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1.0 Project Description
The proposed project is the replacement of the Green Garden Road Bridge
carrying S.R. 3016 over Raccoon Creek. The site is located in the municipality of
Hopewell Township, Beaver County, for the Pennsylvania Department of
Transportation (PENNDOT), District 11-0 (see location map Appendix A.)
2.0 Site Data
2.1 Existing Structures-Although there are structures upstream and
downstream of the project site they are not in the immediate vicinity
and therefore not affected by the project.
2.1.1 Green Garden Road Bridge – The existing structure is a
steel beam concrete filled grid deck bridge with a normal
span(s) between abutments of 120’, an average under
clearance of approximately 8.6’ and a skew of 90°. The
bridge is 25.5’ wide and there are no sidewalks on the
structure. The low chord elevation of the bridge ranges from
772.19’ to 773.02’.
2.2 Proposed Structure- The proposed structure is a single span steel
plate girder with an c/c span length of 126+’-0 3/4” and a width of
59.375’ The cross section allows for three 12’-0” lanes, one in either
direction, as well as a turning lane and 10’-0” shoulders. The single
span alternative uses seven beams due to max overhang and spacing
restrictions. The depth of the beams is 4.77’ and the estimated depth
for the super elevation is 0.8275’ which gives an overall depth of
5.597’. The low chord elevations of the structure are 776.16’ and
774.20’. Since the bottom chord of the proposed structure will be
raised, the average under clearance will increase by approximately
3.0’.It should also be noted that upon completion of the proposed
structure the existing structure will be removed. Line and grade
approval was acquired on 8/19/03.
2.3 Environmental Habitats- Raccoon Creek is listed in the Pennsylvania
Department of Transportation design manual, Part 2, Appendix 10.A as
a stockable warm water and trout stream; it is also listed as a warm
water fishery in PA code, Title 25, Chapter 93, water quality standards.
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2.4 Flood History- The federal emergency Management Agency (FEMA)
Flood Insurance Study (FIS) states that flooding occurred on Raccoon
creek in February 1975 and was estimated to be a 10 year flood,
according to available high water marks.
2.5 Drift and Ice- There is no evident build up of drift or ice during field
views. It should be noted however, that under extreme levels of flow
there could be significant accumulation and the hydraulic opening
could decrease in size thereby increasing the backwater elevations.
The computer programs used to model the hydraulics of the channel
do not take into account obstruction of the hydraulic opening and
therefore are not representative of high discharge events with
significant debris accumulation.
2.6 Streambed and Stream bank Stabilization- The streambed and
banks of Raccoon Creek can be characterized as stable.
2.7 Factors affecting Water stages- There are no flood control structures
on the Raccoon Creek water-shed. However, there are two large
impoundments; the J.C. Bacon Dam on service creek and the Raccoon
Creek Park Dam on Traverse Creek which may partially control 34.8
square miles of the watershed.
3.0 Hydrologic Analysis
A detailed hydrologic investigation was completed for the 1981 FEMA FIS. The
FIS lists peak discharges in cubic feet per second for the 10, 50, 100, 500 year
storms as well as the drainage area in square miles for Raccoon Creek which is
given as 150 sq. mi. The flood flows were estimated by the Log-Pearson Type III
frequency method based on the 50 years of available data for the site. These
flows were also adjusted to account for the aerial difference between the gauging
station and the area of study. The 10, 50, 100, and 500 year discharges were
then plotted against their frequency of occurrence and a best fit line was added
to interpolate additional recurrence frequencies which include the 1, 1.25, 2, 5,
and 25 year floods.
Green Garden Road (S.R. 3016) is considered a minor arterial in the area of the
project site. The minimum design flood for the structure is the 25 year flood
according to PENNDOT S.O.L. 431-99-11 however, the 100 year flood is also
considered due to the fact that the structure lies within a detailed FEMA study
area.
