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Properties of sediment from Newnans Lake, Florida

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Title:
Properties of sediment from Newnans Lake, Florida
Series Title:
UFLCOEL
Creator:
Gowland, Jason E
Mehta, Ashish J
University of Florida -- Coastal and Oceanographic Engineering Dept
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Gainesville Fla
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Coastal & Oceanographic Engineering Dept. of Civil & Coastal Engineering, University of Florida
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English
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xiii, 39 p. : ill. ; 28 cm.

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Sediment transport -- Florida ( lcsh )
Soil erosion -- Florida ( lcsh )
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bibliography ( marcgt )
technical report ( marcgt )
non-fiction ( marcgt )

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Includes bibliographical references (p. 31-32).
Statement of Responsibility:
by Jason E. Gowland and Ashish J. Mehta.

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UFL/COEL-2002/012

PROPERTIES OF SEDIMENT FROM NEWNANS LAKE, FLORIDA
by
Jason E. Gowland and
Ashish J. Mehta
Submitted to: Environmental Consulting & Technology Inc. Gainesville, FL 32606-5004

September 2002




UFL/COEL-2002/012

PROPERTIES OF SEDIMENT FROM NEWNANS LAKE, FLORIDA
By
Jason E. Gowland and Ashish J. Mehta

Submitted to:
Environmental Consulting &Technology Inc.
Gainesville, FL 32606-5004
Department of Civil and Coastal Engineering
University of Florida Gainesville, FL 32606

September 2002




TABLE OF CONTENTS
TABLE OF CONTENTS ................................................................................... ii
SY N O P SIS .................................................................................................... iv
LIST OF TABLES ........................................................................................... viii
LIST OF FIGURES ........................................................................................... ix
LIST OF SYMBOLS ......................................................................................... xi
1. INTRODUCTION ........................................................................................ I
1. 1 L ake L ocation ............................................ ............................................ 1
1.2 Sampling and Measurements ....................................................................... 2
2. SEDIMENTARY PARAMETERS .................................................................... 5
2.1 Sample Densities ..................................................................................... 5
2.2 Organic Content ...................................................................................... 6
Z 3 G rain Size ............................................................................................... 6
2.4 Sorting Coefficient .................................................................................... 6
Z S p H ...................................................................................................... 6
3. SEDIMENT ERODIBILITY ......................................................................... 8
3.1 E credibility ............................................................................................... 8
3.2 Settling Velocity ...................................................................................... 9
3.3 Turbidity, Water Quality and Dredging .......................................................... 10
4. R E SU L T S ................................................................................................... 13
4.1 Sediment Properties ................................................................................. 13
4.1.1 Organic Content .................................................................................... 13
4.1.2 D ensities .............................................................................................. 14




4.1.3 Grain Size......................................................................... 17
4.1 .4pH .............................................................................. 17
4.1.5 Ranges and Mean Values of Parameters ............................................18
4.2 Erosion Rate....................................................................... 20
4.3 Settling Velocity ................................................................... 22
4.4 Zonal Dependence of Erosion and Settling Parameters ............................ 25
5. CONCLUSIONS........................................................................ 23
6. REFERENCES .........................................................................25
APPENDIX: A METHOD TO CALCULATE RESUSPENSION ........................ 27
A. 1 Approach.......................................................................... 27
A. 2 Examples of Calculation............................................................ 28




SYNOPSIS
Selected sediment properties, erodibility characteristics and settling velocity have been determined for forty-five bottom sediment samples collected at Newnans Lake in Florida, as part of an investigation to assess the role of wave-induced sediment resuspension in affecting water quality in the lake. Sediment properties include (wet) bulk density, dry (bulk) density, particle (or granular) density, organic content (loss on ignition), grain size distribution and pH. Erosion characteristics include the erosion rate constant and the critical bed shear stress required for calibrating the erosion rate function. The settling velocity was measured in a column under quiescent conditions over a range of initial suspension concentrations.
Overall the sample pore fluid was found to be acidic with pH between 4.7 and 6.4, and the organic content varied between 13% and 58%. The bed material consisted of a silty-sand (median size range 0.014 mm to 0.39 mm). The sorting coefficient, a measure of the spread of sizes in a distribution, varied between 1.3 to 4.1, implying the presence of graded (i.e., nonuniform) sediment.
The ranges of density values were found to be as follows: bulk density 1,005 kg/m3 to 1,242 kg/m3, dry density 19 kg/m3 to 407 kg/m3 and particle density 1,150 kg/m3 to 2,597 kg/m3. Dry densities of comparable magnitudes have been reported previously in the lake. Trend lines for the variation of these densities with organic content appear to be consistent with those for organic-rich sediments from other water bodies in Florida.
In the upper, unconsolidated bed layer the mean density was 1,036 kg/m3 at a mean depth of 0.15 m below the bed-water interface, whereas in the consolidated layer beneath, it was 1,078 kg/m3 at 0.79 m depth. Densities of this range are consistent with the occurrence of a fluid-like, low shear strength, organic-rich material at the bottom.




Erosion rates of sediment beds prepared in the Particle Erosion Simulator (PES) ranged from nil to 1.6 g/m2/s, corresponding to an applied shear stress range of 0.06 Pa to 0.3 Pa. For the erosion rate function, the rate constant was found to vary between 0.45 gIN-s and 2.06 gIN-s, and the critical shear stress for erosion between 0.062 Pa and 0.120 Pa. These values are consistent with organic-rich sediments from elsewhere in Florida. At a given shear stress, the error band associated with erosion rate was on the order of 15%.
Settling velocities ranged from 1.7x10-6 m/s to 8.7xlOA4 mis, which are consistent with those found for similar sediments in Florida. The mean error associated with these values was on the order of 20%. Tests in the settling column indicated a dependence of the settling velocity on the organic content and suspension concentration. However, inasmuch as the lake in comparatively small, and one in which suspension concentrations exceeding 0.1 kg/in3 are likely to occur during wind episodes, the above range should. be considered to be adequately representative of the settling rate in the lake.
As part of an effort to examine spatial variability in the bottom sediment properties, selected parameters (organic content, bed and particle densities, median diameter, erosion rate constants and median settling velocity) have been sorted into three zones exposed bottom, outer open water and inner open water. These parameters, which are interdependent by virtue of their definitions and the fact that all are derived from the same set of samples, collectively indicate that sediment in the inner open water area has the highest organic content, the lowest (wet) bulk, dry (bulk) and particle densities and the lowest particle size. At the opposite end is the exposed bottom, with the outer open water area in-between. The erosion rate parameters imply that the material in the organic-rich inner open water area is more erodible than in the outer open water area having a lower organic content. Note, however, that the water depth also




matters since wave penetration will be less in the inner area than the outer one, the degree erosion of weaker material in deeper areas will be tempered by lower wave-induced bed shear stresses there.
Assuming that the PES erosion data are applicable to the lake without any model to prototype scaling, and settling of eroded material does not occur under assumed constant wind conditions, example plots of the rate of erosion as a function of wave-induced bottom velocity and wing duration have been presented for illustrative purposes.
Turbidity in the lake appears to results from wash load and suspended bed material load. In general, the former is due to light-weight organic matter that remains in suspension irrespective of the wind speed. In contrast, bed material load is characteristically a strong function of the wind speed, inasmuch as wind correlates with bed erosion via waves and waveinduced bed shear stress. For modeling turbidity in the lake it will be essential to collect calibration/validation data on wind, waves and suspended sediment concentration on a synoptic basis. This can be achieved by installing a self-recording tower assembly in the open-water area with a pressure sensor (for waves), a vertical array of optical backscatter sensors (for suspended sediment profiling) and a wind anemometer over a period of several (at least six) months.
If the exposed bottom continues to remain as such, in due course, as the surficial organic matter is oxidized and desiccation occurs, it is most likely that the bed strength will increase significantly. In that case the presently determined erodibility indices will become inapplicable, and the erosion rate function will have to be re-calibrated.
Dredging the soft bottom sediment down to a level (where the density of the deposit is high enough to better withstand resuspension, both due to its higher consolidated state and




because at deeper levels surface wave penetration in attenuated) can be expected to improve water quality, e.g., as defined by the Trophic State Index.




