Exploring: Reservoir Capacity And Sedimentation Of The Fena Valley Reservoir Guam

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The Fena Valley Reservoir, located in southern Guam, is the primary source of water for the United States Naval Base Guam and nearby village residents. At full capacity, the reservoir surface area extends approximately 0.30 mi2, and drains a watershed area of about 5.88 mi2

Since the construction of the Fena Dam in 1951, sediment has accumulated in the reservoir and reduced its storage capacity. The reservoir was surveyed previously in 1973, 1979, and 1990 (Kennedy Engineering Inc., 1973; Curtis, 1984; Nakama, 1992) in order to estimate the loss in storage capacity. To determine the current storage capacity, the United States Geological Survey in cooperation with the United States Navy resurveyed the bathymetry of the reservoir in February 2014. This project is part of a comprehensive study by the Pacific Water Science Center to investigate the future of water resources on Guam (http://hi.water.usgs.gov/studies/guam-serdp/).

Two methods were used to survey reservoir bathymetry. The first involved use of an integrated Acoustic Doppler Current Profiler and Global Positioning System (ADCP-GPS) (using a Sontek M9 HydroSurveyor ADCP). Horizontal positions were provided by the GPS and bed elevations were computed from ADCP-measured depths and known reservoir water surface elevations at the time of survey. Navigation and data processing were accomplished using Sontek's HydroSurveyor software package. Speed of sound measurements were made with a Conductivity-Temperature-Depth (CTD) profiler and were integrated into the bathymetric survey. The Sontek M9 ADCP is a 5-beam system with 50 degree swath, thus providing increased spatial coverage over single-beam sonar without the additional complexity [e.g. Inertia Measurement Unit (IMU)] of multi-beam sonar equipment.

Sediment deposits in the Fena Valley Reservoir at the mouth of the Imong River, Guam, 2014

Sediment deposits in the Fena Valley Reservoir at the mouth of the Imong River, Guam, 2014

Reservoir bathymetry was also measured with a multi-frequency, single-beam sonar system with the capability for sub-bottom profiling, i.e. mapping the thickness of deposited sediment within the reservoir. Concurrent with the ADCP surveying, the BSS+3 sub-bottom profiler was deployed (Specialty Devices, Inc.), along with differential GPS (Trimble R10) for positioning. This system was developed specifically for reservoir bathymetry and sediment mapping, and the methods have proven successful for a range of reservoir surveying applications (Dunbar and others, 1999, Obhiambo and Boss, 2004, Anderson and others, 2013). Mapping the sediment deposit with sonar provides an independent check of volume changes computed from historical surveys, which used different methods and may contain much more uncertainty than surveys using modern techniques. The BSS+3 system measures water depth with a 200 kHz transducer and sediment thickness with two additional lower-frequency transducers. Thus, the system provides redundancy with an ADCP survey for reservoir bathymetry as well as sub-bottom profiling capabilities.

Preliminary analyses of the bathymetric data indicate that the reservoir currently has 6,916 acre-ft of storage capacity. The original 1951 reservoir storage capacity was estimated at 8,365 acre-ft. Thus, between 1951 and 2014, the total storage capacity decreased by 1,449 acre-ft (a loss of 17 percent of the original storage capacity), and an average decrease of 23.0 acre-ft/yr. The historical surveys will also be digitized and compared to maps of sediment thickness generating using the sub-bottom profiler. These data sets will be analyzed along with hydrologic information, and reservoir sediment data to identify sedimentation trends and processes in more detail and how those trends may affect water resources in years to come.