How can we obtain the calculation method of the maximum force（F) of the cattle when it forced to get back?

The cattle was grazing on the riverbank. It is tied by a rope which is fixed to the top of the wall. We assume the wall is considered to be rigid. Then the water was rising and submerged the wall eventally. The cattle was forced to get back(not run, so no impact load), the wall leaned with an angle of 30 degrees when the cattle made great efforts(The maximum force is F). Let us suppose (that) the wall leaned by the maximum force（F). And the the wall just rotate and without any parallel motion. The soil is homogeneous. The other parameters of soil are also known and can be representd by letters. The height of the water level and wall can also be representd by letters, such as h, hi, hj and hk. How can we obtain the calculation method of the maximum force（F) of the cattle when it forced to get back? The maximum force is always horizontal.（http://screenshot.net/4l99wt9）