Global sediment yields from urban and urbanizing watersheds

Streams with urban watersheds are almost universally subject to degradation, largely driven by changes to flow and sediment inputs from the watershed. However, the impact of urbanization on sediment yields of urban watersheds is poorly understood. We undertook a comprehensive review of global responses of fine-grained and coarse-grained sediment yields to different phases of urbanization and compared them to a long-standing conceptual model. The summarized yields showed a great deal of variability, but were consistent with the widely-used conceptual model for watersheds with active construction. Importantly, however, the yields for established urban areas tended to be higher than previously assumed, and tended to remain higher than background levels. This is most likely because the urban drainage network has a very high sediment transport efficiency and because the increased runoff in urban watersheds is very effective at eroding the available sediment sources (mainly infill development, urban decay and renewal, and gravel surfaces in parks and gardens). The updated model provided here will assist in informing the extent to which sediment supply to stormwater drainage systems and urban streams needs to be addressed to assist the protection and restoration of streams in urban watersheds.