DAR File No. 40175

This rule was published in the February 15, 2016, issue (Vol. 2016, No. 4) of the Utah State Bulletin.

Natural Resources, Water Rights

Rule R655-11

Requirements for the Design, Construction and Abandonment of Dams

Notice of Proposed Rule


DAR File No.: 40175
Filed: 01/30/2016 12:28:22 PM


Purpose of the rule or reason for the change:

The current rule requires clarification and an update to current industry practice.

Summary of the rule or change:

Section R655-11-4A is updated to current industry practice. Section R655-11-4B is clarified and updated to current industry practice. Section R655-11-5A is clarified and updated to current industry practice. Section R655-11-6A is clarified and updated to current industry practice. Section R655-11-6D is clarified and updated to current industry practice. Section R655-11-7C is clarified and updated to current industry practice.

State statutory or constitutional authorization for this rule:

  • Title 73, Chapter 5a

Anticipated cost or savings to:

the state budget:

No cost involved--Clarification of processing does not require a dollar figure.

local governments:

No cost involved--Clarification of processing does not require a dollar figure.

small businesses:

No cost involved--Clarification of processing does not require a dollar figure.

persons other than small businesses, businesses, or local governmental entities:

No cost involved--Clarification of processing does not require a dollar figure.

Compliance costs for affected persons:

No cost involved--Clarification of processing does not require a dollar figure.

Comments by the department head on the fiscal impact the rule may have on businesses:

No fiscal impact--Clarification of processing does not require a dollar figure.

Michael Styler, Executive Director

The full text of this rule may be inspected, during regular business hours, at the Division of Administrative Rules, or at:

Natural Resources
Water RightsRoom 220
SALT LAKE CITY, UT 84116-3154

Direct questions regarding this rule to:

  • Marianne Burbidge at the above address, by phone at 801-538-7370, by FAX at 801-538-7467, or by Internet E-mail at marianneburbidge@utah.gov

Interested persons may present their views on this rule by submitting written comments to the address above no later than 5:00 p.m. on:


This rule may become effective on:


Authorized by:

Michael Styler, Executive Director


R655. Natural Resources, Water Rights.

R655-11. Requirements for the Design, Construction and Abandonment of Dams.

R655-11-4A. Inflow Design Hydrograph Determination.

A) In Utah, the IDF for all High and Moderate Hazard Dams will be the more critical SEF. It will be necessary to calculate both the 72 hour , general SEF using HMR49 [ / ] with USUL as well as the 6 hour local SEF using HMR49 [ / ] with USUS. These precipitation values need not exceed values calculated using HMR49 exclusively. Both of these hydrographs must be routed through the reservoir to determine which one represents the most extreme event.

B) Once the critical SEF has been determined, it must be compared to a flood generated by the 100 year, 6 hour (for local storms), or 100 yr, 24 hour (for general storms) precipitation applied on a saturated watershed. If the routed 100 year event, including appropriate allowances for freeboard, is more critical than the SEF it must be used as the minimum IDF. This 100 year flood should also be used as the IDF for all Low Hazard Dams.


R655-11-4B. Freeboard Requirements.

All high and moderate hazard dams must have a normal freeboard above the crest of the principal spillway capable of 1) routing the IDF, 2) containing the maximum wave action , 3) containing the combined precipitation and wind event detailed in this paragraph and 4) the normal freeboard will be no less than three feet. Wave action will be determined considering site wind-duration and fetch control characteristics. Wave action includes wave height, [ and ] maximum runup, and [ as well as ]reservoir setup against the embankment slope. Unless otherwise justified by specific data acceptable to the State Engineer, [ an extreme ] the maximum wave action will be based on a wind velocity (fastest mile) over land of 100 miles per hour[ should be considered ]. In addition, while routing the 100 year precipitation event through the spillway, sufficient residual freeboard must be available to control wave action from a fetch controlled 50 miles per hour wind. [ In no case will the normal freeboard be less than three feet for high and moderate hazard dams. ] Low hazard dams must have sufficient freeboard to allow the spillway to route the applicable 100 year flood. The State Engineer may reduce the three feet minimum freeboard requirement for low hazard dams based upon a review of the relative increase in risk associated with this reduction.


