Glossary

A fish ladder (or fish way, fish pass) is a structure designed to allow fish the opportunity to migrate upstream and continue their function as part of the river ecosystem. Critical components to determine when a ladder is necessary are the water depth below the blockage, the water velocity, and the type of species that need to pass. Many design factors are looked at by engineers when designing and placing a structure. Every blockage in a river represents a unique situation and challenge, and each fish ladder is therefore carefully designed and placed. Each species has different physical characteristics that need to be taken into consideration when designing passage facilities. Flows, energy dissipation, resting areas, drop between pools, attraction velocities, entrance eddies, and space in pools are just some of the factors that need to be considered when designing a ladder.

A flood that reaches its peak flow in ashort length of time (hours or minutes) after thestorm or other event causing it. Often characterizedby high velocity flows.

The maximum stage or elevationreached or expected to be reached by the waters ofa specific flood at a given location.

The length of time a stream is aboveflood stage or overflowing its banks.

Actions taken immediately before orduring a flood to protect human life and to reduceflood damages such as evacuation, emergencysandbagging and diking, and provision ofassistance to flood victims.

The process of predicting theoccurrence magnitude and duration of animminent flood through meteorological andhydrological observations and analysis.

A statistical expression of theaverage time period between floods equaling orexceeding a given magnitude. For example, a100-year flood has a magnitude expected to beequaled or exceeded on the average of once everyhundred years; such a flood has a one-percentchance of being equaled or exceeded in any givenyear. Often used interchangeably with 'recurrenceinterval.'

The portion of the floodplain outsideof the floodway but still subject to flooding. Sometimes referred to as 'floodway fringe.'

An officialmap of a community, issued or approved by theFederal Emergency Management Agency, FederalInsurance Administration, on which the boundariesof the floodplain and special flood hazard areashave been designated. This map is preparedaccording to the best flood data available at thetime of its preparation, and is superseded by theFlood Insurance Rate Map after more detailedstudies have been completed.

An official mapof a community issued or approved by the FederalEmergency Management Agency, Federal InsuranceAdministration, that delineates both the specialhazard areas and the risk premium zones applicableto the community.

A zone identified on aFlood Insurance Rate Map (FIRM) as subject to aspecified degree of flood, mudslide (i.e., mudflow) or flood-related erosion hazards, to which aparticular set of actuarial rates and floodplainmanagement requirement applies.

A study, funded by theFederal Emergency Management Agency, FederalInsurance Administration, and carried out by anyof a variety of agencies and consultants, todelineate the special flood hazard areas, base floodelevations, and NFIP actuarial insurance ratezones. The study is based on detailed site surveysand analysis of site-specific hydrologiccharacteristics.

Temporary inundation ofnormally dry land areas from the overflow ofinland or tidal waters, or from the unusual andrapid accumulation or runoff of surface watersfrom any source. The rise in water may be causedby excessive rainfall, snowmelt, natural streamblockages, wind storms over a lake or anycombination or such conditions.

A graph showing the relationship ofwater surface elevation to a specific location, thelatter generally expressed as distance above themouth of a stream of water flowing in an openchannel. It is generally drawn to show surfaceelevation for the crest of a specific magnitude offlooding, but may be prepared for conditions atany given time or stage.

Keeping flood waters away fromspecific developments or populated areas by theconstruction of flood storage reservoirs, channelalterations, dikes and levees, bypass channels, orother engineering works.

The issuance and dissemination ofinformation about an imminent or current flood.

Any normally dry land area that issusceptible to being inundated by water from anynatural source. This area is usually low landadjacent to a river, stream, watercourse, ocean orlake.

The operation of a programof corrective and preventive measures for reducingflood damage, including but not limited to floodcontrol projects, floodplain land use regulations, floodproofing of buildings, and emergencypreparedness plans.

General term applied to thefull range of codes, ordinances and otherregulations relating to the use of land andconstruction within floodplain limits. The termencompasses zoning ordinances, subdivisionregulations, building and housing codes, encroachment laws and open area (space) regulations.

Any combination of structural andnonstructural additions, changes, or adjustments toproperties and structures which reduce or eliminateflood damage to lands, water and sanitaryfacilities, structures, and contents of buildings.

The channel of a watercourse and thoseportions of the adjoining floodplain required toprovide for the passage of the selected flood(normally the 100-year flood) with an insignificantincrease in the flood levels above that of naturalconditions. As used in the National FloodInsurance Program, floodways must be largeenough to pass the 100-year flood without causingan increase in elevation of more than a specifiedamount (one foot in most areas).

The folding floodwall consists of a flood barrier which is hinged along the bottom so that it can be lowered to a horizontal position to form a walk, or to fit flush with existing ground or pavement. A floodwall in Monroe, Louisiana is based on this concept.

The part of the shore lying between the crest of the seaward berm (or upper limit of wave wash) and the water's edge, that is ordinarily traversed by the up rush and backrush of the waves.

A factor of safety expressed in feet abovea design flood level for flood protective or controlworks. Freeboard is intended to allow for all of theuncertainties in analysis, design and constructionwhich cannot be fully or readily considered in ananalytical fashion.

The period of time the structure performs as intended. Performance can be expressed in terms of benefits obtained versus the cost of maintenance.