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Flood Management: Hard Engineering

Dams & reservoirs

Dams are huge walls built across rivers. Areservoir (artifcial lake) is ormed behind

them. Floodwater is caught by the dam,which prevents ooding downstream. !hewater is released as a steady ow throughoutthe year. Apart rom ood protection damsalso have other benefts such as electricityproduction using hydropower generated byturbines built into the dam. !he reservoir canprovide water or irrigation and it can also beused or recreational activities such assailing.

"owever, dams are very e#pensive to buildand the reservoir created behind them oods land, orcing people to move, destroyingarmland and disturbing wildlie. !he dam may also a$ect wildlie in the river e.g. bypreventing salmon rom migrating upstream. Another disadvantage o dams is that theytrap sediment behind them, which can cause increased erosion downstream or even leadthe dam to ail. A break dam e.g. due to an earth%uake could cause a serious disaster.

Channel Modifcation

&hannels can be modifed in various ways to makethem more e'cient and able to transport water more%uickly. For e#ample, meanders can removed bybuilding artifcial cutthroughs, or the channel can bedredged to make it deeper and improve its dischargecapacity. !he shape and roughness o the channel canalso be modifed e.g. but building artifcially smooth

banks out o concrete or by removing large bouldersrom the riverbed.

*aking the water ow aster in a river reducesooding because the water will ow downstream more%uickly instead o building up in one place until itoverows.

+ne e#tra beneft o channel straightening it that itimproves navigation. "owever, it may also causeproblems such as increasing the risk o ooding anderosion rates downstream as well as disturbing wildliehabitats.

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Diversion Channels

Diversion channels or spillways are

artifcial river channels that takethe water elsewhere i the waterlevel in the river is too high. ateris normally diverted around animportant area or to another river. !hey prevent ooding becauseriver discharge is reduced. !hespillways oten have gates that canbe opened and closed to controlthe release o water.

"owever, an increase in dischargewhere the diverted river -oinsanother river (or re-oins the sameone) could cause ooding belowthat point. Also, i spillways areoverwhelmed, water will oodareas not used to ooding, which could cause even bigger problems.

Levee and Floodplain Modifcation

Artifcial levees or embankments can be builtalong the sides o the river to stop itoverowing so easily. !hey allow the river tohold a greater discharge and the oodplain tobe built on.

 !he height o the oodplain can also be raisedby dumping material on it.

Although levees can work %uite well, likedams, theyre e#pensive, and i breached theycan cause worse oods than i they werentthere, e.g. water can ow down the steepslopes o the levees on the other side more%uickly than normal. Also they can prevent

water rom draining back into the river.