Updated 9 Mar 2010
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Tidal Trench
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Tidal Trench on the Old Mill Stream
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Looking South to Mill
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In Sep 2007, damage caused by blockage of an old arm of the River Stour
was repaired at Bear Mead near Wimborne in Dorset.
A trench was restored, reconnecting the river with the Old Mill Stream
by excavating and deepening 68 metres of the former channel.
The object was to re-introduce flowing water into an old, 2 mile long
Mill Stream which had become heavily silted up.
Also to help store water in the Mill Stream during river flood and
drain it back afterwards.
A strong flow has been observed through the trench both Northward
and Southward, hence the description "tidal".
The battle between instant local rain and delayed distant rain is
fascinating.
After observing three seasonal cycles of flood and drought, and lowering the
trench bottom in stages to 1 metre below surrounding flood level,
the results appear encouraging.
The trench is now visited by otters.
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Looking North to river
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FLOW: |
South into Mill Stream |
North into River |
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LEVEL: |
of River |
100m up Mill Stream |
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RAINFALL: |
per Day |
River rising and filling the Mill stream reservoir.
FILL flow, 16 Jan 2008, 0841 hrs, R=356, MS=325.
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Meeting of the waters. The silt-laden Stour meets the much clearer
Mill Stream. Near balance, with slight flow into the Stour.
BALANCE flow, 29 Nov 2009, 1150 hrs, R=292, MS=293.
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River falling and Mill stream reservoir draining back into it.
DRAIN flow, 18 Jan 2008, 1549 hrs, R=267, MS=308.
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From Post 01, otter stone, river.
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From Post 01, otter stone, bridge.
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From Post 04, scale.
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From Post 11.
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From Post 17.
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Drain flow in trench, river=225,
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Mill Stream=275, 1630 hrs,
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27 Feb 2010
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The Tidal Trench is 68 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 250 cms above datum
(as of Aug 09).
This prevents flow in either direction except under the following
circumstances:
- The River >250 and >the Mill Stream, producing
FILL flow.
- The Mill Stream >250 and >the River, producing
DRAIN 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.
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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
FILL flow, due to country-wide rainfall that flooded much of middle
England.
FILL flow has the advantage of speeding storage of river water in the Mill
Stream reservoir when the river approaches flooding.
DRAIN flow assists drainage from the Mill Stream reservoir when flooding
is over.
The Mill Stream is unusual in that there are four inflowing brooks 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 at a
level of 250, as is evident from clear water flowing into the silted river
at the North end of the trench.
After 820 days of testing, the tidal trench was found to have:
To Feb 2010
No flow: 75%
DRAIN: 20%
FILL: 5%
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Days with flow in trench
DRAIN FILL DRAIN FILL
-----Note 1-----
Dec 07 4 1 Dec 09 22 9
Jan 08 8 4 Jan 10 27 4
Feb 4 0 Feb 19 3
Mar 4 0 Mar na na
Apr 0 0 Apr na na
May 0 3 May na na
Jun 0 0 Jun na na
Jul 0 0 Jul na na
----Note 2------
Aug 0 0 Aug na na
Sep 1 0 Sep na na
Oct 3 0 Oct na na
Nov 9 3 Nov na na
Dec 08 9 2 Dec 10 na na
Jan 09 11 2
Feb 14 2
Mar 6 0
Apr 2 0
May 0 0
Jun 4 0
----Note 3------
Jul 1 0
Aug 5 0
Sep 0 0
Oct 2 0
Nov 13 8
TOTAL 168 41
Ratio 4.1
Notes
1 Hump set at 300
2 Hump lowered to 270
3 Hump lowered to 250
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OTTERS in the Trench
The trench has now become a highway for wildlife.
From its excavation, otter footprints and spraint
were seen.
7 Jan 2010, Jane Adams
(Co-ordinator of
Nature Watch Corfe Mullen) placed a large stone at the bottom
of the trench, about 3 metres from the river.
9 Feb 2010, two large otter spraints were seen balanced
on top of the stone. Otters were using the stone as
a territory marker.
5 Mar 2010 a multitude of prints appeared
on the silt delta where the trench meets the river.
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30 Nov 2009 Stour flows into the Old Mill Stream
Looking North, R=350, depth=1 metre.
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1 Dec 2009 Flow continues down the Old Mill Stream
Looking South, R=343, depth=93 cm.
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3 Dec 2009 Flow leaving Old Mill Stream into
Julian's Stream.Looking East, R=306.
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Two-directional "tidal" flow in the Old Mill Stream.
Flow reversed 10 times due to 241 mms rain in 31 days,
causing 3 floods.
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Flow in the old Mill Stream
Work in summer 2009 took the level of the trench bottom down to 250,
ready for the wet season. River or Mill Stream levels above
250 produce two-directional "tidal" flows shown in the graph.
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1,4,7 Other streams (OS) taking local rain into
the Mill Stream (MS) cause a rise in level even when there is a flow into
the river.
2. Inflow from the river and OS pulls the
MS upwards.
3,6 Zero local rain and drop of river causes
the exponential decay of MS.
5. River and MS levels nearly balanced, with
little flow either way.
