Types of floods

Riverine flooding

Riverine flooding includes:

Riverine flooding includes: There is often no sharp distinction between river floods, flash floods, alluvial fan floods, ice-jam floods, and dam-break floods that occur due to structural failures or overtopping of embankments during flood (or other such as landsliding, rockfalling, etc.) events. Nevertheless, these types of floods are widely recognised and helpful in considering not only the range of flood risk but also appropriate emergency preparedness and responses.

In general, the river floods are caused either by rainfall of extra-tropical or frontal character, as experienced in temperate latitudes, or by large tropical atmospheric depressions with moisture-laden winds, moving from a maritime environment onto and across a land mass (for instance, seasonal monsoons in Asia and line squalls on the west coast of Africa). Rainfall in these events is generally widespread and can be heavy. The level of flooding can be high, and is influenced by topographic features.

River floods

Overbank flooding of rivers and streams is the most common type of flood event. River (riverine) flood plains (Fig. 1) range from narrow confined channels in the steep valleys of hilly and mountainous areas, and wide, flat areas in the plains and low-lying coastal regions. The amount of water in the floodplain is a function of the size of the contributing watershed and topographic characteristics such as watershed type and slope, and climatic and land-use characteristics.

Consequently, the magnitude and extent of a river flood depends upon the size of the catchment area of the river (contributing watershed), the topography, soil conditions and vegetation, and the weather conditions involved. Size of catchment area usually governs the character of flooding as well as the type of meteorological event, or events, which are capable of inducing extreme floods.

For instance, river flow on very large rivers (such as the Nile, the Danube or the Rhine) is relatively slow to change in the downstream reaches (Fig. 3a); floodwaters will generally be a combination of many rainfall events occurring over a wide area, sometimes augmented by melted snow. In large river basins flooding is usually seasonal and peak discharges can be reached and maintained over days or weeks.

Flood types Flooding in large rivers usually results from large-scale weather systems that generate prolonged rainfall over wide areas. These same weather systems may cause flooding in hundreds of smaller basins that drain to major rivers. Small rivers and streams are susceptible to flooding from more localized weather systems that cause intense rainfall over small areas.

The principal characteristics of river floods are their relatively slow build-up, which in river systems is usually seasonal.

However, the shape of the catchment area has a considerable effect on the peak water discharge in a river or stream (Fig. 3). The rounder the area and the more uniform routes the water takes to the point in question (Fig. 3b), the more the water tends to arrive simultaneously, increasing the possibility of an extreme flood peak (hydrograph B of Fig. 3c). As a rule, round and small catchment areas, which are commonly found in the upper reaches of rivers and in the mountains produce a quickly rising hydrograph after intense (torrential) rainfall. Thus, the flood peak at a given location is in general very pronounced. In longer and wider catchments the run-off is spread better over time (Fig. 3a), as is mostly encountered in flat terrains at the lower reaches of rivers. The hydrograph rises relatively slowly and then flattens out (hydrograph A of Fig. 3c). The water arrives at a given point gradually, even if rainfall is intense. The characteristics of a catchment area and its hydrograph, such as hydrograph A of Fig. 3c, can also result in the land being submerged for a long time.

However, if the rainstorm progresses over a long catchment area towards the point in question in such a manner that it adds more and more water to the flood peak, a situation can develop which is as precarious as the one seen in the hydrograph B of Fig. 3c.

Flash floods

"Flash flood" is a term widely used by flood experts and the general population. However, there is no single definition, and a clear means to separate flash floods from the rest of the spectrum of riverine floods does not exist. Floods of this type are particularly dangerous because of the suddenness and speed with which they occur. They develop in a basin following the occurrence of one or more previously mentioned storm types, especially if the catchment slope is conductive to acceleration of run-off rather than its attenuation.

Flash floods are events with very little time occurring between the start of the flood and the peak discharge (hydrograph B of Fig. 3c). They are often associated with a short time between the storm incidence and the arrival of the flood wave, which is not always the case; and are of short duration with relatively high peak discharge.

Flash floods are characterized by a rapid rise in water level, high velocity, and large amounts of debris. They are capable of tearing out trees, undermining buildings and bridges, and scouring new channels. Major factors in flash flooding are the intensity and duration of rainfall and the steepness of watershed and stream gradients. The amount of watershed vegetation, the natural and artificial flood storage areas, and the configuration of the streambed and floodplain are also important.

Flash floods are often associated with isolated and localised intense rainfall. In some regions, severe and destructive flash floods occur very infrequently in any one of a large number of small catchments within a given region. Efficient surveillance, warning and protection against the hazard are therefore difficult. In other regions, flash floods occur annually on the same river; warning in these cases is more a matter of timeliness. Because the warning time is invariably limited, the flash floods are now the main cause of weather-related deaths.

Flash floods may result from the failure of a dam or the sudden break-up of an ice jam. Both can cause the release of a large volume of water in a short period of time. Flash flooding in urban areas is an increasingly serious problem due to removal of vegetation, paving and replacement of ground cover by impermeable surfaces that increase runoff, and construction of drainage systems that increase the speed of runoff.

Alluvial fan floods

Alluvial fans are deposits of rock and soil that have eroded from mountainsides and accumulated on valley floors in a fan-shaped pattern. The deposits are narrow and steep at the head of the fan, broadening as they spread out onto the valley floor. As rain runs off steep valley walls, it gains velocity, carrying large boulders and other debris. When the debris fills channels on the fan, floodwaters spill out and cut new channels. The process is then repeated, resulting in shifting channels and combined erosion and flooding problems over a large area.

