Piute Dam Rehabilitation Project

BACKGROUND
Piute Dam, owned by the Piute Reservoir and Irrigation Company, is located on the Sevier River about 9 miles south of Marysvale, Utah.  The dam was constructed in the early 1900’s by pioneer farmers to provide an irrigation water supply.  Piute Dam is a rolled earth fill and hydraulic fill (puddle core) structure.  It is 90 feet high with a crest length of 1,200 feet and a storage capacity of 72,000 acre-feet.

DAM SAFETY DEFICIENCIES/PROBLEMS
Evaluations indicated that Piute Dam did not meet current dam safety standards. Problems with the dam included:

• Toe Drains – There were concerns that the existing drain system, consisting of rock drain and wood stave pipes, did not meet critical filter criteria and a chimney drain was not present in the dam.
• Spillway Capacity – The existing spillway could only pass 38% of the design flood.
• Outlet Works – The original guard gate was inoperable and the control gate structure was unstable under earthquake conditions.
• Upstream Stability – Concerns were raised regarding rapid drawdown with the steep upstream slope and potential liquefaction of soil materials if an earthquake occurred.

REHABILITATION DESIGN FEATURES
Following several years of research, field studies, engineering analyses, and coordination with all involved parties, Franson Noble Engineering designed creative rehabilitation solutions that brought the dam into compliance with current dam safety standards, while saving the client millions of dollars.  Features included:

• New Internal Drain System – The most unique and creative feature of the drain system was the use of a 455-ft long by 30-ft deep, bio-polymer slurry trench at the dam toe to install an internal drain filter and collection system.  This state-of-the-art construction method had not previously been used for dam rehabilitation in the area and represented a cost savings of $2 million.  Other key elements of the system include: drain material densification, temporary sheet piling cutoff walls, geomembrane- geotextile barrier, and a chimney drain.
• Emergency Spillway – A four-bay fuse plug was designed to protect the dam by “failing” or eroding when breached and pass the design flood of 106,000 cfs.  Four 50-ft wide bays include incrementally higher crests so that fuse plugs will sequentially overtop and erode, based on flood severity.  This was done so outflows will more closely match flood inflows, which should significantly reduce potential liability to the owner.  Other unique features include: a 30-foot deep sheet piling cutoff wall, concrete training walls, sloped fuse plug core zone, and battered concrete walls.
• Outlet Works – Remains of the original guard gate and the existing control gate were demolished and removed.  New hydraulic controlled guard and control gates were then installed.  Another significant design feature includes extensive lab modeling of the outlet works to optimize configuration of a unique stilling basin within the outlet tunnel.
• Upstream Embankment – Existing riprap was removed and the downstream slope was flattened from a 2:1 slope to 2.5:1 with a 40-foot wide berm at the maximum section to improve stability.
• Instrumentation – Rehabilitation included installation of improved instrumentation, such as automated vibrating wire piezometer and weir monitors for remote observations.

CONSTRUCTION
Construction of the dam was completed in two construction periods from 2002 through 2005.  The total project cost was $8.2 million, which is the largest rehabilitation project completed to date under the Utah Dam Safety program.  Unique and innovative engineering design created savings of an estimated $4 to $6 million compared to standard construction methods that had been proposed during preliminary design stages.

Piute Dam