Development of Tidal Channels Following Restoration
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Following exploratory work (Hood 2002 channel allometry), Hood 2014 natural tidal channel model presents a Tidal Channel Reference Model that predicts equilibrium channel geometry, based on marsh area, and identifies reduction of some channel metrics with increasing wave energy and decreasing tidal range. Restoration of tidal channel geometry following River Delta Restoration appears to occur naturally following historical unplanned dike breaches. However, post restoration conditions may have different topographies than newly forming delta islands. The natural pattern of channel maintenance amid sediment deposition observed in natural marshes (e.g. Eilers 1974) may not be sufficient to form new channels with natural geometry.
Restoration sites may have compacted soils, reduced organic matter, ditch and drainage tile networks, altered topography, subsided elevations, and remnant levee systems, each potentially affecting channel development. Linear topographic features, left over from human development or restoration may capture flows, limiting the formation of meanders compared to a naturally formed system and reducing channel length and area. If a post restoration channel is different than in naturally formed marsh, habitat services may also be different. Both the degree to which differently shaped channel systems provide different services, as well as the effects of system and site level alterations on channel formation are largely unknown.
Restoration actions may employ tillage, ditch filling, or channel excavation in an effort to accelerate development of natural channel geometry, but also affecting project costs. These efforts may facilitate or inhibit channel development, qualitatively change outcomes, or may simply be a waste of effort due to strong natural forces that rework the site following restoration. Hood has proposed that development of channel metrics may follow a sigmoidal curve, such that channel formation is slow at first, speeds up as erosion forces become concentrated in emerging channel networks, and then slows down as the channel becomes increasingly able to convey flows. This suggests that some initial partial excavation of channels may greatly increase the rate of channel formation, increasing the provision of access to fish in restored marsh systems.
Notes[edit]
- Rate of channel development describes the relationship between island size and rate of channel development.
- River flow during flood events that overtop marsh island levees may increase scour and channel formation within otherwise blind tidal channel networks.
- Hood 2015 provides channel allometry models for all of Puget Sound and suggests that channel attributes are affected by tidal range and wave exposure, and that sediment starvation may be causing island deterioration in delta ecosystems.
- Fuller et al 2014 observed low slopes within subsided restoration sites, potentially affecting channel formation drivers compared to naturally formed delta islands with continuous subtle slope.
- Fuller et al 2014 observes limited channel formation due to 'bathtub' effect of subsided sites with limited connections, resulting in constriction of ebb flow to excavated channels at site boundary.
- Fuller et al 2014 observed slow downcutting due to compacted layers of unknown origin, perhaps from plowing on moist soils.
- Fuller et al 2014 observed formation of straight channels following restoration that follow equipment traffic patterns during construction.
Sinuous channels formed well in loose borrow ditch fill following restoration.
Red Salmon Slough tidal channels form through backward evolution of nick points.