Alluvial fan floods can cause greater damage than typical riverine flooding because of the high velocity of flow, the amount of debris carried, and the broad area affected. Floodwaters typically move at velocities of 5 to 10 metres per second due to steep slopes and lack of vegetation.

Human activities often exacerbate flooding and erosion problems on alluvial fans. Roads act as drainage channels, carrying high velocity flows to lower portions of the fan, while fill, levelling, grading, and structures can alter flows patterns.

Ice jam floods

Flooding caused by ice jams is similar to flash flooding. Ice jam formation causes a rapid rise of water at the jam and extending upstream. Failure or release of the jam causes sudden flooding downstream.

The formation of ice jams depends on the weather and physical conditions in river channels. Ice jams are most likely to occur where the channel slope naturally decreases, where culverts freeze solid, at headwaters of reservoirs, at natural channel constructions such as bends and bridges, and long shallows where channels may freeze solid.

Ice jams floods can occur during fall freeze-up from the formation of frazil ice, during midwinter periods when stream channels freeze solid forming anchor ice, and during spring break-up when rising water levels from snowmelt or rainfall break existing ice cover into large floating masses that lodge at bridges and other constructions. Damage from ice jam flooding usually exceeds that caused by open water flooding. Flood elevations are usually higher than predicted for free-flow conditions and water levels may change rapidly. The force of ice impacting buildings and other structures can cause additional physical damage.

Dam break floods

Dam failures can occur as a result of structural failures, such as progressive erosion of an embankment or overtopping and breaching by a severe flood. Earthquakes may weaken dams. Disastrous floods caused by dam failures, although not in the category of natural hazards, have caused great loss of life and property damage, primarily due to their unexpected nature and high velocity floodwater.

Local drainage or high groundwater levels

Local heavy precipitation may produce flooding in areas other than delineated floodplains or along recognizable drainage channels. If local conditions cannot accommodate intense precipitation through a combination of infiltration and surface runoff, water may accumulate and cause flooding problems. During winter and spring, frozen ground and accumulations of snow may contribute to inadequate drainage and localized ponding.

Flooding problems of this nature generally occur in areas with flat gradients, and generally increase with urbanisation which speeds the accumulation of floodwaters because of impervious areas. Shallow sheet flooding may result unless channels have been improved to account for increased flows.

High groundwater levels may be of concern and can cause problems even where there is no surface flooding. Basements are susceptible to high groundwater levels. Seasonally high groundwater is common in many areas, while in others high groundwater occurs only after long periods of above-average precipitation.

Fluctuating lake levels

Water levels in lakes can fluctuate on a short-term, seasonal basis, or on a long-term basis over periods of months or years. Heavy seasonal rainfall can cause high lake levels for short periods of time, and snowmelt can result in higher spring levels. Long-term fluctuations are a less-recognised phenomenon that can cause high water and subsequent flooding problems lasting for years or even decades.

While all lakes may experience fluctuations, water levels tend to vary the most in lakes that are completely landlocked or have inadequate outlets for maintaining a balance between inflow and outflow. These lakes, commonly referred to as closed-basin lakes, are particularly susceptible to dramatic fluctuations in water levels over long periods of time, as much as 1 to 3 metres.

Fluctuations of lake water levels over a short period of time, initiated by local atmospheric changes, tidal currents, or earthquakes, are known as "seiches". These, free or standing wave oscillations of the surface of water in an enclosed basin are similar to water sloshing in a bathtub.

Coastal flooding

Devastating floods can occur as a result of extreme wind storms (typhoons, hurricanes or tropical cyclones). The Indian sub-continent (Bay of Bengal), and countries in Asia and the Pacific are all typically subject to such events. Catastrophic flooding from rainfall is often aggravated by wind-induced surge and low atmospheric pressure along a coastline (Fig. 4), which causes a rise in sea level and inundation of coastal and inland areas. Rainfall intensities are high and the area of the storm is wide; the combination of these factors can produce extreme flood discharge in both small and large river basins, which can be maintained at high levels by a coastal discharge.

Storm surges occur when the water level of a tidally influenced body of water increases above the normal astronomical high tide. Storm surges commonly occur with coastal storms caused by massive low-pressure systems with cyclonic flows that are typical of tropical cyclones, northeasters, and severe winterstorms. Other factors influencing storm surge intensity are:

Storm surges generated by coastal storms are controlled by the following four factors:

The other causes of coastal flooding are tsunamis, the large seismic sea waves, impulsively generated by shallow-focus earthquakes.

Estuarine floods

Commonly caused by a combination of sea tidal surges caused by storm-force winds.

Catastrophic floods

Caused by a significant and unexpected event e.g. dam breakage, or as a result of another hazard (e.g. earthquake or volcanic eruption).

For example: Tropical Storm Alberto, the famous 1994 storm, produced heavy flooding across Georgia, Alabama and northwest Florida and created between 400-600 million dollars worth of damage in the Southeastern US in 1994 United States Dollars.

Other types

Regional Floods

Regional floods are caused by snow melt, and annual phenomena like the Malaysian monsoons and the yearly Nile River overflow. The storms overload the rivers. The floods can happen faster and be more serious if the ground is frozen or already saturated with water.

Storm Surge Floods

Storm Surges are huge waves that come onto the beaches and beyond, flooding the shore. These are mostly caused by hurricanes and large storms.