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Floods resulting from persistently wet weather and thawing of snow case studies

In terms of water flow, the Mississippi is the sixth largest river in the world. Its annual average flow rate is 14,000 cubic metres per second and it discharges into the Gulf of Mexico 580 cubic kilometres of fresh water per year. The greatest flows occur in the period March to May and the least in the period August to October. A large proportion of the United States is drained by this river.

To protect against flooding, which would otherwise occur frequently, the Mississippi River is constrained by levees (embankments) all the way from the State of Missouri to the sea. This barrier, much of it concrete, has isolated the river from a lot of the surrounding countryside, but without it and other means of dealing with excess water in the river, low-lying cities such as New Orleans could not exist.

Tragically, some of the levees around New Orleans were breached on 29 August 2005 when they failed to withstand the battering imposed by the waves and a storm surge generated by Hurricane Katrina. Flood depths reached six metres in places and more than 1,000 people died.

Despite levees, spillways, reservoirs, pumping stations and other constructions, flooding does still occur. The flood-control system proved incapable of containing the flood of 1973, for example, and again proved inadequate in the summer of 1993, when abnormally high rainfall over central parts of the United States caused extensive flooding of the upper and middle Mississippi and lower Missouri rivers. The city of St Louis was particularly badly affected. Discharges of water from the Mississippi into the Gulf of Mexico in the late summer of 1993 were abnormally high. A study using satellite imagery showed that water from the Mississippi spread out far and wide off the States of Louisiana, Texas and Alabama. Indeed, some of the Mississippi water passed through the Florida Strait into the Atlantic.

There was rain, more rain and even more rain in northern California soon after Christmas 1996. From 29 December 1996 to 4 January 1997, depression after depression from the central Pacific brought rain to northern California. As the air was unusually warm, a consequence of the precipitation was that large amounts of snow melted. During the week of the storms, 61 cm of rain was recorded. Significant flooding occurred in northern California and southern Oregon and 43 counties were declared disaster areas. Flooding occurred rapidly because soils became saturated and amounts of snowmelt were large. Flood-control reservoirs could not cope, as their storage capacity was no more than moderate because of near-normal rainfall and run-off prior to the onset of the severe weather. Levee failure occurred on several rivers.

After the United Kingdom’s snowy winter of 1947 came the thaw. In many parts of the British Isles in February and early March 1947, deep drifts of snow, some five metres or more deep, caused villages and hamlets to be cut off for days on end. Then, on 10 March, warm air and rain edged into south-west England and advanced across the British Isles. By 13 March, floods were widespread. Vast areas of Fenland, the Severn Valley and other parts of Britain were submerged. At Selby, Yorkshire, three-quarters of all the houses were under water. A severe south-westerly gale on 16 March drove water ahead and caused dykes in the Fens of eastern England to burst. Warm air and rain are much more effective at thawing large quantities of snow than sunshine. The albedo of old snow (proportion of light reflected) is about 55%. The albedo of fresh snow is about 80%.

On 9 and 10 April 1998, just before Easter, prolonged heavy rain fell over a wide area of Wales and central England. In the Midlands of England, there was serious flooding, with Northampton and Leamington Spa badly affected. In these Easter Floods, as they have come to be known, 4,500 homes were inundated, five people died and the estimated cost of the damage exceeded £350 million. The culprit was a slow-moving depression centred over Brittany. North of it, lying east-west across the Midlands, there were two parallel fronts which were almost stationary, producing prolonged heavy rain that fell on a catchment that was already saturated.

snowflakeIn coastal regions, flooding may be caused not only by a storm surge but also by a combination of high tide and high river level. An example of the latter occurred in South Wales in 1979. On 26 and 27 December of that year, falls of rain exceeded 100mm in many places, especially over the hills. Extensive flooding occurred on the 28th. Cardiff was badly hit when the River Taff in spate met a high tide from the Bristol Channel. Hundreds of homes and offices near the city centre were inundated. Flooding with sea water is bad enough, not least because the salt in the water is left behind when the water evaporates, but the flooding in Cardiff on this occasion was particularly unpleasant, as the water welled up into streets and houses through the sewers.