Optimizing surface model techniques for digital representation of river channels
Project ID: 2834
Principal Investigator: Kurt Wille
Research Topic: Water Resource Data Analysis
Funded Fiscal Years: 2012 and 2013
Keywords: bathymetric survey, linear transformation, surface modeling, sonar, gis
The accurate portrayal of river bathymetry is vital to a variety of numerical modeling efforts conducted by Reclamation, such as hydrodynamics, sediment transport, vegetation, and geomorphic analyses. These modeling efforts typically require a continuous bathymetric surface representation that is spatially interpolated from a collection of surveyed elevation points acquired with SONAR.
Hydraulic models are most sensitive to the geometric representation of the channel and floodplain surface. The overall quality of this bathymetric surface is tied directly to 1) the accuracy and spatial arrangement of the surveyed data, and 2) the methods used to spatially interpolate the surveyed data into a continuous surface.
This research proposal seeks to answer the following two questions:
1. What is the most appropriate method of surface modeling to accurately represent river channel bathymetry? Recent literature identifies one method of data transformation and claims that using this methodology improves overall representation of river channel geometry and features. This methodology should be explored and if appropriate, will be incorporated into ArcGIS customizations readily usable by Reclamation personnel.
2. What is the optimal method for collecting channel surveys regarding point density and boat pattern when using SONAR? There is little guidance in the literature regarding data collection methods for SONAR surveys in rivers. After the channel survey is completed, these data must be processed in such a way that the features of the river channel are well represented in a surface model. This is critical because the modeled surface is the primary input to hydraulic models and also provides input to geomorphic analyses investigating erosion and deposition locations and quantities. The accuracy of these analyses depends largely on true representation of channel bathymetry. With improved representation of channel bathymetry, uncertainty in our analyses is reduced.
Need and Benefit
Questions regarding optimal methods for collecting bathymetric survey data and appropriate spatial interpolation of surveyed data have presented themselves on a regular basis to Reclamation staff as they prepare and execute project funded river studies. Reclamation personnel lack the time and funding to fully explore and quantify optimal methods for surveying and mapping channel bathymetry while performing project work.
Over the past few years academic research has shown improved results in modeling the surface of river channels using a linear transformation of the survey data. This creates a situation whereby the river is represented as a linear feature and is placed in either a north-south or east-west orientation. With this orientation, differing trends can be applied to the interpolation schemes. This transformation is necessary if a trend is to be applied because topography changes much more dramatically in the lateral direction than in the longitudinal direction. Without the transformation, an appropriate trend cannot be applied. After the interpolation has taken place, the data are transformed back into the original coordinate system, preserving the trends and interpolation scheme applied during the transformation. Currently, no commercially available solution exists to transform surveyed point data into a curvilinear orthogonal coordinate system that references data along and perpendicular to flow in the river channel.
When collecting river channel bathymetry using a boat and SONAR, questions arise as to how to navigate the boat to best capture the features of the channel. As in most cases, this research will focus on those rivers where a boat and motor can be used. With an unlimited budget and time on the river, a complete bank to bank survey could be obtained. However, budgets and time are often very limited, posing the need to optimize the data collection, minimizing time spent gathering data while maximizing the quality of the survey. For example, some patterns of data collection are thought to underestimate channel capacity, while other patterns are thought to show a preference for deeper portions of the channel.
Two-dimensional (2D) hydraulic modeling is increasingly being utilized for detailed hydraulic studies, including modeling in-river structures such as fish screens and weirs, modeling bed and bank erosion, and modeling aquatic habitat. Increasing demands on model resolution and accuracy place an ever increasing demand on channel surveys and the representation of the channel surface. The most critical input to any hydraulic model is the geometric representation of the channel and floodplain surface. Incorrect representation of the modeling surface can result in a misrepresentation of channel hydraulics, leading to erroneous conclusions. One common error in the channel surface is a reduction in channel capacity due to a misrepresentation of the channel margins, where we often lack SONAR data. A reduction of channel capacity will misrepresent channel velocities and the discharge at which the flow exceeds bank height and begins to flood adjacent lands.
The results of this research will be directly applied to projects requiring detailed representation of bathymetry. Future data collection efforts will have been optimized, making the best use of time and resources spent in the field. Future surface modeling will benefit from the determination of the most appropriate method of surface modeling in GIS. This interpolation method will be programmed in ArcGIS for efficient future application. Hydraulic engineers across Reclamation will be able to utilize the river surveying methodologies explored in this research and determined to best optimize survey effort. GIS specialists will be able to take advantage of tools created as a result of this research that will provide the best surface model of a river channel, its banks, and floodplain.
Surface modeling customizations delivered as an easily shared and fully documented ArcGIS desktop add-in.
Hardcopy of Technical session presentation.
This information was last updated on December 12, 2013
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