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wo_7520.doc - USDA Forest Service

     7520 Page 1 of 14

     FOREST SERVICE MANUAL

    NATIONAL HEADQUARTERS (WO)

    WASHINGTON, DC

    FSM 7500 WATER STORAGE AND TRANSMISSION

    CHAPTER 7520 DAM PLANNING, INVESTIGATION, AND DESIGN

Amendment No.: 7500-2011-3

Effective Date: May 13, 2011

Duration: This amendment is effective until superseded or removed.

Approved: JAMES M. PEÑA Date Approved: 05/11/2011

     Associate Deputy Chief, NFS

    Posting Instructions: Amendments are numbered consecutively by title and calendar year. Post by document; remove the entire document and replace it with this amendment. Retain this transmittal as the first page(s) of this document. The last amendment to this title was 7500-2011-2 to FSM 7510.

    7520 14 Pages New Document

    7520 10 Pages Superseded Document(s) by

    (Amendment 7500-2000-1, 9/11/2000) Issuance Number and

     Effective Date

Digest:

    7520 - Changes categorization of dams from A, B, C, and D and low, medium, and high hazard to low, significant, and high hazard potential classification throughout chapter. Recodes, reorganizes, and revises direction and makes minor technical and editorial changes throughout the chapter.

7520.2 - Establishes code, caption, and adds cross-reference to 7502.

    7520.3 - Establishes code, and recodes to this section caption and policy statement previously set out in section 7521.03.

WO AMENDMENT 7500-2011-3 7520 EFFECTIVE DATE: 05/13/2011 Page 2 of 14 DURATION: This amendment is effective until superseded or removed.

    FSM 7500 - WATER STORAGE AND TRANSMISSION

    CHAPTER 7520 - DAM PLANNING, INVESTIGATION, AND DESIGN

Digest--Continued:

    7520.5 - Establishes code, caption “Definitions,” and sets forth terminology specific to this chapter.

7521 - Changes caption from “Scheduling” to “Scheduling of Dam Management and

    Engineering.”

    7521. 03 - Removes code, and recodes caption and policy statement to 7520.3.

7523.05 - Removes code, caption, and obsolete terms.

    7524 thru 7524.22 - Removes captions and direction and recodes to section 7525. Establishes captions and sets forth direction for “Review and Approval of Proposed Projects.”

    7525 thru 7525.49 - Removes captions and direction and recodes to 7526. Recodes to this section captions and direction previously set out in section 7524.

7525.43 - Changes caption from “Earthquake Design” to “Seismic Design” and revises direction.

    7526 - Establishes code and recodes to this section caption and direction previously set out in section 7525.

WO AMENDMENT 7500-2011-3 7520 EFFECTIVE DATE: 05/13/2011 Page 3 of 14 DURATION: This amendment is effective until superseded or removed.

    FSM 7500 - WATER STORAGE AND TRANSMISSION

    CHAPTER 7520 - DAM PLANNING, INVESTIGATION, AND DESIGN

    Table of Contents

    7520.2 - Objectives .................................................................................................................4 7520.3 - Policy ........................................................................................................................4 7520.5 - Definitions ................................................................................................................4 7521 - SCHEDULING OF DAM MANAGEMENT AND ENGINEERING ......................... 4 7522 - DESIGN RECORDS ............................................................................................ 5 7523 - PLANNING .......................................................................................................... 5 7524 - REVIEW AND APPROVAL OF PROPOSED PROJECTS .................................. 5 7524.1 - Environmental Review ..............................................................................................6 7524.2 - Engineering Review of Dam Designs ........................................................................6

    7524.21 - Independent Engineering Review .........................................................................6

    7524.22 - Design Approval ..................................................................................................6 7525 - GENERAL DESIGN STANDARDS ..................................................................... 7 7525.1 - Hydrology and Flood Routings ..................................................................................7

    7525.11 - Incremental Damage Analysis ..............................................................................7 7525.2 - Hazard Assessment....................................................................................................8 7525.3 - Spillway Sizing .........................................................................................................8

