Stream Restoration Design Hydrology

Stream restoration projects often include the physical reconstruction of channel cross sections, profiles, or realignment in order to provide ecological succession. The channel's width, depth, and slope are adjusted to convey a design discharge using analog, empirical, or analytical techniques. It has become common practice for restoration projects to be designed for a single flow rate that represents a channel forming discharge that creates an idealistic equilibrium alluvial channel. The channel forming discharge is estimated based upon the channel's bankfull discharge, effective (sediment transport) discharge, or by a specific flow frequency. The channel forming discharge or its surrogate is thought to have a geomorphic influence equivalent to the full range of all flows at a specific location resulting in a stable self-formed channel. However, creating a stable bankfull channel does not necessarily address other fluvial processes and functions. Aquatic species in natural channels have adapted to require a wide range of flow regimes, and created channels need to have suitable forms for low and high flow processes. Restored channels should have profile features and cross sections that create low flow habitats, enable fish passage during migratory seasons with suitable velocity and depth, and should have bed material suitable for benthic species and fish spawning. Stream restoration projects should also consider the role of large floods that exceed the bankfull discharge. Projects designed only for bankfull discharges neglect the assessment and potential adjustment of overbank riparian wetland and floodplain ecological processes, such as plant seeding and germination, nutrient exchange, woody debris relocation, sediment distribution, arid the stimulation of fish migration. Large flood flows may cause channel aggradation, degradation, channel migration, cut of meanders, and avulsions that damage or supercede restoration projects. Urban stream projects should consider flood water profiles, property impacts, utility interruptions, and stream crossings at culverts and bridges. Many watersheds do not have static hydrology due to land use changes, loss of storage, floodplain encroachments, and climate change. Designers should consider future stream flow rates and not depend solely on the past discharges that create historic channel regimes. This paper advocates use of multiple hydrology criteria to facilitate the understanding of both biological and geomorphic processes. Restoration projects should consider seasonal biological and habitat needs during low instream flows, plus the impact and accommodation of larger than bankfull flood flows.