Suppose the waterfall is a metre thick. Then you can count the waterfall. The volume of the waterfall: 15x6xa=90a, then let's calculate the thickness of a=1cm=0.01m, then the volume is 0.9 cubic meters.
According to the height of the waterfall, the amount of water left from 6m is about 0.6 seconds, then the traffic is about 0.9x0.6 = 0.54 cubic/sec, that is 1944 cubic/hour. At this time select the pump flow of 2000 tons/hour, the head of the pump around 10m, at this time the power of the pump is about 110Kw.
When a=1mm=0.001m, also according to this algorithm, then the pump flow is 194 tons/hour. At this time select the pump flow 200 tons/hour, head 10m around the pump, at this time the power of the pump is about 11-15kw.
Specific selection of what pump can be selected according to the actual situation of submersible pumps, or centrifugal pumps (centrifugal pump should be noted that the pump can not be placed above the waterfall, because the centrifugal pump is not so high suction, put at the top of the suction not water).