Preliminary Hydrologic and Hydraulic Report
SR 3016 over Pohopoco Creek – Monroe County, PA
Table of Contents
Certification Statement……………….............................................................................3
Executive Summary…...………………............................................................................4
Project Description ………...............................................................................................5
Existing Conditions…..………………..............................................................................5
Proposed Conditions…..………………............................................................................6
Field Investigation...…...………………............................................................................7
Regulatory Agency. ………...............................................................................................7
Hydrologic Analysis...........................................................................................................8
Hydraulic Analysis...........................................................................................................10
Risk Assessment...............................................................................................................14
Summary Data Sheet.......................................................................................................15
Preliminary Cost Estimate..............................................................................................17
APPENDIX A.................................................................................................Photographs
APPENDIX B.................................................................FEMA Mapping & Information
APPENDIX C.................................................................................... Drainage Area Map
APPENDIX D....................................................................................................Hydrology
APPENDIX E........................................Existing Hydraulics, X-Secs, & Stream Profile
APPENDIX F...................................... Proposed Hydraulics, X-Secs, & Stream Profile
APPENDIX G..............................................................................Flood Frequency Curve
APPENDIX H................................................................................Stage Discharge Curve
APPENDIX I………………………………………………………………...TS&L Plans
APPENDIX J…………………………………………….…Floodplain Delineation Plan
February 2006
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Preliminary Hydrologic and Hydraulic Report
SR 3016 over Pohopoco Creek – Monroe County, PA
CERTIFICATION STATEMENT
I, Andrea H. Reede, do hereby certify that to the best of my knowledge, information, and belief,
that the information contained in the accompanying plans, specifications, and reports has been
prepared in accordance with accepted engineering practice, is true and correct, and is in
conformance with Title 25, Chapter 105 of the rules and regulations of the Department of
Environmental Protection.
Andrea H. Reede, P.E.
P.E. Registration # PE-056076-E
February 2006
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Preliminary Hydrologic and Hydraulic Report
SR 3016 over Pohopoco Creek – Monroe County, PA
EXECUTIVE SUMMARY
This hydraulic study includes an in-depth analysis of SR 3016 over Pohopoco Creek near the
town of Merwinsburg, Pennsylvania. The existing structure consists of an open grate steel deck
supported by steel stringers with a 28 foot clear span and founded on masonry substructure units.
The roadway alignment, both vertical and horizontal, will remain unchanged. There is an
existing mill race structure approximately 120’ upstream of SR 3016, which has been included in
the hydraulic model in this report.
There are no residential areas within the proposed work area upstream or downstream. The
structure replacement project will result in minor increases in velocity and increases in water
surface elevation upstream of the new proposed structure for the 25-year and 100-year storms as
detailed in the Hydraulic discussion. The increases in velocity and water surface elevations will
not result in a threat to property or hazard of life as all the impacts are contained within
PENNDOT existing or proposed right-of-way.
February 2006
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Preliminary Hydrologic and Hydraulic Report
S.R. 3016 over Pohopoco Creek – Monroe County, PA
PROJECT DESCRIPTION
The Pennsylvania Department of Transportation is proposing to replace the existing
structure for SR 3016 over Pohopoco Creek. The existing structure is an open grate steel
deck supported by steel stringers with a 28 foot clear span and founded on masonry
substructure. The proposed structure is a precast concrete twin cell box culvert consisting
of two 15’-0” by 6’-0” cells which will each include 8” fish baffles and be depressed to
promote aquatic wildlife.
Upstream Structures
(Photo 1, furthest
upstream show, other
similar)
Project Location
(Photo 3 & 4)
Downstream
Structure
(Photo 2)
Figure 1 - Project Location Map
EXISTING CONDITIONS
The existing structure consists of an open grate steel deck supported by steel stringers
with a 28 foot clear span and founded on masonry substructure units. The maximum
underclearance is 3.58 feet and the width of the bridge is 38.20 feet. The bridge is
skewed 90 degrees from the centerline of SR 3016.
