Predictive design flows for the Waigani catchments. Papua New Guinea

Abstract

Two catchment area within Port Moresby in Papua New Guinea called the Waigani Urban and Waigani Natural (rural) were studied for their design Peak discharges. The urban catchment has a total catchment area of 7.7 Km2 whilst the natural catchment has a total catchment area of 7.4 Km2. Both having similar geology, lithology, soils, vegetation (except in residential areas), climate and channel slopes making it an ideal location to study the influence of urbanisation on the design peak flows. Whilst attempting to use a variety of methods for determining design peak discharges, the Rational Method was finally chosen to predict design peak flows for return periods of 2, 5, 10, 50 and 100 year. The Rational Method was chosen over other methods because during the period this study was conducted, Port Moresby had been experiencing prolonged dry periods except on one occassion (29th January 1992) flood data was collected for both catchment. Rainfall events during the study period weren't sufficient for any resonable size flood measurements to be made. However, the rainfall and flow measurements collected during the rainstorm of 29th January 1992 together with other data collected during the dry period forms the basis of this study. It was found that the time of concentration for the urban catchment is half that of the natural catchment and the current urban storm flows are approximately 3-fold that of the natural catchment channel provided the current situation remains for some years to come. Also given the current senario a 10 + year design peak discharge can cause substantial economic burden to the city council and also there is possibility of potential damage and danger to properties and health further down stream. As no urban runoff studies have been done for the area, the design peak discharges obtained can be seen as representative of the Waigani area, with runoff coefficient C values of 0.37 for the Urban catchment and 0.16 for the Natural catchment. Hence, the design peak flows ranges from 13.8 m3/s for a 2 -year return period, 1 hour rainfall intensity of 42 mm/h to 31.2 m3/s for a 100-year return period, 1 hour rainfall intensity of 95 mm/h for the Natural catchment and from 33.2 m3/s for a 2 - year return period, 1 hour rainfall intensity of 42 mm/h to 75.2 m3/s for a 100 - year return period, 1 hour rainfall intensity of 95 mm/h for the Urban catchment.