Updated 08 Jun 2011

Planting a new oakwood in Dorset

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Photo 062


New Mill Stream, Nov 2007

An old silted-up arm of the River Stour is deepened and re-connected to the river. Heavy rain in Nov 2007 raised the river by 7 feet, sufficient for water to cross a hump in the connecting trench and begin flowing down 2 miles of the old stream bed, filling the Mill Sream 'reservoir'. This has happened 11 times since Jul 2003, a total of 20 days. In this time, the river has flooded 4 times. A mile down the stream bed, the water flow leaves the old stream arm and finds an existing alternative route back to the river near Merley Hall Farm to the South of Wimborne along "stream #5". When the river drops below 300, the flow reverses, out of the Mill Stream, over the hump at 300 and back into the River. The Mill Stream 'reservoir' begins to be emptied again.
see also: TIDAL TRENCH to control flooding. Unblocking, Tidying, Theory & early results and Results during flooding

62n Heavy rain which ended 3 months of drought in
Nov 2007, nearly causing the River Stour to flood.

The Cause

62o. 84 mms of rain in 5 days caused the Dorset Stour at Bear Mead to rise 210 cms and the water table 145 cms, from a 13-month lowpoint to 10 cms short of flooding.

The Effects

As the upstream river rose to level A, water entered the trench. At level B, water crested the 'hump' and flowed down to start filling the Mill Stream reservoir. The reservoir slowly rose to level C, at which point it flowed into Stream #5 and emptied into the downstream river at D.

As the river fell below level B, water ceased entering the reservoir via the trench. If the reservoir itself was above B, reverse flow took place along the trench into the river. When the reservoir was also below B, no flow ttok place along the trench in either direction.

As the reservoir fell further below C, flow along stream #5 was also cut off. With no flow along trench or #5, the reservoir emptied more slowly, as water seeped into the water table. However, this was also high because of the near-flood, and the reservoir emptied more slowly than usual.

The river Stour flows over clay near Bear Mead. Easy run-off causes the river to rise quickly in response to rain, and also fall quickly when the rain ceases. This is accentuated when the water table is also high.

60k. River below 130, connecting trench empty.

60f. River below 130, not high enough to enter the connecting trench.

62a. River at 292, not high enough to cross 'hump' in trench

62h. River higher than 300, water pours along trench.

62d. White water forms along the trench.

62c. Fills area near concrete bridge.

62i. Rises and fills more of area.

62g. Fills area in SE of field 3.

62b. Fills DL's part of Mill Stream.

62e. Flows under NT bridge, 1 mile downstream.

62f. Flows leaves old Stream bed, turns right into Stream 5.

62k. Stream 5 after last bridge.

62m. Mill Stream re-enters Stour near Eyebridge, fair flow at MS=68.

62j. Stream 5 re-enters the Stour 2 miles down stream.

62L. When river drops below 300, flow reverses, out of Mill Stream, over hump at 300 and back into River. The Mill Stream 'reservoir' begins to be emptied again.

62r. Taken 6 months after 62L, the new grass has altered the view.

62p. Taken 26 Nov 07, the Mill Stream with a bare spoil heap, a real eyesore.

62q. Taken 27 May 08, 6 months later, showing the great healing power of Nature.

62s. Taken 08 Jun 2011, 3½ years later, with dense vegetation.

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