The River Thames at Windsor

Romney Lock and Weir

This article looks at Romney Lock as photographed in the past, then illustrates the workings of a modern lock and its associated weir.

The same area of the Lock Cut in the early 1900s, looking upstream towards Windsor and a pensive schoolboy sits on the bank.
Windsor Castle is just visible in the distance.

An Introduction to the Worklngs of a Lock

Locks are the means whereby river traffic can move between the different levels of one river reach to another, either lifted to a higher reach or lowered to the next, past each weir. All locks comprise a main stream, fed by water tumbling over the weir, and a 'lock cut' for navigation. These were all, in the main, man-made several centuries ago. Romney Lock dates from the 1790s.

Weirs are designed to maintain river levels regardless of how much water is in the river system. They are primarily intended to provide a constant river level for navigation in times of drought and flood. When very little water is flowing, the weir slows down the amount of water passing down into the next 'reach' or section of river, whilst in times of heavy rain or a winter thaw, significantly larger volumes of water can be passed on downstream.

Controlling the Flow of the River

By raising and lowering the various sluice gates at the weir, the flow of water down the river is carefully controlled. In past times, the lock keepers would keep in touch with their colleagues up and down stream to ensure a constant and controlled flow, although these days automation and centralised control has been introduced.

It is only in times of severe flood that the system will be unable to cope, because, once the valley is full, it is full, and no amount of human engineering can prevent flooding if water flow is massively increased, as can happen following extended torrential rain throughout the valley for weeks on end, or if a sudden Spring thaw releases the stored up waters of the Winter snows, as was the case in March 1947. See The River Thames Floods Windsor in 1947.

At times of high water flow in the river system, extra sluices can be opened in the weir to pass more water downstream. In times of drought, water can be held back to ensure the river remains sufficiently deep for navigation.

Here the sluice gates are closed with just a small flow of water over the top maintaining the river level upstream.

Water level monitor.
The level of each reach is monitored so that adjustments can be made to water flow throughout the system

There is a limit to the amount of water that the system can handle. In the Windsor area, a maximum flow of 275 cubic metres per second can be accommodated. Any additional water MUST backup, causing a flood. The degree of flooding is directly related to how much extra water is flowing into Windsor reach and for how long, both from upstream and from the smaller streams that drain the valley on either side. In recent years a peak of almost 350 cubic metres per second was experienced in February 1997 and the river level rose several feet. The worst flooding that Windsor has experienced was in March 1947, see The River Thames Floods Windsor in 1947. In recent times high water levels were also experienced at New Year 2003 both upstream of Maidenehad and down stream of Windsor, but both these towns were protected by the newly completed Flood Relief Channel. See also our Story of the Floods of 2003.

How Locks Work

Weirs and their associated locks are in effect 'steps' in the river as it flows to the sea. With lock gates at each end, boats traveling upstream, for example, would enter the lock when it is 'empty' (at the level of the lower reach). When the gates are closed the lock is filled, and the river traffic lifted by the pressure of water from upstream. When the level of the lock is the same as the upstream level, the higher gates can be opened and boats can proceed on their way.

This sudden demand on water to fill the lock causes the level directly upstream of the lock to fall temporarily by several inches, and at Romney Lock a small 'waterfall' adjacent to the lock has been seen to temporarily stop flowing as water is redirected into the lock.

Once the lock is filled, in about 6 minutes, the upper gates are opened and the boats proceed. At this point vessels travelling down stream can now enter the lock and be lowered to the next reach by emptying the lock.

When the sluice gates are first opened, especially if they are opened a little too enthusiastically initially, the in-rush of water is quite impressive. Indeed boats can be damaged unless care is taken. It is for this reason that the sluices are only partially opened at first, and not fully opened until the level within the lock has risen significantly and the pressure differentials are less so water turbulence is reduced. Similarly, care must be taken that all craft within the lock are free to rise with the water level. If a vessel is trapped or restrained in any way, perhaps by the bow or stern being wedged in the structure of the lock gates themselves, it is possible for the water to rise sufficiently to pour into the boat and sink it!

The controls for the lock. A similar controls are positioned at the opposite end of the lock. The large wheel opens the gates when the water level on both sides is equal. To the side are the sluice controls, to fill the lock from the upstream level, or to empty the lock into the downstream reach.

Romney Lock Dimensions

Length 257 feet 7 inches (78.51 metres)
Width 24 feet 5 inches (7.44 metres)

The Flood Relief Scheme for Windsor and Maidenhead

We have included on The Royal Windsor Web Site an article about the Flood Relief Scheme which is designed to increase the river's capacity from above Boulters Lock in Maidenhead, to below Black Potts Railway Bridge, downstream of Romney Lock. This scheme is designed to accelerate the transfer of water from above Maidenhead to below Windsor, bypassing the several notorious river bottlenecks such as Windsor Bridge and Black Potts Bridge.

Whilst Windsor has not suffered a major flood since 1947, they are not a rare occurence. Indeed, studying earlier records shows a pattern of significant flooding every twenty years or so... we have just been lucky! Without wishing to appear pessimistic, it can only be a matter of time, and it is best that Windsor residents are fore-warned - and fore-armed.

We would welcome additional material and comment on this subject.

We have also prepared an article about the major flooding of Windsor in March, 1947, based on the extensive knowledge of Gordon Cullingham who was closely involved throughout the event. His comments are worthy of note.

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