LIST OF TABLES
Table 4.1 Measured organic content, bulk, dry and granular densities, 25, 50 (median)
and 75 percentile grain sizes, sorting coefficient and pH for all samples ...................... 13,14
Table 4.2 Comparison of dry densities for sample pairs (from two studies) in mutual
proxim ity ................................................................................................... 17
Table 4.3 Dependence of particle size parameters on organic content ............................... 18
Table 4.4 Minimum, maximum and mean values of parameters listed in Table 4.1 ............... 19
Table 4.5 Mean and standard deviation of characteristic properties of unconsolidated
bed layer for the three zones ............................................................................ 20
Table 4.6 Mean and standard deviation of characteristic properties of consolidated
bed layer for the three zones ............................................................................. 20
Table 4.7 Erosion rate constant and critical shear stress values ...................................... 21
Table 4.8 Representative settling velocities .............................................................. 24
Table 4.9 Mean and standard deviation of erosion parameters and median settling velocity for the
three lake zones unconsolidated ...................................................................... 26
Table 4. 10 Mean and standard deviation of erosion parameters and median settling velocity for
the three lake zones consolidated ..................................................................... 26
Table A. I Calculation of wave-induced bottom velocity ............................................... 34




LIST OF FIGURES
Figure 1. 1 Location of Newnans Lake in Florida..............................................1
Figure 1.2 Topographic map of Newnans Lake and floodplain (courtesy: USGS).
The open-water area in June/July 2001 was 16 km2....................................... 2
Figure 1.3 Lake level (stage) during 1995-1998 (Nagid, 1999), and in July 2001
when sampling was carried out............................................................ 3
Figure 1.4 Open-water coring and exposed bottom sampling sites in the lake.................. 3
Figure 3.1 Schematic view of Particle Erosion Simulator (based on Tsai and Lick, 1986)......8 Figure 4.1 Variation of bulk, dry and particle densities with organic content. Lines are
based on Ganju (2001)................................................................... 15
Figure 4.2 Variation of (wet) bulk density with depth. Boxes are meant to identify density ranges. Pavg denotes mean density ........................................... 16
Figure 4.3 Cumulative size distributions (percent passing though sieve by weight versus
grain diameter) of five selected samples with varying organic content ....................18
Figure 4.4 Lake region divided into three sub-areas ......................................... 19
Figure 4.5 Erosion rate plot as a function of (applied) shear stress and approximate
bounds of the effect of varying organic content. M denotes the erosion rate constant.......22 Figure 4.6 Erosion rate plots for three samples (AC3-c, A14-u and U22-c) highlighting
the effect of increasing organic content in increasing the erosion rate .....................23
Figure 4.7 Variation of settling velocity with suspended sediment concentration. Curves
correspond to 10% OC and 60% OC bounds and 3 5% OC ............................... 25
Figure A. 1 Bed material concentration increase (in the absence of settling) at 8 M/s
wind speed in the inner and outer open water areas. Low water condition ................ 35
Figure A.2 Bed material concentration increase (in the absence of settling) at 9 m/s
wind speed in the inner and outer open water areas. Low water condition................... 36
Figure A.3 Bed material concentration increase (in the absence of settling) at 10 m/s
wind speed in the inner and outer open water areas. Low water condition .................36
Figure AA4 Bed material concentration increase (in the absence of settling) at 9 m/s
wind speed in the inner and outer open water areas. High water condition ................ 37




Figure A.5 Bed material concentration increase (in the absence of settling) at 10 M/s
wind speed in the inner and outer open water areas. High water condition ................37
Figure A.6 Bed material concentration increase (in the absence of settling) at 8 m/s
wind speed in the presently exposed area under high water condition ..................... 38
Figure A.7 Bed material concentration increase (in the absence of settling) at 9 m/s
wind speed in the presently exposed area under high water condition..................... 38
Figure A. 8 Bed material concentration increase (in the absence of settling) at 10 m/s
wind speed in the presently exposed area under high water condition..................... 39




LIST OF SYMBOLS

a wave amplitude (m)
aw settling velocity coefficient
bw settling velocity coefficient
c suspended sediment concentration (kg/m3)
C depth-mean value of c (kg/m3)
CD conductivity [(At)"'/cm] or (pS/cm)
CL chlorophyll-a (mg/m3)
CR cation ratio
d size or diameter of particle (m or mm)
do amplitude of wave particle excursion at the bottom (m)
d25 diameter of 25 percent of sample (mm)
dso median size diameter (mm)
d75 diameter of 75 percent of material (mm)
fw wave friction factor
g gravity (m2/s)
h height of water above bed (m)
I+ activity of hydrogen ions (mol/L)
k wave number (1/m)
k, bottom roughness coefficient (m)
K mass diffusion coefficient (m/s2)
m mass (g)
mw settling velocity coefficient




M erosion rate constant (g/N-s)
MA dry mass after ignition (g)
MD dry mass (g)
Mw mass of wet sample (g)
nw settling velocity coefficient
OC organic content (%)
PP primary productivity (mg-C/m2-hr)
So sorting coefficient
t time (s, min or h)
T wave period (s); also turbidity (JTU)
TN total organic nitrogen (mg/L)
TP total phosphate (mg/L)
TSI Trophic State Index
Um wave-induced velocity at bottom (m/s)
V volume (cm3 or inm3)
wS settling velocity (m/s)
z vertical coordinate (inm)
6 erosion rate (g/m2/s)
AC time-rate of change of C (kg/m3)
At time increment (s, min or h)
v kinematic viscosity of water (m2/s)
p bulk density (kg/m3)
Pavg mean density (kg/m3)




PD dry density (kg/m3)
Ps particle density (kg/m3)
Pw density of water (kg/m3)
o. Standard deviation of any parameter i
applied applied bed shear stress (Pa) Tb bed shear stress (Pa)
Tcr critical bed shear stress (Pa)




1. INTRODUCTION

1.1 Lake Location
The study reported here was undertaken as part of an investigation to assess the role of wind-wave induced resuspension which affects the water quality in Newnans Lake, Florida (Figures 1.1 and 1.2). It is companion to an overall study reported by Environmental Consulting & Technology, Inc (2002) related to an effort to restore water quality in the lake. It is understood that remedial measures as necessary will be carried out to minimize the role of wave-induced resuspension and associated turbidity, which are believed to degrade water quality in the lake.
Figure 1.1 Location of Newnans Lake in Florida.
Newnans Lake, which occurs in north-central Florida (Figures 1.1 and 1.2), had an openwater area of -16 km2 in June/July 2001, when the present suite of (submerged) core samples and exposed bottom samples were collected there (Environmental Consulting & Technology, Inc., 2002). As seen from Figure 1.3, the lake level (or stage) in June/July, 2001 was




considerably lower (-1.1 m) compared to about 20 m for the 1995-1998 period, and almost 2 m below the El Niflo high in February, 1998.

Figure 1.2 Topographic map of Newnans Lake and floodplain (courtesy: USGS). The open-water area in June/July 2001 was -16 km2.
1.2 Sampling and Measurements
Samples were collected at twenty-five sites (Figure 1.4), all pegged by the Global Positioning System to the nearest tenth of a second. At the twenty open-water (submerged) sites a piston core was used, whereas at the exposed sites, surface samples were collected by a shovel. Water depth at each open-water coring location was recorded to the nearest hundredth of a meter (Environmental Consulting & Technology, Inc., 2002).




21.5 I -- ,.
February 1998
95 95 96 96 97 97 98 98 99 99 00 00 01 01
Date
21.5
TFigure 1.3 Lake level (stage) during 1995-1998 (Nagid, 1999), and in July 2001 when samplinghigh
20was carried out.
195 ___ ~ s 200_ _0
19.
Jan- Jut- Jan- Jut- Jan- Jut- Jan- Jul- Jan- Jut- Jan- Jul- Jan- Aug95 95 96 96 97 97 98 98 99 99 00 00 01 01
Date
Figure 1.3 Lake level (stage) during 1995-1998 (Nagid, 1999), and in July 2001 when sampling was carried out.

Figure 1.4 Open-water coring and exposed bottom sampling sites in the lake.




Total core length ranged from 1.05 m to 1.74 m. Each core was placed into a plastic canister, labeled for identification and capped so that its moisture content would be preserved while being transported to the laboratory for analysis. Subsequently, each core was divided into two parts based on inspection (of fluid/solid consistency), with the upper part, ranging between 0.14 m and 0.61 m, considered unconsolidated and the lower 0.61 m to 1.52 m consolidated. Division of core samples as: 1) open-water unconsolidated, 2) open-water consolidated and 3) exposed bottom, was to identify characteristic differences in the erodibility potential of the bottom sediment in these three categories. Accordingly, characteristic sedimentary properties were determined for all forty-five samples.
In the analysis presented, sample designations used are in accordance with samples received and analyzed in the laboratory. The relationships between those designations with the ones in Figure 1.4 are as follows:
F1Il = 5A, F16 =5B; F21 = 5C; F26 =5D; M1 1 = 4A; M16 =4B; M22 =4C; M28 =4D; U10,= 3A; U16 =3B; U22 = 3C; U28 =3D; AC3 = 2A; ACIlI = 2B; AC19 =2C; AC24 = 2D; A14 = IA; A19 =IB; A114 = 1C; A118 =ID; Land 1 = 1329-900; Land 2 = 489-800; Land 3 48-800; Land 4= 1224-200 and Land 5 =5 15-700.




