HYDROLOGIC AND HYDRAULIC REPORT
S.R. 4005, SECTION 006
SEGMENT 0050, OFFSET 0000
OVER TRIBUTARY TO SHAMOKIN CREEK
SNYDERTOWN BOROUGH
NORTHUMBERLAND COUNTY, PA
Pennsylvania Department of Transportation
District 3-0
P.O. Box 218, 715 Jordan Avenue
Montoursville, PA 17754
Prepared by District 3-0 Bridge Unit
HYDROLOGIC AND HYDRAULIC REPORT
S.R. 4005, SECTION 006
SEGMENT 0050, OFFSET 0000
OVER TRIBUTARY TO SHAMOKIN CREEK
SNYDERTOWN BOROUGH
NORTHUMBERLAND 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
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
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7
7
7
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8
8
13
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 single span steel I-beam bridge with a precast reinforced
concrete box culvert, having a clear normal span of 18.0 ft. at the same location. This project is
located on State Route 4005 at Segment 0050 Offset 0000 over a Tributary to Shamokin Creek,
Snydertown Borough, in Northumberland County, Pennsylvania. S.R. 4005 is classified as a rural
minor collector with an average 2005 ADT of 471. The existing structure has a normal clear span of
17.0 ft., a curb-to-curb width of 22.75 ft., a maximum under clear of 5.1 ft., and is on an
approximate skew of 77° with the roadway.
Proposed construction consists of an 18.0 ft. span x 6.0 ft. rise precast reinforced concrete box
culvert placed with a 76° skew. The proposed structure will be approximately 47.98 ft. long,
including the inlet and outlet precast sections, with a curb-to-curb width of 28.0 ft.
Due to the low ADT, a detour will be utilized during replacement of the existing structure.
The Tributary to Shamokin Creek is not listed on the PA Fish & Boat Commission’s listing of
stockable trout waters. However, it is classified in Pennsylvania’s Chapter 93 for Water Quality
Standards as CWF.
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-gaged
watersheds. The hydraulic analysis is performed using the HEC-RAS 3.1.2 computer program.
The Pennsylvania State Programmatic General Permit No. 3 (PASPGP-3) applies to the proposed
activities.
2
WATERSHED CHARACTERISTICS
Area = 0.67 square miles
Shape = irregular
Terrain = rolling hills of forest, fields, pasture
Elevation Range = 1120 to 484 ft.
Land Use = Residential, Forest, Agriculture (Cropland & Pasture)
Forest = 57.8%
The project is not located within a detailed FEMA Study Area and no gage data exists for the
Tributary to Shamokin Creek. Therefore, the hydrologic analysis is performed in accordance with
the procedures in PennDOT Design Manual, Part 2 for non-FEMA and un-gaged watersheds.
WATERSHED DESCRIPTION
The terrain is composed of rolling hills of forest, cropland, pasture, and residential areas.
Elevation ranges from 1120 to 484 ft. Approximately 57.8% of the watershed is forest.
The drainage area is taken from the Riverside, PA quadrangle of the USGS 7.5-Minute Series Maps.
CHANNEL CHARACTERISTICS
Top Width = 30 ft.
Bottom Width = 11 ft.
Bank Height = 4.5 ft.
Alignment = Good – the proposed structure will be constructed on a 76° skew.
Stability = Good
Vegetation on Bank = Grasses downstream, Brush and trees upstream
Channel = Stones and Sediment throughout
Stream Slope = 0.0102 ft. /ft.
Water Quality = Good
High Water – No data exists
CHANNEL DESCRIPTION
The channel has an average top width of 30 ft. and an average depth of 4.5 ft. The average stream
flow width varies due to seasonal changes, with an average width of 11 ft. and normal flow depth
of 12 inches. Minor channel scour is evident at the abutments. No abutment footings are exposed.
3
FLOOD HISTORY
During the 1996 high water event, the water surface elevation overtopped the bridge and roadway.
Based on FEMA, the bridge location is in the special flood hazard area, inundated by the 100-year
flood of Shamokin Creek. The flood zone for this area was determined by approximate methods,
with no flood elevations of flood hazard factors determined. No flood damage was noted for the
September 2004 high water event. The high water mark for the September 2004 event was 2 in.
above the bottom of beam at the inlet. The approximate elevation was 490.12 ft. No flood data
exists for the 1972 flood event.
PROJECT DESCRIPTION
The subject project is located on State Route 4005 at Segment 0050 Offset 0000 over a Tributary to
Shamokin Creek in Snydertown Borough, Northumberland County, PA. The exact location of the
project is presented in Attachment C.
