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Procedural Manual 77-2: Floodplain Management

VI. Procedures


A. Introduction

The general procedure for implementation of NPS floodplain policy is depicted in Figure 1. To implement this policy, a proposed action is to be classified into one of three classes. A Statement of Findings will be prepared if the proposed action falls within the regulatory floodplain applicable to that class of actions. A preliminary floodplain assessment is conducted to determine if the proposed action has a chance of being located in the applicable regulatory floodplain. If there is no chance that the proposed action will be located in the applicable regulatory floodplain, there are no further requirements of this procedural manual.

If there is a chance that the proposed action will be in the applicable regulatory floodplain, then that floodplain must be determined. If it is determined, based on that delineation, that the proposed action will be located in the applicable regulatory floodplain, then additional actions are required as set out in this procedural manual. Flood conditions and associated hazards must be quantified; appropriate actions (an alternative site, or effective mitigation and/or warning and/or evacuation planning) must be taken to manage floodplain conditions and flood hazards; and a formal Statement of Findings must be prepared.

Each of the following sections specifically relates to an item on Figure 1.

B. Determine the Action ClassA determination must be made as to which class the proposed action falls into:Class I Actions include location or construction of administrative, residential, warehouse, and maintenance buildings; non-excepted parking lots; or other man-made features which by their nature entice or require individuals to occupy the site, are prone to flood damage, or result in impacts to natural floodplain values. Class I Actions are subject to the floodplain policies and procedures if they lie within the 100-year floodplain (the Base Floodplain).

Class II Actions include any activity for which even a slight chance of flooding is too great. Class II Actions are subject to the floodplain policies and procedures if they lie within the 500-year floodplain. Examples of Class II Actions are the location or construction of:

  • Schools, hospitals, clinics, or other facilities occupied by people with physical or medical limitations;
  • Emergency services;

    Fuel storage facilities, 40,000 gallons per day or larger sewage treatment plants, and storage of toxic or water-reactive materials, including hazardous materials; and

  • Irreplaceable records, museums, and storage of archeological artifacts.

Class III Actions include Class I or Class II Actions in high hazard areas, which include coastal high hazard areas and areas subject to flash flooding. In high hazard areas, picnic facilities, scenic overlooks, foot trails, and associated day-time parking facilities may be placed within the 100-year floodplain, but these facilities must contain signs informing visitors of flood risk and suggested actions in the event of flooding. Consideration should be given to providing additional levels of flood protection. For other activities, Class III Actions are subject to the floodplain policies and procedures if they lie within the extreme floodplain.

C. Determine the Regulatory Floodplain

Action Class
Regulatory Floodplain
Chance of Flooding during One Year
Chance of Flooding during Fifty Years
Class I
100-year(Base Floodplain)
1%
39%
Class II
500-year
0.2%
10%
Class III
Extreme Flood
0%
0%

D. Perform Preliminary Floodplain AssessmentA preliminary floodplain assessment can be made as part of a general floodplain inventory or on a case-by-case basis. Flood-prone areas subject to or potentially subject to Class I, II, or III Actions should be inventoried and assessed during the preparation of the Resource Base Inventory preceding the General Management Plan or other planning documents. The preliminary assessment can be accomplished using existing floodplain delineations or by conducting a site assessment using qualified hydrologists, engineers, planners, or scientists. Where existing floodplain delineations are not available, the preliminary assessment should be based upon an analysis of such factors as vegetation, topography, soils, geomorphic features and processes, stream type, watershed characteristics, and other factors. The objective is to determine if there is any chance that the site might be flood-prone. If, based upon the preliminary floodplain assessment, it can be determined that there is no chance that the site is subject to flooding, then there are no further requirements under this procedural manual.

The preliminary assessment should also include associated hydrologic factors such as the rate of flood water rise, duration of flooding, likely sediment and debris loads, potential pollution hazards, and hazards associated with ice and/or debris jams. Geologic and geomorphic stability also should be evaluated, including the potential for sediment deposition, bank erosion and channel realignments, alluvial fan processes, mudslides, and other contingencies. In the case of coastal sites, areas susceptible to high tides, storm waves, tsunami, beach erosion, and other factors should be described and evaluated.

