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|Darren Gewant, hydrorgapher, checks the roll sensor on the Stanislaus River in Oakdale, Calif.|
An innovative multitransducer sweep system based on single beam technology enables high resolution bathymetric mapping of shallow waterways.
Gold mining, water flow regulation, watershed land use practices and a variety of other human activities have dramatically altered most rivers in California. As a result, fish and wildlife habitats have been severely degraded, causing sustained inter-decadal declines in native populations and conditions favorable for nonnative species invasions.
As native wildlife populations in the state continue to decrease, habitat restoration has become big business. The restoration of wetlands, rivers and creeks is a multidisciplinary endeavor involving fluvial geomorphologists, hydrologists, engineers, biologists and surveyors.
Because river rehabilitation involves re-engineering terrestrial and aquatic landforms, the keystone data underlying environmental analysis and engineering design is a high-resolution bathymetric map. The map provides the baseline for conceptual design, enabling the altering of topographic contours to obtain the desired ecological functionality. It also allows for detailed modeling and evaluation of design alternatives. No rehabilitation can be performed without a good map. Also, it would be difficult to prioritize among projects without a comprehensive map produced independently of the advocates of individual projects.
Within the last decade, high-resolution bathymetric mapping has become increasingly more sophisticated and popular among hydrographers. Multibeam systems have the capability of providing near 100-percent bottom coverage, resulting in highly detailed contour maps. However, multibeam systems have limitations in the high-flow, shallow environment of small to medium rivers and streams within the foothills of the Sierra Nevada. As a result, service providers have continued to seek alternatives. A project on the Lower Stanislaus River provides insights on a promising solution.
|Sweep systems can be set up in a variety of configurations, depending on the vessel and water body. (Image courtesy of Seafloor Systems Inc.)|
One of the largest tributaries of the San Joaquin River, the Stanislaus historically supported a large population of spring-run Chinook Salmon. However, the run has been decreasing and is now much lower than it was in the past. The United States Bureau of Reclamation (Central California Area Office in cooperation with the Technical Service Center in Denver has been analyzing the Lower Stanislaus River to identify the relationship between river discharge and fall-run Chinook Salmon and Steelhead Trout.
One aspect of this research involves using hydraulic modeling coupled with a high-resolution digital terrain model (DTM) of the river channel and corridor. The DTM is being generated from topographic and bathymetric data collected using LiDAR and swath-bathymetric data collection techniques. To obtain the data, the agency contracted Environmental Data Solutions (EDS) of San Rafael, Calif., a firm that specializes in hydrographic surveying.
EDS originally considered using a multibeam system due to the extraordinary bottom coverage offered by this technology. However, the extremely shallow nature of the river (less than 2 feet deep in some areas) would have required continually demobilizing and remobilizing the sonar equipment, which would have been time-consuming and costly. Instead, EDS began investigating ways to use multiple single-beam transducers to achieve the desired accuracy with less effort in shallower depths.
After comparing different technologies, the firm chose Seafloor Systems Inc.’s Sonarmite BT echosounder for its survey-grade accuracy, compact size and ease of setup. With support from Seafloor Systems, EDS designed and fabricated a four-transducer sweep system to work in shallow-water environments.
|The chief hydrographer receives tech support from Seafloor Systems.|
The 52-mile project reach, from Knights Ferry, Calif., to the downstream confluence with the San Joaquin River at Two Waters, Calif., consisted of multiple snags below and above the waterline, extremely shallow areas with sand bars and exposed bedrock, thick vegetation along the riparian corridor and reaches that contained fast-moving water. EDS required a survey platform that was able to provide accurate bathymetric data while being robust and flexible enough to handle the challenging environment.
The survey crew used a 17-foot Jon boat manufactured by Lowe Inc. and powered by a 50-hp Johnson outboard jet drive. The bathymetric data collection system consisted of three Seafloor Sonarmite fathometers and one Odom Hydrotrac fathometer arranged in a sweep system across the beam of the boat. The three Sonarmites (200 kHz each) were mounted approximately 8 feet apart on a boom across the beam of the vessel for a swath width of 32 feet. The Sonarmite transducers were each mounted at the end of a large spring, which would allow the transducer downrigger to “bend” in the opposite direction of vessel motion if the transducer hit a snag or rock. The Odom fathometer was mounted offset from the boom transducers on the bow of the vessel to allow the helmsman to maneuver the bow transducer into small areas between stands of bank vegetation. The lower frequency of the Odom transducer (20 kHz) assisted in the acoustic penetration of sub-aquatic vegetation.
One of the most important components of the survey system was the heading and roll sensor. Due to the large distances between the outermost transducers and the vessel centerline, a data string describing individual transducer heading and roll (updated at a rate of 10 Hz) was necessary to ensure accurately placed position and vertical data. The survey crew used a Hemisphere GPS VS100 DGPS heading and roll sensor, which provides accurate, reliable heading and position information at high update rates. The system consists of a high-performance GPS engine and two multipath-resistant antennas for GPS signal processing.
The system uses moving base station real-time kinematic (RTK) technology to achieve precise heading and position accuracies. The technology also uses two antennas: One antenna is designated as the primary GPS, while the other is designated as the secondary GPS. Because the distance between the primary and secondary antennas is fixed, the Hemisphere GPS VS100 is able to constrain its search volume when computing GPS satellite positioning data.
