Global sediment yields from urban and urbanizing watersheds

Abstract

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.

Introduction

In our rapidly urbanizing societies, urban streams are becoming increasingly valued for the products and services they provide to humans (fresh water, food, waste disposal), as well as their intrinsic and biodiversity values. However, they are subject to extensive and severe impacts from human use and land use changes, a problem that is encountered globally and known as ‘the urban stream syndrome’(Walsh et al., 2005a). With more than half the world's population now living in urban areas, and with urban populations growing at 2.1% per year (The World Bank, 2014), the degradation of waterways through urbanization has never been greater. Stream restoration is now a multi-billion dollar effort worldwide, with the cost of stream restoration in the US alone exceeding a billion U.S. dollars a year (Palmer et al., 2007).

It has long been recognized that channel morphology is a function of discharge and sediment supply (Mackin, 1948). In the context of urban development, flow regime disturbance has been widely studied as a key driver of the degradation of streams (Booth, 1991, Hammer, 1972, Wolman, 1967), and the role of sediment regime change is receiving increased recognition (Fletcher et al., 2014, O'Driscoll et al., 2010, Vietz et al., 2016, Vietz et al., 2015, Wohl et al., 2015). This dual disturbance of both the flow and sediment regime is analogous to the role of dams in sediment trapping and channel change that has been well understood for several decades (Petts and Gurnell, 2005).

The prevailing and widely-used model of sediment supply from urban watersheds is based on the ‘cycle of urbanization’ (Fig. 1) proposed 50 years ago by Wolman (1967). The three stages described include: a stable or equilibrium condition waterway with a forested or agricultural watershed and modest sediment yields; a period of construction, when bare soil is exposed and sediment yield rapidly rises, and a final stage where the watershed is dominated by urban land cover, streams are stabilized and buried in pipes, and sediment yield further declines to values as low as or lower than in the initial equilibrium stream. The sediment response under established urbanization was represented with particular uncertainty as indicated by the dashed line. Uncertainty was also indicated for forest yields, highlighting the difficulty of measuring or inferring pre-agricultural conditions in areas with a long history of agricultural development.

Very little work has tested or built on this conceptual model, despite recognition of the impact of sediment regime disturbance on morphology and condition of streams in urban watersheds (Bledsoe and Watson, 2001, Chin, 2006, Paul and Meyer, 2001, Vietz et al., 2014). In particular, studies of sediment regimes of established urban watersheds are limited (Chin, 2006). Urbanization impacts on sediment load are highly variable (Vietz et al., 2015), and the question of whether there is a globally ‘common’ response is yet to be thoroughly investigated.

Opportunities for addressing the ‘urban stream syndrome’ (Walsh et al., 2005b) are greatly limited without understanding sediment supply from urban watersheds. Stream characteristics such as bed complexity, hydraulic diversity and the presence of bars and benches, for example, are reliant on sediment and these characteristics, in turn, contribute to the ecological condition of streams. Better understanding sediment supply to streams in urban watersheds may reveal the need for management measures that consider sediment regime restoration alongside activities that address flow regime and water quality (Vietz et al., 2014, Wohl et al., 2015).