USLE cannot be used to compute sediment yield. For instance, for a 1000-km2 basin, only 5% of the soil loss computed by the USLE may appear as sediment yield at the basin outlet. The remaining 95% is redistributed on uplands or flood plains, and it does not constitute a net loss from the drainage basin. Example 15-3. Assume a 600-ac watershed in Fountain County, Indiana. Compute the average annual soil loss by the USLE for the following conditions: Cropland, 280 ac, contour strip-cropped, soil is Fayette silt loam, slopes are 8% and 200 ft long; Pasture, 170 ac, 50% canopy cover, 80% groundcover with grass, soil is Fayette silt loam, slopes are 8% and 200 ft long; Forest, 150 ac, soil is Marshall silt loam, 30% canopy cover, slopes are 12% and 100 ft long. Solution: From Fig. 15-2, R = 185 From Table 15-4, K = 0.38 From Fig. 15-3, LS = 1.4 Value of C for cropland is obtained from local sources. Assume C = 0.12 From Table 15-7, P = 0.25 A = R K SL C P = 2.95 tons/ac/yr From Fig. 15-2, R = 185 From Table 15-4, K = 0.38 From Fig. 15-3, LS = 1.4 No value of P has been established for pasture. Assume P = 1. A = R K SL C P = 1.18 tons/ac/yr From Fig. 15-2, R = 185 From Table 15-4, K = 0.33 From Fig. 15-3, LS = 1.8 No value of P has been established for pasture. Assume P = 1. A = R K SL C P = 0.66 tons/ac/yr The total sheet and rill erosion from the 600-ac watershed is: (280 × 2.95) + (170 × 1.18) + (150 × 0.66) = 1126 tons/yr Example 15-3 Online. Verify this example with online usle 2. [Watershed] soil loss = 1126.406 tons/yr Channel erosion Channel erosion includes gully erosion, streambank erosion, streambed degradation, floodplain scour, and other sources of sediment, excluding upland erosion. Gullies are incipient channels in process of development. Gully growth is usually accelerated by several climatic events, improper land use, or changes in stream base levels. Significant gully activity is found in regions of moderate to steep topography with thick soil mantles. Camp Creek, Oregon. The total sediment outflow from gullies is usually less than sheet and rill erosion. Streambank erosion and streambed degradation can be significant in certain cases. Downstream of a newly constructed dam, "hungry" water will cause streambed degradation. Degradation to bedrock downstream of a sediment retention dam. Changes in channel alignment and/or removal of natural vegetation from streambanks may cause streambank erosion. Methods for determining soil loss due to various types of channel erosion include the following: Comparing aerial photos taken at different times, Performing river cross sectional surveys to determine changes in cross section, Assembling historical data, Performing field studies to evaluate annual growth. Field surveys may provide sufficient data to estimate streambank erosion as follows: S = H L R in which: S = annual volume of streambank erosion, H = average height of bank, L = length of eroded bank, and R = annual rate of bank recession (net rate). Streambed degradation can be estimated as follows: S = W L D in which: S = annual volume of streambed degradation, W = average bottom width of degrading channel reach, L = length of degrading channel reach, and R = annual rate of streambed degradation. Accelerated erosion due to strip mining and construction activities Strip mining and construction activities greatly accelerate erosion rates. Human induced land distrubances have a substantial impact on sediment production. EPA's Best Management Practices (BMP's) are used to control erosion from anthropogenically disturbed sites. The USLE may be used to compute erosion from disturbed sites. Sediment yield In engineering applications, the quantity of sediment eroded at the sources is not as important as the quantity of sediment delivered to a downstream point, i.e., the sediment yield. Sediment yield is calculated by multiplying the gross sediment production by a sediment delivery ratio that varies in the range 0-1. Sediment delivery ratio The sediment delivery ratio (SDR) is largely a function of: sediment source, proximity of sediment source to the fluvial transport system, density and condition of the fluvial transport system, sediment size and texture, and catchment characteristics. The sediment source has an influence on the SDR. Not all sediments originating in sheet and rill erosion are likely