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.
November 29, 2001
Notice of Continuation July 12, 2001
73-5a