R655-11-5A. Geological and Seismic Study.

A review of the seismic or earthquake history of the region will be performed to establish the relationship of the site to known faults and epicenters. This will be based primarily on review of existing maps and technical literature and should include major earthquakes during historic time, epicenter locations and magnitudes, and the location of any major or regional fault traces. Geologic conditions at or near the dam site that might indicate recent fault or seismic activity should be included. Resulting design earthquakes and associated site ground motion parameters will be selected considering all available evidence including tectonic and seismological history. The ground motion parameters to be selected for the site will consist of those that are needed by the analyses that are appropriately selected for design and may include peak accelerations, velocities, displacements, response spectra, and acceleration time histories. Both the Maximum Credible Earthquake (MCE) and the Operating Basis Earthquake (OBE) will need to be investigated for all projects. The MCE should be evaluated [ from the following analyses: ] using both deterministic and probabilistic methods.

1. A deterministic analysis from active faults in the region surrounding the dam will be performed to estimate magnitude and ground motion parameters. High and moderate hazard dams will be evaluated using ground motion parameters that are at least equal to mean plus 1 standard deviation predictions (84th percentile). At the discretion of the State Engineer, these values may be reduced to mean (50th percentile) for moderate hazard dams. Low hazard dams will be evaluated using ground motion parameters that are at least equal to mean (50th percentile) predictions. [ Magnitude estimates will consider ] Evaluation of the impacts on the dam from more than one source, including the potential for multi-segment rupture for segmented faults may be necessary .

2. A probabilistic analysis will be performed. The most recent United States Geological Survey (USGS) Interactive Deaggregation tool found on the USGS website, using a 5,000 year return interval, can be used to identify magnitude and [ peak ]ground motion[ s. ] parameters for high and moderate hazard dams. At the discretion of the State Engineer, a 2,500 year return interval can be used for moderate hazard dams. A 1,000 year return interval can be used for low hazard dams. Site specific evaluations may be performed to define ground motions for [ this ] these events if the methods used and assumptions made are acceptable to the State Engineer. Unless waived by the State Engineer, the minimum earthquake magnitude shall be 6.5.[ At the discretion of the State Engineer, the OBE requirement may be waived. ]

3. The OBE will be determined by probabilistic methods acceptable to the State Engineer and may include the use of the Deaggregation tool on the USGS website with a 200 year return interval. An OBE evaluation is not necessary for a low hazard dam.

4. Regardless of the assigned hazard rating, the seismic design parameters for flood control dams may be reduced at the discretion of the State Engineer, in consideration of unique operating conditions.


R655-11-6A. Factors of Safety.

A. All dams should meet the following criteria for factors of safety under normal loading conditions.



Minimum Factor
Condition of Safety

End of Construction Case--upstream and
downstream slopes 1.3
Steady State Seepage--upstream and downstream
slopes (full pool) 1.5
Instantaneous Drawdown--upstream slope 1.2
Actual Drawdown--upstream slope 1.5


B. All factors of safety should be generated by methodology acceptable to the State Engineer. In undertaking the analysis, the effects of anisotropy should be considered and a ratio of horizontal to vertical permeability of at least nine should be used in the seepage analysis, unless otherwise justified to the satisfaction of the State Engineer. Ratios of up to 100 should be considered if the material types and construction techniques will cause excessive stratification.

C. The strengths used in the stability analysis should be obtained from tests which best model the situation being analyzed.

D. The analysis of the upstream slope stability for actual drawdown should consider drawdown rates which the low level outlets are capable of generating. Actual residual pore pressures should be used.

E. For low hazard dams the State Engineer may waive the requirements of a stability analysis, including a seismic analysis, if it can be demonstrated that conservative slopes and competent materials are used in the dam, and seismic problems (i.e., liquefiable materials, active faults close to the dam) are not present.[

F. Stability evaluations where residual strengths are used must have a minimum factor of safety of 1.3. ]


R655-11-6D. Internal Erosion Control.

A. All dams should have design provisions for controlling internal erosion. In zoned dams all adjacent zones must meet filter criteria with the abutting zones and foundation soils. If filter criteria cannot be met, a transition zone must be provided.

