Tucson chapter of the Arizona Hydrological Society
Tucson Chapter Arizona Hydrological Society


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Tucson Chapter News

June Meeting

Topic: “Urban Recharge”

Speaker: Jeff Kennedy

Schedule Details
Social Half Hour—6:00 PM
Speaker—6:30 PM

May Meeting Summary — Earth Fissures by Todd Shipman

On May 13th, the Tucson Chapter hosted a meeting at the offices of Errol L. Montgomery and Associates. Eleven people attended “Crack in Arizona”, presented by Dr. Todd Shipman, a Senior Research Scientist with the Arizona Geological Survey. Dr. Shipman is currently in charge of the earth fissure mapping program for the state of Arizona. Earth fissures are cracks or faults caused by the compaction of basin fill sediment. Most documented earth fissures form in response to groundwater overdraft; however, natural processes, such as earthquakes, can also cause the formation of earth fissures. In Arizona, the vast majority occur in agricultural areas, with the first reported occurrence was near Picacho Peak in the 1920’s.

There are two main types of earth fissures – shear and tension. Shear earth fissures have a dip-slip component. Displacement can be horizontal, vertical, or both, and the magnitude of displacement can increase with time. Earth fissures caused by tension typically show a surface crack of only a few inches, but are frequently enlarged by erosive forces. Dramatic pictures of this phenomena were presented. Often times, just a single intense storm can open a tension fissure by a few feet in just a couple of hours.

Earth fissures must be distinguished from polygonal cracks. Polygonal cracks have a similar surface expression to earth fissures, but are caused by completely different phenomena. Though the exact cause of their formation is not understood, the current theory prevalent in Arizona states that they form in response to capillary induced tension caused by the drying of a shallow clay body. As such, polygonal cracks are near-surface phenomena that can be safely filled in by land developers, while earth fissures usually hinder development. An example of what happens when a developer attempts to fill in an earth fissure was presented. During just one storm event west of Phoenix, a rural road that was paved over a filled-in earth fissure was washed away as the fissure rewidened. The only safe way to construct roads or canals over earth fissures is to bridge them, as was done during construction of Highway 202 in Phoenix, AZ and the CAP canal.

Earth fissures are one of the few geologic occurrences that happen on a human time scale. As such, mapping earth fissures is challenging. Their dynamic nature necessitates that any map depicting earth fissures be continually updated. Often times, earth fissures have minimal, if any surface expression, and so they are easily missed.

There is still much to be learned about earth fissures. Interactions between the aquifer and water that in transmitted into the ground through earth fissures has yet to be studied in detail. It is thought that earth fissures can aide in recharge, though there is a concern that they could also serve as a direct conduit for pollutants. Earth fissures have been known to close as well, and there is not an understanding of what factors determine the length of time an earth fissure will remain open.

For more information on earth fissures, visit the AZGS’s website at: http:// www.azgs.az.gov/. There you will also find the most recent earth fissure map completed for the State of Arizona, which was released on April 23rd, 2008.

Looking Ahead...

Summer schedule — No Tucson Chapter meetings are planned for July and August.  In September, members are encouraged to attend the Annual Symposium in Flagstaff Arizona. Regular monthly meetings will resume in October.

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Meeting information for the Tucson chapter

Date: Tuesday, June 10

Time
: 6:00 PM

Location
: Offices of Errol L. Montgomery (1550 E Prince Rd, Tucson)

For general information about chapter meetings, click here.

Tucson chapter board members

President
Jeff Gawad
Errol L. Montgomery & Associates

Vice President
Robert McGill
(520) 647-3315

Treasurer
Mike Mahan
Arizona Geological Survey
(520) 770-3500, ext. 211

Secretary
Marla Odom
Errol L. Montgomery & Associates
(520) 881-4912

Director
Dan Guido
Montgomery & Associates
(520) 881-4912

Corporate Board Member
Mike Geddis
Water Management Consultants
(520) 319-0725


Corporate Board Member

Nick Melcher