Design discharge for river restoration

Selecting a design discharge is a critical stage in a wide range of river restoration approaches and tasks but is not straightforward in practice and rarely involves following any of the several standardized procedures suggested in the literature on stable channel design because the data required are simply not readily available for most project sites. This chapter reviews the scientific bases of three popular candidates for representing the geomorphologically important dominant discharge that can be adopted as a design discharge for channel restoration: the bankfull discharge, a discharge of specified recurrence interval, and the effective discharge. The chapter goes on to assess how the strengths and weaknesses, inherent to their derivation and application, play out in practice. Experience shows that effective discharge analysis has considerable potential for further advances in computational methods that could provide improved insights into the morphological significance of an effective range of flows, enabling restorers to incorporate not one but a series of nested design discharges into their restoration plan, enhancing both geomorphological sustainability and ecological integrity. It is increasingly recognized that the primary challenge in selecting a suitable design discharge for river restoration lies in accounting for uncertainty in future flow and sediment regimes, associated with global warming and ongoing changes in watershed land use, by making sufficient allowance for restored channels to adjust within their restored, functional floodplains, while maintaining the dynamic equilibrium necessary to conserve key species and ecosystems.