- Risk Assessment Study & Key Challenges
- Additional Topography Needs
- Engineering Excellence Award Winner
Historically, the Williston Regional WTP had three intakes on the Missouri River, immediately downstream of the U.S. Highway 85 bridge. However, two of these intakes have become buried with sediment from the aggradation, leaving the third as the sole source of potable water for the city and the surrounding region. Currently, plans are being prepared by the North Dakota Department of Transportation (NDDOT) to expand the traffic capacity of U.S. Highway 85 by constructing a new bridge upstream of the existing bridge and demolishing the existing bridge. WAWSA is concerned that the relocation of the U.S. Highway 85 bridge could alter flow characteristics near the water intake structure and allow for additional sedimentation to occur in the area, thereby potentially reducing the operational lifespan of the existing water intake. Understanding the risk to the only remaining water intake for the Williston Regional WTP is critical, as temporarily or permanently losing capacity at this intake would be catastrophic to a five-county region.
WAWSA retained the team of AE2S (Advanced Engineering and Environmental Services, Inc.), KBM, Inc., and WEST Consultants to perform a bathymetric and LiDAR survey of the Missouri River and evaluate the potential for increased risk of sedimentation due to the NDDOT bridge project. The first task of the sedimentation risk assessment study was to review the numerous existing studies and determine the data gaps that existed that would need to be filled in for the sedimentation study. While the U.S. Army Corps of Engineers has previously surveyed cross-sections of the river bottom every several years since the closure of the Garrison Dam, these cross-sections would only be sufficient to complete a one-dimensional sediment transport analysis. However, sedimentation patterns within a river are driven by more complex two- and three-dimensional flow patterns, so a detailed bathymetric map of the river was needed. Over 7.5 miles of the river was surveyed to support an accurate two-dimensional sediment transport study.
Three key challenges had to be overcome due to the immense project scope and the nature of river surveying.
The first challenge was project scheduling. The overall project timeline was such that obtaining data as soon as possible was a necessity. Project authorization occurred in March 2014, but the Missouri River was still covered with ice. AE2S engineers monitored upstream United States Geological Survey (USGS) gauges on an almost daily basis looking for ice-off conditions, as well as coordinated with staff located in Williston to provide visual observations of the river. AE2S also coordinated with USGS staff on their experience and expectations for ice-off timing to provide the surveying crew, boat service, client and project team as much lead time as possible. Ice-off conditions occurred on April 4 and, on April 22, a surveying crew and boat were dedicated full-time to collecting data, which required coordinating multiple flight and travel schedules, as well as arranging a kick-off meeting with the client for the same trip to minimize overall project costs.
The second challenge is that Missouri River water levels can change by several feet within a few days, along with the fact that the water level changes from the upstream to downstream ends of the survey project area. This challenge was overcome by choosing the right tool for the project. AE2S surveyors utilized a Hydrolite single-beam echosounder that collected accurate water depth readings. The echosounder was real-time-linked to a GPS/RTK survey unit to provide accurate horizontal location and real-time river bottom elevations at the boat location. Using this approach, AE2S was able to efficiently collect approximately 30,000 bathymetric survey points.
The final challenge was the inability to see the surface being surveyed. Typical ground topographic surveys have the advantage of the surveyor being able to visually locate the necessary breaklines that form abrupt changes in slope. Underwater surveys do not have this luxury and, while thousands of data points were collected, simply using the mass points to generate a river bottom surface was inaccurate and would cause issues in the eventual two-dimensional sediment transport modeling. To overcome these issues, AE2S’ surveyors, CAD technicians and engineers worked together to interpolate points between the survey points and then manually create breaklines that better represented the river bottom surface.
Through the data collection effort and extensive post-processing, AE2S created a detailed 1-foot contour map of 7.5 miles of the Missouri River. In addition to the river bathymetric survey, AE2S surveyors also used GPS/RTK methods to survey the riverbank location and water elevation throughout the river reach, which proved invaluable later in the project.