    7525.31 - Spillway Sizing Standards....................................................................................8

    7525.32 - Exceptions to Spillway Sizing Standards..............................................................8 7525.4 - Investigation and Design Criteria ............................................................................. 10

    7525.41 - Site Investigations .............................................................................................. 10

    7525.42 - Structural Stability ............................................................................................. 10

    7525.43 - Seismic Design .................................................................................................. 10

    7525.44 - Geosynthetics .................................................................................................... 11

    7525.45 - Outlet Works ..................................................................................................... 11

    7525.46 - Flashboards........................................................................................................ 12

    7525.47 - Access ............................................................................................................... 12

    7525.48 - Instrumentation .................................................................................................. 12

    7525.49 - Additional Design Items .................................................................................... 12 7526 - CHANNEL PLANNING, INVESTIGATION, AND DESIGN ................................ 13 7527 - ACQUISITION OF DAMS BY THE FOREST SERVICE .................................... 14

WO AMENDMENT 7500-2011-3 7520 EFFECTIVE DATE: 05/13/2011 Page 4 of 14 DURATION: This amendment is effective until superseded or removed.

    FSM 7500 - WATER STORAGE AND TRANSMISSION

    CHAPTER 7520 - DAM PLANNING, INVESTIGATION, AND DESIGN

7520.2 - Objectives

See FSM 7502.

    7520.3 - Policy

    Ensure that the design of dams is consistent with the applicable land management plan and applicable feasibility studies, environmental impact assessments, and site investigations. 7520.5 - Definitions

For additional definitions, see FSM 7505 and FSH 7509.11, section 05.

    Design Storm. A specified amount of storm rainfall, described using the areal and

    temporal distribution of the rainfall and used to estimate a design discharge that is

    typically based on a specified recurrence period.

    Design Earthquake. The magnitude of earthquake a dam is designed to resist without

    failing, represented by specification of the free-field ground motion that would be felt at

    the dam due to the seismic event that is used as the basis for earthquake-resistant design.

    Design Maximum Credible Earthquake (MCE). The largest possible earthquake that

    could reasonably occur along recognized faults or within a particular seismic source.

    Horizontal Acceleration. The acceleration horizontally at the point of consideration

    resulting from the design earthquake.

    Incremental Damage Analysis. A comparative study of two floods of different

    magnitude that is used to quantify incremental increases in impacts on human life and

    property.

    Probable Maximum Flood. The flood flow that may be expected from the most severe

    meteorological and hydrological conditions that are reasonably possible in the area.

    Recurrence Period. The interval in which an event will occur once on average. 7521 - SCHEDULING OF DAM MANAGEMENT AND ENGINEERING

    Dam management includes coordination, budgeting, scheduling, planning, and project and construction management for dams. Dam engineering includes all aspects of design and project engineering for dams, including reviews. Schedule dam management and engineering to:

WO AMENDMENT 7500-2011-3 7520 EFFECTIVE DATE: 05/13/2011 Page 5 of 14 DURATION: This amendment is effective until superseded or removed.

    FSM 7500 - WATER STORAGE AND TRANSMISSION

    CHAPTER 7520 - DAM PLANNING, INVESTIGATION, AND DESIGN

    1. Design dams effectively and economically.

    2. Ensure compliance with program objectives, environmental concerns, and public

    safety.

    7522 - DESIGN RECORDS

    Keep design records orderly and current to allow efficient review at any stage. Design records must be complete and understandable, since they may form the basis for later actions such as design changes, modification or addition of appurtenant works, investigation of structural deficiencies, periodic safety evaluations, hazard assessment, development and updating of an EAP, and possible litigation. Scope and detail of the design record will vary with the complexity and risk of the project.

    Design-related correspondence must contain a full presentation of the pertinent facts and analysis and must document the selection of alternatives or other matters affecting the design.

    Final design documentation must include a permanent record of the design analysis and decisions and an index to the design records.

