+-----------------------------------+------+---------------------------+
| ![](media/image1.                 |      | !                         |
| png){width="3.5590277777777777in" |      | [](media/image2.jpeg){wid |
| height="0.6201388888888889in"}    |      | th="0.7076388888888889in" |
|                                   |      | height                    |
|                                   |      | ="0.48680555555555555in"} |
|                                   |      |                           |
|                                   |      | *Agricultural Research    |
|                                   |      | Service*                  |
+-----------------------------------+------+---------------------------+
| **National Sedimentation          |      |                           |
| Laboratory**                      |      |                           |
|                                   |      |                           |
| **Channel and Watershed Processes |      |                           |
| Research Unit**                   |      |                           |
|                                   |      |                           |
| **Oxford, Mississippi**           |      |                           |
|                                   |      |                           |
| **Acoustic Imaging of Sediment    |      |                           |
| Impounded Within**                |      |                           |
|                                   |      |                           |
| **USDA-NRCS Flood Control Dams,   |      |                           |
| Wisconsin**                       |      |                           |
+-----------------------------------+------+---------------------------+
| ![](media/image3                  |      |                           |
| .wmf){width="5.995138888888889in" |      |                           |
| height="3.142361111111111in"}     |      |                           |
+-----------------------------------+------+---------------------------+
| []{.mark}                         |      |                           |
|                                   |      |                           |
| ##### By John A. Dunbar, Pa       |      |                           |
| ul D. Higley, and Sean J. Bennett |      |                           |
+-----------------------------------+------+---------------------------+
| **Research Report No. 30**        | **Au |                           |
|                                   | gust |                           |
|                                   | 20   |                           |
|                                   | 02** |                           |
+-----------------------------------+------+---------------------------+

#  Executive Summary

Since 1948, the USDA-NRCS has constructed nearly 11,000 upstream flood
control dams in 2000 watersheds in 47 states, most with a design life of
50 years. The watershed projects, which represent a \$14 billion
infrastructure, have provided flood control, municipal water supply,
recreation, and wildlife habitat enhancement. Because of population
growth and land use changes through time, sediment pools are filling,
some structural components have deteriorated, safety regulations are
stricter, and the hazard classification for some dams has changed.
Presently, 42 dams in Wisconsin are in need of immediate rehabilitation
at an estimated cost (1999 dollars) of \$3 million.

Before any rehabilitation strategy can be designed and implemented, the
sediment impounded by these dams must be assessed in terms of the
structure's efficiency to regulate floodwaters and the potential hazard
the sediment may pose if reintroduced into the environment. At the
direct request of the USDA-NRCS in Wisconsin, two reservoirs, White
Mound Lake located in Sauk County and Twin Valley Lake located in Iowa
County, were chosen as part of their statewide assessment program. To
this end, this study was undertaken to determine the amount and
distribution of post-impoundment sediment contained within these
selected flood control reservoirs using an acoustic profiling system.

The acoustic profiling system used in the surveys was developed in
collaboration with Specialty Devices, Inc. of Plano, TX. The complete
system consists of one, suitcase-sized, 10 kg, water resistant control
module, a 50 cm long, 15 kg acoustic source array, GPS and differential
correction antennas, and associated cables. The system images the bottom
and sub-bottom sediments with acoustic signals produced at 200, 48, and
24 kHz, deployed from a single Johnboat.

The results from the two surveys were quite similar. The acoustic
response of sediments onlapping the dam face shows three distinct
acoustic layers. The upper-most layer, which appears white on the
multifrequency displays, corresponds to fine-grain, water-rich
sediments. The layer beneath the first, which appears light blue in the
display, preferentially fills the lows and channel axes, and thins and
becomes more restricted upstream. The bottom-most layer deposited on the
dam face, which appears dark blue in the display, is relatively thin in
both reservoirs, but reaches considerable thickness in the channel axes.

At Twin Valley Lake, the sediment isopach map indicates a maximum
thickness of 3.1 m, in a small area along the main channel axis, and a
total volume of post-impoundment sediment of 458,659 m^3^ (371.8
acre-ft). At White Mound Lake, the sediment isopach map indicates a
maximum thickness of 1.4 m (4.6 ft) and a total volume of
post-impoundment sediment of 203,205 m^3^ (164.7 acre-ft). These values
represent conservative estimates of sediment accumulation without
corroboration with ancillary data.

**Table of Contents**

Executive Summary [2](#executive-summary)

Acknowledgments [6](#acknowledgments)

1\. Introduction [7](#introduction)

1.1 Federal Program for Flood Control
[7](#federal-program-for-flood-control)

1.2 Current Status of Small Watershed Program
[8](#current-status-of-small-watershed-program)

1.3 USDA-NRCS Flood Control Structures in Wisconsin
[8](#usda-nrcs-flood-control-structures-in-wisconsin)

1.4 Problem Statement [8](#problem-statement)

2\. Field Sites [10](#field-sites)

2.1 White Mound Lake [10](#white-mound-lake)

2.2 Twin Valley Lake [12](#twin-valley-lake)

3\. Acoustic Survey System [15](#acoustic-survey-system)

