**4. Discussion and Conclusions**

The present investigation explored the progression of local scour around a circular cylinder placed in an erodible bed and the effects of channel blockage ratio *D*/*b* therein. It was established that, under the experimental conditions of this work, equilibrium of sediment removal in the vicinity of the cylinder was reached at 24 h. Beyond this period, changes in the scour formation were observed primarily in the downstream region which did not contribute significantly to the design of the foundation head for local scour.

At *D*/*d*50 < 100, the experimental results of the present investigation indicated that the size of the scour hole decreased with increasing blockage ratio which can likely be attributed to a weakening of the flow field mechanisms due to the sidewall proximity. In order to fully understand this phenomenon, detailed flow measurements in the flow field surrounding the cylinder are required in future work.

When compared with data from the literature, the progression of local scour and the relative scour depth did not appear to be significantly a ffected by changes in the blockage ratio for small values of relative coarseness (i.e., *D*/*d*50 < 100). However, analysis of data with high values of relative coarseness ( *D*/*d*50 > 100) showed that blockage ratio had a greater e ffect on scour depth. Furthermore, comparison of the results of the present investigation with the literature results was particularly limited, since such analysis requires that all scour-governing parameters are identical. Future experimental design corresponding to previously published results would assist in accomplishing similar analysis for a broader range of conditions in future work. The authors recommend that further experimentation on the e ffect of *D*/*b* is required, particularly for a wider range of *D*/*d*50 which approaches field conditions (i.e., *D*/*d*50> 100). In order to achieve this, use of larger-sized flumes with finer bed material is required.

Various predictive methods from the literature were used to estimate the progression of local scour to varying degrees of e fficacy. The scour-predicting equation presented in Williams et al. [19] was shown to provide a better estimation of *dse*/*D* for *D*/*d*50 < 100 than for larger values of relative coarseness, indicating that a scaling factor may be required in order to implement such predictive methods in practical design.

Larger values of *D*/*d*50 are particularly desirable in this venture, since field values of relative coarseness are usually greater than 200. Once the e ffect of sidewall proximity on scour depth has been established at a prototype level, further evaluation can be carried out in order to incorporate this e ffect into scour estimation.

**Author Contributions:** The experimental work in the present investigation was carried out by P.W. The analysis and manuscript compilation were conducted jointly by the authors.

**Funding:** This research was funded by the Natural Sciences and Engineering Research Council of Canada through the Discovery Grant program to Ram Balachandar and Tirupati Bolisetti.

**Conflicts of Interest:** The authors declare no conflict of interest.