2. SEDIMENTARY PARAMETERS

2.1 Sample Densities
All 45 samples, i.e., open water and exposed, were analyzed for densities, organic content, grain size parameters and pH using the same sets of procedures.
Sample bulk density was determined by placing the material in a Pyrex glass beaker of known weight and measuring the volume and weight of wet material. After drying the sample in an oven, the beaker was weighted again to determine the dry density of sediment. Particle (or granular) density was then determined from the known volume and mass balance relationship for water and sediment in the sample. The following analytic description is taken from Mehta et al. (1994).
The wet bulk density p, was determined by dividing sample mass, M,,, by sample volume, V, of the container:
P = M(2.1) Typically, 40 cm.3 of samples were used. After drying the sample in the oven at 1050 C for 24 hours, the dry mass, MD yielded the dry bulk density PD: PD- MD (2.2)
The particle or granular density p, was determined through mass balance:
P, PDPW (2.3)
PD +PW -P
where p,,is water density (1,000 kg/in3).




2.2 Organic Content
The material from density measurement was placed into a Coors porcelain dish for organic content determination. The sample was then heated to 4000 C to determine organic content, OC as loss on ignition (ASTM, 1993): OC = MD -MA X100% (2.4)
MD
where MA is sample dry mass after ignition.
2.3 Grain Size
Grain size distribution was determined by wet-sieving the sample material through a set of sieves and filtering out by vacuum-suction the residual suspension consisting of particles finer than 74 microns (corresponding to #200 Tyler sieve). Suspension concentration (dry mass per unit volume of suspension) was then measured gravimetrically and multiplied by the total volume of suspension to obtain the total dry mass of material finer than 74 microns. The sum of the total coarse grain mass retained on the sieves and the mass of the fine grained residue yielded total mass of the sample wet-sieved, from which the percent passing each sieve could be calculated (ASTM, 1993).
2.4 Sorting Coefficient
The grain size spread can be characterized by the sorting coefficient, S,, according to: S d75 (2.5)
d 25
where d25 and d75 are the grain size of 25th and 75th percentiles of the sample cumulative size distribution, respectively (SPM, 1984).
2.5pH
The pH of the pore fluid in the samples is given by (Faure, 1998):




pH = -og10IIH+] (2.6)
where H+ is the activity of the hydrogen ions. When the hydrogen ion activity is 1x10-7, the sample is said to be neutral with a pH = 7. Therefore, pH < 7 indicates an acidic condition and pH > 7 implies a basic condition.
pH was measured by placing a digital Coming electrolyte pH meter in the sample pore water after removal of material was completed for sedimentary parameters. Pore water for this purpose was obtained by taking a partially consolidated or mixed sample, decanting off the supernatant liquid and allowing it to consolidate. The excess pore water thus released was used for pH determination. Due to their initial "wetness", the exposed samples also yielded to this approach after an overburden was applied.




3. SEDIMENT ERODIBILITY

3.1 Erodibility
Sample erosion rates where obtained through the use of the Particle Erosion Simulator (PES) (Tsai and Lick, 1986).

Figure 3.1 Schematic view of Particle Erosion Simulator (based on Tsai and Lick, 1986).
Samples (of 3 cm to 5 cm height) were placed in suspension inside a 15.2 cm diameter perplex cylinder and allowed to consolidate for up to 96 hours. A porous vertical grid was then oscillated at different angular speeds (rpm), which applied a shear stress on the bed. This shear stress was obtained from the calibration relationship (Rodriguez et al., 1997):

Drive disc Linkage bar Drive rod
Hold-down plate
SGuide bat

Oscillating grid Suspension Sediment bed




applied =.0005914rpm (3.1)
Two to four different shear stresses were applied to each sample to obtain the erosion rate, c. Each shear stress was continued for one hour and the concentration of suspended sediment, c, was taken at 0, 5, 15, 30 and 60 minutes in order to track the approach of the concentration to a steady state value. The erosion rate was then calculated according to: dhc
e h- (3.2)
dt
where h is the water depth in the cylinder and dc/dt is the total time-rate of change of suspension concentration. The corresponding erosion rate function is:
6 = M(Tapptiea Tcr) (3.3)
For each sample, the erosion rate constant, M and the critical shear stress, zrc, were obtained as follows: First, E values calculated from Eq. 3.2 were plotted against the corresponding Tapplied calculated from Eq. 3.1, and a best-fit line was drawn through the data points. The erosion rate constant M was then taken as the slope of the line and the intercept of the line with the Tappldie axis gave the critical shear stress, rr (Mehta et al., 1994; Rodriguez et al., 1997).
3.2 Settling Velocity
The settling column used for the tests was a 2 m high and 100 mm diameter cylindrical perplex tube with sample outlets at 0.05, 0.15, 0.3, 0.55, 0.8, 1.05, 1.3, and 1.55 m. Sample aliquots of 10 cm3 each were withdrawn at these elevations over a selected set of time of intervals, usually 0, 5, 15, 30, 60, 120, and 180 minutes. These samples were analyzed gravimetrically to obtain suspension concentrations at different elevations and times, which were then used to calculate the settling velocity as follows.
I The governing equation for suspended sediment concentration is:




ac a
(wc) = 0 (3.4)
at az
where t is time and z is the vertical elevation coordinate. The settling velocity, w,, is obtained by numerical integration of Eq. 3.4 given the initial condition: c(z, t) = c(z, 0) (3.5)
along with a zero sediment flux condition at the surface (z = h, where h is the height of water in the column) and at the bottom (z = 0). The initial condition was prescribed by the uniform suspension concentration at the beginning of each test. A Matlab computer program named SET1.M was used to solve Eq. 3.4 for w, (Mehta and Li, 2001).
3.3 Turbidity, Water Quality and Dredging
An inspection of the site indicated that turbidity in the lake appears to result from wash load and suspended bed material load. The former is due to light-weight organic (including algal) matter that seemingly remains in suspension irrespective of the wind speed. In contrast, and in general, bed material load is a strong function of the wind speed, and can be modeled in a simple way by extending Eq. 3.4 to include a term for the vertical diffusion of sediment mass. Accordingly, we obtain
ac a K az+ w'c 0 (3.6)
at az az
in which K is the mass diffusion coefficient. The critical difference between Eq. 3.6 and 3.4 arises from the need to specify the erosion flux at the bottom for Eq. 3.6, based on Eq. 3.3. The bed shear stress, applied, required for that purpose must be obtained from measurement of windinduced wave height and period (along with an estimate of the bottom hydraulic roughness) in the lake under different wind conditions.




The solution of Eq. 3.6 for c(z,t), which must be carried out through numerical integration, requires calibration with respect to K using measured time-series of c(z,t) (Mehta and Li, 2001). To that end, therefore, it will be essential to install a self-recording tower assembly in the lake with a pressure sensor (for waves), a vertical array of optical backscatter sensors (for suspended sediment profiling) and a wind anemometer over several (at least six) months to obtain synoptic data required to calibrate and validate the solution of Eq. 3.6 for lake turbidity.
Water quality in the lake can be characterized by the Shannon-Brezonik Trophic State Index (TS1) (Wanielista, 1978):
TSI = 0.18T + 0.008CD + 1.1TN + 4.2TP+ 0.01PP + 0.044CL + 0.39CR + 0.26 (3.7)
where T = turbidity (JTU), CD = conductivity [(At)-'/cm =/S/cm], TN = total organic nitrogen (mg/1), TP = total phosphate (mg/1), PP = primary productivity (mg-C/m2-hr), CL = chlorophylla (mg/m3) and cation ratio CR = [Ca++]+[Mg++]/[Na +]+[K +]. Shannon and Brezonik (1972) provide the following classification based on TSI: 0-3 oligotrophic, 3-7 mesotrophic, 7-10 eutrophic, and >10 hypereutrophic. They reported a value of 15.3 for Newnans Lake, implying that it was hypertrophic in the early 1970s.
As to water quality change in Newnans Lake, we can comment only the role of T in the present context. One can expect that, dredging the soft bottom sediment down to a level where the density of the deposit is high enough to better withstand resuspension, both due to its higher consolidated state and because at deeper levels surface wave penetration in attenuated, would decrease TSI (see also, Baird, 1987; Ryding and Rast, 1989).
In recent years, small, dedicated dredgers have been developed with suction intakes that eschew the use of conventional bed-fluidizing jets which tend to spread the ensuing turbid (and at times contaminated) water. Field and laboratory testing of the basic technique for "sucking"




soft mud in this way have been carried out at, for example, the U.S. Engineer Research and Development Center, Vicksburg, MS (Parchure and Sturdivant, 1997).




