Upcountry tea estate lock-and-spill contour drain engineering: silt retention on 30-degree slopes?

Contour drainage systems on steep highland tea estates (gradients between 25° and 35°) utilize "lock-and-spill" channels cut across hillsides at a gentle 1:120 gradient. The ditch is excavated with alternating level sections ("locks") and deeper rectangular settling basins ("spills"). Torrential monsoonal storm runoff enters the ditch laterally from tea plucking rows, but the flat longitudinal gradient drops water velocity below erosive thresholds. Suspended organic humus and fertilizer salts settle out in the pits, while clarified water spills safely into granite-lined leader drains that discharge down natural rocky ravines.