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TRRP Rehabilitation Sites use a combination of features to encourage a dynamic alluvial channel and to jump-start needed salmon habitats.
Physical channel processes and dimensions respond to changes in flow regimes, sediment regimes, large wood regimes, confinement, slope, grain size, and other controlling parameters. It is important to note that the TRRP philosophy is that rehabilitation site construction is only the beginning, and that the site is expected to evolve over time (the slideshow on our Sven Olbertson site page demonstrates this nicely).
This page briefly outlines a few common features at TRRP rehabilitation sites. Specific constructions of these features will vary and may be placed to interact with other features in complex ways that are not covered here. TRRP has written a design guide that covers these in much more detail.
Evolution of Site Designs
The Trinity River Flow Evaluation Study and the Record of Decision provided an overview of the channel rehabilitation strategy yet few specifics on channel dimensions. TRRP was established as an adaptive management program so that rehabilitation strategies could improve as the Program progressed. Whether by strict Adaptive Management procedures or by general improvements to scientific understanding of river rehabilitation, new information and approaches have been developed since the ROD was signed in 2000, offering substantial improvements in the ROD goals of increasing and improving salmonid habitats.
This evolution has tended to improve the performance of the projects in achieving ROD restoration objectives, including:
- Substantial increase in the number of side channels;
- Inclusion of large wood on the mainstem, in alcoves, in side channels, and on floodplains;
- Encouragement of lateral channel migration by excavation of dredge tailings and/or terraces on the outsides of meander bends;
- Relocation of the mainstem channel into a more sinuous pattern through construction of midchannel islands and/or half-wavelength bends excavated through dredge tailings and/or terraces; and
- Incorporation of macro-topography and micro-topography into newly created floodplains, including high flow scour channels, lower benches for fish habitat and natural riparian regeneration, as well as constructed terraces for spoil materials.
Floodplains are a natural feature of most alluvial river channels, and have important geomorphic and ecological functions. Floodplains support riparian forests and provide off-channel fish habitat during high flows.
Many of the floodplains on the Trinity River were disrupted by dredger mining. Dredgers removed the top soil and mixed and inverted floodplain sediments to a depth of up to 40 ft. The surface expression of former channels as depressions in the floodplains, buried large wood that supports certain species of riparian vegetation, top soil, and topographically higher islands of vegetation were often removed by dredger mining.
Given that the unimpaired Trinity River was a high-energy river during large floods, floodplains were probably topographically diverse rather than large flat surfaces typical of a low gradient, meandering, low-energy river. Whatever their morphology, due to the much larger flow regime prior to the Trinity River Division, the floodplain surfaces were substantially higher in elevation than what is currently appropriate for the smaller ROD flow regime. Therefore, the elevation of the floodplain, as well as the topographic diversity of the floodplain surface, must be designed to the ROD hydrologic regime, including the hydrology imposed by downstream tributaries.
Floodplain design on the Trinity River is predominately focused on the manipulation (lowering) of surface materials, including terraces, dredge tailings, and flattened dredge tailings. The primary design elements within a floodplain are the typical floodplain elevation, floodplain width, and floodplain channels, which may include high flow channels, high flow scour channels, winter baseflow channels, tributary channels, and/or wall-based channels.
The primary objectives of floodplain width design include:
- At minimum, satisfy riparian regeneration mitigation requirements via planting and natural regeneration.
- Maximize riparian planting area via floodplain surfaces and riparian benches.
- Maximize fry and juvenile rearing habitat via side channels, high flow channels, high flow scour channels, alcoves, and other topographically diverse features on the floodplain.
From an ecological perspective, the more floodplain area, the better. However, in nearly all cases, existing surfaces require substantial excavation to create lower elevation floodplains that are scaled to the ROD high flow regime and downstream tributary floods. Therefore, the designs need to analyze tradeoffs between the sometimes competing objectives above.
For details, please read section 5.9 in the TRRP design guide.
Side Channels and Alcoves
Side Channels have been constructed on the Trinity River for fisheries purposes since the 1980s. Early side channels were constructed with little to no engineering. Most of the side channel entrances quickly filled with coarse sediment and were abandoned by the mainstem channel. These early side channel entrance failures resulted in the TRFEFR and ROD focusing on mainstem channel rehabilitation over side channel restoration, and thus only three side channels were identified for construction.
However, secondary channels (side channels, split channels, high flow channels) have become much more common in designs due to their immediate habitat benefits, providing a jump-start to needed salmon habitats. Designs are considered for how the side channel entrance and exit (alcove) may avoid sedimentation.
Recently constructed side channels are now known to contribute substantially to juvenile salmonid habitats (see the Juvenile Fish Habitat page) and several are used by spawning adult salmon (see the Redd Distribution and Abundance page).
Like side channels, alcoves are desirable because of the fish habitat they provide, as well as the geomorphic complexity they offer. However, alcoves too, have a high failure rate due to sedimentation. Alcoves are often designed in combination with high flow scour channels on the back side of bars, contributing to additional topographic diversity on floodplains, while minimizing risk of sedimentation.
For details, please read section 5.8.2 in the TRRP design guide.
Large wood may be used to:
- Create a desired geomorphic effect on the reach and unit scale.
- Create desired habitat cover and complexity.
- Promote channel migration and avulsion.
- Cause local bed mobilization and scour that helps reduce risk of future detrimental riparian encroachment.
- Promote island or medial bar formation.
- Provide a source for large wood recruitment in the future.
For details, please read section 5.11 in the TRRP design guide.
Gravel Bars and Meanders
Constructed bars potentially serve three functions: (1) coarse sediment augmentation to increase and maintain gravel/cobble storage (see Gravel Augmentation), (2) floaw shoaling to encourage channel migration and hydraulic complexity (point bars), and (3) flow splitting to encourage bank erosion and hydraulic complexity (medial bars in split channels). One or more of these functions may be applied to any given channel rehabilitation site depending onthe site-specific objectives. The grain size of constructed bars must therefore consider the size range needed to provide each potential function.
The selection of bar type depends on site specific valley wall confinement, frequency and extent of bedrock control, type of bank materials (bedrock, floodplain sediments, tailings piles, etc.), sediment supply, flow strength and radius of curvature. The size and frequency of lateral bars or point bars in straight or meandering channels without medial bars or islands should be tied directly to radius of curvature and wavelength (a measure of river feature frequency). Point bars or alternative lateral bars should coincide with the apex of channel bends or the proposed sinuosity of the thalweg, respectively. Medial bars or islands must be placed in locations that have an appropriate sediment supply, bankfull width, flow strength, and bank strength to maintain the features. The length of a medial bar should not exceed that of the ½ wavelength associated with riffle-pool spacing in a single thread channel.
For details, please read section 5.4 in the TRRP design guide.