3.1 Acoustic Profiling System [15](#acoustic-profiling-system)

3.2 Survey Procedure [17](#survey-procedure)

3.3 Digital Processing and Interpretation of Collected Acoustic Data
[17](#digital-processing-and-interpretation-of-collected-acoustic-data)

4\. Results [20](#results)

4.1 Acoustic Survey of Twin Valley Lake
[20](#acoustic-survey-of-twin-valley-lake)

4.2 Acoustic Survey of White Mound Lake
[26](#acoustic-survey-of-white-mound-lake)

4.3 Discussion [31](#discussion)

5\. Conclusions [32](#conclusions)

6\. References [34](#references)

**List of Figures**

Figure 2-1. Map of White Mound Lake Park and environs (from Wisconsin
Department of Natural Resources). [10](#__RefHeading___Toc18806778)

Figure 2-2. Orthographic composite photographic of White Mound Lake, WI.
Coordinates are in meters (UTM Zone 15).
[11](#__RefHeading___Toc18806779)

Figure 2-3. The photograph of White Mound Lake taken from the boat ramp
area and facing south. Areas of the lake with 4 m of water depth or less
contain aquatic vegetation that extends from the bottom to the lake
surface as shown in distance. [12](#__RefHeading___Toc18806780)

Figure 2-4. 1967 base map of Twin Valley Lake (from Wisconsin Department
of Natural Resources). [13](#__RefHeading___Toc18806781)

Figure 2-5. Orthographic composite photographic of Twin Valley Lake, WI.
Coordinates are in meters (UTM Zone 15).
[14](#__RefHeading___Toc18806782)

Figure 3-1. Photograph of acoustic profiler control module. All acoustic
profiling and DGPS navigation electronics are contained in a compact,
water-resistant control module. [16](#__RefHeading___Toc18806783)

Figure 3-2. Picture of acoustic profiler transducer array. The profiling
system produces acoustic signals with three widely separated frequencies
(200, 48, and 24 kHz) in rapid succession as the profiles are traversed.
[17](#__RefHeading___Toc18806784)

Figure 3-3. Picture of aquatic vegetation. Both White Mound Lake and
Twin Valley Lake contain vegetation that extends from the water bottom
to the surface in areas of 4 m water depth and less. The vegetation
scatters the 200 kHz acoustic signals. [19](#__RefHeading___Toc18806785)

Figure 4-1. Profile map of Twin Valley Lake acoustic survey. The survey
contains 17 km of profile lines. Profiles 11 and 52, highlighted in red,
are shown as example profiles in accompanying figures. Coordinates are
in meters (UTM Zone 15). [21](#__RefHeading___Toc18806786)

Figure 4-2. Southeast end of Twin Valley Lake axial profile, Line 52
(see Figure 4-1 for line location). Three acoustic intervals are seen
within the sub-bottom in Twin Valley Lake. All three onlap onto the dam
face. Hence, all three must be part of the post-impoundment sediment
fill package. [22](#__RefHeading___Toc18806787)

Figure 4-3. Processed Line 11 with interpretation (see Figure 4-1 for
line location). The multifrequency acoustic profiles show three distinct
sediment layers within the post-impoundment fill.
[23](#__RefHeading___Toc18806788)

Figure 4-4. Contour map of water depth in Twin Valley Lake, WI. Depth is
measured from the water surface at an elevation of 277.4 m (910 ft).
Coordinates are in meters (UTM Zone 15).
[24](#__RefHeading___Toc18806789)

Figure 4-5. Contour map of sediment thickness in Twin Valley Lake, WI.
Sediment thickness is measured from the water bottom to the interpreted
base of post-impoundment fill. Coordinates are in meters (UTM Zone 15).
[25](#__RefHeading___Toc18806790)

Figure 4-6. Profile map of White Mound Lake acoustic survey. Profiles 25
and 50, highlighted in red, are shown as example profiles in
accompanying figures. Coordinates are in meters (UTM Zone 15).
[26](#__RefHeading___Toc18806791)

Figure 4-7. Southeast end of White Mound Lake axial profile, Line 50
(see Figure 4-6 for line location). Three acoustic intervals are seen
within the sub-bottom in White Mound Lake. All three onlap onto the dam
face. Hence, all three must be part of the post-impoundment sediment
fill package. [27](#__RefHeading___Toc18806792)

Figure 4-8. Acoustic profile 25 of the White Mound survey (see Figure
4-6 for line location). (a) Multifrequency display in which 200 kHz is
red, 48 kHz in green and 24 kHz is blue intensity in the RGB image. The
interpreted top and base of the post-impoundment fill are marked with
black lines. (b) Aquatic vegetation blocks the 200 kHz signal, but is
relatively transparent to the 48 kHz signal.
[28](#__RefHeading___Toc18806793)

Figure 4-9. Contour map of water depth in White Mound Lake, WI. Depth is
measured from the water surface at an elevation of 278.4 m (913 ft).
Coordinates are in meters (UTM Zone 15).
[29](#__RefHeading___Toc18806794)

Figure 4-10. Contour map of sediment thickness in White Mound Lake, WI.
Sediment thickness is measured from the water bottom to the interpreted
base of post-impoundment fill. Coordinates are in meters (UTM Zone 15).
[30](#__RefHeading___Toc18806795)

# Acknowledgments

We thank Barbara Lensch and Scott Mueller, both of the USDA-NRCS,
Wisconsin, for providing technical and logistical assistance, and the
USDA-NRCS, Wisconsin, for financial support.

