Assessment of flood early warning and response in an urbanized catchment

Abstract

Rapid urbanization has altered the natural hydrological cycle of the Tub Ma catchment, inducing major floods after heavy rainfall events. For that reason, structural mitigation measures were built, including floodwalls along the channel and a pumping station at the outlet. However, no operational rules were available for operating the river outlet structures and inundation was still common. To prevent or reduce flooding by improving the operational management of the outlet pumping system, an early warning system was required. The objective of this research was to assess whether informed decision-making resulted in an operational pumping strategy that prevents or reduces fluvial flooding. First, the question was asked whether water level monitoring could result in a timely and reliable warning service for an early warning system. The second question was whether timely operation of the outlet pumping station prevented fluvial flooding. For the fulfillment of the data requirements in this thesis, a fieldwork trip was organized first. During a short period of dedicated data survey, water level time series and river geometries were obtained. Water levels from three gauging stations were statistically tested as precursors for fluvial flooding in Tub Ma. Consequently, a hydraulic model was built to assess the flood mitigation capabilities of the outlet pumping station. Lastly, synthetic flood design hydrographs were run through the model for different pumping station operation strategies, with and without notification from a flood early warning system. The results from this thesis demonstrated that monitoring water levels in the upstream tributaries of Tub Ma was a useful information source for an early warning system that forecasts flood waves. This water level based early warning enabled a maximum potential lead time of 13-15 hours for flood mitigation. Nevertheless, connecting flood early warning to the operation of the pumping station did not reduce or prevent flooding. The Tub Ma river had a flow regime of limited baseflow and rapidly rising discharge, and as a result, the discharge capacity of the pumping station ahead of flood waves was physically limited by water level and inflow. Therefore, improved operation of the pumping station was not the solution for preventing or reducing fluvial flooding. On the basis of the results from this research, structural measures were suggested for flood mitigation, like temporary storage, a bypass channel, and measures that slow down the run-off processes. Also, it was recommended to examine the value of a flood early warning system by using 13-15 hours of lead time within a broader scope of disaster risk reduction in Tub Ma.