Denver Sluice The Gaywood Valley Conservation Group met at the car park by Denver Sluices on 19.08.19. when we were given a guided tour by Dan Pollard from the Environment Association of the whole sluice complex starting with the most recent installation and inishing with the oldest. The history and the need for controlled water supply up and down the river system were described in detail. He said initially that the role of the Environmental Association locally was to take part in the Flood and Coastal Risk Management programme of which the Denver Complex is a very important component. At Denver he has to control in a 1000Km of waterways the activities of the ishermen, rowers, canal and cruiser boats, to monitor the water levels to prevent looding in Cambridge and Ely and to ensure that the urban areas of Essex speciically Colchester and Chelmsford receive adequate supplies of water for domestic and industrial use. These two towns do have reservoirs but in the summer these may be down to 30-40% capacity because the average annual rainfall is only 500ml in Essex compared with 800ml in the West so some help with the water supply in Essex is required. This had taken the form of a Water Transfer Scheme whereby surplus water in the Ouse is transferred to Essex. After this general discussion the party moved to the irst structure on the site. The irst structure we saw was only two years old. It was a small sluice which allowed water to enter the Relief Channel from the River Ouse. The functioning of the sluice was fully automatic as the entry was controlled essentially by a ball and valve principle similar to those used in the domestic toilet ie when the rate of water coming through the sluice was high the guillotine sluice gate remained open and when low it was closed. The water was fed through gates and into the Relief Channel as 1.7 cubic m per sec in the summer and 3.2 cubic m per sec in the winter. Its purpose was just to ensure a certain level of water in the Ouse by allowing a small amount of low through this system. It was not designed to cope with large changes in level that might occur as a result of a lood upstream when a larger and more rapid outlow was required. Alongside this was a ish ladder which was of a suficient slope to attract migratory ish like eel, lamprey (an endangered species that is now very scarce) and sea trout where all species travel up river to spawn in the small tributaries. Those migratory species are attracted by the physical down low of water and will automatically climb the low ladder. While the sea trout are able to overcome the bafles in the centre of the slope the eels choose to wriggle up the latter surface made up of rubber matting at the sides of the slope. Halfway up there was a deep pool to allow the ish to rest before completing the ladder. Coarse ish like Perch, Roach, Tench, Pike which are often 3-4 foot long and the newly arrived species Zander would not use the ladder but would go underneath the sluices to migrate upstream. Since the slope was installed ish predators have appeared to the area. These include otters, kingishers, herons and even a seal. The party then moved onto the sluices that controlled the entry of water into the Relief Channel and into the Cut Off Channel. Because technically the countryside is still in a state of drought the water in the Ouse is being held back by the action of the modern large sluices that lead onto the Relief Channel so as to provide water for the farmers and domestic water supplies. At a point just upriver from the lock that leads into the Relief Channel there is an entry of water from the Ouse to the Cut Off Channel which as the upper part of the Relief Channel runs south as far as Mildenhall. This and the Relief Channel down to Downham Market is entirely man made and only completed in 1964. There was an electrically operated lock which allowed boats to move from the River Ouse into the Relief Channel. Although it has been discussed to have a lock below Downham Market opening the Relief Channel to the River Ouse but the number of boats passing through the Denver lock did not justify the further expense. The function of the Cut Off Channel is to provide for the transfer of water to Essex and to make irrigation water available to farmers. In order to send water to Essex water is extracted from the Residual Channel at the Black Dyke via a very wide 100 inch diameter pipe which is sunk to a depth of 100 foot. The water then travels for twelve and half miles at one point underneath Milden hall Air Base to ‘Kennety’ then for a further nine and half miles to ‘Ketterling Green’ when low is directed into the Stour where it runs out to the sea or is directed into the ‘Atterton Resevoir’ for use by Essex and Suffolk. Water is also removed directly from the Stour and into the Hanningield Resevoir. Last August the level in this reservoir was down to 24% capacity making the water transfer an essential strategy especially since there is an anticipated build of 100,000 new homes in Chelmsford. The reduction in industrial activity and the introduction of water metres in domestic building in the Chelmsford area has caused a reduction in current water demand but the arrival of the large number of new houses is going to increase demand. The water transfer scheme does make use of electrical power to drive the sluices and pumps involved in all of this water movement. In the future more of this electrical demand may come from renewable sources. Unfortunately these sources are not constant and a base line supply will still be