4. RESULTS

4.1 Sediment Properties
4.1.1 Organic Content
We note from Table 4.1 that, in the aggregate, the organic content (OC) varies from 13 to 58%, which is representative of typical organic-rich muck found in Florida (Ganju, 2001). The mean value of OC is 41%. Brenner and Whitmore (1998) report similar values in Newnans Lake.

Table 4.1 Measured organic content, bulk, dry and granular densities, 25, 50 (median) and 75 percentile grain sizes, sorting coefficient and pH for all samples.
Organic Densities Grain sizes Sorting
No. content coeff.
Sample no. OC P PD PS d25 dso d75 So pH
(%) (kg/m3) (kg/m3) (kg/m3) (mm) (mm) (mm)
1 1A-c (AI4-c) 17 1061 113 2153 0.220 0.293 0.379 1.3 6.0
2 1B-c (AI9-c) 50 1019 81 1297 0.009 0.030 0.128 3.7 6.1
3 1C-c (AI14-c) 32 1082 140 2414 0.063 0.101 0.296 2.2 6.1
4 1D-c (AI18-c) 13 1114 185 2597 0.248 0.327 0.417 1.3 6.3
5 1A-u (AI4-u) 36 1068 149 1827 0.022 0.053 0.290 3.6 6.3
6 1B-u (AI9-u) 50 1025 58 1769 0.009 0.029 0.124 3.7 6.1
7 IC-u (AI14-u) 49 1040 112 1556 0.009 0.031 0.136 3.8 6.3
8 1D-u(AI18-u) 35 1108 218 1985 0.029 0.061 0.291 3.2 6.2
9 2A-c (AC3-c) 23 1148 257 2353 0.168 0.227 0.310 1.4 6.0
10 2B-c (AC11-c) 35 1063 108 2393 0.029 0.061 0.291 3.2 6.1
11 2C-c (AC19-c) 37 1062 106 2407 0.019 0.049 0.275 3.8 6.0
12 2D-c (AC24-c) 37 1092 156 2430 0.019 0.049 0.278 3.8 6.0
13 2A-u (AC3-u) 48 1041 68 2455 0.010 0.034 0.146 3.9 5.9
14 2B-u(ACl l-u) 56 1005 19 1316 0.008 0.017 0.072 2.9 6.3
15 2C-u(AC19-u) 44 1009 66 1150 0.011 0.040 0.182 4.1 6.2
16 2D-u(AC24-u) 52 1016 70 1289 0.009 0.026 0.112 3.5 6.1
17 3A-c (U10-c) 52 1047 117 1676 0.009 0.026 0.113 3.5 6.1
18 3B-c (U16-c) 42 1029 67 1750 0.013 0.043 0.210 4.0 6.0
19 3C-c (U22-c) 48 1018 54 1523 0.010 0.033 0.142 3.9 6.1
20 3D-c (U28-c) 21 1092 188 1964 0.189 0.254 0.336 1.3 5.7
21 3A-u (U10-u) 52 1020 54 1577 0.009 0.025 0.106 3.5 6.2
22 3B-u (U16-u) 52 1022 39 2240 0.009 0.025 0.107 3.5 6.3
23 3C-u (U22-u) 53 1021 36 2383 0.009 0.023 0.101 3.4 6.3
24 3D-u (U28-u) 36 1072 123 2422 0.020 0.050 0.290 3.8 6.1
25 4A-c (M1 -c) 33 1089 152 2408 0.052 0.089 0.294 2.4 5.9
26 4B-c (M16-c) 50 1044 73 2500 0.009 0.029 0.124 3.7 6.1




Table 4.1 (continued)
Densities Grain sizes Sorting
No. Organic coeff.
Sample no. content, OC P PD PS d25 d5o d75 So pH
(%) (kg/m3) (kg/m3) (kg/m3) (mm) (mm) (mm) 27 4C-c (M22-c) 46 1011 67 1205 0.010 0.037 0.163 4.1 6.4
28 4D-c (M28-c) 25 1091 160 2308 0.143 0.198 0.306 1.5 5.8
29 4A-u (M1 l-u) 45 1048 82 2357 0.010 0.039 0.175 4.1 6.0
30 4B-u (M16-u) 50 1022 38 2400 0.009 0.028 0.123 3.7 6.2
31 4C-u (M22-u) 58 1019 32 2333 0.008 0.014 0.058 2.7 6.3
32 4D-u (M28-u) 45 1072 122 2452 0.010 0.039 0.175 4.1 5.9
33 5A-c (F1 l-c) 27 1096 158 2541 0.123 0.173 0.304 1.6 6.0
34 5B-c (F16-c) 50 1044 75 2400 0.009 0.028 0.123 3.7 6.1
35 5C-c (F21-c) 42 1099 166 2484 0.013 0.042 0.206 4.0 6.2
36 5D-c (F26-c) 32 1072 124 2396 0.067 0.107 0.296 2.1 5.9
37 5A-u (F1 I-u) 50 1043 75 2304 0.009 0.030 0.128 3.7 6.1
38 5B-u (F16-u) 51 1041 71 2333 0.009 0.027 0.117 3.6 6.1
39 5C-u (F21-u) 53 1022 37 2381 0.009 0.022 0.097 3.3 6.2
40 5D-u (F26-u) 55 1013 23 2308 0.008 0.019 0.082 3.1 6.1
41 1329-900(Landl) 22 1151 253 2469 0.110 0.230 0.290 1.6 5.4
42 489-600 (Land2) 46 1069 121 1969 0.071 0.120 0.330 2.2 5.0
43 48-800 (Land3) 34 1138 240 2227 0.190 0.390 0.510 1.6 5.1
44 1224-200 (Land4) 19 1242 407 2477 0.120 0.260 0.490 2.0 5.8
45 515-700(Land5) 27 1181 312 2339 0.140 0.310 0.500 1.9 4.7
Note: Exposed bottom samples 41-45 were analyzed ahead of the remainder.
4.1.2 Densities
For illustrative purposes, three density representations are plotted in Fig. 4.1 as functions
of organic content. The corresponding lines have been plotted using relationships derived by
Ganju (2001) for similar (lacustrine and estuarine) sediments from Florida. These comparisons
imply that the sediment in Newnans Lake is similar to what is found elsewhere in Florida with
respect to the dependence of densities on OC. Note that in Ganju's case the particle density was
found to vary with organic content per Eq. 4.1 below. According to this equation, sediment with
no organics would have a particle density of 2,650 kg/m3 and the particle density would decrease
with increasing organic content:




p,= -16.5(OC) +2650 (4.1)
The corresponding bulk density was found to vary according to:
p = 1568e-S(c) 0.9(OC) + 1114 (4.2)
From the bulk density and particle density curves the dry density was then calculated through use of the mass balance relation:
PD = (P -Pw)P, (4.3)
(p, -PJ)
* Bulk
3000.0 U Dry
2500.0 A E 4.1
M A2000.1A Particle
-, 2000.0 AAA
1500.0
A. Particle
"A Density
1000.0
E4 Curve
Eq. 4.32 Bulk
500.0 -,-Bl
5 Density
0.0 Curve
* Dry
0.0 20.0 40.0 60.0 80.0 Density
Organic Content (%) Curve
Figure 4.1 Variation of bulk, dry and particle densities with organic content. Lines are based on Ganju (2001).
In general, bed density in the lake increases with depth below the bed-water interface. This trend is evident from Figure 4.2, in which data on the (wet) bulk density from Table 4.1 are plotted against the mean height of the core sample (u = upper, "unconsolidated"; c = lower, "consolidated"). Notwithstanding wide-ranging densities that are found both in the u-samples
and in the c-samples, there is an evident mean trend of increasing density (from 1,036 kg/m3 to 1,078 kg/m3) with mean depth increasing from 0.15 m to 0.79 m below the bed-water interface.




