Illinois EPA Clean Lakes Program%3A Clean Lakes Study Review and Implementation Report for Lake Hillsboro, Montgomery County, Illinois

              5201 South Sixth St. Rd., Springfield, Illinois 62703
Illinois EPA Clean Lakes Program
Clean Lakes Study Review and Implementation Report
for Lake Hillsboro, Montgomery County, Illinois
July 2007
City of Hillsboro
Illinois EPA
USDA NRCS
Prepared and Submitted by: with Assistance from:
Table of Contents
Introduction and Phase 1 Study Review 1
Description and Analysis of Restoration Alternatives 4
Restoration Alternative: Shoreline Stabilization 5
Summary of Findings and Proposed Actions for Shoreline Stabilization 8
Restoration Alternative: Sediment and Nutrient Control Basin 10
Restoration Alternative: Sediment Removal 13
In-Lake Sedimentation Survey 13
Sediment Removal Options 16
Summary of Findings and Proposed Actions for Sediment Removal 18
Additional Management Considerations 23
Aeration and Destratification 23
Watershed Best Management Practices 24
Water Quality Monitoring 25
Educational Programs 26
Fisheries Management 26
Prioritized Recommendations for Lake Hillsboro 27
List of Figures
Figure 1. Shoreline Erosion Survey Plan 7
Figure 2. Examples of Riprap Shoreline Protection 9
Figure 3. Location of Proposed Sediment & Nutrient Control Basin Enhancement 11
Figure 4. Sediment Survey Plan 15
Figure 5. Proposed Sediment Removal Plan 20
List of Tables
Table 1. Estimate of Probable Cost to Stabilize Eroded Shoreline 9
Table 2. Estimated Loading Reductions for Shoreline Stabilization 10
Table 3. Estimate of Probable Costs for Sediment and Nutrient
Control Basin Enhancement 12
Table 4. Estimated Loading Reductions for Sediment and Nutrient Control Basin 13
Table 5. Sedimentation Survey Results 14
Table 6. Recommended Dredging Areas and Quantities 19
Table 7. Estimate of Probable Sediment Removal Costs 21
Table 8. Estimated Loading Reductions for Proposed Sediment Removal 23
Appendix
Sediment Survey Cross-Sections
Laboratory Analyses of Sediment Core Samples
Representative Photographs
Clean Lakes Study Review and Implementation Report
for Lake Hillsboro, Montgomery County, Illinois
Introduction and Phase I Study Review
In April 2007, HDR│Cochran & Wilken, Inc. (HDR│CWI) entered into a contract
with the City of Hillsboro to review the completed Phase 1 Diagnostic/Feasibility Study
of Lake Hillsboro and provide recommendations for the implementation of restoration
alternatives for extending the usable lifespan of the lake. The primary objectives of the
Phase 1 Report recommendations were intended to reduce sediment and nutrient
delivery to the lake, improve aquatic habitat and overall water quality, and improve
recreational opportunities (Zahniser Institute, undated). A summary and review of the
Phase 1 Diagnostic/Feasibility Study of Lake Hillsboro previously completed is provided
in the following paragraphs.
The Phase 1 Clean Lakes Study reported that Lake Hillsboro is a 108 acre
reservoir located in the central portion of Montgomery County, Illinois. The major
tributary that feeds into the lake is an unnamed tributary of Middle Fork Shoal Creek.
Lake Hillsboro is managed and owned by the City of Hillsboro, and used for recreation
purposes such as swimming, boating, fishing, hiking, and camping. The lake is also
used as a supplemental water supply for the City of Hillsboro. The Lake Hillsboro
watershed is approximately 4,470 acres and is comprised primarily of agricultural land
uses such as cropland and pasture (85 percent).
The Phase I Clean Lakes Study reported limnological data for the purpose of
identifying water quality impairments. Three in-lake monitoring locations were
established with Site 1 located at the northwestern end of the lake near the water supply
intake, Site 2 located in the center of the lake, and Site 3 located upstream at the
southeastern end of the lake. Specific water quality impairments identified in the study
included turbid water, high phosphorus levels, low dissolved oxygen levels, and internal
nutrient release (Zahniser Institute, undated). In order to provide a better understanding
of the water quality impairments present at Lake Hillsboro, select water quality data
from the previous study are summarized briefly in the following paragraphs.
2
Secchi transparency depth measurements at Lake Hillsboro were consistently
less than two feet during the Phase 1 monitoring period (May 2001 - April 2002). Poor
water clarity or turbidity was attributed to excessive suspended solids and algal
populations within the water column. The Secchi readings during the 2001 – 2002
sampling period averaged approximately 20 inches at Site 1 near the dam to 15 inches
at Site 3 located in the upper portion of the lake.
The lake experienced low dissolved oxygen (DO) levels at Site 1 during extended
periods of the summer in 2001. During the summer stratification period, anoxic
conditions (i.e., DO concentrations less than 1.0 mg/l) were present at Site 1 at water
depths greater than 9 to 11 feet. The surface summer DO concentrations at Site 1
ranged from 6 to 18 mg/l. In addition to turbid water and low DO concentrations,
phosphorus and nitrogen concentrations were elevated. Samples collected during the
Phase 1 period indicated that high total phosphorus (TP) and total nitrogen (TN)
concentrations were present throughout the lake. The water quality standard for total
phosphorus of 0.05 mg/l was exceeded at Lake Hillsboro during the entire monitoring
year with a mean TP concentration of approximately 0.3 mg/l during the summer. The
inorganic nitrogen levels (nitrate-nitrite and total ammonia nitrogen concentrations)
greatly exceeded 0.30 mg/l, a threshold known to be sufficient to stimulate excessive
algal growth.
The Phase 1 Report indicated that Lake Hillsboro was eutrophic during 2001, but
the Trophic State Index (TSI) was not specifically reported. After re-evaluating the
water quality data, it is more likely that the lake was exhibiting hypereutrophic conditions
during the productivity season (May – September 2001). Based on the graphs in the
report, the annual mean values were estimated to be 20 inches for Secchi transparency
at Site 1 and 17 inches at Site 3, 50 ug/L for Chlorophyll a at Site 1 and 60 ug/l at Site 3,
and 300 ug/l for total phosphorus at each site. According to Carlson’s TSI, a Secchi
depth of 20 inches would correspond to eutrophic conditions (TSI ~70), whereas
Chlorophyll a concentration of 60 ug/l (TSI >70) and a total phosphorus concentration of
300 ug/L (TSI ~85) would correspond to hypereutrophic conditions. TSI values between
50 and 70 indicate eutrophic conditions and values greater than 70 indicate
hypereutrophic conditions. Based on the estimated TSI values, Lake Hillsboro exhibited
3
consistent hypereutrophic conditions at Site 3 and eutrophic to hypereutrophic
conditions at Site 1.
