University of Mississippi University of Mississippi

eGrove eGrove

Honors Theses Honors College (Sally McDonnell Barksdale Honors College)

2011

Hydrogeologic Analyses for an Earthen Dam Proposal at the Hydrogeologic Analyses for an Earthen Dam Proposal at the

University Field Station University Field Station

Maria Theresa Brown

Follow this and additional works at: https://egrove.olemiss.edu/hon_thesis

Recommended Citation Recommended Citation Brown, Maria Theresa, "Hydrogeologic Analyses for an Earthen Dam Proposal at the University Field Station" (2011). Honors Theses. 1958. https://egrove.olemiss.edu/hon_thesis/1958

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HYDROGEOLOGIC ANALYSES FOR AN EARTHEN DAMPROPOSAL AT THE LFNIVERSITY FIELD STATION

byMaria T. Brown

A thesis submitted to the faculty of The University of Mississippi in partial fulfillment ofthe requirements of the Sally McDonnell Barksdale Honors College.

Oxford

May 2011

Approved by

^^^isor: Associate Promisor Joel KuszmaulJReader: Associate Professor Adnan Aydin

Reader: Professor John O’Haver

ACKNOWLEDGEMENTS

1 would like to acknowledge the work of my team members and the assistance of my

professors.

For the initial site characterization, my classmate Jon Sochovka led a student team

consisting of Charlie King, Drew Haight, and me. During this process. Dr. Holt provided

fimi guidance to direct us in our initial investigations.

Similarly, for the dam feasibility study and dam proposal, Steven Tidwell led Caleb

James, Michael Magee, and me. Dr. Kuszmaul left room for our creative problem-solving

but also worked alongside our Teaching Assistant Steven Fox to provide constructivecriticism of our work.

None of this work would have been possible without the collaboration of the individualslisted above.

11

ABSTRACT

MARIA T. BROWN: Hydrogeologic Analyses for an Earthen Dam Proposal at theUniversity Field Station

(Under the direction of Joel Kuszmaul)

In response to a request for a new dam at the University of Mississippi Field

Station (UMFS), BTJM Engineering prepared an initial feasibility assessment and

continued investigations until a full construction plan was completed. This report details

the hydrogeologic analyses involved in this process.

Initial site investigations included mapping surficial geology, creating a generalized

stratigraphic column, mapping the potentiometric surface, and developing a conceptual and

quantitative understanding of the local hydrologic budget. This data informed decisions

regarding the location of a new dam, which was finally designed with full considerations

of the local hydrogeology. After the dam was designed, analysis of the seepage, water

level response to storm events, water supply, and inundation potential were performed.

Ill

TABLE OF CONTENTS

LIST OF FIGURESINTRODUCTIONCHAPTER I: SITE CHARACTERIZATIONCHAPTER II: RESERVOIR SELECTION AND CHARACTERISTICSCHAPTER III: HYDROLOGIC ANALYSES OF PROPOSED DAM....BIBLIOGRAPHYAPPENDIX A. SOIL ANALYSESAPPENDIX B. RESERVOIR CHARACTERISTICSAPPENDIX C. WATER LEVEL RESPONSE

V

14131926324451

IV

LIST OF FIGURES

Figure I-1. Location of the UMFS within Lafayette County, Mississippi.Figure I-1. UMFS Soil MapFigure 1-2. UMFS Stratigraphic ColumnFigure 1-3. Soil Sampling MapFigure 1-4. UMFS Conceptual Hydrogeologic ModelFigure 1-5. UMFS Potentiometric Surface MapFigure 2-1. Proposed Reservoir and Dam LocationFigure 3-1. Flownet through the embankmentFigure 3-2. Flownet under the embankmentFigure 3-3. Inundation Map

2678101114202023

V

INTRODUCTION

BTJM Engineering has been contracted by the University of Mississippi to design

the construction specifications for a dam to be built at the Field Station (UMFS). The

client expects the dam to provide both a new area to conduct research and a source of

water to the other research ponds during the dry summer months. In response to initial

feasibility studies and client feedback, a location for a new reservoir was chosen, and a

dam was designed specifically for this site.

Site Background

The University of Mississippi Field Station is located approximately 11 miles

northeast of the Oxford campus (Figure I-l). Established in 1985, the 740-acre property

is used primarily for experimental activities and research in various disciplines including

biology, geology, engineering, toxicology and pharmacology (UMFS Staff, 1997).

The site was originally a spring-fed marsh area that was developed into a minnow

fann, supplying the baitfish industry. The farming operation was ended in 1980, and the

majority of ponds were reclaimed when the site was purchased by the University in 1985

and converted into a research station. Originally used only for the biology department,

the scope of studies at the UMFS was broadened, in 1995, to include geology, toxicology,

engineering, and limnology.

The UMFS contains an existing dam that forms Old Bramlett pond. This dam,

which is a short distance upstream from the proposed location of the new reservoir, was

Figure 1-1. Location of the UMFS within Lafayette County, Mississippi.

2

built by the employees of the UMFS and is composed of materials from the hillside

above the pond. The materials were excavated, transported, and placed using a tractor and

then compacted by repeatedly driving the tractor over the soil. The pond, which is spring-

fed and unlined, holds water year round according to Mark Baker, the resident director of

the UMFS (personal communication, 2011). According to UMFS employees, the only

major problem with the existing dam is the presence of beavers on site. The activities of

these creatures cause the employees to have to check and unblock the spillways daily.

However, removing the beavers is not an option due to the nature of the biological studies

at the UMFS.

