Northern Arizona University

Senior Capstone Project Spring 2006

Hydrological Analysis

Hydrological Analysis

This hydrological analysis was completed with the sole purpose of proving that the water surface elevation under the bridge will never reach an elevation in which it could put this bridges structure or foundation in jeopardy.  If the water surface elevation were to reach this critical height, our design would need to account for scour.  According to FHWA’s Hydraulic Engineering Circular No. 18, scour is “Erosion of streambed or bank material due to flowing water,” (HEC-18, pg 36).  For this situation scour could erode the soil around the bridge’s foundations making the structure more vulnerable to failure.

A 42 inch Corrugated Metal Pipe (CMP)  runs under Amberwood St. to the north.  This culvert is managed by and was approved by the City of Flagstaff as an adequate CMP, sufficient of carrying the necessary amount of water away from this area south of Amberwood St.  This culvert drains all of the water from lot 176, and the neighboring lots that border lot 176.

Using the Haestad Methods program this 42 inch CMP was found to be able to carry a maximum of 77 cubic feet per second (cfs) under full flow conditions.  Full flow conditions occur when the CMP is slightly less that totally full.  The 77 cfs was then used as the basis for which all other values would be compared to; meaning the maximum amount of water expected from this small basin should be less than 77 cfs.

The following step proved that the water surface elevation under the bridge would never reach the same elevation as the lowest part of the foundation, of the bridge.  This was done by obtaining a cross-section under the bridge with the use of AutoCAD.  Manning’s Equation was then used to calculate a water surface elevation if the flow under the bridge reached 77cfs.  This assumes that the 2 acre basin above the bridge is contributing to all of the flow that eventually runs through the 42 inch CMP.  With a flow of 77cfs a water height of 8.3 inches would be reached in the middle of the channel.  For the lowest foundation on the south side of the bridge to be reached, a water surface level of 8 feet would need to be met.  A flow of 12,430 cfs would need to be exceeded for the water surface elevation to reach this height.  Since the initial value of 77 cfs and 12,000 cfs are not even on the same order of magnitude scour is not a factor in our abutment or foundation design.