• TABLE OF CONTENTS
HIDE
 Front Cover
 Title Page
 Acknowledgement
 Table of Contents
 List of Tables
 List of Figures
 List of symbols
 Abstract
 Introduction
 Sediment resuspension
 Data analysis
 Sediment resuspension potentia...
 Conclusions
 Core thickness and bed densiti...
 List of references
 Biographical sketch






Group Title: Technical report – University of Florida. Coastal and Oceanographic Engineering Program
Title: Wind induced wave resuspension and consolidation of cohesive sediment in Newnans Lake, Florida
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Permanent Link: http://ufdc.ufl.edu/UF00075318/00001
 Material Information
Title: Wind induced wave resuspension and consolidation of cohesive sediment in Newnans Lake, Florida
Physical Description: xv, 70 leaves : ill. ; 29 cm.
Language: English
Creator: Gowland, Jason E
Publication Date: 2002
 Subjects
Subject: Civil and Coastal Engineering thesis, M.E   ( lcsh )
Dissertations, Academic -- Civil and Coastal Engineering -- UF   ( lcsh )
Genre: bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis (M.E.)--University of Florida, 2002.
Bibliography: Includes bibliographical references (leaves 67-69).
Statement of Responsibility: by Jason E. Gowland.
General Note: Printout.
General Note: Vita.
Funding: This publication is being made available as part of the report series written by the faculty, staff, and students of the Coastal and Oceanographic Program of the Department of Civil and Coastal Engineering.
 Record Information
Bibliographic ID: UF00075318
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved, Board of Trustees of the University of Florida
Resource Identifier: aleph - 002852855
oclc - 50799599
notis - ANY3944

Table of Contents
    Front Cover
        Front Cover
    Title Page
        Title Page
    Acknowledgement
        Acknowledgement
    Table of Contents
        Table of Contents 1
        Table of Contents 2
    List of Tables
        List of Tables 1
        List of Tables 2
    List of Figures
        List of Figures 1
        List of Figures 2
    List of symbols
        Section 1
        Section 2
        Section 3
        Section 4
        Section 5
    Abstract
        Abstract 1
        Abstract 2
    Introduction
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
    Sediment resuspension
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
    Data analysis
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
        Page 39
    Sediment resuspension potential
        Page 40
        Page 41
        Page 42
        Page 43
        Page 44
        Page 45
        Page 46
        Page 47
        Page 48
        Page 49
        Page 50
        Page 51
        Page 52
        Page 53
        Page 54
        Page 55
        Page 56
        Page 57
        Page 58
        Page 59
        Page 60
        Page 61
        Page 62
    Conclusions
        Page 63
        Page 64
    Core thickness and bed densities
        Page 65
        Page 66
    List of references
        Page 67
        Page 68
        Page 69
    Biographical sketch
        Page 70
Full Text











WIND INDUCED WAVE RESUSPENSION AND CONSOLIDATION
OF COHESIVE SEDIMENT IN NEWNANS LAKE, FLORIDA






by






JASON E. GOWLAND


2002
















WIND INDUCED WAVE RESUSPENSION AND CONSOLIDATION OF COHESIVE
SEDIMENT IN NEWNANS LAKE, FLORIDA
















By

JASON E. GOWLAND


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF ENGINEERING

UNIVERSITY OF FLORIDA

2002


'ARCHIVES










ACKNOWLEDGMENT

I would like to thank all those before me that have contributed to the knowledge I

have acquired throughout my studies. This includes everyone who has and passed along

his or her spirit, a piece of his or her soul, or a fundamental piece of knowledge in order

to make the world we live in a better place.

















TABLE OF CONTENTS

page

ACKNOWLEDGMENT.......................................................................... ii

LIST OF TABLES ................................................................ ............. v

LIST OF FIGURES................................................................... .......vii

LIST O F SY M B O LS................................................................................ ix

ABSTRACT ....................................................xiv

CHAPTER

1 IN TR O D U CTIO N ............................................................................ .. 1

1.1 Problem Statem ent .............................................................................................. 1
1.2 Lake Characteristics ............................................................................................ 5
1.2.1 G eology ........................................................................................................ 5
1.2.2 B iology......................................................................................................... 6
1.2.3 O organic M matter ............................................................................................. 7
1.3 Objectives and Tasks .......................................................................................... 7
1.4 Thesis O outline ..................................................................................................... 8

2 SEDIMENT RESUSPENSION ...................................................... ...........

2.1 B background ......................................................................................................... 9
2.2 W ind R record ..................................................................................................... 10
2.3 Wave Height and Period Calculation.............................................................. 12
2.4 Method To Determine Suspension Concentration................................................ 13
2.4.1 Conservation of Sediment Mass ............................................... ............ 13
2.4.2 U pw ard D iffusion ................................................................................... 14
2.4.3 E erosion ....................................................................................................... 16
2.4.4 D position .................................................................................................. 17
2.4.5 C consolidation ............................................................................................ 19

3 DATA ANALYSIS......................................................................... 23

3.1 Introduction ....................................................................................................... 23


-- I









3.2 B bottom Sam ples................................................................................................ 23
3.3 Sediment Parameters......................................................................................... 24
3.3.1 D ensities........................ ............................................................................ 24
3.3.2 Organic Content............................................................................................ 26
3.3.3 Grain Size..................................... .................................29
3.4 Settling V elocities............................................................................................ 31
3.5 Erosion Param eters ........................................................................................... 33
3.6 C consolidation .................................................................................................... 35
3.6.1 Self-weight Consolidation ....................................................................... 35
3.6.2 Overburden Consolidation...................................... ................................. 36

4 SEDIMENT RESUSPENSION POTENTIAL 40

4.1 Modeling Newnans Lake Resuspension........................................... ........... ... 40
4.2 Resuspension Modeling.................................................................................. 41
4.2.1 Model Calibration and Validation ............................................. ........... 41
4.2.2 Effect of Wind Speed and Location on Resuspension................................ 44
4.2.3 Effect of Dredging on Resuspension .......................................................... 57
4.3 C consolidation .................................................................................................... 60

5 CONCLUSIONS 63

5.1 Sum m ary................................................... ....................................................... 63
5.2 C onclusions.......................... ............................................................................. 63
5.3 Recommendations for Future Work............................... .............................. 64

APPENDIX CORE THICKNESSES AND BED DENSITIES...........................66

LIST OF REFERENCES........................................................................68

BIOGRAPHICAL SKETCH ...................................................................71
















LIST OF TABLES


Table page

2.1 Mean wind speeds and directions at 10 m elevation (from NCDC)..................11

3.1 Sample locations coordinates...................................................................24

3.2 Unconsolidated material densities and organic content ................................27

3.3 Consolidated material densities and organic content ......................................28

3.4 Mean organic content and densities for inner, outer and exposed area ................29

3.5 Sample, d25, dso, d7s, and sorting coefficient .................................................30

3.6 Settling velocity param eters ................................. .............................. 33

3.7 Coefficients of consolidation and compressibility for overburden consolidation
.................................................... ............. .............. ....... 39

4.1 Model calibration and validation input parameters............................................42

4.2 Mean sedimentary and erosion parameters for inner, outer and exposed areas.........46

4.3 Inner open water low water input parameters .............................................47

4.4 Inner open water area mid-depth concentrations at 600 min; low water .............49

4.5 Inner open water high water input parameters ..............................................50

4.6 Inner open water area mid-depth concentrations at 600 min; high water ..............51

4.7 Outer open water high water input parameters ..........................................52

4.8 Outer open water area mid-depth concentrations at 600 min; high water ..............53

4.9 Exposed area model input parameters at high water .....................................54









4.10 Exposed area mid-depth concentrations at 600 min; high water ......................55

4.11 Consolidated bed properties..................................................................58

4.12 Dredged bottom model input parameters................. .....................................59

4.13 Dredged lake mid depth concentration at 600 min..................................... 60

4.14 Model input parameters for 100-year self-weight consolidation simulation..........62

















LIST OF FIGURES


Figure pge

1.1 Location of Newnans Lake in north-central Florida.................................... 2

1.2 Topographic map of Newnans Lake floodplain, courtesy of U.S. Geological Survey..3

1.3 Five-year monthly stage of Newnans Lake taken from Nagid (1999) and ECT (2001)
.........................................................................................................5

2.1 Inflow into lake basin with controlled outflow ...........................................9

2.2 Wind wave resuspended sediment is transported to outflow creek during draw-down
................................................................ ...................................... 10

2.3 Satellite photo of Prairie Creek in southwest corer of Newnans Lake ................11

2.4 Schematic diagram showing the water column and suspended sediment concentration
profile ......................................................................14

2.5 Increased neutral diffusivity at the bed surface assuming no change in fluid density
with depth ........... ....................................... ...............15

2.6 Variation of wave-induced water particle horizontal velocity with depth......... ....16

2.7 Reduced Coordinate System (a) Material or Langrangian coordinate system at time,
t=0; (b) Spatial or Eulerian coordinates at time, t; (c) Reduced coordinates at time, t
........................................................................................... ............. 2 0

3.1 Approximate locations for piston core barrel sampling along with depth
contours ............................................... ........... ....... ..... .... ...........25

3.2 Settling velocity plots for lake sediment samples.........................................32

3.3 Erosion rates vs. shear stresses..............................................................34

3.4 Self-weight consolidation of sample 4-c-u ...............................................35

3.5 Consolidation for an overburden stress of 25 kN/m2......................................37










3.6 Consolidation for an overburden stress of 50 kN/m2 ........ .................. .........38

3.7 Determination of bed compressibility for overburden consolidation .................38

4.1 Depth contours showing inner cores inside 0.3 m contour...............................41

4.2 Calibration run, with 90 minutes to reach 4 kg/m3 field concentration shown by blue
dot. Left plot is log-linear; right is linear scale............................................. 42

4.3 Validation run, with 90 minutes to reach 2 kg/m3 field concentration shown by blue
dot. Left plot is log-linear; right is linear scale ..............................................42

4.4 Resuspension at 8 m/s wind in the inner open water area at low water .................48

4.5 Inner open water area mid-depth concentration evolution at different wind speeds,
low w ater ...................................................... .................... ........... ....... 48

4.6 Inner open water area suspended sediment evolution at high water. Resuspension
occurred 10 m/s wind speed only ................................................... ..........50

4.7 Outer open water area mid-depth concentration evolution at different wind speeds,
high w ater............................................ .... ..... ................... .............. .. 53

4.8 Exposed area mid-depth concentration evolution at different wind speeds, high water
................... ........................................ ........................................... 5 5

4.9 Mid-depth suspended sediment concentration in the three sub-areas at 8 m/s
w ind.............. ...................................................................................... 56

4.10 Mid-depth suspended sediment concentration in the three sub-areas at 9 m/s
w ind................................................................................... ................. 56

4.11 Mid-depth suspended sediment concentration in the three sub-areas at 10 m/s
w ind............... ................. ... .. .... ..... ...... ................... ... ........... 57

4.12 Mid-depth suspended sediment concentration evolution with time; dredged
bottom ..................................................... ................. ...... ........... ......... 60

4.13 Self-weight consolidation modeling: variation of density profile with time. Model
output interval is 1 hour....................................................... ............ ........61

4.14 100-year self-weight consolidation of bed material with each line representing 10
years ........... .......................... .............. ............. .......................62

A.1 Core densities for unconsolidated and consolidated samples at the respective mean
elevations of the sub-lengths............................................................... 67