The aquatic plant community (i.e., macrophytes and phytoplankton or algae) and
other biological resources within Lake Hillsboro appeared to be unbalanced during the
Phase 1 sampling period. The aquatic macrophyte survey included terrestrial, wetland,
and some aquatic species. While several emergent and floating aquatic macrophyte
species were noted, submerged species were not observed during the survey. Based
on the water clarity at Lake Hillsboro, these findings were not surprising and indicate
that the aquatic macrophyte population could be improved. The phytoplankton
population during the Phase 1 period was dominated by cyanophytes (nuisance blue-green
algae), which have the ability to out-complete other more beneficial algal groups
and can cause taste and odor problems for the public water supply. Several blue-green
algal species reached “bloom” densities (greater than or equal to 1 million units per liter)
in samples that were collected during the summer of 2001. This is supported by
elevated chlorophyll levels during the summer productivity period. These imbalances in
the aquatic plant populations can impact fish habitat and spawning areas within the
lake.
The hydrologic budget suggests that inputs (i.e., inflow from runoff and
precipitation) to the lake were overestimated and outputs (i.e., outflow from spillway
discharge, water supply withdrawal, and evaporation) were underestimated. The Phase
1 total inputs and outputs of water during the May 2001 – April 2002 monitoring period
were calculated at 19,256 and 8,461 acre-feet, respectively. Actual rainfall totals for the
Phase 1 period (50.6 inches) strongly suggest that the initial Phase 1 hydrologic
estimates (18,595 acre-feet from the watershed and tributaries and 661 acre-feet from
precipitation that fell directly onto the lake) were significantly overestimated. Re-calculated
hydrologic inputs based on Phase 1 precipitation totals were estimated to be
approximately 4,147 acre-feet for watershed runoff and tributaries and 455 acre-feet for
direct precipitation to the lake.
Nutrient (i.e., phosphorus and nitrogen) and sediment budgets were also
developed by Zahniser during the Phase 1 monitoring period. The initial Phase 1
nutrient budgets suggest that most of the incoming phosphorus (82 percent) and
4
nitrogen (74 percent) is retained and assimilated into biomass (i.e., algal blooms).
Similarly, almost all the sediment (93 percent) entering the lake is retained and
deposited at the bottom of the lake. It is important to note that the overestimates in the
initial hydrologic budget inputs likely skewed and offset subsequent nutrient and
sediment budgets, which were presumed to be based on the hydrologic budget.
As noted in the Phase 1 Report, shoreline erosion can increase overall lake
sedimentation, impact aquatic plant and fish habitat, and reduce the value of shoreline
property. The results of the initial Phase 1 shoreline erosion survey were reviewed and
reconfirmed in the field. The 2007 survey noted an approximate total of 10,000 linear
feet of eroded shoreline. Severely (bank heights >8.0 feet) and moderately (bank
heights >3.0 feet and <8.0 feet) eroded shoreline accounted for 1,508 and 2,855 linear
feet, respectively. These findings differ from the Phase 1 Study results, which indicated
that there were approximately 300 feet of severe and 2,522 feet of moderate shoreline
erosion present at Lake Hillsboro. The 754 feet of shoreline noted in the Phase 1
Report as being protected with riprap was observed to be 1,492 feet plus an additional
578 feet were found to be protected with other materials.
Previous estimates of volume loss throughout the lake have been calculated by
NRCS and Hurst-Rosche Engineers using bathymetry, original lake bottom contour
mapping, and depth finding equipment. Based on a sediment survey conducted by the
NRCS in 1995 and bathymetric mapping performed by the Zahniser Institute in 2002,
the Phase 1 Study reported that Lake Hillsboro has lost approximately 36 to 40 percent
of its original capacity due to sedimentation. In order to more accurately determine the
amount and location of sediment deposition throughout the lake, an updated
sedimentation survey was completed by HDR│CWI in May 2007. Results of the
sedimentation survey showed that only 15% of the lake’s total original capacity has
been lost to sedimentation. It is possible that previous estimates of volume loss may
have been overestimated.
Description and Analysis of Restoration Alternatives
Pursuant to the information collected during the Phase 1 study period, potential
alternatives for water quality improvement were developed by Zahniser and presented
5
within the Phase 1 Report (undated). The primary alternatives with the most significant
cost implications recommended for implementation by the City of Hillsboro included: 1)
Sediment Removal (dredging) to be performed in the south end of lake (161,333 cy), at
an estimated cost of $1,613,330 and in several smaller coves at an estimated cost of
$60,000 to $150,000 per cove; 2) Shoreline Stabilization of approximately 10,484 linear
feet of eroded shoreline (including slight erosion) at a total estimated cost of $431,360,
or stabilization in smaller annual increments of 1,500 feet each year; 3) Installation of an
Aeration System near the water intake structure at an estimated cost of $200,000; and
4) Construction of a Wetland Detention System east of South (actually Smith) Road at
an estimated cost of $250,000.
Other alternatives that were less significant in cost, were to be implemented in
cooperation with NRCS/SWCD, or dependent on future land owner cooperation
included: 1) Stream Bank Stabilization; 2) Conservation Practices in the Watershed (i.e.
riparian buffers, grassed waterways, filter strips, conservation tillage, etc.); 3)
Construction of Shallow Water Wetlands and/or Storm Water Wetlands in the
watershed; 4) Lake Education Programs to enhance public understanding and
perception of lake and watershed water quality relationships; and 5) Reviewing and
Updating City Ordinances (related to the lake).
Since the alternatives that are directly within City control (i.e., on City property)
are the most feasible for immediate implementation, additional field work and analysis
was completed by HDR│CWI in order to adequately evaluate and prioritize those
alternatives best suited for reducing sedimentation, improving water quality, increasing
water quantity, and extending the usable life span of the lake.
Restoration Alternative: Shoreline Stabilization
The uncontrolled erosion of shoreline areas is a source of sediment and nutrient
loadings to Lake Hillsboro. There are a number of factors that contribute to shoreline
erosion, including easily erodible shoreline soil types, exposure to waves generated by
prevailing winds, fluctuating water levels, and a lack of near shore aquatic macrophytes
and/or rock breakers. Shoreline erosion impairs lake usage and access by adding
turbidity and decreasing lake storage capacity. The loss of shoreline soils may also
6
jeopardize the stability of infrastructure, such as bridges, roads, docks, etc. In addition,
shoreline loss reduces the extent and value of shoreline property and the overall
aesthetic appeal of the lake.
