Tidal Trench on the Mill Stream

10-19 Jan 2008.

In Sep 2007 a 100 metre trench was excavated joining the River Stour with the Mill Stream at Bear Mead near Wimborne in Dorset. The object was to re-introduce flowing water into the 2 mile long MS which had become silted up. Also to help store water in the MS during river flood and drain it afterwards. A strong flow has been observed through the trench both Northward and Southward, hence the description "tidal". After observing a full cycle of flood and drought, the results appear admirable. see also: TIDAL TRENCH to control flooding. Unblocking, Tidying, Theory & early results and Results during flooding

River rising and filling the Mill stream reservoir. South flow, 16 Jan 2008, 0841 hrs, R=356, MS=325.

River falling and Mill stream reservoir draining back into it. North flow, 18 Jan 2008, 1549 hrs, R=267, MS=308.

How it works

The Tidal Trench is 100 metres long, 1 meter wide and 1 meter deep. It connects the River Stour with the Old Mill Stream. The Mill Stream is an old arm of the Stour, now silting up but acting as a reservoir around 3 kms long, 10 metres wide and 50 cms deep, holding around 15,000 cubic metres. The trench bottom has a high point or hump 300 cms above datum. This prevents flow in either direction except under the following circumstances: The River >300 and >the Mill Stream, producing Southwards flow. The Mill Stream >300 and >the River, producing Northwards flow. The rate of flow is proportional to the difference in levels. When flow occurs but levels are equal, an interesting oscillation occurs along the trench, triggered by small level changes in the fast-flowing river. Flow will usually occur during the winter, when river and Mill Stream levels are often high enough. However 5 July 2007 would have produced a good South flow, due to country-wide rainfall that flooded much of middle England. South Flow has the advantage of speeding storage of river water in the Mill Stream reservoir when the river approaches flooding. North flow assists drainage from the Mill Stream reservoir when flooding is over. The Mill Stream is unusual in that there is only one inflowing brook for rainwater collected in nearby hills, and two outflow points which only work for levels above 300. As a result the Mill Stream was usually stagnant. The tidal trench now allows heavy rainfall to find its way to the river, as is evident from clear water flowing into the silted river at the North end of the trench.

Compiled, formatted, hyperlinked, encoded, and copyright © 2008, John Palmer, All Rights Reserved.