6. A temporary pedestrian bridge is being designed in Bath for pedestrians to cross the river Avon. A contractor has been employed and the engineering company has decided to support the bridge using cylindrical columns with either a 0.3m or a 0.5m diameter. The bridge has to withstand the hydrodynamic force which occurs when there is a maximum design flow rate of 70 m/s. The contractor is considering two different options which are shown in Figure Q6a and b. Note that the river is flowing into the page. i. Three of the smaller bridge piers (D=0.3m; Figure Q6a), or DIII DIII ii. Two of the larger bridge piers (D=0.5m; Figure Q6b). DIII DIII DIII DIII Figure Q6a) River cross section for option 1 (three bridge piers) DIll Figure Q6a) River cross section for option 2 (two bridge piers) (a) Assuming the river Avon has a rectangular cross section where the channel width = 40 m and a constant design water depth = 4 m, calculate the velocity of the flow. (b) Ignoring surface effects and only considering the part of the object that is fully submerged (shaded part on Figures Q6a and Q6b), determine the total drag force for each of the contractor's two options. Which option would result in the smallest force? Take the kinematic viscosity of water to be v = 10-6 ms-1. (c) Which component of the total drag is dominant in this example?