HYDROLOGIC AND HYDRAULIC REPORT
S.R. 4014, SECTION 017
SEGMENT 0020, OFFSET 0836
OVER BLACKWATER RUN
ELKLAND TOWNSHIP
SULLIVAN COUNTY, PA
Pennsylvania Department of Transportation
District 3-0
P.O. Box 218, 715 Jordan Avenue
Montoursville, PA 17754
HYDROLOGIC AND HYDRAULIC REPORT
S.R. 4014, SECTION 017
SEGMENT 0020, OFFSET 0836
OVER BLACKWATER RUN
ELKLAND TOWNSHIP
SULLIVAN COUNTY
“We do hereby certify 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 the standards and requirements
of the Department of Environmental Protection.”
Hydrologic/Hydraulic Analysis and Assessments, Plans and Specifications
, P.E.
Signature
District Bridge Engineer
Date
TABLE OF CONTENTS
Attachments:
A
B
C
D
E
F
G
PERMIT APPLICATION
ACT 14 NOTIFICATION AND PROOF OF RECIEPT
DETERMINATION OF HISTORIC/ARCHEOLOGICAL SITES
COMPLETED AND APPROVED SUPPLIMENT NO. 1 FORM
SITE PLAN
LOCATION MAP
PROJECT DESCRIPTION AND NARRATIVE
Introduction and Project Description
Watershed Characteristics
Watershed Description
Channel Characteristics
Channel Description
Flood History
Project Description
Temporary Channel
Hydrologic Analysis
Hydraulic Analysis
Stormwater Management
Flood Plain Management Analysis
Risk Assessment
Erosion and Sediment Control
Wetland Involvement
Alternative Analysis
Conclusion
Summary Data Sheet
H
I
J
K
L
M
N
O
P
Q
2
3
3
3
3
4
4
4
5
6
6
7
7
7
7
7
8
9
COLOR PHOTOGRAPHS
ENVIRONMENTAL ASSESSMENT FORM
EROSION AND SEDEMENTATION CONTROL PLAN
DRAINAGE AREA MAP
HYDROLOGIC ANALYSIS
FLOOD FREQUENCY CURVE
STAGE DISCHARGE CURVE (EXISTING CONDITIONS)
STAGE DISCHARGE CURVE (PROPOSED CONDITIONS)
HYDRAULIC ANALYSIS (EXISTING CONDITIONS)
HYDRAULIC ANALYSIS (PROPOSED CONDITIONS)
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INTRODUCTION AND PROJECT DESCRIPTION
The intent of this report is to present information to the PA Department of Environmental
Protection, the PA Fish and Boat Commission, and any other agencies, for the purpose of obtaining
a Water Obstruction and Encroachment Permit. This study will provide information to support the
fact that the replacement of the existing bridge and implementation of a new structure will not
cause any excessive mean velocities that result in scour or intolerable conditions.
The project involves replacement of the stone masonry arch with a precast reinforced concrete box
culvert, having a clear normal span of 14.0 ft. at the same location. This project is located on State
Route 4014-017 at Segment 0020 Offset 0836 over Blackwater Run, Elkland Township, in Sullivan
County, Pennsylvania. S.R. 4014 is classified as a rural local road with an average 2004 ADT of 86.
The existing structure has a normal clear span of 12.0 ft., a curb-to-curb width of 12.5 ft., an
under clear of 6.0 ft. and is on an approximate skew of 50° with the roadway.
Proposed construction consists of a 14.0 ft. x 7.0 ft. pre-cast reinforced concrete box culvert
placed with a 30° skew. The proposed structure will be approximately 83 ft. long with a curb-tocurb width of 22.0 ft. The culvert bottom will be depressed 1.0 ft. below the streambed to provide
an effective waterway opening of 14.0 ft. x 6.0 ft.
Due to the relatively low ADT, a detour will be established during replacement of the existing
structure.
Blackwater Run, a tributary to Elk Creek, is listed on the PA Fish & Boat Commission’s listing of
stockable trout waters and is classified as a high quality coldwater fishery.