LIST OF SYMBOLS


a settling velocity parameter

a, erosion rate parameter

av bed compressibility

A constant

b settling velocity parameter

br erosion rate parameter

c celerity

Cf, cv coefficient of consolidation

C concentration

C1 limiting free settling concentration

CD conductivity

CL chlorophyll-a

CR cation ratio [Ca++]+[Mg++]/[Na+]+[K+]

d water depth

do wave particle orbital diameter

d25 diameter of 25 percent of sample

dso median size diameter

d75 diameter of 75 percent of material

e void ratio










ei initial void ratio

ee ultimate void ratio

E total energy (one wavelength/unit crest width)

Ek, Ep kinetic and potential energies respectively

E average wave energy per unit surface area

F, net flux

Fs settling flux

Fe erosion flux

fp peak spectral frequency

fw wave friction factor

g gravity

h height of water above bed

hb bed depth

H+ activity of hydrogen ions

H wave height

Hso height of bed at 50 % consolidation

k wave number

ko initial permeability

kh hydraulic conductivity

ks bottom roughness parameter

K diffusion coefficient

K, neutral diffusivity coefficient

L wavelength










m mass, or settling velocity parameter

M erosion rate constant

MA dry mass after ignition

MD dry mass

Mw mass of wet sample

n porosity, or settling velocity parameter

ot output time step

OC organic content

p excess pore water pressure

pw static pore water pressure

PP primary productivity

Ro beginning of consolidation

Rloo end of consolidation

Ri gradient Richardson number

So sorting coefficient

t time

tt consolidation durration

tso time at 50 % consolidation

T wave period, time period, also turbidity

TN total organic nitrogen

TP total phosphate

TSI Trophic State Index

u horizontal velocity










Um water particle horizontal velocity

U wind speed

Ua wind stress factor

V volume

Vs velocity of the solids

Vsolids volume of the solids

Vv volume of the voids

Vw water seepage velocity

ws settling velocity

Wsf free settling velocity

x fetch

xi bed density parameter

z, zi vertical elevation coordinate

Az incremental depth

ao non-dimensional empirical coefficient

ae erosion rate parameter

cXw wave diffusion coefficient

8o non-dimensional empirical coefficient

v constant

S non-dimensional scale for depth

e erosion rate

E non-dimensional wave energy













1r






V

V
rim

v







p

PB

PD


PD

Ps

Pf

Pw






rb, Tapplied

Zs

VDep




X


bed density parameter

reduced coordinates

initial volume of solids

modified bed height

kinematic viscosity of water

non-dimensional frequency

spatial or Eularian coordinate

density

bulk density

dry density

average dry density

particle density

fluid density

density of water

wave frequency

effective stress

applied bed shear stress

critical bed shear stress

deposition shear stress

erosion rate parameter


non-dimensional fetch















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Engineering

WIND INDUCED WAVE RESUSPENSION AND CONSOLIDATION OF COHESIVE
SEDIMENT IN NEWNANS LAKE, FLORIDA

By

Jason Eric Gowland

August 2002


Chairman: Ashish J. Mehta
Major Department: Civil and Coastal Engineering

Newnans Lake in north-central Florida has been slated for restoration of water

quality and, in 2001-2002, drought brought the lake down to record low levels. Since

most of the material at the lake bottom is detrital organic matter, it remains light-weight

and is easily resuspended, thus raising turbidity and degrading water quality. In this

study, wind-wave induced resuspension of the bed material was modeled to determine the

wind speed required to increase turbidity.

Resuspension of the bed material was found to become significant at -8 m/s wind

for the low stage of summer, 2001. At the mean lake stage, 1.1 m higher than the low

level, the required speed was in excess of 10 m/s. The drought exposed some of the bed

material to sun and air, which consequently acquired a higher strength and lower erosion

rate as compared to the submerged bed in the open water area.

In order to consolidate the bed to densities found -1.5 m below the bed surface, a

model for self-weight consolidation had to be run for a duration on the order of 100









years. It was found that, as a potential measured for restoration, dredging the

unconsolidated material, about 0.3 m in the open water area, would further reduce the

likelihood of resuspension, requiring a wind of -11 m/s at the mean stage.














CHAPTER 1
INTRODUCTION

1.1 Problem Statement

Wind-wave induced suspended fine-grained sediment, while a natural

phenomenon, can lead to potential problems associated with nutrient and contaminant

release and transport of suspended matter to contiguous water bodies, which may be

undesirable. For example, in Florida, Lake Okeechobee and Lake Apopka are two lakes

that have had problems with nutrient loading and suspended particulate matter. Baird

(1987) notes that observations of the first algal blooms in Lake Apopka occurred in the

late 1940's and early 1950's, after which rooted aquatic vegetation began to disappear

and game fish populations declined. These algal blooms are linked to heavy loadings of

nutrients, nitrogen and phosphorous from bordering muck farms, citrus groves,

wastewater treatment plants, and various other non-point sources. Another well-

documented example of this type is Lake Onodoga, New York, considered "America's

dirtiest lake" (Effler, 1996). The cause of water quality degradation in this case has been

linked to the production of soda ash by the Solvay Process Company along the western

bank of the lake, which began in 1884.

In the present study, a straightforward, 1D-vertical modeling of wave-induced

suspended sediment has been applied to examine the potential for fine, organic-rich

sediment re-suspension in Newnans Lake in north-central Florida (Fig. 1). This lake,

located about 8 km east of Gainesville, Florida, and approximately 28 km2 in size, has

received much attention in recent years due to legislation enacted as Florida Statutes









Section 373.453 related to water quality and restoration. The St. Johns River Water

Management District, Palatka, Florida and the Florida Fish and Wildlife Conservation

Commission oversee the Newnans Lake region and have been interested in the dynamics

of the system as a whole for many years. In December of 1990, the Fresh Water Fish

Commission placed Newnans Lake on the lake restoration priority list. A 1993 report by

the Game and Fresh Water Fish Commission (Holcomb, 1993) anticipated that the

Newnans Lake restoration project would cost in the range of eight to13 million dollars.


Figure 1.1 Location of Newnans Lake in north-central Florida.


-Newnans
Lake


'Ow
























































Geological Survey.









As seen from Fig. 1.2, Hatchet Creek and Little Hatchet Creek are two streams

that spill into Newnans Lake in the north. Prairie Creek carries water from the lake south

to Paynes Prairie. In 1966, a flow control dam was constructed at Prairie Creek near

State Road 20 to reduce seasonal water level fluctuations in the lake. In 1976,

modifications were made to the earthen dam to reintroduce limited draw-down events in

order to improve the condition of the lake with respect to algae and weed control.

In a shallow lake environment such as Newnans, nutrients stored in the sediment

may be re-suspended into the water column. This resuspension can be a significant

contributing factor to high nutrient concentrations and plant biomass in the system. As

the biomass grows, and eventually decays, a bio-cycle develops that accumulates organic

sediment in the system. Gottgens and Montague (1987) notes that excessive growth of

aquatic plants can lead to interference with the use of the lake for domestic, recreational,

navigational, or agricultural (aquacultural) purposes. Furthermore, if draw-down

occurred after some wave induced re-suspension event the organic sediment discharge

could be considerably greater than for draw-down with low suspended sediment

concentrations. Nagid (1999) surmised that at Newnans Lake there would be an

increased probability of sediment resuspension as lake stage decreased. This observation

shall be tested in the present study based on wave-induced resuspension modeling, and

knowledge of lake level variation in recent years (Fig. 1.3).






5




21.5 i
21 --- --- February 1998 El
1 Typical 1995-1998 level eNiuao high
E 20.5
20
S19.5
19 August 2001 low
18.5 I I I
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 Five-year monthly stage of Newnans Lake taken from Nagid (1999) and ECT
(2001).

1.2 Lake Characteristics

1.2.1 Geology

Holly (1976) noted that Newnans Lake is surrounded by sediments of the

Hawthorne Formation on the west, north and east, with (Eocene) Ocala Limestone

formation to the south extending into the Paynes Prairie area. The lake began some 8,000

years BP as a dry grassland due to low water table associated with a low sea level. From

then to about 5,000 years BP it was transformed into a "grass swamp" surrounded by dry

oak forest or shrub, and in the past 5,000 years has been in its present status as a shallow,

cypress fringed, eutrophic lake. Water quality in the lake can be characterized by the

Shannon-Brezonik Trophic State Index (TSI) (Wanielista, 1978):

TSI = 0.18T +0.008CD+ .1TN +4.2TP+O.O1PP+0.044CL +0.39CR +0.26 (1.1)

where T = turbidity (JTU), CD = conductivity [(Q)-l'/cm =iS/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 hypereutrophic in the early 1970s. Brenner and

Whitmore (1998) have given more recent data on the total organic nitrogen and total

phosphate along with radioisotopes found in the cores for dating.

1.2.2 Biology

Gottgens and Montague (1987) explained data on the phytoplankton found in the

lake. The dominant forms were blue-green algae with several green algae as

subdominants. In a seasonal study of the lake, a shift in the size distribution of the

phytoplankton was noted. Smaller forms appeared to dominate in the fall and early

winter, while larger forms were most common in the summer. Brenner and Whitmore

(1998) include a detail summary of the diatom taxa with depth in the lake.

A survey of macrophytes indicated the following species: emergents and floating

leaved, free floating, and submersed plants. Included in the survey were cattail, American

lotus, cypress, water hyacinth, pickerel weed, alligator weed, coontail, southern naiad,

and hydrilla. Midges, segmented worms and snails were also found, which can provide

valuable data on the contamination history of the lake through paleontological

investigations of their chitin body parts, which preserve well in the sediments (Gottgens

and Montague, 1987). Nagid (1999) proposed that a macrophyte coverage of 50% of the

lake area would be necessary to significantly reduce chlorophyll concentrations, which

would maintain a clearer and more stable lake and reduce sediment resuspension. The

Florida Fish and Wildlife Conservation planted maidencane and giant bulrush in efforts

to achieve reduced nutrient availability, refuge, and spawning substrate for sport fish.









1.2.3 Organic Matter

Microorganisms and meiofauna are also found in the lake at the soil-water

interface with high organic content. Within a few millimeters of the interface, oxidative

metabolism decays the organic material; however, deeper into the mud layer oxygen is

unavailable and only anaerobic metabolism occurs. Bacteria at the interface, which

secrete adhesive substances, may influence the dynamics of the system by increasing

shear strength and reducing erodibility. Meiofauna may also have the same effect by

secreting mucus that may bind the sediment.

Wetzel and Likens (2000) note that organic matter in an aquatic ecosystem ranges

from dissolved organic compounds to large aggregates of particulate organic matter and

includes both living and dead material. Most of this material is detrital organic matter

from dead organisms, while some may be unicellular algae with cell walls of hydrated

silica embedded into an organic matrix.

1.3 Objectives and Tasks

The objective of this study was to assess the potential role of wind-induced waves

in causing fine sediment resuspension in the lake. Several tasks were completed in order

to achieve this objective. These include

Analyzing bottom sediment samples from the lake to determine sedimentary
parameters used in the re-suspension modeling.

Laboratory testing of the samples to determine erosion rates, settling velocity
for deposition and consolidation of the deposit.

Predicting wave height and period for wave-induced resuspension based on
wind record.

Modeling the system to predict the evolution of suspended sediment
concentration as a function of wind speed.









*Conclusions based on the modeling related to the response of turbidity in the
lake to wind forcing.


