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Floodway. The channel of a river or watercourse and the adjacent land areas that must be reserved to dis- charge the one-percent-probability flood without cumulatively increasing the water surface elevation more than a designated height, generally one foot. Flood Boundary and Floodway Map. An official map of a community, issued or approved by the Federal Emergency Management Agency, on which floodplain and floodway boundaries have been designated. Hydrograph. A graph that charts the passage of water as a function of time. It shows flood stages, depicted in feet above mean sea level or gage height, plotted against stated time intervals. Hydrology. The science of the behavior of water in the atmosphere, on the earth's surface, and underground. Hydrodynamic Loads. As flood water flows around a structure at moderate-ta-high velocities it imposes loads on the structure. These loads consist of frontal impact by the mass of moving water against the struc- ture, drag effect along the sides of the structure, and eddies or negative pressures on the structure's down- stream side. Hydrostatic Loads. Those loads or pressures resulting from the static mass of water at any point of flood water contact with a structure. They are equal in all directions and always act perpendicular to the surface on which they are applied. Hydrostatic loads can act verticallv on structural members such as floors, decks, and roofs, and can act laterally on upright structural members such as walls, piers, and foundations. Infiltration. The flow of fluid into a substance through pores or small openings. The word is commonly used to denote the flow of water into soil. Mean Sea Level. The average height of the sea for all stages of the tide, usually determined from hourly height observations over a nineteen-year period on 76 an open coast or in adjacent waters having free access to the sea. New Construction. Structures on which construction or substantial improvement was started after the ef- fective date of a community's floodplain management regulations. One-HulIdred-Year Flood. See Special Flood Hazard Areas. Permeability. The property of soil or rock that allows passage of water through it. Regulatory Floodway. Any floodway referenced in a floodplain ordinance for the purpose of applying floodway regulations. Special Flood Hazard Areas. Areas in a community that have been identified as susceptible to a one- percent or greater chance of flooding in any given year. A one-percent-probability flood is also known as the lOO-year flood or the base flood. Special Flood Hazard Areas are usually designated on the Flood Hazard Boundary Map (FHBM) as Zone A. After de- tailed evaluation of local flooding characteristics, the Flood Insurance Rate Map (FIRM) will refine this categorization into Zones A, AO, A1-30, and Vl-30. Trallspiration. The process by which water vapor es- capes from a plant through its leaf system and enters the atmosphere. Watershed. An area from which water drains to a single point; in a natural basin, the watershed is the area contributing flow to a given place or a given point on a stream. Water lllbIe. The uppermost zone of water saturation in the ground.