![]() Flash flood forecasting: an ingredients-based methodology. Now Cell III is developing its first precipitation aloft. It is still raining at ‘X’ but now the rainfall is from Cell II, and heavy rain from Cell II is descending from aloft. Cell I's rainfall is continuing but it is now nearly dissipated, while Cell II is entering late maturity. Cells III, IV, and now V are still immature. Heavy rain continues at ‘X’ while Cell II is maturing and developing a downdraft. (b) About 15 min later, Cell I's updraft is dissipated, and it is now dominated by downdraft. The hatched contours are radar reflectivity, in units of dBz, which is related to the rainfall rate. Cell I is mature, with both updrafts and downdrafts, and heavy rain is about to commence at point ‘X.’ Cells II, III, and IV are still developing, and have only updrafts. (a) At this time, there are four numbered thunderstorm cells in various stages of development. The ensuing flash flood was responsible for 77 fatalities and $550 million (in 1999 dollars) in damage.įigure 2. ![]() The result was torrential rainfall concentrated near Johnstown, with amounts exceeding 400 mm. Thunderstorms forming in northwestern Pennsylvania moved southeastward, only to be replaced by newly formed thunderstorms, a process that went on for several hours. The infamous Johnstown, Pennsylvania flash flood of 19–20 July 1977 was produced by such a system. A succession of thunderstorms results when new thunderstorms pass repeatedly over the same place while the overall system of thunderstorms is very nearly stationary. A single thunderstorm cell is unlikely to produce enough rainfall to cause a flash flood, so the typical flash flood is the result of several thunderstorms moving successively over the same area, known as ‘training’ thunderstorms ( Figure 2), because it resembles the passage of cars in a train. Most flash floods associated with rainfall are produced by thunderstorms that is, deep, moist convection. A comprehensive review of the efficacy of NWS FFG systems, utilizing NWS Storm Data ( ) for verification of FFG values, appears in Clark, Gourley, Flamig, Yang, and Clark (2014). The approach used to estimate FFG varies considerably between RFCs. ![]() For FFG purposes, a specific amount of rain is needed to produce a given amount of runoff based on estimates of current soil moisture conditions as maintained by soil moisture accounting models. This process requires that prior hydrologic analyses be made to determine the peak streamflow (or normalized as peak runoff with respect to basin averaged peak flow) that corresponds to bankfull conditions-this index value is known as threshold runoff. The NWSRFS FFG operation used to compute FFG is a computationally efficient process that iteratively runs the operational hydrologic model, including snow accumulation and melt, over a basin until the precipitation needed to reach bankfull flow (or runoff) is reached. FFG is the amount of rainfall needed over a prescribed area, for a given duration to bring small streams to bankfull, flood conditions (see Sweeney, 1992 Sweeney & Baumgardner, 1999). The FFG System includes techniques and programs for computing FFG values. ![]() Adams III, in Flood Forecasting, 2016 3.4.5 Flash Flood Guidance (FFG) System More specifically, we examine the climatic settings of the flash floods considered, analyzing the distribution of event runoff coefficients and assessing the role of antecedent saturation conditions in controlling the magnitude of extreme flash floods. In the second part of the chapter, we provide an analysis of the runoff properties of a number of extreme flash floods that occurred in Europe since 1994. Furthermore, we present an overview of scientific investigations carried out in different hydro-climatic settings to characterize the runoff-generation processes occurring under intense rain rates. The first part of the chapter provides general concepts regarding the hydrological mechanisms controlling catchment and hillslope runoff response to intense precipitation. This contribution aims to provide a review of the hydrological mechanisms driving hillslope runoff response to intense rainfall and to characterize runoff response from selected extreme flash floods in Europe. Despite their scientific and social importance, the fundamental processes triggering a flash-flood response are poorly understood. As such, they are characterized by short response time and have the potential to severely impact and damage communities in different climatic settings all over the world. Zoccatelli, in Treatise on Geomorphology, 2013 Abstractįlash floods are typically associated with short, high-intensity rainstorms.
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