In 2007, HDR│CWI reconfirmed the initial shoreline erosion survey completed by
Zahniser during the Phase 1 study period in order to assist the City of Hillsboro with
overall prioritization of needs. The 2007 survey found a total of 9,996 linear feet (1.9
miles) of shoreline along the lake to have some degree of erosion. Severely (bank
heights >8.0 ft) and moderately (bank heights >3.0 ft and <8.0 ft) eroded shoreline
accounted for 1,508 and 2,855 linear feet, respectively, while slightly (bank heights >1.0
ft and <3.0 ft) eroded shoreline accounted for 5,633 linear feet (see Figure 1).
Approximately 2,000 feet of riprap, concrete, and wood has been placed in an attempt
to stabilize the eroded shoreline and reduce erosion. Most of the stabilized locations
appeared to be successful measures. In addition to the general observations and
measurements, the areas exhibiting the greatest need of stabilization were documented
and photographed (see Representative Photographs in Appendix). One area that
should be noted is located on the west side of the lake beneath the golf course country
club. The bank heights in this area are drastic (approximately 25 to 30 feet high) and
unstable. This area should be monitored closely and may need measures put in place
at the top of the bank in addition to shoreline stabilization at the toe to prevent future
mass wasting events that could be detrimental to any structures located close to the
edge of the bank.
Over the 89-year history of Lake Hillsboro, approximately 36,500 tons of soil has
been eroded from the shoreline areas. These shoreline erosion estimates equate to an
average annual loading of approximately 410 tons/year. The total tons of delivered soil
were calculated using a dry unit weight of 100 pounds per cubic-ft for undisturbed,
native soil densities. The estimated loading to Lake Hillsboro based on the 2007
shoreline survey was estimated by extending the eroded bank into the lake at a
projected slope of 3:1 (3 foot horizontal to 1 foot vertical) to form a typical triangular end
area. Then, the length of the eroded shoreline in linear feet was multiplied by the
projected end area for each degree of classification of erosion.
0 87.5 175 350 525 700
Feet ± Figure 1. Shoreline Erosion Survey Plan
Legend
Slight Erosion (<3')
Moderate Erosion (>3' - <8')
Severe Erosion (>8')
Existing Stabilization
Lake
Hillsboro
2005 ISGS Aerial Photograph
8
Summary of Findings and Proposed Actions for Shoreline Stabilization
As previously mentioned, there have been segments of shoreline at Lake
Hillsboro that have been stabilized with riprap revetments and off-shore riprap,
breakwater structures. Despite these minor shoreline stabilization efforts, a significant
amount of eroded shoreline within the lake is in need of protection and stabilization.
Technology has resulted in the development of many products to control erosion and
older methods have been improved. The following were considered when deciding the
best approach for stabilizing Lake Hillsboro shoreline: riprap (both crushed stone and
rounded glacial stone) with filter rock or filter fabric, erosion mats, plastic and natural
geowebs, gabions, and native plantings.
For the impacted areas with moderately and severely eroded shorelines, riprap is
the recommended alternative to address the erosion problems at Lake Hillsboro. The
advantages of riprap include its reliable longevity, ease of installation and relatively
inexpensive cost over large areas. All riprap should be installed using either filter stone
or filter fabric to prevent washout from behind the installed riprap. Shoreline
stabilization work on this scale will require a Joint Application Permit from the Army
Corps of Engineers, the Illinois Environmental Protection Agency, and the Illinois
Department of Natural Resources, particularly for riprap placed as fill material beneath
the normal water level.
Riprap should be placed along the undercut bank of the shoreline two feet below
and two feet above normal pool (spillway elevation) at a 2:1 slope. Once the toe of the
slope is protected from further undercutting by structural stabilization methods, the
eroded slope will gradually slough until a state of equilibrium is reached. If a shallow,
gently sloping littoral zone is present, an offshore breakwater may be feasible and
would allow natural establishment of native plant species to provide additional soil
stabilization and nearshore habitat. Representative photographs of shoreline
stabilization options are provided in Figure 2. The estimated cost of riprap stabilization
using A-grade, gradation RR4 broken stone riprap with filter fabric is approximately $60
per linear foot installed. Estimates of probable costs for shoreline stabilization at Lake
Hillsboro are listed in Table 1.
9
Figure 2. Examples of Riprap Shoreline Protection
Shoreline Revetment Offshore Breakwater
Table 1. Estimate of Probable Cost to Stabilize Eroded Shoreline
Task
Description
Length of
Shoreline
Stabilization
Method
Estimated
Cost
Severe Erosion Areas (> 8') 1,508 LF Riprap ($60/LF) $90,480
Moderate Erosion Areas (3' - 8') 2,855 LF Riprap ($60/LF) $171,300
Mobilization & Site Prep. $20,000
Estimated Shoreline Stabilization Cost $281,780
Estimating Contingency (5%) $14,089
Engineering Design and Permitting (15%) $42,267
Total Estimate of Probable Cost for Shoreline Stabilization $338,136
Aside from physical stabilization of moderately and severely eroded shoreline,
areas that have been identified as slightly eroded should be closely monitored. This will
prevent severe erosion in the future and allow for prioritization of areas that will need to
be stabilized at a later date. In some cases, it is possible for extensive shoreline
erosion to occur in a very short period of time; however, mass wasting events could be
prevented by closely monitoring susceptible areas.
Loading reductions from the proposed shoreline stabilization actions were
estimated using the Illinois EPA’s Spreadsheet program entitled “Estimating Pollutant
Load Reductions for Nonpoint Source Pollution Control BMP’s.” Table 2 summarizes
10
the estimated annual loading reductions from the stabilization of prioritized shoreline
areas.
Table 2. Estimated Loading Reductions for Shoreline Stabilization
Classification of
Eroded Shoreline
Linear
Feet
Sediment
(tons/yr)
Phosphorus
(Lbs/yr)
Nitrogen
(Lbs/yr)
Moderate (3 to 8 ft. bank heights) 2,8 55 29 1 29 1 58 2
Severe (> 8 ft. bank heights) 1,508 385 385 769
Total 4,363 676 676 1,351
Restoration Alternative: Sediment and Nutrient Control Basin
Sediment and nutrient control basins can be an effective measure for controlling
and isolating sediment and nutrients prior to being transported further downstream in a
lake system. Construction of a wetland system with a detention pond to be located
east of South Road was suggested in the Phase I Report in order to reduce
sedimentation and nutrient loadings; however, a similar type of system is currently in
place south of Smith Road (Figure 3). Currently, this location is owned by the City and
acts as a sediment detention basin with a wetland area.
As a result of in-lake sediment and water depth measurements, it was
determined that the area south of Smith Road has: 1) approx. 3,000 cu. yds. of
accumulated sediment; 2) approx. 40 percent less capacity than it’s original storage
capacity; 3) water depths of only 3 feet or less; and 4) sediment thickness
measurements over 2 feet in some areas. The sediment deposition pattern indicates
that the existing roadway embankment with a bridge opening has functioned effectively
over the 90 year life of the lake. As sediment continues to be transported into the lake,
the relative sediment trapping efficiency of the roadway embankment and existing
bridge opening will gradually decrease unless corrective measures are taken.