1.4 Thesis Outline

Chapter 2 discusses the resuspension modeling approach and the determination of

wave height and period from wind record. Chapter 3 explains procedures for the

analyses of the sediment samples and interpretation of data for model input parameters.

Resuspension and consolidation modeling and results based on the findings are presented

in Chapter 4. Chapter 5 conclusions are outlined. References and a biographical sketch

follow.















CHAPTER 2
SEDIMENT RESUSPENSION

2.1 Background

Shallow water-bodies such as Newnans Lake may become very turbid during a

severe wind-induced wave event due to resuspension at the water mud interface or bed.

When modeling resuspension in a system such as this, model calibration must be done

using events that characterize the limits relative to the degree of resuspension. One limit

is defined by the period when there is little or no wave action and most of the material

remains at the bed (Fig. 2.1).

At the opposite end there is the condition after a storm wherein high wind and

wave action resuspends a significant amount of the bed sediment. Following such an

event a draw-down could discharge material out of the system, which in turn may prove

to be harmful to the receiving waters.


Figure 2.1 Inflow into lake basin with controlled outflow.


Drainage
Drainage Calm surface

Lake water -
Soft ...
bottom'
SFlow Outfl to
Con creek
Control









In connection with the above potentialities, this chapter introduces the modeling

of resuspension during wind events, and outlines the mathematical correlation between

laboratory-derived data and what is physically occurring in the system. The schematic

diagram of Fig. 2.2 is a visualization of the resuspension process.

The following sections help in the understanding of the associated transport

phenomena relevant to Newnans Lake.

2.2 Wind Record

The National Climate Data Center (NCDC) wind data for Gainesville Region

Airport (latitude 29 41 23 longitude 820 16 19 ) for the year 2001 are summarized in

Table 2.1. These data will be used as a basis to make estimates of characteristic wave

heights and the corresponding periods. Observe that the most common direction for the

wind was in the vicinity of 2500, toward the Prairie Creek (Fig. 2.3) direction.


Figure 2.2 Wind wave resuspended sediment is transported to outflow creek during draw-
down.


i ..... Wave activity
































te photo of Prairie Creek in southwest corer of Newnans Lake.


Table 2.1 Mean wind speeds and directions at 10 m elevation (from NCDC).
Wind (for period) Wind (2-min max.) Wind (5-s max.)
Period Speed Direction Speed Direction Speed Direction
(m/s) (deg) (m/s) (deg) (m/s) (deg)
January 3.1 310 16.1 250 19.2 270
February 3.4 270 14.8 240 18.3 240
March 3.6 270 17.9 250 23.2 320
April 3.3 270 13.9 330 17.9 330
May 3.1 250 13.4 250 16.5 250
June 2.8 250 20.1 240 23.2 220
July 2.5 270 20.1 290 28.6 290
August 2.4 240 13.4 120 19.2 120
September 2.6 90 13.0 120 17.4 160
October 2.9 40 10.7 40 13.4 20
November 2.8 40 11.6 320 14.8 320
December 2.7 300 13.0 270 15.2 280
Year 3.0 270 20.1 240 28.6 290


Figure 2.3 Sate









2.3 Wave Height and Period Calculation

The wave energy transferred to the bottom suspends the bed material. The total

energy of a wave, E (in one wave length, per unit crest length), is the sum of the kinetic

energy, Ek, and the potential energy, Ep:

pwgH2L pwgH2L pwgH2L
E = E + E 16 16 (2.1)
E 16 16 8

where H is the height of the wave, pw is the density of water and L is the wavelength.

The specific energy, E is the total average wave energy per unit surface area

given by
2
E pgH
E = (2.2)
L 8

The wave celerity, c relates to the wave period T and length L by

L
c = (2.3)
T

Relating the celerity to water depth, h and wavelength, L is an iterative method involving

the following wave dispersion relationship:


c= tanh(L (2.4)


From this equation we can determine the wave number k (= 2z7/L), hence the wave length

L, for a given wave frequency, cy (= 2;r/T).

Young and Verhagen (1996) describe fetch-limited wave growth in finite-depth

water from experiments and data collected at Lake George in Australia. Non-

dimensional charts for the prediction of wave energy and peak frequency from non-









dimensional fetch and depth parameters can be found in that work. The equation for non-

dimensional wave energy, e, is:

g2E
W= (2.5)
U4

where Uis the wind speed. The corresponding non-dimensional frequency, v, is:

fU
S- (2.6)
g

wherefp is the frequency of the spectral peak. The non-dimensional fetch, X, is

=Ugx (2.7)
U2

with x being the fetch length. Finally, the non-dimensional scaling factor for depth, 3, is

gh
1= g (2.8)
U2

From the charts an estimate of the wave energy is obtained and converted to wave

height through

H = 4E-5 (2.9)

The wave period is the inverse of the frequency of the spectral peak, i.e., 1/fp.

2.4 Method To Determine Suspension Concentration

Figure 2.4 is a schematic diagram showing the water column, the suspended

sediment concentration profile C(z), bed material being eroded/deposited and the net

sediment flux.

2.4.1 Conservation of Sediment Mass

With reference to Fig. 2.4, vertical suspended sediment transport can be

mathematically expressed by the wave-mean settling-diffusion equation:









t- zK -a + w C (2.10)
at 8z az

which is a particular case of the general mass conservation equation for vertical sediment

transport (Mehta et al., 1984), where K is the diffusion coefficient, ws is the sediment

dC
settling velocity, and C is the suspended sediment concentration. Thus, Fe =K- is the
dz

diffusive flux and Fs= -wsC is the settling flux.

The boundary conditions for solving the Eq. 2.10 are: zero net sediment flux at

the water surface, z=h, and at the bottom of the water column, z=0:


K-+wC _, = -F, (2.11)
zC

where F, is the net resuspension flux, which is equal to the difference between the

settling and erosion fluxes at the bed.

<--- .. --

jh



FC F( C()






Figure 2.4 Schematic diagram showing the water column and suspended sediment
concentration profile.
2.4.2 Upward Diffusion

The diffusion coefficient, K, is modeled by the suspended sediment concentration

gradient and is expressed as:










K= K, (2.12)
(1+ aoRi,)

where K, is the neutral diffusion coefficient for non-stratified flows, ao and Po are non-

dimensional empirical coefficients and Ri is the gradient Richardson number defined as



Sp (au u/ z)2
R (u(2.13)


In the above equation, u is the horizontal velocity, pf is the fluid density, and g is

gravitational acceleration.

For wave motion, the neutral diffusivity is obtained from:

K 2 sinh2 kz
K, = a,aH2 (2.14)
2sinh2 kh

where aw is a non-dimensional wave diffusion coefficient and H is the wave height.

Note that the above equations are obtained from the wave coordinate system

where z is measured from the water surface and is shown in Fig. 2.5.


Z
A wave height, H





depth, h

P=Pw


Sbed surface
K,

Figure 2.5. Increased neutral diffusivity at the bed surface assuming no change in fluid
density with depth.









2.4.3 Erosion

Wave action erodes and entrains particles from the bed surface into the water

column. For fine sediment this erosion takes place when the imposed fluid shear stress

exceeds the inter-particle bond strength (or shear strength) of the bed. The resistance of

the bed to erosion is sensitive to water content, pore water chemical composition,

sediment composition (i.e., clay mineralogy and organic content), and the age and the

structure of the bed and its stress history. Once the wave-induced shear stress Tb is greater

than the shear strength Ts, erosion occurs at a linear rate.

The change of concentration in the water column dC (or AC in difference form)

due to erosion can be determined from


dC = (rb -s)dt (2.15)
h

where M is an empirical rate constant and dt (or At) is the time-step for the calculation.

The erosion rate constant M is used to determine the net flux in equation 2.19 and is

described further in Chapter 4 with the corresponding model input parameters.

z

amplitude, a = H/2




P=Pw water particle
horizontal
depth, h velocity, u,


bed surface


Figure 2.6 Variation of wave-induced water particle horizontal velocity with depth.









The applied shear on the bed is a function of wave friction factor fw and the

horizontal velocity amplitude of the water particle, u,, at depth h. The horizontal

velocity of the water particle can be expressed as

cosh [k(z + h)]
u = ack (2.16)
sinh(kh)

where a is the wave amplitude (= H/2).

The wave friction factor can be obtained from (Jonsson, 1966):


f 0= 0.4 2- (2.17)
(2 k,

in which the water particle orbital diameter do = umT/r, and k, is the bottom roughness

parameter. Equation 2.17 must satisfy the requirement 4< doks<40. The applied shear

stress can then be found from
2
Tb =w m (2.18)
2

2.4.4 Deposition

When material settles out of suspension it is deposited onto the bed surface. This

process may also occur during erosion, and has been observed in laboratory tests. This is

so because once material is eroded into suspension it may also settle out to redeposit on

the bed surface. The settling flux F, is determined by the concentration of suspended

sediment and the settling velocity corresponding to that concentration. Laboratory tests

show that an increase in suspension concentration increases the settling flux F, so that an

equilibrium condition (equality of erosion and deposition) may be reached at a given

applied shear stress after a sufficiently long duration.









The erosion flux Fe is determined from the applied shear at the bed surface, while

the vertical diffusion coefficient determines how much material may be vertically

transported into the water column through (turbulent) diffusive processes. The difference

between the erosion and settling flux is the net flux or the amount of material being

suspended into concentration. The net flux can be determined by the equation


wC-0 1 b (b ZTDp)

F = 0 TDep < b <:s (2.19)
M (rb ,) (b > ,)


In Eq. 2.19 if the bed shear stress, Tb is less than are equal to the deposition shear stress,

TDep then the net flux would be the settling velocity multiplied by the concentration at the


bed surface, C -. with a probability of deposition factor, 1 b- (Krone, 1962). The


required settling velocity is obtained from

w"f C < C
w = a_ C _C, (2.20)
(C b C>C

for concentration C less than or greater than the limiting free settling (velocity

wsy)concentration, C1 = 0.1 kg/m3. Note that a, b, n and m are sediment-specific

coefficients.

The net erosion flux should be zero when the applied shear is greater than the

deposition shear but less than the shear strength, rs. If the deposition stress is less than

the applied shear stress the net flux is determined from the empirical erosion parameter

M.









2.4.5 Consolidation

As noted, for an applied shear stress below the critical shear stress deposition of

suspended matter will occur. As sediment accumulates, the self-weight of the particles

may crush the structure of the inter-particle contacts, which decreases the void volume

and increases the resistance of the bed to erosion.

For modeling consolidation, the general assumptions for the following equation

based on the conservation of water mass the equation of continuity for water are that the

bed is saturated and that the flow in the bed can be described by Darcy's Law:

-(pnV)+ (pn)= O (2.20)
84 at

in which p, is water density, V, is the water seepage velocity, n is the porosity and is

the vertical coordinate. Equation 2.20 is also called the Biot Equation for water and

solids. Considering the conservation of mass of the solids and assuming the solids

incompressible, the equation of continuity for the solids is:

S[p,(1- n)V]+ [p,1-(n)] =0 (2.21)
at

where p, in particle density and V, is the seepage velocity of the solids. The momentum

balance for water flow is:

khP = -n(V, V,)p g (2.22)


where kh is the hydraulic conductivity and p is the excess pore water pressure.