Author contact information:

+---+------------------------+--------------------+--------------------+
|   | John A. Dunbar         | Paul D. Higley     | Sean J. Bennett    |
|   |                        |                    |                    |
|   | Department of Geology  | Specialty Devices, | USDA-ARS           |
|   |                        | Inc.               |                    |
|   | Baylor University      |                    | National           |
|   |                        | 1104 Summit Avenue | Sedimentation      |
|   | P.O. Box 97354         |                    |                    |
|   |                        | Suite #104         | Laboratory         |
|   | Waco, TX 76798-7354    |                    |                    |
|   |                        | Plano, TX 75074    | P.O. Box 1157      |
|   | john_dunbar@baylor.edu |                    |                    |
|   |                        | sdipdh@aol.com     | Oxford, MS 38655   |
|   |                        |                    |                    |
|   |                        |                    | sjbe               |
|   |                        |                    | nnett@ars.usda.gov |
+---+------------------------+--------------------+--------------------+

#  1. Introduction

# 

## **1.1 Federal Program for Flood Control**

In response to devastating floods of the 1930's and 1940's, Congress
enacted legislation for the construction of flood control dams on small
tributary streams. The Flood Control Act of 1944 (PL-534) authorized 11
projects in the United States. Since 1948, more than 3,400 flood control
dams have been constructed in the 320 subwatershed projects covering
more than 35 million acres in 12 states (Caldwell, 1999).

In 1954, Congress enacted the Watershed Protection and Flood Prevention
Act (PL-566), commonly referred to as the Small Watershed Program
(Caldwell, 1999). Since that time, more than 6,300 flood control dams
have been constructed in 47 states as well as Puerto Rico and the
Pacific Rim, covering over 109 million acres.

The Pilot Watershed Program provided the transition between PL-534 and
PL-566 (Caldwell, 1999). More than 400 flood control dams were
constructed in 62 projects in 33 states, covering almost 3 million
acres. In addition, the RC&D Program has provided technical and
financial assistance to local sponsors for the planning, designing, and
construction of more than 200 flood control dams since the 1960's.

In total, the U.S. Department of Agriculture, Natural Resources
Conservation Service (USDA-NRCS) and its cooperators have constructed
nearly 11,000 flood control dams in 47 states. More than \$8.5 billion
(1997 dollars) of federal funds and over \$6 billion of local funds have
been invested in these projects since 1948. This \$14.5 billion
infrastructure provides over \$1 billion in benefits annually.

The primary purposes for these structures were to prevent flooding and
to protect watersheds. Other dams were built or have evolved into
structures for water management, municipal and industrial water supply,
recreation, and the improvement of fish and wildlife, water quality, and
water conservation. Local sponsors were to provide leadership in the
program and secure land rights and easements for construction. The
USDA-NRCS was to provide technical assistance and cost-sharing for the
construction of these dams.

Flood control dams typically consist of an earthen embankment 6 to 20-m
high with a principal spillway made of concrete pipe 0.3 to 1.8-m wide
(Caldwell, 1999). Because the dams were built on small streams in the
upper reaches of watersheds, upstream drainage areas range from 1.6 to
16 km^2^. The majority of these dams were planned and designed for a
50-year service life. The inlet pipe of the principal spillway is placed
at an elevation that would provide water retention for the design storm
and storage for sediment accumulation. Each reservoir also has an
emergency or auxiliary spillway for safe conveyance of water around the
embankment when runoff rates exceed storage capacity.

## **1.2 Current Status of Small Watershed Program**

At present, more than half of the dams constructed are older than 34
years and more than 1,800 will reach their 50-year design life within
the next 10 years (Caldwell, 2000). A rapid survey conducted in April
1999 by the USDA-NRCS revealed more than 2,200 dams in need of immediate
rehabilitation at an estimated cost of more than \$540 million. The
primary issues of dam rehabilitation are: replacement of deteriorating
components, change in hazard classification, reservoir sedimentation,
failure to meet dam safety regulations, failure to meet resource needs
of the watershed, inadequate land and water rights, inadequate community
benefits, and the potential transfer of responsibility. Common
approaches to address rehabilitation typically involve dredging the
reservoir to remove accumulated sediment, raising the dam to increase
storage capacity, and removing or decommissioning the dam.

## **1.3 USDA-NRCS Flood Control Structures in Wisconsin**

Because of the dustbowl era and through the 1950's, rainfall in western
Wisconsin caused severe soil erosion, stream sedimentation, and
flooding, which also adversely impacted stream trout populations. Flood
control structures and conservation practices built and promoted by the
USDA-NRCS helped mitigate these problems. Since the inception of the
PL-566 program, 83 dams have been constructed in Wisconsin, with 3
additional dams constructed under the related programs. These structures
provide over \$3.5 million in annual benefits in the form of flood
control, recreation, and erosion control, according to the USDA-NRCS.

Presently, 42 dams in Wisconsin are in need of immediate rehabilitation
at an estimated cost (1999 dollars) of \$3 million. Twelve of these dams
require hazard classification upgrades while the remaining 30 dams
require an extension of their service life, restoration, and repair.

