Date of Award

12-2011

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Civil Engineering

Advisor

Khan, Abdul A

Committee Member

Aziz , Nadim M

Committee Member

Testik , Firat Y

Abstract

The present study was aimed at measuring the scour hole profile at bridge piers under clear water scour. Scour tests were performed under both steady state flow and under stepped hydrographs. Multiple smaller flood events were run to determine how flood history affects scour hole development. Various equilibrium scour depth equations and proposed temporal scour models were compared. The similarity of the scour hole profiles with time and flow history were evaluated.
Experiments were conducted in a 14.8 m long, 1.19 m wide, 1.22 m deep rectangular flume with a model bridge pier. A uniform sediment bed (d50= 1.5 mm) was used throughout the study. Scour hole profiles were measured using a light sensor. Three steady state flow experiments with a constant flow depth and different bed shear stresses and velocities were run. These three flow conditions were later used to model eight unique stepped hydrographs.
The order of flood events was found not to effect the scour depth or scour hole shape. The scour hole maintained the same non-dimensional, longitudinal similarity regardless of flow history or time for both steady and unsteady flows. This finding indicated steady state scour evolution models can be used to model scour under stepped hydrographs.
Several equilibrium scour depth equations were evaluated and it was found the CSU equation (FHWA, 2001) was just as accurate as several proposed temporal scour models. Best-fit power and logarithmic temporal scour depth equations were obtained from each steady state test. These equations along with models proposed by Melville and Chiew (1999) and Chang et al. (2004) were found to predict steady state temporal scour evolution with reasonable accuracy.
Using the method of superposition, these methods were used to predict scour depths under stepped hydrographs. For stepped hydrographs, the scour development for each event (or each step) followed the temporal scour evolution under steady flow conditions. It was found that the order and frequency of events did not change the scour development pattern. It was determined that the power and logarithmic function accurately predicted final scour depths (± 10% and ± 12%, respectively) and temporal scour evolution. The models proposed by Melville and Chiew (1999) and Chang et al. (2004) were found to provide no greater accuracy in predicting final scour depths than the CSU equation (FHWA, 2001). More research is needed on temporal scour evolution under steady flow conditions in order to predict scour during floods.

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