3

CHAPTER I: SITE CHARACTERIZATION

(CO-AUTHORS: C. KING, S. SOCHOVKA, S. TIDWELL)

Regional Geology and Hydrology

The UMFS is located within an outcrop of the Meridian, which is regionally

composed of rust red, medium to coarse-grained sands with subordinate clay layers.

According to Attaya (1951), the Meridian is white sand in the upper portion which grades

downward into a rusty-brown or rusty-red sand, cross-bedded to evenly stratified with

lighter colored sands, extremely well sorted, and uniform in its properties. This formation

was deposited during the early Eocene in a shallow marine environment and now serves

as the primary groundwater source for the UMFS and surrounding areas.

The aquifer in the Meridian is called the Meridian-Upper Wilcox aquifer and has

a varying thickness from 50 to 200 ft with well water levels 50 to 200 ft below the

surface. The groundwater is soft and cool, between 58 and 64 degrees, due to its shallow

depth. The ground water has a low pH below seven and low concentrations of dissolved

solids (Moyse and Swann, 1997).

The Wilcox Group, which underlies the Meridian, is gray, poorly bedded clay

with pyrite, lignite, low plasticity and some slickensides (Moyse and Swann, 1997).

Local Geology

The UMFS is located on the flood plain and valley walls of Bay Springs Branch, a

tributary of Puskus Creek, which flows into the Little Tallahatchie River. The UMFS has

a total relief of 140 ft, ranging from a high elevation of 540 ft above mean sea level

(AMSL), to a low elevation of 400 ft. The local geology is dominated by sands, silts, and

clays. Field mapping showed that surface exposures at the site consist of four soil units:

1) Upper Sand, SC, 2) Silt Loam, ML, 3) Middle Sand, SC, and 4) Lower Clay, CL

(Figure 1-1). The Wilcox and the Lower Sand were not exposed at the surface. Based on

data gathered by Moyse and Swann (1997), as well as data from our own well, the Upper

Sand extends from highest elevations to 470 ft, the Silt Loam from there to 440 ft, the

Middle Sand to 410 ft, the Lower clay to 385 ft. Lower Sand to 360 ft, with the Wilcox

extending to an undetermined depth below these units (Figure 1-2).

To further refine previous investigations, hand samples were taken at nine

locations near the proposed locations for the dams (Figure 1-3). Sieve analysis was

conducted on nine soil hand samples retrieved from the site area (Appendix A). This

series of analyses allowed definitive classification of the soil units. Two units, clayey

sand and clay, were expected to be found at their respective depths. The conditions onsite

revealed a heterogeneous mixture of both layers. The slopes and valley floor contain

appreciable amounts of fines (5-75%) (Appendix A). Appendix A contains the test

results for compaction and Atterberg limit tests, as well as the results from the sieve and

hydrometer analyses. The samples that are classified as sands were 2, 3, A-3, and A-4

(Appendix A). These are classified as SP, SW, SC or SM, and SC or SM, respectively.

The last two samples are suspected to be non-plastic and ftirther tests will be needed to

classify these samples. The clay samples were 1, 4, A-1, A-2, A-5, and A-6 (Appendix

A). The classifications for these samples are CL, CL or ML, CL or ML, CL, CL or ML,

and CL-ML. Three of these samples were suspected to be non-plastic and will need

5

0 0.5 1 Milesi. JX X X X X X

Figure 1-1. UMFS Soil Map.

6

500ftVo;:IV}A

*.

iytev1V;:v/Ai●; i.'i

●●●●i

Upper Sand:.

v.V5.● :

M:●

\

480ft*. };;; ●\V>1V

460ftSilt LoamT3cfU

LO440ftc

fD *.■

:"D

Middle SandO) 420ft:

::

400ftLower Clay

1380ft

Lower Sand●?.

1:1:

360ftm Wilcox GroupmgggggggggggSggm

Figure 1-2. UMFS Stratigraphic Column.

7

v.Vv::VV 500ft; U; -r\ :

fI

r

●V J

Upper SandI ii

*»v/.●VJ

480ft:s'

Ir<r

>.i

460ftSilt LoamT3CfD

LD440ft

fU:

●AU-vV*’■DMiddle Sand

s%T a*;«s5ci-<f

●v*V?'mCD »V.

420ft>-f

Vi1

400ftLower Clayrn^mm

I380ft5>

'jrs Lower Sand%

w●'t' \ 1

i 360ftm Wilcox Group[ 1

Figure 1-2. UMFS Stratigraphic Column.

7

Allies

Figure 1-3. Soil Sampling Map.

8

j

further tests to classify these samples. The silty clay layer is estimated to be 5 ft below

the current ground surface and is where the dam’s foundation will be located.

Local Hydrostratigraphic Units

The upper aquifer is a perched, unconfined aquifer consisting of the top soil and

Middle Sand, SC. The upper aquifer recharges the streams through seepage and

discharges numerous springs at the contact with the Lower Clay (Figure 1-4).

The lower aquifer is a confined aquifer, which is located within the Lower Sand

and between the Lower Clay and the clays of the Wilcox Group. This aquifer is a very

coarse to medium grained sand (Swann and Moyse, 1997). Some of the ponds were

created within this aquifer by digging into the CL within the Wilcox Group.

UMFS Potentiometric Surfaces

The potentiometric surface map (Figure 1-5) illustrates the water table surface at

the field station. Groundwater flow is roughly to the northeast in the southern boundary

and to the southeast within the northern portion, overall groundwater flow is to the east.

