Extended Abstract

4.2 Flash flood forecasting and warningA flash flood can be defined as: “a flood that rises and falls quite rapidly with little or noadvance warning, usually as a result of intense rainfall over a relatively small area”.Flash floods are distinguished from other types of flooding by the short timescalesover which flood-producing rainfall occurs (generally less than 6 hours) and the smallspatial scales (generally less than 1000 km 2 ) of drainage basins in which floodingoccurs. Most flash floods occur at night and, as would be expected, produce the mostdamage in urban areas. Aside from intense rainfall and small net storm motion,factors that contribute strongly to flash flooding are low permeability or saturated soils,impervious ground surfaces, and steep slopes. Failure of small to medium-sized dams,including debris dams, contributes significantly to the fatalities and damageassociated with flash floods.The purposes of flash flood forecasting are to provide flash flood watch and warning.Flash flood watching is to (1) inform the public of hydro-meteorological conditionswhich may cause flooding when the flooding is neither certain nor imminent; (2)provide advance notice on flooding possible within 36 hours; and (3) enable decisionmakers to monitor conditions more closely and elevate flood mitigation resources to ahigher state of readiness.Flash flood warning is to (1) issue when flooding is imminent; (2) provide advancenotification of short-fused flood events which require immediate action to protect livesand property; (3) allow customers and partners to take immediate mitigation actionssuch as evacuation to higher ground.The physical processes causing flash floods and river floods are not much different;however, predictive uncertainties tend to be greater for flash floods than for riverfloods. This is partly due to errors in rainfall data which tend to average out over thelarger spatial and temporal scales associated with river floods. In addition, predictivemodels for river floods can be calibrated using streamflow observations at forecastpoints. Most regions where flash flood predictions are required do not havestreamflow gages, even rain gages. The historical sediments disaster and flash floodevents and their corresponding rainfall record are also hard to collect.Both lumped and distributed models can be used in hydrological modeling of flashflood. Due to averagely Mean Area Precipitation (MAP), the high peak is very hard tobe predicted by lumped models. With increasing of availability of geographic data andradar rainfall data, distributed or semi-distributed hydrological models are adoptedwidely in practice. The statistical model would be based on developing the frequencydistribution of flooding at ungaged locations based on retrospective QPE data.The requirements on QPE/QPF for flash flood prediction could be described as(1)improved accurate high–resolution seamless hourly QPE; (2) hourly nowcasting; (3)hourly high quality seamless QPF.QPE and nowcasting are very important for flash flood warning. They refer to theestimation of rainfall up to the current time using multiple sensors (radar, rain gauges,-158-