Combining Eqs. 2.20 and 2.21 and inserting Eq. 2.22 results in:

a kh ap 1 De
S -L I Q? (2.23)
8a pg aQ 1+eDt









where De/Dt is the material derivative of the void ratio, e [= n/(1-n)], and can be

expressed as

De Be Be
De= e + V (2.24)
Dt at s a

Note that for self-weight consolidation the total pressure, Otot,,, is equal to the

excess pore water pressure, p, plus the static pore water pressure, pw, plus the effective

stress, o'or (o=p + pw + o'). Finally expressing Eq. 2.23 in terms of the reduced

coordinates, q, we obtain the general equation for self weight consolidation also known

as the Gibson Equation:

L[ kh da' Be] P p d ( kh )ae e (2.24)
a7 p,,(l+e) de arq p, de l+e rI at

This is valid for the coordinate system shown in Figure 2.7.





-(zo, t) = o =
Zo Present Volume
z Initialbed of
A bed 8 height 877 solids
height
77



(a) (b) (c)

Figure 2.7 Reduced Coordinate System (a) Material or Langrangian coordinate system at
time, t=-; (b) Spatial or Eulerian coordinates at time, t; (c) Reduced coordinates at time, t.

For self-weight consolidation, to simplify the solution of the Gibson Equation

2.24, Been and Sills (1981) apply three basic assumptions:

(1) A linear relationship between the void ratio and the effective stress.










Ca'=A- (2.25)
a,

where A is a constant and the compressibility, av is also assumed constant.

(2) A linear relationship between the permeability and the void ratio.

kh = pko (1+ e) (2.26)

where ko is the initial permeability.

(3) A constant coefficient of consolidation, cy

kh do-'
Cf = -w(l ) = constant (2.27)
Sp, (1 + e) de

These assumptions reduce Eq. 2.24 to:

de d2e
d- c d (2.28)
dt f dq 2

The boundary conditions for the solution to the above equation are at the surface

the effective stress is zero and the void ratio at the surface remains constant:

e(zo,t)= ei (2.29)

where e, is the void ratio at the surface which is therefore equal to the initial void ratio,

eo. For the bottom boundary condition, void ratio at the bottom, z = 0, can be expressed

as

de(O, t)
= av(ps -p,)= ,v (2.30)
drq

where /fl is a constant. The initial condition is

e(z,0) = e, (2.31)

Using Laplace transform, the solution to Eq. 2.28 with the above boundary conditions,

Eqs. 2.29 and 2.30, and the initial condition, Eq. 2.31 one obtains









2n+1 7q
e(, t)= e,- 1 -2 2 exp 2- 1 2 c f t (2.32)
IL n=1 2n + 1 2 77,


From experimental observations, the effective stress at the surface is found not to be zero

but instead increases with the time of consolidation. Therefore, the void ratio at the

surface must decrease with consolidation. To realistically simulate consolidation an

imaginary overburden is included and its height is considered to depend on the difference

between the initial void ratio, ei and the ultimate void ratio, ee at the surface (Been and

Sills, 1981). The modified height of the bed, ,, is the sum of the original height and the

imaginary overburden height or

.m = r, + (e ee) (2.33)

Then, using 1m. instead of r7, in Eq. 2.32, the modified solution becomes:

2n +1 ;
cos--- -2
e(q,t)= e, ,1- cs/ -21 2 2r ex[- (2n+1 f t2 (2.34)
7lmo ni 2n+1 2 77.
I ( 2)















CHAPTER 3
DATA ANALYSIS

3.1 Introduction

Laboratory analysis of field data was done mainly at the Department of Civil and

Coastal Engineering's Coastal and Oceanographic Engineering Laboratory and the Soils

Laboratory. From these analyses, input parameters were obtained for modeling purpose

and are given in Section 3.6.

As noted earlier, much of the field data, in the form of bottom cores and (five)

shovel samples, were collected in the summer of 2001 when the lake was at an unusually

low level due to drought. These samples were analyzed for sedimentary parameters,

settling velocity parameters, erosion parameters and consolidation parameters for

modeling purposes. Each sample location was classified as consolidated and

unconsolidated during field sampling (Appendix).

3.2 Bottom Samples

Core samples were taken with a piston core barrel at transects across the lake

open water area (ECT, 2001). Core lengths averaged 1.41 m below top of mud layer

(Appendix). Sample location coordinates are given in Table 3.1 and are shown in Fig.

3.1. Samples in the exposed area were collected with a hand shovel.

Bathymetry was obtained over the lake during the time of sampling and the

contours are an estimate from locations and depths given in ECT (2001) for the lake at

low water level. Based on these contours the lake is conveniently divided into three









distinct areas. The inner open water area is deeper than 0.3 m, the outer open water area

which is shallower than 0.3 m and the exposed area is above the water level at the time of

sampling.


Table 3.1 Sample location coordinates
Sample ID Lat Long
5A 29.662041 -82.232417
5B 29.662001 -82.225155
5C 29.661946 -82.217213
5D 29.662360 -82.209655
4A 29.652611 -82.232754
4B 29.652658 -82.224720
4C 29.653002 -82.215039
4D 29.652733 -82.206271
3A 29.641728 -82.234122
3B 29.641823 -82.224667
3C 29.642107 -82.215023
3D 29.641954 -82.205985
2A 29.630943 -82.244785
2B 29.630899 -82.232208
2C 29.631073 -82.219799
2D 29.631156 -82.212113
1A 29.622649 -82.243081
1B 29.622811 -82.235576
1C 29.622838 -82.227713
1D 29.622891 -82.221490


3.3 Sediment Parameters

3.3.1 Densities

Determination of the wet bulk density, p was done by weighing a sample of mass,

Mw, and dividing by the volume, V, of the container. Typically, Pyrex beakers were


used to give a known volume:









M
p -
V


* Open-water coring sites
* Exposed bottom
sampling sites


(3.1)





















Land 4


Figure 3.1 Approximate locations for piston core barrel sampling along with depth
contours.









After drying the sample in the oven at 1050 C for 24 hours, the dry mass, MD was

taken to determine the dry bulk density pD:


P MD (3.2)
PD V

Particle or granular density ps could then be determined through mass balance

(Mehta et al.., 1994):


P = PDPW (3.3)
PD +Pw -P

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

3.3.2 Organic Content

The samples were then heated to 4000 C to determine organic content, OC, as loss on

ignition:

MD -M
OC= D A x100% (3.4)
MD

where MA is sample dry mass after ignition.

Tables 3.2 and 3.3 provide the densities and the organic content of (upper)

unconsolidated and (lower) consolidated samples. The upper core was placed inside a

watertight plastic canister and labeled u. The lower core (c) representing consolidated

material. Grain sizes are given in Table 3.4, and in Table 3.5 the density and organic

content values for the exposed area are given, based on the mean values of all five "land"

samples shown in Fig. 3.1.









T~h1 ~ 9I11rA11cnAirhat~rd mnitPri21 dlFnnitie~n 2nd AtrP~nic. content


.U I JI --UILLUV II- -IU VIIII --U -- -- r--- ------
Sample PD Ps OC
(kg/m3) (kg/m3) (kg/m3 (%)
1A-u 1068 149 1827 36
1B-u 1025 58 1769 50
IC-u 1040 112 1556 49
1D-u 1108 218 1985 35
2A-u 1041 68 2455 48
2B-u 1005 19 1316 56
2C-u 1009 66 1150 44
2D-u 1016 70 1289 52
3A-u 1020 54 1577 52
3B-u 1022 39 2240 52
3C-u 1021 36 2383 53
3D-u 1072 123 2422 36
4A-u 1048 82 2357 45
4B-u 1022 38 2400 50
4C-u 1019 32 2333 58
4D-u 1072 122 2452 45
5A-u 1043 75 2304 50
5B-u 1041 71 2333 51
5C-u 1022 37 2381 53
5D-u 1013 23 2308 55
Minimum 1005 19 1150 35
Average 1036 75 2042 48
Maximum 1108 218 2455 58


Table 32









Table 3.3. Consolidated material densities and organic content
Sample p Po Ps OC
(kg/m3) (kg/m3) (kg/m3) (%)
1A-c 1061 113 2153 17
1B-c 1019 81 1297 50
1C-c 1082 140 2414 32
1D-c 1114 185 2597 13
2A-c 1148 257 2353 23
2B-c 1063 108 2393 35
2C-c 1062 106 2407 37
2D-c 1092 156 2430 37
3A-c 1047 117 1676 52
3B-c 1029 67 1750 42
3C-c 1018 54 1523 48
3D-c 1092 188 1964 21
4A-c 1089 152 2408 33
4B-c 1044 73 2500 50
4C-c 1011 67 1205 46
4D-c 1091 160 2308 25
5A-c 1096 158 2541 .27
5B-c 1044 75 2400 50
5C-c 1099 166 2484 42
5D-c 1072 124 2396 32
Minimum 1011 67 1205 13
Average 1069 127 2160 36
Maximum 1148 257 2597 52









Table 3.4 Mean organic content and densities for inner, outer and exposed area.
Organic Bulk Dry Particle
Content Density Density Density
Area
OC p po ps
(%) (kg/m3) (kg/m3) (kg/m3)
Inner 51 1025 53 2025
Outer 47 1048 96 2059
Exposed 30 1156 267 2296


3.3.3 Grain Size

Grain size distribution was determined by wet-sieving the sample material

through a set of Tyler sieves and filtering out by vacuum-suction the residual suspension

consisting of particles finer than 74 microns the #200 sieve. The 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 so that the percent passing

each sieve could be calculated.


---~------~---









Table 3.5 Sample, d25, dso, d75, and sorting coefficient
Sample d25 dso d75s So
(mm) (mm) (mm)
1A-c 0.220 0.293 0.379 1.3
1B-c 0.009 0.030 0.128 3.7
1C-c 0.063 0.101 0.296 2.2
1D-c 0.248 0.327 0.417 1.3
1A-u 0.022 0.053 0.290 3.6
IB-u 0.009 0.029 0.124 3.7
1C-u 0.009 0.031 0.136 3.8
1D-u 0.029 0.061 0.291 3.2
2A-c 0.168 0.227 0.310 1.4
2B-c 0.029 0.061 0.291 3.2
2C-c 0.019 0.049 0.275 3.8
2D-c 0.019 0.049 0.278 3.8
2A-u 0.010 0.034 0.146 3.9
2B-u 0.008 0.017 0.072 2.9
2C-u 0.011 0.040 0.182 4.1
2D-u 0.009 0.026 0.112 3.5
3A-c 0.009 0.026 0.113 3.5
3B-c 0.013 0.043 0.210 4.0
3C-c 0.010 0.033 0.142 3.9
3D-c 0.189 0.254 0.336 1.3
3A-u 0.009 0.025 0.106 3.5
3B-u 0.009 0.025 0.107 3.5
3C-u 0.009 0.023 0.101 3.4
3D-u 0.020 0.050 0.290 3.8
4A-c 0.052 0.089 0.294 2.4
4B-c 0.009 0.029 0.124 3.7
4C-c 0.010 0.037 0.163 4.1
4D-c 0.143 0.198 0.306 1.5
4A-u 0.010 0.039 0.175 4.1
4B-u 0.009 0.028 0.123 3.7
4C-u 0.008 0.014 0.058 2.7
4D-u 0.010 0.039 0.175 4.1









Table 3.5 (continued) Sample, d25, dso, d75, and sorting coefficient
Sample d25 dso d75 So
(mm) (mm) (mm)
5A-c 0.123 0.173 0.304 1.6
5B-c 0.009 0.028 0.123 3.7
5C-c 0.013 0.042 0.206 4.0
5D-c 0.067 0.107 0.296 2.1
5A-u 0.009 0.030 0.128 3.7
5B-u 0.009 0.027 0.117 3.6
5C-u 0.009 0.022 0.097 3.3
5D-u 0.008 0.019 0.082 3.1

3.4 Settling Velocity

The settling column for measuring the settling velocity of initially suspended

sediment was a 2 m high, 100 mm diameter cylindrical tube with outlets at 0.05, 0.15,

0.3, 0.55, 0.8, 1.05, 1.3, and 1.55 m. Water samples (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. The purpose of these withdrawals was to determine the suspended sediment

concentration, C, as a function of elevation and time during the settling process.