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Preliminary Hydrologic and Hydraulic Report
S.R. 3016 over Pohopoco Creek – Monroe County, PA
No major reservoirs (existing or proposed) are situated along this waterway which would
impact water stages. A small pond exists just upstream of the existing structure which
does not significantly affect flow through the project site. This pond allows flow to enter
through a small channel approximately 300 feet upstream from the existing structure and
is then allowed to rejoin the main stream approximately 200 feet downstream. At some
time this pond appears to have been used to power a small mill structure located just
upstream of the existing bridge. See Appendix A for details on the mill structure and
upstream pond. Field visits have verified that this structure is deteriorated and no longer
used in this capacity. No flood control projects or any other controls are currently in
place within the project study area for Pohopoco Creek.
Directly upstream of the existing structure is a garage/mill structure which spans
Pohopoco creek. The structure has a 35’ span across Pohopoco Creek with a maximum
underclearance of 3.7’ and a width of building along Pohopoco Creek of 18.2 feet. The
building is at an approximate 90 degree skew to Pohopoco Creek.
Besides the garage/mill structure, there are two additional structures upstream of the
project site. The structure furthest upstream along Pohopoco Creek is found just
downstream of the confluence with Poplar Run, and consists of a 36” pipe with a stone
headwall. The next structure upstream of SR 3016 upstream consists of two 36” diameter
corrugated metal pipes placed side by side. Both upstream structures carry local roads
across Pohopoco Creek.
The downstream structure consists of a 39′-0″ single span concrete T-beam bridge which
carries SR 3009 across Pohopoco Creek. The structure has a significant skew of 30° –
45° with respect to the centerline of the road and is comprised of two 8 to 10 foot lanes
with no shoulders. This structure is located approximately 3000 feet downstream from
the project site. Photographs of existing structures are located in Appendix A.
PROPOSED CONDITIONS
The conceptual TS&L examined a single cell box culvert and a rigid frame option as well
as the twin cell box culvert option. The single cell box culvert was eliminated due to the
increased depth of the top slab which infringes on the water surface elevation. The rigid
frame option was considered unfeasible due to the differential settlement concerns
between abutment footings. The proposed structure is a precast concrete twin cell box
culvert consisting of two 15’-0” by 6’-0” cells. Each cell will include 8” fish baffles and
be depressed to promote aquatic wildlife. The box culverts will have a low chord
elevation of approximately 969.11 feet. In addition to this the box culverts will be
designed with 8” fish baffles in accordance with BD-632, including concrete apron
design for scour protection. The roadway alignment, both vertical and horizontal, will
remain unchanged.
ENVIRONMENTAL CONCERNS
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Preliminary Hydrologic and Hydraulic Report
S.R. 3016 over Pohopoco Creek – Monroe County, PA
Pohopoco Creek is classified as a Cold Water Fishery north of the bridge and is listed as
High Quality and a Cold Water Fishery south of the bridge according to Pennsylvania
Code Title 25, Environmental Protection, Chapter 93, Water Quality Standards.
Pohopoco Creek is also listed in DM2 Chapter 10 as a stockable warm water and trout
stream.
Environmentally, six (6) wetlands of the U.S. have been identified within the Study Area,
which extends 150 feet on both approaches to the bridge as well as 50 feet in an upstream
and downstream direction. Pohopoco Creek is a continuously flowing creek and is not
intermittent.
Pohopoco Creek appears to have stable and highly vegetative banks both upstream and
downstream of the proposed bridge replacement. There is no evidence of shifting or drift
of the creek centerline. Problems associated with ice and debris on the proposed
structure are not a concern due to the significant year round flow of the creek and because
of the large freeboard being provided with the twin cell box culvert being proposed.
FIELD INVESTIGATION
There is no recorded flood history for this portion of Pohopoco Creek. In addition, a site
visit was performed by HNTB engineers on April 25th, 2003 in order to initially assess
the project site and gather structural and hydraulic information. At this time there was no
indication or evidence of previous highwater marks near the existing bridge. On the day
of the site visit, no nearby building occupants were able to be reached for questioning.
Follow-up inquires with the Chestnuthill Township did not indicate any history of
flooding in the project area.
Monroe County Maintenance was also contacted to potentially provide details of any
flooding history at this location. Ed Possinger of that group indicated that there has been
no flooding at this location during his tenure.
REGULATORY AGENCIES
This culvert is located in the township of Chesthuthill, which has been included in
FEMA’s flood insurance study (FIS) and flood insurance rate maps (FIRM) program.
