Development of tidal channels following restoration
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Following exploratory work (Hood 2002, Hood (2014), has developed 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 restoration appears to occur naturally. 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.
- 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.
Delta Strategy Analysis
The uncertainties in how this topic affects delta restoration has resulted in its inclusion in the ESRP River Delta Adaptive Management Strategy. This three criteria analysis should build off the analysis above, and supports development of learning projects.
How site conditions affect channel development following restoration is largely unknown. Channels affect marsh function for fish, which is a fundamental motivation for restoration. Channel drainage affects gas exchange in soils between tidal cycles likely affecting benthic biota, and primary production.
Natural breaches and old restoration sites provide a chronosequence for observation of channel formation following restoration, but may not provide the replication to isolate the range of factors anticipated to affect channel formation. A range of sites are available to observe the mechanics of channel formation over time, and projects can be designed to provide replication of treatments. High project costs may reduce willingness to conduct controlled experiments.
Redevelopment of natural channels is a priority objective for delta restoration, and actions to stimulate channel formation increase project costs. Other design actions (or inaction) may have unintended consequences for channel formation by affecting hydrodynamics. Refinement of this postulate would affect design and funding policy.