The existing structure is a single span steel I-beam bridge built in 1938 with a 77° skew and a curbto-curb width of 22.75 ft. The structure has a normal clear span of 17.0 ft. and exhibits a maximum
under clear of 5.1 ft. Photographs of the project site are presented in Attachment J.
The proposed structure is an 18.0 ft. span x 6.0 ft. rise precast reinforced concrete box culvert
placed on a skew of 76° with a guide rail to guide rail width of 28.0 ft.
The disturbed channel at the inlet and outlet will be reconstructed using R-8 rock. The inlet and
outlet channel’s final grade and cross section will provide a defined channel upstream,
downstream, and through the structure.
The following list of work items is proposed:
Replace the existing single span steel I-beam bridge with the proposed precast reinforced concrete
box culvert. The inlet and outlet wings of the proposed structure will be precast sections.
Rock protection will be placed at all wings.
A detour will be used during construction of the bridge replacement project. The low traffic
volume on S.R. 4005 makes a detour the most feasible alternative for maintaining traffic.
TEMPORARY CHANNEL
During construction the stream flow will be pumped around the project. Details of the filter bag
and concrete barrier are in the construction plan (Attachment E). No Erosion and Sedimentation
Control Plan is required for this project since the impacts will be less than 5000 SF.
4
HYDROLOGIC ANALYSIS
The project is located in a non-FEMA area and within an un-gaged watershed. Therefore, the
hydrologic analysis is performed in accordance with the procedures in PennDOT Design Manual, Part
2 for non-FEMA and un-gaged watersheds.
The drainage area was calculated to be 0.67 mi.² using the Riverside, PA USGS map. The NFF
method is used in obtaining the peak discharge values for the watershed. HEC-1 and PSU-IV are
used as comparative methods only. The results are summarized below.
2 year
HEC-1
PSU-IV
NFF
222 cfs
---
2.33
year
-62 cfs
82 cfs*
5 year
10 year
25 year
50 year
337 cfs
99 cfs
130 cfs*
465 cfs
138 cfs
182 cfs
691 cfs
202 cfs
266 cfs
908 cfs
261 cfs
343 cfs
100
year
1180 cfs
331 cfs
436 cfs
500
year
1917 cfs
549 cfs
718 cfs
* Values extrapolated using a best fit curve.
The HEC-1 method results in higher flows as compared to the PSU-IV and NFF methods. All three
sets of results are utilized in HEC-RAS to determine the validity of the data. The NFF method is
utilized since the high water elevations compare closely to the high water event data of the stream
during flooding.
Utilizing soils maps, an average CN value of 74.7 is calculated for the entire drainage area
composed of rolling hills of forest, cropland, pasture, and residential areas. The SCS methodology
is used in the HEC-1 model in WMS v. 8.0. The NRCS Type-II 24-hour storm is input for the
precipitation, with the following rainfall amounts for Northumberland County:
2 Year
5 Year
10 Year
25 Year
50 Year
100 Year
500 Year
2.9 in.
3.5 in.
4.1 in.
5.1 in.
6.0 in.
7.1 in.
10.4 in.
The drainage area centroid is approximately located at latitude 40° 53' 00" and longitude 76° 40'
00". The area is also located in Region 2 according to Plate 1 of PSU-IV. Percent of forest is
estimated to be 57.8% from the land use maps in the WMS computer software. The standard
deviation is determined to be 0.292 as per Plate 2. Plate 3 represents the skew coefficient as
0.400.
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.
5
HYDRAULIC ANALYSIS
Flow depths, overtopping frequency, and water velocities for the existing and proposed conditions
are determined using the HEC-RAS 3.1.2 computer program. A field study is performed to obtain
existing stream and roadway cross-sections and profiles. Manning’s ‘n’ roughness coefficients are
determined from field observations.
Existing Conditions
- The hydraulic opening of the existing structure is 17.0 ft. wide with a maximum depth of 5.1 ft.,
for a total area of 59.27 ft². Roughness Coefficient is 0.045 for the upstream channel, 0.100 for
the upstream overbanks, 0.035 for the downstream channel, and 0.060 for the downstream
overbanks. The existing low chord elevation at the inlet is 489.90 ft. The overall low chord of the
existing bridge, located at the outlet, is 488.42 ft.
- The 25-year flow of 266 cfs results in a water surface elevation of 488.49 ft. that is approximately
1.41 ft. below the existing low chord at the inlet.
- The 100-year flow of 436 cfs results in a water surface elevation of 489.70 ft. that is
approximately 0.20 ft. below the existing low chord at the inlet. The roadway is not overtopped by
the 100-year flow of the Tributary to Shamokin Creek. However, the bridge will be overtopped by
the Shamokin Creek flood waters, as approximated by FEMA flood maps.