In addition to the river channel bathymetry, floodplain topography was also needed to evaluate sedimentation conditions during flood conditions. Because sedimentation is less sensitive to overbank geometry, the initial project plan to save costs was to use a proprietary 5-meter Digital Elevation Model (DEM) for the overbank geometry that had successfully been used in previous Missouri River studies.
After merging the bathymetric data with the proprietary DEM, it became apparent that the proprietary DEM was lower than surveyed riverbank elevations by up to 10 feet, and further investigation revealed that the DEM was created prior to the Missouri River’s 2011 flood. The DEM was then compared to the Williston LiDAR data collected in 2012 that partially covered the project area. The surface comparison indicated that the 2012 LiDAR provided a much better match to the surveyed waterbank elevation. Further, the difference between the 2012 LiDAR data and the proprietary DEM was not consistent, indicating that it would not be appropriate to simply raise the proprietary DEM surface by a set elevation. Interestingly, the surface comparison suggests that the sedimentation that occurred during the 2011 flood drastically altered the floodplain topography in the project area.
Given that the 2012 Williston LiDAR data did not provide sufficient coverage and there was no reasonable method to adjust the proprietary DEM, AE2S discussed with WAWSA the necessity of collecting new LiDAR data of the project area, which the client agreed was a necessary item to limit the uncertainty in the study conclusions. AE2S then contracted with KBM, Inc., of Grand Forks, N.D., to provide new high-resolution LiDAR data for the entire project area.
On July 8, KBM performed the flight with AE2S providing ground targets to provide vertical and horizontal control. Horizontal and vertical control for the survey was based on previously-set WAWSA project control monuments. The monuments’ horizontal and vertical data was determined by occupying each control point for a minimum of two hours with a GPS receiver. The data taken from the GPS receiver was then submitted to the National Geodetic Survey (NGS) via the Online Positioning User Service (OPUS) to refine the vertical and horizontal locations, and then post-processed to ensure accuracy of the points.
During the flight, AE2S surveyors occupied a single location with a GPS unit based on the same horizontal and vertical control system, and submitted that data to KBM to provide the best possible vertical and horizontal control for the LiDAR data. Ultimately, KBM provided AE2S with a 3-foot grid of over 20 square miles of the Missouri River floodplain, along with 3-inch pixel aerial orthophotography, with an accuracy sufficient to generate 1-foot contours of the entire floodplain. KBM also provided AE2S with water-edge 3D breaklines at the time of the flight.
A final challenge remained – merging the bathymetric survey data with the high-resolution LiDAR data, which is where the riverbank survey completed in April proved invaluable. The water level during the LiDAR flight was 4 feet higher than when the bathymetric survey was completed, so AE2S used the April riverbank line combined with the LiDAR riverbank line along with significant manual manipulation of the TIN triangles between the two breaklines to fill in the gap between the two data sets. This data merge created a single composite surface of the river bathymetry and floodplain topography that was necessary for the two-dimensional sediment transport analysis. Quality control for the merged surface was provided by creating contours of the composite surface, as well as visually inspecting several transects of the composite surface to confirm that the data did not have any errors that would affect the sediment transport analysis.
The Missouri River Bathymetric and LiDAR Survey project utilized a combination of methods to construct a complete topographic map for over 7.5 miles of the Missouri River and 20 square miles of adjacent property, as well as provide high-resolution photography for the same area. A survey of this size and complexity required a coordinated effort between several agencies and engineering consultants for successful completion. The results of the survey will ultimately be used to make critical infrastructure planning decisions that will help ensure a reliable water supply source for Williston and the five-county area that is one of the most rapidly growing populations in the country thanks to the Bakken shale oil play.
The Missouri River Bathymetric and LiDAR Survey Project received a 2015 Engineering Excellence Award from the American Council of Engineering Companies (ACEC) of North Dakota in the Surveying and Mapping Technology Category.
Jon Lefers is project engineer and Todd Norton is survey project manager with Advanced Engineering and Environmental Services (AE2S), Inc., a civil/engineering/environmental firm in the Upper Midwest. For more information, visit www.ae2s.com.