From the perspective of the potential for erosion, it is highly unlikely that material below
-0.10-0.20 m from the interface would erode in this lake (see, e.g., Hwang, 1989). Also, observe in Figure 4.2 that the beds prepared in the PES (for erosion testing) via self-weight consolidation had densities mainly in the range of the upper cores samples. Higher densities did not result due to lack of sufficient self-weight of the beds in the device. Hence we conclude that higher densities in the consolidated layer have resulted from overburden.
0 Core data (upper layer) UCore data (lower layer) 0 PES Bed
-~0.0 I- --
S-0.4 pa -1,036k& m3
'0
0 -0.8 00 _____-1.2 ,V=1,7 gnI
1000 1040 1080 1120 1160
Bulk density (kg/in3 )
Figure 4.2 Variation of (wet) bulk density with depth. Boxes are meant to identify density ranges. Pavg denotes mean density.
The densities of core samples obtained in a 1997 study (Brenner and Whitmore, 1998) are compared with those from the present study (2001) in Table 4.2. The two stations in each of the five pair were in proximity to each other, although they were occupied at different times. Inasmuch as the stations were not aligned in the strict sense, it is not surprising that the densities differ. Nevertheless, there appears to be an order of magnitude similarity in values, and to that extent one may conclude that the two sets of values are mutually consistent and seemingly reflect the stability (non-erodibility or immobility) of the bottom deposit.




Table 4.2 Comparison of dry densities for sample pairs (from two studies) in mutual proximity
1997 study Present (2001) study
Core no. Dry density Core no. Dry density
(kg/rn3) (kg/n3)
16-IV-97-1 41 5C-u (F2 l-u) 37
16-IV-97-2 41 4C-u (M22-u) 32
16-IV-97-3 45 2C-u (AC 19-u) 66
16-IV-97-4 83 IB-u (A19-u) 58
4.1.3 Grain Size
Based on the d25, d50 and d75 sizes of the cumulative distribution in Table 4.3 we observe that the material is mainly in the very fine to medium sand size range (0.0625 to 0.50 mm according to the Wentworth classification). The occurrence of low d25 values, e.g., 0.008 mm (8 Am) also suggests the presence of silt-sized material, which can arise from two sources: 1) finesized organic matter, and 2) clays.
In Figure 4.3, five grain-size distributions are plotted to illustrate the dependence of size on OC. From Table 4.3 we note that the particle size decreases significantly as OC increases from 13 to 58%; e.g., d75 from 0.42 to 0.06 mm. At the same time, the sorting coefficient S, increases from 1.3 to 2.7, suggesting that the organic material exhibits greater heterogeneity than the inorganic component. However, this trend is not consistently observed. For example, note the seemingly anomalous decrease in S, from 4.1 to 2.7 even as OC increased from 45% to 58%. It is likely that the source, age and extent of degradation of organic matter are contributors to this behavior.
4.1.4pH
Table 4.1 indicates that, as one might expect, the pore fluid pH was fairly uniform and acidic, with a mean of 6 and a range of 4.7 to 6.4.




4.1.5 Ranges and Mean Values of Parameters
As far as a statistical representation of the measured/derived parameters is concerned,
given the paucity of data, only the minimum, mean and maximum value of parameters in Table
4.1 are summarized in Table 4.4. These imply the following mean trends: 1) High organic
content (41%), 2) fluid mud-like or low shear strength sediment (bulk density 1,064 kg/m3), 3)
composite material lighter than clay or sand (granular density 2,123 kg/m3), 3) silt to fine sandsized particle aggregates (median size 0.092 mm), 4) non-uniform (or graded size distribution
with sorting coefficient 3.0; SPM, 1986) and 5) acidic pore water (pH = 6.0).
100 1-d-c(13%)
90 K----90 U (AI18-c)
80
70 X A 2-a-c(23%)
S60 (AC3-c)
- 50 X 3-d-u(36%)
C
40 (U28-u)
30 -- 4-d-u(45%)
20 a (M28-u)
10 ) 4-c-u(58%)
0 (M22-u)
0.01 0.1 1
Diameter (mm)
Figure 4.3 Cumulative size distributions (percent passing though sieve by weight versus grain diameter) of five selected samples with varying organic content.
Table 4.3 Dependence of particle size parameters on organic content Grain sizes
Organic content Sorting
Sample OC d25 d5o d75 coefficient
no. (%) (mm) (mm) (mm) So
1D-c (AI18-c) 13 0.248 0.327 0.417 1.3
2A-c (AC3-c) 23 0.168 0.227 0.310 1.4
3D-u (U28-u) 36 0.020 0.050 0.290 3.8
4D-u (M28-u) 45 0.010 0.039 0.175 4.1
4C-u (M22-u) 58 0.008 0.014 0.058 2.7




Table 4.4 Minimum, maximum and mean values of parameters listed in Table 4.1
Organic Densities Grain sizes Sorting
Value content d5 d75 coefficient pH
(%) (kg/m3) (kg/m3) (kg/m3) (mm) (mm) (mm) S.
Minimum 13 1005 19 1150 0.008 0.014 0.058 1.3 4.7
Mean 41 1064 119 2123 0.051 0.092 0.223 3.0 6.0
Maximum 58 1242 407 2597 0.248 0.390 0.510 4.1 6.4
It is interesting to examine the spatial variability in OC, p, PD, ps and d50; in Fig. 4.4, the lake area sampled has been divided into three sub-areas exposed, outer open water and inner open water. Referring to Fig. 1.4, these zones encompass the following sampling sites: Exposed area Landl through 5; outer open water area F 11, F26, M 11, M28, U10, U28, AC3, AC24, A14 and AI18; and inner open water area- F16, F21, M16, M22, U16, U22, ACll, AC19, A19 and A114. In Tables 4.5 and 4.6, mean and standard deviation of the above, state-related (as opposed to transport-related), parameters are given.
N I km
Exposed Y
a|'ea

Figure 4.4 Lake region divided into three sub-areas.




Table 4.5 Mean and standard deviation of characteristic properties of unconsolidated bed layer for the three zones
Organic Bulk Dry Particle Median
content density density density diameter
OC (Coc) P (crp) PD (apD) p, (ap') d5o (Aso)
(%) (kg/m3) (kg/m3) (kg/r3) (mm)
Exposed 30(11) 1156(63) 267(105) 2296(210) 0.26(0.10)
Outer 45 (8) 1050 (30) 98 (56) 2098 (411) 0.04 (0.01)
Inner 52(4) 1023 (11) 51(27) 1986 (491) 0.03 (0.01)
Table 4.6 Mean and standard deviation of characteristic properties of consolidated bed layer for the three zones
Organic Bulk Dry Particle Median
content density density density diameter
Lc (aoc) p (ap) PD (qaoD p (OP,) d5o (0aO)
(%) (kg/m3) (kg/mn (kg/rn) (mm)
Exposed 30(11) 1156(63) 267(105) 2296(210) 0.26(0.10)
Outer 28(11) 1090 (28) 161 (43) 2283 (280) 0.17 (0.10)
Inner 43 (7) 1047 (29) 94 (36) 2037 (531) 0.05 (0.02)
The parameters, which are interdependent by virtue of their definitions and the fact that
all are derived from the same set of samples, collectively indicate that sediment in the inner open
water area has the highest organic content, the lowest (wet) bulk, dry (bulk) and particle densities
and the lowest particle size. At the opposite end is the exposed bottom, with the outer open water
area in-between. The erosion rate parameters imply that the material in the inner open water area
is more erodible than in the outer open water area. Note, however, that the water depth also
matters since wave penetration will be less in the inner area than the outer one, the degree
erosion of weaker material in deeper areas will be tempered by lower wave-induced bed shear
stresses there.
4.2 Erosion Rate
Values of the erosion rate constant M and the critical stress rr corresponding to Eq. 3.3
(for each sample) are given in Table 4.7. These values are comparable to those found for similar
organic-rich fine sediments in Florida (Mehta and Parchure, 2001).