required. As part of lood control water from the Wissey,, Little Ouse and Lark are fed into it. Farmers obtain water for irrigation from all parts of the freshwater system. Water is released into the large ditches that run through their land by gravity feed and it is then pumped back into the river by the farmers using the same pumps as they used to pump it onto the land. Regulation and recording of the volumes used is by inspectors and now by drones. Last year farmers were in fact warned that this summer was going to be a dry one. This year they responded by growing more cereals which require less water than potatoes, sugar beet or maize. As it happened the summer was not as dry as forecast. The other important function at Denver is lood control. Here there is the large three sluices on the Ouse called the Head Sluice which open water from the River Ouse to the Relief Channel which runs down to Saddlebow where there are seven sluices gates which are referred to as the Tail Sluice. In 1939 there was extremely bad looding in Ely which led to the suggestion of a relief Channel running alongside the River Ouse but the onset of the Second World War in 1939 prevented any construction. There was more interest after the war when in 1947 there was again bad looding at Ely. In 1948 the lood protection was started and inally inished in 1964 at the cost of £10 M which would be £130M if measured in todays money. The idea would be to open the Head Sluices to release the water into the Relief Channel if a lood was threatened up river. It meant that within one hour of a potential lood in Cambridge and in ifteen minutes in Ely the danger would be avoided by opening the Upper Sluice then the Tail Sluice opened to release the water into the River Ouse where large increases in height of the water would not cause any looding in Kings Lynn. The rise and fall of the tide here far exceeds the likely rise of water from the Bottom Sluices being opened. In 2010 and 2012 staff were on 8 hour watches to ensure that the water levels were monitored constantly and the Head and Tail sluice gates adjusted to take into account of heavy rainfall leading to a likely lood. The staff could not rely on weather forecasting as often the storms would be very local so they had to monitor the rainfall and water levels constantly during these times. There is also electronic assistance and alarms do go off if the river levels rise to dangerous levels. In Ely for instance there is only a range of 30cm of river level above which loods will occur there. The oldest sluices were built in the 1600s as a result of the efforts of the Duke of Bedford who wanted to prevent the salt water looding the land if it was pushed up by high tides assisted by strong winds as well as to reclaim the Fens for use by agriculture. The installations were carried out by a Dutch engineer called Cornelius Vermuy. In 2013 there was a tidal surge generated by a high tide and a strong north wind. This caused the levels of water below the sluices to rise well above ground level. The sluices are designed such that a rising tide of saline water from below the sluices will cause the V doors on the down side of the sluices to slam shut and a falling tide for them to open. In addition there are guillotine gates on the upriver non-saline side which can be closed to maintain the level of the water in the river but opened at a time of a lood but then will only release that water at low tide when the V gates are open. This is why it is essential to have sluice gates opening onto the Relief Channel that are independent of the state of the tide. There is also a lock on one side of these sluice gates which allows the boats to move from the River Ouse to the tidal River Ouse. Three hundred years before the 2013 tidal surge there was a very high tide which actually damaged the sluices and saline water penetrated the River Ouse and the Fens. In 2013 there was the same high tide but all the V gates and guillotine held so the river and land was safe. The V doors used to be lined with oak but are now lined with an African hard wood called Ekki. On closing in response to a rising tide they will close with a noise like a thump - an ‘Ecky thump’ as described by the Goodies and reminded for us by Mike. Further down the tidal river there is the entry of Old Bedford River and the parallel New Bedford Cut which take lood waters from Bedford only so as to stop looding there. The water is also released onto the land at Walney as part of lood control and to the great beneit of the wading birds there at the RSPB site. A lock at the entry of the Bedford River to the tidal Ouse allows boats to transfer from the Ouse to the Bedford River which then takes them to the Midland water ways. Silt can be a problem on the tidal river with levels building up below the sluice such that it impeded the low of the water. This accumulation is reduced or prevented by opening the guillotine gates of the sluices allowing water to rush out when the silt is suspended and carried downstream. Mixing the saline with fresh water reduces the salinity from 30,000 ppm salt as in the Wash to 5,000 but it can be as much as13,000 ppm if the low of fresh water is very slight. The reduced salinity below the sluice did not stop a ive foot Sturgeon being seen swimming up to that point but unfortunately Dan was unable to get his camera out in time to photograph it and show us but we believed him. The group were very grateful to him for giving us an interesting and detailed description of the very complex activities of the Denver Complex.