Introduction

In June 1999, while conducting routine maintenance on the Central Arizona Project (CAP) aqueduct in the Phoenix metropolitan area, workers noticed that the water level in the canal was visibly higher in a section that crossed through north Scottsdale near the McDowell Mountains. A follow up investigation revealed that the canal was subsiding, or sinking, in a 1 mile section at a rate of approximately 0.2 feet per year. Already nearly 1.5 feet of free board had been lost. Freeboard is the height between the top of the water surface in the canal and the top of the canal lining. Minimum freeboard is required to avoid the risk of overtopping the canal, especially during emergency shut-down procedures. Future subsidence was expected to reduce delivery capacity of the canal by 9% in 2003 and 18% in 2005. Because 80% of the CAP deliveries (approximately 1.2 million acre-feet) pass through this section of the canal on its way to farms, cities and other users in central and southern Arizona, something had to be done soon.

An assessment of the problem by the Central Arizona Water Conservation District (CAWCD) recommended several alternatives to address the situation. It was decided to raise the canal lining now to prevent future delivery restrictions followed by a comprehensive subsidence investigation to assess the area, magnitude, rate, and causes of the subsidence as well as develop long-term solutions. The canal lining was raised in June 2002 and the Subsidence Study Project began in January 2003.

Aerial View of Subsidence Area
Oblique Aerial View of Subsidence Area

Rebar re-enforcement
Rebar re-enforcement was placed in the subsidence affected area prior to cement placement

Testing of procedures
Testing of procedures were conducted in an unaffected area of the canal in March 2002

Canal Retrofit Project

The canal retrofit project was conducted by CAP’s Engineering and Maintenance Departments. The project began in March 2002 with testing of equipment and procedures, and was completed in June 2002 when peak summer month deliveries are conducted. The repair work was conducted over a 1.5 mile section of the canal where the most severe subsidence was experienced. Equipment and procedures were used to raise the lining without having to lower the canal water level and reduce deliveries to downstream customers. The concrete was poured on the adjacent 2:1 side slope to the top of the maintenance road. The work was conducted on a fast-track schedule at a cost of $350,000.

Raising the Lining
Special techniques were developed to
raise the lining without having to lower
the canal water level.

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 Overview

Hydrogeologic Conditions of Project Area

The Subsidence Study Project area is located in the Paradise Valley Sub-basin of the Salt River Valley. These sedimentary basins are part of the Basin and Range Physiographic Province of Arizona. Each basin is bounded by uplifted mountain blocks comprised primarily of dense igneous and metamorphic rocks, while the valleys are filled with less dense alluvial basin-fill deposits comprised of a layered sequence of clay, silt, sand and gravel. Groundwater is present in the alluvial sedimentary deposits and is pumped by municipalities and other groundwater users. Subsidence in these sedimentary basins occurs in response to excessive groundwater pumping. When water levels decline in underground aquifers containing substantial fine-grained deposits, such as clay and silt, slow drainage of these fine-grained deposits can occur. When this water is removed, the pore pressure that assists in supporting the sedimentary deposits decreases, leading to compaction. The fine-grained deposits compact in-elastically in that the subsidence caused by compaction of fine-grained units is permanent and cannot be restored even if water levels rise at some later date.

Diagram of Basin Margin
Conceptual Diagram of Basin Margin
Showing Buried Pediment Surface
and Fault Scarp
(click image to enlarge)

Satellite View of Subsidence Study Project Area
Satellite View of Subsidence Study Project Area

General Geologic Cross-Section of a Typical Basin and Range Valley
General Geologic Cross-Section of a Typical Basin and Range Valley

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 Approach

The Subsidence Study Project was designed to take a comprehensive approach to quantify existing subsidence, determine the causes and mechanisms, predict future subsidence, and evaluate risk, so that appropriate corrective actions can be implemented. With assistance from the local municipalities and the Arizona Department of Water Resource (ADWR) , CAWCD initiated the project to using a multi-disciplinary approach coupled with the latest advances in technology. Cooperation with the municipalities and agencies were required to obtain hydrologic data needed for this multi-year investigation. CAWCD retained the services of an earth-science consultant, GeoTrans, Inc., as the principal investigator for the project team that includes Tetra Tech, Inc. and HydroGeophysics, Inc.

