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History lesson

To save Derby from fl ooding the city's 70-year old defences are being given a major overhaul.Christina Taylor reports.

The 2m deep floods that hit Derby 73 years ago were the worst in the city's history. As the foul water receded Derby's city fathers swore they would never allow a repeat, and ordered the construction of flood storage capacity and hard defences.

Over the decades, though, the 1930s flood retention ponds have become clogged and the walls are crumbling. Meanwhile, changes in climate and land use are increasing flows in local watercourses. The estimated cost of an extreme flood within the city centre is £104M.

So to ward off the threat of inundation, Derby City Council (DCC) has proposed £10M of defence improvement works under the Markeaton catchment area management plan.

Derby's ood defences include an old culvert, built in the 1870s, and the 2.2km long Markeaton Brook overflow interceptor culvert, also known as the Northern flood culvert, built in the 1930s. This was designed to cope with a 1% probability event as defined in the mid-1930s.

'The problem is that what constituted a 1% probability storm event in 1935 is not the same as a 1% probability event today, ' says DCC land drainage and flood defence manager Ian Frearson.

About 75km of open watercourses and 25km of culverted streams feed the Markeaton and Mackworth brooks, which converge on Derby's north western fringes, 3km upstream of their junction with the River Derwent in the city centre. From the point of confluence, the Markeaton Brook ran in the 1870 culvert.

As part of the 1930s flood control works, stilling ponds were excavated on the Markeaton and Mackworth brooks about 100m upstream of their confluence. Each stilling pond has a flat crested, notched weir to regulate downstream flows.

Overflow spillways in each pond take flood flows into the Northern flood culvert, which carried them around the city and into the Derwent.

Over the last 40 years the probability of extreme storm events has risen, says Frearson, with flow peaks rising far faster than they did 70 years ago. The brooks can reach full spate, delivering 50m 3/sec, within 13 hours of heavy rainfall starting, leaving too little time to issue flood warnings or erect demountable defences.

The increased speed at which floods arrive has been caused by a dramatic rise in the amount of land dedicated to arable farming, estimated at 50% of land within the catchment. This has increased soil erosion, resulting in silt accumulations of up to 2m depth in the brooks and stilling ponds.

Their ood storage capacity has been slowly pared back to ineffective levels. Now, water is overtopping the flat crested weirs into Markeaton brook and on to the city instead of being diverted into the Northern culvert. This local flooding raises the spectre of another major flood.

To tackle the problem, the stilling ponds and adjoining sections of the Markeaton brook are to be dredged. The £200,000 de-silting works are being carried out by contractor Land & Water.

Dredged material is being left on local park land. After a six month drying period, the silt will be spread and initially left to reseed naturally.

Meanwhile, to improve flow control from the stilling ponds, the original flat crested weirs are to be replaced with slotted or notched 'letter box' weirs to reduce flows entering Markeaton brook to a maximum of 6m 3/sec. The £500,000 contract for these works is out to tender, with an April start planned.

Since the water level relative to the current flow control structures will be reduced by the dredging, weir height for normal flow rates will be lowered to maintain fast flow speeds and help minimise future silt deposits.

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