Archive for the ‘Flooding and Flood Damage’ Category

Geohazards: Rivers and Floods

Wednesday, November 7th, 2007

Hazard Characterization

Floods are high-water stages where water overflows its natural or artificial banks onto normally dry land, such as a river inundating its floodplain. Floods occur at more or less regular intervals in riverbeds and floodplains but also beyond them. Besides storm surges the two main types of floods are river flood and flash flood. Floods occur as natural phenomena when the volume of river runoff is so high, and the riverbed too small to contain the water masses. Floods, or high water stages, are most regular in springtime. Strong floods happen irregularly, in so-called re-occurrence intervals of 10, 50 or 100 years. But these intervals are only statistical averages. Heavy summer rainfalls can also lead to floods.

Floods have become an increasing problem for man’s environment since urbanization has altered natural drainage ways, straightened and even relocated river beds within their natural flood prone areas. Also impacting runoff rates of flood prone areas is the habit of increased soil sealing (asphalt parking areas, highways, commercial urban development, etc.) leading to a higher than normal flood hazard. This occurs as rainwater more quickly runs off directly into the streams and the water mass inflow via paved drainage ways and underground pipes to rivers, storm water runoff which is no longer delayed by natural soil retention induces “flash flood” like storm events.

Flash floods are the fastest-moving types of floods. A flash flood is a specific type of flood that appears and moves quickly across the land, with little warning. Heavy rainfall concentrated over an area, thunderstorms, hurricanes and/or tropical storms cause most flash flooding. Dam failures can also cause flash flood events. When a dam or levee breaks, a gigantic quantity of water is suddenly discharged downstream, developing strong destructive forces which may reach elevations previously undamaged by flood waters.

Flash floods can contribute to river floods, or can be caused by river floods, for example if an embankment collapses. Flash floods can happen anywhere but are mostly bound to river and stream drainage areas and are thus integrated into most government agency flood zone maps and the delineation of flood zones.

Risk Management

The most important part of flood risk identification and management is the flood-prone area delineation (extent). Flood-prone areas are those areas subject to inundation as a result of flooding with certain known frequency. The determination of a flood prone area requires considerable collation of historical data, accurate digital elevation data, and hydrologic discharge data calculated along a number of cross-sections located throughout a watershed. In addition to taking past flood events into account, it could be possible to derive river flood prone areas by area elevation modelling with the assistance of satellite imaging. Research institutions can develop a “flood prone area map” based on digital terrain models, river runoff, flood data and climate models. These are available to the public.

Flood Zone Determination

Flood Zone Determinations are usually provided by the state and local government engineering departments. Certificates of Elevation are provided by Professional Engineers, Professional Land Surveyors or Registered Architects from the private sector on a fee basis.

Flood Insurance

Flood insurance is recommended in flood prone areas. There are two types of coverage, structural and contents. Renters can buy contents coverage only. Flood insurance is available through the National Flood Insurance Program (NFIP) and information about this insurance is available through you local insurance agent. Don’t wait for the next flood there is a 30 day waiting period for coverage to take effect.

Dominick Amari , P.G.