Proposed Conditions
- Proposed conditions (18.0 ft. span x 6.0 ft. box culvert with one foot of bottom depression and 6”
x 6” haunches at all corners of the box) provide a hydraulic opening of 89.75 ft². The hydraulic
opening is increase by 51.4%, or 30.48 ft². Roughness Coefficient is 0.011 for the structure surface
and 0.035 for the natural stream bed through the culvert. The proposed inlet low chord elevation
at the box inlet is 489.55 ft. The overall low chord of the proposed box culvert, located at the
outlet, is 489.15 ft. The overall low chord will be raised 0.73 ft.
- The proposed conditions pass the 25-year design storm with a headwater elevation of 487.83 ft.,
which is 1.72 ft. below the low chord elevation at the inlet of the proposed structure.
- The 100-year flow results in a water surface elevation of 488.98 ft. that is approximately 0.57 ft.
below the proposed low chord at the box culvert inlet. The roadway is not overtopped by the 100year flow. However, the bridge will be overtopped by the Shamokin Creek flood waters, as
approximated by FEMA flood maps.
- The proposed structure reduces the 25-year water surface elevation by 0.66 ft. and the 100-year
water surface elevation by 0.72 ft. immediately upstream of the structure.
6
STORMWATER MANAGEMENT
The proposed bridge will be constructed on a 76° 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 the Tributary to Shamokin Creek will not be impacted due to the replacement of this
structure.
Currently, no stormwater management plan exists for this area in Snydertown Borough. The
borough 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.
FLOODPLAIN MANAGEMENT ANALYSIS
The project is not a part of a FEMA Flood Insurance Study for Snydertown Borough; therefore, it is
not a detailed FEMA study area. NFF is used to obtain the design flows. The 25-year and 100-year
storms are modeled using HEC-RAS 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 Borough 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 488.49 ft. to
487.83 ft. The 100-year water surface is reduced from 489.70 ft. to 488.98 ft. The construction of
the proposed structure should reduce the risk of flooding in the adjacent area.
Given the rural location of the structure and the low ADT, no adverse economic or social impacts
are expected with the replacement of this structure.
EROSION AND SEDIMENT CONTROL
An erosion and sedimentation plan will not be used on this project. Impacts will be less than 5000
SF.
WETLAND INVOLVEMENT
Wetlands are not present in the proposed worksite location.
7
ALTERNATIVES ANALYSIS
The alternatives to replacing this structure are to do nothing (no build alternative) or to
rehabilitate the existing structure. The existing single span steel I-beam bridge, built in 1938, is in
poor condition due to severe deterioration of the abutments. The existing structure has a curb-tocurb width of 22.75 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.
CONCLUSION
The proposed reinforced concrete box culvert will replace the existing single span steel I-beam
bridge. 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. No fill below ordinary high water will be placed, and no
cultural resource or endangered species impacts or conflicts is expected.
Proper erosion and sediment pollution control measures will be employed during construction. The
Pennsylvania State Programmatic General Permit (PASPGP-3) applies to the proposed activities.
In view of the hydrologic studies, approval is recommended for the proposed structure
replacement.
8
Existing Conditions - 25 year flow
Upper Reach
Lower Reach
Station
W.S. Elevation
Velocity
(ft.)
(ft./s)
1035
489.32
1.61
1020
489.29
1.59
1005
489.28
961
489.22
930
915
Station
W.S. Elevation
Velocity
(ft.)
(ft./s)
450
487.68
3.27
435
487.65
3.16
1.54
420
487.60
3.52
1.47
405
487.59
3.11
489.19
1.28
390
487.48
3.91
489.17
1.23
375
487.42
3.99
885
489.14
1.21
360
487.30
4.49
870
489.12
1.21
345
487.28
3.85
840
489.09
1.17
330
487.20
4.11
810
489.08
0.99
315
487.16
3.67
795
489.07
1.12
300
487.10
3.88
765
489.03
1.60
270
487.01
3.80
735
488.99
1.89
255
487.01
3.21
705
488.95
1.75
240
486.96
3.40
690
488.94
1.73
225
486.91
3.62
675
488.92
1.69
210
486.86
3.71
645
488.91
1.49
195
486.83
3.52
615
488.89
1.22
165
486.70
3.70
585
488.88
1.32
150
486.57
4.23
570
488.86
1.49
135
486.52
3.71
555
488.82
1.99
120
486.39
4.16
540
488.60
3.57
105
486.35
3.63
525
488.49
4.02
75
486.33
2.12
60
486.32
1.91
30
486.25
2.52
15
486.17
3.19
Bridge
9
Existing Conditions - 100 year flow
Upper Reach
Lower Reach
Station
W.S. Elevation
Velocity
Station
W.S. Elevation
Velocity
(ft.)
