Flash flood hydrologic processes of a small headwater catchment in Hong Kong

Abstract

With frequent heavy monsoon rainfalls and steep slopes, Hong Kong suffers flash floods almost every year. For studying the features of flash floods, two automatic rainfall gauges (one under a tree and the other in the open) and one automatic water level data-logger were set up in a small catchment in the New Territories of Hong Kong, in the summer of 2007. Since then, several heavy rainfall events and related flash flood hydrographs have been recorded. This paper presents the results from analyzing these events. The study reveals that the features of canopy interception of rainfall can be linearly represented. By separating baseflow from the storm hydrographs, it was found that the direct runoff depth ratio to the total rainfall depth in this small catchment is about 2%. Finally, a rainfall-runoff model, TOPMODEL, has been applied to simulating the flash flood processes. At a one-minute time step, the model displays the capability of representing the very rapid increase of discharge on the rising limb of the flood hydrographs in the small catchment.