Downstream reduction of rural channel size with contrasting urban effects in small coastal streams of southeastern Australia

摘要: Abstract Although most streams show a downstream increase in channel size corresponding to flood discharges, those flowing off the Illawarra escarpment of New South Wales marked reduction size, accompanied by down-stream frequency their lower reaches. Within confined and steeply sloping valleys foothills, bed bank sediments are relatively coarse uncohesive, channels increasing discharge downstream. However, once these emerge into more open rural at slopes extensive floodplains formed fine cohesive sediment, there is dramatic size. This decrease apparently results from sudden decline slope associated stream power, nature alluvium, its retention on banks dense cover pasture grasses, availability an floodplain carry displaced floodwater. Under conditions floodwaters very frequently spill out over channel-flow becomes unimportant component total peak discharge. emphasizes importance as part system. In situations where urban development has increased runoff reduced available area effective floodplain, this alluvium rapidly entrench cross-sectional 2–3 times. Minor man-induced alteration maintenance appears trigger enlargement.