Scour and scour-related complication have been widely established as the most prevalent cause of bridge pier failure in North America (Melville and Coleman 2000; Wardhana and Hadipriono 2003; Foti and Sabia 2011). When a structure such as a bridge pier or abutment is introduced into a flow environment, the flow structures which are induced cause sediment removal around its base. As these flow structures increase in size and intensity, so too does the removal of sediment (better known as scouring action), effectively removing the lateral support provided to the foundation. If scouring proceeds up to and beyond the point in which the foundations are exposed or undermined, then failure of the footing or foundation in tension is likely to follow (Melville and Coleman 2000). The case of the Schoharie Creek bridge collapse, which occurred in upstate New York in 1987, is an example of such a failure, which also resulted in total collapse of many of the bridge's spans as well as the deaths of 10 motorists (LeBeau and Wadia-Fascetti 2007). As a result, determination of foundation head (the depth to which footings should be placed) is prescribed by several design standards, and is chiefly calculated on the basis of empirical equations which have been experimentally determined over the past several decades. It has been shown that these equations have a tendency to over-predict this depth, leading to unnecessarily high construction and material costs (Ettema et al. 1998). This is due to the development of these equations, which is largely based on experimental results which were obtained under varying conditions. For example, the difficulty in obtaining perfect geometric, kinematic and dynamic similitude between a prototype in the laboratory and a model in the field lies primarily in the inability of a model to properly scale for sediment size without altering inter-particle forces existing in the sediment (Heller 2011; Ettema et. al. 1998). This is an example of a scale effect in scour modelling, and has been previously investigated. However, there are other lesser-known scale effects in scour experimentation which have been shown to affect experimentally-obtained results. These include failure to obtain results under controllable conditions, such that conclusions drawn are on the basis of defined influences, as well as the effects of wall interference due to channel blockage. A series of experimentation was carried out at the Ed Lumley Centre for Engineering Innovation at the University of Windsor in Windsor, Ontario in order to isolate the effects of several non-dimensional parameters affecting scour. These include relative flow depth or flow shallowness, h/D, where h is mean flow depth and D is pier diameter, and blockage ratio, D/b, where b is channel width. The effects of flow shallowness on relative scour depth, dse/D (where dse is maximum scour depth measured from the front of the pier at equilibrium) were investigated through two subsets of tests. In each test, the majority of scour-influencing parameters, including those non-dimensional parameters upon which many scour prediction methods are based, were held constant and the only such parameter which changed was flow shallowness. Each subset was completed for a different sediment size in a flume with a width of 1.22 m, a height of 0.84 m, and a length of 10 m. Each test was carried out for 48 hours, which was determined through prior experimentation to be sufficient time to equilibrium under the conditions required. It was determined that for each subset considered independently, relative scour depth increases with flow shallowness before reaching constancy over the range of h/D between 2.0 and 3.2. However, the influence of flow shallowness on relative scour depth was also present for values of h/D greater than 3.2. Furthermore, when comparing tests with similar values of h/D in different sediment sizes, the effects of sidewall interferences are more prevalent in tests done in finer sediment. Bed formations were also shown to be affected by these parameters; bed ripples appeared to have formed on primary scour deposits in finer sediment, and primary deposit length is greater in all tests of finer sediment with constant flow shallowness. The results of several investigations were compared in order to draw conclusions on the effects of channel blockage on scour geometry. It was determined that sidewall proximity does influence scouring action, particularly in finer sediment. When tests with constant relative coarseness, sediment size, flow shallowness and flow intensity but different blockage ratios in finer sediment were compared, the extent of scour was deeper, wider, and longer in the test with a blockage ratio of 10 percent compared with the test with a blockage ratio of 5 percent. However, two tests with constant relative coarseness, sediment size, flow shallowness and flow intensity in the coarser sediment showed the opposite effect; that lesser blockage results in greater scour. Therefore, the influence of sediment size, or some alternate sediment-related parameter, is greater than the effects of channel blockage alone. A comparison of three tests performed further confirms that some additional sediment-related parameter influences scour geometry. In all three tests, flow shallowness, relative coarseness and flow intensity are constant. However, one of the tests was performed in coarser sediment, with a blockage ratio of 10 percent. The other two tests were performed in finer sediment, with blockage ratios of 5 percent and 15 percent respectively. Experimental results showed that scour for the test with the coarser sediment was greater than either of the tests in the finer sediment, despite one of these having a higher blockage ratio. In conclusion, analysis of experimental results obtained in scour investigations has indicated that when isolated, the effects of flow shallowness on scour geometry are constant over a certain range of h/D. Furthermore, the influence of channel blockage is dependent on both blockage ratio and sediment size, when all other scour-defining parameters are held constant. However, the influences of blockage ratio can change depending on sediment size, which indicates that there is some other sediment-related factor to which changes in scour can be attributed.
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