to enter the transport system. Sediments originating in channel bank erosion are more likely to be delivered to downstream points. The amount of sediments delivered to downstream points will depend on the ability of the fluvial transport system to entrain and hold on to the sediment particles. Silt and clay particles can be transported much more readily than sand particles. High catchment relief often indicates both high erosion and high SDR. High channel density is an indication of an efficient transport system and, therefore, of a high SDR. Estimation of sediment delivery ratios The SDR is the ratio of sediment yield to gross sediment production. Estimates of sediment yield can be obtained by reservoir sedimentation surveys. Alternatively, sediment yield can be evaluated by a direct measurement of sediment load at the point of interest. Estimates of gross sediment production can be obtained with the USLE. When warranted, this estimate can be augmented by field measurements of gully and channel erosion. Regional SDR equations can be derived with data. The simplest SDR prediction equation is based solely on drainage area, as shown in Fig. 15-4. This figure shows that SDR varies in inverse proportion of the 1/5 power of the drainage area. The greater the drainage area, the smaller the catchment relief, and the greater the chances for sediment deposition within the catchment. Rough estimates can be obtained from Fig. 15-4, but caution is recommended for more detailed studies. An example of a regional SDR equation: SDR = 31,623 (10 A)-0.23(L/R)-0.51 B-2.79 in which: SDR = sediment delivery ratio, A= drainage area, in sq. mi., L/R = ratio of catchment length to relief, B = weighted mean bifurcation ratio, defined as the ratio of number of streams of a given order to the number of streams in the next higher order. Empirical formulas for sediment yield Statistical analysis has been used to develop regional equations for the prediction of sediment yield. The Dendy and Bolton formula is a good example of a sediment yield equation. Sediment yield vs drainage area Dendy and Bolton studied sedimentation data for about 1500 reservoirs, ponds, and detention basins. They used reservoirs with drainage areas greater than 1 sq mi. For drainage areas between 1 and 30,000 sq mi, Dendy and Bolton found that the annual sediment yield per unit area was inversely related to the 0.16 power of the drainage area: S/SR = (A/AR)-0.16 in which: S = sediment yield in tons per acre per sq mi, SR = reference sediment yield corresponding to 1 sq mi area, equal to 1645 tons/yr, A = drainage area in sq mi, and AR = reference drainage area, equal to 1 sq mi. Sediment yield vs mean annual runoff Dendy and Bolton studied sedimentation data from 505 reservoirs having mean annual runoff data. Annual sediment yield per unit area was shown to increase sharply as mean annual runoff Q increased from 0 to 2 in. Thereafter, for mean annual runoff from 2 to 50 in, annual sediment yield per unit area decreased exponentially. This lead to the following equations for sediment yield: For Q ≤ 2 in: S/SR = 1.07 (Q/QR)0.46 For Q > 2 in: S/SR = 1.19 e -0.11(Q/QR) in which: QR = reference mean annual runoff, equal to 2 in. Dendy and Bolton further combined the equations for sediment yield in terms of drainage area and mean annual runoff into the following: For Q ≤ 2 in: S/SR = 1.07 (Q/QR)0.46 [1.43 - 0.26 log(A/AR)] For Q > 2 in: S/SR = 1.19 e -0.11(Q/QR) [1.43 - 0.26 log(A/AR)] For SR = 1645 tons/yr, QR = 2 in, and AR = 1 sq mi, the Dendy and Bolton equations reduce to: For Q ≤ 2 in: S = 1280 Q0.46(1.43 - 0.26 log A) For Q > 2 in: S = 1965 e -0.055Q (1.43 - 0.26 log A) with Q in in., A in sq. mi., and S in tons/sq. mi./yr. These equations should be used with caution. In certain cases, local factors such as soils, geology, topography, land use, and vegetation may have a greater influence on sediment yield than either mean annual runoff or drainage area. Example 15-4. Calculate the sediment yield by the Dendy and Bolton formula for a 150-sq mi watershed with 3.5 in of mean annual runoff. Solution: The application of the Dendy and Bolton formula leads to: S = 210,000 tons/yr. Example 15-4 Online. Verify Example 15-4 with online dendy and bolton. Sediment yield = 210,123.47 tons/yr. Go to Lecture 17B.

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