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flooding, or the direction of flow, because shallow flood- ing is not readily analyzed in relation to more serious channel flooding Flood Sez>?riJy. Flood severity is determined first by the amount of water runoff to be conveyed through the watershed Flooding is most likely to occur during times of heavy rainfall or snowmel~ when the amount of runoff is higher Soil characteristics, ground and surface water star. age, and vegetation also influence flood levels. Soil per, meability determines how much surface water can be ab- sorbed rather than adding to runoff Water runoff that col- lects in surface depressions will he released gradually into the ground and atmosphere and not contribute to flooding Likewise, ground water that collects in cavities beneath the earth's surface helps reduce runoff and flood- ing Finally, vegetation slows the rate of water runoff by holding moisture on leaves and in roots, and then releas- ing it to the air through evaporation and tran5piration Thus, if a specific site has little permeable soiL no water storage capacity; and sparse vegetation, water either from a storm or from higher ground will flow directly across the surface of the site, adding to a build-up of water that may be in excess of the nearest stream's capac- ity. Obviously. the bigger this hypothetical site or region the greater the flooding impact throughout the water- shed Coastal Flooding Coastal flooding, on the other hand, has little to do with the movement of ,vater through a watershed Rather, it is due to the effects of severe ocean-based storm systems. Hurricanes, tropical storms, and extratropical storms such as "northeasters" are the principal causes, with flooding occurring when storm tides are higher than the normal '. "'.,: :.":::'::,.~~~u.n.d)\'at~~:~~~~.~>/'" . -' " ,., .". ..-.......:.::;.. .. Transpiration . Variable soil porosity, as illustrated above, influ- ences the degree of surface water ahsorption and runoff Vegetation, left, holds water in leaves and roots, releasing it grad- ually through transpira- tion and irifiltration. Water absorbed by the soil, below, is held as ground water in cavities below the water table. ~I~~~II~')I ~~~ ~;~%.~tri i%}&~~ r _ lilllIU_ 'W - . Coastal storms cause dam- j age through the combi1U!d ...... effects of wind, rain, wave . wash, storm surges, scour, and battering by debris. ~ "~@ ';If '_, "'" ,;20:),'" " 4A t;;;~'A" (:.';;l\\,'// L _ _ ~..x/*< .f(1ii$ t'.'" ~. ".. .,~J",~,,~ ~~:~ -.v,_ ~ >d'-j;Jr I!!""' v.~~ if.VQ." . "., - 81 , ~ c o ~ ..Ii ~ 13