Table 4.7 Erosion rate constant and critical shear stress values Critical Shear Erosion Rate Critical Shear Erosion Rate
Sample Stress, Tcr Constant, M Sample Stress, Tcr Constant, M
No. no. (Pa) (g/N-s) No. no. (Pa) (g/N-s)
1 1A-c (AI4-c) 0.115 0.59 24 3D-u (U28-u) 0.090 1.27
2 1B-c (AI9-c) 0.072 1.77 25 4A-c (Ml 1-c) 0.094 1.17
3 1C-c (AI14-c) 0.096 1.13 26 4B-c (M16-c) 0.072 1.77
4 ID-c (AI18-c) 0.120 0.45 27 4C-c (M22-c) 0.077 1.63
5 1A-u (AI4-u) 0.090 1.27 28 4D-c (M28-c) 0.105 0.88
6 1B-u (AI9-u) 0.073 1.77 29 4A-u (M1 -u) 0.079 1.59
7 IC-u (Al14-u) 0.079 1.74 30 4B-u (M16-u) 0.072 1.77
8 1D-u (AI18-u) 0.092 1.24 31 4C-u (M22-u) 0.062 2.06
9 2A-c (AC3-c) 0.107 0.81 32 4D-u (M28-u) 0.079 1.59
10 2B-c (ACI l-c) 0.092 1.24 33 5A-c (F11 -c) 0.102 0.95
11 2C-c (AC19-c) 0.089 1.31 34 5B-c (F16-c) 0.072 1.77
12 2D-c (AC24-c) 0.089 1.31 35 5C-c (F21-c) 0.083 1.49
13 2A-u (AC3-u) 0.075 1.70 36 5D-c (F26-c) 0.096 1.13
14 2B-u (AC11 -u) 0.065 1.99 37 5A-u (F11 -u) 0.072 1.77
15 2C-u (AC19-u) 0.080 1.56 38 5B-u (F16-u) 0.071 1.81
16 2D-u (AC24-u) 0.070 1.85 39 5C-u (F21-u) 0.068 1.88
17 3A-c (U10-c) 0.070 1.85 40 5D-u (F26-u) 0.066 1.95
18 3B-c (U16-c) 0.083 1.49 41 1329-900 (Landl) 0.108 0.77
19 3C-c (U22-c) 0.075 1.70 42 489-600 (Land2) 0.077 1.63
20 3D-c (U28-c) 0.110 0.74 43 48-800 (Land3) 0.093 1.20
21 3A-u (U10-u) 0.070 1.85 44 1224-200 (Land4) 0.112 0.66
22 3B-u (U16-u) 0.070 1.85 45 515-700 (LandS) 0.102 0.95
23 3C-u (U22-u) 0.071 1.81
In Figure 4.5, the erosion rate is plotted as a function of applied shear stress. The data
points encompass the measured range of values of organic content (13 to 58%). Bracketing lines
denote very approximate bounds characterized by OC = 10% and 60%. The 35% divide is also
approximate. Observe that as OC increases from 10 to 60%, Mincreases from 0.45 to 2.06 g/N-s,
whereas rcr decreases from 0.12 Pa to 0.065 Pa (although this latter variation is not entirely
certain, given the limited number of data points). These changes, taken together with Eq. 3.3,
imply that the higher the OC value, the larger the erosion rate. At a given shear stress, the error
band associated with the erosion rate is found to be on the order of 15% (corresponding to
90% values).
In Figure 4.5, the observed increase in the erosion rate constant with increasing OC is
consistent with the observations of Mehta and Parchure (2001) for organic-rich sediments. As




noted, these data signify a loss of bed strength against erosive forces with increasing organic content due to an overall decrease in cohesion. The plot of Figure 4.6 is included to further highlight this trend. The dependence of M and Tcr on the zone of the lake is discussed later.
0.3 I
45-60 60% OCbound -- / 35% OC line
0.25 t4F... ..
025-45 ,1-=20 /010-25 _M 2.6W/ K
E 0. -- ---0.0
'Z 0. 1 .. .. - - 10% OC bound0 '+--- -. --'-M =_0.45
0 0.05 0.1 0.15 0.2 0.25 0.3
Applied Shear Stress (Pa)
Figure 4.5 Erosion rate plot as a function of (applied) shear stress and approximate bounds of the
effect of varying organic content. M denotes the erosion rate constant.
4.3 Settling Velocity
The settling velocity, w,, values (calculated from measuring the suspension concentration in the settling column) are given in Table 4.8. These velocities corresponding to the suspended sediment concentration value of 0.1 kg/m3. The choice of this concentration is based on the assumption that, higher concentrations are likely to occur only episodically. It is also the concentration below which settling tends to be largely free of the effects of inter-particle collisions, and therefore represents the characteristic free-settling value (Mehta and Dyer, 1990).




0.3 7
- 0.25 .- 2-a-c(23% OC)
E (AC3-c)
- 0 .2 .. ... ..
48% OC pop
m 0.15 1-a-u(35% OC)
C Zcr = 0.075 Pa 4A 35% OC (AI4-u)
.or0. 1 -'
S0.05 -A 3-c-c(48% OC)
I 23% OC (U22-c)
0 0.05 0.1 0.15 0.2 0.25 0.3
Shear Stress (Pa)
Figure 4.6 Erosion rate plots for three samples (AC3-c, A14-u and U22-c) highlighting the effect of increasing organic content in increasing the erosion rate.
In general, inasmuch as fine-grained sediment settling velocity depends on concentration, it is instructive to examine the nature of this dependence as well as the effect of organic content. This is done is Figure 4.7, in which the variation of w, with concentration, c, follows trends characteristic of fine sediment described by the equation (Mehta and Li, 2001): W awC "(4.4)
- (bw2 + C2)m(
in which aw, bw, nw and mw are sediment-specific coefficients. At lower concentrations, as c increases w, also increases because increasing inter-particle collisions lead to the formation of larger flocs (or aggregates) with increasing size. Beyond a certain peak value, which in the present case is observed to depend on the concentration, hindered settling decreases w, as c increases further.




The effect of increasing OC, in general, is to reduce the settling velocity because the organic component tends to be considerably lighter than the inorganic component (see Figure 4.1). Note that the 10% OC and 60% OC curves are very approximate bounds based on Eq. 4.4. For both cases aw = 0.7, nw = 1.4 and mw = 2.5. The respective values of bw are 1.6 and 10.0. The 35% OC line is also approximate.
Table 4.8 Representative settling velocities

Sample
no.
1A-c (AI4-c) 1B-c (AI9-c) IC-c (AI14-c) 1D-c (AI18-c) 1A-u (AI4-u) 1B-u (AI9-u) 1C-u (All14-u) 1D-u (AI18-u) 2A-c (AC3-c) 2B-c (AC1 1-c) 2C-c (AC19-c) 2D-c (AC24-c) 2A-u (AC3-u) 2B-u (AC 11 -u) 2C-u (AC19-u) 2D-u (AC24-u) 3A-c (U10-c) 3B-c (U16-c) 3C-c (U22-c) 3D-c (U28-c) 3A-u (UO10-u) 3B-u (U16-u) 3C-u (U22-u)

Settling velocity
(m/s)
7.9x10 1.6x104 5.0x104 8.7x 104 4.2x104 1.6x104 1.7x104 4.4x104 6.7x104 4.4x10-4 4.0x104 4.0x10 4 1.9x104 4.0x10-5 2.7x104 1.2x104 1.2x104 3. 1x10 4 1.9x104 7. 1x104 1.2x104 1.2x104 9.8x10"

Sample
no.
3D-u (U28-u) 4A-c (M 1 -c) 4B-c (M1 6-c) 4C-c (M22-c) 4D-c (M28-c) 4A-u (M 1 -u) 4B-u (M16-u) 4C-u (M22-u) 4D-u (M28-u) 5A-c (Fl -c) 5B-c (F 16-c) 5C-c (F21-c) 5D-c (F26-c) 5A-u (F I1-u) 5B-u (F16-u) 5C-u (F21-u) SD-u (F26-u) 1329-900 (Landl) 489-600(Land2) 48-800(Land3) 1224-200(Land4) 515-700(Land5)

Settling velocity
(m/s)
4.2x10 4.8x10 1.6x10 2.3x10 6.4x104 2.5x 10 1.6x104 1.7x10 2.5x 104 6.0xl0 1.6x104 3.1x104 5.0x10 1.6x10 1.4x104 9.8x10-5 5.9x105 1.0x104 1.0x104 1.0x104 7.5x104 3.0x1 0

Note that the sediment in the settling column tests was not deflocculated because of the presence of high amount of organic matter, and because for sediment transport purposes the settling velocity of the sediment in the form it occurs naturally is desired. Based on the data one may accordingly conclude that the sandy material, if it at all erodes, settles out rapidly and is not

