BOARD01548

• Board Meeting Date: 1/25/2005
• Description: Proposed Revisions to the Rules and Regulations for Dam Safety and Dam Construction
• Board Meetings - Doc Type: Memo

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Alternative methods for determination of IDF and Spillway size

Site Specific Hydrometeorolog1c Analysis consisting of current scientific methods to determine the
probable intensity and duration of an extreme storm for the drainage basin for the dam. This study
generally results in a 10 to 30 percent reduction in the PMP value. The results vary with basin size,
elevation and storm duration and in some cases the resulting storm is greater than PMP values.
Fourteen site-specific studies have been approved since 1992; three are currently being reviewed;
and one is in progress.

Incremental Damage Analysis (IDA) consisting of a determination of an IDF less than the
minimum requirements associated with PMP methodology. The IDA study consists of a comparison
of the downstream damage resulting from design flood without a dam and damages as a result of a
failure of the dam due to overtopping during the design flood event. Nineteen IDA's have been
approved since 1999; six resulted in reduction in the PMP requirements.

III. Proposed Revisions to Current Dam Safety Rules

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The use ofHMR's to determine IDF's generally provides conservative engineering values, a
reasonable outcome in the protection ofIife and property. The Colorado Water Conservation Board
and the State Engineer recognized lack of extreme precipitation data for the area above 7500 ft.
elevation. The CWCB and SEO initiated a study to identify extreme precipitation events and use that
information in the determination of reasonable IDF's above 7500 ft elevation. During this same
period 14 site-specific hydrometeorologic studies and 19 IDA's were completed and approved by the
State Engineer.

The results of those studies and additional evaluation of meteorological methods has resulted in
proposed revisions to the existing rules. The proposed revisions include a 10% reduction in the PMP
values derived from the HMR procedures, and additional reductions based on the average elevation
of the contributing drainage basin. The result of these reductions in PMP is a reduced IDF and thus a
reduction in the spillway size necessary to protect the dam from overtopping. The proposed
revisions are summarized below.

New Dam Construction

INFLOW DESIGN FLOOD REQUIREMENTS
(Without Adjustment for Elevation)

Hazard Classification Bil!h Sil!nificant Low NPH
Dam Size
. Large .9PMP .75 (.9 PMP) 100 YR 50YR
SmaIl .9PMP .50 (.9 PMP) 100 YR 25YR
Minor .50 (.9 PMP) 100YR 50YR 25YR