Drainage Overview - Maricopa County Department of Transportation

3. WATERSHED FEATURES
3.1 Topography and Geology
The study area slopes north and south to large alluvial washes that generally convey flow to the
southeast. The White Tank Mountains are located south of the study area. A land form slope
analysis map is provided in Figure 3. The map shows the land slopes mildly north of these
mountains.
The Surficial Geology around the White Tank Mountains, Central Arizona (AZGS, 1991) maps
and report delineate the distribution and age of alluvial deposits within the study area. According
to the surficial geology maps, most of the study area south of Wash 1 contains younger deposits,
which may indicate areas of occasional to frequent flooding. The remainder of the study area is
typically defined as areas where only entrenched channels are subject to flooding, although other
areas of low relief could become susceptible to flooding with relatively minor shifts in
depositional patterns. The AZGS maps pertaining to the study area have been included as
Appendix TM3-03.
3.1.1 Land Subsidence and Earth Fissures
ADWR has mapped an active land subsidence feature in the West Valley area based on
measurements obtained between 2004 and 2010. The map entitled Land Subsidence in
Western Metropolitan Phoenix (ADWR, 2010) indicates that there has been up to 2 cm (0.8
inches) of subsidence within the study area east of 195 th Avenue. South of the study area,
up to 6 cm (2.4 inches) of subsidence has been documented. The extent of the land
subsidence is included in Appendix TM3-04. Land subsidence in Arizona typically occurs
due to groundwater drawdown. While subsidence is not considered to be a significant issue
within the study area at this time, as water demand changes with future development, the
increased potential for land subsidence should be considered when building infrastructure.
Based on a review of the Earth Fissure Map of Maricopa County, Arizona (AZGS, 2009),
there are no earth fissures mapped within the study area. No surface evidence of fissures has
been found, but this conclusion does not guarantee that hidden or future earth fissures are
not present. According to the Wittmann ADMP McMicken Dam Project Alternatives
Analysis (FCDMC, 2008), the northern end of McMicken dam, including the spillway, is in
an area of low to moderate fissure risk. See Section 3.5.4 for further details.
3.2 Soils
The National Resources Conservation Service (NRCS) assigns soil map unit components to
hydrologic soil groups to broadly indicate soils groups that have similar runoff characteristics.
The locations of these hydrologic soil groups within the study area are shown in Figure 4. Most
of the study area falls within Hydrologic Soil Group B: soils that have moderately low runoff
potential when thoroughly wet. These areas typically have a large proportion of sands and allow
unimpeded transmission of water through the soil. As shown in Figure 4, there are limited
regions that fall within Hydrologic Soil Group C: soils with moderately high runoff potential.
Soils in Group C typically have between 20 to 40 percent clay and less than 50 percent sands.
Water movement through these soils is expected to be somewhat restricted.
091337137, 2011-018, TT005 Maricopa County Department of Transportation
Technical Memorandum 3 Deer Valley Parkway Feasibility Study
Drainage Overview 8 April 2012