Managing Urban Sogginess: Implications of Saturation Excess Runoff and Shallow Groundwater Flow for Stormwater Management

Abstract:

Knowing which runoff process(es) – infiltration excess, saturation excess, and shallow groundwater runoff – predominate in a watershed is critical for the success of hydrological management. Each runoff process defines a unique hydrologic system, e.g. relevant flow paths, scales, and boundaries, which form the basis of hydrologic analysis and planning. Misattributing hydrological outcomes to particular runoff processes can cause a mis-match of management actions to this system, undermining the success of management. In urban areas, where runoff is generally assumed to be generated via infiltration excess, this assumption has driven a large suite of codified planning and analysis tools. The assumption of infiltration excess in urban areas has influenced definitions of risk (typically measured with event-based metrics), policies (often conditioned on extent of impervious surfaces and thresholds), and management practices (typically focused on increasing infiltration). Yet a simple analysis of global groundwater and bedrock data suggests that 24% of cities globally are situated over shallow water tables or shallow bedrock. In these settings, surface runoff and shallow groundwater flows can interact, resulting in complex hydrologic processes that may be poorly addressed using these conventional management approaches. For example, urban runoff policies focused on increasing infiltration could exacerbate high groundwater and flooding in urban areas dominated by saturation excess runoff or shallow groundwater flow.

We review the interaction of different runoff processes with elements of in urbanization and urban water management, identifying the challenges posed by saturation excess and shallow groundwater flow for management of urban runoff. Using Seattle, Washington, USA and Perth, Western Australia as contrasting case study areas, we offer guidance for adaptation of 'risk-based' approach for managing urban runoff across saturation excess or shallow groundwater flow dominated areas. While no urban runoff management approach can ‘restore’ an urban catchment to pre-developed hydrologic conditions, better knowledge of pre-existing hydrologic processes in urban areas can improve effectiveness of policy and management approaches.