1 t




a major contributor to sediment transport in the lake, except perhaps under significantly episodic conditions, and that too for very short durations in comparison with fine sediment resuspension.
In the data collected, the mean, experimental error band (covering 90% of values) for the settling velocity at any concentration is estimated to be +20%. For practical purposes, the settling velocities given in Table 4.8 should suffice as representative values for the lake.
10-1 Data points lying bdTheen 10% and'35% OC lines
Data points i Tng be tweenl 5% and 60a% V lines.
10 -2 ....- ..
....................................................... ......... ... .. .. .
1 .......................... U q. ......i,.......... ..... ....... .... ..................... ........
= 10-/9 c bound? L
10- 0 II
0
- 0 0 W -S.,
io0. ... -S35% OC line 600/afc bound
10-2 10-1 10 101 102
Concentration (kg/m3)
Figure 4.7 Variation of settling velocity with suspended sediment concentration. Curves correspond to 10% OC and 60% OC bounds and 35% OC.
4.4 Zonal Dependence of Erosion and Settling Parameters
Based on the erosion and settling related data, mean values of transport-related parameters M and z- (characterizing erosion rate) and w, (charactering deposition) are given in Table 4.9 for the unconsolidated layer and in Table 4.10 for the consolidated layer. In each table, the data are separated by the zone or area exposed, outer open water and inner open water. Observe that for the unconsolidated layer, as the organic content increases moving from the




exposed area to the inner open water area, the erosion rate constant M increases, which implies
increasing potential erodibility with increasing organic fraction. The corresponding trend of
decreasing erosion shear strength r is in consonance with that of M. However, the variation in z,
in this case can be shown to be less important in comparison with that of M in influencing the
rate of erosion (Mehta and Parchure, 2000). The settling velocity exhibits a trend of decreasing
value with increasing OC, which reflects the effect of decreasing density with increasing OC.
Note, however, that given the 20% error band, the variation in w, with OC can be considered to
be only marginally significant. The consolidated layer does not show a similarly consistent trend
with OC.
Table 4.9 Mean and standard deviation of erosion parameters and median settling velocity for the three lake zones unconsolidated
Erosion rate constant Bed shear strength Median settling velocity OC (Uoc) M (UM) -; ('.O) w, (ows)
(%) (gIN-s) (Pa) (m/s)
Exposed 30(11) 0.87 (0.35) 0.08 (0.013) 2.3xlO4 (2.5x104)
Outer 45 (8) 1.61 (0.25) 0.078 (0.009) 2.4x104 (1.3x104)
Inner 52 (4) 1.82 (0.13) 0.07 (0.005) 1.3x104 (0.7x10"4)
Table 4.10 Mean and standard deviation of erosion parameters and median settling velocity for the three lake zones consolidated
Erosion rate constant Bed shear strength Median settling velocity OC (Goc) M (GM) r, (a.) w. (a.)
(%) (g/N-s) (Pa) (m/s)
Exposeda 30 (11) 0.87 (0.35) 0.08 (0.013) 2.3x104 (2.5x104)
Outer 28 (8) 0.99(0.38) 0.10 (0.014) 5.8x104 (2.0x104)
Inner 43 (4) 1.53 (0.23) 0.08 (0.008) 2.9x104 (1.2xl0a)
aSame values are reported for unconsolidated and consolidated layers since sampling was done with a shovel and associared depths were uncertain.




5. CONCLUSIONS
The following observations are derived from this study:
1. For the forty-five samples, the extracted sample pore fluid was found to be acidic
with pH between 4.7 and 6.4.
2. The organic content (measured as loss on ignition) varied between 13% and 58%.
The corresponding mean values for the exposed, outer open water and inner open water were: 30%, 45% and 52% for the unconsolidated layer and 30%, 28% and
43% for the lower, consolidated layer.
3. The bottom material consisted of a silty-sand (median size range 0.014 mm to
0.39 mm). The corresponding median sizes for the exposed, outer open water and inner open water were: 0.26 mm, 0.04 mm and 0.03 mm, respectively, for the unconsolidated layer, and 0.26 mm, 0.19 mm and 0.05 mm, respectively, for the
consolidated layer.
4. Overall, the sorting coefficient, a measure of the spread of sizes in a distribution,
varied between 1.3 to 4.1, implying the presence of graded (non-uniform)
sediment.
5. The ranges of density values were found to be as follows: bulk density 1,005
kg/m3 to 1,242 kg/m3, dry density 19 kg/m3 to 407 kg/m3 and particle density 1,150 kg/m3 to 2,597 kg/m3. Dry densities of comparable magnitudes were reported previously by Brenner and Whitmore (1998) in the lake. Trend lines for the variation of these densities with organic content appeared to be consistent with those found for sediments from several water bodies in Florida, and analyzed by
Ganju (2001) and by Brenner and Whitmore (1998) in Newnans Lake.




6. In the upper, unconsolidated layer mean density was 1,036 kg/m3 at a mean depth
of 0.15 m below the bed-water interface, whereas in the consolidate layer it was 1,078 kg/m3 at 0.79 m depth. Densities of this range are consistent with a fluidlike, low shear strength, organic-rich bottom.
7. Erosion rates of sediment beds prepared in the Particle Erosion Simulator ranged
from nil to 1.6 g/m2/s, corresponding to an applied shear stress range from 0.06 Pa to 0.3 Pa. For the unconsolidated layer, whose properties (as opposed to those of the undisturbed lower, consolidated layer) are relevant to bottom erodibility, the corresponding mean values by zone were: exposed 0.10 Pa, outer open water 0.08 Pa and inner open water 0.07 Pa. For the erosion rate function, the critical shear stress for erosion varied between 0.065 Pa and 0.120 Pa, and the rate constant between 0.45 g/N-s and 2.06 g/N-s. For the unconsolidated layer the mean values were: exposed 1.0 g/N-s, outer open water 1.6 g/N-s and inner open water 1.8 g/N-s. These values are consistent with organic-rich sediments
from elsewhere in Florida.
8. Settling velocities ranged from 1.7x10-6 m/s to 8.7xlO4 m/s, which are consistent
with those found for similar sediments in Florida. Tests in the settling column indicated a dependence of the settling velocity on the organic content and suspension concentration. Taking the settling velocity at 0.1 kg/m3 as a characteristic value, the following zonal variation was obtained for the unconsolidated material: exposed -- 2.7xlO4 m/s, outer open water -- 2.4xlO4 m/s, inner open water -- 1.3x10"4 m/s. The corresponding values for the




consolidated material were: 2.7x104 m/s, 5.8x10-4 m/s and 2.9x10"4 m/s,
respectively.
9. Turbidity in the lake appears to results from wash load and suspended bed
material load. In general, the former is due to light-weight organic matter that remains in suspension irrespective of the wind speed. On the other hand, bed material load is a strong function of the wind speed, inasmuch as wind correlates with bed erosion via waves and wave-induced bed shear stress. For modeling turbidity in the lake it will be essential to collect calibration/validation data on wind, waves and suspended sediment concentration on a synoptic basis. This can be achieved by installing a self-recording tower assembly in the open-water area with a pressure sensor (for waves), a vertical array of optical backscatter sensors (for suspended sediment profiling) and a wind anemometer over several (e.g.,
three or more) months.
10. Assuming that: 1) the PES erosion data are applicable to the lake without any
model to prototype scaling, and 2) settling of eroded material does not occur under assumed constant wind conditions, example plots of the rate of erosion as a function of wave-induced bottom velocity and wing duration have been presented
purely for illustrative purposes.
11. If the exposed bottom continues to remain as such, in due course, as the surficial
organic matter is oxidized and desiccation occurs, it is most likely that the bed strength will increase significantly. In that case the presently determined erodibility indices will become inapplicable, and the erosion rate function will
have to be re-calibrated.




12. Dredging the soft bottom sediment down to a level where the density of the
deposit is high enough to better withstand resuspension, both due to its higher consolidated state and because at deeper levels surface wave penetration in attenuated, can be expected to improve water quality, e.g., as defined by the
Trophic State Index.




