Wednesday, March 25, 2009

Surficial Geologic and Flood Hazard Assessment of Rainbow Valley

Announcement From the AZGS:

The Arizona Geological Survey’s (AZGS) newest map product, "Surficial Geologic Map and Flood Hazard Assessment, Rainbow Valley, Maricopa County, Arizona", DGM – 71, displays the surface geology of Rainbow Valley on three map sheets at 1:24,000 map-scale; the three sheets encompass portions of 12 USGS 7.5 minute quadrangles.

The accompanying 11-page report includes a _Flood Hazard Assessment_ that pigeonholes map units into high, intermediate, or low flood hazard areas. The three flood hazard areas encompass more than 250 sq. miles of Rainbow Valley and environs: Flood Hazard High (76.29 sq. miles), Intermediate (129.76 sq. miles), and Low (48.25 sq. miles).

Within flood area categories, e.g., Flood Hazard High, units are arranged in order of decreasing frequency of flooding. For example, Qycr – Modern channels of Waterman Wash, has the highest flood frequency of the seven geologic units in the Flood Hazard High zone; Qyaf – Potentially active alluvial fans, has the lowest flood frequency in that same category.

This first flood hazard assessment will provide land-use planners of Maricopa County with a blueprint for directing future development in Rainbow Valley.

Rainbow Valley is a checkerboard of agricultural fields drained by Waterman Wash, which debouches into the Gila River to the north. AZGS mapping shows the valley floor and adjacent alluvial fans comprise a complex of fluvial sediments including channel and sheetflow deposits, terrace and bar deposits, alluvial fans, and sparse eolian sands, ranging in age from modern, through the Holocene or late Pleistocene, to early Quaternary or late Tertiary.

AZGS geologists Philip Pearthree, Ann Youberg, and Todd Shipman co-authored the work. Funding was provided by the Flood Control District of Maricopa County.

DGM-71, maps and report, are available on CD-ROM at the State Map & Bookstore, 416 W. Congress, Ste#100, Tucson, AZ, 85701, for $15.00.

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