8,10. Flow into the MS only from the river pulls the
MS up. Flow out via Julians stream (JS) prevents MS from rising above
about 320.
9,11 Heavy rain reverses fall of river and starts
filling MS via OS.
12 Continued heavy rain pushes river into flood.
Heavy inflow into MS pushes it up to 330. Dry period allows river to drop
and MS to drain into it.
13 River drops further, JS cuts off at 288, MSE at
about 280 and MS at 250.
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Comparison of areas leads to the conclusion that inflow from river and
other streams are approximately equal.
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Julian's stream (JS), 1 km long, leaves MS at SY997994, flows under
Julian's Rd, to SZ007992 near Merley Hall farm where it joins the
Stour. About 80% of water in full MS flows down JS into Stour, preventing
MS from rising above about 320.
See photos 62f, 62k, 62m, 62j.
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Lengthwise profile along 68 metres of ditch.
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Pipe bottom defines lowest effective level.
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Post bot top wdth
01 253 398 95
02 255 392 90
03 254 393 100
04 254 388 120
05 253 383 130
06 252 387 120
07 249 379 105
08 247 372 125
09 251 366 115
10 246 353 95
11 253 355 100
12 248 345 105
13 243 348 100
14 244 329 100
15 250 340 110
16 242 317 100
17 240 305 115
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Post bot top wdth
18 237 302 115
19 238 288 110
20 231 281 120
21 214 279 130
22 213 283 125
23 210 277 125
24 213 283 115
25 213 283 110
26 212 279 110
27 212 282 115
28 213 287 115
29 218 285 115
30 205 280 115
31 212 286 120
32 209 284 105
33 214 282 95
34 219 280 105
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Post bot top
CBrN 227
Cpipe 237 332
CBrS 238
MS
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MILL STREAM FLOW: |
S(outh) into Mill Stream |
N(orth) into River Stour |
**=Balanced (zero) flow |
01 NOVEMBER 2009
196 mm (350cm) (250cm)
Date Rain River MillStream
1 21 - -
2 0 - -
3 4 173 256
4 1 - -
5 0 210 253N
6 5 - -
7 0 160 250
8 0 - -
9 0 - -
10 6 145 248
11 2 147 250
12 13 - -
13 20 163 270N
14 13 277 278**
15 11 329 296S
16 5 333 315S
17 0 255 282N
18 0 200 265N
19 0 175 260N
20 6 167 258N
21 10 - -
22 12 277 275N
23 13 303 292S
24 0 290 285**
25 12 292 289**
26 3 289 285**
27 2 212 275N
28 14 180 -
29 23 292 293N
30 0 357 320S
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DECEMBER 2009
150 mm (350cm) (250cm)
Date Rain River MillStream
1 0 343 318S
2 22 260 293N
3 16 306 300S
4 1 354 323S
5 11 260 290N
6 14 284 294N
7 8 328 308S
8 6 347 320S
9 6 361 330S
10 0 322 307S
11 0 245 283N
12 0 227 276N
13 0 217 271N
14 0 210 267N
15 2 206 267N
16 2 206 266N
17 1 208 266N
18 0 - -
19 0 196 262N
20 2 192 262N
21 2 - -
22 4 - -
23 9 - -
24 15 - -
25 0 222 272N
26 5 - -
27 2 - -
28 0 205 271N
29 11 - -
30 11 275 281N
31 0 292 288N
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78 JANUARY 2010
72 mm (350cm) (250cm)
Date Rain River MillStream
1 0 - -
2 0 - -
3 0 - -
4 0 182 261N
5 1 - -
6 2 175 258N
7 0 174 256N
8 0 170 256N
9 0 - -
10 0 - -
11 0 165 253N
12 0 - -
13 9 - -
14 0 179 259N
15 1 - -
16 18 231 276N
17 11 305 291S
18 0 353 322S
19 0 221 276N
20 6 203 270N
21 0 207 266N
22 24 - -
23 0 295 286S
24 0 306 -
25 0 214 268N
26 0 198 264N
27 0 190 262N
28 0 187 261N
29 0 - -
30 0 - -
31 0 184 256N
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79 FEBRUARY 2010
89 mms (350cm) (250cm)
Date Rain River MillStream
1 0 181 256N
2 0 - -
3 7 - -
4 1 218 256N
5 4 215 257N
6 4 220 259N
7 0 204 256N
8 0 - -
9 0 184 255N
10 0 - -
11 0 175 252
12 0 170 251
13 0 168 251
14 0 166 251
15 0 165 250
16 4 - -
17 4 177 253
18 9 183 257N
19 0 280 265S
20 0 242 263N
21 5 197 257N
22 11 261 265N
23 7 290 280S
24 6 - -
25 6 232 -
26 0 239 266N
27 11 225 275N
28 10 286 277N
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80 MARCH 2010
mm (350cm) (250cm)
Date Rain River MillStream
1 0 291 282S
2 0 216 267N
3 0 186 263N
4 0 186 261N
5 0 180 260N
6 0 175 260N
7 0 173 260N
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Compiled, formatted, hyperlinked, encoded,
and copyright © 2008, John Palmer,
 All Rights Reserved.
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