The FEMA maps indicate that this culvert is included in a detailed study area (Zone AE).
The detailed study was completed only for the 100-year storm and included cross
sections
1000 feet upstream of the bridge and 700’ downstream. Since this study was completed
500 feet upstream and downstream from SR 3016 the water surface elevations have been
compared to our analysis to make sure it is consistent with the detailed study.
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Preliminary Hydrologic and Hydraulic Report
S.R. 3016 over Pohopoco Creek – Monroe County, PA
The design criteria for local roadways per Strike-off-Letter 431-99-11 is the 10-year
storm. However, for DEP’s Chapter 105 obstruction and encroachment permits the
design criteria for rural locations is the 25-year storm.
Line, Grade approval was granted on November 21, 2003 from PENNDOT.
HYDROLOGIC ANALYSIS
Pohopoco Creek is formed approximately two and one quarter miles north of the project
site in the Pocono Mountains. From this point the creek flows in a generally south /
southeasterly direction through Sun Valley until it reaches SR 3016 in Merwinsburg, Pa.
During its travel, the flow divides nearly 1000 feet upstream of the project site and
converges again approximately 1000 feet downstream. The topography of the immediate
area is that of mountainous terrain.
No major reservoirs (existing or proposed) or flood control projects are situated along
this waterway. There is a small pond just upstream of the existing structure which does
not significantly affect flow through the project site. This pond allows flow to enter
through a small channel approximately 300 feet upstream from the existing structure and
is then allowed to rejoin the main stream approximately 200 feet downstream. At some
time this pond appears to have been used to power a small mill structure located just
upstream of the existing bridge. See Appendix A for pictures of the mill structure and
upstream pond. Field visits have verified that this structure is deteriorated and no longer
used in this capacity.
The drainage area to the structure under SR 3016 is 5.05 square miles. These drainage
areas are shown on the USGS Quadrangles (Broadheadsville and Pocono Pines
Quadrangle) in Appendix C – Drainage Area Map.
Pohopoco Creek has been included in a FEMA detailed study for Chestnuthill Township.
However, the analysis was only completed for the 100-year storm and the flow values
used for this analysis were determined using the drainage area of 7.7 square miles at the
confluence with Sugar Hollow Creek (approximately 8000 feet downstream of the project
site). This flow data has been included in Table 1 below.
In addition to this Pohopoco Creek does have a stream gage located at Kresgeville which
is approximately 5 miles downstream. The drainage area for this gage is 49.90 square
miles which is greater than acceptable drainage area size for use in comparison as
prescribed in Strike-off Letter 431-99-11. However, from this data it was determined that
the most severe flood event in recent history occurred during Hurricane Ivan in 2004.
The U.S.G.S. National Flood Frequency Program, Techniques for Estimating Magnitude
and Frequency of Peak Flows for Pennsylvania Streams (2000), was also used to
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Preliminary Hydrologic and Hydraulic Report
S.R. 3016 over Pohopoco Creek – Monroe County, PA
determine the peak flows for the structure. This program uses regression equations for
estimating peak stream flows for ungaged streams based on recorded streamflow gaging
stations throughout the state of Pennsylvania. This program takes into account the size of
the drainage area, percentage of forest area, percentage of urban development, percentage
of carbonate area and percentage of controlled area. The computer output and
assumption used for this program has been included in Appendix D- Hydrology.
The PSU-IV method was also used to determine the peak flows for the bridge. The PSUIV method is a Pennsylvania regional regression model that is based on the Log Pearson
III equation. The model is valid for use in watersheds between 0.50 sq-mi and 150 sq-mi.
According to PennDOT’s Strike-off-Letter 431-99-11 for limited access highway, PSUIV should only be used for comparison for drainage areas between 0.5 sq-mi and 1.5 sqmi. The computer output has been included in Appendices D – Hydrology.
Table 1: Flood Information
(For calculations see Appendix D)
Year Storm
Drainage Area
Qlow
Q2.33
Q10
Q25
Q50
Q100
Q500
PSU-IV Flow
Rate (cfs)
5.05 sq. mi.
119
237
427
617
759
917
1559
USGS NFF Flow
Rate (cfs)
5.05 sq. mi.