## **1.4 Problem Statement**

At the direct request of the USDA-NRCS in Wisconsin, two reservoirs,
White Mound Lake located in Sauk County and Twin Valley Lake located in
Iowa County, were chosen for this study. As part of their statewide
assessment program, the USDA-NRCS are interested in determining the
volume of sediment deposited within each of these lakes and the spatial
distribution of the impounded sediment, since loss of storage capacity
is an integral part of the assessment and rehabilitation process.

In the absence of previously collected survey data, acoustic technology
offers the possibility of mapping the subsurface distribution of
sediment within reservoirs. In previous work, Bennett et al. (in press),
Dunbar et al., (2001), and Bennett and Dunbar (in press) used a
multifrequncy acoustic system for sub-bottom profiling. The system can
comprise up to five acoustic transducers with operating frequencies of
200, 24, 24, 12, and 3.5 kHz, a receiving hydrophone, and a signal
processor that controls the acoustic profiling, data collection and
processing, and navigational systems. This portable system can be
deployed from Johnboats, and it works extremely well in shallow-water
environments with clay and silt-rich sediments typical of flood control
reservoirs.

The goal of the present report was to determine the current water
storage capacity and the amount and distribution of post-impoundment
sediment contained within two selected flood control reservoirs in
Wisconsin. To achieve this end, acoustic profiling surveys were
conducted in order to image the subsurface reservoir fill.

# 2. Field Sites

## **2.1 White Mound Lake**

White Mound Lake is located in west-central Sauk County, about 13 miles
southwest of Reedsburg, WI. Located within White Mound County Park
(Figure 2-1), the lake area is approximately 104 acres (Figure 2-2) and
it has a maximum depth of about 4.9 m (16 ft). White Mound Lake gets its
name from a village once located on the southern end of the park, named
for the mounds of white limestone in the region. A boat ramp is
available at the location. The reservoir impounding White Mound Lake is
structure number 15 on Honey Creek, which has a watershed drainage area
of about 4,474 acres. Honey Creek flows eastward from White Mound Lake
into the Wisconsin River.

  -----------------------------------------------------------------------
  ![](media/image4.png){width="3.6194444444444445in"
  height="5.579861111111111in"}

  []{#__RefHeading___Toc18806778 .anchor}Figure 2-1. Map of White Mound
  Lake Park and environs (from Wisconsin Department of Natural
  Resources).
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image5.png){width="5.995138888888889in"
  height="5.186805555555556in"}

  []{#__RefHeading___Toc18806779 .anchor}Figure 2-2. Orthographic
  composite photographic of White Mound Lake, WI. Coordinates are in
  meters (UTM Zone 15).
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image6.png){width="6.18125in" height="2.5236111111111112in"}

  []{#__RefHeading___Toc18806780 .anchor}Figure 2-3. The photograph of
  White Mound Lake taken from the boat ramp area and facing south. Areas
  of the lake with 4 m of water depth or less contain aquatic vegetation
  that extends from the bottom to the lake surface as shown in distance.
  -----------------------------------------------------------------------

## **2.2 Twin Valley Lake**

Twin Valley Lake is located in Iowa County approximately 6 miles north
of Dodgeville, WI. Located within Governor Dodge State Park (Figure
2-3), the lake area is approximately 152 acres and it has a maximum
depth of about 9.8 m (32 ft). Twin Valley Lake was constructed in 1967
for wildlife habitat, recreation, and flood control. The reservoir
impounding Twin Valley Lake is structure number 15 on Mill Creek (Figure
2-4). From Twin Valley Lake, Mill Creek flows eastward, northward, and
then westward (counterclockwise rotation) into the Wisconsin River.

  -----------------------------------------------------------------------
  ![](media/image7.png){width="5.9840277777777775in"
  height="4.797916666666667in"}

  []{#__RefHeading___Toc18806781 .anchor}Figure 2-4. 1967 base map of
  Twin Valley Lake (from Wisconsin Department of Natural Resources).
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image8.png){width="5.99375in" height="5.678472222222222in"}

  []{#__RefHeading___Toc18806782 .anchor}Figure 2-5. Orthographic
  composite photographic of Twin Valley Lake, WI. Coordinates are in
  meters (UTM Zone 15).
  -----------------------------------------------------------------------

# 3. Acoustic Survey System

## **3.1 Acoustic Profiling System**

The acoustic profiling system used herein was developed in a
collaboration between Baylor University and Specialty Devices, Inc.,
Plano, TX (SDI; Dunbar, et al., 1999; Figures 3-1 and 3-2). The system
was designed for sediment surveys of large water supply reservoirs, but
the commercialized version has been used to survey lakes, harbors, and
rivers as well. The prototype electronics module includes one computer
that controls the acoustic profiling, another that controls navigation,
a built-in computer monitor, and a differential GPS navigation system.
All of these components are contained in one, suitcase-sized
water-resistant box. The prototype module weighs 15 kg. In the
commercialized version, both subsurface profiling and navigation
operations are controlled by a single, faster computer, resulting in an
electronics module that is smaller and lighter. Power for the
electronics and acoustic source is supplied by a 12-volt marine battery.
Depending on the number and type of acoustic transducers used, the sound
source may weigh between 15 and 30 kg. Hence, even though the system was
designed for large reservoir surveys, deployment in small floodwater
retention reservoirs is logistically practical.