The upper and lower aquifer are considered to be in equilibrium, and contribute equally

to the potentiometric surface of the UMFS. MMRJ Well No. 1 is screened within both the

upper and lower aquifer, so its potentiometric surface may not be representative of the

system. MMRI wells 2, 3,4, and 5 were screened within just the lower aquifer, and the

piezometers are screened in the upper aquifer. The gradient is much higher in the

northeastern portion of the site (with a potentiometric surface difference of approximately

40 ft over less than 0.25 mi) than the overall smaller gradient.

//)

)}

$IsI

UI o> X02

(994l i

mo

,'-M< 0^> c:<T3 .! ■

rv~\ ■ U>

u m(Li 'PV 0>''o. '' 9t

cnfOnu4>cc

if

((U \\

>o. /y f

iipue$ ueipiJdvV)

pues IiUMOT ;

ABOpUBSUJBUI

jaArtOiajppUAiM\S

Figure 1-4. UMFS Conceptual Hydrogeologic Model.

10

/,

tPotentiometric SurfaceContours

Ponds/ Creeks

SpringCl. = 10 ft.

Miles

Figure 1-5. UMFS Potentiometric Surface Map.

11

UMFS Conceptual Hydrogeologic Model

Figure 1 -4 shows a conceptual model of water flow through the site. Assuming

that the groundwater table divide mimics the topographic divide, the basin is an isolated

system receiving water only from precipitation and releasing flowing water only at the

outlet. Artificial losses and gains into the system were deemed negligible.

Evapotranspiration (ET) acts as a nonpoint water loss over the entire area of the

watershed. Water is expected to enter the system solely via precipitation.

Some of this rainwater will infiltrate the surficial deposits and percolate through

the Meridian. This water will encounter a low permeability clay layer within the Meridian

and form a perched water table above it. This perched aquifer feeds the site’s springs.

The Lower Clay allows for vertical groundwater leakage from the perched aquifer

into a lower sand layer. This produces a confined aquifer, which has lateral groundwater

flow toward the outlet. The ponds on site were dug until the top of the lower aquifer was

reached; thus, their surfaces represent the aquifer’s potentiometric surface.

Water that exceeds the soil’s infiltration capacity will runoff and directly enter

streams and ponds.

12

CHAPTER II: SITE SELECTION AND RESERVOIR CHARACTERISTICS

Site Selection

BTJM proposed two feasible sites for a new reservoir and dam. Both would

involve removing the existing dam and enlarging the existing reservoir of Old Bramlett

These were chosen because they would:pond.

create new ponded areas in which aquaculture and limnology studies could be

conducted.

have favorable geologic conditions for a dam and its associated reservoir.

not be likely to flood or endanger neighboring properties.

not interfere with ongoing research, especially where buildings or infrastructure

have been constructed on site.

● not endanger Bay Springs Church.

The client chose the larger reservoir (Figure 2-1) with the intention that it may be

used for water supply to the existing small ponds. At this time, it is anticipated that the

proposed dam in the selected site would pose minimal threat to human life, critical

infrastructure, and economic assets in the event of a dam failure. Thus, it seems likely

that it would be considered a low hazard dam according to MDEQ Regulation LW-4.

However, unforeseen circumstances may result in classifying this design as a significant

hazard dam. Watershed runoff for Probable Maximum Precipitation (PMP, 41.2 in in

Lafayette County, MS) was computed to determine the required discharge based on these

13

0.5 1 Mites0 AI

Figure 2-1. Proposed Reservoir and Dam Location.

14

classifications (Appendix B). MDEQ requires that a low hazard dam must be designed to

control 35% of the PMP without overtopping. Similarly, a significant hazard dam must

be capable of withstanding runoff from 50% of the PMP.

Unfortunately, this location will disturb existing wetlands. Environmental impact

will be greatest during construction when the reservoir area and embankment site will

have to be cleared. Overall environmental impact will be countered by contracting

construct wetlands in a nearby area. Pending a response from the Vicksburg District of

the U.S. Army Corps of Engineers, a 404 permit may be necessary to begin construction.

Reservoir Specifications

The new reservoir (Figure 2-1) would approximately triple the size of Old

Bramlett pond. The reservoir would be have a drainage area of 180 acres and would be

store approximately 190 acre-ft of water at its maximum elevation of420 ft.

Water-tightness of the reservoir bottom was initially a concern. In the northern

part of the field station, ponds have had leakage issues that have necessitated the

installation of a bentonite layer to enable them to hold water. However, the existing

(unlined) Old Bramlett pond on this area of the site has not had any problems holding

water year-round (M. Baker, Field Station Manager, personal communication, 2011).

Field observations have verified that the water table is very near the surface, so water loss

into groundwater may be minimal. Even so, further analysis was required to determine

whether the gradient caused by impounding a reservoir will raise groundwater seepage

rates to unacceptable levels. Initial estimates indicate that seepage through the proposed

dams may be insignificant (See Chapter III).

to

15

Previously conducted water balance studies were used to compute expected

reservoir fill time (Appendix B). If only surface water flows contribute to the

the minimum expected contribution from the drainage area

reservoir,

is 20 in/yr (per unit drainage

area). If all precipitation that is not evaporated from the basin contributes, the maximum

contribution is 35 in/yr. (This condition assumes that water that infiltrates into the ground

IS recovered through spring flow.) Under these assumptions, it is expected that it will take

5-8 months to fill the reservoir. Actual fill time may differ slightly because of the

uncertainties in this estimate. The groundwater/surface water interactions at the site are

not well understood except that the groundwater seeps out in the form of a spring on the

western edge of Old Bramlett.