While the previously proposed system may enhance the trapping efficiency of the
current basin south of Smith Road, it is likely that enhancing this area by removing
sediment and increasing the diversity of native wetland plants would have a similar
benefit. It would also be beneficial to further deepen this area by removing several feet
of the underlying native soils. The added depth would provide increased trapping
efficiency and extend longevity for the sediment and nutrient control basin.
Location of Sediment and Nutrient Control Basin
0 87.5 175 350 525 700
Meters ± Figure 3. Location of Proposed Sediment &
Nutrient Control Basin Enhancement
Smith Road
Smith Road
USGS 1974 Topographic Map
12
Probable tasks and costs for enhancement of the area south of Smith Road are
listed in Table 3. The proposed sediment removal effort excludes the wetland area so
that sediment trapping is enhanced and the opportunity for nutrient uptake is increased.
Table 3. Estimate of Probable Sediment and Nutrient Control Basin Enhancement
Sediment Removal
Work Task Quantity
Estimated
Cost Range
Basin Enhancement ($8.00/cy) 7,610 cu.yds. $60,880
Dredge Mobilization and Demobilization 1 Lump Sum $35,000
Polymers / Flocculants for Effluent Water ($1.00/cy) 1 Lump Sum $7,610
Construct Sediment Retention/Dewatering * Facility 1 Lump Sum $25,000
Native Plantings 1 Lump Sum $10,000
Subtotal $138,490
Estimating Contingency (10%) $12,849
Engineering and Permitting (15%) $19,274
Illinois EPA Permit Fees (i.e. dredging, stormwater) 1 Lump Sum $2,109
Total Estimated Cost for Basin Enhancement $172,721
Probable Site Reclamation Cost * 1 Lump Sum $25,000
* This cost is included only if the sediment basin enhancement work is completed as a stand alone
project. If completed with lake dredging efforts, the dredged sediment could be placed in one storage
facility.
Since sedimentation rates in most Illinois agricultural watersheds have typically
decreased over the past 10 to 15 years as a result of the implementation of various
conservation practices (e.g., grass waterways, vegetative buffers, conservation
farming), the rate of future sediment deposition is likely to decrease. By selectively
removing accumulated sediment in the basin and extending the depth several feet
below the original bottom depth, the longevity and function of the current sediment and
nutrient basin would be greatly increased. Estimated loading reductions for sediment
removal and future sediment and nutrient trapping are listed in Table 4. Loading
reductions for sediment removal activities were estimated using the mean sediment,
phosphorus and nitrogen concentrations listed in the Phase I Report (Zahniser,
undated) and a typical mean dry bulk density for sediment (40 lbs/CF (1,080 lbs/CY)).
Loading reductions based on the functionality of a sediment and nutrient control basin
on a yearly basis were calculated using the sediment and nutrient inflows reported by
Zahniser (undated).
13
Table 4. Estimated Loading Reductions for Sediment and Nutrient Control Basin
Parameters Units
Sediment Removal
Loading Reductions
Sediment and Nutrient
Basin Loading Reductions *
Volume Removed CY 7,610 NA
Sediment Lbs/CY 1,080 NA
Phosphorus mg/kg 1,232 NA
Nitrogen mg/kg 8,553 NA
Sediment Lbs 8,218,800 3,680,013
Tons 4,109 1,840
Phosphorus Lbs 20,669 15,066
Tons 10 46
Nitrogen Lbs 143,495 91,023
Tons 72 8
* Assumes a 50% trapping efficiency.
Restoration Alternative: Sediment Removal
In-Lake Sedimentation Survey
In May 2007, an in-lake sedimentation survey of Lake Hillsboro was completed
by HDR│CWI in order to determine the extent of sediment deposition and loss of water
storage capacity. Existing water depths and accumulated sediment thicknesses were
measured along various cross-section transects and points throughout the lake. Figure
4 displays locations of lake segments, transect lines, and sediment core samples. The
original capacity, existing capacity, and percent capacity loss were estimated using the
survey data and are listed in Table 5. Cross sections that show existing water depth
and sediment deposition measurements for the surveyed transects are in the Appendix.
It is important to note that two locations have functioned to prevent a large
amount of sediment from entering the lake. One location is the small area south of
Smith Road at the southern end of the lake. This area has trapped sediment and has
allowed a delta to form in a way that has resulted in a small established wetland in the
southeast corner. The second location that has benefited the lake is a very large area
upstream of the lake that appeared to be a reservoir at one time (i.e., Big Four
Reservoir). According to aerial photography, it is likely that sediment deposition has
occurred upstream of the lake in this area. Without these two areas historically
functioning as sediment traps, the amount of accumulated sediment in Lake Hillsboro
would be much higher and capacity loss would be more significant.
14
Table 5. Sedimentation Survey Results
Segment
Original
Capacity
Existing
Capacity
Sediment
Deposited
Percent of
Capacity Loss
(cubic yards of water) (cubic yards)
2A 18,295 9,697 8,598 47.0%
3A 67,030 41,815 25,215 37.6%
4A 83,938 61,275 22,663 27.0%
7A 151,479 117,197 34,282 22.6%
8A 129,721 104,683 25,038 19.3%
9A 146,754 119,980 26,773 18.2%
14A 428,336 369,486 58,850 13.7%
23A 725,127 657,838 67,289 9.3%
24A 91,588 84,051 7,537 8.2%
SubArea A 1,842,268 1,566,023 276,246 15.0%
5B 6 66 4 13 2 53 38.0 %
6B 5,113 4,097 1,016 19.9%
SubArea B 5,779 4,510 1,269 22.0%
10 C 10,3 42 8,5 40 1,8 02 17.4 %
11C 36,168 31,793 4,375 12.1%
SubArea C 46,510 40,334 6,177 13.3%
15 D 24,5 72 23,7 18 8 55 3.5 %
16D 50,086 47,684 2,402 4.8%
SubArea D 74,658 71,401 3,257 4.4%
21 E 1,9 06 1,7 46 1 60 8.4 %
22E 14,526 13,218 1,308 9.0%
SubArea E 16,432 14,964 1,468 8.9%
18F/ 20F 5 76 4 44 1 32 23.0 %
19F 1,256 975 282 22.4%
17F 5,934 4,896 1,038 17.5%
SubArea F 7,191 5,871 1,320 18.4%
13 G 5,4 14 4,0 57 1,3 57 25.1 %
12G 32,696 25,823 6,874 21.0%
SubArea G 38,110 29,880 8,231 21.6%
Total 2,039,388 1,738,327 301,061 14.8%
!(
!(
Figure 4. Sediment Survey Plan
2A
3A
4A
7A
8A
9A
14A
23A
24A 15D
16D
10C
11C
13G
12G
22E
21E
20F
18F
17F
19F
6B
5B
0 312.5 625 1,250 1,875 2,500
Feet ±
Legend
!( Sediment Core Locations
Transect Lines
2+89A
13+99A
8+07A
1+96H
35+37A
22+45A
28+27A
49+02A
64+19A
2+59B
0+79B
2+92C
6+02C
3+56D
6+33D
4+85E
1+93E
1+62F
0+93FF
3+58F
0+90F
6+14G
3+00G
2005 ISGS Aerial Photograph
#1
#2
2A Lake Segments
16
As expected, the 2007 sedimentation survey indicated that the southern end and
inlets of the lake had been impacted to the greatest degree, although several coves
appear to be impacted as well. The accumulated sediments within these areas have
impacted lake water quality through increased turbidity, reduced water depths, internal
nutrient recycling from resuspension and/or anaerobic decomposition, which has had
subsequent adverse effects on the aquatic plant communities and fish population. In
addition, excessive sedimentation and shallow water depths have reduced lake access
and overall recreational usage. The survey suggested that Lake Hillsboro contains
approximately 300,000 cubic yards of accumulated sediment and has experienced an
average sediment deposition rate of approximately 3,345 cubic yards per year over the
89 year life of the lake. Using the original volume estimate determined by HDR│CWI,
this rate of sediment deposition has accounted for a water storage capacity loss of 14.8
percent over the history of Lake Hillsboro.