The governing equation for suspended sediment concentration is:

aC a
(w C) =0 (3.7)
at 8z

where t is time and z is the vertical elevation coordinate (Mehta and Li, 2001). The

settling velocity ws is then be obtained from the equation above given the initial

condition:

C(z,t)= C(z,0) (3.8)

along with zero sediment flux conditions 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 computer program











named SET1.M written in MATLAB solved Equation 3.7 to find w, (Mehta and Li,

2001). In Fig. 3.2 the data (points) are plotted along with computed curves for different

OC.


1.00E-01


1.00E-02


1.OOE-03


S1.00E-04


1.00E-05



1.00E-06
0


.01


Concentration (kg/m3)


Figure 3.2 Settling velocity plot for lake sediment samples.

The settling velocity curves of Fig. 3.2 are used to determine the settling velocity

parameters for model input. Ten percent organic content is the top curve and increasing

the organic content by ten for each additional curve one ends with the sixty percent

organic content at the lower end. Each curve is represented mathematically on a log-log

scale by the following equation:



aC"
w =(C2 (3.9)
(C' +b )"

where a, b, n and m where determined to be 0.7, 7.9, 2.5, and 1.4, respectively and a free

settling limiting concentration was 0.1 kg/m3 with a velocity of 9x10"7 m/s, all for the


-- -- -- I I- -L- l- -l ^ --i i--- -I I T I -- r- 1- i_ l -





E L2 l -- - 3- i "1 r-

- -- --- -- ---- - -,-- L 121 -- -L L
-4






E rgan-c- it -,
- -- -- -
1 j a1m, o.









curve for 50% organic content, which is where most of the unconsolidated sample

averages fall.

Modeling was done based on the average values of organic content in the inner,

outer, and exposed areas corresponding to the curves for that organic content and

associated coefficients a, b, n and m. These are listed in Table 3.6.

Table 3.6 Settling velocity parameters.

Organic Settling Velocity Parameters
Content
(%) a b m n
10 0.7 1.6 2.5 1.4
20 0.7 2.4 2.5 1.4
30 0.7 3.7 2.5 1.4
40 0.7 5.8 2.5 1.4
50 0.7 7.9 2.5 1.4
60 0.7 10 2.5 1.4

3.5 Erosion Parameters

Sample erosion rates were obtained using the particle erosion simulator (PES)

(Tsai and Lick, 1986). The PES is essentially a 15.2 cm diameter perplex cylinder in

which suspended sediment concentration can be monitored over time. In the present tests,

samples are mixed with water from the lake inside a cylinder and allowed to settle and

then consolidate for up to 72 hours. A porous vertical grid was then oscillated at

different angular speeds (rpm), which effectively applied a shear stress on the bed. This

shear stress was obtained from the calibration relationship (Rodriguez et al., 1997):

applied =0.0005914rpm (3.9)

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

erosion rate s. Each shear stress was applied for one hour and concentration of










suspended sediment, C was taken at 0, 5, 15, 30 and 60 minutes. The erosion rate (flux)

was then calculated according to:

dC
F =h d (3.10)
dt

where h is the water depth and dC/dt is the time-rate of change of suspension

concentration. The corresponding erosion rate function is:

F, =M( ppied -r) (3.11)


For each sample, the erosion rate constant, M and the shear strength, r, were

obtained as follows: First, Fe values calculated from Eq. 3.10 were plotted against the

corresponding Tappiied calculated from Eq. 3.9, 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 Tappied axis gave the shear strength, r, (Mehta et al., 1994).

The data indicated that the erosion rate constant increased with increasing organic

content, while the shear strength decreased (Fig. 3.3).


0.3

" 0.25

0.2

S0.15

S0.1

W 0.05

0


0


0.05 0.1 0.15 0.2 0.25 0.3
Shear Stress (Pa)

Figure 3.3 Erosion rate vs. shear stress.


/ M=2.06 g/.s

60% O-ganic Content /

Ts=0.12 Pa --- '

--- =0.062 Pa 10% Or ganic Conter


** -






35


Following Fig. 3.3, the erosion shear strength was assumed to vary linearly with

the organic content by the following equation:

r, = -0.00129(OC)+ 0.138 (3.12)

Bounds of this equation are: rs= 0.062 Pa at 60% OC and 0.12 Pa at 10% OC.

3.6 Consolidation

3.6.1 Self-weight Consolidation

In self-weight consolidation tests carried out, the fall in the bed height was

measured as a function of time. Initially, settling occurred during which the bed-water

interface fell rapidly. Consolidation was considered to begin when there was a step-

decrease in the rate of fall. The results from one such test are given in Fig. 3.4. The height

Ro = 7.62 cm at the beginning of consolidation and Rloo = 4.57 cm at the end.


10000


10 100 1000
Time (min)

Figure 3.4 Self-weight consolidation of sample 4-c-u.


R=7.62 dmc:



RIO = 4.57 cm,:









The bulk density of the bed material after self-weight consolidation showed a

range similar to that of unconsolidated bed from core samples in Newnans Lake. As

noted, Been and Sills (1981) assumed a constant value of the coefficient of consolidation

equal to 8.2x10-3 mm/s. This value was used in the resent study to estimate bed

compressibility, av for self-weight consolidation with an initial uniform void ratio of e,=

65 and a final void ratio at the surface ee= 35. Note that for definitional purposes, the

void ratio e is

V, H H,
e i= V = = (3.13)
Vsolids Hs

where V, is the volume of the voids, Vso3ids is the volume of the solids, H is the bed height

and Hs is the height of the solid phase.

Bed compressibility was then assumed to obtain the best-fit curve for the data in

Fig. 3.4.

3.6.2 Overburden Consolidation

Two consolidation tests were run with overburden stresses of 25 and 50 kN/m2

(Figs. 3.5 and 3.6). The Terzaghi consolidation equation (e.g., Lambe and Whitman,

1969) is

c U aU e o-, (3.14)
SaZ2 at at

and can be derived from Eqs. 2.20 and 2.21. The coefficient of consolidation is estimated

from oedometer test according to

c 0.197(H50 /2)2 (3.15)
t50










in which Hso and tso are the height and time at 50% consolidation, respectively. The time

ts0 was determined by Casagrande's log method (Lambe and Whitman, 1969).

Permeability values calculated from Eq. 3.16 (Davidson, 1998) were found to be

very low (Table 3.7):


kh = c, w (3.16)
1+ eso


In Eq. 3.16, c, is the coefficient of consolidation determined by Eq. 3.15 and av is the bed

compressibility (which was determined graphically as the slope of the line in Fig. 3.7),

eso is the void ratio at 50% consolidation and Yw is the unit weight of water. The high

deflections that occurred under overburden were probably due to high velocity of the

solids and high seepage velocity of the water.

The results from the oedometer tests are given in Fig. 3.7 in order to determine

bed compressibility for the applied stresses of 25 and 50 kN/m2. Determination of H50,

tso and es0 are shown in Fig. 3.5 and 3.6. Results are summarized in Table 3.7.


0

-0.1
R o=-0.16
-0.2

-0.3

I -0.4

-0.5
R oo00-0.5E
-0.6

-0.7


0.1 1 t oo 10 100 1000 10000
t5o= 0.4 mm
Time (min)



Figure 3.5 Consolidation for an overburden stress of 25 kN/m2


-I-l-~f--,---. -II. rl---------- -- l-- llII, r--.---r, r



-I- -- C-- C--- - - - - - I (-- I~ CI -- (---l-( C



..Rso, Q:38 _?I1~'~ '''
-; -- IIrl -~C- ~ -~i~ -- ; 7 7~i i i I T 7 C~'''~' I~~~ ~~~ '''














-0.65
R o=-0.66 c]





-0.75
E





-0.85
Roo=-0.86





-0.95


t so=0.9 min


10 twoo

Time (min)


,...,.-,


1000


10000


Figure 3.6 Consolidation for an overburden stress of 50 kN/m2.


18.0

16.0

14.0

12.0

10.0

8.0

6.0- 0.25m F/kN

4.0

2.0

0.0
0 10 20 30 40 50 60

Pressure (kN/m2)




Figure 3.7 Determination of bed compressibility for overburden consolidation.


Table 3.7 Coefficients of consolidation and compressibility for overburden
consolidation.


t-- r rn rr
9 I I I I I


Rso',=2-0:7,61 r InI



[-'i i J ~ I -Ill


I II I t-I I rI IIl I I lI II


'''


----
n
















The results in Table 3.7 were used to estimate input parameters for modeling self-weight

consolidation. Note that the density in the oedometer were very close to densities of the

consolidated bed material in the lake, while densities of the unconsolidated material were

closer to those of the self-weight consolidated beds in the PES. Note also that at the lake

bed, incremental material may have deposited after self-weight consolidation of the

previously deposited material was complete. Hence, this additional load amounts to an

overburden.















CHAPTER 4
SEDIMENT RESUSPENSION POTENTIAL

4.1 Modeling Newnans Lake Resuspension

This chapter examines the resuspension potential of the bed material as a function

of the location in the lake area and water level. Model input parameters are given for the

Visual Fortran and MATLAB programs developed by Mehta and Li (2001). A uniform

depth and uniform mud layer thickness are assumed throughout the lake with an

estimated limiting wave height for resuspension of 0.2 m. This wave height was chosen

based on the limiting horizontal velocity needed to erode material at the assumed depth.

The lake was divided into three distinct regions based on bathymetry data

available from ECT (2001). From Fig. 4.1 we note that the inner open water area was

assumed to be that greater depth than 0.3 m, while the outer open water area was

shallower than the 0.3 m. The third area was the exposed, and will be submerged if and

when the lake rises by 1.1 m to its estimated mean stage. In the exposed area much of the

pore water has escaped due to desiccation.

Bed consolidation was modeled to fit laboratory test results and then the site-

specific parameters thus determined were used to estimate the time to reach densities of

consolidated core samples.












Land3

Open-water coring sites
Exposed bottom1.0 km
sampling sites
Exposed area




Lan

Land4

3








d5




Figure 4.1 Depth contours showing inner cores inside 0.3 m contour.

4.2 Resuspension Modeling

4.2.1 Model Calibration and Validation

The resuspension model was calibrated and then validated using data collected at

the lake as part of the study. For a 0.05 m wave height and 1 s period under a wind speed

of 3 m/s, the concentration was found to be 4 kg/m3 at a water depth of 0.25 m. These

data were used for calibration. Table 4.1 gives the model input parameter, and model

results are given in Fig. 4.2. For further details on modeling including parameter

definitions see Section 4.2.2.

The model was then validated based on data collected under the same wave

conditions at a site where the water depth of 0.3 m and where the concentration was 2


I






42


kg/m3. Table 4.1 gives the model input parameter, and model results are given in Fig.

4.3.


