SEDHYD-2023, Sedimentation and Hydrologic Modeling Conference

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Quantifying Sediment Deposition Within Constructed River Restoration Sites Using Repeat Aerial Lidar

The Middle Rio Grande and its aquatic habitat have been heavily modified since the expansion of European-based American settlers into New Mexico and southern Colorado during the late 1800s. Hydrologic and geomorphic effects of dams, land use, infrastructure constraints, and channelization have altered habitat for the native Rio Grande Silvery Minnow (silvery minnow). The silvery minnow was listed as endangered in 1994 and occupies only about 7 percent of its historical range, an indicator of the ecological deterioration of the Rio Grande ecosystem. Primary stressors to silvery minnow populations and life cycle include decreased river flow and loss of river-floodplain connectivity. Restoration projects attempt to improve lateral connectivity by creating low-velocity habitat for retention of semi-buoyant eggs during spawning and development of larvae into juvenile fish.

About 300 restoration sites have been constructed since 1998 to mechanically lower the floodplain using a variety of techniques. Restoration methods include bank lowering, side channels, backwater areas, embayments, spoils placement, and vegetation clearing. These sites often require a progressively larger discharge to become inundated based on field observations. In this study, we use four sets of LiDAR data (2010, 2012, 2017, and 2018) to analyze geomorphic change. We determine erosion and deposition detection limits for each pair of LiDAR years using stable surface comparisons such as parking lots and roads. Results are organized by construction interval (before, during, or after), feature type, and geomorphic reach. The elevation change within each site is represented by a Cumulative Distribution Function (CDF) rather than a single value.

Most sites have a median elevation change within the detection limits, indicating that erosion or deposition cannot be generally classified relative to the uncertainty of the LiDAR data. However, the spatial distribution of elevation change is important, and a median value is not sufficient to represent a site. The number of sites exceeding the deposition detection limit increases significantly when applying the 75% or 90% exceedance elevation change values from the CDF rather than the median. Elevation change maps demonstrate that deposition is often concentrated to areas where flow enters restoration sites, such as near the bank or a side channel inlet. This may cause the site to be disconnected and require larger flows to fully inundate the site. Our analysis also indicates there are differences between feature types. The spoils placement and embayment sites tend to have the most deposition, followed by side channel and bank lowering sites. Vegetation clearing and backwater areas tend to have the least deposition. The results inform recommendations for future restoration projects during the different phases of planning, construction, monitoring, and analysis.

Nathan Holste
Bureau of Reclamation
United States

D. Nathan Bradley
Bureau of Reclamation
United States

Colin Byrne
Bureau of Reclamation
United States

 



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