Feature: At the Forefront of Wetland Restoration
Professional Surveyor Magazine - April 2008
Edgar Salire and Chris Wang
Most people know that wetlands are steadily declining nationwide and that we are losing their numerous biological, ecological, environmental, and recreational benefits. California alone has seen a drastic decline caused by human-related activities. Only recently has government policy change halted further destruction of wetlands. As a prime example, several governmental agencies led by the U.S. Army Corps of Engineers, the California Coastal Conservancy, and the San Francisco Bay Conservation and Development Commission are undertaking wetland restoration on the former Hamilton Army Airfield.
The Hamilton Wetlands Restoration Project is located in the city of Novato, California, about 25 miles north of San Francisco. The former airfield was active until the early 1970s, and then in the 1990s it was closed as part of the Base Realignment and Closure Program. The area was originally a natural wetland but was diked off from San Francisco Bay waters in the late 1800s. Today, the project footprint covers 988 acres of wetlands, home to several protected species such as the clapper rail, salt harvest mouse, and the burrowing owl. The cost to restore the historical wetlands will exceed several hundred million dollars.
This project is multifaceted in that it will not only create a wetland but also recycle dredge spoils from the Port of Oakland harbor-deepening project, which is normally disposed of in a landfill or an offshore site. The dredge materials will be enclosed by a series of constructed perimeter levees, thereby creating a "bathtub" effect. Once the dredge material is imported, it will be sculpted to create a northern seasonal wetland and a southern tidal wetland. Survey instruments installed at predetermined intervals will guide the bulldozers as they sculpt the site to design elevations.
Surveying is incorporated in the design, construction, and monitoring phases of the Hamilton Wetland Restoration Project. The data collected are incorporated in civil design, hydrologic/ hydraulic design, and geotechnical engineering to document prior, existing, and future conditions. Surveying tells the story of the wetland restoration process by documenting many construction activities ongoing at any given time.
The Hamilton survey team and survey consultants have deployed a multitude of surveying technology in the forms of RTK GPS and traditional surveying. To optimize effectiveness, survey crews and remote sensing technology were deployed to complete construction activities. High-resolution orthophoto imagery was developed and used to produce a project basemap. Derived from the rectified imagery and composed of a feature map and a digital terrain model (DTM), this is a key component in making project design decisions.
Hydrologically, survey data forms a direct input into the design and has proved instrumental in developing a self-sustaining wetland design, thereby preventing short-circuiting of design water flow between the southern seasonal and northern tidal wetland. Survey contributions to geotechnical engineering have come in the form of monitoring of ground response and impact on existing infrastructures such as pipelines and residential and commercial buildings.
In construction surveying for this project, data acquisition is used to verify if as-built levee elevations match with flood design heights specified from hydrologic/hydraulic engineers. Construction quality assurance has been used to estimate quantity takeoffs including verification of excavation and removal of low-level site contaminants such as DDT (dichloro-diphenyl-trichloroethane) and PAH (polycyclic aromatic hydrocarbon), calculate excavation and fill volumes, or even isolate UXO (unexploded ordinance) areas for safety.
Mapping Is Important
The project base map is a very important component of the Hamilton Wetland Restoration Project. As an update to the 1997 base map, an aerial survey was conducted to provide up-to-date survey information and high-resolution imagery to develop a DTM and a topographic feature map. Furthermore, this data was used to update the map using newer horizontal and vertical datums, principally converting to NAD1983 and NAVD1988 in the California state plane projection. The project deliverables included color digital orthos in Tiff format containing 0.4-foot ground sample distance (pixel size) and a composite photo in MrSid format.
A series of controls were set up for the flight path network for the project. At the request of the project delivery team, the National Geodetic Survey (NGS) reviewed the numerous survey control installations. Many of the controls set onsite were incorporated into the NGS's database for GPS survey datasheets.
Construction activity after each construction season reflected outdated areas on the basemap. After understanding the potential impacts and delays of using obsolete data to planning, design, and construction, the project delivery team deemed it necessary to expend resources to keep it current. However, updating these areas was challenging because it involved making sure the replaced survey data integrated correctly into the existing DTM network. The updated points were primarily acquired from sub-centimeter RTK GPS or other traditional survey methods performed by survey consultants and the Corps of Engineers in-house survey crew. The Corps of Engineers GIS section served as the lead in the periodic survey updates on the basemap using a combination of CAD or GIS-related software.
Surveying also played a role in geotechnical instrumentation. A series of geotechnical studies conducted by URS and Arup, two large engineering firms, evaluated the subsurface conditions through field and laboratory data. One of the deliverables was to evaluate any ground movement induced on the neighboring residential and commercial properties using specialized geotechnical instruments and survey markers. Surveyed instruments in the form of settlement plates and settlement markers used to evaluate the foundation consolidation were installed on all the perimeter levees and the experimental Test Fill. Survey data collected from the Test Fill was used to calibrate soil deformation predictions from wetland and levee effects.
The overall monitoring program was used to validate that the geotechnical models are within specified threshold limits and to evaluate foundation consolidation for staged levee construction. Specialized survey instruments called pipe displacement markers (PDM), designed by the Corps of Engineers, were used to monitor and evaluate movement of a 52-inch outfall pipeline about 30 feet from the levee outboard toe. This pipeline is owned and operated by the Novato Sanitary District and functions several months a year to discharge treated effluent into the San Pablo Bay. The pipeline parallels the two levee systems for approximately three miles with PDMs installed at approximately 200-foot increments. The intent of the monitoring program is to collect soil deformation movement caused from the levee construction and to modify construction activities accordingly to mitigate further movement. Because the predicted movement is relatively small, high survey accuracy is required. The resulting survey data collected is reviewed under the direction of a California registered professional land surveyor.