It is also expected that the reservoir would have losses both to groundwater

seepage (see Chapter III) and to maintaining streamflow. A bucket test estimated the

discharge of the stream (at the pipes exiting Old Bramlett dam) after a period of heavy

rainfall in April 2010 to be about 1-2 gpm. This provides some insight to normal

streamflow conditions, but no long term data is available. Accordingly, comparisons were

made between the drainage area of interest and the nearest downstream gage (Little

Tallahatchie River at Etta, 07268000) using direct area ratios. The 90% of instantaneous

expected to be below 1 cfs (450 gpm, USGS, 2010).

Monthly precipitation data (NOAA, 2000) and ET data (Ahn and Tateishi, 1994)

were used to evaluate the potential for seasonal diying-out of the reservoir (Appendix B).

assumed that the reservoir would retain all precipitation falling in its

basin except that which is lost to ET. For June, July, and August, ET exceeds

precipitation, but the volume of water loss from each reservoir is less than 10% of the

discharge rates are

Once again, it was

16

reservoir storage capacity. Thus, even if these estimates are significantly flawed, the

proposed reservoir is likely to hold water year-round. The change in water level caused

by this volume change depends on the starting water level. Due to the nature of the

reservoir terrain, volume increases non-linearly with increasing stage. Using a digital

elevation model (DEM);(MAR1S, 2011), a volume versus stage curve (Appendix A) was

produced for further analysis regarding expected seasonal water levels.

Water Supply Calculations

In response to client requests that the reservoir be utilized to supply the

downstream ponds with water, an analysis was performed to determine the volume of

water that could be supplied downstream during the dry summer months.

To allow the reservoir to be used for research, it is preferred that the water level

not be dropped below approximately 415 ft AMSL, which corresponds to a volume of 2.6

million ft^ Assuming a full reservoir at the beginning of the diy period, the water level

remaining in the reservoir after ET losses until the month of August is 6.8 million

which corresponds to a water level of about 423 tf AMSL. Thus, if the water level is

allowed to drop to 415 ft AMSL over the course of the summer, the water available for

release downstream is 4.2 million ft̂ , or about 96 acre-ft.

The volumes of the ponds downstream were estimated for reference. “Small

Ponds” refers to the collection of small ponds along Bay Springs Branch in the north

central area of the field station. “Larger Ponds” refers to the separate group of larger

ponds downstream (east) of the small ponds. Surface areas of the existing ponds

downstream from the proposed dams were estimated from a digitized ponds layer created

17

from aerial photography of the site. Volume of water in these ponds was then estimated

by assuming a uniform depth of 4 ft in all ponds.

Surface Area Depth Volume(acres) (ft) (acre-ft)

Small Ponds

Larger PondsTotal

5.3 4

15.9 4

21.2 4

21

64

85

Without more information regarding the efficiency of the water transfer and the

expected initial water levels in the ponds, no definite statement on the adequacy of this

water supply should be made. However, it seems likely that the proposed reservoir could

at least supply the small ponds during the siunmer.

ip

18

CHAPTER III: HYDROLOGIC ANALYSES OF PROPOSED DAM

Seepage

To check that the reservoir is likely to fill and hold water, rough calculations for

Ihe condition of a full reservoir were conducted using flownets. Two separate scenarios

were considered: 1) seepage occurring through the dam (Figure 3-1) and 2) seepage

occurring under the dam (Figure 3-2).

In scenario 1, the complicated cross-sections of the proposed designs were

simplified by making the assumption that most of the head loss will occur as water moves

across the clay core. This is a conservative assumption; it underestimates the distance

over which head is lost and thus overestimates hydraulic gradient and seepage. However,

the clay layer underlying the embankment was considered completely impermeable, an

assumption which is likely to cause some underestimation of seepage. This was corrected

by considering seepage through the foundation separately in scenario 2. Discharge was

computed as:

Kph ̂Q = f

where Q is the rate of seepage discharge, K is hydraulic conductivity, p is the number of

flow tubes in the flownet, h is the total head loss, f is the number of squares in each flow

tube, and L is the effective length of the dam. To obtain a conservative estimate of

seepage, the K of the clay core was assumed to be 10 ̂ ft/day, which is a high value for

clay (Fetter, 2001). Likewise, the foundation material, which is composed of a clay at the

19

Clay Core I

-i

f

7:A .

15 ft /

/ r

i-7-_“

■f/

I - Filter Layer»

.--l- .1, r

Clay foundation assumed impermeable.

Figure 3-1. Flownet through the embankment.

If

I

;i

Figure 3-2. Flownet under the embankment.

20

base of the dam and sand with approximately 50% fines along the abutment

considered to have a K of 10 “ ft/day, a value consistent with silty sand (Fetter, 2001).

Although the seepage in the abutments will be considerably lower than that of the

center of the dam, a conservative seepage estimate was obtained by counting the entire

length of the dam (660 ft along the top center from abutment to abutment) as having the

same (maximum) seepage rate as the center of the dam. Using this method and summing

the two scenarios, the maximum expected seepage rate through the dam is 4 ftVday. This

amounts to approximately 0.03 acre-ft/yr, which is insignificant compared to the 300

acre-ft/yr of minimum expected watershed contributions (Appendix B).