Sediment Removal Options
The major alternatives for removing sediment accumulation from within Lake
Hillsboro include extended water level drawdown, mechanical dredging, and hydraulic
dredging.
a. Lake Water Level Drawdown and Compaction
Lowering the water level and allowing the sediment to dry and consolidate is an
alternative for restoring lost water depths in some lakes. However, this treatment
alternative is generally a limited solution for excessive sediment deposits. In order to
assure optimum drying and compaction, the water level would have to be substantially
lowered for a sufficient period of time. According to a study completed by Fox et al.
(1977), approximately 170 days of exposure to drying conditions would produce a
sediment consolidation ranging from 7 to 50 percent, with water losses ranging from 40
to 50 percent. It would be anticipated that the sediment found in the upper arms of Lake
Hillsboro would fall in the median range, and could thus be expected to consolidate
approximately 25 percent. However, some of the anticipated consolidation could be
reversed after refilling the lake and re-saturating the sediment.
17
In order to effectively reduce sediment volume in the upper end of Lake Hillsboro,
water levels would have to be lowered approximately ten feet or more. A drawdown
extended well into the spring or even into the summer would be difficult to implement
and would negatively impact the recreational uses of the lake. Most importantly, Lake
Hillsboro is an alternate public water supply and an extended drawdown could be
detrimental to public need, particularly if a drought occurred during the anticipated
refilling period.
b. Mechanical Dredging
There are several methods of mechanical dredging or excavation. The lake can
be dredged at normal pool with a dragline, or the water level could be lowered enough
to allow low ground pressure excavation equipment into the dry lakebed. There are
several advantages to dry lakebed excavation as compared to hydraulic or dragline
dredging, such as the elimination of excessive turbidity or resuspended solids, and a
smaller quantity of material to remove due to consolidation and compaction. However,
there are many disadvantages and problems that could be encountered. The length of
time required for the sediment to dewater and consolidate sufficiently enough to support
excavation equipment may take longer than expected if frequent rainfall events occur.
Therefore, this option is not considered feasible, since watershed drainage would likely
cause flooding problems within the dredging area and because Lake Hillsboro is used
as an alternate public water supply that needs to remain at full pool for public need.
Another method of mechanical dredging could be accomplished with a dragline
while the lake water level is at normal pool. This is accomplished by extending
excavating equipment from shore, or by mounting the equipment on a barge. This
method is most practical for small lakes or when a large quantity of rocks or debris is
anticipated. Furthermore, the removal of accumulated lake sediment in this manner is
inefficient and can leave high percentages of material behind. Offsite placement of the
sediment is also very inefficient and labor intensive, since it must be handled several
times. Once the sediment is removed from the lake, it must be placed on a barge or a
truck and transported to the retention site. This repeated handling is generally not cost
effective, and can result in sediment losses during transfer. Equipment access for the
removal and placement of dredged sediment would also have a negative impact on the
18
lake shoreline. Therefore, mechanical dredging with a dragline would not be considered
a feasible sediment removal method for Lake Hillsboro.
c. Hydraulic Dredging
Hydraulic dredging involves a centrifugal pump mounted on a pontoon or hull,
which uses suction to pull the loose sediment off the bottom and pump it through a
polyethylene pipeline to a sediment retention area. Generally, a cutterhead is added to
the intake of the suction line in order to loosen the accumulated or native sediment for
easy transport and discharge. A slurry of sediment and water, generally ranging
between 10 and 15 percent solids (by weight), can be pumped for distances as much as
10,000 to 15,000 feet (or more) with the use of a booster pump(s). The efficiently
pumped sediment slurry must be discharged into a suitably constructed earthen dike-walled
containment area with adequate storage capacity. The sediment containment or
retention area must be properly designed to allow sufficient retention time for the
sediment particles to settle, and allow the clear decant or effluent water to flow through
the outlet structure back to the lake.
One of the advantages of hydraulic dredging is the efficiency of sediment
handling. The removal, transport, and deposition are performed in one operation, which
minimizes expenses and potential sediment losses during transport. Another
advantage is that the lake does not have to be drained, and most areas can remain
open for public use. Most hydraulic dredges are considered portable and are easily
moved from one site to another. They are extremely versatile and are capable of
covering large areas of the lake by maneuvering with their spud anchorage system and
moving the discharge pipeline when necessary.
Summary of Findings and Proposed Actions for Sediment Removal
Hydraulic dredging is recommended to remove the accumulated sediment in the
upper portions of Lake Hillsboro. Sediment removal at Lake Hillsboro will provide an
effective improvement in water quality and recreational benefits by removing nutrient
rich, accumulated sediment. The removal of sediment will also improve and expand
aquatic habitat for fish, macro-invertebrates, and other aquatic organisms. A maximum
dredge cut depth of at least 10 feet (or when the underlying hard, original lake bottom is
19
reached) is strongly recommended in order to provide additional storage volume, extend
the useful lifespan of the lake and provide a long-term water quality benefit to the lake.
The percentage of capacity loss for each sub-area and the relationship between
water depth and sediment thickness were used to prioritize sub-areas for potential
dredging. After eliminating those segments with adequate existing water depths (i.e.,
more than 8 to 10 feet), minimal sediment deposition (i.e., less than 2 feet) and minimal
capacity loss, preliminary sediment removal recommendations were developed (Table
6). Dredging of six lake segments, including three segments in the main body and three
segments within small coves, is recommended to restore an overall capacity of 3%.
