Study of Sediment Deposits for Reservoir Dredging Tiao J. Chang1, Travis D. Bayes2, Scott McKeever3 Abstract Many reservoirs in the country are aging and dredging has been the most common means to maintain continuing function of varied reservoirs. This study is to investigate geographic distribution of sediment deposits in a reservoir to gather information to develop the guidance for future dredging programs or other similar reservoirs. Charles Mill Lake, located near Mansfield, Ohio, was constructed on the Black Fork of the Mohican River in 1935 by the U.S. Army Corps of Engineers. The lake has a surface area of 1,350 acres with a maximum water depth of 34 feet at conservative pool level, i.e., 997 feet mean sea level. The watershed that supplies the lake is approximately 217 square miles. The reservoir was constructed for controlling floods, maintaining and enhancing the recreational opportunities, and improving the quality of life for residents and users of all the facilities of the watershed. It has been reported that significant sediment deposition has gradually reduced the effectiveness of the man-made reservoir over the years. The sediment deposits in the lake has affected flood control and preservation of natural resources, including recreation, navigation, and water quality. Based on the recent topographical survey and the original topographical map of the lake, sixty sampling locations were selected to geographically represent the lake. A gravity corer, two feet long and three inches in diameter, was used to collect sediment samples. These samples were first classified by color and then the general physical characteristics of the sediment deposits were recorded in a field book. Each sample of sediment deposit was then bagged and marked with its geographical location by a global positioning system (GPS) and physical landmarks in conjunction with a detailed map of the area. Samples were dried and analyzed by mechanical sieve analysis to develop a particle-size distribution curve for all sampling locations. Grain sizes of sediment deposits at sampling locations were assumed to be geographically referenced variables. The variables were interpolated to obain a regional distribution using the spline method. This analysis of sediment grain sizes and texture classification resulted in grids of raster-based values that express the distribution by spatial images. These images and analysis were used to develop the guidance for future dredging programs. Introduction In calm waters particles that are more dense than water will fall out at a constant velocity, where the force of downward motion is equal to the drag force resisting motion. This velocity is called the settling velocity. The force causing the downward motion on a sphere can be solved by 1 Professor, Civil Engineering Department, Ohio University, Athens, OH 45701 Research Assistant, Civil Engineering Department, Ohio University, Athens, OH 45701 3 Manager of Special Projects, Muskingum Watershed Conservancy District, New Philadelphia, OH 44663 2 Fs where, Fs d s g = = = = = d 3 s g 6 , the force causing the downward motion [ML-2T-1]; particle diameter [L]; particle density [ML-3]; density of water [ML-3]; acceleration due to gravity [ML-2]. The drag force is equated to C Dd 2 v 2 , Df 8 where, Df = the total drag force due to friction and the particle form [ML-2T-2]; CD = drag coefficient [dimensionless]; v = settling velocity [LT-1]. Stokes’ law for spherical particles is derived by setting these two forces equal to one another. When this is done the settling velocity for a sphere is equal to v s gd 2 , 18 since CD 24 , Re and Re where, vd , = the coefficient of absolute viscosity [FL-2T]; Re = the Reynolds Number of the flowing water [dimensionless]. Most sediment particles are not spherical and most streams are slightly turbulent, but Stokes’ law still gives a good representation of the settling velocities of the particles in the water. The density, volume, and shape are all factors that determine the settling velocity of the particle. Different particles of different shape and size can have the same settling velocity, or hydraulic equivalence. However, in general, the size or diameter of the particle carries the most weight in determining the settling velocity. The particles’ settling velocity increases dramatically with a small increase in particle diameter. In other words, the larger particles will settle out faster. Turbulence of a stream also affects the particle size that is deposited. If the average velocity of the current is constant and fluctuations in the current’s turbulence are small, the sediment will be well graded. Soil particles that are small enough to stay suspended will do so, while the larger particles fall out. On the other hand, when the current changes in velocity or turbulence, the large particles have deposited earlier, so the range of particle size is small. In an ideal reservoir, the sediment deposits would be predictable; the coarser soil particles would be deposited near the entrance stream and the finest material near the dam. The velocity of the stream decreases, depositing finer material as the flow gets closer to the dam. The depth of the sediment would generally be the greatest near the dam as can be seen in Figure 1. Figure 1. Ideal Reservoir Sedimentation However, Charles Mill Lake is not an ideal reservoir due to the location - on the edge of the glaciated region of Ohio. The reservoir has a shallow slope while it resides in several valleys so that it has an irregular shape (Figure 2). Because of this, the amount of sediment deposits and the size of the sediment are not easily predicted. Background Reports have been made that the sediment deposition in the lake is gradually filling the lake. The residents of the area have complained about poor navigability. In addition, the reservoir has reduced its effectiveness in controlling floods. Charles Mill Lake is located four miles east of Mansfield, Ohio in Richland and Ashland counties. Charles Mill Lake receives its inflow from the Black Fork Creek, located on the north of the lake. The water from Charles Mill Lake ultimately discharges to the Ohio River via the Muskingum River. The dam holding Charles Mill Lake was constructed in 1935 for the purpose of flood control. The Muskingum Watershed Conservancy District (MWCD) owns the lake and surrounding land and is responsible for the conservation management and recreational activities. The dam is owned and operated by the U.S. Army Corps of Engineers. Charles Mill Lake contains three natural lakes, Mifflin, Bell, and Mud, which existed before the dam was built. The lake also includes fourteen islands. The average depth of the lake has decreased by about one foot in the last 63 years. This is a decrease of about 20% of the volume of the lake. The watershed draining into Charles Mill Lake is 217 mi2. Table 1 lists other facts about Charles Mill Lake and its