The system images the bottom and sub-bottom sediments with up to five
widely separated acoustic frequencies at a time. For large reservoirs,
with water depths of 5 to 50 m and sediment fill thickness of 1 to 10 m,
acoustic transducers with central frequencies of 200, 48, 24, 12, and
3.5 kHz are used. During acquisition the system collects traces using
each transducer independently in rapid succession in round-robin
fashion. The high-frequency signals provide a sharp image of low-density
fluid mud at the water bottom, whereas the low-frequency signals
penetrate many meters of sediment to image the base of sediment fill,
even in areas of high sediment accumulation. The limitation of the 12
and 3.5 kHz transducers is that they require a minimum of 3 to 5 m of
water depth to produce usable records while adding significantly to the
weight of the sound source (50 kg). The 24 and 48 kHz transducers
require 1 to 2 m of water depth to produce usable records. For the
present applications, acoustic transducers with central frequencies of
200, 48, and 24 kHz were used (Figure 3-2).

In surveys of water supply reservoirs with boat ramps and significant
water depth, the profiling system is deployed from a 24-ft pontoon boat.
As is typical of USDA-NRCS flood control reservoirs, boat access to
White Mound Lake and Twin Valley Lake were limited to Johnboats. The
sound source was suspended from a sling over the side of the boat,
holding the transducers at a constant depth below the surface and in a
fixed position relative to the GPS antenna. Control over the transducer
location is required to achieve precision water depth information and
sub-meter horizontal positioning accuracy.

## 

  -----------------------------------------------------------------------
  ![](media/image9.png){width="3.9993055555555554in"
  height="5.634722222222222in"}

  []{#__RefHeading___Toc18806783 .anchor}Figure 3-1. Photograph of
  acoustic profiler control module. All acoustic profiling and DGPS
  navigation electronics are contained in a compact, water-resistant
  control module.
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image10.png){width="5.448611111111111in"
  height="2.6145833333333335in"}

  []{#__RefHeading___Toc18806784 .anchor}Figure 3-2. Picture of acoustic
  profiler transducer array. The profiling system produces acoustic
  signals with three widely separated frequencies (200, 48, and 24 kHz)
  in rapid succession as the profiles are traversed.
  -----------------------------------------------------------------------

## **3.2 Survey Procedure**

Conventional sediment surveys are conducted by collecting data along a
series of parallel profiles at a set spacing that provides adequate
spatial coverage, plus a number of tie lines to insure consistency in
interpretation between profiles. During the surveys, the pre-planned
lines and the current boat location are displayed on the system monitor
as the boat is navigated. DGPS corrections are received from U.S. Coast
Guard beacons so that the location of the boat can be determined with
sufficient accuracy to guide it down the line in real-time. For both
surveys presented herein, strong correction signals were available from
a station in Milwaukee.

The present survey was conducted on May 30 and 31, 2002 by John Dunbar
and Paul Higley. Barbara Lensch and Scott Mueller, both of the
USDA-NRCS, Wisconsin, provided technical and logistical assistance.

## **3.3 Digital Processing and Interpretation of Collected Acoustic Data**

The main difference between conventional bathymetric surveying and
subsurface profiling is that subsurface profiling involves more
extensive digital processing and interpretation of the collected data.
During post-survey interpretation, recordings of the frequencies are
combined to produce a single, false-color cross-section through the
water column and sediment. In the present application, the red component
of the RGB display was set to the intensity of the 200 kHz signal, the
green component was set to the intensity of the 48 kHz signal, and the
blue component was set to the intensity of the 24 kHz signal.

The task of the user is to identify the water bottom and the base of
post-impoundment sediment and to trace these surfaces along each
profile. This is again accomplished using the program *Depthpic*. The
user traces the water bottom and sub-bottom surfaces by drawing on the
displayed acoustic data using the computer mouse. The depth and
horizontal position of each interpreted point is exported to a file that
can be read by commercial mapping programs.

The interpreted base of post-impoundment sediment was identified by
locating the deepest stratal surface (acoustic horizon) onlapping the
face of the dam. This base of sediment was then traced continuously from
deposits on the face of the dam out into the reservoir and up the
tributary arms. Once this was done, the same basal surface was
identified and traced on each of the crossing profiles. For both
surveys, a velocity of 1477 m/s was used to represent the average
velocity of both water and sediment in the conversion from sound travel
time to depth. This corresponds to the speed of sound in fresh water at
65° F and to near bottom sediments with a composition intermediate
between clay and sand.

An additional surveying difficulty was caused by the occurrence of thick
aquatic vegetation in areas of the reservoirs with 4 m of water depth or
less (Figure 3-3). During the surveys, the weeds fouled the outboard
propeller and transducer array. It was discovered during
post-processing, that the vegetation efficiently scatters high frequency
sound, making the 200 kHz data useless in heavily vegetated areas.
Fortunately, the vegetation was nearly transparent to the 48 kHz signal.
Hence, the water bottom was traced using the 48 kHz data alone in the
vegetated areas.

