S.R. 0366 over Tributary 3 to Pucketa Creek
Camp Joann Bridge Replacement
Westmoreland County, PA
Hydraulic Approach
This project involves the replacement of an existing structure carrying S.R. 0366 over The
Unnamed Tributary to Pucketa Creek (Tributary 3 to Pucketa Creek). The replacement structure
will be placed at the existing location of the current structure. The existing structure is a
concrete slab bridge with a right angle span 15’-6” and a vertical clearance of 6’-9”. The
proposed structure is a precast reinforced concrete box culvert with a right angle span of 16’-0” x
7’-0”. A hydraulic analysis would not be required for the proposed opening if the hydraulic
opening is equal to or greater than the existing opening. We are providing a water surface profile
from the FEMA Flood Study Report showing the performances of the existing opening on the
Unnamed Tributary to Pucketa Creek. With the larger clear right angle span of 16.0’ and an
increase in the vertical clearance to 7.0’ the proposed opening will not increase the water surface
elevations.
Since the replacement bridge will be constructed along the current alignment of S.R. 0366 and an
appropriate detour is not achievable, a temporary roadway needs to be constructed. The
temporary structure needs to pass the 2.33-year storm event; however, an increase in the 100year water surface elevations (WSEL’s) above the existing WSEL’s or above the existing
channel banks, is not acceptable.
Westmoreland County currently has a Flood Insurance Study; however, we were unable to obtain
any detailed information from FEMA or from the engineering consultant who prepared the study.
The FIS contained the following discharges for Pucketa Creek Tributary 3:
10-year storm
50-year storm
100-year storm
500-year storm
340 cfs
515 cfs
590 cfs
740 cfs
Since the FIS did not address the 2.33-year storm event, we computed the discharges using PSUIV and compared the values to those reported in the FIS. The following are the discharges as
computed using PSU-IV:
2.33-year storm
50-year storm
127.64 cfs
448 cfs
For the hydraulic analysis, we used the FIS discharge for the 100-year storm event and the PSUIV discharge for the 2.33-year storm event. The existing conditions were analyzed for both the
2.33 and 100-year storm events. Using HEC-RAS 3.0, a hydraulic analysis was completed for
the existing structure and it was found that the 100-year flood event passed with no overtopping.
Therefore, an analysis was not performed for the proposed structure.
Once the existing conditions were modeled and analyzed, the temporary roadway was modeled.
Because the temporary structure only needs to pass the 2.33-year storm, but cannot raise the
WSEL’s for the 100-year storm, two temporary models were generated. The first model
determined how many pipe culverts were needed to pass the 100-year storm without raising the
WSEL’s above the existing banks. Several alternates were modeled and analyzed including
evaluating different types of pipe culverts, such as concrete and corrugated metal pipes. It was
determined that 5–72” corrugated metal pipe (C.M.P.) culverts are adequate to carry the 100-year
storm without raising the WSEL’s above the channel banks.
Once the 100-year storm criteria were satisfied, we then analyzed the temporary structure for the
2.33-year storm event. It was found that 1-72” C.M.P. would carry the storm, but that WSEL’s
rose above the existing channel banks. This would require temporary flowage easements, which
is undesirable for the size of this project. We determined that 2-72” corrugated metal pipes will
be adequate to carry the 2.33-year storm during construction. During construction, 3 of the 572” C.M.P. culverts will be sand bagged to the normal flow elevation so that if a 100-year storm
event would take place, the sand bags will collapse and allow the flow to be carried though the
pipes.
The two (2) pipes in the channel will maintain normal stream flow during temporary roadway
construction and removal. Normal stream flow will be shifted to the two (2) pipes on the
southeast side of the channel during construction of the proposed structure. See the attached
HEC-RAS outputs in the Hydraulic Analysis section for both the existing conditions and the
proposed temporary crossing. A disk with the HEC-RAS runs can be found at the back of this
report.