Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...

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Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment 4.0 AFFECTED ENVIRONMENT This section describes the current physical, biological, ecological, economic, and social environment for the watershed and other areas of the project impact. This provides the context for determining the effects of alternatives. Some conditions will be constant throughout the evaluated life of the project, while others will be subject to change because of social, economic, and political influences. 4.1 CLIMATE The Sonoran Desert has a continental climate with great variability of both diurnal and seasonal temperatures. It is characterized by a summer monsoon season with light rains in the winter. The average monthly temperatures range from a low of 45.9° F in January to a high of 106.3° F in July. Average monthly rainfall ranges between 0.03 inches in June to 1.01 inches in July, with annual rainfall reaching 6.56 inches in an average year. 4.2 GEOLOGY The Powerline, Vineyard Road, and Rittenhouse FRS (PVR) study area is near the junction of the Mexican Highland and Sonoran Desert sections of the Basin and Range physiographic province, just southwest of the boundary between the Basin and Range and Transition Zone physiographic provinces. The boundary between these two provinces is marked by the western edge of the Superstition and Goldfield mountains, which rise abruptly above the valley floor. These mountain ranges also mark the eastern margin of the down-dropped Phoenix Basin, which underlies much of the Phoenix metropolitan area and extends westward to the Sierra Estrella and White Tank Mountains. The Phoenix Basin formed between approximately 8 and 15 million years ago after cessation of the volcanic activity that formed the bulk of the Goldfield and Superstition mountains. The Phoenix Basin is subdivided into several sub-basins. The study area is within the Mesa- Chandler sub-basin. On the west, this sub-basin is partially separated from the central Phoenix Basin by the bedrock highs of the Tempe Butte area. The Mesa-Chandler sub-basin is bounded by the Goldfield and Superstition mountains on the north and northeast, by Mineral Mountain on the east, and by the Santan Mountains on the southwest. Alluvial fans and pediments have developed along the Superstition and Goldfield mountain fronts during the last approximately five million years of relative tectonic stability. The pediments are broad, gently sloping erosional surfaces that consist of shallow bedrock covered with a thin veneer of alluvial fan deposits. Bedrock is often exposed in drainages that have cut through the thin alluvial deposits or is exposed in inselbergs (knobs and small hills of bedrock that rise above the alluvial fan surfaces, such as Hawk Rock). Farther west of the mountain fronts, the basin is dominated by a thick sequence of alluvial fan deposits. Alluvial fans are gently sloping masses of alluvium deposited by streams and washes that flow from mountains onto the floor of a valley. As streams emerge from the mountains, they diverge into multiple channels, water begins to percolate into the subsurface, flows diminish and sediment carried by the streams is deposited. Coarser materials (e.g., boulders, cobbles and gravel) tend to be deposited near the apex of fans, whereas finer-grained materials (e.g., sand, silt and clay) USDA- NRCS Page 4-1 January 2013 Kimley-Horn and Associates, Inc.
Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment are transported to the distal portions of fans. As gradients flatten toward the distal portions of fans, sediments become finer grained and less permeable, and drainage channels often converge. The Powerline, Vineyard Road, and Rittenhouse FRS have been the subject (particularly Powerline and the northern 1/3 of Vineyard Road FRS) of previous geohazard investigations and analyses including studies as part of this planning study. The Powerline and Vineyard Road FRS are located in a region of local land subsidence. The purpose of the geohazard investigations were to evaluate potential impacts of ground subsidence and earth fissuring on the existing structures and on the selection and subsequent development of rehabilitation designs and/or replacement alternatives for flood protection afforded by the Powerline, Vineyard Road and Rittenhouse Flood Retarding Structures (FRS). The results, findings and conclusion of the studies were used by the project team in its development and evaluation of dam rehabilitation and/or replacement alternatives. There are known earth fissures in the project area near Hawk Rock and one known earth fissure located immediately downstream of the Powerline FRS embankment at about Station 115+45. There also is a high probability that the earth fissure near Station 115+45 extends beneath the existing embankment. It is suspected that subsidence patterns at the Powerline FRS will continue into the future, potentially causing further development of fissures in this area. The studies have developed, through various techniques and methods, earth fissure risk zone mapping for the PVR structures. Figure C-8 in Appendix C is the earth fissure risk zones prepared for the PVR structures. As depicted in the risk zone map, Powerline FRS has portions of the dam within all four zones. Vineyard Road FRS is within two zones, and Rittenhouse FRS falls within one zone (the lowest probability zone). The zones are defined below. In general terms, the earth fissure risk delineating process has evolved to include 4 general earth FRZs that are broadly similar from site to site, although the exact definitions of the FRZs vary from one site to another. The numbering system used to identify the four zones has changed through time and from site to site, so a better understanding of the level of risk is provided by the actual definition than by the zone number. The four FRZ definitions can be summarized as follows: • Zone 1: An area with a known earth fissure(s), a high probability for the presence of earth fissures without surface expression, or a high probability for the future development of earth fissures. • Zone 2: An area that in the future is likely to experience tensional ground strain that could cause earth fissures, yet no earth fissures have been identified to date. An area with a moderate probability for the future development of earth fissures. • Zone 3: An area that has or is likely to experience some tensional ground strain, though not great enough to result in earth fissures. An area with a low to moderate probability for the future development of earth fissures. • Zone 4: An area that has not, and is not expected to, experience tensional ground strain. An area with a low probability for the future development of earth fissures. The key terms used in the FRZ definitions are the presence of known fissures or the high, moderate, low to moderate or low probability for the future development of earth fissures. USDA- NRCS Page 4-2 January 2013 Kimley-Horn and Associates, Inc.
Page 1 and 2: DRAFT SUPPLEMENTAL WATERSHED PLAN A
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Powerline Flood Retarding Structure
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Signal Butte Rd Meridian Rd Ironwoo
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POWERLINE FRS TYPICAL SECTION
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Figure 22 Figure 21 - Conceptual Mu
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Exhibit A-2 - Emergency Spillway Di
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Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...

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