(ft./s)
1035
490.21
(ft.)
(ft./s)
1.27
450
488.61
3.28
1020
490.20
1.26
435
488.61
2.91
1005
490.20
1.24
420
488.60
3.02
961
490.18
1.14
405
488.60
2.62
930
490.17
0.99
390
488.02
6.25
915
490.17
0.97
375
487.97
4.47
885
490.16
0.94
360
487.99
3.83
870
490.16
0.92
345
487.98
3.18
840
490.15
0.90
330
487.96
3.00
810
490.14
0.93
315
487.52
5.88
795
490.14
0.99
300
487.44
4.74
765
490.12
1.31
270
487.34
4.67
735
490.11
1.52
255
487.33
3.93
705
490.09
1.51
240
487.26
4.20
690
490.08
1.57
225
487.20
4.41
675
490.07
1.61
210
487.16
4.38
645
490.06
1.49
195
487.12
4.09
615
490.05
1.35
165
487.00
4.06
585
490.03
1.51
150
486.94
3.95
570
490.01
1.70
135
486.88
3.76
555
490.00
1.92
120
486.68
4.90
540
489.85
3.26
105
486.65
4.20
525
489.70
4.10
75
486.63
2.62
60
486.61
2.36
30
486.54
2.78
15
486.47
3.40
Bridge
10
Proposed Conditions - 25 year flow
Upper Reach
Lower Reach
Station
W.S. Elevation
Velocity
Station
W.S. Elevation
Velocity
(ft.)
(ft./s)
1035
489.24
(ft.)
(ft./s)
1.76
450
487.68
3.27
1020
1005
489.21
1.75
435
487.65
3.16
489.18
1.69
420
487.60
3.52
961
489.11
1.66
405
487.59
3.11
930
489.05
1.48
390
487.48
3.91
915
489.03
1.44
375
487.42
3.99
885
488.97
1.45
360
487.30
4.49
870
488.94
1.47
345
487.28
3.85
840
488.88
1.47
330
487.20
4.11
810
488.86
1.15
315
487.16
3.67
795
488.84
1.34
300
487.10
3.88
765
488.78
1.99
270
487.01
3.80
735
488.67
2.54
255
487.01
3.21
705
488.59
2.48
240
486.96
3.40
690
488.54
2.48
225
486.91
3.62
675
488.51
2.41
210
486.86
3.71
645
488.46
2.08
195
486.83
3.52
615
488.43
1.61
165
486.70
3.70
585
488.40
1.70
150
486.57
4.23
570
488.36
2.01
135
486.52
3.71
555
488.26
2.72
120
486.39
4.16
540
487.87
4.88
105
486.35
3.63
525
487.83
4.16
75
486.33
2.12
60
486.32
1.91
30
486.25
2.52
15
486.17
3.19
Culvert
11
Proposed Conditions - 100 year flow
Upper Reach
Lower Reach
Station
W.S. Elevation
Velocity
Station
W.S. Elevation
Velocity
(ft.)
(ft./s)
1035
489.82
(ft.)
(ft./s)
1.67
450
488.61
3.28
1020
1005
489.81
1.66
435
488.61
2.91
489.79
1.63
420
488.60
3.02
961
489.76
1.51
405
488.60
2.62
930
489.73
1.31
390
488.02
6.25
915
489.72
1.27
375
487.97
4.47
885
489.70
1.23
360
487.99
3.83
870
489.69
1.22
345
487.98
3.18
840
489.68
1.20
330
487.96
3.00
810
489.66
1.15
315
487.52
5.88
795
489.65
1.25
300
487.44
4.74
765
489.62
1.72
270
487.34
4.67
735
489.59
1.95
255
487.33
3.93
705
489.57
1.84
240
487.26
4.20
690
489.55
1.86
225
487.20
4.41
675
489.54
1.86
210
487.16
4.38
645
489.52
1.70
195
487.12
4.09
615
489.51
1.46
165
487.00
4.06
585
489.48
1.62
150
486.94
3.95
570
489.46
1.83
135
486.88
3.76
555
489.41
2.41
120
486.68
4.90
540
489.03
4.74
105
486.65
4.20
525
488.98
4.44
75
486.63
2.62
60
486.61
2.36
30
486.54
2.78
15
486.47
3.40
Culvert
12