Table 4.1 Model calibration and validation input parameters.
Parameter Calibration Validation
h 0.25 0.3
hb 0.3 0.3
ngrids 25 30
stt 0 0
ett 300 300
dt 60 60
Ott 30 30
C1 0.1 0.1
Wsf 9x10-7 9x107
a 0.7 0.7
b 7.9 7.9
m 2.5 2.5
n 1.4 1.4
ao 0.5 0.5
f3o 0.33 0.33
H 0.05 0.05
T 1 1
a, 0.35 0.35
ps 2059 2059
p 1201 1201
pw 1000 1000
TDep 0.08 0.08
f 0.049 0.057
S1 1
xi 0 0
PD 96 96
0Pe 0 0
ae 0.08 0.08
f 0 0
smax 0.2 0.2
ar 18.2 18.2
b, 0.5 0.5
zi 0.1 0.1
Ci 4x10-3 4x10-
zi 0.2 0.2
Ci 4x10- 4x10-






































l0. 10L2 10-. 10 10' 102 0 5 10 15 20 25

Suspended Sediment Cone. (kg/m3) Suspended Sediment Cone (kg/m3)

Figure 4.2. Calibration run, with 90 minutes to reach 4 kg/m3 field concentration shown
by blue dot. Left plot is log-linear; right is linear scale.



3 III0.35



0.25 .... .

0.05 -... ................................ ............... .. ...............
0.2 . . .



0.15 ..........



0.3 -. .........
0 2 i . -- .. - -
01 ... ... I C..........








103 10- 101 10i 0 10 20 30 40 50
Suspended Sediment Cone. (kg/m) Suspended Sediment Cone. (kg/m)

Figure 4.3. Validation run, with 90 minutes to reach 2 kg/m3 field concentration shown
by blue dot. Left plot is log-linear; right is linear scale.









4.2.2 Effect of Wind Speed and Location on Resuspension

Selecting a water depth of h = 1.5 m and a mud thickness hb = 0.3 m, the water

column was divided into a number of vertical grids, ngrds= 15. The starting time of

simulation was sit= 600 min and ending time et= 600 min. The time step for calculation

dt= 60 and the output time step ot= 60 min. So for 600 min the model gave the suspended

sediment concentration profile at 60 min intervals.

Settling velocity parameters were given based on the test outlined in Chapter 3,

with the limiting free settling concentration C1= 0.1 kg/m3. The corresponding limiting

free settling velocity wsf = 9.0x10-7 m/s. The parameters for the curve identifying median

values for organic material were found to be a= 0.7, b= 7.9, m= 2.5 and n= 1.4.

The wave diffusion constant, aw= 0.35, was determined through the model

calibration and validation. Stabilized diffusion parameters ao= 0.5 and lo= 0.33 were

used throughout the modeling process.

The wave height H= 0.202 m and period T= 1.85 s were selected for the selected

8 m/s wind speed. A wave height of 0.209 m and 1.9 s period were used for the 9 m/s

wind speed, and 0.223 m and 2 s, respectively, for the 10 m/s wind speed.

Mean values of the unconsolidated sediment parameters was used for the inner

open water area were: the granular density Ps = 2,025 kg/m3, bed bulk density p= 1,201

kg/m3 and water density pw= 1,000 kg/m3. The bottom sediment was considered as a bed

and the critical stress for deposition rDep= 0.07 Pa. The bed shear strength for erosion, r,

is modeled by the following equation:

r, = a,(D- (D (4.1)









where 0 = PD / p, and ,e is the minimum value of 0. We will assume =0 and ae=

0.07 based on the laboratory results.

The wave bottom friction coefficient fw, which changes with wave height, was

determined for the conditions above to be 0.029, 0.027, and 0.023 respectively for the 8,

9 and 10 m/s wind speed. Assuming a linear bed density variation with depth, the bed

density parameters Q= 1 and xi= 0 to satisfy Eq. 4.5:


PD D hb i (4.5)


where hb is bed depth, pD is the bed average dry density and Az is the incremental depth

from the bed surface. The erosion rate constant is related to ,s according to (Li and

Mehta, 2001):

M = s.x exp(-arsbr) (4.6)

Based on laboratory test results (Table 4.2), Smax=0.2, ar=17.8, and br=0.5 were selected.

The initial concentration input was based on field information; for i=1, elevation

zi = 0.25 m, concentration Ci = 4.0x10-3 kg/m3, and for i=2, zi= 0.1 m and Ci = 4.0x103

kg/m3, where 4.0x10-3 kg/m3 is wash load which does not to settle out. The model input

data are summarized in Table 4.3.

Modeling at low water (depth of 1.5 m) (Fig. 4.4) indicated that material erodes at

a wind speed of 8 m/s and higher. This wind speed is estimated to produce waves of 0.2

m and a wave period of 1.85 s. Once this speed is attained erosion may take place. In

Fig. 4.5, the mid-depth concentration is plotted for each wind speed as a function of time.

In each case, erosion is rapid at the beginning; however, as time progresses the effect of

settling/deposition increases and the concentration approaches a constant value as the









rates of erosion and deposition approach equality and an equilibrium condition is

established. The results are summarized in Table 4.4.

Table 4.2 Mean sedimentary and erosion parameters for inner, outer and exposed areas.
Organic Bulk Dry Particle Bed Shear Erosion
Content Density Density Density Strength Rate Constant
Core OC p pD Ps rs M
Area Nos. (%) (kg/m3) (kg/m3) (kg/m3) (Pa) (g/N-s)
Inner 5-b-u 51 1041 71 2333 0.071 1.81
5-c-u 53 1022 37 2381 0.068 1.88
4-b-u 50 1022 38 2400 0.072 1.77
4-c-u 58 1019 32 2333 0.062 2.06
3-b-u 52 1022 39 2240 0.07 1.85
3-c-u 53 1021 36 2383 0.071 1.81
3-d-u 36 1072 123 2422 0.09 1.27
2-b-u 56 1005 19 1316 0.065 1.99
2-c-u 44 1009 66 1150 0.08 1.56
2-d-u 52 1016 70 1289 0.07 1.85
Average 51 1025 53 2025 0.07 1.80

Outer 5-a-u 50 1043 75 2304 0.072 1.77
5-d-u 55 1013 23 2308 0.066 1.95
4-a-u 45 1048 82 2357 0.079 1.59
4-d-u 45 1072 122 2452 0.079 1.59
3-a-u 52 1020 54 1577 0.07 1.85
2-a-u 48 1041 68 2455 0.075 1.7
1-a-u 36 1068 149 1827 0.09 1.27
1-b-u 50 1025 58 1769 0.073 1.77
1-c-u 49 1040 112 1556 0.079 1.74
1-d-u 35 1108 218 1985 0.092 1.24
Average 47 1048 96 2059 0.08 1.60

Exposed Landl 22 1151 253 2469 0.108 0.77
Land2 46 1069 121 1969 0.077 1.63
Land3 34 1138 240 2227 0.093 1.2
Land4 19 1242 407 2477 0.112 0.66
Land5 27 1181 312 2339 0.102 0.95
Average 30 1156 267 2296 0.10 1.00










Table 4.3 Inner o en water low water input parameters.
Wind 8 9 10
Speed (m/s) (m/s) (m/s)
h 1.5 1.5 1.5
hb 0.3 0.3 0.3
ngrids 15 15 15
stt 0 0 0
ett 600 600 600
dt 60 60 60
Ott 60 60 60
C1 0.1 0.1 0.1
wsf 9x10-7 9x107 9x107
a 0.7 0.7 0.7
b 7.9 7.9 7.9
m 2.5 2.5 2.5
n 1.4 1.4 1.4
ao 0.5 0.5 0.5
flo 0.33 0.33 0.33
H 0.202 0.209 0.223
T 1.85 1.9 2
a, 0.35 0.35 0.35
ps 2025 2025 2025
p 1201 1201 1201
pw 1000 1000 1000
rDep 0.07 0.07 0.07
fw 0.029 0.027 0.023
1 1 1
xi 0 0 0
PD 53 53 53
.e 0 0 0
ae 0.07 0.07 0.07
1 0 0 0
sma 0.2 0.2 0.2
ar 17.8 17.8 17.8
br 0.5 0.5 0.5
zi 0.5 0.5 0.5
Ci 4.0x103 4.0x10- 4.0x10
zi 1 1 1
Ci 4.0xl0"3 4.0xlO-3 4.0x10-3


































5 10
Suspended Sediment Cone. (kg/m3)


Figure 4.4 Resuspension at 8 m/s wind in the inner open water area at low water.





7.00E+00

6.00E+00 --

a 5.00E+00 -

a 4.00E+00 -.. -+- 8 (m/s)
| .. 9 (m/s)
S3.00E+00 -A 10 (m/s)

o 2.00E+00 -O .

1.00E+00

O.OOE+00


0 100 200 300 400
Time (min)


500 600 700


Figure 4.5 Inner open water area mid-depth concentration evolution at different wind
speeds, low water.









Table 4.4 Inner open water area mid-depth concentrations at 600 min; low water.
Water Wave Wave Organic Concentration
Depth Height Period Content at 600 min
(m) (m) (s) (%) (kg/m3)
1.5 0.202 1.85 51 2.4
1.5 0.209 1.9 51 4.2
1.5 0.223 2 51 6


Since Newnans Lake was at a very low state in Summer 2001, a suspension

estimate for the normal stage was made to examine whether dredging efforts would be

feasible for decreasing the potential for resuspension, thus improving the water quality.

The new modeling depth was changed to 2.6 m based on stage data given in Figure 2.1.

The new depth changed the wave height and period slightly, and also affected the wave

friction factor. At this depth the number of grid points (ngrids) was taken as 26. The new

wave heights for 8, 9 and 10 m/s wind speeds were 0.236 m, 0.234 m and 0.255 m,

respectively with respective 1.99 s, 2.00 s, and 2.10 s. These conditions yielded wave

friction factors of 0.051, 0.044, and 0.038, respectively. Table 4.5 summarizes the model

input parameters.

Figure 4.6 shows that during the high water condition a 10 m/s wind speed is

needed to suspend bed material in the inner open water area because of the increase in

water depth. Lines corresponding to the 8 and 9 m/s wind speeds are not plotted because

no material was resuspended. See also Table 4.6.









Table 4.5 Inner o en water high water input parameters.
Wind 8 9 10
speed (m/s) (m/s) (m/s)
h 2.6 2.6 2.6
hb 0.3 0.3 0.3
ngrids 26 26 26
Stt 0 0 0
ett 600 600 600
dt 60 60 60
ott 60 60 60
Cl 0.1 0.1 0.1
Wsf 9x10-7 9x10-7 9x10-7
a 0.7 0.7 0.7
b 7.9 7.9 7.9
m 2.5 2.5 2.5
n 1.4 1.4 1.4
ao 0.5 0.5 0.5
flo 0.33 0.33 0.33
H 0.226 0.234 0.255
T 1.9 2 2.1
ao, 0.35 0.35 0.35
ps 2025 2025 2025
p 1201 1201 1201
pw 1000 1000 1000
tDep 0.07 0.07 0.07
fw 0.051 0.044 0.038
1 1 1
xi 0 0 0
PD 53 53 53
e o 0 0 0
ae 0.07 0.07 0.07
p 0 0 0
Smax 0.2 0.2 0.2
a, 17.8 17.8 17.8
b, 0.5 0.5 0.5
zi 0.5 0.5 0.5
Ci 4.0x10- 4.0x103 4.0x103
zi 1 1 1
Ci 4.0x10-3 4.0x10-' 4.0x10











1.20E-01

1.00E-01

8.00E-02

6.00E-02

4.00E-02

2.00E-02

0.OOE+00


-A- 10 (m/s)


0 100 200 300 400 500 600 700
Time (min)

Figure 4.6 Inner open water area suspended sediment evolution at high water.
Resuspension occurred 10 m/s wind speed only.

