Runoff peak frequency curves were derived from runoff flows observed in an urban test catchment and from runoff flows simulated for storms selected from two rainfall records. The first record, of a shorter length, was recorded in the test catchment. The second rainfall record, of a greater length, was recorded at a station located 10.6 km west of the test catchment. Comparisons of peak frequency curves derived from observations and both sets of simulations indicated a good agreement. As the sewer system became surcharged, the rate of flow increase with the increasing return period diminished.