I agree with esshup. A steep sided trench even built in clay bottom quickly gathers sediment into the bottom of the "funnel". The steep uncompacted sides of the trench does tend to slough. If a bottom center trench is built the sides should be sloped at least 2:1 and compacted to minimize sloughing.

aka Pond Doctor & Dr. Perca Read Pond Boss Magazine -
America's Journal of Pond Management