US Army Corps of Engineers
Albuquerque District

Scrutinizing Sediment Deposits at Cochiti Lake

Published Jan. 16, 2012

The U.S. Army Corps of Engineers’ Cochiti Dam, near Albuquerque, N.M., is one of the 10 largest earthen dams in the United States. The dam is maintained and operated by the Corps’ Albuquerque District. More than five miles in length, the dam comprises 65 million cubic yards of earth, and its reservoir is a popular recreational site, providing camping, boating and fishing.

An important purpose for the dam is sediment control. Erosion within the watershed is a major problem in this arid region, exacerbated by historic land use and wildfires. Sediment transported in the Rio Grande is deposited on the riverbed as water flow diminishes. Known as “aggradation,” this results in an elevation increase of the riverbed. Unless constrained by levees, aggradation can cause widening of the river channel and overflowing of the riverbank.

Ironically, “degradation,” river scouring and lack of sediment deposition, is a problem of concern in the Albuquerque area. Monitoring and management of fluvial sedimentation is a principle responsibility of both the Corps and the U.S. Bureau of Reclamation. Along with other dams in the watershed, Cochiti Dam serves as a “sediment trap” and is thus a critical component to region-wide sediment management efforts.

Paradoxically, the effectiveness of the dam for sediment control has led to a serious issue confronting the reservoir—sediment deposition is reducing reservoir storage capacity and causing significant aggradation upstream within the Rio Grande channel. Monitoring sediment volume, spatial distribution and rate of deposition is of paramount concern to the District. Consequences for the operation and life expectancy of Cochiti Dam and Reservoir are at stake.

Because any hydrologic modeling and proposed remedial measures must be based on accurate, empirical geospatial information, the District contracted the services of Wilson & Company, Engineers & Architects, to perform professional survey, mapping and geospatial information services to support management of Cochiti Dam and Reservoir. The Area of Interest, or AOI, includes the region occupied by the dam and reservoir, Cochiti Pueblo lands below the dam and the Rio Grande and White Rock Canyon several miles upstream of the reservoir.

Of particular interest to the District was the potential to use aerial photography acquired before construction of the dam in 1965. If available, photogrammetric techniques could be used to digitize features, such as the historic river channel, and compile a Digital Terrain Model (DTM) of pre–dam topography for comparative analysis. This pre-dam data would provide baseline geospatial information against which post-dam data and bathymetry could be compared. The District hoped the comparison would yield visual, graphical and quantitative information about the volume and spatial distribution of sediment deposition since the dam was completed.

Does historical geospatial data exist?

The University of New Mexico had done preliminary research on the availability of historical aerial photography and discovered photography was acquired in October 1963 by the U.S. Forest Service. Wilson & Company photogrammetists determined this photography was suitable for pre-dam mapping of the AOI. The photography was collected with a calibrated metric aerial camera and had the requisite coverage and scale for photogrammetric mapping.

The District and Wilson & Company were also aware of additional historic geospatial data obtained over several years by the Corps, as well as other images stored in Wilson & Company archives.

Using historical data to see the big picture

The pre-dam aerial photography supported DTM compilation for 5-foot contour accuracy, when combined with camera calibration data and ground control in an aerotriangulation solution. Aerial negatives were scanned for use in digital photogrammetric workstations. Existing ground control used to support aerial mapping in 2000 and 2004 was photo transferred to the pre-dam photography. It was determined that additional ground control was required around the periphery of the AOI.

In 2010, Wilson & Company dispatched survey crews to derive coordinates for additional photo identifiable features. These features needed to be identifiable at the time of survey and on the pre-dam photography, creating a unique challenge for the photogrammetists and surveyors.

DTM, infrastructure and hydrologic features like roads, the river channel and dams, were digitized for the AOI from the 1963 photography and for the dam and reservoir area from later photography. This mapping provided data for baseline geospatial information.

One interesting input was from July 1987 when Cochiti Reservoir filled to the highest pool elevation in its history. At 5,435 feet above sea level, this elevation was below design maximum pool elevation of 5,482 feet, but considerably higher than normal pool elevation of 5,335 feet. Digital orthophotos were created from aerial photography taken at the time to document the event.

Delivery to the Corps

Final deliverables included a temporal geo-database that included survey control, contours, DTM, ArcTINs, bathymetry, infrastructure, water and hydrology data, as well as orthophotos of multiple dates. Metadata was carefully prepared to thoroughly document the multiplicity of dates, scales, resolutions, ground control, technologies and methodologies utilized for this composite geospatial project.

Quantitative comparisons show maximum sediment accumulation of 75 feet occurs in the river upstream of the reservoir, increasing river baseline elevation significantly over historic conditions. Sediment deposition continues upstream for several miles before tapering off. These figures refer to change in river surface elevation, since only this can be derived from aerial photography. The river depth averages 3 to 5 feet in most of the AOI and is not significant relative to total sediment depth.

Use of information by the District

In addition to providing geospatial information, analysis of these temporal DTM’s by District hydrologists will clarify spatial deposition patterns of sedimentation. Various hydrology-oriented software applications exist to further quantify and visualize this phenomenon. Hydrologic flow modeling and forecasting can be augmented. Additional ancillary geospatial data such as sediment core sampling and historic land use can be geo-referenced and interactively viewed. This data will provide a base layer for a program being designed by the District for use by all resource management agencies involved with the Rio Grande and its watershed above and below Cochiti Dam.

Recent events affecting resource

During the summer of 2011, a wildfire ravaged the watershed upstream of Cochiti Reservoir. The fire consumed 150,000 acres of the Santa Fe National Forest, Bandelier National Monument and surrounding Pueblo and private lands. Subsequent torrential rains caused flash floods that flushed sediment and debris into the Rio Grande and Cochiti Reservoir. These floods caused considerable damage and compounded the effect of previous sediment deposition.

Wilson & Company collected data from the flooding that will provide valuable new information for ongoing geospatial mapping. 

 

About the authors: John Peterson is the District's Geospatial Coordinator.  Daniel Paulsen is with Wilson & Company, Engineers & Architects.