N/A
N/A
365
555
737
960
1680
FEMA Flow Rate
(cfs)
7.7 sq. mi.
N/A
N/A
N/A
N/A
N/A
1100
N/A
Based on engineering judgment, a low flow condition was calculated by taking fifty
percent of the 2.33 year event. This low flow condition was used to assess the channel
during normal flow conditions and determine whether a fish channel would be needed.
For future scour analysis, the 500-year flow rate was determined by increasing the 100year flow rate by a factor of 1.7 in accordance with Design Manual Part 4, Part A,
Section 7.2.3.
The peak flows used for the analysis of these structures were the 25-year and 100-year
frequency storm events as shown in the table above. The peak flows for the NFF analysis
are very similar to the flows calculated using the PSU-IV methodology. PSU-IV flows
were slightly more conservative for the smaller storms (25-year storm) and very similar
for the 100-year storm. Therefore, the PSU-IV flows were used and considered valid
because of their consistency with the FEMA flows.
October 2005
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Preliminary Hydrologic and Hydraulic Report
S.R. 3016 over Pohopoco Creek – Monroe County, PA
HYDRAULIC ANALYSIS
Hydraulic calculation for the existing and proposed conditions along Pohopoco Creek
was performed using the U.S. Army Corps of Engineers HEC-RAS version 3.1.3
program. HEC-RAS supports one-dimensional, steady flow, water surface profiles
calculations. The HEC-RAS analysis was performed along Pohopoco Creek from
approximately 500 feet upstream and downstream of the proposed bridge crossing.
The Manning’s roughness coefficients (n values) were initially obtained from the FEMA
Flood Insurance Study (FIS) for the Township of Chestnuthill, Monroe County,
Pennsylvania. A field reconnaissance trip was also performed on April 25th, 2003 to
validate the study’s findings. Utilizing page 13 of the FIS, it was initially determined that
Manning’s roughness coefficients are as follows:
nchannel (initial) = 0.035 – 0.045
noverbank (initial) = 0.080 – 0.090
Then, in executing the hydraulic analysis for the existing structure in conjunction with
field verification, the roughness coefficients were adjusted in order to calibrate the model
and provide more realistic water surface profiles for the site. More specifically, the
calibration was done to agree more closely with observed and recorded (FEMA FIS)
water surface elevations. The following calibrated n values were used in the existing and
proposed HEC-RAS models:
nchannel (calibrated) = 0.055
noverbank (calibrated) = 0.095
Through hydraulic analysis, it was found that subcritical and supercritical flows are
present within 500 feet both upstream and downstream of the existing structure.
Therefore, mixed flow analysis in HEC-RAS was used for analysis.
It is proposed that the existing structure be replaced with a precast concrete twin cell box
culvert consisting of two 15’-0”x 6’-0” cells that allow the 100-year storm to pass with at
least 6 inches of freeboard. Per DM2 Chapter 10, the 10-year storm was used for this
analysis of flow under SR3016 which is classified as a rural local road. In addition the
25-year storm was also analyzed in accordance with the DEP’s Chapter 105 requirements
for rural locations. Based on this analysis the roadway profile is only overtopped in the
event of a 500-year storm.
The proposed structure has negligible impacts to the water surface elevations and
velocities in Pohopoco Creek. Riprap will be installed upstream and downstream of the
proposed extensions to reduce the effects of erosion per BD-632. Table 2 and Table 3
show a comparison of the existing and proposed velocities and the water surface
elevations for the design storm and the 100-year storm. Referencing Table 3, a decrease
in velocity of 1.58 ft./sec. can be seen between existing and proposed conditions for the
100-year storm.
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Preliminary Hydrologic and Hydraulic Report
S.R. 3016 over Pohopoco Creek – Monroe County, PA
In addition, an analysis was performed on the creek in order to determine the need for a
fish channel within the culverts. Engineering judgment was used to approximate the flow
for such a condition. Referencing Table 1, a flow equivalent to fifty percent of the 2.33year storm was used for low flow conditions. Based upon this flow, a fish channel is not
needed because the flow depth through the structure was greater than 9 inches.