    7523 - PLANNING

    In the planning stage, consider alternate sites and structural types to meet objectives, maximize use of on-site or local materials, and minimize project costs and social and environmental impacts (FSM 1950). Constraints may include minimum stream flow requirements, fish passage and protection measures, sluice gates to pass sediment downstream to protect and maintain channel and aquatic habitats, water rights, land management plan standards and guidelines, project funds, the dam’s hazard potential classification, and environmental issues such as wetlands. The environmental analysis must consider these constraints as well as the benefits of reservoir storage. Consider flood storage for Forest Service-operated dams when the potential benefits exceed additional construction costs.

    Gather data needed to plan the design and assess design alternatives from available maps, records, and site reconnaissance. Field surveys and geological and geotechnical foundation investigations may be necessary, depending on the complexity and risks of the project. 7524 - REVIEW AND APPROVAL OF PROPOSED PROJECTS

    Plans and designs for all new dams and modification or repair of existing dams on National Forest System lands must have written approval from the authorized officer prior to construction. Approval must be based on administrative (7524.1) and engineering (7524.2) review.

WO AMENDMENT 7500-2011-3 7520 EFFECTIVE DATE: 05/13/2011 Page 6 of 14 DURATION: This amendment is effective until superseded or removed.

    FSM 7500 - WATER STORAGE AND TRANSMISSION

    CHAPTER 7520 - DAM PLANNING, INVESTIGATION, AND DESIGN

Coordinate with appropriate Federal and State agencies to ensure that:

    1. Project plans are consistent with policies in FSM 7503.

    2. Project plans include securing any needed water rights or permits from the State for

    water development or use (FSM 2540).

    3. The project is consistent with the applicable land management plan.

    4. Construction of the project can be effectively coordinated with other National Forest

    System activities in the area.

    5. The emergency action plan (EAP) is prepared, when required, during the design phase

    (FSM 7515 and FSH 7509.11, ch. 50).

    7524.1 - Environmental Review

    Ensure that the scope and complexity of the environmental analysis (FSM 1950) and administrative work are appropriate and have been completed for each project. Protection and Mitigation measures should be developed pursuant to authority granted via FLPMA (FSM 2710 and FSM 2720) and the Federal Power Act (FSM 2770) and to protect fish and wildlife habitat (FSM 2630).

    7524.2 - Engineering Review of Dam Designs

    The regional director of Engineering shall review and approve the designs for dams to be operated by the Forest Service or the holder of a special use authorization. The scope of engineering review of a dam design varies with the complexity, extent, and hazard potential classification of the dam, as well as the level of involvement of a Federal or State dam safety agency. See FSM 7504 for additional direction on design review and approval. 7524.21 - Independent Engineering Review

    All dams with a high or significant hazard potential classification must have an independent engineering review conducted by a qualified engineer (FSM 7505) not directly involved in the design process. Independent engineering reviews of dams operated by the holder of a special use authorization may be conducted by qualified Forest Service engineers, Federal and or State agencies, or professional engineering consultants not employed by the primary design firm. 7524.22 - Design Approval

    Designs of new dams and rehabilitation of dams with a high or significant hazard potential classification must be accomplished by a qualified engineer with professional experience consistent with the requirements for analyzing the size of the structure and the hazard potential classification involved.

WO AMENDMENT 7500-2011-3 7520 EFFECTIVE DATE: 05/13/2011 Page 7 of 14 DURATION: This amendment is effective until superseded or removed.

    FSM 7500 - WATER STORAGE AND TRANSMISSION

    CHAPTER 7520 - DAM PLANNING, INVESTIGATION, AND DESIGN

    Plans and specifications for new dams and rehabilitation of dams operated by the holder of a special use authorization must be approved in writing by the regional director of Engineering before construction commences. The approval documentation must contain any terms and conditions for project implementation. Design changes made after the initial review or during construction must be submitted to the regional director of Engineering for written approval at least 3 days prior to implementation.