Water Level Response to Storm Events

To check that the design storm events would not overwhelm the reservoir’s

capacity, simple numerical modeling of the events was conducted. The 35 % PMP, 50 %

PMP, and 80 % PMP (the design storm, which has a 1.6 factor of safety with respect to

discharge required of a significant hazard dam) were each modeled separately.

These models assume an initial water level of 420 ft , which corresponds to the base of

the spillway. Initial flow out of the reservoir is neglected (at t - 0 hours, Qout = 0).

Inflows (Qin) are modeled as constant throughout the storm, a simplification that ignores

effects of both infiltration and water travel time from the edges of the basin. Only

outflows through the over-the-crest spillway were considered. The low-level outlet is

intended for use only when draining the spillway below normal operating level and

such has not been considered in these scenarios.

For each time step, the reservoir response was calculated as follows:

1. Net inflow was calculated: Qnet = Qin - Qout-

, was

as

21

2. Accumulated volume was computed as: V: = QnetO: -1|) + Vi

3. Height above the spillway base was computed based the observed stage-volume

relationship for the water levels involved: H = 6.2 x 10'^ (V)

4. A new Qout for use in the next time step was computed using the new flow area

and Manning’s equation.

The models (Appendix C) show that the spillway discharge rates would equilibrate to

the inflow rates within 9 hours for these events. In addition, the freeboard never drops

appreciably below 3 ft (the desired minimum emergency freeboard). Thus, models

demonstrate that for the design storms the reservoir capacity will not be overwhelmed,

3nd the dam will not overtop.

Inundation Map

An inundation map (Figure 3-3) was created using a simplified empirical procedure

outlined by the Washington State Department of Ecology (2007):

The volume of material eroded from the dam was estimated to be about 1400 yd^:

Vn, = 250(BFF)

where BFF is Breach Formation Factor (BFF) was calculated:

BFF = VwHw

where is the volume of water stored in the reservoir (200 acre-ft) at the water surface

elevation under consideration (424 ft).

Assuming a rectangular breach, a breach width, Wb, of 27 ft was calculated:

27V_

0.77

W,=

H,(C+i|5)

22

1

Figure 3-3. Inundation Map.

23

f

where Hb is the height of the dam breach (19 ft), C is the crest width (14 ft), and S is a

embankment slope factor (6).

The breach formation time is estimated as 0.5 hours based on:

t = 0.036(VJ"'^

From tabulated data, peak discharge (Qp) is 7,700 cfs for an embankment made of

erosion resistant materials with a surface area of 35 acres and water height of 19 ft.

Tabulated data was used to predict the discharge attenuation coefficients (Cx)

downstream of the dam breach for a 200 acre-ft reservoir. The discharge at some point

downstream, Qx, is determined by:

Qx ~ CxQp

The cross-sectional area. Ax, required to pass the discharge can be calculated:

Ax = Qx/vX

where Vx is the representative velocity determined using tabulated data. Supporting cross-

sections were drawn at various points along the downstream to determine the depths and

widths necessary to attain the required cross-sectional area. A summary of these results is

shown below.

Velocity(ft/s)

AttenuationCoefficient

Discharge(cfs)

7700

Maximum

Depth (ft)

Width AverageDepth (ft)

Distance

(mi)

Elevation Slope(ft/mi)

Land Area

IftlIftl _type

4.20.0 400 18 Grass 4.5 1.00 1710 5.2 410

0.93 5.21.0 382 18 Woods 2.6 7160 2750 7.9 525

16 Woods 0.91 7010 2800 6.6 3.7370 2.5 754

2.0 365 13 Woods 2.7 0.90 6930 2570 4.9 754 3.4

Woods2.5 360 18 2.6 0.89 6850 2640 4.9 951 2.8

The inundation map stops at the point where the released water would flow into

Puskus Lake. After entering the lake the water would encounter the Puskus Dam, a low

24

hazard dam with approximately 20 ft of freeboard. It is expected that the Puskus Dam

may have sufficient volume to retain the flow. However, even if the failure of the

proposed dam causes the failure of Puskus Dam, the impact is expected to be low.

The map indicates that a shed on site will be the only building destroyed. Research

on the northern reach of Bay Springs Branch will also be compromised. Off site, the

nearest buildings are approximately 1000 ft north and 20 ft higher in elevation than the

inundated area. The remaining area is wooded. Thus, a failure of the dam is expected to

produce no loss of human life or critical infrastructure.

25

i

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1

26

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2011)

Mississippi Department of Environmental Quality (MDEQ), 2005, Dam Safety

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by The Mississippi Mineral Resources Institute at The University of Mississippi

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28

Mississippi.

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Station Normals of Temperature, Precipitation and Heating and Cooling Degree

Days 1971 -2000: Climatography of the United States, No. 81

National Resources Conser\'ation Service, 2011, NRCS Custom Web Soil Survey for

Lafayette: http:/Avebsoilsurvey.nrcs.usda.gov/app/HomePage.htm

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Mechanics and Foundations Division. ASCE. Vol. 6. SM6. 1879-1892.

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2011)

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http://waterdata.usgs.gov/ms/nwis/uv?site_no=07268000, 2011)

U.S. Army, 1997, FM 5-410 Military Soils Engineering: Washington, DC, U.S.

Government Printing Office

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U.S.

U.S.

29

U.S. Amiy Corps of Engineers, 1995, EM 1110-2-1901 Seepage Analysis and Control for

Dams: Washington, DC, U.S. Government Printing Office.