GPS Receiver for Ground Elevation Measurements
GPS Receiver Set Up Along Canal for Ground Elevation Measurements

Project Plan

The project plan for the investigation uses a two-phased approach: an initial phase (Phase A) that involves data acquisition, analysis, and monitoring, and a final phase (Phase B) that consists of supplemental data acquisition and computer modeling. Phase A tasks include:

Determine the Magnitude and Rates of Subsidence

  • Map spatial dimensions and rates using INSAR
  • Develop long-term monitoring network using high precision GPS
  • Re-survey monuments on set frequency to determine subsidence rates

Collect Existing Hydrogeological Data

  • Driller logs and well construction information
  • Down-hole geophysical logs and pumping test data
  • Hydrogeologic cross-sections
  • Groundwater models and other information
  • Develop a Graphical Information System (GIS) database for managing and analyzing data

Conduct Gravity Survey

  • Collect existing gravity station data
  • Perform targeted gravity readings in area of concern
  • Analyze data to determine basin structure and relationship to earth fissuring

Evaluate Existing and Planned Groundwater Flow Models

  • Obtain a recently completed groundwater flow model from the City of Scottsdale
  • Acquire the ongoing Northeast Aquifer Model once completed by the City of Phoenix
  • Assess existing models for suitability of generating more refined model capable of 3-dimensional compaction modeling

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 Results

 Subsidence Mapping

The magnitude and rates of subsidence were evaluated using orbiting satellites to image the ground surface from space, and to determine precise ground elevations at discrete points using land-based surveying methods. Radar images obtained from the Center for Space Research in Austin, Texas were acquired with the assistance of ADWR for multi-year timeframes. The radar mapping employs a technology called Interferometric Synthetic Aperture Radar (InSAR). The InSAR method produces a colored image depicting ground elevation changes between two radar images taken at different times. The color bands are produced by the phase-shifted radar signal reflected from the ground that is caused by the increased return path length from the time of the first image. Each color band of the fringe pattern, represents a constant change in ground elevation caused by the phase shift. Web links discussing the technology and uses are provided below. The InSAR image of the Pool 23 and 24 portions of the CAP aqueduct shows several en-echelon subsidence depressions, called bowls, having subsidence rates up to 0.35 feet per year.

InSAR Image showing subsidence bowels along CAP aqueduct
InSAR Image showing subsidence bowels along CAP aqueduct

Ground-based surveying using Global Positioning Satellite (GPS) was used by Tetra Tech, Inc. to establish survey control from bedrock and stable areas, and develop a network of monuments along the canal. A total of 55 monuments comprise monitoring network. The network was established in May 2003 and then re-surveyed in December 2003 to yield preliminary subsidence rates up to 0.37 feet per year. Additional surveys will be used to verify the subsidence rates and to evaluate changes over time.

GPS Survey Monitoring Network Showing Subsidence
The GPS Survey Monitoring Network Showing Subsidence
Rates determined for the later half of 2003

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 Gravity Survey and Analysis

A gravity survey was conducted for the project to image the subsurface bedrock topography beneath the alluvial basin-fill deposits. Variations in the gravity field caused by substantial bulk-density contrast between the lower density basin-fill deposits and the higher density bedrock provides an unobtrusive method of imaging bedrock topography through gravity measurement surveys. It is well known that bedrock topographic variations in subsidence areas can lead to differential subsidence and earth fissure development. One purpose of the gravity study was to identify areas along the canal where earth fissure development was most likely to occur, so that additional monitoring and canal strengthening can be planned.,/

Gravity field overlay on INSAR Subsidence Map
Gravity field overlay on INSAR Subsidence Map. Subsidence features appear along margin of basin where the main fine-grained unit, called the Middle Alluvial Unit (MAU), thins and is closer to land surface. INSAR data provided by Dr. Sean Buckley of the Center for Space Research, Austin, Texas.