    Designs submitted for dams to be operated by the holder of a special use authorization must be prepared and signed or stamped by a licensed engineer with professional experience consistent with the requirements for analyzing the size of the structure and the hazard potential classification involved.

    7525 - GENERAL DESIGN STANDARDS

    The level of design detail increases from preliminary to final design and with increasing size, value, and hazard potential classification of the structure. The level of detail of the geological and geotechnical investigation, design, design reports, and drawings must be adequate to construct the dam within acceptable safety parameters for its size and hazard potential classification. Utilize current U. S. Army Corps of Engineers or Bureau of Reclamation standards and procedures for dams in planning, investigating, and designing dams on National Forest System lands.

    Appropriate design standards for tailings dams include “Design and Evaluation of Tailings Dams,” EPA 530-R-94-038, and State of Idaho Rule 37.03.05. Federal or other State agencies with jurisdiction or overlapping jurisdiction may impose additional or more stringent standards for tailings dams on National Forest System lands.

    7525.1 - Hydrology and Flood Routings

    At a minimum, utilize standards and procedures established by the Bureau or Reclamation, U.S. Army Corps of Engineers, or Natural Resources Conservation Service for determining the design storm event and sizing the spillway. Any appropriate Federal and State standards may be used case by case. Conduct site-specific evaluations to develop the inflow design hydrographs. 7525.11 - Incremental Damage Analysis

    Incremental damage is the additional damage resulting from flooding with overtopping dam failure versus flooding without overtopping dam failure. Perform an incremental damage analysis to determine flood damage and potential loss of life resulting from the sudden release of water stored in the reservoir.

    In addition to flows from the reservoir, consider flows from tributaries downstream from the reservoir up to the point where no further significant incremental damage would occur.

WO AMENDMENT 7500-2011-3 7520 EFFECTIVE DATE: 05/13/2011 Page 8 of 14 DURATION: This amendment is effective until superseded or removed.

    FSM 7500 - WATER STORAGE AND TRANSMISSION

    CHAPTER 7520 - DAM PLANNING, INVESTIGATION, AND DESIGN

7525.2 - Hazard Assessment

    Determine the hazard potential classification of a dam (high, significant, or low, according to FSM 7512.1) based on flood damage from a breach of the dam in clear weather up to a point downstream from the dam where the flood is contained within the 100-year flood channel. In conducting a hazard assessment, consider the reservoir full to the crest of the uncontrolled spillway, and consider and document reasonably foreseeable downstream development as well as existing development. Calculate the time required for complete breach to occur for the type of dam and size of the dam using generally accepted methods.

    7525.3 - Spillway Sizing

    7525.31 - Spillway Sizing Standards

    In general, size the spillway so that it can accommodate the inflow design flood shown in exhibit 01. An inflow design flood that is smaller than the applicable inflow design flood listed below is acceptable if the results of an incremental damage analysis show that failure of the dam would cause insignificant incremental damage to property and no additional threat to human life (7525.32).

    7525.31 Exhibit 01

    Required Inflow Design Flood for New Dams and Dam Modifications

    Hazard Potential Classification Inflow Design Flood

    HIGH PMF*

    SIGNIFICANT 1/2 PMF

    LOW 100-YEAR RP**

    * Probable maximum flood.

    ** Recurrence period.

7525.32 - Exceptions to Spillway Sizing Standards

    The discharge produced during the failure of dams with small reservoirs is often less than the expected discharge for the probable maximum flood (PMF). For these dams, the inflow design flood generally should not exceed the peak discharge resulting from a dam failure because any

WO AMENDMENT 7500-2011-3 7520 EFFECTIVE DATE: 05/13/2011 Page 9 of 14 DURATION: This amendment is effective until superseded or removed.

    FSM 7500 - WATER STORAGE AND TRANSMISSION

    CHAPTER 7520 - DAM PLANNING, INVESTIGATION, AND DESIGN

    additional consequences resulting from larger flood events can be attributed to natural flooding. Include an appropriate freeboard value based on the potential for waves, wave run-up, future dam settlement, and future dam maintenance to ensure the dam safely withstands the selected inflow design flood.

