R655.  Natural Resources, Water Rights.

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

R655-11-4.  Hydrologic Design.

In order to arrive at an Inflow Design Hydrograph or Inflow Design Flood (IDF) more representative of actual conditions in Utah, the State Engineer has commissioned, or has been involved in, numerous studies to supplement the National Oceanic and Atmospheric Administration’s (NOAA) Report entitled “Hydrometeorological Report No. 49 (HMR49) - “ Probable Maximum Precipitation Estimates, Colorado River & Great Basin Drainages". The results of most of these studies are used to better identify soil conditions, discharge coefficients, and unit hydrograph parameters. The results of two of the studies are used directly to refine the calculation of the design rainfall values. Both studies were completed by Donald Jensen of the Utah Climate Center and are entitled, "2002 Update for Probable Maximum Precipitation, Utah 72 Hour Estimates to 5,000 sq. mi. - March 2003" (USUL) and " Probable Maximum Precipitation Estimates for Short Duration, Small Area Storms in Utah - October 1995 "(USUS). All of HMR49, Table 1, page 4 of USUL, and Table 15, pages 74-75 of USUS are hereby incorporated by reference. All High Hazard and Moderate Hazard dams in Utah must use the precipitation values obtained from the use of all three publications. To avoid confusion, precipitation values obtained from HMR49 exclusively will be referred to as the Probable Maximum Precipitation (PMP), while those obtained from using HMR49 in conjunction with USUL or USUS, will be referred to as the Spillway Evaluation Precipitation (SEP). The resulting hydrographs generated will be referred to as the Probable Maximum Flood (PMF) and the Spillway Evaluation Flood (SEF) respectively.


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

    A) In Utah, the IDF for all High and Moderate Hazard Dams will be the SEF. It will be necessary to calculate both the 72 hour SEF using HMR49/ USUL as well as the 6 hour SEF using HMR49/ USUS. 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 dams must have a normal freeboard above the crest of the principal spillway capable of containing the maximum wave action considering site wind-duration and fetch control characteristics.  Wave action includes wave height and maximum runup, as well as reservoir setup against the embankment slope.  Unless otherwise justified by specific data acceptable to the State Engineer, an extreme 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.  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.


R655-11-4C.  Spillways.

    In designing the spillway for a dam to pass the IDF, the State Engineer will consider the use of a principal spillway in conjunction with emergency spillways.  The principal spillway must be designed so that no structural damage will occur during passage of the IDF.  Emergency spillways, including Fuse Plug Spillways, may be designed so that some damage may be expected during use provided the anticipated damage does not represent a threat to the dam. Sunny day failure modeling of Fuse Plug Spillways may be required to determine if they are creating an additional unacceptable risk.  Overtopping of the dam will not be considered as an emergency spillway on earthfill dams, unless it can be demonstrated that the dam is protected from erosion, and the duration of overtopping will not saturate the dam and reduce its stability.


R655-11-4D.  Infiltration Rates.

The State Engineer will accept an IDF using SEP values in conjunction with soil moisture conditions representative of historical maximums.  If the design engineer is using infiltration rates which represent something less than saturated conditions, information should be submitted to justify the lower soil moisture selection.


R655-11-4E.  Flood Routing.

     A.  In routing the IDF through the reservoir, the initial water surface should reflect conservative estimates which would exist at the time of the flood event.  Unless documentation can be provided to the contrary, it should be assumed that all low level outlets are closed during routing of the IDF.  For dams receiving inflow from pipelines and supply canals, it should be assumed these additional sources are operating at capacity during the flood event.  In the event the spillway is gated or has "stop logs", which are only allowed on existing dams, documentation must be provided to show the gates are automated or operational procedures are in place to insure that the gates can be opened or the stop logs removed in a timely manner.

    B.  The SEF can be routed so the maximum water surface is at an elevation equal to the lowest point on the crest of the dam with no residual freeboard.

C. In generating the IDF, the basin characteristics used and the parameters used to generate the unit hydrograph should be based on the best information available. Unit hydrographs generated from historical records or calibrated watersheds should be used, where data is available, rather than using synthetic procedures.


R655-11-4F.  Incremental Damage Assessment for High and Moderate Hazard Dams.

     The State Engineer may, at his discretion, accept an IDF less than the SEF based on the results of an Incremental Damage Assessment (IDA) which shows that failure of the dam would cause insignificant incremental damage to property and no additional threat to human life.  The State Engineer may consider the use of early warning systems in evaluating the threat to human life.  In requesting the acceptance of an IDF determined from an IDA, documentation must be furnished that the owner of the dam is aware that the design reflects something less than the SEF and they are willing to accept the additional liability.  In no case will the State Engineer approve an IDF generated by something less than the applicable 100 year flood event.  The resulting selected IDF, based on the IDA, should be reported as a percent of the SEF.


R655-11-4G.  Historical Records.

     In some cases it may be appropriate to use historical streamflow records to generate a 100 year flood.  If these records are used as a basis for the IDF, they should be accompanied by the Synthetic IDF established by using the 100 year precipitation.  Following a review of the data, the State Engineer will make a determination of which flood will be used as the IDF.


KEY:  dams, earthquakes, floods, reservoirs

November 29, 2001

Notice of Continuation July 12, 2001


Effective Date: 12/10/2003