• TABLE OF CONTENTS
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 Front Cover
 Title Page
 Table of Contents
 Synopsis
 List of Tables
 List of Figures
 List of symbols
 Main
 Reference
 Appendix














Group Title: UFLCOEL
Title: Properties of sediment from Newnans Lake, Florida
CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00091057/00001
 Material Information
Title: Properties of sediment from Newnans Lake, Florida
Series Title: UFLCOEL
Physical Description: xiii, 39 p. : ill. ; 28 cm.
Language: English
Creator: Gowland, Jason E
Mehta, Ashish J
University of Florida -- Coastal and Oceanographic Engineering Dept
Publisher: Coastal & Oceanographic Engineering Dept. of Civil & Coastal Engineering, University of Florida
Place of Publication: Gainesville Fla
Publication Date: 2002
 Subjects
Subject: Sediment transport -- Florida   ( lcsh )
Soil erosion -- Florida   ( lcsh )
Genre: bibliography   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Includes bibliographical references (p. 31-32).
Statement of Responsibility: by Jason E. Gowland and Ashish J. Mehta.
 Record Information
Bibliographic ID: UF00091057
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 50761678

Table of Contents
    Front Cover
        Front Cover
    Title Page
        Page i
    Table of Contents
        Page ii
        Page iii
    Synopsis
        Page iv
        Page v
        Page vi
        Page vii
    List of Tables
        Page viii
    List of Figures
        Page ix
        Page x
    List of symbols
        Page xi
        Page xii
        Page xiii
    Main
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
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        Page 11
        Page 12
        Page 13
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        Page 18
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        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
    Reference
        Page 31
        Page 32
    Appendix
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
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        Page 39
Full Text




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 PSIS ...................................................................................................iv

LIST OF TABLES........................................................................................viii

LIST OF FIGURES......................................................................................ix

LIST OF SYMBOLS..................................................................................... .. xi

1. INTRODUCTION.................................................................................... ....

1.1 Lake Location.................................................................... .............. .. 1

1.2 Sampling and Measurements.....................................................................2

2. SEDIMENTARY PARAMETERS............................................................... 5

2.1 Sample Densities................................................................................. ...5

2.2 Organic Content.....................................................................................6

2.3 Grain Size............................................................ ........................... 6

2.4 Sorting Coefficient............................................................... ................ 6

2.5pH ........................ .... ........................ ............................. 6

3. SEDIMENT ERODIBILITY.......................................................... ........ 8

3.1 E rodibility ........................................................................................... .. .8

3.2 Settling Velocity.................................................................................

3.3 Turbidity, Water Quality and Dredging.........................................................10

4. RESULTS.............................................................................................13

4.1 Sediment Properties ............................................................................ 13

4.1.1 Organic Content................................................................................13

4.1.2 Densities...............................................................................................14









4.1.3 Grain Size........................................................................................ .... 17

4.1.4 pH ................................................................................................... 17

4.1.5 Ranges and Mean Values ofParameters...................................................... 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., non-

uniform) 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 g/N-s and 2.06 g/N-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 8.7x104 m/s, 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/m3 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 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 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. 1 Calculation of wave-induced bottom velocity............................................34









LIST OF FIGURES

Figure 1.1 Location of Newnans Lake in Florida......................................................

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 G anju (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, AI4-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 35% 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 A.4 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 [(Ag)y'/cm] or (LS/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)

f+ 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 m3)

ws settling velocity (m/s)

z vertical coordinate (m)

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)

oi 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.




Ncwnans
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 open-

water 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 ofNewnans 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 - ----
21 _February 1998
1 Typical 1995-98 level El Niiio high
20.5 .m
E 20 ---------------------
19.5
19 i- August 2001 low
18.5 -
Jan- Jul- Jan- Jul- Jan- Jul- Jan- Jul- Jan- Jul- Jan- Jul- Jan- Aug-
95 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:

F11 = 5A, F16 = 5B; F21 = 5C; F26 = 5D; M11 = 4A; M16 = 4B; M22 = 4C; M28 = 4D;

U10 = 3A; U16 = 3B; U22 = 3C; U28 = 3D; AC3 = 2A; AC11 = 2B; AC19 = 2C; AC24 = 2D;

AI4 = 1A; AI9 = 1B; AI14 = 1C; AI18 = ID; Land 1 = 1329-900; Land 2 = 489-800; Land 3 =

48-800; Land 4 = 1224-200 and Land 5 = 515-700.








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, Mw, by sample

volume, V, of the container:

p M (2.1)
V

Typically, 40 cm3 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 Po:


S MD (2.2)
V

The particle or granular density ps was determined through mass balance:


,= PDPW (2.3)
Po +Pw P

where p, is water density (1,000 kg/m3).








