A model for the dual drainage system in a flat 20 hectare slum in Indore, India was developed to evaluate the factors which influence drainage performance. Performance was defined by the depth, extent, duration, and frequency of flooding during the 1994 monsoon. This paper reports on the conceptual problem of model validation, particularly for "flooding" or "not flooding". The model's predictions for maximum depth during verification were reasonable, as 72% of maximum depth predictions were within +/- 0.10 m of observed values. The predictions for the binary variable of flooding at a point were, however, poor. The model gave good predictions of freeboard violation, where the freeboard level was set at 50 mm below the lowest kerb. The paper offers some intuitive reasons as to why prediction of freeboard violation may be expected to be better than prediction of flooding. The authors conclude that where models cannot be validated for flooding prediction, it may be better to redefine performance criteria and validate the model for freeboard violation. (C) 1999 Published by Elsevier Science Ltd on behalf of the IAWQ. All rights reserved.
