HomeMy WebLinkAboutCV Subsidence Monitoring Program R5--ool
' �" ' ^�•: Memorandum
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Cityof Palm Desert
... a 3yof';;:�°' Office of the Assistant City Manager
To: Honorable Mayor, Members of the City Council, & City Manager
From: Sheila R. Gilligan, Assistant City Manager
Subject: Letter from C.V. Water District Relative to Coachella Valley Subsidence
Monitoring Program.
Date: August 24, 2000
Attached is a copy of the subject letter which Mayor Crites would like considered under Reports and
Remarks during the City Council meeting of August 24t.
It will need to be added by unanimous vote as only four Councilmembers will be in attendance.
SHEILA R. GILLIGAN
ASSISTANT CITY MANAGER LTY COUNCIL ACTION:
n 't'ROVED DENIED
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\MEMORANDUMsrRic�
DATE: July 18, 2000 FILE: 0643.56
TO: Board of Directors
FROM: Steve Robbins, Assistant to the General Manager
RE: Coachella Valley Subsidence Monitoring Program
The purpose of this project authorization is to authorize the General Manager-Chief Engineer to
enter into a joint funding agreement with the U. S. Geological Survey(USGS) for preparation of
Phase III of the Coachella Valley Subsidence Study.
The original study was begun in 1996 to study the possible consequences of declining
groundwater levels in relation to land subsidence. Phase I of the study gathered and analyzed
existing data to evaluate the potential and possible distribution of subsidence, reviewed past
surveys for possible signs of subsidence and established a network of monitoring points.
Study results indicated that long term declines in water levels of sufficient magnitude to induce
land subsidence have occurred in portions of the Coachella Valley. Subsidence at network
monument locations has occurred to varying degrees. Fourteen of the seventeen monument
locations indicate cumulative subsidence measurements ranging from -0.2 to -0.5 feet. Where
data were available, historical subsidence was plotted with time and compared to water level
changes in nearby wells. In general, subsidence occurred during periods of water level decline
and rebound occurred during intervening periods of water level recovery. The magnitude of
these subsidence determinations were near or within the range of uncertainty (+/-0.2 ft) for
measurements made under these conditions, and therefore did not unequivocally indicate that
subsidence has occurred. However, the fact that timing of the subsidence measurements
correspond with water level changes indicated that land subsidence is probably occurring and
that a significant part of the subsidence has occurred since 1991, about the time when water
levels began declining below their previously recorded low levels.
Phase II of the program consisted of two components;
1. Determine the extent and magnitude of change in land surface elevations in the lower
Coachella Valley by making repeat measurements using geodetic standards for GPS
surveying.
2. Obtain a semi-quantitative determination of land surface displacements areally by creating an
image- an interferogram - that is a composite of two satellite synthetic aperture radar(SAR)
images. The direction(up or down) and magnitude (in the direction of the radar signal) of
changes in land surface are determined by contrasting data taken at varying intervals of time
anywhere from a few months to 5 years.
A draft of the Phase II report indicates that "GPS results revealed that small land-surface
changes occurred throughout the lower Coachella Valley during 1996-98. Of the 11
monuments examined for land-surface-elevation changes, 7 indicated subsidence and 4
indicated uplift. Generally, land subsidence occurred in the northern and western margins of
the lower Coachella Valley and ranged from 0.02 to 0.14 ft, ± 0.07 ft and uplift generally
occurred near Coachella and ranged from 0.02 to 0.19 ft, ± 0.07 ft."
The draft report also indicated that "InSAR revealed that land subsidence occurred in at least
three locations in the Coachella Valley during the period 1996-98, ranging in magnitude from
about 0.08 to 0.23 ft. Two of these areas are near Palm Desert, and one is near Lake Cahuilla.
The two subsidence areas in the Palm Desert area are the largest in geographic extent and in
magnitude. Both of these areas correspond to areas of significant ground-water development,
as indicated by the number of wells and the volume of water produced during the period 1996-
98."
Phase III of the program consists of four main elements:
1. Extension of the existing GPS network to the Palm Desert area.
2. Execution of a GPS survey of the entire subsidence monitoring network,
3. Acquisition of SAR imagery to prepare one or more interferograms representing different
periods, and
4. Obtainment of ground-water-level data and ground-water-production data from the district to
assess the relationship to changes in land-surface elevation during 1998-2000.
The program is estimated to cost $106,550. Cost to the district will be $61,150. USGS would
supply $45,400 towards the study.
This is the third phase of a long-term subsidence monitoring program that will continue into the
future as long as water levels in the valley continue to decline.
C- • ' i
Prepared by:
Steve Robbins
Assistant to the General Manager
Approved by:
Tom Levy
General Manager-Chief E gineer
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