Table 4.6 Inner open water area mid-depth concentrations at 600 min; high water.
Water Wave Wave Organic Concentration
Depth Height Period Content at 600 min
(m) (mS)(s) (%) (kg/m3)
2.6 0.226 1.9 51 4x10-3
2.6 0.234 2 51 4x10-
2.6 0.255 2.1 51 lx10-1


Model input parameters for the outer open water area are given in Table 4.7.

Note that the high water depth changed to 1.25 m, because the mean depth inside the 0.3

m contour was 0.15 m at low water. Note also that due to this very shallow low water

depth, resuspension at low water was not examined for the outer pen water area. Results

for high water are shown in Fig. 4.8.


/


--A -A- ____ _____ _____ ____


1


-

-









Table 4.7 Outer o en water high water input parameters.
Wind 8 9 10
speed (m/s) (m/s) (m/s)
h 1.25 1.25 1.25
hb 0.3 0.3 0.3
ngrids 26 26 26
Stt 0 0 0
ett 600 600 600
dt 60 60 60
Ott 15 15 15
C1 0.1 0.1 0.1
wsf 9x10-7 9x 107 9x10-7
a 0.7 0.7 0.7
b 7.9 7.9 7.9
m 2.5 2.5 2.5
n 1.4 1.4 1.4
ao 0.5 0.5 0.5
fo 0.33 0.33 0.33
H 0.226 0.234 0.255
T 1.9 2 2.1
a. 0.35 0.35 0.35
ps 2059 2059 2059
p 1201 1201 1201
pw 1000 1000 1000
TDep 0.08 0.08 0.08
f, 0.021 0.018 0.016
S 1 1 1
xi 0 0 0
PD 96 96 96
Pe 0 0 0
ae 0.08 0.08 0.08
P 0 0 0
Sma 0.2 0.2 0.2
ar 18.2 18.2 18.2
br 0.5 0.5 0.5
zi 0.5 0.5 0.5
Ci 4.0x10- 4.0x103 4.0x10
zi 1 1 1
Ci 4.0x10-3 4.0x10- 4.0x103











1.20E+01

1.00E+01 -

S8.00E+00 -
S --- 8 (m/s)
S6.00E+00 --- 9 (m/s)
|- 10(m/s)
I 4.00E+00 -

2.00E+00

0.00E+00
0 100 200 300 400 500 600 700
Time (min)

Figure 4.7 Outer open water area mid-depth concentration evolution at different wind
speeds, high water.


Table 4.8 Outer open water area mid-depth concentrations at 600 min; high water.
Water Wave Wave Organic Concentration
Depth Height Period Content at 600 min
(m) (m) (s) (%) (kg/m3)
1.25 0.226 1.9 47 7
1.25 0.234 2 47 7.9
1.25 0.255 2.1 47 10


From Fig. 1.3 estimate of mean lake stage, the exposed area was assumed to have

a water depth of 1.1 m. Waves generated by the fetch (3,000 m) and mean depth for the

open water area were used. The wave bottom friction factor values were found to be

0.027, 0.022, and 0.018 for 8, 9 and 10 m/s wind, respectively. Since the exposed area

had higher dry densities due to exposure, an erosion rate constant value of M = 1.1 g/N-s

was used with a bed shear strength rs = 0.1 Pa based on the mean values listed in Table

4.2.











Table 4.9 Exposed area model input parameters at high water.
Wind 8 9 10
speed (m/s) (m/s) (m/s)
h 1.1 1.1 1.1
hb 0.3 0.3 0.3
ngrids 11 11 11
Stt 0 0 0
ett 600 600 600
dt 15 60 60
Ott 60 60 60
C1 0.1 0.1 0.1
wsf 4.0x10 4.0x105 4.0x10-
a 0.7 0.7 0.7
b 3.7 3.7 3.7
m 2.5 2.5 2.5
n 1.4 1.4 1.4
ao 0.5 0.5 0.5
fo 0.33 0.33 0.33
H 0.226 0.234 0.255
T 1.9 2 2.1
a, 0.35 0.35 0.35
ps 2296 2296 2296
p 1201 1201 1201
pw 1000 1000 1000
TDep 0.1 0.1 0.1
S 0.019 0.017 0.014
S 1 1 1
xi 0 0 0
Po 267 267 267
0e 0 0 0
ae 0.1 0.1 0.1
p 0 0 0
max 0.2 0.2 0.2
ar 16.75 16.75 16.75
br 0.5 0.5 0.5
zi 0.5 0.5 0.5
Ci 4.0x10-3 4.0x10- 4.0x10-
zi 1 1 1
Ci 4.0x10'3 4.0x103 4.0x10-











9.00E+00

8.00E+00

7.00E+00

S6.00E+00 -

a 5.00E+00 --- 8 (m/s)
S.-- 9 (m/s)
S4.OOE+00 -
au JV 2.00 10 (m/s)
S3.00E+00

2.00E+00

1.00E+00
1.00E+00 ---- ------ --

0.00E+00
0 100 200 300 400 500 600
Time (min)

Figure 4.8 Exposed area mid-depth concentration evolution at different wind speeds, high

water.


Table 4.10 Exposed area mid-depth concentrations at 600 min; high water.
Water Wave Wave Organic Concentration
Depth Height Period Content at 600 min
(m) (m) (s) (%) (kg/m3)
1.1 0.226 1.9 30 1.2
1.1 0.234 2 30 7
1.1 0.255 1 30 8.1


Conclusions can be drawn relative to the potential for sediment resuspension in

the three sub-areas of the lake. Figures 4.10, 4.11 and 4.12 summarize plots of the time-

evolution of mid-depth concentration. All plots are for the high water condition. The

outer pen water area seems to have the greatest suspension concentration for the lake as a

whole at -1 kg/m3.












S1.00E+01



~ 1.00E+00



S1.00E-01



1.00E-02



j 1.OOE-03


-- Inner Area
-u- Outer Area
-*- Exposed Area


0 100 200


300 400


500 600


Time (min)


Figure 4.9 Mid-depth suspended sediment concentration in the three sub-areas at 8 m/s
wind.


1.00E+01



1.00E+00



-1.00E-01



1.00E-02



1.00E-03


z .- I -; z

------------ li--- -I-------------- -- -------

---- - -

/ l - - - - - - - - -


-- Inner
Area



-- Outer
Area



SExposed
Area


0 100 200 300 400 500 600

Time (min)


Figure 4.10 Mid-depth suspended sediment concentration in the three sub-areas at 9 m/s
wind.


-- --


-- ------ -------------- ----------- ---- -
- - - -- - - - -
---"" ------ --------'---- ------
- - -- -- - - - - - -




--- -
- - - - -











1.00E+01


S 1.00E+00






1.OOE-02

01.00E-03
Sl.OOE-03


-i


x

-k
- - - - - - --
- -- -- -- -- ------- -


- --


-i_- _-_ -- _- _- -- -


0 100 200 300 400 500 600

Time (min)

Figure 4.11 Mid-depth suspended sediment concentration in the three sub-areas at 10 m/s
wind.

4.2.3 Effect of Dredging on Resuspension

To improve water quality in the lake, one proposal calls for dredging the

unconsolidated bed layer in order to decrease the resuspended sediment in the water

column. Assuming the 0.3 m unconsolidated layer has been removed a new depth of 2.9

m at high water can be used. This new depth would increase the wave height slightly; the

new input parameters are shown in Table 4.12. Since the newly exposed bed material

will then have properties associated with the consolidated layer, the mean organic content

of the consolidated samples may be used to determine the input parameters. From this

organic content the shear strength for erosion is obtained from a linear equation based on

laboratory results:

r, =-0.001290C+0.138 (4.2)

The erosion rate constant can also be determined through the mean organic

content from:


Inner
Area


-.- Outer
Area


-Expose
d Area









M = 0.0360C-0.015


From these relationships, rs and M were determined to be 0.09 Pa and 1.27 g/N-s,

respectively. These gave a rDep = 0.09, ae = 0.09, and an a, =16. The mean consolidated

bed parameters are given in Table 4.11, and model input parameters in Table 4.12.


Table 4.11 Consolidated bed properties.
Sample p pD Ps Organic Content
(kg/m3) (kg/m3) (kg/m3) (%)
1A-c 1061 113 2153 17
1B-c 1019 81 1297 50
1C-c 1082 140 2414 32
1D-c 1114 185 2597 13
2A-c 1148 257 2353 23
2B-c 1063 108 2393 35
2C-c 1062 106 2407 37
2D-c 1092 156 2430 37
3A-c 1047 117 1676 52
3B-c 1029 67 1750 42
3C-c 1018 54 1523 48
3D-c 1092 188 1964 21
4A-c 1089 152 2408 33
4B-c 1044 73 2500 50
4C-c 1011 67 1205 46
4D-c 1091 160 2308 25
5A-c 1096 158 2541 27
5B-c 1044 75 2400 50
5C-c 1099 166 2484 42
5D-c 1072 124 2396 32
Minimum 1011 67 1205 13
Average 1069 127 2160 36
Maximum 1148 257 2597 52


(4.3)









Table 4.12 Dredged bottom model input parameters.
Wind 10 11 12
speed (m/s) (m/s) (m/s)
h 2.9 2.9 2.9
hb 0.3 0.3 0.3
ngrids 29 29 29
Sit 0 0 0
ett 600 600 600
dt 60 60 60
ott 60 60 60
C1 0.1 0.1 0.1
wsf 4.2x106 4.2x10-6 4.2x10-
a 0.7 0.7 0.7
b 5.8 5.8 5.8
m 2.5 2.5 2.5
n 1.4 1.4 1.4
ao 0.5 0.5 0.5
Ao 0.33 0.33 0.33
H 0.26 0.292 0.305
T 2.17 2.34 2.45
a,, 0.35 0.35 0.35
ps 2160 2160 2160
pm 1201 1201 1201
pw 1000 1000 1000
rTep 0.09 0.09 0.09
S 0.04 0.029 0.025
1 1 1
xi 0 0 0
PD 127 127 127
,e 0 0 0
a, 0.09 0.09 0.09
p 0 0 0
smax 0.2 0.2 0.2
ar 16 16 16
br 0.5 0.5 0.5
zi 0.5 0.5 0.5
Ci 4.0x103 4.0x10- 4.0x103
zi 1 1 1
Ci 4.0x10- 4.0x10- 4.0x10-














__ __ _A_
,n


/ ,*


.,a___
3/


--11 (m/s)
-- 12 (m/s)


0 100 200 300 400 500 600 700
Time (min)

Figure 4.12 Mid-depth suspended sediment concentration evolution with time; dredged
bottom.

From Fig. 4.12 and Table 4.13 we observe that dredging the lake would have a

positive affect on reducing turbidity. Larger wind speeds would be needed to acquire the

wave height necessary to significantly resuspend the bed material at the new depth.

Specifically, by taking out the 0.3 m unconsolidated bed material a wind speed of 11 m/s

would be needed to erode bed material.

