Flood Barrier

Flood barrier

A flood barrier is also known as a storm surge barrier or a surge barrier and is a particular kind of floodgate. It is designed as a means of preventing a spring tide or storm surge from flooding protected areas behind a barrier. Surge barriers are usually part of more extensive flood protection systems that consist of levees (commonly known as dykes), floodwalls, and other natural geographic elements and constructions. A surge barrier can let water pass through under ordinary circumstances. Whenever a storm surge is expected, however, the barrier is able to be closed. This is done by various types of gates.

The Moveable Flood Barrier

Moveable flood barriers can be positioned at the mouths of rivers which are adjacent to a bigger lake, an ocean or a watercourse. These barriers can protect people against water level rises in the lake, sea or watercourse. Moveable barriers can be of numerous different designs. Examples of a moveable flood barrier are at Rotterdam that comprises two floating rounded steel gates, and the Thames Barrier that comprises 9 concrete piers and 10 mobile ports situated within the riverbed. When an imminent high water level is forecast the gates of the Thames Barrier can be turned 90 degrees to form a barrier.

The Non Moveable Flood Barrier

A non moveable flood barrier is a solid barrier that separates the water both outside and inside. Barriers like this are sometimes constructed wide so that the top of it can also be utilised for additional purposes such as a highway. An example of this is the Afsluitdijk found in the Netherlands where the North Sea is separated from a bay. The 32 kilometre long barrier has a highway on top of it.

Another form of flood barrier is a soil embankment, providing protection against certain water level rises. Soil embankment flood barriers can be constructed out of coarse materials, fillers and geo textile fabric. Water pipes on the outsides of the embankments must close manually or have check valves to prevent any leakages inside the flood barrier.

The Largest Flood Barrier in the World

The Netherlands has the largest flood defence project found worldwide, called the Delta Works. This comprises several surge barriers, with the largest measuring 5.6 miles (9 kilometres) long. The Eider Barrage is found in Germany near Tonning on the Eider River. The main purpose of this flood barrier is to protect the coastline from severe storm surges from the North Seas.

The Thames Barrier

The second largest flood barrier in the world is the Thames Barrier which is found downstream in central London. The purpose of this movable flood barrier is in preventing exceptionally high storm surges and tides coming in from the North Sea and flooding London. It only needs to be closed or raised when it is high tide. At low tide it can then be lowered. This releases the water backing up behind it. The northern bank of the barrier is found in Silvertown while the southern bank is found in Charlton. The Thames Barrier was built after sections of London and the Thames Estuary were flooded by the North Sea in 1953.

London is susceptible to flooding. This is due to storm surges that are created by low pressures within the Atlantic Ocean. This storm surge will sometimes track eastward past northern Scotland and into the North Sea. This surge tide funnels its way from the North Sea to the Thames Estuary. If spring tide coincides with this storm surge the Thames Estuary can experience water levels that are dangerously high. Downstream flows combined with these high levels of water means that a flood barrier is of paramount importance.

Rising Water Levels in London

Over time, the threat of flooding in London has increased because of the slow but constant rise in the water level through the centuries. The water level rises 8 inches (or 20 cm) every 100 years. Not only that, but post glacial rebound has caused Britain to ‘tilt’ (downwards in the east and south, and upwards in the west and north.) The Thames flood of 1928 saw fourteen people die, and the flood of 1953 saw 307 fatalities. The issue of building a flood barrier became of prime importance.

Charles Draper devised the idea of rotating gates. The New Charlton site was chosen due to the river chalk being strong enough for supporting the barrier, as well as because the banks were relatively straight. Work began on the flood barrier in 1974, and by 1982 construction was largely completed. The flood defences were strengthened and raised for eleven miles downriver. HM Queen Elizabeth II officially opened the Thames Barrier on the 8th of May, 1984. The total cost of the flood barrier was about £534 million, with another £100 million for the river defences.

The Thames Barrier is constructed across a 570 yard (520 metre) wide stretch of river. The flood barrier splits the river into two 100 ft (30 metre) and four 200 ft (61 metre) navigable spans. In addition, there are four smaller non navigable channels as well between two abutments and 9 concrete piers. Each floodgate across the flood barrier openings is a circular segment in cross section. The floodgates work by rotating and are elevated for allowing ‘underspill’. This allows the operator to control the upstream levels as well as enable a full one hundred and eighty degree rotation when doing maintenance work.

All of the floodgates are hollow and comprised of steel that is up to 1.6 inches (40 millimetres) thick. When the gates are submerged they fill up with water and empty when they come back out of the water. The four big central gates weigh about 3,700 tonnes and are 66 ft (20.1 metres) high. There are also 4 radial gates on the riverbanks which can also be lowered. These gates are about 100 ft (30 metres) wide. Unlike the six main gates, these radial gates are non navigable. This non moveable flood barrier system certainly helps in protecting London from floods.

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