Hydrologic and hydraulic analyses are performed for the existing and proposed structures. No
detailed FEMA study is available for this site. Therefore, the hydrologic analysis is performed in
accordance with the procedures in PennDOT Design Manual, Part 2 for non-FEMA and un-gauged
watersheds. The hydraulic analysis is performed using the HY-8, version 6.1 computer program. All
hydrologic and hydraulic computations are included as Attachments L, P, and Q of this report.
The Pennsylvania State Programmatic General Permit No. 2 (PASPGP-2) applies to the proposed
activities.
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WATERSHED CHARACTERISTICS
Area = 1.89 square miles
Shape = irregular
Terrain = forest, fields, pasture
Elevation Range = 1720 to 1350 ft.
Land Use = Forest, Agriculture (Pasture& Meadow)
Forest = 55%
The project is not located within a detailed FEMA Study Area and no gage data exists for
Blackwater Creek. Therefore, the hydrologic analysis is performed in accordance with the
procedures in PennDOT Design Manual, Part 2 for non-FEMA and un-gauged watersheds.
WATERSHED DESCRIPTION
The terrain is composed of rolling hills of forest, meadows, and pasture. Elevation ranges from
1720 to 1350 ft. Approximately 55% of the watershed is forest.
The drainage area is taken from the Overton, PA quadrangle of the USGS 7.5-Minute Series Maps. A
drainage area map is included in this report as Attachment K.
CHANNEL CHARACTERISTICS
Top Width = 18.0 ft.
Bottom Width = 10.0 ft.
Bank Height = 6.0 ft.
Alignment = Good – the proposed structure will be constructed on a new 30o skew.
Stability = Good
Vegetation on Bank = Grasses, Brush
Channel = Highly Vegetated, Sediment
Stream Slope = 0.0204 ft. /ft.
Water Quality = Good
High Water = No high water mark available
CHANNEL DESCRIPTION
The channel has an average top width of 18.0 ft. and an average depth of 6.0 ft. The average
stream flow width varies due to seasonal changes, with an average width of 10.0 ft. and normal
flow depth of 9.0 inches. There is no embankment erosion occurring immediately upstream or
downstream of the structure. Concrete jackets exist at the flow line at each end of the stone
masonry arch to stabilize the structure due to past scour.
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FLOOD HISTORY
The exact high water mark at the site is unknown.
PROJECT DESCRIPTION
The subject project is located on State Route 4014 at Segment 0020 Offset 0836 over Blackwater
Run in Elkland Township, Sullivan County, PA. The exact location of the project is presented in
Attachment F.
The existing structure is a stone masonry arch built in 1912 with a 50° skew and a curb-to-curb
width of 12.5 ft. The structure has a clear span of 15.0 ft., a normal span of 12 ft., and exhibits an
under clear of 6.0 ft. Photographs of the project site are presented in Attachment H.
The proposed structure is a 14.0 ft. x 6.0ft. precast reinforced concrete box culvert placed on a
skew of 30° with a guide rail to guide rail width of 22.0 ft.
The disturbed channel at the inlet and outlet will be reconstructed using R-8 rock choked with R-4
rock. The inlet and outlet channel’s final grade and cross section will provide a defined channel
upstream, downstream, and through the structure. The culvert will include baffles spaced evenly
with heights of eight inches each.
The following list of work items is proposed:
Replace the existing stone masonry arch with the proposed reinforced concrete box culvert. The
inlet and outlet of the proposed structure will be pre-cast sections.
The bottom slab of the box culvert will be depressed one foot below the existing streambed.
Natural aggradation over time will provide a natural streambed to allow for fish passage and
minimize the impact to the streambed environment.
Rock protection will be placed at all wings.
A detour will be used during construction of the bridge replacement project. The availability of the
detour and low traffic volume on S.R. 4014 makes a detour the most feasible alternative for
maintaining traffic.