Approximately 880 gravity stations were obtained from Arizona State University from two uncompleted M.S Thesis studies. These stations were supplemented by 57 stations from a regional database and 142 new stations taken near the canal to increase station density in areas of concern. After reducing the data and accounting for the regional gravity field, data analysis was conducted by generating gravity contour maps, evaluating the horizontal gradient component, and conducting 2-dimensional gravity models. Preliminary results indicate low gravity values over the central portion of the Paradise Valley Basin and high gravity values over much of the McDowell Mountain area. The gravity field variations correlate well with the basin structure determined from widely–spaced deep boreholes in the area. Steep gravity gradients are present adjacent to the McDowell Mountains suggesting the presence of a fault-bounded and/or eroded edge to a buried pediment surface. The canal crosses the edge of the buried pediment surface several times in this area. Future work will include refined mapping of the pediment edge, so that canal strengthening can be planned where needed.

Map showing the intensity of the horizontal gravity gradient
Map showing the intensity of the horizontal gravity gradient. Reds indicate steep gradient areas and greens indicate shallow gradient areas.

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 Reconnaissance Search For Earth Fissures

In January 2003, an earth fissure was located on the north edge of the Pool 24 area subsidence bowl. The fissure is 575 feet long, about 3 feet wide, and about 6 feet deep. It occurs in an area of steep gravity gradients, which marks the general location of the pediment edge. A reconnaissance level search for other fissures along the canal was conducted in April 2003 to document baseline conditions. Low altitude aerial photographs were taken in the early morning hours to utilize sun shadows to enhance ground features that may be caused by earth fissures. Stereoscopic analysis of 123 photos along a 15 mile section of the canal did not reveal the presence of any new fissures; however, recent land development may have obscured signs of developing earth fissures.

Reconnaissance Survey Image Map
No additional earth fissures were located along
the CAP canal during the reconnaissance survey

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 Hydrogeologic Data Acquisition and Analysis

Available driller logs, well construction information, groundwater level and pumping data, borehole geophysical logs, hydrostratigraphic cross-sections, groundwater flow models and geophysical surveys were acquired from ADWR and the local municipalities. GeoTrans, Inc. imported the data into a Graphical Information Systems (GIS) database for managing and analyzing the data. The database contains approximately 4,300 wells for the project area, of which 127 pumped groundwater in 2000. Approximately 4,800 groundwater level measurements were obtained from the City of Scottsdale and approximately 200 measurements were obtained from the City of Phoenix. The data was used to produce groundwater level contour maps for specific time frames and summaries of groundwater pumping from 1984 to 2000. Approximately 75,000 acre-feet of groundwater is pumped from the sub-basin annually. The data shows that the basin is comprised of three hydrostratigraphic units called the upper, middle and lower alluvial units (UAU, MAU, LAU). In 1966 groundwater levels were at the top of the MAU and have decreased over time desaturating the fine-grained MAU, which appears to be compacting and causing the ground surface to subside. Preliminary results of predictive groundwater modeling scenarios indicate continued lowering of the groundwater surface over time at current groundwater extraction rates.

groundwater flow modeling results graph
Preliminary groundwater flow modeling results showing potential
declining water levels in the MAU at continued 2001 pumping rates

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 Phase B Plan

Phase B of the project will be conducted from 2004 to 2006 and consist of tasks designed to more fully quantify subsidence rates, and hydrogeologic conditions of the area. Phase B tasks include:

  • Determine the Magnitude and Rates of subsidence
    • Continued monitoring using INSAR and GPS surveying
  • Conduct Additional Gravity Survey (optional)
    • Collect additional gravity data and perform 3-dimensional gravity modeling
  • Conduct Additional Geophysical Surveys (optional)
    • Test seismic reflection and electrical resistivity techniques to assess their use in defining the pediment edge so that canal strengthening to withstand earth fissuring can be planned
  • Conduct Groundwater Flow/Subsidence Modeling
    • Refine the existing groundwater flow model to predict future impacts from current groundwater pumping rates and other proposed water management strategies.
    • Develop a subsidence model to assess future ground movements in response to desaturation of fine-grained units within the aquifer system.

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 Summary

The Subsidence Study Project is intended to be a multi-year project because subsidence rates are generally small and take several years to quantify. The results presented here are preliminary as additional data obtained through the course of the investigation will provide more information on the subsidence rates, causes, and possible solutions. Subsidence impacts have required CAWCD to make repairs on the canal and the preliminary rates detected thus far indicate that additional repairs and modifications will be needed to mitigate the possibility of structural impacts and/or reduced delivery capacity to downstream customers.

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