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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 Dd 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.
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