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):

M -M
OC= D AX100% (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, So, according to:


S = 5 (2.5)
d25

where d2s 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=-loglo[H+] (2.6)

where IH is the activity of the hydrogen ions. When the hydrogen ion activity is 1x107, 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):


Oscillating grid









rapplie =.0005914rpm (3.1)

Two to four different shear stresses were applied to each sample to obtain the erosion

rate, E. 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:

dc
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:

e = M(TappTie Tcr) (3.3)

For each sample, the erosion rate constant, M and the critical shear stress, Tcr, 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 Tapplied

axis gave the critical shear stress, cr (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.

SThe governing equation for suspended sediment concentration is:








ac a
(w ) = 0 (3.4)
at az

where t is time and z is the vertical elevation coordinate. The settling velocity, ws, 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

e 9c ( 8 8c
ac- a K-+wc =0 (3.6)
at azz 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 wind-

induced 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 (TSI) (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 [(/t)-'/cm =/LS/cm], TN = total organic nitrogen

(mg/1), TP = total phosphate (mg/1), PP = primary productivity (mg-C/m2-hr), CL = chlorophyll-

a (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 Po p, 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 1C-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(ACll-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 (M11-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 d5s 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 (M11-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 (F11-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(F1l-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 (LandS) 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 organic 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-0. 8(OC -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)P (4.3)
(, -Pj


Bulk

3000.0 M Dry
2500.0
A Eq.4.1 A Particle
2000.0-
1500.0 -- .Pa
A. Particle

A-A- Bulk
1000.0 Density
Eq. 4.3 Eq. 4.2 Bulk
500.0
Density
0.0............ Curve
0.0Dry
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, unconsolidatedd"; 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.

O Core data (upper layer) E Core data (lower layer) 0 PES Bed
I~ 0.0 I------------o----



| -0.4 pa = 1,036 k m3
* ------- -- -- -- -.^ -
I -08--- ---.-- ---------------- -----

0 __n__________________

pvg = 1,078 k& r
-1.2
1000 1040 1080 1120 1160
Bulk density (kg/m3)


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/m3) (kg/m3)
16-IV-97-1 41 5C-u (F21-u) 37
16-IV-97-2 41 4C-u (M22-u) 32
16-IV-97-3 45 2C-u (AC19-u) 66
16-IV-97-4 83 IB-u (AI9-u) 58


4.1.3 Grain Size

Based on the d25, dso 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

Im) also suggests the presence of silt-sized material, which can arise from two sources: 1) fine-

sized 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 So

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 So 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.

4.1.4 pH

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 sand-

sized 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
90 (AI18-c)
80 -
S70 / / -- 2-a-c(23%)
60 (AC3-c)
a- 50 I 3-d-u(36%)
C
S40 (U28-u)
30 K 4-d-u(45%)
20
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 d2s dso d7s 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
Vaue content coefficient pH
value C P PD p d25 dso d75
(%) (kg/m3) (kg/m3) (kg/m3) (mm) (mm) (mm) So
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, po, Ps and dso; 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 F11, F26, M11, M28, U10, U28, AC3, AC24,

AI4 and AI18; and inner open water area F16, F21, M16, M22, U16, U22, AC11, AC19, AI9

and AI14. In Tables 4.5 and 4.6, mean and standard deviation of the above, state-related (as

opposed to transport-related), parameters are given.

i 1km


Exposed
area


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
Lake zone
OC (coc) p () Po (Cpo) p, (ac) dso (dso)
(%) (kg/m3) (kg/m3) (kg/m3) (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
Lake zone
OC (Coc) P (ap) PDo ( Ps (CO d5o (0djo)
(%) (kg/m3) (kg/mr (kg/mr) (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 Tcr 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, T,, 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 (M11-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 (M11-u) 0.079 1.59
7 1C-u (AI14-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 I-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%, M increases from 0.45 to 2.06 g/N-s,

whereas Tcr 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 cr on the zone of the lake is discussed later.



0.3
0.3 OC (%1 I
0 60% OC bound---/ 35% OC line
0.25 245-6045
S*10-25-45 "Z_-M= 2.06 gN-s
S0.2 10-25

0.15 .. .

