Dam safety technical notes

These dam safety resources aid in assessing potential inundation area, design performance goals, and design storm construction.

  • DSO Technical Note 1: Dam break inundation analysis and downstream hazard classification — This document provides assistance in conducting a dam break inundation analysis and in assessing the downstream hazard posed by a dam failure.
    • Tech Note 1 spreadsheets and guidance – Dam breach calculation spreadsheet - This zip file contains Microsoft Excel spreadsheets using the equations from technical note 1, including calculations for both the dam breach hydrograph and downstream routing of the dam breach flood. Template spreadsheets and worked examples are provided, in both metric and U.S. customary units, along with a separate set of instructions and tips (Instruct.doc) for using the spreadsheets.
  • DSO Technical Note 2: Selection of design performance goals for critical project elements — Critical project elements are those whose failure could result in dam failure and an uncontrolled release of the reservoir. This technical note provides guidance in selecting design/performance goals for critical project elements. It has application in design of proposed dams and in evaluating the adequacy of existing dams.
    • Tech Note 2 spreadsheets and guidance - This zip file contains a Microsoft Excel spreadsheet using the summary table from technical note 2, appendix B. Its purpose is to allow the tally of the effects of a dam breach on downstream structures, using a rating scheme. This tally is then used to calculate the downstream hazard probability, and from this the associated Design Step.

      The correct Design Step allows the determination of the Design Storm precipitation, as outlined in technical note 3, data which is used to calculate the Inflow Design Flood (IDF) off the impoundment’s watershed, into its reservoir, and through its emergency spillway. In effect, the Design Step is used to size the emergency spillway.

      A template spreadsheet is provided, allowing the user to input data concerning various downstream facilities and structures. This input is automatically converted to a summation of the total consequence rating points for the project, subsequently used to calculate the Design Step. A separate worksheet is provided within the Excel workbook, with instructions for using the calculation spreadsheet.
  • DSO Technical Note 3: Design storm construction — This document provides engineering guidance for development of design storm hyetographs, the standard form of precipitation data input used in computing an Inflow Design Flood (IDF) with rainfall-runoff computer models.  Additional calculation tools to develop an IDF are provided in the two files below:
    • Precipitation Calculator — This zip file contains an executable file that installs the Precipitation Calculator to perform design rainfall depth calculations using gridded rainfall data. This software is a complete redesign of the prior precipitation depth calculator, but there is no change to the gridded data itself, and should essentially produce the same results as the previous calculator at any given geographic coordinate in the existing database. NOTE: This 25 MB file may take time to download on slower Internet connections.
    • Storm Hyetographs – This zip file contains additional materials supporting Technical Note 3, including storm hyetographs, Design Step estimating spreadsheet, freeboard estimator, PMP estimation graphs (1-hr and 24-hr), NRCS Storm calculation information, and general instruction for using the Precipitation Calculator.

The following hot links open maps that are excerpted and adapted from HMR-57 (NWS, 1994) for 1-hour and 24-hour PMP.  These files are also included in the Storm Hyetographs download:

  • PMP 1-hr — 1-hour local storm PMP, inches
  • PMP 24-hr — 24-hour general storm PMP, inches

Recommended protocols for using NRCS curve numbers (CNs) with the dam safety short design storm:

  • Short storm curve numbers document and spreadsheets — This zip file contains recommended protocols for using NRCS CNs with the dam safety short design storm (file named T-storm CN's.pdf), plus Microsoft Excel spreadsheets to perform the calculations.