Hydraulic Geometry: A Geomorphic Design Tool for Tidal Marsh Channel Evolution in Wetland Restoration Projects

Abstract

Empirical hydraulic geometry relationships for tidal marsh channels are a practical geomorphically based design tool that can assist in the planning of tidal wetland restoration projects. This study provides hydraulic geometry relationships for predicting the depth, width, and cross-sectional area of mature tidal channels as functions of contributing marsh area or tidal prism. The relationships are based on data from San Francisco Bay coastal salt marshes ranging in size from 2 to 5,700 ha. These hydraulic geometry relationships refine and expand on earlier relationships. Relationships for mature marshes can be used to predict the direction and rate of evolution of a channel in an immature or perturbed marsh system. Channel evolution data for three youthful tidal channels, ages 4 to 13 years, suggest that the channels are converging on their predicted equilibrium morphology. Two channels are eroding in response to significant increases in upstream tidal prism. They have enlarged first by deepening, in one case after 13 years to beyond the predicted equilibrium depth, and then widening through slumping of the channel banks. The third channel, a new one forming in a depositional mudflat, is converg-ing on its equilibrium morphology after 13 years but will likely take several decades to equilibrate.

Changes in Coastal Sediment Dynamics Due to Managed Realignment

“…In the long-term, the tidal prism tends to decrease due to accumulation of sediment (e.g. Warm Springs, San Francisco Bay, Williams et al 2002) and saltmarsh development in the realignment site. However, this process may be slow or prevented in sediment starved systems.…”

“…Hughes 2002), which vary with the scale of the system, tidal range, salinity, vegetation, and sediment characteristics (e.g. Williams et al 2002). The required field data rarely exist (Williams et al 2002;Vandenbruwaene et al 2012), and often values defined for other areas are applied for systems largely different (e.g.…”

“…When projects cause major changes in tidal prism, erosion due to the growth and evolution of the drainage system can be considerable and persistent, as observed for over 13 year in Warm Springs, San Francisco Bay (Williams et al 2002). In contrast, the length and cross-sectional area of channels fronting the 8 ha CRT project in the Scheldt estuary (Belgium) reached equilibrium within 4 years (Vandenbruwaene et al 2012).…”

“…After the rapid adjustment phase, the rate of morphological change is reduced (Figure 1c, e), but can persist for over a decade (e.g. Williams et al 2002). The changes that follow will depend on the concentration of suspended sediment, whether the estuary is ebb or flood dominated and the rate of sedimentation in relation to sea-level rise.…”

“…Major flood events are responsible for most of sediment exchange between rivers, estuaries and the open coast (e.g. Williams et al 2002) and key pulses of morphological change.…”