While a 3% overall gain may appear to be minimal, capacity restored in each of the
targeted segments ranges from 17.5% to 47%. A proposed sediment removal plan is
illustrated in Figure 5.
Table 6. Recommended Dredging Areas and Quantities
Lake Segment
Amount of Sediment
to Dredge (cu. yds.)
Capacity
Restored
2A 8,598 47.0%
3A 25,215 37.6%
4A 22,663 27.0%
5B 1,016 38.0%
20F 132 17.5%
13G 6,874 21.0%
Total 64,498 3.2%
The sediment removed from the lake should be placed in an upland containment
and dewatering site. This site should consist of flat or gently sloping land within two
miles of the lake that drains back into the lake (i.e., within the watershed). Preferably,
the land should be owned by the City of Hillsboro. If the City of Hillsboro does not own
or have access to land within the lake’s watershed to construct the detention facility, a
cost effective option would be for the City to enter into an agreement with a cooperative
landowner (e.g., purchase, lease, etc.) that would allow a retention site(s) to be
constructed for storage and dewatering of dredged sediment.
Figure 5. Proposed Sediment
Removal Plan
2A
3A
4A
7A
8A
9A
14A
10C
11C
13G
12G
20F
18F
17F
19F
6B 5B
0 325 650 1,300 1,950 2,600
Feet ±
Legend
Transect Lines
Sediment Removal Areas
2+89A
13+99A
8+07A
0+79B
0+90F
3+00G
2A Lake Segments 2005 ISGS Aerial Photograph
Small Coves to be Dredged
21
The land utilized for sediment storage and dewatering could benefit from the
reclaimed topsoil if the dried sediment is graded properly after the dredging project has
been completed. For a project of this size, one to two years after dredging is complete
is a normal time period to allow sufficient drying time prior to site grading and
reclamation. The dredged sediments can also be land applied and beneficially reused
as fertile agricultural soil and/or fill. Further evaluation and analysis will be required to
find a suitable location(s) for a detention site(s).
The laboratory analyses of two sediment core samples collected by HDR│CWI
indicated a very high percentage of fine grained silt and clay particles (see Appendix),
which typically remain in suspension for long periods of time and re-suspend easily
when agitated in shallow water. Although preliminary laboratory analysis indicated no
elevated chemical constituents, a polymer or flocculent will likely be required during
dredging in order to satisfy Illinois EPA effluent discharge requirements for total
suspended solids (TSS) at the sediment storage and dewatering site. The flocculent
would be a minimal additional cost to the overall dredging project.
The scope of engineering services for a typical dredging project include planning,
design, permitting, bid document preparation and coordination of potential bidders, and
construction phase oversight. Table 7 summarizes work tasks that would be required
for a sediment removal project and an opinion of the estimated costs for completion
based on removal of an estimated 64,498 cubic yards from Lake Hillsboro.
Table 7. Estimate of Probable Sediment Removal Costs
Sediment Removal
Work Task Quantity
Estimated
Cost Range
Dredging Lake Segments ($5.00/cy) 64,498 cu. yds. $322,490
Dredge Mobilization and Demobilization 1 Lump Sum $50,000
Polymers / Flocculants for Effluent Water ($0.50/cy) 1 Lump Sum $32,249
Construct Sediment Retention/Dewatering Facility * 1 Lump Sum $75,000
Subtotal $479,739
Contingency (10%) $47,974
Engineering and Permitting (15%) $71,961
Illinois EPA Permit Fees (i.e. dredging, stormwater) 1 Lump Sum $4,725
Total Estimated Cost for Dredging $604,399
Probable Site Reclamation Cost 1 Lump Sum $50,000
* Approximately a 10 acre site
22
The preliminary estimate of probable cost for dredging all sediment within the
impacted areas is $604,399. This includes higher costs that may be incurred due to
longer pipeline distances and logistical challenges when dredging coves that are not in
close proximity to the targeted sub-areas. It is important to note that if multiple small
sediment storage sites are required, then overall project costs will be higher. Due to the
variability in selection of a sediment dewatering site, costs for the land required for the
retention site are not included with the opinions of probable cost listed in Table 3. The
probable cost for site grading and reclamation is $50,000 and would likely be completed
one to two years after dredging under a separate engineering/construction contract.
More accurate costs can be determined after a detailed engineering design has been
completed and prior to actual project implementation by requesting bids from several
appropriately qualified dredging contractors.
The removal of approximately 64,498 cubic yards of accumulated sediment from
Lake Hillsboro would restore approximately 13.0 million gallons or more than 3 percent
of the lake’s original 1918 volume. According to the Phase 1 Report, the City of
Hillsboro uses an average of 1.3 million gallons of water per day for the public water
supply. Removing the recommended volume of accumulated sediment would provide
an additional 10 day supply of water for Hillsboro and the surrounding communities.
This could be valuable considering Lake Hillsboro is used as an alternate water supply
and can only be used for a limited amount of time due to its size.
Loading reductions for sediment removal were estimated using the mean
sediment phosphorus and nitrogen concentrations listed in the Phase I Report
(Zahniser, undated) and a typical mean dry bulk density for sediment (40 lbs/CF (1,080
lbs/CY)). Table 8 lists the estimated internal loading reductions from the removal of in-lake
sediments (outlined in Table 6). These load reductions are substantial and will
greatly benefit the water quality at Hillsboro by reducing turbidity and reducing nutrient
availability under anoxic conditions.
23
Table 8. Estimated Loading Reductions for Proposed Sediment Removal
Parameters Units
Estimated Loading
Reductions
Volume Removed CY 64,498
Sediment Lbs/CY 1,080
Phosphorus mg/kg 1,232
Nitrogen mg/kg 8,553
Sediment Lbs 69,657,840
Tons 34,829
Phosphorus Lbs 175,183
Tons 88
Nitrogen Lbs 1,216,183
Tons 608
Additional Management Considerations
Aeration and Destratification
Installation of an aeration system near the water intake structure was
recommended in the Phase I Report in order to add oxygen to the lower portions of the
lake. Artificial circulation of lakes during the summer thermal stratification period is a
practice commonly used to improve water quality conditions (i.e., increase dissolved
oxygen concentrations). The primary improvements in water quality that may be
attained as a result of artificial aeration or circulation are reduced nutrient loading from
bottom sediments, improved ecological diversity due to reduced blue-green algae
dominance, increased oxygen levels, and chemical oxidation of substances in the entire
water column.