To generate water depth maps, the interpreted depths and horizontal
positions for each profile were exported from *Depthpic* to an ASCII
file of x, y, z values. These data were combined with points defining
the shorelines of each lake that were digitized from orthographic
composite photographs. The combined data were read by *Surfer*™, grided
using a triangulation and linear interpolation procedure, and contoured.
The contour maps generated by *Surfer*™ were then exported to a drafting
program to make the final figures presented herein. The sediment
thickness maps were generated in the same way, except that the sediment
isopach thickness (difference between the depth to the base of sediment
and the water depth) was exported directly from *Depthpic*. Volumes were
computed by trapezoidal rule applied to the interval grids.

  -----------------------------------------------------------------------
  ![](media/image11.png){width="4.572222222222222in" height="2.59375in"}

  []{#__RefHeading___Toc18806785 .anchor}Figure 3-3. Picture of aquatic
  vegetation. Both White Mound Lake and Twin Valley Lake contain
  vegetation that extends from the water bottom to the surface in areas
  of 4 m water depth and less. The vegetation scatters the 200 kHz
  acoustic signals.
  -----------------------------------------------------------------------

# 4. Results

## **4.1 Acoustic Survey of Twin Valley Lake**

The survey of Twin Valley Reservoir was conducted on May 30, 2002. The
survey began by collecting track lines across the main body of the
reservoir near the dam at 25 m spacing. Line-spacing was then switched
to 50-m for the tributary arms, in order to complete the survey in one
day (Figure 4-1).

The acoustic response of sediments onlapping the dam face show three
distinct acoustic layers (Figure 4-2). The upper-most layer appears
white on the multifrequency displays, indicating scattered returns from
all three frequencies. Such a response commonly corresponds to
fine-grain, water-rich sediments. This layer has a nearly uniform
thickness, 0.3 to 0.4 m, throughout the reservoir and appears to drape
bottom topography. The layer beneath the first appears light blue in the
false-color display, indicating returns from the 48 and 24 kHz signals,
but not the 200 kHz. This interval preferentially fills the lows and
channel axes, is 0.6 m thick near the dam, and thins upstream (Figures
4-2 and 4-3). In the upper reaches of the tributary arms, the light blue
layer is only present at the base of the channel axes. The bottom-most
layer deposited on the dam face appears dark blue, indicating returns
from only the 24 kHz signal. This interval is relatively thin (0.1 to
0.2 m) over most of the reservoir, but reaches considerable thickness in
the channel axes.

The resulting map of the water depth indicates a maximum depth of 10.2 m
(33.4 ft) near the dam and a total current water capacity of 2,089,500
m^3^ (1693.6 acre-ft; Figure 4-4). The sediment isopach map indicates a
maximum thickness of 3.1 m, in a small area along the main channel axis,
and a total volume of post-impoundment sediment of 458,659 m^3^ (371.8
acre-ft; Figure 4-5).

  -----------------------------------------------------------------------
  ![](media/image12.wmf){width="5.936111111111111in"
  height="6.845138888888889in"}

  []{#__RefHeading___Toc18806786 .anchor}Figure 4-1. Profile map of Twin
  Valley Lake acoustic survey. The survey contains 17 km of profile
  lines. Profiles 11 and 52, highlighted in red, are shown as example
  profiles in accompanying figures. Coordinates are in meters (UTM Zone
  15).
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image13.wmf){width="5.6618055555555555in"
  height="6.256944444444445in"}

  []{#__RefHeading___Toc18806787 .anchor}Figure 4-2. Southeast end of
  Twin Valley Lake axial profile, Line 52 (see Figure 4-1 for line
  location). Three acoustic intervals are seen within the sub-bottom in
  Twin Valley Lake. All three onlap onto the dam face. Hence, all three
  must be part of the post-impoundment sediment fill package.
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image14.png){width="5.997222222222222in"
  height="4.995833333333334in"}

  []{#__RefHeading___Toc18806788 .anchor}Figure 4-3. Processed Line 11
  with interpretation (see Figure 4-1 for line location). The
  multifrequency acoustic profiles show three distinct sediment layers
  within the post-impoundment fill.
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image15.wmf){width="5.909722222222222in"
  height="6.842361111111111in"}

  []{#__RefHeading___Toc18806789 .anchor}Figure 4-4. Contour map of water
  depth in Twin Valley Lake, WI. Depth is measured from the water surface
  at an elevation of 277.4 m (910 ft). Coordinates are in meters (UTM
  Zone 15).
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image16.wmf){width="5.911111111111111in" height="6.83125in"}

  []{#__RefHeading___Toc18806790 .anchor}Figure 4-5. Contour map of
  sediment thickness in Twin Valley Lake, WI. Sediment thickness is
  measured from the water bottom to the interpreted base of
  post-impoundment fill. Coordinates are in meters (UTM Zone 15).
  -----------------------------------------------------------------------

##  

## **4.2 Acoustic Survey of White Mound Lake**

White Mound Reservoir was surveyed on May 31, 2002. The entire survey
was conducted with a 25-m track line spacing (Figure 4-6). Acoustic
intervals similar to those seen in Twin Valley Lake were observed in
White Mound Lake onlapping onto the dam face (Figure 4-7) and thinning
upstream (Figure 4-8). The resulting maps indicate a maximum water depth
of 8.6 m (28.3 ft) and a current water storage capacity of 1,462,460
m^3^ (1,185.4 acre-ft; Figure 4-9). The sediment thickness map indicates
a maximum thickness of 1.4 m (4.6 ft) and a total volume of
post-impoundment sediment of 203,205 m^3^ (164.7 acre-ft; Figure 4-10).