TEMPORARY CHANNEL
During construction the stream flow will be transported around the project site via a temporary
rock-lined channel. Details of the channel are in the Erosion and Sediment Control Plan. The
Erosion and Sediment Control Plan is currently pending.
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HYDROLOGIC ANALYSIS
The project is located in a non-FEMA area and within an un-gauged watershed. Therefore, the
hydrologic analysis is performed in accordance with the procedures in PennDOT Design Manual, Part
2 for non-FEMA and un-gauged watersheds.
The drainage area was calculated to be 1.89 mi.² using the Overton, PA. USGS map. HEC-1 is used
in obtaining the peak discharge values for the watershed. This method is the procedure
recommended in PennDOT’s Strike-Off Letter 431-99-11 “Criteria for Applicability of Hydrologic
and Hydraulic Methodologies” dated April 21, 1999. PSU-IV is used as a comparative method only.
The results are summarized below.
2 year
HEC-1
PSU-IV
96 cfs
--
2.33
year
-146 cfs
5 year
10 year
25 year
50 year
400 cfs
226 cfs
524 cfs
307 cfs
708 cfs
434 cfs
870 cfs
547 cfs
100
year
1040 cfs
677 cfs
500
year
1473 cfs
1064 cfs
Approximately 0.62 mi.² of the drainage area creates flow that discharges into Maple Lake. Only a
portion of the flow from this drainage area impacts the flow at the studied structure. Therefore,
using engineering judgment, 10% of the flow from this sub-basin is added to the flow from the
remaining 1.27 mi.² basin to determine the HEC-1 peak discharges. This assumption is valid, based
on a comparison with the PSU-IV method.
Utilizing soils maps, a CN value of 74.1 is calculated for the entire drainage area composed of
rolling hills of forest, meadows, and pasture. The SCS methodology is used in the HEC-1 model in
WMS v. 7.0. The NRCS Type-II 24-hour storm is input for the precipitation, with the following
rainfall amounts for Sullivan County:
2 Year
5 Year
10 Year
25 Year
50 Year
100 Year
500 Year
2.9 in.
3.5 in.
4.0 in.
4.7 in.
5.3 in.
5.9 in.
7.4 in.
The drainage area centroid is approximately located at latitude 41° 31' 00" and longitude 76° 36'
45". The area is also located in Region 3 according to Plate 1 of PSU-IV. Percent of forest is
estimated to be 55% from the land use maps in the WMS computer software. The standard
deviation is determined to be 0.271 as per Plate 2. Plate 3 represented the skew coefficient as
0.348. The PSU-IV model is used as a comparative model only.
The Pennsylvania Department of Transportation Design Manual, Part 2 in Section 10.12 and StrikeOff Letter 439-99-11 (April 21, 1999:page 15) specifies a 25-year design flow with consideration
given to the rural location of the project site and the improbability of property damage. The 100year flow was also checked. The hydrologic analysis is presented in Attachment L.
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HYDRAULIC ANALYSIS
Flow depths, overtopping frequency, and water velocities for the existing and proposed conditions
were determined using the HY-8 version 6.1 computer program. The HY-8 computer program is a
viable hydraulic modeling method since the structure being replaced is isolated with no present or
expected future development in close proximity. A field study was performed to obtain existing
stream and roadway cross-sections and profiles. Manning’s ‘n’ roughness coefficients were
determined from field observations.
Existing Conditions
- The hydraulic opening of the existing structure is 12.0 ft. wide with a maximum depth of 6.0 ft.
for a total area of 56.5 ft². Roughness Coefficient is 0.035 for the channel bottom and for the
structure top and sides. The existing low chord elevation at the inlet is 1356.64 ft. Overtopping of
the approach roadway will occur prior to the structure overtopping. The hydraulic analysis is
presented in Attachment P.
- The 25-year flow of 708 cfs results in a water surface elevation of 1358.64 ft. that overtops the
approach roadway by approximately 0.82 ft.
- The 100-year flow of 1040 cfs results in a water surface elevation of 1359.64 ft. that overtops the
approach roadway by approximately 1.82 ft.