Based on the Phase 1 water quality data, Site 1 (located near the dam and water
supply intake) at Lake Hillsboro appears to thermally stratify at approximately 9 to 13
feet from June through mid September. The City of Hillsboro has reported some taste
and odor issues associated with the public water supply; however, Glenn Shoals Lake is
the primary water supply. In most public water supply lakes, taste and odor
occurrences have been linked to high counts of blue-green algae within the lake.
Aeration and destratification, as previously mentioned, can be effective in improving
ecological diversity by restructuring the algae populations (i.e., reducing blue-green
algal dominance and allowing for a more diverse and desirable algal population) and
24
improving water quality by increasing dissolved oxygen concentrations and limiting
nutrient release from anoxic bottom sediments. Given the public water supply issues,
an artificial aeration or circulation system could be useful within the deep basin adjacent
to the water supply intake. The limited use of the water intake at Lake Hillsboro and the
fact that dominance of blue-green algae could possibly be reduced with other measures
recommended in this report make artificial aeration a lower priority than other
alternatives currently under consideration. However, water quality and phytoplankton
counts should be continuously monitored and re-evaluated at a later date to determine if
installation of an aeration and destratification system should be prioritized in the future.
Watershed Best Management Practices
As a proactive solution, increased soil conservation practices in the watershed
should be promoted to reduce and minimize erosion and sediment delivery to the lake.
This is especially important due to Lake Hillsboro being designated on the 303(d) list of
impaired waters for elevated phosphorus levels. Since much of the phosphorus
entering the lake is bound to sediment, any action that involves reducing or controlling
sediment delivery will also control phosphorus levels. The local NRCS and Montgomery
County SWCD have assisted the City of Hillsboro and various watershed stakeholders
over the years in the implementation of various watershed best management practices
(BMPs). BMPs serve to control erosion and runoff, resulting in reduced sediment and
nutrient loading. Specific BMPs mentioned in the Zahniser Phase 1 Report included
conservation practices such as field borders, riparian buffers and conservation tillage.
As an example, allowing vegetative buffers to grow around areas of the golf course
adjacent to the lake, if not already in place, would reduce nutrients transferred to the
lake. Excess fertilizer carried in runoff is most likely the main contributor to the elevated
phosphorus levels in the lake. Another possibility may consist of a modification to the
structure that previously contained Big Four Reservoir, southeast of Lake Hillsboro, in
order to create a sediment and nutrient control basin and wetland.
Although implementation of BMPs throughout the watershed may be difficult to
quantify as a cost item, cooperative efforts directed towards the continued
implementation of conservation practices throughout the watershed should be a high,
25
long-term priority. In order to facilitate planning and implementation of specific BMPs
strategically placed to provide maximum load reductions, development of a watershed
implementation plan for Lake Hillsboro and Glenn Shoals Lake is recommended. This
plan would be based on the US EPA’s “Nine Elements of a Watershed Plan” and would
encompass both watersheds with emphasis on areas closest to the lakes and
tributaries. An estimated cost of $50,000 is recommended for completion of a combined
Lake Hillsboro and Glenn Shoals Lake Watershed Implementation Plan that will serve to
reduce sediment and nutrient loadings to both lakes and provide a vehicle for obtaining
future funding through the US EPA Nonpoint Source Pollution Control Program (Section
319).
Water Quality Monitoring
As restoration alternatives are implemented, it is important to have methods to
measure progress. The City of Hillsboro is encouraged to conduct water quality
monitoring to assess sources of pollutants and evaluate changes in water quality.
Water quality monitoring will provide data and information to assist the City of Hillsboro
in making future lake and watershed management decisions. For water bodies such as
Lake Hillsboro where eutrophication and sediment are major problems, the US EPA
(2005) recommends the following indicators to measure progress in reducing pollutants:
Eutrophication:
· Phosphorus loads
· Number of nuisance algal blooms
· Transparency of lake or Secchi transparency depth
· Frequency of taste and odor problems in water supply
· Hypolimnetic dissolved oxygen in lake
Sedimentation:
· Total suspended solid concentrations and loads
· Raw water quality at drinking water intake
· Frequency & degree of dredging of impoundments & water supply intakes
26
At a minimum, water quality should be measured through the Illinois EPA’s
volunteer lake monitoring program. Depending on the projects implemented and grants
awarded, it is likely that additional post-implementation water quality monitoring will be
required. The most beneficial data, however, will result from comprehensive
measurement of water quality indicators on a consistent basis at more than one
location. The approximate cost of a sampling program would be $15,000 per year.
It should be noted that the rate in which nutrient loads can be reduced is often
dependent on the nature of the pollutants. Unlike pathogens, which tend to die off
quickly once the source is removed, management practices and erosion controls
designed to reduce phosphorus loadings often do not have immediate observed effects.
In cases where phosphorus is the problem, it may take years to observe a response to
watershed management practices.
Educational Programs
Informational and educational (I/E) programs improve and increase the public’s
reception to and awareness of various lake and watershed issues. Many I/E programs
can be implemented throughout the community to get people involved in protecting their
lake. Developing a pamphlet or brochure that summarizes the Lake Hillsboro
restoration alternatives is one means of informing the public and should be strongly
considered. The proposed brochure is intended to inform and educate the public about
watershed and lake issues and encourage conservation practices among current and
future generations. A total cost of approximately $5,000 to $10,000 should be allocated
for this purpose.
Fisheries Management
According to a status report submitted by IDNR in 2002, Lake Hillsboro provides
good fishing opportunities and the fish population has remained relatively stable over
the years. The fishery will benefit from improved water quality (as a result of reduced
sediment and nutrient inputs), continued stocking, and current fishing regulations;
however, it is recommended that several fish attractors be installed to further enhance
fish habitat and improve fishing opportunities. These structures could be installed in
27
conjunction with shoreline stabilization and would cost approximately $5,000 to $10,000
including planning, engineering, and technical consultation.
Prioritized Recommendations for Lake Hillsboro
In summary, HDR│CWI recommends implementation of the restoration
alternatives listed below in order to improve and increase the quantity and quality of
water for the public water supply, improve water quality for lake aesthetics, and support
a more balanced aquatic habitat for the fish population and surrounding ecosystem. In
addition to these benefits, alternatives implemented will reduce phosphorus loadings to
the lake and contribute to satisfying TMDL requirements.