The proposed structure over Pohopoco Creek will have minor impacts to the water
surface elevations and velocities. The change in velocities and water surface elevations
for the 25-year and 100-year storm extend minimally upstream and downstream and all
adverse effects are contained with existing PENNDOT right-of-way. Table 2 and Table 3
compare existing vs. proposed velocities and water surface elevations.
October 2005
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Preliminary Hydrologic and Hydraulic Report
S.R. 3016 over Pohopoco Creek – Monroe County, PA
Table 2: Comparison of Existing and Proposed HEC-RAS Results
(Precast Concrete Twin Cell Box Culvert – Design Storm 25-yr. storm)
RS
Location
Existing
25-yr.
W.S.
Elev.
(ft.)
Proposed
25-yr. W.S.
Elev. (ft.)
Difference
(ft.)
23
975.32
975.32
0.00
(500’ U/S)
22
974.84
974.84
0.00
21
974.24
974.24
0.00
20
973.25
973.25
0.00
19
972.77
972.77
0.00
18
972.30
972.30
0.00
17
971.83
971.83
0.00
16
970.32
970.31
-0.01
15
969.95
969.78
-0.17
14
969.67
969.28
-0.39
13.5 Obstruction from Cantilever Building
12.8
969.63
969.28
-0.35
12
969.45
967.87
-1.58
11.1
969.11
967.54
-1.57
10.5 Prop Bridge
9.9
966.86
966.76
-0.10
9
966.01
966.01
0.00
8
965.40
965.40
0.00
7
964.34
964.34
0.00
6.5
963.34
963.34
0.00
6
963.14
963.14
0.00
5
961.97
961.97
0.00
4
961.05
961.05
0.00
3.5
960.38
960.38
0.00
3
960.33
960.33
0.00
2
959.24
959.24
0.00
1
958.53
958.53
0.00
(500’ D/S)
Existing
Velocity
(ft./sec.)
Proposed
Velocity
(ft./sec.)
Change in
Velocity
(ft./sec.)
5.34
5.34
0.00
5.54
6.15
6.65
5.04
4.88
5.54
7.31
5.27
6.04
5.54
6.15
6.65
5.04
4.88
5.52
7.35
5.74
7.11
0.00
0.00
0.00
0.00
0.00
-0.02
+0.04
+0.47
+1.07
5.35
4.89
4.31
6.18
9.38
4.96
+0.83
+4.49
+0.65
7.11
6.83
6.40
6.58
4.97
4.53
5.90
3.34
5.42
5.05
7.06
5.60
6.84
6.40
6.57
4.97
4.53
5.90
3.34
5.42
5.05
7.06
-1.51
+0.01
0.00
-0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.00
6.28
6.27
-0.01
Table 3: Comparison of Existing and Proposed HEC-RAS Results
(Precast Concrete Twin Cell Box Culvert - 100-yr. storm)
RS
Existing
Proposed
Difference
October 2005
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Existing
Proposed
Change in
Preliminary Hydrologic and Hydraulic Report
S.R. 3016 over Pohopoco Creek – Monroe County, PA
Location
100-yr.
W.S.
Elev.
(ft.)
100-yr.
W.S. Elev.
(ft.)
(ft.)
23
976.04
976.04
0.00
(500’ U/S)
22
975.40
975.40
0.00
21
974.78
974.78
0.00
20
974.68
974.68
0.00
19
973.21
973.21
0.00
18
973.71
973.73
+0.02
17
972.17
972.25
+0.08
16
971.64
971.01
-0.63
15
971.46
970.54
-0.92
14
971.34
970.15
-1.19
13.5 Obstruction from Cantilever Building
12.8
971.24
969.88
-1.36
12
971.15
968.90
-2.25
11.1
970.88
968.49
-2.39
10.5 Prop Bridge
9.9
967.28
967.16
-0.12
9
966.57
966.57
0.00
8
965.81
965.81
0.00
7
964.80
964.80
0.00
6.5
963.77
963.77
0.00
6
963.57
963.57
0.00
5
962.31
962.31
0.00
4
961.81
961.81
0.00
3.5
961.29
961.29
0.00
3
961.23
961.23
0.00
2
959.59
959.59
0.00
1
959.03
959.03
0.00
(500’ D/S)
October 2005
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Velocity
(ft./sec.)
Velocity
(ft./sec.)
Velocity
(ft./sec.)
