Dam construction & modification guidance

Project planning and approval

Dam safety guidelines

• Part I – General Information and Ownership Responsibilities — This document contains general information about statutory authority, dam safety activities, and dam owner responsibilities for completed projects.
• Part II – Project Planning and Approval — This document provides information on the specific activities and actions that are involved in project planning and in the plan review and construction permit process. An overview of the planning process is provided along with a list of permits and approvals that are normally involved. Submittal requirements and obligations of dam owners during project construction, operation, maintenance, and inspection are identified.
• Suggested table of contents for dam safety engineering reports — For engineers with limited experience submitting engineering reports to us, we provide a suggested table of contents to share a framework for compiling the information requested in the guidelines (part II). A condensed table of contents is provided, along with an annotated version to provide more details about each section of the report.
• Supplemental guidance for construction inspection plans — The construction inspection plan is a required submittal for the dam construction permit and approval of the plans and specifications for construction or modification of a dam. Guidance is offered for major sections of the inspection plan and target lengths for the various sections, a matrix of construction activities and inspections and quality assurance testing, and critical stages of construction that we are particularly interested in observing. An example construction inspection plan is included.

Dam safety technical notes

• 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.
  • 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.
• 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.
  • Design Step calculation spreadsheet — 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.
• Technical note 3: Design storm construction — This document provides engineering guidance for development of design storms for use in computing inflow design floods using rainfall-runoff computer models for spillway design.
  • Gridded data and rainfall calculator 04/28/2020 — This zip file contains updated materials supporting Technical Note 3 to perform the dam safety design storm calculations. This file has a calculator for rainfall depths plus required gridded data sets.  NOTE: This 25 MB file may take time to download on slower Internet connections.  Once you have the files, you will need to run the setup.exe to start the installation wizard to install the necessary files on your computer.
  • Storm hyetographs, spreadsheets, and documents 04/28/2020 — This zip file contains updated materials supporting Technical Note 3 to perform the dam safety design storm calculations. This file has storm hyetographs and other spreadsheets. Includes instructions and usage tips. Updated in 2020.
  • 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.

The following maps are excerpted and adapted from HMR-57 (NWS, 1994) for 1-hour and 24-hour PMP:

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

Engineering design for dams & spillways

Dam safety guidelines

• Part IV - Dam Design and Construction — This document outlines a rational engineering approach to address the majority of cases. Individual projects may pose unique problems that require specialized measures. Where engineers face such cases, it will be their responsibility to demonstrate that their approach satisfactorily addresses the pertinent engineering issues.

Technical tools & materials

• Hydraulic computations for open spillways and riprap-lined channels — Updated Oct. 2021. This zip file of Microsoft Excel spreadsheets includes a stage-discharge curve for the spillway, drawdown from critical flow to supercritical uniform flow in steep channels, and stability calculations for loose riprap. New features include provisions for using metric units, and an algorithm for estimating energy losses at the spillway overflow. Template spreadsheets and worked examples are provided, along with a set of instructions and guidance (Instruct_Spillway_2021.PDF) for using the spreadsheets. We recommend reading the instructions before using the spreadsheets.
• Hydraulic computations for drop inlet spillways  — Updated May 2022. This zip file of Microsoft Excel spreadsheets includes a stage-discharge curve for the spillway, flow surges and pressure fluctuations in the outlet pipe, air vent sizing, and downstream tailwater conditions. New features include provisions for using metric units. Template spreadsheets and worked examples are provided, along with a set of instructions and guidance (Instruct_DropIn_2022.PDF) for using the spreadsheets. We recommend reading the instructions before getting into the spreadsheets.

Design details:

• Design details for conduits through earth embankments
• Design details for air vents for drop inlet spillways