U.S. Army Corps of Engineers, 2003, EM 1110-2-1902 Slope Stability: Washington, DC,

U.S. Government Printing Office.I

i

30

(i

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Reservoir Characteristics Methods:

Drainage area w as determined using watershed delineation and geometry calculation on

ArcGIS. Dam length was estimated on the same digital map.

Nomial. emergency, and o\ ertop capacities represent water impounded in the reservoirwhen water level is at 420 ft, 421 ft, and 424 ft , respectively as determined by volume

calculations performed in ArcGIS using a digital elevation model.

Required discharges w ere computed based on MDEQ regulations, which dictate that a

low hazard dam must be designed to control 35% of the probable maximum precipitation

(PMP, 41.2 inches in Lafayette county) without overtopping. Similarly, a significant

hazard dam must be capable of withstanding runoff from 50% of the PMP. It was

assumed that the rain tell in one equally distributed event over a 24-hour period and that

infiltration was negligible due to this event quickly overwhelming the infiltration

capacity of soils. (This is a conservative assumption; the actual runoff will be slightly lessdue to infiltration.)

Drainage Area(acres)

181

Dam Length at Base 440(ft)

Dam Length on Top 660(ft)

Normal Capacity(acre-ft)

190

Emergency Capacity(acre-ft)

225

Overtop Capacity(acre-ft)

Low Hazard DischargeRequirement

(cfs)Significant Hazard Discharge

Requirement(cfs)

340

110

156

Appendix B. 1. Reservoir characteristics.

44

I ime ot Concentration:

The lime ol eoneeniralion (t,) is the lime il lakes for water to travel from the most

hydraulically distant point (i.e. the point with the longest flow path) to the reser\oir.

1 he Kirpich equation, an empirical formula relevant for small watersheds (less than 200

acres; C hin. 2006) such as the one in question (180 acres) calculates k (in minutes) as:

gO.W5

\\ here L is the tlow length in meters and S is the average slope along the flow path. Thelongest How path in the dam's basin is 530 m with a slope of 0.079, resulting in a k of 6.3min.

t. =0.019c

1 lowe\ er, it is recommended (Chin, 2006) that the k obtained from the Kirpich equation

be multiplied b\- 2 for general o\ erland How to account for ground roughness. Thus, thefinal t^ computed for our basin is 12.6 min.

Map shows the longest flow path, which has L = 530 m and S =0.079.

Appendix B.2. Basin time of concentration.

45

Water Balance CalculationsV

During pre\ ious site investigation, three independent analyses of the field station annual

water budget \\ ere conducted using similar methodologies. The reasoning of Brown’s

water budget is included below for reference. A fourth investigation using monthly

precipitation and e\ apotranspiration (ET) data was added for this report in attempt to

quantify the expected seasonal fluctuations in water availability.

Results from four independent analyses of field station water budget:

NOAA/Ahn and TateishiMagee, TidwellBrown James

inches %

54 100

inches inches

57.65

inches %

55 100

% %

58 100 100Precip.

24 40

15 26

20 35

28 51 35.1227.5 50

5.5 10

22 40

61ET

Groundwater

Surface Water

5 9

22 40

Effective

Precip.(precip. - ET)

35 60 22.53 3927.5 50 27 49

As seen in the table, uncertainty exists in the precise quantity and distribution of water

available. To obtain a probable range of watershed contributions, the lowest and highest

of these estimates were used. If only surface water flows contributed to the reservoir, the

minimum expected contribution from the drainage area is 20 inches/year. If all

precipitation that is not evaporated from the basin contributes, the maximum contribution

is 35 inches/year.

Appendix B.3. Water balance to determine contributions to reservoir.

46

Explanation for Hro\\n W ater Bud‘>of*

Assiimiiiy thai ilic eroiiiuluatcr table divide mimies the topographic divide, the basin is

an isolated s\ stem reeei\ iiig u ater onlv from preeipitation and releasing flowing water only at the

outlet. .Xi tificial losses and gams into the svstem were deemed negligible. ET acts as a nonpoint

uater lo.ss o\ er the entire area ot'the watershed.

.Assuming stead\ state, the w ater balanee equation is:

Qi' “ Qs ’ Oi.w * Qi 1

where is the \olume of water entering the watershed by preeipitation over a specified time

peritHl, Qs is the basin's o\erall stream diseharize, Qe.w is the basin’s groundwater diseharge, Qet

is the water lost \ ia I I . I)i\ iding these by watershed area (,A«) and solving for groundwater flow,we obtain:

flow =qi> - qs - qriwhere q Q .A

Stream output (qs) was obtained \ ia an area ratio eomputation with data from the

dow nstream I S(iS gage of l ittle Tallahaiehie River at Etta, MS (LSGS, 2010, Little Tallahatchie

River at l ata, ,\1S .Annual Statistics, L'.S. Geological Survey, retrieved November 5,2010,ailable at: http: w aterdaia.usizs.uo\ nwis annual'). Average annual discharge at that gage was

HSO ft' s w ith a contributing drainaize area of 520 miA Using a direct area ratio and a watershed

area of 2.7 mi\ Qs was estiniated to be 4.5 ftVs. with qs = 22 in/yr. Gaffin and Lower)' (1996, A

Rainfall Climatology of the NW'Sl'O Memphis County Warning Area, National Oceanic andAtmospheric .Administration. retrie\ed Novembers. 2010. available at:

littp:/ w w w .srh.noaa.go\ meg ?n-rainelim) report qp for northern Mississippi as 55 in/yr. Hanson

( 1 991 ) repm ts an a\ erage annual ET (q, ,) of 25-30 in for the area. Using these values, qcw is

c'xpeeted to be 3 S in yr. In total, appro.ximately 45-55 “o of outflow is evapotnmspiration (ET).