5.57
5.57
0.00
6.57
7.01
7.67
5.77
5.66
6.52
4.98
4.43
5.08
6.57
7.00
7.68
5.75
5.59
6.18
7.10
6.08
7.09
0.00
-0.01
+0.01
-0.02
-0.07
-0.34
+2.12
+1.65
+2.01
4.88
4.53
4.32
7.23
9.01
5.87
+2.35
+4.48
+1.55
9.14
7.45
7.85
7.18
5.76
5.19
6.54
3.61
5.34
5.64
9.15
7.43
7.45
7.84
7.18
5.76
5.19
6.54
3.61
5.34
5.64
9.15
-1.71
0.00
-0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
7.03
7.03
0.00
Preliminary Hydrologic and Hydraulic Report
S.R. 3016 over Pohopoco Creek – Monroe County, PA
RISK ASSESSMENT
The hydraulic analysis of the proposed structure indicates a decrease in the 100-year
maximum water surface elevation by 2.23 feet just upstream from the proposed bridge
crossing. This decrease in water surface elevation extends approximately 200 feet
winding both upstream and downstream of the existing / proposed structure location. All
land directly adjacent to the creek and near the bridge in this 400 foot zone will
experience a minor reduction in water surface elevation. The amount of land that will
now be available for use due to this decrease in water surface elevation will not constitute
a significant nor measurable amount. See Table 2 and Table 3 for WSE’s and channel
velocities at additional locations.
It is not anticipated that this change in water surface elevation will have an appreciable
impact on bank-denning animals or nesting fowl, nor will it require a revision to FEMA
FIRM’s for the area.
Flow regime changes should not spark any changes in erosion and deposition balance.
To prevent such instances, final design drawings will need to implement measures to be
taken during the life of the contract to control water pollution through the use of berms,
dikes, jute matting, soil retention blankets, mulches, grasses, and other erosion control
devices and methods. Permanent and temporary erosion and sedimentation control
measures shall be provided in accordance with the contract drawings, details and
specifications.
In order to prevent scour at the culvert, riprap protection will be designed and
implemented per BD-632.
In summary, there are minimal changes in water surface elevation associated with the
various design storms. In particular, the 100-year storm maximum water surface
elevation decreases by 0.61 feet as a result of the proposed structure. Ample clearance
will be provided between the water surface and the low chord of the structure for the
design 100-year storm (6 inch freeboard). The proposed culvert span length is the critical
factor controlling the proposed water surface elevations. By increasing this span length,
the water surface elevation was reduced as well as the velocity within the culvert. In
addition, temporary cofferdams during construction will be required for placement of the
precast culvert sections. Flow will need to be diverted for a short time while culvert
installation takes place. While the responsibility of the contractor, the impact of these
temporary structures will be minimal.
October 2005
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Preliminary Hydrologic and Hydraulic Report
S.R. 3016 over Pohopoco Creek – Monroe County, PA
SUMMARY DATA SHEET
U.S. Army Corps of Engineers Nationwide Permits: Chapter 105 Joint Permit (COE
Section 404 Permit)
Wetland Impact: Wetland mitigation is not anticipated as the overall acreage impacted
is expected to be less than 0.05 acres (approximate); however, stream mitigation may be
required and will be determined during Final Design
Quantity of Fill Material below Ordinary High Water Elevation: No new fill will be
required below the ordinary high water elevation for the proposed structure
October 2005
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Preliminary Hydrologic and Hydraulic Report
S.R. 3016 over Pohopoco Creek – Monroe County, PA
Insert Summary Data Table
October 2005
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Preliminary Hydrologic and Hydraulic Report
S.R. 3016 over Pohopoco Creek – Monroe County, PA
Preliminary Cost Estimate
(Precast Reinforced Concrete Twin Cell Box Culvert)
Element / Item
Demolition and Site Preparation
Structure
Roadway
Total Bridge Replacement Cost
Mobilization (5%)
Clearing and Grubbing (1%)
Contingencies (15%)
Total Others
Grand Total
Total Cost
$ 25,000.00
$ 219,352.00
$ 39,623.00
Subtotal
$ 283,976.00
$ 14,198.00
$ 2,839.00
$ 42,596.00
$ 59,633.00
$ 343,700.00
October 2005
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