Promotion and implementation of watershed BMPs in conjunction with
development of a watershed implementation plan, water quality monitoring, installation
of fish habitat structures, and distribution of an education brochure are items that will
enhance the outcome of major restoration alternatives. The major restoration
alternatives should be prioritized in the following order: 1) the stabilization and
protection of approximately 4,363 linear feet of eroded shoreline at an estimated cost of
$338,136, 2) enhancement of the sediment and nutrient control basin by removing an
estimated 7,610 cubic yards of sediment at a preliminary estimated cost of $197,721
(not including land acquisition costs for sediment storage, if required), and 3) the
removal of approximately 64,498 cubic yards of accumulated sediment at a preliminary
estimated cost of $654,399 (not including land acquisition costs for sediment storage, if
required). Although an aeration and destratification system is considered to be a lower
priority than the alternatives listed above, it should be considered for future
implementation. Continuous management and implementation of restoration
alternatives for Lake Hillsboro and its watershed can improve lake water quality,
enhance aesthetic and recreational opportunities, enhance water storage capacity, and
provide habitat for game fish and other wildlife. HDR│CWI would be pleased to assist
the City in the design and implementation of these measures in an ongoing effort to
restore Lake Hillsboro and extend the useable lifespan for many decades to come.
APPENDIX
Sediment Survey Cross-Sections
Laboratory Analyses of Sediment Core Samples
Representative Photographs
Lake Hillsboro Sedimentation Survey
Cross Section 2+89A
-15.0
-10.0
-5.0
0.0
0 10 30 80 138 197 308 392 441 461
Distance (Ft.)
Water/Sediment Depth
(Ft.)
Original Bottom Water Depth (Ft.)
Cross Section 8+07A
-12.0
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
0 6 44 112 194 263 331 408 457 501 508
Distance (Ft.)
Water/Sediment Depth
(Ft.)
Original Bottom Water Depth (Ft.)
Cross Section 13+99A
-15.0
-10.0
-5.0
0.0
0 7 20 51 102 177 243 297 331 360 392 412 426
Distance (Ft.)
Water/Sediment Depth
(Ft.)
Original Bottom Water Depth (Ft.)
Lake Hillsboro Sedimentation Survey
Cross Section 22+45A
-20.0
-15.0
-10.0
-5.0
0.0
0 2 31 99 177 248 317 367 428 455 479 492 504
Distance (Ft.)
Water/Sediment Depth
(Ft.)
Original Bottom Water Depth (Ft.)
Cross Section 35+37A
-25.0
-20.0
-15.0
-10.0
-5.0
0.0
0 11 19 40 69 87 143 199 238 327 368 398 410 423
Distance (Ft.)
Water/Sediment Depth
(Ft.)
Original Bottom Water Depth (Ft.)
Cross Section 28+27A
-20.0
-15.0
-10.0
-5.0
0.0
0 7 23 34 41 80 127 191 261 302 363 398 417 430 442
Distance (Ft.)
Water/Sediment Depth
(Ft.)
Original Bottom Water Depth (Ft.)
Lake Hillsboro Sedimentation Survey
Cross Section 49+02A
-25.0
-20.0
-15.0
-10.0
-5.0
0.0
0 7 16 30 52 78 142 212 273 320 421 511 545 569 574
Distance (Ft.)
Water/Sediment Depth
(Ft.)
Original Bottom Water Depth (Ft.)
Cross Section 64+19A
-30.0
-25.0
-20.0
-15.0
-10.0
-5.0
0.0
0 6 22 40 66 104 236 356 476 558 603 653 674 692 712
Distance (Ft.)
Water/Sediment Depth
(Ft.)
Original Bottom Water Depth (Ft.)
Lake Hillsboro Sedimentation Survey
Cross Section 0+79B
-8.0
-6.0
-4.0
-2.0
0.0
0 8 34 53 78 94
Distance (ft)
Water/Sediment Depths
(ft)
Original Depth (ft) Water Depth (ft)
Cross Section 2+59B
-12.0
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
0 22 55 107 155 176 185
Distance (ft)
Water/Sediment Depths
(ft)
Original Depth (ft) Water Depth (ft)
Lake Hillsboro Sedimentation Survey
Cross Section 6+02C
-20.0
-15.0
-10.0
-5.0
0.0
0 7 14 25 39 93 137 253 307 325 347 378
Distance (ft)
Water/Sediment Depths (ft)
Original Depth (ft) Water Depth (ft)
Cross Section 2+92C
-12.0
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
0 13 32 39 71 100 133 182 211 230 238 246
Distance (ft)
Water/Sediment Depths
(ft)
Original Depth (ft) Water Depth (ft)
Lake Hillsboro Sedimentation Survey
Cross Section 3+56D
-20.0
-15.0
-10.0
-5.0
0.0
0 8 21 46 79 132 183 252 291 318 324
Distance (ft)
Water/Sediment Depths
(ft)
Original Depth (ft) Water Depth (ft)
Cross Section 6+33D
-20.0
-15.0
-10.0
-5.0
0.0
0 22 31 73 117 183 247 315 364 424 435 446 454
Distance (ft)
Water/Sediment Depths
(ft)
Original Depth (ft) Water Depth (ft)
Lake Hillsboro Sedimentation Survey
Cross Section 1+93E
-12.0
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
0 15 39 57 84
Distance (ft)
Water/Sediment Depths
(ft)
Original Depth (ft) Water Depth (ft)
Cross Section 4+85E
-25.0
-20.0
-15.0
-10.0
-5.0
0.0
0 10 25 52 107 138 166
Distance (ft)
Water/Sediment Depths
(ft)
Original Depth (ft) Water Depth (ft)
Lake Hillsboro Sedimentation Survey
Cross Section 0+90F
-12.0
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
0 44 59
Distance (ft)
Water/Sediment
Depths (ft)
Original Depth (ft) Water Depth (ft)
Cross Section 0+93FF
-12.0
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
0 19 69
Distance (ft)
Water/Sediment
Depths (ft)
Original Depth (ft) Water Depth (ft)
Lake Hillsboro Sedimentation Survey
Cross Section 1+62F
-15.0
-10.0
-5.0
0.0
0 49 96
Distance (ft)
Water/Sediment
Depths (ft)
Original Depth (ft) Water Depth (ft)
Cross Section 3+58F
-20.0
-15.0
-10.0
-5.0
0.0
0 40 70 98 115 125 138 140 147
Distance (ft)
Water/Sediment
Depths (ft)
Original Depth (ft) Water Depth (ft)
Lake Hillsboro Sedimentation Survey
Cross Section 1+96H
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
0 18 86 164 251 293 324
Distance (ft) Water/Sediment Depths (ft)
Original Depth (ft) Water Depth (ft)
Cross Section 3+00G
-20.0
-15.0
-10.0
-5.0
0.0
0 8 17 50 77 103 118 125
Distance (ft)
Water/Sediment Depths
(ft)
Original Depth (ft) Water Depth (ft)
Cross Section 6+14G
-25.0
-20.0
-15.0
-10.0
-5.0
0.0
0 19 27 36 74 150 215 243 268 278 286
Distance (ft)
Water/Sediment Depths
(ft)
Original Depth (ft) Water Depth (ft)
Laboratory Analyses of Sediment Core Samples
Representative Photographs
1
2
3
4
5
6
7
8