Modelling of runoff from landslide affected streams

Abstract

The computer model Runoff Routing Program RORB is used to determine flood characteristics in three small Hawkes Bay streams flowing within the same catchment. Comparisons are made between the characteristics found for two of these streams which have been heavily affected by landslides and debris flows and the third, largely unmodified, stream. Within the low flow regime experienced during the study period, the characteristic hydrograph fitting parameters determined in the program show a small difference between the unmodified and the modified streams. This is interpreted as showing a decrease in storage in the landslide affected streams compared to the unmodified stream. In addition, other landslide induced differences in stream response, such as shorter delay time between rainfall and commencement of discharge, were closely modelled by the program. Using the characteristic hydrograph parameters determined by the program the flood events that caused the landslides were simulated. The hydrographs produced possessed steep rise and fall rates, high peak discharge values and large flow volumes all of which are consistent with the necessary conditions for landslide and debris flows to occur in these catchments. The estimated stage height of the calculated peak discharge matched well with the height of the physical damage observed in the affected stream channels. This technique could be extended to other ungauged flow events. The model is shown to work well in small catchments as well as the larger catchments it has mostly been applied to previously and that it models the streams sufficiently closely to detect changes to the streams characteristics, caused by the landslides. By comparing the effects of landslides that were detected by the model with the known effects of urbanisation, it is shown that the model can be used as a tool to forecast the effects of urbanisation of a catchment.