Extremes of water supply
are of greatest concern to people, because both lives and property may be
endangered. In times of flood, swollen rivers and surging seas may cover
roads, homes and entire towns. Even the arid South-west is not immune to
flooding. Unstable sandy banks cavein under the force of floodwaters,
which may shift the course of river channels or widen them substantially
during a single storm event. Houses located far from a channel may
suddenly be endangered.
When a major flood is imminent, hydrologists, meteorologists and engineers work together to predict when the waters will rise over the riverbanks or levees, how much area will be flooded, and how long the flood will last. These warnings give people time to move equipment and stored goods and reinforce flood protection measures.
Forecasts often are based on complex mathematical models that use data from river gauge readings from which hydrologists estimate flow rate. Water from other streams and reservoirs in the flood path also are considered in forecasting downstream flooding. For ungauged watersheds, hydrologists make predictions considering watershed area, length of stream channels, soil types, slopes and land uses.
Mapping flood-prone areas provides fair warning of the risks of building in such places and should discourage unwise land use. Communities use such maps to plan and design public structures and highways. These maps also are used in drafting zoning and building regulations to prevent land use that could increase the risk of flooding. The federal government uses the maps to determine those eligible for federal flood insurance.
Flood hazard maps may show several zones according to estimated frequency of flooding. Starting at the streambank, where water overflows every few years, a cross section across a valley indicates land subject to flooding at a long -term predicted frequency such as once every 100 years (100-year flood plain).
Hydrologists use statistical analyses of streamflow data and computer models of the effects of various combinations of meteorological and hydrological conditions to prepare these maps. Aerial photographs and soil maps also are useful for defining flood-prone areas.
Hydrologists provide information needed by engineers who design, construct and operate spillways, bridges, culverts and reservoirs for controlling floods. For such projects, hydrologists estimate peak height, discharge rate and total volume for a chosen level of protection such as a 50-year flood. With this knowledge, flood hazards and drainage can be reduces in four ways: 1) reducing peak flow by storage in reservoirs; 2) confining flow with levees, floodwalls or pipers; 3) expediting flood flows by improving channels; and 4) diverting waters through bypasses, floodways or culverts.
modification of upstream land uses can reduce rates and volume of storm runoff. Hydrologists design terraces or detention ponds, or recommend land management practices to maintain forests and grasslands in key areas.