    An inflow design flood that is smaller than the applicable inflow design flood in 7525.31, exhibit 01, is acceptable if the results of an incremental damage analysis show that failure of the dam would cause insignificant incremental damage to property and no additional threat to human life. Utilize “Federal Guidelines for Dam Safety: Selecting and Accommodating Inflow Design Floods for Dams,” April 2004, as amended, in making this determination.

    In determining whether an exception to spillway sizing standards is appropriate, consider the level of flood protection provided by the dam for populations downstream. In sizing the spillway, consider local site conditions, such as the potential for debris in the watershed, the orientation and potential for plugging of the spillway, and any other factors that affect safe operation of the spillway. Under no circumstances should the spillway capacity be reduced below the applicable minimum threshold in exhibit 01.

    7525.32 Exhibit 01

    Minimum Spillway Capacity

    For Incremental Damage Analysis

    Hazard Potential Inflow Design Minimum Spillway

    Classification Flood Capacity

    High PMF* 1/2 PMF

    Significant 1/2 PMF 100-Year RP**

    Low 100-Year RP 50-Year RP

    * Probable maximum flood.

    ** Recurrence period.

    Existing dams which can accommodate at least 75 percent of the inflow design flood do not need to be modified to increase spillway capacity to the minimum requirements in 7525.32, exhibit 01, until the dams undergo major reconstruction.

    New spillways will be located in a manner that does not prevent ground access to the dam or its appurtenances and affords maximum exposure to sunshine during the shortest day of the year. If an existing spillway is blocked by ice or snow during the winter months, determine the angle of the sun at that location during the shortest day of the year and remove all obstructions, such as vegetation or boulders, of the sun at that angle.

WO AMENDMENT 7500-2011-3 7520 EFFECTIVE DATE: 05/13/2011 Page 10 of 14 DURATION: This amendment is effective until superseded or removed.

    FSM 7500 - WATER STORAGE AND TRANSMISSION

    CHAPTER 7520 - DAM PLANNING, INVESTIGATION, AND DESIGN

7525.4 - Investigation and Design Criteria

    Apply the site investigation and design criteria in 7525.41 through 7525.48 to new dams, enlargement/improvement of existing dams, and for dams considered structurally unsafe. Any variance from these criteria must have prior written approval from the regional director of Engineering.

    7525.41 - Site Investigations

    Investigate and test a potential dam site to the extent appropriate for the size and hazard potential classification of the dam and for possible geological features that influence the risk of dam failure. Consult with an experienced professional engineer and a geologist to formulate an appropriate geotechnical and geological investigation of potential dam sites. 7525.42 - Structural Stability

    Current structural guidelines for dams developed by the U. S. Army Corps of Engineers and the Bureau of Reclamation may be used in designing side slopes made of earth for dams with a low hazard potential classification. Analyze the embankment stability for dams with a significant or high hazard potential classification to ensure that all pertinent static and dynamic loading conditions will not exceed allowable shearing stresses in the embankment or foundation. Ensure that the factors for safety and allowable shear stresses used in the design are appropriate for the construction and operating conditions for the dam.

    7525.43 - Seismic Design

    Determine the design earthquake for new dams and dams undergoing major modifications based on the dams’ hazard potential classification, as shown below:

    Hazard Annual Recurrence Period Exceedance

    Potential Exceedance in Years* Probability

    Classification Probability in 50 years

     -3Low 2 x 10 500 10%

     -4Significant 4 x 10 2,500 2%

     -4High 2 x 10 5,000 or MCE** 1%

    * Multiply the recurrence period in years by 0.75 to obtain the design earthquake recurrence period for

    existing dams. For example, for a dam with a low hazard potential classification, 500 years x 0.75 = a

    375-year design earthquake recurrence period.

    ** Use of the 5000 year event or maximum credible earthquake (MCE) is left to the discretion of the

    regional director of Engineering. Establish ground motion associated with the MCE using a deterministic

    approach.

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