In addition to monitoring the aforementioned existing geotechnical instruments on the perimeter levees, the Hamilton project delivery team has taken extra precaution in verifying ground subsurface predictions on structures outside the project limits. Adhesive reflective targets installed on selected residential home windows are surveyed at predetermined dates. Additionally, selected ground monuments are also surveyed and monitored for movement between the homes and the project. This monitoring program will end in 2010.
However, long-term monitoring will not end upon the groundbreaking of dredge import for wetland construction. The wetlands and associated features will be surveyed and monitored long after the project is completed to verify that features have met design intent developed during the planning phases. The collected survey data will determine settlement and habitat elevations, which directly impact plant and wildlife survival. For example, the northern seasonal wetland is designed to protect small wildlife from predators by creating islands during high tide and provide land bridging during low tides.
Challenges in the Mud
Bordered by San Pablo Bay, the Hamilton Airfield is characterized primarily by soft marine clays known as bay mud. This varies in thickness between 40 to 70 feet across the site, creating a so-called "bowl of Jell-O." The western project perimeter contains adjacent residential and commercial buildings. Similarly, soft marine clays underlie the northern and eastern project perimeter.
Establishing permanent survey controls on the project for high-accuracy monitoring would require installing through the soft bay mud into stiff alluvium, which can range to 70 feet in depth. This was considered an option because the existing permanent controls were located outside the project site and would require considerable traversing using traditional survey methods. However, these were found to be costly to install. Team members and consultants considered the various heavy construction and future planned activities that may damage or dislocate the permanent project control. The project delivery team determined that a permanent project control onsite was not needed now because of future project uncertainties but would be reconsidered later.
Monitoring a 52-inch-diameter outfall pipe paralleling constructed levees and other project features over soft soil proves challenging. Understanding the survey requirements is important to prevent construction-related effects leading to leakage or rupturing in the pipeline. This translates to high accuracy and repeatability challenges when horizontal monuments and vertical benchmarks are often not near the monitoring area. Thus, calibrated survey instruments with high precision and accuracy and experienced land surveyors familiar with working in soft subsurface conditions are important in obtaining quality data.
The baseline surveys are the most important aspect of the monitoring program for the project, which incorporated several datums. The vertical datums were NGVD (National Geodetic Vertical Datum) 1929, the NAVD (North American Vertical Datum) 1988, and the tidal datums. Although they appear as only a simple conversion, the common misuse and/or confusion could potentially cause major problems. Datums can vary by several feet, which could potentially cause flood overtopping of a levee, a component of levee failure. This could translate to schedule delays and higher project costs if caught late in the project. To avoid confusion between vertical datums, Bill Firth, PE, a technical specialist in hydraulic engineering for the U.S. Army Corps of Engineers, developed a simple conversion sheet between the vertical datums (tidal and land).
Early data collected for this project prior to 2000 have horizontal coordinates in NAD1927 and a vertical datum in NGVD 1929 in U.S. survey foot. Newer acquired land survey data are horizontally in NAD1983 and vertically in NAVD1988 in U.S. survey foot. All older land-based datums and tidal datums are converted to meet the aforementioned datum to avoid confusion and detrimental effects.
GIS Enters the Picture
The Hamilton Wetland Restoration Project contains a wealth of data, and if properly managed, it can serve as a valuable resource. The team has incorporated its collected field, design, and survey data into its long-term data management program, interfacing this with ESRI's ArcGIS software. An abundance of field, laboratory, and survey data compiled from geotechnical studies and field exploration exists. The Corps plans to integrate software such as Gint, database software that stores and outputs engineering diagrams for its subsurface data compatible with ArcGIS. Such examples are part of the Corps of Engineers' adaptive management plan to effectively store, organize, and visualize collected data on a GIS.
Mapping-grade GPS equipment has been used to delineate borrow and excavation limits or boring locations for this project. Corps of Engineers field technicians use the Trimble Geo-XH or Geo-XT, with differential post processing at the sub-foot or sub-meter accuracy. Other managed data include photos taken with a GPS-fitted camera and annotated with GPS coordinates, notes, and orientation. Ricoh and Geospatial Experts manufacture the camera and software, respectively. Processed photos can be overlain on a web-based image/ USGS quadrangle map, exported as an ESRI shapefile, or viewed on Google Earth. This allows the Corps of Engineers to organize their photo inventory on a georeferenced map or GIS rather than storing the photos on a desktop folder with little or no descriptions as to where they are geographically. In addition, this allows for an easy display of information in a format that is understandable to other project members or managers that may be unfamiliar with the project site.
The Corps of Engineers in San Francisco is currently purchasing the Leica 1200 series, a multi-function total station and GPS. Because the restoration site is located in an area primarily with open sky, GPS is ideal for sub-centimeter- level surveying for quality assurance. The project will rely on Leica's RTKmax network, a permanent control network that allows real time kinematic correction through the internet via integrated cell phone transmission.
This is just one the many surveying technologies that will come into play as the U.S. Army Corps of Engineers and others undertake this massive project to return a closed military base to its former glory as a wetland.
About the Authors
Chris Wang, PE, CMS is a civil engineer and mapping scientist in the geo-sciences section of the U.S. Army Corps of Engineers in San Francisco. He received his M.S. in Geotechnical Engineering from the U. of California, Berkeley. He is responsible for implementation of the field GPS and survey equipment and provides geotechnical support to the Hamilton Wetland Restoration Project.
Edgar V. Salire, PE, BCEE is a civil engineer and a board-certified environmental engineer in the geo-sciences section of the U.S. Army Corps of Engineers in San Francisco. He received his Master of Engineering in Geotechnical Engineering from Arizona State U. He is the lead geotechnical engineer for the Hamilton Wetland Restoration Project.
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