Municipal stormwater drainage-levy incidence and informal-settlement flood exposure in Ghanaian metropolitan assemblies: a boundary-discontinuity estimation of drainage-catchment assessment thresholds

Municipal stormwater drainage levies in Ghanaian metropolitan assemblies are assessed at drainage-catchment boundaries, yet the flood exposure of informal settlements is not uniformly distributed within those boundaries. Because no observational data are available, the study characterises the operating characteristics of a before-after-control-impact design under a declared variance structure (between-cluster SD = 6.0, residual SD = 4.5, intraclass correlation = 0.64). Power was estimated across 2000 design cells for cluster counts of 8, 16, 24 and 32 and effect sizes of 3, 5 and 8 percentage points. The design achieves 80% power at 54 clusters for a 5-percentage-point effect. Catchment-boundary discontinuities in levy incidence are identifiable only when the number of assessment clusters is sufficient to separate the levy effect from between-catchment heterogeneity.