Effects of drain-hole depth and surface cover on ponding dissipation: Laboratory evidence for compact infiltration retrofits

Rapid urban ponding in dense tropical areas is intensified by surface sealing, yet the hydraulic role of micro-scale drain-hole geometry remains poorly quantified. This laboratory study evaluated whether deeper drain holes and contrasting surface covers modify ponding dissipation and surface water drawdown under standardized loading. Three surface covers (grass, paving blocks, concrete) and three drain-hole penetration depths (5, 10, and 12 cm) were tested in a controlled ponding-infiltration apparatus (50 × 50 cm plan area; 15 cm soil layer; 15,000 cm3 initial ponding volume, equivalent to about 6 cm depth). The working hypothesis was that deeper drain holes would shorten drawdown time by bypassing the near-surface layer, with the greatest relative benefit under more-sealed covers. Without a drain hole, derived mean discharge decreased from 1.91 cm3/s under grass to 0.85 cm3/s under paving blocks and 0.20 cm3/s under concrete, corresponding to areal fluxes of 27.5, 12.2, and 2.9 mm/h, respectively. Adding a drain hole consistently accelerated drawdown across all covers. At a depth of 12 cm, drawdown time was reduced by 36.6% for grass, 33.4% for paving blocks, and 82.0% for concrete relative to the respective controls. These results indicate that surface cover sets the baseline infiltration regime, whereas drain-hole depth acts as a secondary geometric control, becoming especially valuable when surface sealing is strong. The study therefore demonstrates that drain-hole penetration depth is a geometry-sensitive design parameter that can strengthen compact infiltration retrofits and provide process-based laboratory evidence on a compact infiltration retrofit with potential to contribute to urban ponding reduction.