Effect of bottom undulation for mitigating wave-induced forces on a floating bridge

A model for oblique wave interaction with a floating bridge in the presence of a submerged breakwater and a trench is developed. The breakwaters and trenches are considered to be of trapezoidal and circular types. The performance of these undulations in bottom bed are analyzed in detail for reflecting waves and reducing resulting forces on bridge. The analysis is performed by utilizing solutions derived with the aid of modified mild-slope equation in the undulated bottom region and of eigenfunction expansion in uniform bottom region. Significant changes are found in wave reflection and forces due to consideration of these undulations in bottom bed as compared to those in the case of uniform bottom everywhere in fluid region. The present study shows certain range of wave incident angles at which the breakwaters and trenches perform effectively in reflecting waves and thereby reducing forces. A complementary connection is justified between reflection of waves and consequent forces on bridge. Moreover, critical wavenumbers and angles where zero reflection occurs are found in shallow and intermediate water depth. The critical values are found to lead an increase in forces and for these values appropriate gap lengths between bridge and breakwater or trench, width of breakwater and trench are explored. Phase shift in reflection and forces is observed due to the inversion of undulated profiles. Within a periodic value of the distance between the breakwater/trench and the floating bridge, optima in wave reflection occurs. In addition, the maxima in wave reflection is associated with the minima in wave forces acting on the floating bridge. The findings from the present model are likely to be useful in understanding the role of breakwaters and trenches in engineering applications. (C) 2019 Elsevier B.V. All rights reserved.