Earlier 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 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 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 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.