6. REFERENCES

ASTM, 1993. Annual Book of ASTM Standards. American Society of testing and Materials, Philadelphia, PA.
Baird, R. A., 1987. A preliminary assessment of phosphorous precipitation-inactivation as a restoration technique in Lake Apopka. Project report, Orange County Environmental Protection Department, Orlando, FL.
Brenner, M., and Whitmore, T. J., 1998. Historical sediment and nutrient accumulation rates and past water quality in Newnans Lake. Final Report, Department of Fisheries and Aquatic Sciences, University of Florida, Gainesville.
Burt, T. N., 1986. In: Estuarine Cohesive Sediment Dynamics, A. J. Mehta ed., Springer-Verlag, Berlin, 251-265.
Environmental Consulting & Technology, Inc., 2002. Bathymetry and sediment thickness surveys of Newnans Lake, Project 99B250. Report for ECT Project No. 990765-0400, Gainesville, FL.
Faure, G., 1998. Geochemistry, Prentice Hall, Engelwood Cliffs, NJ.
Ganju, N. K., 2001. Trapping organic-rich fine sediment in an estuary. M, S. thesis, University of Florida, Gainesville.
Hwang, K.-N., 1989. Erodibility of fine sediment in wave-dominated environments. MS. Thesis, University of Florida, Gainesville.
Jonsson, I. G. 1966. Wave boundary layers and friction factors. Proceedings of the Tenth Confrence on Coastal Engineering, Vol 1. ASCE, New York, 148Mehta, A. J., and Dyer, K. R., 1990. Cohesive sediment transport in estuarine and coastal waters. In: The Sea Volume 9: Ocean Engineering Science, B. Le Mehaute and D. M. Hanes eds., Part B, Ch. 23, Wiley, New York, 814-840.
Mehta, A. J., Lee, S. C., Vinzon, S. B., and Abreu, M. G., 1994. Analyses of some sedimentary properties and erodibility characteristics of bottom sediments from the Rodman Reservoir, Florida. Report UFL/COEL/MP-94/03, Coastal and Oceanographic Engineering Department, University of Florida, Gainesville.
Mehta, A. J., and Li, Y., 2001. Principles and process modeling of fine grained sediment transport. OCP 6297 Lectures, University of Florida, Gainesville.
Mehta, A. J., and Parchure, T. M., 2000. Surface erosion of fine-grained sediment revisited. In: Muddy Coast Dynamics and Resource Management, B. W. Flemming et al. eds., Elsevier, Amsterdam, 55-74.




Nagid, E. J., 1999. A limnological assessment of Lake Newnan, Florida, August 1997-July 1998. M S. thesis, University of Florida, Gainesville.
Parchure, T. M., and Sturdivant, C. N., 1997. Development of a portable innovative contaminated sediment dredge. Final Report CPAR-CHL-97-2, Construction Productivity Research Program, U.S. Army Engineer Research and Development Center, Vicksburg, MS.
Ryding, S.-O., and Rast, eds. 1989. The Control of Eutrophication of Lakes and Reservoirs. Parthenon, Park Ridge, NJ.
Rodriguez, H. N., Jiang, J., and Mehta, A. J., 1997. Determination of selected sedimentary properties and erodibility of bottom sediments from the Lower Kissimmee River and Taylor Creek-Nubbin Slough Basins, Florida. Report UFL/COEL-97/09, Coastal and Oceanographic Engineering Department, University of Florida, Gainesville.
Shannon, E. E., and Brezonik, P. L., 1972. Eutrophication analysis: A multivariate approach, Journal ofSanitary Engineering Division, ASCE, 89(1), 37-57.
SPM, 1984. Shore Protection Manual. U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS.
Tsai, C. H., and Lick, W., 1986. A portable device for measuring sediment resuspension. Journal of Great Lakes Research, 314-321.
Wanielista, M. P., 1978. Stormwater Management. Ann Arbor Science, Ann Arbor, MI.
Young, I. R., and Verhagen, L. A., 1996. The growth of fetch limited waves of finite depth. Part
1. Total energy and peak frequency. Coastal Engineering, 29, 47-78.




APPENDIX: A METHOD TO ESTIMATE RESUSPENSION

A.1 Approach
From Eq. 3.2 we note that the rate of erosion can be calculated as AC
c=h- AC(A.1) At
where AC is the increment in (depth-mean) suspended sediment concentration over time increment At, and h is the water depth. Hence, noting that in Eq. 3.3 Thppjid = rb is the bed shear stress in the lake, we can calculate AC from AC = -(b v)At (A.2)
h
In Eq. A.2 -r, is obtained from
2
Tb f.P m(A.3) where fw is the wave friction factor, Pw is the fluid (nominally water) density and Umn is the amplitude of the wave-induced orbital velocity at the bottom. This velocity is obtained from the linear wave theory as (SPM, 1984) ao
um a (A.4)
sinh(kh)
In Eq. A.4 a is the wave amplitude (i.e., one-half the wave height) and a (=27r/T, where T is the wave period) is the wave frequency. The friction factor is obtained from
0 =.4 d, -' (A.5)
!2k. )




in which do is the amplitude of wave particle excursion at the bottom and k, is the bottom roughness coefficient. Equation A.5 is valid for 4 < dIk,< 40 (Jonsson, 1966). The value of d, is obtained from

do, = Urn
U

(A.6)

A.2 Examples of Calculation
The wave amplitude a and period T depend on the wind speed. Selecting speeds of 8, 9 and 10 m/s, the method of Young and Verhagen (1996) can be used to calculate the corresponding wave height and period. In addition the following parameters will be selected:
Wind fetch = 3,000 m (a representative maximum value), bottom roughness k, = 0.001 m (typical), h = 1.5 m (mean depth low water condition; see Fig. 1.3) and Pw = 1,000 kg/m3. Values of M and rs for the open water area are taken from Table 4.9. For the high water condition, selected to be 1.1 m above the low water (Fig. 1.3), the water depth in the open water area becomes 2.6 m, while the exposed area is conveniently assumed to be submerged to 1.1 m depth. The resulting peak (over a wave-period) values the wave orbital velocity at bottom (u,,) are gven in Table A. 1.

Table A. 1 Calculation of wave-induced bottom velocity
Wave
Wind speed amplitude, a
(m/s) Area Water level (M)
8 Open water Low 0.10
9 Open water Low 0.10
10 Open water Low 0.11
8 Open water High 0.11
9 Open water High 0.12
10 Open water High 0.13
8 Exposed High 0.09
9 Exposed High 0.10
10 Exposed High 0.11

Wave period, T
(s)
1.85 1.90
2.00 2.00 2.05 2.10 1.70
1.84 1.96

Peak waveinduced velocity, u,,
(m/s)
0.11 0.12 0.14 0.05 0.06 0.07 0.14 0.16 0.20




Let us vary the (wind) duration At from 0 to 300 min. We then obtain the relationships of Figures A.1-A.3 between wind speed, wind duration and increase in suspended sediment concentration in the inner and outer open water areas at low water. Note that since the initial (bed material load) concentration is assumed to be nil, increase in concentration is equal to the concentration itself. Note also that the resulting concentrations are higher than what would actually occur, because the effect of settling is not included in this calculation.

---Outer
--- Inner

50 100

250

150
Time (min)

Figure A. 1 Bed material concentration increase (in the absence of settling) at 8 m/s wind speed in the inner and outer open water areas. Low water condition.
In Figures A.4-A.5 results for high water are given. Note that in this case, due the deeper water, no resuspension occurred at 8 m/s, which is therefore not reported. Also, the less erodible bed material in the outer open water area did not resuspend even at 9 m/s. The corresponding plots for the presently exposed area are given in Figures A.6-A.8 for all three wind speeds.

zJ
U7
m w

300




~'2.5
E
0
*1.5
000.5
0

0 50 100 150 200 250
Time (min)

Outer
* Inner

300

Figure A.2 Bed material concentration increase (in the absence of settling) at 9 m/s wind speed
in the inner and outer open water areas. Low water condition.

50 100

200

250

Outer Inner

300

Time (min)
Figure A.3 Bed material concentration increase (in the absence of settling) at 10 rn/s wind speed
in the inner and outer open water areas. Low water condition.

I
-U
-U-
if- _________ __________ ________--U
U-




0.06

c'0.05
*~0.04
0
..0.03
C
*0.02
0
o0.01
0

200

300

Outer U Inner

400

Time (min)
Figure A.4 Bed material concentration increase (in the absence of settling) at 9 rn/s wind speed
in the inner and outer open water areas. High water condition.

0.3 S0.25
~0.2
0
0.15 *0.1
C
0
c 0.05
0

100 200 300
Time (min)

-.-- Outer
a Inner

400

Figure A.5 Bed material concentration increase (in the absence of settling) at 10 m/s wind speed
in the inner and outer open water areas. High water condition.

A
U
7
-E
U
N if
-U
U
7
U
-U--
if
3
U
-U




E 2.5
2
0)
S1.5
o 0.5
0

0 50 100 150

Exposed]

200 250 300

Time (min)
Figure A.6 Bed material concentration increase (in the absence of settling) at 8 m/s wind speed
in the presently exposed area under high water condition.

3.5 C) 3
p2.5
0 2 1.5
0. 5
00.

-- Exposed

0 50 100 150 200 250 300
Time (min)
Figure A.7 Bed material concentration increase (in the absence of settling) at 9 m/s wind speed
in the presently exposed area under high water condition.




0 50 100

150 200

- Exposedl

250 300

Time (min)
Figure A.8 Bed material concentration increase (in the absence of settling) at 10 m/s wind speed
in the presently exposed area under high water condition.

IA