The amount of weight actually supported by this style of arch is critical during the out going tides. The piston in this diagram is simply filling with water at a low pressure during the incoming tides which will be pumped out at a very high pressure during the out going tides. It is vital that the weight this arch can support is greater than the entire float because, as a tide goes out the weight of the float shifts to the arch and pressurizes the water in the piston and cylinder pump forcing the water out. During the outgoing tides the weight of the float will shift and rest on the arch, and the weight of the float is crucial to pump pressure.
The math here is simple:
The equations mentioned here should be helpful in selecting the proper materials, precalculating pressures and structural stresses in order to avoid rupturing lines, toppling etc..