I he rest of tliis iTnvs out as groundwater (5-15%) or surface w'atcr (40%) at the single dischargepoint located just east of the BFS.

To confirm that this water budget is reasonable, the hydraulic conductivity ofthe area can

be calculated from q^. The outflow coefficient, a. can be obtained by (Gelhar and Wilson, 1974):‘tcu’

\s ●

a\

a =(H-ho)

atershed (410 fl) and h„ is the head at the outlet (399 ft),

qow. a is expected to be 0.023-0.061 yf.

where is the average head in the w

Using minimum and maximum

ransmissivity ff) is ealeulated:

where f) is a geometry term for which w'c used 3.0 on the suggestion of Gelhar and Wilson

(1 974). Calculated transmissivity is 2.6 - 7.0 in Vs.

1 lydraulic conductivity (K) can be obtained by dividing T by average aquifer thickness(b):

K = i> .3

Using an aquifer thickness of 60 ft, hydraulic conductivity was calculated as 3.6 x 10

9.7 X I 0 in/s, a reasonable value for a silty sand (Fetter, 2001).This lends credibility to the waterbudget approximations.

Appendix B.4. Sample water balance justification.

47

Reser\()ir I'ill 1 iinc C alculations:

Rcscia oir fill lime \\ as estimated using the upper and lower bounds (determined in

budget ealeulaiions) ot'evpeeted watershed contributions, shown here as eftective

preeiiiiiation. It w as assumed tliai each part of the watershed equally contributed, so

elTeeii\ e annual preeipilation was multiplied by drainage area to obtain an annual volume

ol w ater entering the reser\ oir. Rough fill time estimates were then made by dividing thetotal reseiAoir N cdume b\ the annual available water volume.

water

181Drainage .\rea (aeres)

Minimum Time

35l-.lTeeti\e Preeipilation (in vr)

Precipitation \ olume (aere-tiyr’)

time (years)

I'ill time (months)

■ 1

527

0.38

5

Maximum Time

20l-iffeetive Precipitation (in yr)

Precipitation Volume (aere-fL'yr)

Fill time (years)

l-'ill time (months)

301

0.67

8

Appendix B.5. Reservoir Till time calculations.

48

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SI c

■o | on so O

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I i 1111118- 811 ̂iiiisuumsfVl' >T ^ 'I 2

2: 3C. Q.£ ™o*x O

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■5 S>rc *-tz c/5

^ 3CO ^

3

i? i?●E g

csc

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iliilillllirr.r M of 5 ;3 in JH pj ^ m w g

S<

S s s- M

c

Q. 'C

£ 3QJ C3.CO TOC w c.

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CO ro

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I- yroroo mCO r^ CO^ ^ CO *sf 05 o

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LT5 CO cnUD^ rn m

mro LHo> CD Q. om Oc ro OO p CnJ

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S8§§§||§i|||5°'ss'3S? = ssSj

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CIDCT5 «DOLno'Domcj5vX5 lDOLn »DCO^ CT5

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49

I

\ olunic versus Sta^e

I

10,000,000

8,000,000n

6,000,000CD

Z3O 4,000,000>

2,000,000

0 *

416 418412 414410 420

Water Level (ft)

Stage Reservoir

Volume (ft^)(ft)

420 7,980,000

5,940,000

4,110,000

2,590,000

1,350,000

510,000

41S

417

415

413

412

0410

Appendix B.7. Volume versus stage calculations.

50

Qout,

original Qiu't (cfs) \ oliime (ft')(cfs)

h (ft of

fIo>\ in Qout. new (cfs)

spillway)0.12 8

Volume

(acre-ft)

lime(Jin (cfs)

(ti)

51 10 0 108,000

382.000

5.^8.000

(164.000

760.000

832.000

884.000

021.000

048.000

066.000

070.000

080.000

005.000

000.000

1.002.000

1.004.000

1.006.000

1.007.000

1,008.000

1.008.000

1.008.000

1.009.000

1.009.000

1.009.000

1.009,000

1.009,000

1.009,000

1,009,000

1,009.000

1.009.000

1.009.000

1,009.000

1,009.000

1,009.000

1,009,000

1,009,000

1,009,000

1,009,000

1,009,000

1,009,000

1,009,000

1,009,000

1,009,000

1,009,000

1.009,000

1,009,000

1,009,000

1,009,000

1,009,000

0.(1 1 109231 10 0.241020.5 s12401 10 0.341 .0 8715561 10 0.4140 70●>17701 10 0.472.0 54-■'619811 10 0.52

0.55“0 40

2089I 103.0 81 2021951 10 0.57SO3.5~n1001 10 0.594.0 05 15r)1031 10 0.604.5 100 10-n1050.611 105 0 103231060.621 105.5 105231080.621 10 1066.0 4231080.621 10 1086.5231090.621 107.0 108 *)231090.631 107.5 109231090.(i31 10S.O 100231100.631 108.5 10023no0.63I 109.0 no 0231100.630.5

10.01 10 1 10 0

231100.631 10 1 10 0231100.6310.5 1 10 1 10 0231100.631 1 .0 1 10 1 10 0231 100.631 1 .5 1 10 1 10 0231100.6312.0

12.513.0

1 10 1 10 0231100.631 10 1 10 0231100.631 10 1 10 0231 100.6313.5 1 10 1 10 0231 100.6314.0