Table 4.13 Dredged lake mid-depth concentrations at 600 min.
Water Wave Wave Organic Concentration
Depth Height Period Content at 600 min
(m) () (s) (%) (kg/m3)
2.9 0.26 2.17 36 4x10-3
2.9 0.292 2.34 36 0.08
2.9 0.305 2.45 36 0.16

4.3 Consolidation

For modeling consolidation (Mehta and Li, 2001), based on a series of laboratory

tests, Been and Sills (1981) recommend a mean coefficient of consolidation is 8.2x103

mm2/s as being suitable for fine sediment settlement and associated density predictions.

The initial bed height was taken as 0.076 m. The initial (uniform) void ratio ei and the


1.80E-01
1.60E-01
1.40E-01
E
S1.20E-01
1.00E-01
E 8.00E-02
6.00E-02
0 4.00E-02
2.00E-02
n nnF+nn











final void ratio ee at the surface were selected as 65 and 35, respectively. The best-fit bed


compressibility av was found to be 1.0 for a 48-hour consolidation duration (tt) with a


model output interval (ot) of 1 hour [see Mehta and Li (2001) for symbol definitions].


Model outputs along with data comparison are shown in Fig. 4.13.


0.08


dt = 1 (hr)
0.07
t final = 48 (hr)

0.06 I = initial density profile
F = final density profile

0.05


0.04


0.03
F

0.02


0.01


0 I I I I I I II



re .102 104 106 Se ens 108 og:1030 1032 103o 1036d i 10proie wih time.














Brenner and Whitmore (1998).
Brenner and Whitmore (1998).







62


Table 4.14 Model input parameters for 100-year self-weight consolidation simulation.

Parameter Value

di 1.3

ei 25

ee 5

cf 8.00x10-9

a, 0.0025

ps 2025

tt 876000
ot 87600

ngrids 13

1.4

dt = 87600 (hr)
1.2- Brenner and Whitmore
(1998) t final= 876000 (hr)
I = initial density profile
1 F = final density profile
1,078 kg/m3 average bulk density o
S consolidated samples Appendix A
0,o

[0.6 -

F

0.4

10 1 0
0.2 ye rs y s


0 IL I I I
1030 1040 1050 1060 1070 1080 1090
Density (kg/m3)


Figure 4.14 100-year self-weight consolidation of bed material with each line
representing 10 years.















CHAPTER 5
CONCLUSIONS

5.1 Summary

Newnans Lake in north-central Florida has been placed on the restoration priority

list for the past twelve years, and has had a hyper-eutrophic rating since the early 1970's.

A significant cause of this problem is believed to be resuspension of bottom "muck" in

the lake under the action of episodic wind waves. The objective of this study was to

model wind-wave induced resuspension of organic-rich fine sediment in the lake in order

to determine the correlation between wind speed and suspended sediment load. Such an

assessment would facilitate the development of strategies for lake restoration. In order to

meet this objective, bottom sediment samples and some suspended sediment

concentration measurements in the lake were analyzed and laboratory tests on sediment

erosion, settling and consolidation were conducted in the laboratory. Results from these

studies were then used to run a 1-D vertical model on wave-induced resuspension as well

as a model for self-weight consolidation in order to simulated suspended sediment load as

a function of location and water level in the lake and wind speed and bed strength. In

addition, the effect of dredging the bottom in reducing resuspension was examined.

5.2 Conclusions

The following main conclusions have been derived:

Newnans Lake bed material is highly organic (13-58%) with very low bed
densities (1,009-1,148 kg/m3).









A settling velocity function was developed to account for differences in the
organic content of the suspended matter. It was found that the higher the
organic content the lower the settling velocity.

Bed strength and erosion rates showed dependence on organic content. As the
organic content increased the bed shear stress needed to resuspend material
decreased.

Significant resuspension of bed material is likely when wind speed exceeds -8
m/s.

Since the inner open water area is deeper than the outer open water area,
resuspension there is less likely, especially at the "normal" lake stage than
during the low water depth as occurred in Summer 2001. A wind speed of
around 10 m/s is required to resuspend the bed material in the inner area under
a normal water level.

Dredging out the unconsolidated top layer of the bed, which is on the average
-0.3 m in thickness, would reduce the likelihood of resuspension.

5.3 Recommendations For Future Work

It seems 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 better calibrate and validate modeling for lake turbidity.

Accurately modeling the strength of the bed will require the use of fully non-

linear Gibson's equation for consolidation.

Spectroscopy may be used to determine the structure of the organic material and

sediment that remains after loss of ignition. The USGS has begun a data base on sites in

South Florida, especially in Florida Bay and other restoration projects to determine

nutrients and elements in the pore water (http://sofia.usgs.gov/).
















APPENDIX
CORE THICKNESS AND BED DENSITIES

Data on core locations and associated water depth, bottom sediment thickness,

core length and sub-lengths of the unconsolidated and consolidated segments for each

sample is given in Table A. 1.

Table A.1 Core locations and data

Core
Water Sediment Core C
Depth Thickness Length Sun-lengt Sample
Latitude Longitude (m) (m) (m)) No.
29.630943 -82.244785 0.06 1.63 1.3 0.25 2AU.
1.05 2AC
29.630899 -82.232208 0.28 3.34 1.74 0.22 2BU
1.52 2BC
29.631073 -82.219799 0.15 4.43 1.71 0.26 2CU
1.45 2CC
29.631156 -82.212113 0.10 1.45 1.05 0.23 2DU
0.82 2DC
29.641728 -82.234122 0.07 1.67 1.29 0.14 3AU
1.15 3AC
29.641823 -82.224667 0.22 3.28 1.66 0.37 3BU
0.99 3BC
29.642107 -82.215023 0.52 3.33 1.50 0.30 3CU
1.20 3CC
29.641954 -82.205985 0.05 1.44 1.12 0.16 3DU
0.96 3DC
29.622649 -82.243081 0.01 1.31 1.27 1AU
1AC
29.622811 -82.235576 0.08 1.54 1.15 0.40 1BU
0.75 1BC
29.622838 -82.227713 0.17 1.58 1.21 0.23 1CU
0.98 1CC
29.622891 -82.221490 0.29 1.38 1.22 0.24 1DU
0.98 1DC
29.662041 -82.232417 0.05 1.80 1.27 0.40 5AU
0.87 5AC













Table A. 1 Core locations and data (continued)
29.662001 -82.225155 0.07 3.03 1.63 0.22 5BU
1.41 5BC
29.661946 -82.217213 0.14 3.64 1.76 0.29 5CU
1.47 5CC
29.662360 -82.209655 0.06 1.84 1.22 0.61 5DU
0.61 5DC
29.652611 -82.232754 0.05 2.75 1.50 0.22 4AU
1.28 4AC
29.652658 -82.224720 0.27 4.03 1.64 0.35 4BU
1.29 4BC
29.653002 -82.215039 0.46 4.44 1.59 0.38 4CU
1.21 4CC
29.652733 -82.206271 0.04 1.62 1.45 0.15 4DU
__1.30 4DC


Core densities for unconsolidated and consolidated

mean elevations of the sub-lengths are plotted in Fig. A.1.


samples at the respective


* Core data (upper layer) U Core data (lower layer) O PES Bed Density





"____ ml



-07Q n- ------------------ ------------
Pvg= 1,036 kmkg



pav = 1,078 kgn3


-10.
1000


1040


1080
Bulk density (kg/m3)


1120


1160


Figure A. 1 Core densities for unconsolidated and consolidated samples at the respective
mean elevations of the sub-lengths.


u.



-0.4

-0.8
0
o -0.8


/\ f\


i


I #
















LIST OF REFERENCES

Been, K., and Sills, G. C., 1981. Self weight consolidation of soft soils: an experimental
and theoretical study. Geotechnique, 31 (4), 519-535.

Brenner, M., and Whitmore, T. J., 1998. Historical sediment and nutrient accumulation
rates and past water quality in Newnans Lake. Final Report, St. Johns Water
Management District, Palatka.

Burt, T. N., 1986. Field settling velocities of estuary muds. In: Estuarine Cohesive
Sediment Dynamics, A. J. Mehta ed., Springer-Verlag, Berlin, 251-265.

Davidson, J. L., 1998. Soil Mechanics Laboratory Manual. Department of Civil and
Coastal Engineering, University of Florida.

Department of the Army. 1984. Shore Protection Manual. U. S. Government Printing
Office, Washington, DC.

Effler, S. W., 1996. Limnological and engineering analysis of a polluted urban lake.
Springer-Verlag, New York.

Environmental Consulting and Technology, Inc., (ECT), 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, Englewood Cliffs, NJ.

Federico, A. C., Dickinson, K. G., Kratzer, C. R., and Davis, F. E., 1981. Lake
Okeechobee water quality studies and eutrophication assessment. Technical Publication
#81-2, Resource Planning Department, West Palm Beach, FL.

Ganju, N. K., 2001. Trapping organic-rich fine sediment in an estuary. M. S. thesis,
University of Florida, Gainesville.

Gottgens, J. F., and Montague, C. L., 1987. Orange, Lochloosa, and Newnans Lake: A
survey and preliminary interpretation of environmental research data. Final Report,
University of Florida, Gainesville.

Holcomb, D. E., 1993. Study 6262 Newnans Lake restoration. Completion Report
Statewide Lake Restoration, Bureau of Fisheries Management.










Holly, J. B., 1976. Stratigraphy and sedimentary history of Newnans Lake. M. S. thesis,
University of Florida, Gainesville.

Jonsson, I. G., 1966. Wave boundary layer and friction factors. Proceedings of the 10h
Coastal Engineering Confrence, Vol. 1 ASCE, New York.

Jorgensen, S. E., 1980. Lake management. Pergamon Press, New York.

Krone R. B., 1962. Flume studies of the transport of sediment in estuarial shoaling
processes. Final Report, Hydraulic Engineering Laboratory and Sanitary Engineering
Research Laboratory, University of California, Berkley.

Lambe T. W. and Whitman R. V., 1969. Soil mechanics, Wiley, New York.

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.

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.

Tsai, C. H., and Lick, W., 1986. A portable device for measuring sediment resuspension.
Journal of Great Lakes Research, 12 (4) 314-321.

Wanielista, M. P., 1978. Stormwater Management. Ann Arbor Science, Ann Arbor, MI.






69


Wetzel, R. G., and Likens, G. E., 2000. Limnological analyses. Springer-Verlag, New
York.

Young, I. R., 1997. The growth rate of finite depth wind-generated waves. Coastal
Engineering, 23, 181-195.

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, 47-77.















BIOGRAPHICAL SKETCH

Jason Eric Gowland was born in Winter Garden, Florida, husband to Monica Erin

Henry, the son of Jan and Deborah Gowland and brother to Jessica Gowland. The author

attended Vero Beach High School and was a member of two undefeated Class 6A

football teams. As a member of the Vero Beach High School track team, he earned a

spot as All Cape Coast Conference for pole vaulting. Enrolling at the University of

Central Florida as a Lead Scholar and Florida Academic Scholar he helped in Central

Florida's transition to become a Division A program for two years then transferred to the

University of Florida to pursue a degree in civil engineering. After graduating with a

Bachelor of Science in Civil Engineering from the University of Florida, the author

proceeded to continue his education in the Department of Civil and Coastal Engineering

for a master's degree. Upon completion of a master's degree the author plans to practice

as a professional in his field in hopes to admire the world's natural beauty that he has

spent a lifetime learning to understand and build upon.




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