Flood­ing

Northwest Arkansas Democrat-Gazette - - NORTHWEST ARKANSAS - Source: Staff re­port

The changes in land use associated with ur­ban de­vel­op­ment af­fect flood­ing in many ways. Here are a few from a U.S. Ge­o­log­i­cal Sur­vey fact sheet:

■ Re­mov­ing veg­e­ta­tion and soil, grad­ing the land sur­face and con­struct­ing drainage net­works in­crease runoff to streams from rain­fall and snowmelt. As a re­sult, the peak dis­charge, vol­ume and fre­quency of floods in­crease in nearby streams.

■ Changes to stream chan­nels dur­ing ur­ban de­vel­op­ment can limit their ca­pac­ity to con­vey flood­wa­ter. Roads and build­ings con­structed in flood-prone ar­eas are ex­posed to in­creased flood haz­ards, in­clud­ing in­un­da­tion and ero­sion, as new de­vel­op­ment con­tin­ues.

■ Streams are fed by runoff from rain­fall and snowmelt mov­ing as over­land or sub­sur­face flow. Floods oc­cur when large vol­umes of runoff flow quickly into streams and rivers.

■ The peak dis­charge of a flood is in­flu­enced by many fac­tors, in­clud­ing the in­ten­sity and du­ra­tion of storms and snowmelt, the to­pog­ra­phy and ge­ol­ogy of stream basins, veg­e­ta­tion and the hy­dro­logic con­di­tions pre­ced­ing storms.

■ Land use and other hu­man ac­tiv­i­ties in­flu­ence the peak dis­charge of floods by mod­i­fy­ing how rain­fall and snowmelt are stored on and run off the land sur­face into streams.

■ In un­de­vel­oped ar­eas such as forests and grasslands, rain­fall and snowmelt col­lect and are stored on veg­e­ta­tion, in the soil col­umn or in sur­face de­pres­sions. When this stor­age ca­pac­ity is filled, runoff flows slowly through soil as sub­sur­face flow. In con­trast, ur­ban ar­eas, where much of the land sur­face is cov­ered by roads and build­ings, have less ca­pac­ity to store rain­fall and snowmelt. Con­struc­tion of roads and build­ings often in­volves re­mov­ing veg­e­ta­tion, soil and de­pres­sions from the land sur­face. The per­me­able soil is re­placed by im­per­me­able sur­faces such as roads, roofs, park­ing lots, and side­walks that store lit­tle wa­ter, re­duce in­fil­tra­tion of wa­ter into the ground, and ac­cel­er­ate runoff to ditches and streams.

■ Dense net­works of ditches and cul­verts in cities re­duce the dis­tance that runoff must travel over­land or through sub­sur­face flow paths to reach streams and rivers. Once wa­ter en­ters a drainage net­work, it flows faster than ei­ther over­land or sub­sur­face flow.

■ With less stor­age ca­pac­ity for wa­ter in ur­ban basins and more rapid runoff, ur­ban streams rise more quickly dur­ing storms and have higher peak dis­charge rates than do ru­ral streams. In ad­di­tion, the to­tal vol­ume of wa­ter dis­charged dur­ing a flood tends to be larger for ur­ban streams than for ru­ral streams.

■ Sed­i­ment and de­bris car­ried by flood­wa­ter can con­strict a chan­nel and in­crease flood­ing. This haz­ard is great­est up­stream of cul­verts, bridges or other places where de­bris col­lects.

■ Com­mon con­se­quences of ur­ban de­vel­op­ment are in­creased peak dis­charge and fre­quency of floods. Typ­i­cally, the an­nual max­i­mum dis­charge in a stream will in­crease as ur­ban de­vel­op­ment oc­curs.

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