The John Martin Sluice at Welmore Lake

Heritage Lottery FundEarlier in the summer I visited the John Martin Sluice at Welmore Lake.  It is located at the point where the River Delph joins the New Bedford River, and can be accessed via a bridleway from Salters Lode.  This is the most northerly part of the Ouse Washes flood storage area (see here for a simple description of how the Ouse Washes work).  The location of the John Martin Sluice in relation to the Denver Sluice Complex can be seen in the diagram below (bottom left hand corner).

The Denver Sluices (Source – Environment Agency)

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Welmore Lake Pumping Station, opened 2010

The John Martin Sluice serves two purposes.  As levels rise in the Old Bedford/River Delph, water flows onto the Ouse Washes. This water spreads northwards across the washes, and is held back by the sluice. Water is discharged by gravity through the sluice when levels in the Old Bedford/Delph are higher than in the New Bedford River. In spring, gravity drainage is sometimes not sufficient to attain the desired water level on the Ouse Washes. When this happens the electrically powered pumping station is put into operation to pump the remaining surplus downstream of the sluice. The second role of the sluice is to stop tidal surges from the Tidal River entering the washes. This saline water would have an adverse environmental impact.

The current sluice, completed in 1999, was named after John Martin, a local landowner who over the years has made a large contribution to water management in the area.  It has three sets of gates and has a 50% greater discharge capacity than the previous 1933 sluice.

The John Martin Sluice at Welmore Lake

The John Martin Sluice at Welmore Lake

The old sluice was located 70 metres upstream of the outfall into the tidal river, which led to silt building up in front of the gates and stopping them from opening. To reduce the build-up of silt the current sluice is positioned closer to the tidal river and is also fitted with silt jetting equipment. However, in spite of these measures, frequency, duration and depth of flooding in the Ouse Washes is increasing, causing problems for both people and the environment, such as flooding of the A1101 at Welney and shortage of breeding grounds for wading birds.

There are multiple reasons for this increase in flooding.  One factor is the large amount of silt in the tidal river, which creates higher riverbed levels, resulting in higher water levels.  This reduces gravity drainage from the Ouse Washes, thereby causing water to drain more slowly than it did in the past.

Twice a day silt is carried in on the tide from The Wash. The outgoing tide travels more slowly than the incoming tide, and this causes silt to be deposited on the bed of the Tidal River. Good freshwater flows are needed to flush the silt out but, particularly during periods of winter drought, the silt accumulates.

The Ouse Washes in flood   Bill Blake Heritage Documentation, All Rights Reserved

The Ouse Washes in flood
Bill Blake Heritage Documentation, All Rights Reserved

The Environment Agency is responsible for managing this issue, but it is far from easy. Dredging to remove the silt is one option. In 2007 the Environment Agency employed consultants who found that 185,000 cubic metres of silt would need to be removed over a distance of 10km downstream of Welmore Lake. This is equivalent to the volume of ten football pitches filled to a depth greater than the height of the goalposts. Not only is this costly (estimated to be in the region of £4-5 million), but it is also not a long-term solution as silt can very quickly accumulate. It was found that in the same stretch of river between April and August 2007, 100,000 cubic metres of silt settled. There is also the problem of disposal of the dredged material, as well as significant environmental impacts.  (It is thought that the disturbance of aquatic ecosystems through dredging affects biodiversity and could reduce fish numbers.)

After several years of low river flows and increased silt build-up, last year’s high rainfall provided a well-needed flush of the system.  Bed levels of the Tidal River around Denver have now returned to circa the 2002 figures. In the future, rising sea levels caused by climate change are likely to have an increasing impact on how quickly water can drain off the washes, and so the problem of flooding is likely to increase.

Whilst the John Martin Sluice in itself works effectively, it can be seen that there are wider issues that reduce the rate at which the Ouse Washes can drain. This is an on-going problem for which there is no simple solution.

We would be very interested to hear your thoughts on the management of this complex system.

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How it all works

Heritage Lottery FundMy name is Anna Growns and, like Peter Stroud (see here for his previous post) I am also working as a summer placement volunteer for the Ouse Washes Landscape Partnership Scheme.  Now in my second week I think I can safely say that I am beginning to get a feel for it all!  The scheme brings together a wide variety of projects, and it is fascinating learning more about what makes the Ouse Washes such a unique landscape.  There is so much to find out, and I am becoming aware of how little I really know about the area where I live – my reading list grows by the day!

I am currently focusing on water management in the area, which is of particular interest to me as I am hoping to work towards a career in flood risk management.  On Monday I visited the Denver Sluice Complex.  The morning started with a talk given by Dan Pollard, who works for the Environment Agency, based at the Denver Sluice Complex.  His job involves monitoring river levels and adjusting the sluices accordingly.  I also met John Martin, a local farmer who owns land immediately adjacent to the Ouse Washes.  He was involved in both the 1987 refurbishment of the Denver Sluice, and the Welmore Lake Sluice (which is now also known as the John Martin Sluice).

It was interesting to hear about the potential conflicts between those who use the area; farmers, conservationists, anglers and boaters, amongst others.  I will discuss these issues in more detail at a later date, but for now all I’ll add is that there is no perfect solution to managing the area, but perhaps by working together to understand the problems, a fairer outcome could be achieved.

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Dan Pollard (Environment Agency) explaining the complexity of managing the Denver Sluice Complex and its many waterways.

Anyway, I mustn’t get side-tracked – back to Denver Sluices!  Denver Sluice is just one of several in the area that make up the Denver Sluice Complex.  They play a vital role in controlling river levels, and are successful in stopping the low-lying fenland from flooding.  I suppose before discussing the sluices it makes sense to begin with a wider look at the function of the Ouse Washes.  The diagram below shows the area from Earith in the south to Downham Market in the north.  The Ouse Washes lie between the New Bedford River to the east, and the Old Bedford/River Delph to the west.  As previously mentioned in this post, these channels were created as part of Vermuyden’s scheme to drain the fens.

Ouse Washes overview

Ouse Washes overview: geography and names of main channels. Source: http://www.ousewashes.info/maps/washes-lrg.jpg

The first channel, the Old Bedford River, was cut in 1630, and the second, the New Bedford River or Hundred Foot Drain, was constructed 20 years later.  As can be seen from the diagram, this considerably shortens the route that water takes from Earith to Downham Market on its journey to the sea (where previously the water would have followed the course of the Great Ouse River to the east), thereby diverting water from the surrounding fenland and discharging it more quickly.

The purpose of having two parallel channels was to create a huge flood storage area, i.e. the Ouse Washes, which protects the surrounding land from flooding.  To put it very simply, when there is too much water in the River Great Ouse the water is allowed to flow onto the washes, normally through the Earith Sluice and the Old Bedford River, and stored there until it can be discharged.  The following diagram explains in more detail how this works (see also this previous post):

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Schematic layout of the Ouse Washes flood system. It also shows the relatively few crossings over the washes. Source: Environment Agency and http://www.ousewashes.info. Click on the map to enlarge.

Next time I will look in more detail at particular aspects of how this system works.  More information on water management in the Ouse Washes can be found at the following useful websites: ousewashes.org and ousewashes.info