The system contains an integrated electronic tilt sensor and gyro. Because the instrument knows the approximate inclination of the secondary antenna with respect to the primary antenna, the tilt sensor constrains the RTK heading solution beyond the volume associated with just a fixed antenna separation.
Position data was supplied by a Leica GPS1200 Series RTK GNSS system. Project primary control points were set at strategic locations along the river corridor, and the base station was moved to corresponding points depending on what reach was being surveyed on a particular day.
|Fifty-seven miles of changing conditions and adaptability: the Lower Stanislaus Survey Reach.|
Since loss of RTK lock was a concern during the vessel mobilization process due to the heavy riparian vegetation, the EDS crew decided to use GPS antenna quick-release mounts on top of each of the outermost transducers. These mounts would allow them to move the primary RTK GPS antenna to the transducer that was located the farthest out in the river channel and, therefore, the farthest away from the vegetation canopy. As a result of this “fluid configuration,” precise survey notes outlining GPS antenna locations became a key part of this survey.
Potential loss of GPS lock was more of a concern with regard to vertical control. Even though a short loss of lock was acceptable for the horizontal (ï‚£ 1 meter), any loss of lock would send the vertical component outside the bounds of acceptable error.
Bank to bank survey coverage was attempted for over 52 miles of the river using multiple longitudinal transects throughout the entire survey reach where possible. The survey plan originally called for integrating the RTK-derived geoid separations (known as an RTK Tides file) into Hypack, a hydrographic survey data planning, collection and processing software, and getting an elevation fix on each transducer. This survey configuration was abandoned in favor of physically surveying the water surface and using an algorithm in Hypack that interprets the water surface elevation (WSE) profiles between survey points (known as the Tides Adjustment program).
The WSE survey consisted of using an RTK rover rod with a stadia rod attached to it. The survey crew would stop the survey vessel, anchor it and survey a point on the channel bottom. The depth of the water column would be recorded with the survey point and then added to the bottom elevation in order to calculate the WSE at that point. This technique also provided RTK-derived channel bottom survey points as a QA/QC check on the bathymetric data.
WSE points were collected as much as possible--especially in areas where rapid WSE changes occurred, such as at the head and toe of rapids, riffles and fast glides as well as transition points into pools and glides.
River flows were managed through controlled releases at the Goodwin Dam located approximately 3.90 miles upstream of the Knights Ferry end of the survey footprint. These controlled releases were part of an adaptive management program that aimed to control downstream water quality and habitat resources.
To account for changes in the river stage during any particular survey day, at least three vented pressure transducers were installed at the upstream end, midpoint and downstream end of each day’s survey reach. The resultant time series of each stage gage was then converted into a Hypack Tides File and integrated into the data reduction process for that particular reach.
To reduce the raw bathymetric survey line from each transducer into an XYZ dataset, each survey line was imported into the Single Beam Editor in Hypack. The Single Beam Editor enabled all of the survey variables and ancillary datasets (water level time series and, if necessary, roll values, sound velocity data and sensor offsets) to be reviewed and applied to each survey line.
Each survey line was then reviewed in detail, and erroneous bathymetric points (spikes and other outliers) were deleted or interpolated into the survey line. RTK position and elevation quality values were also reviewed. Once each line was reviewed and edited, all of the lines for each survey day were then exported into a Hypack Edited file. The edited file was then reviewed in the Hypack Shell for position and elevation quality.
Each survey trackline was then plotted and checked against QA/QC data, which was in the form of RTK shots taken directly on the channel bottom. Cross line checks (where two separate fathometer tracklines cross) were also scrutinized and compared for an entire day’s worth of surveying using Hypack’s Cross-Check Statistics program.
The combination of innovative technology and experience generated a high-quality dataset on a river that would have been difficult to survey otherwise. Some gaps resulted from extremely shallow areas, large snags within the channel and areas where the riparian canopy was simply too thick for GPS lock. Once these data gaps were identified and re-surveyed (using a total station where GPS lock couldn’t be obtained), the resulting XYZ dataset provided channel-toe to channel-toe coverage with survey lines spaced at 8-foot intervals along the majority of the river’s 52-mile reach.
In addition to comprehensive survey coverage, the field effort was required to provide vertical accuracy to U.S. Army Corps of Engineers Class 1 standards. A third-party topographic survey crew surveyed various unknown sub-reaches throughout the project area to ensure that the bathymetric dataset achieved this goal.
The Lower Stanislaus River survey has formed the basis for numerous hydrodynamic computer modeling efforts that have been used in many habitat restoration and flood control projects. Subsequent hydrographic surveys have benefitted from lessons learned on the Stanislaus, coupled with ongoing improvements in shallow-water survey technology and the platforms on which these systems are deployed.
In the five years since this survey was completed, acquiring bathymetric data in inland, shallow-water bodies has become more common place through the emergence of off-the-shelf systems. However, this does mean that the process has become easier. When the environment can change around the next bend in the river, adaptability in both equipment and mindset is key.
For more information about EDS, visit www.fielddata.biz.
For more information about the equipment and software used in this project, visit:
Seafloor Systems - www.seafloorsystems.com
Hypack - www.hypack.com
Hemisphere GPS - www.hemispheregps.com
Leica Geosystems - www.leica-geosystems.us