  -----------------------------------------------------------------------
  ![](media/image17.wmf){width="5.184027777777778in"
  height="5.686111111111111in"}

  []{#__RefHeading___Toc18806791 .anchor}Figure 4-6. Profile map of White
  Mound Lake acoustic survey. Profiles 25 and 50, highlighted in red, are
  shown as example profiles in accompanying figures. Coordinates are in
  meters (UTM Zone 15).
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image18.wmf){width="6.0784722222222225in"
  height="6.7444444444444445in"}

  []{#__RefHeading___Toc18806792 .anchor}Figure 4-7. Southeast end of
  White Mound Lake axial profile, Line 50 (see Figure 4-6 for line
  location). Three acoustic intervals are seen within the sub-bottom in
  White Mound Lake. All three onlap onto the dam face. Hence, all three
  must be part of the post-impoundment sediment fill package.
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image19.png){width="5.486111111111111in"
  height="7.146527777777778in"}

  []{#__RefHeading___Toc18806793 .anchor}Figure 4-8. Acoustic profile 25
  of the White Mound survey (see Figure 4-6 for line location). (a)
  Multifrequency display in which 200 kHz is red, 48 kHz in green and 24
  kHz is blue intensity in the RGB image. The interpreted top and base of
  the post-impoundment fill are marked with black lines. (b) Aquatic
  vegetation blocks the 200 kHz signal, but is relatively transparent to
  the 48 kHz signal.
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image20.wmf){width="5.433333333333334in"
  height="6.028472222222222in"}

  []{#__RefHeading___Toc18806794 .anchor}Figure 4-9. Contour map of water
  depth in White Mound Lake, WI. Depth is measured from the water surface
  at an elevation of 278.4 m (913 ft). Coordinates are in meters (UTM
  Zone 15).
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image21.wmf){width="5.433333333333334in"
  height="6.014583333333333in"}

  []{#__RefHeading___Toc18806795 .anchor}Figure 4-10. Contour map of
  sediment thickness in White Mound Lake, WI. Sediment thickness is
  measured from the water bottom to the interpreted base of
  post-impoundment fill. Coordinates are in meters (UTM Zone 15).
  -----------------------------------------------------------------------

##  

## **4.3 Discussion**

The SDI acoustic profiling system produced interpretable images of the
water bottom and the base of sediment fill throughout both reservoirs.
The major uncertainties and sources of error in the surveys are
associated with error in the speed of sound and potential error in
identifying the base of post-impoundment sediment. The speed of sound in
fresh water varies about 0.6 % due to a 5° F temperature change from 65°
F. Hence, variation in the water velocity with the average temperature
would not contribute to more than 1 to 2% error in water volume
estimates. Unconsolidated sediments near the water bottom range in
velocity from 1530 m/s for pure sand to 1430 m/s for pure clay. Hence,
it is possible, though unlikely, that sediment thickness estimates could
be in error by as much as 3 to 4%, because of uncertainty in the speed
of sound in the sediments.

Misidentification of the stratal surface marking the base of
post-impoundment sediment could result in significant error in the
estimated total sediment volume. In reservoirs characterized by water-
and clay-rich sediment fill, the acoustic contrast between the
pre-impoundment soils and alluvium and the overlying low-density fill is
easily identified and mapped. The sediments in the Twin Valley and White
Mound Reservoirs appear to be predominantly composed of silt and clay
and produce reasonably distinct acoustic contrast with the
pre-impoundment material. Tracing the basal surface of sediments
deposited on the dam face provides an additional check on the correct
identification of the base of post-impoundment fill in these surveys.

As discussed in Bennett et al. (in press) and Bennett and Dunbar (in
press), the critical interpretation in sedimentation assessment is to
accurately discriminate between the pre-impoundment material and the
post-impoundment sediment deposition. This discrimination is made more
easily with ancillary data, obtained through coring activities, such as
the physical, textural, and chemical characteristics of the sediment or
through the use of isotopic signatures of elements or radioactive
emissions of Cesium-137 to facilitate dating specific sediment horizons.
In the absence of these data, the maps of sediment thickness presented
herein represent conservative estimates of sediment accumulation rates.

#  5. Conclusions

Since 1948, the USDA-NRCS has constructed nearly 11,000 upstream flood
control dams in 2000 watersheds in 47 states, most with a design life of
50 years. The watershed projects, which represent a \$14 billion
infrastructure, have provided flood control, municipal water supply,
recreation, and wildlife habitat enhancement. Because of population
growth and land use changes through time, sediment pools are filling,
some structural components have deteriorated, safety regulations are
stricter, and the hazard classification for some dams has changed.
Presently, 42 dams in Wisconsin are in need of immediate rehabilitation
at an estimated cost (1999 dollars) of \$3 million. Twelve of these dams
require hazard classification upgrades while the remaining 30 dams
require an extension of their service life, restoration, and repair.

