Procedural Manual 77-2: Floodplain Management
Action: Any Federal activity, including, but not limited to, acquiring, managing, and disposing of Federal lands and facilities; facilitating human occupation or visitation; providing Federally undertaken, financed, or assisted construction and improvements; and conducting Federal activities and programs affecting land use, including but not limited to, water and related land resources planning, and regulating and licensing activities.
Alluvial Fan: The land counterpart of a delta; fan assemblage of sediments marking the place where a stream moves from a steep gradient to a flatter gradient and suddenly loses its transporting power. Typical of arid or semiarid climates, but not confined to them.
Base Flood: That flood which has a one percent or greater chance of occurring in any given year (also known as the 100-year flood). This term is used by the National Flood Insurance Program to indicate the minimum level of flooding to be used by a community in its floodplain management regulations.
Base Floodplain: The 100-year floodplain.
Channel: A natural or artificial watercourse of perceptible extent with a definite bed and banks to confine and conduct continuously or periodically flowing water.
Coastal High Hazard Area: The area usually confined to the beach in front of high bluffs or the crest of primary or foredunes, where wave impact is the most significant inducing factor. Also includes those areas subject to tsunamis.
Critical Action: Activities for which even a slight chance of flooding is too great. Examples of critical actions include schools, hospitals, fuel storage facilities, irreplaceable records, museums, and storage of archeological artifacts.
Extreme Flood: The flood considered to be the largest in magnitude possible at a site. Methods for determining extreme floods for a basin or area of concern include, but are not limited to, Probable Maximum Flood, Q Extreme, and Paleoflood Determinations. The purpose of using these methods is to delineate the area beyond which there is no risk of flooding.
Extreme Floodplain: The area inundated during an extreme flood.
Flash Flood: The flood that occurs in a short time interval (minutes to hours) following the a causative event, and for which there is insufficient time for persons on-site to become aware of the flood and safely evacuate.
Flood or Flooding: A general and temporary condition of partial or complete inundation of normally dry land areas from the overflow of inland and/or tidal waters and/or the unusual and rapid accumulation of runoff of surface waters from any source.
Floodplain: The lowland and relatively flat areas adjoining inland and and coastal waters, including flood-prone areas of offshore islands including flood-prone areas of offshore islands and and (including at a minimum), that area subject to temporary inundation by a regulatory flood.
Geomorphically Active Zones: Zones where flood-induced erosion, sediment deposition, or sediment transport processes present a clear hazard to structures or facilities, or present a danger to persons occupying the site. Geomorphically active zones include hillslopes subject to mass-failure if undercut by floods, arroyo banks, eroding river terraces, alluvial fans, and beaches.
High Hazard Areas: Those portions of riverine or coastal floodplains nearest the source of flooding or areas subject to flooding events which are so unexpected, violent, or otherwise devastating that human lives are placed in immediate and grave danger. High Hazard Areas include, but are not limited to, areas subject to flash flooding; areas where floodwaters exert their maximum force; areas behind unsafe or inadequate levees; areas below dams known to be structurally unsound; areas from which escape would be difficult; areas near or on alluvial fans; and coastal high hazard areas.
Hydraulic and Hydrologic Hazards: Hazards to human life or property caused by the conditions of flow (e.g., deep water; high velocities; debris loads; ), or by the characteristics of flooding (e.g., rate of flood rise; rapidity of response to causative events).
Mitigation: Measures that serve to minimize the potential of flooding or the adverse impacts of actions in floodplains. These measures may be used for either present or planned actions. Examples include:
- dikes and/or conveyance ditches built to divert and/or carry flood flows away from the site;
- dams built to impound flood flows above the site and designed to release the captured waters at a slow, moderate rate which will not result in flooding of the site;
- modification of structures to provide sufficient elevation above the flood crest (e.g., placing structures on columns, walls, piles, or piers);
- restoration of watershed conditions to eliminate accelerated runoff caused by soil compaction, poor vegetation cover, or the unnatural conveyance of water by roads, ditches, or trails;
- closure of the area to the public during seasons which historically produce flood events;
- replacement or compensation for lost natural floodplain values;
- development of an adequate flood warning system which monitors one or more physical parameters (e.g., rainfall, runoff, streamflow) and provides warning of an impending flood to visitors and park personnel with adequate time to permit evacuation; and
- signs, high water indicators, and other information indicating that a site is flood-prone and suggesting appropriate actions in the event of flooding.
Natural Floodplain Values: Attributes of floodplains which contribute to ecosystem quality, including, but not limited to, soils, vegetation, wildlife habitat, dissipation of flood energy, sedimentation processes, and ground water (including riparian ground water) recharge. Periodic disturbance of natural floodplain soils and geomorphic and vegetation attributes by floods also contributes to ecosystem quality.
Practicable: Capable of being done within existing constraints. The test of what is practicable depends upon the situation and includes consideration of pertinent factors such as local environmental conditions, cost, or technology.
Preliminary Floodplain Assessment: A subjective, reasoned determination by a qualified professional of the existence of flood potential at a site. Preliminary floodplain assessments are based upon an analysis of site characteristics including hydrology, channel type and capacity, geomorphology, topography, vegetation, climatic data, watershed characteristics, and existing published information.
Recurrence Interval: The average time interval between occurrences of a hydrological event of a given or greater magnitude.
Regulatory Floodplain: The specific floodplain that is subject to regulation by Executive Order 11988 and this procedural manual. For Class I Actions, the Base Floodplain (100-year flood) is the regulatory floodplain; for Class II Actions, the 500-year floodplain is the regulatory floodplain; for Class III Actions, the Extreme floodplain is the regulatory floodplain.
Restore: To re-establish a setting or environment in which the natural functions of the floodplain can again operate.
Structures: Walled or roofed buildings, including mobile homes, and gas or liquid storage tanks that are primarily above ground.Procedural Manual 77-2: Floodplain Management Table of Contents | RM#77 Table of Contents