B. All filter and drainage zones in a dam must meet criteria acceptable to the State Engineer.

C. In designing filter zones where dispersive clays or broadly-graded materials exist, special considerations may be imposed by the State Engineer.

D. All [internal]chimney filter and drainage zones will have a minimum width of three feet per zone [to facilitate construction]unless waived by the State Engineer. Wider zones are encouraged[,]. Chimney drains may be vertical or inclined, but inclined drains may require additional width. [especially i] In active seismic areas filter widths must be at least twice the predicted lateral deformation resulting from an earthquake.

E. Proper filtering and drainage is essential in all dams where cracking from differential settlement, hydraulic fracturing, or earthquake shaking is possible. Chimney, blanket, and toe drains are considered to be standard design measures. Justification must be provided if these features are not included in the design. Other filter and drainage features may also be appropriate.


R655-11-7C. Outlet Details.

A. All outlets shall have a trash rack to prevent clogging.

B. All outlets connected directly to a downstream pipeline shall have an emergency bypass valve.

C. All outlets shall have a suitable energy [dissipator]d issipater at the discharge end to prevent erosion of the downstream channel.

D. All outlets will be placed on a concrete cradle or encased in concrete unless specifically exempted by the State Engineer in writing. All conduits made of plastic materials will be fully encased. The sequencing and construction methods for secure placement of the conduit to prevent movement during pressure testing and concrete placement must be included in the design documents.

E. All outlets, with the exception of ungated outlets, shall have an operating gate or a guard gate on the upstream end.

F. All outlets shall have seepage control measures to reduce the potential for piping along the conduit. Common methods may include locating the outlet conduit in bedrock and installing a conduit filter drain to intercept seepage. Where possible, the outlet should penetrate the chimney drain so it acts as the conduit filter drain. Where an individual filter drain is used, it must have sufficient lateral extent to also protect against localized embankment cracking as well as seepage along the conduit. The use of cutoff collars is not an approved method.

G. Outlets encased or cradled in concrete should have battered sides to facilitate compaction against the [ encasement ] concrete, unless approval is given by the State Engineer to place the conduit in a trench.

H. Every attempt should be made to locate the outlet on bedrock or consolidated materials. In the event this is not possible, consideration should be given to articulating the outlet to allow for settlement.

I. Outlet gates and valves can be either mechanically or hydraulically operated. In either case the hydraulic lines or mechanical stems must be adequately protected from debris, wave action, settlement, and ice damage. Buried stems should be encased in an oil-filled pipe supported on pedestals. No catwalks or similar access structures will be allowed on reservoirs where freezing occurs or significant floating debris is present. All outlets which are operated with [ electrical ] motorized equipment must have back-up [ generating ]capability or a manual bypass system capable of being operated in a reasonable amount of time.

J. All outlets shall be properly vented. A vent pipe and air manifold around the perimeter of the conduit immediately downstream of the gate will be required unless waived by the State Engineer. The air supply lines should be conservatively sized for the anticipated flows and protected in the same manner as the outlet control lines or stems.

K. All operators and supporting equipment for outlet controls should be properly protected and secured. Particular attention needs to be given to protection from vandals and unauthorized operation. All outlet controls should be clearly marked as to which way the gates and valves operate so that overloading of a closed gate or valve should not occur.

L. Outlet controls should be accessible when the spillways are in use.


KEY: dams, earthquakes, floods, reservoirs

Date of Enactment or Last Substantive Amendment: [October 24, 2012]2016

Notice of Continuation: April 14, 2011

Authorizing, Implemented, or Interpreted Law: 73-5a


Additional Information

More information about a Notice of Proposed Rule is available online.

The Portable Document Format (PDF) version of the Bulletin is the official version. The PDF version of this issue is available at https://rules.utah.gov/publicat/bull-pdf/2016/b20160215.pdf. The HTML edition of the Bulletin is a convenience copy. Any discrepancy between the PDF version and HTML version is resolved in favor of the PDF version.

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For questions regarding the content or application of this rule, please contact Marianne Burbidge at the above address, by phone at 801-538-7370, by FAX at 801-538-7467, or by Internet E-mail at marianneburbidge@utah.gov.  For questions about the rulemaking process, please contact the Division of Administrative Rules.