What is the core regulatory intent behind designating a 'floodway' within a floodplain, beyond simply restricting development?

A 'floodway' is the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation by more than a designated height, typically zero net rise or a maximum of one foot. The 'base flood' is a flood event with a one percent chance of being equaled or exceeded in any given year, often referred to as the 100-year flood. This floodway is a specific, often narrower, component of the broader 'floodplain,' which is the land area susceptible to being inundated by floodwaters from the base flood. The portion of the floodplain outside the floodway is called the 'flood fringe,' where floodwaters are generally shallower and move more slowly. Beyond simply restricting development in a high-risk area, the core regulatory intent behind designating a floodway is to preserve the natural and unimpeded conveyance of floodwaters. 'Conveyance' refers to the ability of the river channel and adjacent areas to efficiently carry flood flows. This primary objective ensures that the existing capacity for water flow is maintained during a base flood event. Any 'encroachment,' which means placing fill material, constructing structures, or undertaking other development that obstructs flood flow within the floodway, reduces the available cross-sectional area for water to move. The critical consequence of such obstructions is an increase in flood elevations and velocities. Reduced conveyance forces floodwaters to back up upstream, causing higher water levels, and to accelerate through the constricted area, increasing pressure and erosive forces. Therefore, the strict regulations for floodways, often prohibiting any new development or fill that would result in a net increase in flood elevation, are designed to prevent artificial increases in flood heights and associated hazards for existing and future properties both upstream and downstream of the floodway. By maintaining the natural 'hydraulic function,' or the way water naturally flows and moves within the system, the designation safeguards public safety, protects structures from exacerbated flood damage, and avoids transferring flood risks to other areas or properties. It ensures that human activities do not worsen the natural flood hazard.