Before any rehabilitation strategy can be designed and implemented, the
sediment impounded by these dams must be assessed in terms of the
structure's efficiency to regulate floodwaters and the potential hazard
the sediment may pose if reintroduced into the environment. At the
direct request of the USDA-NRCS in Wisconsin, two reservoirs, White
Mound Lake located in Sauk County and Twin Valley Lake located in Iowa
County, were chosen for this study. As part of their statewide
assessment program, the USDA-NRCS are interested in determining the
volume of sediment deposited within each of these lakes and the spatial
distribution of the impounded sediment, since loss of storage capacity
is an integral part of the assessment and rehabilitation process. To
this end, this study was undertaken to determine the current water
storage capacity and the amount and distribution of post-impoundment
sediment contained within two selected flood control reservoirs in
Wisconsin using an acoustic profiling system.

Two field sites were chosen for this study. The first site is White
Mound Lake, located in White Mound County Park in west-central Sauk
County, about 13 miles southwest of Reedsburg, WI. The lake area is
approximately 104 acres and it has a maximum depth of about 4.9 m. The
reservoir impounding White Mound Lake is structure number 15 on Honey
Creek, which has a watershed drainage area of about 4,474 acres. Honey
Creek flows eastward from White Mound Lake into the Wisconsin River. The
second site is Twin Valley Lake, located in Governor Dodge State Park in
Iowa County, approximately 6 miles north of Dodgeville, WI. The lake
area is approximately 152 acres and it has a maximum depth of about 9.8
m (32 ft). The reservoir impounding Twin Valley Lake is structure number
15 on Mill Creek, which flows eastward, northward, and then westward
into the Wisconsin River.

The acoustic profiling system used in the surveys was developed in
collaboration with Specialty Devices, Inc. of Plano, TX (SDI; Dunbar, et
al., 1999). The complete system consists of one, suitcase-sized, 10 kg,
water resistant control module, a 50 cm long, 15 kg acoustic source
array, GPS and differential correction antennas, and associated cables.
A 12-volt marine battery provides power for the system. The system was
originally intended to be used for sediment surveys of large water
supply reservoirs, but the commercialized version has been used to
survey lakes, harbors and rivers as well. The system images the bottom
and sub-bottom sediments with acoustic signals produced at 200, 48, and
24 kHz, deployed from a single Johnboat. All collected data were
post-processed to amplify the acoustic signals at depth (spherical
divergence) and to remove reverberations or multiple sound waves due to
the shallow water depth (predictive deconvolution). Once processed, the
acoustic data can be interpreted and subsurface stratigraphic horizons
can be identified with the aid of false-color coding of the frequency
data, although the presence of thick aquatic vegetation scattered some
of the high frequency sound.

The results from the two surveys were quite similar. The acoustic
response of sediments onlapping the dam face shows three distinct
acoustic layers. The upper-most layer appears white on the
multifrequency displays, indicating scattered returns from all three
frequencies. Such a response commonly corresponds to fine-grain,
water-rich sediments. The layer beneath the first appears light blue in
the false-color display, indicating returns from the 48 and 24 kHz
signals, but not the 200 kHz. This interval preferentially fills the
lows and channel axes and thins upstream. In the upper reaches of the
tributary arms, the light blue layer is only present at the base of the
channel axes. The bottom-most layer deposited on the dam face appears
dark blue, indicating returns from only the 24 kHz signal. This interval
is relatively thin in both reservoirs, but reaches considerable
thickness in the channel axes.

At Twin Lake, the sediment isopach map indicates a maximum thickness of
3.1 m, in a small area along the main channel axis, and a total volume
of post-impoundment sediment of 458,659 m^3^ (371.8 acre-ft). At White
Mound Lake, the sediment isopach map indicates a maximum thickness of
1.4 m (4.6 ft) and a total volume of post-impoundment sediment of
203,205 m^3^ (164.7 acre-ft). These values represent conservative
estimates of sediment accumulation without corroboration with ancillary
data.

# 6. References

Bennett, S.J., C.M. Cooper, J.C. Ritchie, J.A. Dunbar, P.M. Allen, L.W.
Caldwell, and T.M. McGee, in press, Assessing sedimentation issues
within aging flood control reservoirs in Oklahoma. *Journal of the
American Water Resources Association*.

Bennett, S.J., and J.A. Dunbar, in press, Physical and stratigraphic
characteristics of sediments impounded within flood control reservoirs,
Oklahoma. *Transactions, American Society of Agricultural Engineers*.

Caldwell, L.W., 1999, Rehabilitating our nation's aging small watershed
projects. Presented at the *Soil and Water Conservation Annual
Conference*, Aug. 8-11, Biloxi, MS.

Caldwell, L.W., 2000, Good for another 100 years: The Rehabilitation of
Sergeant Major Creek Watershed. Presented at the *Association of State
Dam Safety Officials*, September 28-30, 2000, Providence, RI.

Dunbar, J.A., P.M. Allen, and S.J. Bennett, 2001, Acoustic imaging of
sediment impounded by a USDA-NRCS flood control dam, Oklahoma. *USDA-ARS
National Sedimentation Laboratory Research Report No. 22*, 54pp.

Dunbar, J.A., P.M. Allen, and P.D. Higley, 1999, Multifrequency acoustic
profiling for water reservoir sedimentation studies. *Journal of
Sedimentary Research*, 69, 521-527.