Advanced Photogrammetry Techniques as a Tool for Evaluating Sediment Gradations
Hydraulic and sediment modeling efforts and monitoring programs in gravel and cobble dominated rivers almost always require the collection of bed material sediment samples through a variety of intensive field methods, including pebble counts and volumetric sampling of surface and subsurface materials. Recent advances in photogrammetric techniques for evaluating grain size distributions may significantly reduce the costs and time associated with field sampling and offsite laboratory sieving. This proposal addresses the following question:
* Can photogrammetric methods accurately predict bed material gradations to better represent physical conditions of the river while saving time and money in modeling and monitoring programs?
Without additional research into advanced photogrammetric techniques, Reclamation-wide efforts to understand sediment issues and improve water delivery can not benefit from application of the potentially most efficient and lowest cost methods.
Need and Benefit
Bed material is a key component of sediment transport modeling, evaluating downstream changes in sediment sizes, incipient motion analyses, and sediment size monitoring. Bed material sediment samples are frequently used in Reclamation studies to address questions related to:
* Sedimentation of irrigation canals and its impacts on water delivery
* Restoration and maintenance of spawning gravels
* Aggradation and degradation of river beds
* Local scour around irrigation infrastructure
* Sustainability of in-stream structures
At present, the most common methods for sediment sampling in gravel and cobble dominated streams include collecting pebble counts and volumetric sampling of the surface and subsurface material. Pebble counts offer a tedious means of evaluating surface sediment size and are limited in the spatial distribution within which they can represent the size gradations of bar and bed material. This method poorly represents true conditions and may incorporate bias into the analysis. Volumetric samples in gravel and cobble dominated streams often require onsite sieving of the largest materials and removal of up to 150 pounds of gravels smaller than 32 millimeters for laboratory sampling. This labor-intensive approach provides high accuracy but only represents a localized portion of an individual gravel bar. Furthermore, only three to four sites can be sampled each day per team, depending on the distance to vehicle access and the number of trips needed to transport equipment and gravel material for laboratory sampling.
Photogrammetric sediment sampling offers a unique opportunity to sample a much greater spatial area at a lower cost and in a faster manner than traditional means. Additionally, the digital image processing removes biases and errors related to human participation that are present in traditional pebble count methods. Bias leads to a lack of confidence in study results and makes management decisions difficult and unclear. Recent advancements in processing of photogrammetric sampling suggests that digital imaging of sediment samples may be able to perform with precision equivalent to a pebble count in less than one sixth of the time (Graham et al. 2004).
While the advancements in photogrammetry appear to have some application across all aspects of sediment assessment, the most promising appears to be related to evaluating changes in sediment gradations resulting from habitat restoration actions. In particular, in response to our partners, Reclamation is performing field evaluations as a tool to determine the effectiveness of habitat complexity design projects. Standard field evaluation techniques provide an opportunity to collaborate with our partners by providing mechanisms to determine post-construction effectiveness of projects designed by Reclamation. Potential parameters for field evaluation include specific habitat elements, such as sediment size gradations. Currently, baseline conditions and their relevant ecological indicators are determined prior to project construction through intensive field data collection efforts and analyses, such as sediment sampling. Field evaluation is scheduled throughout the design process and then at specific intervals following project completion. In lieu of intensive sediment sampling efforts, repeated photogrammetric sampling may be able to accomplish similar objectives at a much reduced cost to Reclamation. Photogrammetric sampling as an evaluation tool may ultimately provide the necessary data to determine the effectiveness of Reclamation projects at achieving restoration goals and gain added support for our continued participation in these efforts.
Bed material sampling provides invaluable data for river studies across all regions of Reclamation. Application of photogrammetric techniques can assist Reclamation managers and planners in making more informed decisions, reducing costs associated with sediment studies, and accelerating sch
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