14.51 10 1 10 0 231 100.631 10 1 10 0 231100.631 5.0 1 10 1 10 0 231100.6315.5 1 10 1 10 0 231100.631 6.0 1 10 1 10 0 231 100.6316.5 1 10 1 10 0 231100.6317.0 1 10 1 10 0 231100.6317.5 1 10 1 10 0 231100.6318.0 1 10 1 10 0 231100.6318.5 1 10 no 0 231 100.6319.0 1 10 1 10 0 231 100.6319.5 1 10 1 10 0 231 100.6320.0 1 10 1 10 0 231100.6320.5 1 10 1 10 0 231100.6321 .0

21 .522.022.523.023.524.0

1 10 1 10 0 231 100.631 10 1 10 0 231100.631 10 1 10 0 231100.631 10 1 10 0 231100.631 10 1 10 0 231 100.631 10 1 10 0 231 100.631 10 1 10 0

Appendix C. 1 . Predicted water level response in the 35% PMP event.

51

h(ftof

Qnct (cfs) Volume (ft^) flow in Qout, new (cfs)

spillway)

Qout,

Time (h) Qin (cfs) ori«inal(cfs)

Volume

(acre-ft);/

6140,17281,000

536,000

745,000

904,000

1,018,000

1,098,000

1,153,000

1,189,000

1,214,000

1,230,000

1,241,000

1,248,000

1,252,000

1,255,000

1,257,000

1,259,000

U60,000

1,260,000

1,260,000

1,261.000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

1,261,000

0 1560.0 1 5612400.3314 1420.5 15617680.4640 1 161 .0 1 5621920.561 .5 6S SS1 56231120.63022.0

2 ̂

641 56251260.681 12 44156261360.721263.0 30156271420.74136 20.3.5 1 56281470.76142 144.0 156281500.77147

150

152

153

154

155

155

94.5 156281520.775.0 6156291530.785.5 156 4291540.7836.0 156291550.78■>6.5 156291550.787.0 156 1291560.787.5 1156291560.781568.0 0156291560.79156S.5 0156291560.791569.0 0156291560.791569.5 0156291560.7915610.0 156 0291560.7910.5 156156 0291560.791 1 .0 156156 0291560.791561 1 .5 156 0291560.7915612.0 156 0291560.79 !15612.5 156 0291560.7915613.0 156 0291560.7913.5 156156 0 291560.7914.0 156 0156 291560.7914.5 156156 0 291560.7915.0 156156 0 291560.7915.5 156156 0 291560.7916.0 156 156 0 291560.7916.5 156156 0 291560.7917.0 156156 0 291560.7917.5 156156 0 291560.7918.0 156 156 0 291560.7918.5 156156 0 291560.7919.0 156 156 0 291560.7919.5 156156 0 291560.7920.0 156156 0 291560.7920.5 156156 0 291560.7921.0 156 156 0 291560.7921.5 156156 0 291560.7922.0 156156 0 291560.7922.5 156156 0 291560.7923.0 156 156 0 291560.7923.5 156156 0 291560.7924.0 156 0156

Appendix C.2. Predicted water level response in the 50% PMP event.

52

h (ft of

N oliime(ft') flow in Qout. new (cfs)

spillway)0.28

0.53

30450.000

846.000 S3

Qout,

ori'^inal Qiiet (cfs)(cfs)

\olumc

(acre-ft)rime (h) Qiii (cfs)

10

1.146.000

1.354.000

1,401,000

1,577.000

1.630.000

1.663,000

1.682,000

1.604.000

1.701.000

1,706,000

1.708.000

1,710.000

2 50

250

250

250

250

2 50

2 50

2 50

2 50

250

2 50

250

250

250

250

250

250

2 50

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250O () 01930 2200.5261340.71S3 1671 .0311740.841 .5 134 1 16342020.932.0 1~4362200.98s 202 4S372321.022203.0 30382391.043.5 232

230

243

IS392431.054.0 1 1392461.064.5392481.065.0 246

24S

240

240

240

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

250

4392491.06392491.060.0 1392491.070.5392501.071 .-71 1,0007.0 1392501.071,711,000

1.712.000

1,712.000

1.712.000

1.712,000

1.712.000

1.712.000

1,712.000

1.712.000

1.712,000

1,712,000

1,712.000

1.712.000

1.712,000

1.712.000

1.712,000

1,712,000

1.712.000

1,712,000

1,712,000

1,712,000

1,712,000

1,712,000

1,712,000

1,712.000

1,712,000

1,712,000

1,712,000

1,712,000

1,712,000

1,712,000

1,712,0001,712,000

1,712,000

7.5 0392501,070S.O392501.07S.5 0392501.07'^).0

‘).5

0392501.070392501.071 0.0

10.5

1 1 .0

0392501.070392501.070392501,070392501.0712.0

12.5

13.0

13.5

14.0

14.5

15 0

15.5

16.0

16.5

17.0

0392501,070392501.070392501.070392501.070392501.070392501.070392501.070392501.070392501.070392501.070392501.071 7.5 0392501.07I 8.0 0 392501.0718.5 0 392501.0719.0 0 392501.0719.5 0 392501.0720.0

20.5

21.0

250 0 392501.07250 0 392501,07250 0 392501.0721.5 250 0 392501.0722.0

22.5

23.0

23.5

24.0

250 0 392501,07250 0 392501.07250 0 392501.07250 0 392501.07250 0

Appendix C.3. Predicted water level response in the 80% PMP

53