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1.0 Project Description
The proposed project is the realignment of McClintock Rd. S.R. 2033 in order to
increase safety at the intersection of McClintock Rd. and Jack’s Run Rd. S.R.
0048. The site is located in the municipality of White Oak Township, Allegheny
County, which lies in 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 neither affect nor are affected by the project.
2.1.1 McClintock Road Bridge – The existing structure is a
reinforced concrete slab bridge with a normal span between
abutments of 16’-2” and average under clearance of
approximately 8.0’ and a skew of 60°. The bridge is 24’-3”
wide and there are no sidewalks on the structure. The low
chord elevation of the bridge ranges from 875.05’ to 875.12’
This existing structure will be removed during construction.
2.2 Proposed Structure- The proposed structure is a box culvert, with a
18.0’ span and 9.0’ rise. The length of the culvert is approximately
84.0’ which allows for improved turning radiuses at the intersection.
2.3 Environmental Habitats- Jack’s Run is not listed in the Pennsylvania
Department of Transportation design manual, Part 2, Appendix 10.A as
a stockable warm water and trout stream.
2.4 Flood History- The federal emergency Management Agency (FEMA)
Flood Insurance Study (FIS) states that flooding occurred on Jack’s Run in
June, 1972 and was estimated to be slightly greater than the 100 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 some accumulation of debris which may reduce
hydraulic capacity of the structure and increase the backwater
elevations. The computer programs used to model the hydraulics of
the channel do not take into account obstruction of the hydraulic
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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 Jack’s Run can be characterized as stable. The streambed
consists of gravel and small to medium sized cobbles. The stream
banks have little or no exposed soil or slumping.
2.7 Factors affecting Water stages- There are no flood control structures
on the Jack’s Run watershed.
3.0 Hydrologic Analysis
In 1978 there was a detailed hydrologic investigation completed for the borough
of White Oak, Allegheny County, Pennsylvania. However, the portion of Jack’s
Run where the project site is located has only been studied by approximate
methods therefore the study does not give flow values or the drainage area. The
drainage area for the project site was determined to be 1.39 sq. mi using both the
Department of Defense software program Watershed Modeling System (WMS)
as well as a planimeter and the McKeesport Quadrangle 7.5 minute series
topographic map. Peak discharges in cubic feet per second for the 10, 50, 100,
500 year storms were determined by using the U.S.G.S Water-Resources
Investigations Report 00-4189. 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.
McClintock Road (S.R. 2033) is considered an “other collector system”.
Therefore, the minimum design flood for the structure is the 10 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 FEMA study area.
Peak flows for Jack’s Run are as follows:
Flood Frequency
(YEARS)
1
1.25
2
5
10*
25
50
100
500
Peak Discharge
(CFS)
192
206
237
313
373
507
646
793
1215
Notes:
1. * 10 year frequency = Minimum Design Storm according to PENNDOT
S.O.L. 431-99-11
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2. Values in bold are from U.S.G.S. Water-Resources Investigations Report
00-4189 Calculations. All other values are interpolated
For the Flood frequency curve as well as the stage-discharge frequency curve
for Jack’s Run see appendix C.
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 structure accommodates both the design flood (10 year
frequency) as well as the 100 year storm.
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. Although the bottom chord of the
proposed structure has not been raised, the “Bridge open area” has
increased from 137.7 sq. ft. to 162.00 sq. ft. according to the HEC-RAS
modeling. The additional “bridge open area” of the proposed structure
is due to the fact that the proposed structure has a slightly greater rise
as well as a greater span when compared to the existing structure.
The results found while modeling the proposed structure are identical
to the existing structure. The design flood water levels, as well as the
channel velocity, are unchanged upstream of the project site. The
hydraulic conditions downstream of the structure are unchanged form
the existing condition.
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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 replicate the backwater
elevations of the existing structure while adhering to the constraints for the road
profile.
The proposed structure has a slightly improved hydraulic opening due to
the fact that there is additional under clearance and a greater span length. 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
793cfs. and reaches an elevation of 873.18 ft. The 100-year flood stage for the
proposed structure is, according to the HEC-RAS modeling, nearly identical to
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 $275,000 and
has no impact on wetlands.
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 pre-cast arch culvert structure. This alternative was not chosen
because it was more expensive than the chosen alternative while providing no
additional benefit.