E. Delineate the Regulatory Floodplain

If the proposed action is determined to be in a potentially flood-prone site, the regulatory floodplain that corresponds to the applicable Action Class should be delineated on a map of sufficient scale to meet all planning needs. Or, if a map is unavailable, some determination of regulatory flood elevation relative to the proposed action should be made. The delineated floodplain should correspond to the elevation, on the land's surface, and location of the maximum extent of inundation by the regulatory flood. Flood magnitudes (e.g., 100-year; 500-year) should be determined by commonly accepted flow-frequency analysis procedures, regional hydrologic analysis procedures, or hydrologic modeling procedures. The extreme flood magnitude should be determined by any one of several accepted extreme flood procedures. The elevation of the regulatory flood should be determined using commonly accepted procedures, implemented by qualified professionals. All methods should be properly referenced.

F. Develop Information on Flood Conditions and Hazards

In addition to delineating the regulatory floodplain, floodplain management decisions are to be based upon information on the hydrologic and geomorphic processes and hazards associated with the location of the proposed activity. Flood hazard information should be developed and should include an analysis of flooding frequency at the proposed activity site, the probability of flooding over the planned project life, and the hydraulic attributes associated with the regulatory flood at the proposed activity site (flood depth and velocity). Further descriptions and evaluations of the hydrologic factors outlined in Section VI D will be made to fully assess hazards of those contingencies.

G. Design Actions to Manage Flood ConditionsAfter determining the Action Class and Regulatory Floodplain and evaluating hydrologic, hydraulic, and geomorphic hazards associated with the site of the proposed action, it is necessary to take effective actions to protect floodplain natural and cultural resources, and mitigate flood hazards to human life and property. Those actions will include selection of an alternative (non-floodplain) site, structural or other forms of mitigation, and/or flood warning and evacuation plans. The following guidance is provided in developing appropriate floodplain management actions.Alternative site determinationsNon-flood-prone sites should be identified and evaluated for all proposed actions when it is determined that the action will occur in a regulatory floodplain. If practicable alternative sites are identified, it is NPS policy as set forth in DO #77-2, the Executive Order, and this procedural manual to give preference to locating, or relocating, the proposed action at an alternative site outside and not affecting the regulatory floodplain. In the event that an alternative site outside and not affecting the regulatory floodplain is selected, no further requirements under this manual apply. When pre-existing actions are relocated from a regulatory floodplain to an alternative site, restoration of floodplain values should be factored into the relocation action.MitigationMeasures may be applied to proposed actions to provide for their location in the regulatory floodplain when it is documented in a Statement of Findings that practicable alternative locations are not available and the importance of the location clearly outweighs the policies set forth in Section II of this procedural manual. Generally the mitigation should provide protection up to the level of the applicable regulatory floodplain. Mitigation measures may also be applied if other substantive management considerations exist which clearly favor locating an action in a regulatory floodplain. Mitigation may consist of any combination of seasonal closure, structural flood protection measures, specific actions to minimize impacts to floodplain natural resource values, effective flood warning, and flood evacuation.Structural flood protection measures must be professionally engineered to effectively manage flood hazards as quantified according to Section VI-F of this procedural manual. Effective structural flood proofing is very difficult to achieve for geomorphically unstable sites such as alluvial fans and active beach zones.Flood warning and evacuation planning must be designed and determined to be adequate to manage flood hazards as quantified according to Section VI-F of this procedural manual. Effective flood warning and evacuation planning is very difficult to achieve for flash flood-prone, high hazard locations.

Some mitigation measures may result in the modification of floodplains to such an extent that the elevation of the regulatory flood will be affected. In these situations, additional hydraulic analyses may be required to quantify flood hazards under mitigated conditions.

Procedural Manual 77-2: Floodplain Management Table of Contents | RM#77 Table of Contents
update on 02/05/2004  I   http://www.nature.nps.gov/rm77/pm77_2/procedures.cfm   I  Email: Contact Us
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