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Peak flows for Raccoon Creek are as follows:
Flood Frequency
(YEARS)
1
1.25
2
5
10
25*
50
100
500
Peak Discharge
(CFS)
5139
5295
5639
6374
6793
7906
8949
9765
11475
Notes:
1. * 25 year frequency = Minimum Design Storm according to PENNDOT
S.O.L. 431-99-11
2. Values in bold are from 1981 FEMA FIS. All other values are interpolated
For the Flood frequency curve as well as the stage-discharge frequency curve
for Raccoon Creek see appendix B.
4.0 Hydraulic Analysis
4.1 Methodology- Using the calculated discharges, hydraulic effects of
existing and proposed conditions was studied. Calculations were
performed using U.S. Corps of Engineers HEC-RAS river analysis
system software, version 3.0.1
Given the site data and hydrologic data, the HEC-RAS models provide
flood profiles, cross-section data, and channel velocities. This software
was used to compare hydraulic models for the existing and proposed
to determine if there were any changes in back water elevations or
channel velocity that would occur due to the project. The HEC-RAS
output for the existing and proposed conditions are presented in
Appendices D and E.
4.2 Existing Condition Model-The HEC-RAS model shows that the
existing condition does not accommodate the 10, 50, and 100 year
storms as a negative freeboard value is calculated for all cases. The
500 year storm is similarly not accommodated by the structure with the
deck elevation being only approximately 3” above the water elevation.
4.3 Proposed Condition Model- The existing condition HEC-RAS model
was modified to include the geometric and profile adjustments of the
proposed structure conditions. The bottom chord of the proposed
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structure has been raised an average of 3.0’ in elevation, and the
“Bridge open area” has increased from 817.19 sq. ft. to 1238.54 sq. ft.
according to the HEC-RAS modeling. The additional clearance of the
proposed structure is due partly to the fact that the location of the
bridge is at a higher elevation and the superstructure is shallower
compared to the existing structure.
The results found while modeling the proposed structure are generally
similar to the existing condition with the exception of the proposed
structure accommodating the 10 year storm while providing 2.06’ of
freeboard. This is an improvement over the existing condition which
cannot accommodate the 10 year storm and has 0.83’ of the lower
chord inundated by flood water. There is also a decrease in most
backwater elevations for the 10, 50, 100, and 500 year storms at river
stations upstream of the proposed structure while any increases are
considered negligible. The hydraulic conditions downstream of the
structure are unchanged form the existing condition.
5.0 Risk Assessment or Analysis
The proposed structure was designed to minimization the risk of future
flooding at the site. The opening size was chosen to reduce the backwater
elevations while adhering to the constraints for the road profile.
The proposed structure has an improved hydraulic opening due to the fact
that there is additional under clearance while maintaining the width of the
opening as well as the removal of the pier. The performance of the proposed
structure is very similar to the existing structure and does not produce a greater
risk of flooding. The 100 year flood discharge is 9,765 cfs. and reaches an
elevation of 776.16. The 100-year flood stage for the proposed structure is,
according to the HEC-RAS modeling, approximately 1.17’ below that of existing
and therefore does not present any increase in flood risk compared to the
existing condition. It should also be noted that neither the 100 nor 500 year flood
overtop the structure.
The proposed structure has a capital cost of approximately $3.8 million.
The proposed structure also impacts wetlands in the project area. It has been
determined that the project will reduce the current wetland area by approximately
0.04 acres. This includes the 0.42 acres that the roadway realignment will impact
as well as the replacement of 0.38 acres that is to be constructed adjacent to the
existing wetland area. However, there will be additional investigation performed
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in order to reduce the impact to the wetlands at the project site. See field view
notes Appendix H
The proposed structure is the smallest opening, according to the study
performed, recommended in order to maintain or improve upon the existing
condition.
Due to the fact that there is no “significant” encroachment as defined by
23 CFR 650 subpart A Sec.650.105 there was no risk analysis required for the
project.
6.0 Alternative Proposal
There was also a second proposal investigated. This alternative proposal
consisted of a two-span spread box beam structure. This alternative was not
chosen because of the obstruction in the channel caused by the pier. The
existing structure has a pier and during high discharge floods there is a
significant amount of debris buildup that severely diminishes the hydraulic
performance of the structure. This problem was alleviated by removing the
obstruction to the opening and using a single span structure.