watershed. Table 1. Facts and figures of physical features of Charles Mill Lake and Charles Mill Lake Watershed Lake Length 20,700 feet 6,300 meters Lake Breadth 6,200 feet 1,900 meters Original Average Depth 5 feet 1.5 meters Current Average Depth 4 feet 1.2 meters Maximum Depth 34 feet 10.4 meters Original Volume 11,369 acre-feet 14,034,230 km3 Current Volume 8,129 acre-feet 10,034,678 km3 Water Surface Area (normal pool) 1,339.5 acres 5.42 km2 Shoreline Length 34 miles 53.5 km Lake Elevation (normal pool) 997.1 feet (MSL) 304.0 meters Lake Elevation (spill way) 1020.0 feet (MSL) 311.0 meters Watershed Area 217 mile2 562 km2 Methodology Digitized topography and hydrography of the study area was downloaded from the Ohio Department of Administrative Services (DAS) Geographic Information System Support Center web page and converted for use in ArcView GIS. This data were 1:24,000 scale Digital Line Graphs (DLG). An original topographic map of the land, before the dam was built, was obtained and digitized. Once the map was digitized, the elevation point data were interpolated over the whole lake to create a continuous grid, or a raster image. The recent topographic survey of Charles Mill Lake was obtained from the Army Corps of Engineers (ACOE). The ACOE surveyed the lake in 1998 with cooperation with the MWCD using a Digital Global Positioning System (DGPS) and sounding equipment. These data points were also interpolated to create a raster image. These two images are shown in Figure 3. Analysis of these two topographic images, indicated that most of the sediment was being deposited in the original channel, especially in the north section of the lake. From this ideal sampling locations were formed. With these ideal sample locations, samples were collected (Figure 4). Sixty samples were taken in July of 1998 covering the whole lake. The samples were retrieved with a gravity corer that could collect a core two feet long and three inches in diameter. Each sample was described in the field book and the location was taken down on the map and with a GPS. The description included the sample number, color, general soil classification, location, length of core, and time taken. The sample was bagged and marked for identification later. Moisture content testing and mechanical sieve analysis was conducted on each sample. The results were input into the coordinates along with the sample’s general description. With GIS, the data was analyzed further using a spline interpolator. A particle-size distribution curve was developed for all sixty samples. From the curves the D10, D30, D50, D60, and D90 values were interpolated and used for data points at the respective locations. The D10 is the particle diameter that corresponds to 10% finer on the particle-size distribution curve. In other words, 90% of the particles, by weight, are larger than the D10 value. The particle-size distribution curves were used again to find the percentages of gravel, coarse sand, fine sand, and silt/clay. The sizes of the soil classifications are listed in Table 2. Table 2. Soil Classification Sizes Soil Classification Gravel Coarse Sand Fine Sand Silt/Clay Minimum Size (>) 0.0787 in 0.0157 in 0.00295 0.00000 in Maximum Size () 0.0787 in 0.0157 in 0.00295 in The spline interpolation imposes that the surface must pass exactly through the data points. In addition, the cumulative sum of the squares of the second derivative terms of the surface, taken over each point on the surface, must be a minimum. The spline fits a mathematical function to a specified number of points, while passing through the sample points. Since it can be assumed that the sediment deposit data are continuous and only moderate variations occur between the data points, this method seemed to be the best method. Results Once the interpolations were completed, the four grids of soil classification percentages were added together. This produced a grid that should be continuously 100% for perfect interpolation. Only 2.6% of the grid contained more than 5% error. Only 0.4% of the grid contained more than 10% error. This validated the interpolation of the sediment deposits. The sediment contained a small amount of gravel. This makes sense because stream would have to maintain a rapid current to keep gravel suspended. Since the gradient of the stream is small, the flow is moderate. The sediment contains an average of 4.4% gravel. The maximum percentage of gravel is about 50% near the bridge and in the southwest side of the lake (Figure 5). The bridge constricts the only connection from the north section to the south section of the lake. This causes scour during drawdown of the reservoir and flooding. The smaller particles are eroded while the larger particles remain. The average D90 for the sediment is 0.070 inches, where the D90 for downstream of the bridge is 0.30 inches (Figure 6). On the opposite side of the spectrum, the average D10, or effective size, for the sediment is 0.0046 inches. The smallest effective sized sediment deposits are located near the entrance of the Black Fork (Figure 7). This is due to the stream velocity slowing before entering the lake. Once the stream enters the lake, the water becomes very calm because of the constriction reducing flow to the southern section. Downstream of the bridge constriction the effective size increases. The percent of silt and clay are the greatest near the entrance of the Black Fork, around 20% (Figure 8). An unusual result is the percentage of silt and clay decreases as the sediment nears the dam. The uniformity coefficient (Cu) and the coefficient of gradation (Cc) are equal to Cu D60 D10 Cc 2 D30 . D60 * D10 When the uniformity coefficient is greater than four, the soil is usually gravely, while if it is greater than six then it is sandy. The main sandy area is in the middle of the southern section of the lake (Figure 9). Most of the sediment qualifies as non-uniform since its uniformity coefficient is less than four. When the coefficient of gradation is between one and three, the soil is considered well graded. This soil consists of a large range of particle sizes. Figure 10 shows the area upstream of the bridge is not well graded, but the area downstream of the bridge is well graded. Figure 11 is the depth of the sediment deposited in the lake since 1935. The original channel in the southern section has the deepest amount of sediment. Because of the sedimentation, the bottom of the lake seems to have become smoother and flatter. The majority of the sediment seems to be just upstream and downstream of the bridge. From this analysis, MWCD can develop a working knowledge of how Charles Mill Lake has changed in the last 63 years. With this knowledge, the dredging program can be guided to produce a lake that is effective in serving the community by controlling floods and supplying recreation. In the future, dredging of other lakes can benefit from the methodology laid down from this study.