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Reach Y Restoration Feasibility Assessment 1: Final Draft

Summary: 
Reach Y Restoration Feasibility Assessment
Primary Contact: 
Rights: 
Unknown
Status: 
Ongoing

By River Run Consulting, March 2007
EXECUTIVE SUMMARY

This report examines opportunities for restoring geomorphic and ecosystem function of the Truckee River between East McCarran Blvd. and near the Truckee Meadows Water Reclamation Facility (TMWRF), herein referred to as Reach Y. The report was prepared for the Cities of Reno and Sparks and TMWRF, who are seeking opportunities to improve water quality and the ecosystem of the Truckee River, especially opportunities for early implementation.

Reach Y is an extraordinarily complex geomorphic environment. There is a distinct transition in fluvial geomorphic processes within the reach. Channel dimensions and hydraulic characteristics are far different in the upper end of the reach than in the lower end. Prior to human disturbance, riparian ecosystems were likely also very different, dominated by shrubs and trees at the upstream end and herbaceous, meadow-forming species at the lower end.

The river has been highly modified by human activities throughout the reach. Straightening, channel enlargement, removal of obstructions and levee construction, undertaken to improve the channel for agriculture and flood control, have resulted in significant incision (lowering of the channel bed through erosion) throughout most of the reach. Channel response to the initial disturbance continues today, with relatively high rates of bank erosion and lateral instability. In the absence of artificial stabilization, the channel will continue to erode, with the eventual formation of a new floodplain at a lower elevation.

Due to the inherent geomorphic complexity within Reach Y, the river has responded to human disturbance in different ways in different locations. Incision has been more rapid in some areas than in others. Streambank erosion and lateral stability subsequent to incision have also occurred in different ways and at different rates throughout the reach.

To be effective, restoration measures implemented for this reach should recognize its inherent geomorphic and ecosystem complexity, which have important implications both for the potential characteristics of the restored system as well as its stability. Restoration measures should also be designed with an understanding of the effects of human disturbance, and should be capable of accommodating continuing channel adjustment to past human disturbance.

Restoration of the floodplain, through benching or similar techniques, represents the best opportunity to restore functional riparian ecosystems. However, this alternative will magnify many of the factors that tend to enforce instability and dynamics:
-Construction of a floodplain or benching will reduce the ability of the channel to transport coarse sediment, thus promoting coarse sediment deposition in the reach and subsequent lateral instability;
-Excavation of the existing streambanks will make likely make them more erosive in the short-term, until they are stabilized by vegetation;
-Constructed floodplains or benches will also be susceptible to erosion until stabilized by vegetation.

Assurance of stability for this alternative will therefore require a relatively high degree of engineering, and thus high cost. Throughout much of the reach, establishing functional native vegetation communities would also require intensive and expensive revegetation and erosion control techniquies, as well as a long-term commitment to maintenance of planted materials.
Because this reach has a high potential for substantial erosion, it is extremely sensitive to modifications of the channel upstream for flood control or restoration. The success of any restoration treatment in Reach Y will require careful integration with upstream restoration and flood control strategies, with consideration of potential changes in sediment transport and flood magnitude. We recommend that restoration in most of Reach Y be implemented in conjunction with upstream channel improvements.

The most feasible project for short-term implementation is bank stabilization in the vicinity of TMWRF, which can be implemented prior to other flood or restoration alternatives. Due to flood control constraints, bank stabilization is the sole feasible riparian restoration alternative in this location, and could be designed to integrate effectively with any restoration alternative upstream. Riparian habitat, aquatic habitat, and water quality improvements provided by bank stabilization would be limited. However, our analysis of geomorphic evolution in this area suggests that bank erosion is likely to continue in this area, even though erosion rates may be relatively slow. Bank stabilization may be required to protect infrastructure.

For entire document, please find initial figures as attachments below, and others listed in Documents and Reports or Resources Sections as subsequent Reach Y volumes (for the 21 figures shown as pdfs).

AttachmentSize
Final Draft 3-07.pdf679.77 KB
Figure 1.pdf517.23 KB
Figure 2.pdf1.15 MB
Figure 3.pdf923.76 KB
Figure 4.pdf487.53 KB
Figure 5.pdf487.48 KB