Proposed Conditions
- Proposed conditions (14.0 ft. x 7.0 ft. box culvert with one foot of bottom depression and 6”x 6”
haunches at all corners of box) provide a hydraulic opening of 83.75 ft². Roughness Coefficient is
0.035 for the structure as per Strike-Off Letter 431-03-12, dated July 15, 2003. The proposed low
chord elevation is 1357.14 ft. The hydraulic analysis is presented in Attachment Q.
- The proposed conditions pass the 25-year design storm with a headwater elevation of 1357.96 ft.,
which is 0.82 ft. above the low chord elevation of the proposed structure.
- The 100-year flow results in a water surface elevation of 1359.06 ft. that overtops the proposed
approach roadway by approximately 1.24 ft.
- The proposed structure reduces the 25-year water surface elevation by 0.68 ft. and the 100-year
water surface elevation by 0.58 ft.
STORMWATER MANAGEMENT
The proposed bridge will be constructed on a 30° skew. The project will produce very little to no
change in the watershed hydrology. The project will improve the hydraulics within the project
area, and Blackwater Run will not be impacted due to the replacement of this structure.
There is currently no stormwater management plan for this area in Elkland Township. The
township and county have been notified of the proposed project in accordance with PA ACT 14,
P.L. 834. No objections have been raised concerning this project.
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FLOODPLAIN MANAGEMENT ANALYSIS
The project is not a part of a FEMA Flood Insurance Study for Elkland Township; therefore, it is not
a detailed FEMA study area. HEC-1 is used to obtain the design flows. The 25-year and 100-year
storms are modeled using HY-8 for both the existing and proposed conditions. The models show
that the proposed structure would reduce both the 25-year and 100-year storm’s water surface
elevations, as shown in the Hydraulic Data Table on the Summary Data Sheet.
The Township and County officials have been notified and no concerns have been raised.
RISK ASSESSMENT
The proposed structure will reduce the flood risk of both the 25-year and the 100-year flood. The
Proposed structure reduces the 25-year water surface elevation at the inlet from 1358.64 ft. to
1357.96 ft. The 100-year water surface is reduced from 1359.64 ft. to 1359.06 ft. The
construction of the proposed structure should reduce the risk of flooding in the adjacent area.
Given the remote rural location of the structure and the low ADT, no adverse economic or social
impacts are expected even if the approach roadway floods during a high water event.
EROSION AND SEDIMENT CONTROL
An erosion and sedimentation plan will be used on this project. This plan and approval letter is
currently pending.
WETLAND INVOLVEMENT
Wetlands are not present in the proposed worksite location. The Environmental Assessment Form
(E.A. Form) is attached in Attachment I.
ALTERNATIVES ANALYSIS
The alternative to replacing this structure is to do nothing (no build alternative) or rehabilitation of
the existing structure. The existing stone masonry structure, built in 1912, is in poor condition due
to severe deterioration on the masonry work. The existing structure has a curb-to-curb width of
12.5 ft. This width is considered substandard and unsafe by current highway design standards. The
rehabilitation alternative will prove to be costly and will not provide any benefit relative to
providing a safer roadway or improving the hydraulic capacity of the structure. Replacing this
bridge will be less costly, provide a wider and safer roadway crossing with a longer design life, and
improve the crossing’s hydraulic capacity. Therefore, the replacement alternative is selected
because it provides the best cost-benefit ratio to the Commonwealth’s taxpayers.
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CONCLUSION
The proposed reinforced concrete box culvert will replace the existing stone masonry arch.
Hydraulic computations show that an improvement in the hydraulic capacity for both the 25-year
and 100-year flow events will occur. The proposed structure will have no adverse impacts and will
reduce the risks to public safety. There is no fill below ordinary high water, and there is no
expected cultural resource or endangered species impacts or conflicts.
Proper erosion and sediment pollution control measures will be employed during construction. The
Pennsylvania State Programmatic General Permit (PASPGP-2) applies to the proposed activities.
In view of the hydrologic studies, we recommend approval of the proposed structure replacement.
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