0.1 '-10%OCbound
o t s' 00
1 0
0.05 ...--
0 ----- --M= 0.45 gN-s

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, ws, 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

I 0.25 --- -
0.25 ----- -+-- 2-a-c(23% OC)
-- 0.2 ----
I48% OC
M 0.15 ---- : -- ---- 1-a-u(35% OC)
Scr= 0.075 Pa 35% OC (AI4-u)
.u 0.1 -- -
2005 -. A- 3-c-c(48% OC)
W 0.05
---23% OC (U22-c)
0
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, AI4-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):

ac""
w, = (b (4.4)
(b 2 + C m


in which aw, bw, nw and mw are sediment-specific coefficients. At lower concentrations, as c

increases ws 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 (AI14-u)
1D-u (AI18-u)
2A-c (AC3-c)
2B-c (AC1 -c)
2C-c (AC19-c)
2D-c (AC24-c)
2A-u (AC3-u)
2B-u (AC11-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 (U10-u)
3B-u (U16-u)
3C-u (U22-u)


Settling velocity
(m/s)
7.9x104
1.6x104
5.0x104
8.7x104
4.2x104
1.6x104
1.7x104
4.4x104
6.7x10l
4.4x104
4.0x104
4.0x104
1.9x104
4.0x10-5
2.7x104
1.2x104
1.2x104
3.1x104
1.9x104
7.1x104
1.2x104
1.2x104
9.8x10"5


Sample
no.
3D-u (U28-u)
4A-c (M11-c)
4B-c (M16-c)
4C-c (M22-c)
4D-c (M28-c)
4A-u (M11-u)
4B-u (M16-u)
4C-u (M22-u)
4D-u (M28-u)
5A-c (Fll-c)
5B-c (F16-c)
5C-c (F21-c)
5D-c (F26-c)
5A-u (F11-u)
5B-u (F16-u)
5C-u (F21-u)
5D-u (F26-u)
1329-900 (Landl)
489-600(Land2)
48-800(Land3)
1224-200(Land4)
515-700(Land5)


Settling velocity
(m/s)
4.2x104
4.8x104
1.6x104
2.3x104
6.4x104
2.5x10"
1.6x104
1.7x10-
2.5x104
6.0xl04
1.6x104
3.1x104
5.0x104
1.6x104
1.4x104
9.8x10-5
5.9x10-5
1.0x104
1.0x104
1.0x104
7.5x104
3.0x104


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 Data points lying between 10% and 35% OC lines
Data points ljingi hb'elween 3 5",, and 611%t OC line%

1 0 ................................. ..... .. .. . .. ............ .. . .
S10 2

01 -0




35I OC Oline 60/ Of bound o |



10-2 10-1 1300 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 ofErosion and Settling Parameters

Based on the erosion and settling related data, mean values of transport-related

parameters M and -s (characterizing erosion rate) and ws (charactering deposition) are given in

Table 4.9 for the unconsolidated layer and in Table 4.10 for the consolidated layer. In each table,
-I 0 # SI
,'~ . *.' -- I










the data are separated by the zone or area exposed, outer open water and inner open water.

1Observe that for the unconsolidated layer, as the organic content increases moving from the
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 rs (characterizing erosion rate) and ws (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 rs 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
Lake zone
OC (Uoc) M (aM) ,; (a,) w, (ow,)
(%) (g/N-s) (Pa) (m/s)
Exposed 30(11) 0.87 (0.35) 0.08 (0.013) 2.3x104 (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 (0.7x104)



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
Lake zone C (oc) M () r, (a,) w, (a,,)
(%) (g/N-s) (Pa) (m/s)
Exposed 30 (11) 0.87 (0.35) 0.08 (0.013) 2.3x104 (2.5x10"4)
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 (1.2xl04)
aSame values are reported for unconsolidated and consolidated layers since sampling was done with a shovel and
associated 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 fluid-

like, 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 8.7x104 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 -- 2.7x104 m/s, outer open water -- 2.4x104

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
Conference on Coastal Engineering, Vol 1. ASCE, New York, 148-

Mehta, 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 of Sanitary 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
e=h h (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 Tappied = b is the bed shear

stress in the lake, we can calculate AC from

M
AC ( = -(rb -)At (A.2)
h


In Eq. A.2 r is obtained from

2
Tb = fwPw (A.3)

where fw is the wave friction factor, pw is the fluid (nominally water) density and Um 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.4)
Ssinh(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

= /O.4 i (A-0.75
=0.4 d 0' 75(A.5)
2k ,









in which do is the amplitude of wave particle excursion at the bottom and ks is the bottom

roughness coefficient. Equation A.5 is valid for 4 < do/ks< 40 (Jonsson, 1966). The value of do is

obtained from


U=
do =


(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 ks = 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 Ts 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 wave-
induced
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.


zI

U

L if:


300













S2.5
E


0
1.5



o 0.5

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 m/s wind speed
in the inner and outer open water areas. Low water condition.


I-'

-U
--U-
U--











0.06


S0.05

0.04
c-
* 0.03
I--


0
o 0.01

0


200


300


- Outer
U- Inner


400


Time (min)

Figure A.4 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.


0.3

m 0.25

0.2
0
o
0.15

* 0.1

0 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





I-U












E 2.5

2
0

I 1
o
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

iCi 3

p 2.5

C 2
o2
S1.5


S0.5


-I- 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.


r
~


f
























0 50 100


150 200


-*- Exposed


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'




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