Photo, City of Scottsdale. Looking west at Black Hills Dam with Fraesfield Mountain on the right. Upstream to the right.
Source materials of black hills earthen dams Maricopa county, aZ

Earthen dams abound in the U.S. – by one account there may be as many as 80,000. Arizona is home to 228 earthfill and rockfill jurisdictional* dams, one of which, the Black Hills Dam, is owned by the City of Scottsdale.

The City recently contracted AZGS to examine the dam and report on: 1) the nature of materials comprising the dam; and 2) the source of those materials. The full results of that study are in Brian Gootee’s “Geological Characterization of the Black Hills Dam, Scottsdale, Arizona”, AZGS Open-File Report-09-01 (LINK). It is Scottsdale’s plan to remove the dam and return the surrounding terrain to pre-dam conditions.

Blacks Hills Dam is 110-m long at its base – 131-m long at the crest, six- to seven-m high, with a base 40-m wide. It is composed of coarse sand, gravel and cobbles, with minor silt and trace clay. Gootee constructed a surficial geologic map (FIG MAP) and concluded that dam materials were derived from local surficial deposits.

FIG 2: Surficial Geologic map of Black Hill Tank dam area.

The dam surface is defaced by small, regularly-spaced (2- to 4-m spacing) gullies, 0.2- to 2-m wide and 0.1- to 1-m deep, oriented transverse to the long axis of the dam. (Such gullies are common on earthen dams and may potentially constitute a threat to dam integrity.)

Gootee identified several options for locally redistributing dam materials, most notably to disperse them between a terrace and bedrock high situated upstream from the dam and topographically above 100-year flood levels.

* A Jurisdictional Dam, as defined by the Arizona Department of Water Resources (ADWR), is an artificial barrier for the impounding or diversion of water either 25 feet or more in height or having a storage capacity of more than 50 acre-feet.

Acknowledgments

We thank Ravi Murthy and Nicole Spence-Gibson of ADWR’s Dam Safety Section for providing statistics on Arizona’s earthen dams.

Full Citation

Gootee, B., 2009, Geological Characterization of the Black Hills Dam, Scottsdale, Arizona, AZGS Open-File Report-09-01, 9 p.

 

ARTICLE AUTHOR:



Brian Gootee
Arizona Geological Survey

Tucson, AZ
brian.gootee@azgs.az.gov


 

 

 












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