Flooding on Dorset Stour - Averages.

This page shows monthly and yearly average RAINFALL and WATERTABLE for Bear Mead. Also an estimated RIVER FLOW RATE for a range of river heights, MASS TRANSPORT caused by rain, and pH readings. Location of Bear Mead: 2,100 metres upstream of Julians Bridge, Wimborne Minster, Dorset, UK. and 1,090 metres upstream of Eyebridge.

Have a look at the following: Daily report. Numbers for past records. Graphs for past records.

Rainfall at Blandford, Dorset in mms per month 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 Average Jan 132 29 119 114 164 76 15 129 162 44 108 98 124 128 55 26 161 99 Feb 43 48 8 111 120 86 112 12 36 94 83 111 41 50 18 59 105 67 Mar 94 57 57 84 51 63 34 69 25 46 146 56 43 51 47 77 75 63 Apr 75 91 88 54 35 58 27 102 88 152 88 54 52 80 69 24 4 67 May 8 22 41 77 38 46 47 65 38 64 22 107 44 37 31 101 111 53 Jun 97 38 64 15 5 44 87 82 53 21 21 35 47 44 33 24 109 48 Jul 66 71 60 35 26 14 12 53 11 54 64 49 50 45 60 40 132 50 Aug 28 114 29 39 10 67 146 20 95 41 23 36 14 86 59 52 86 56 Sep 68 127 158 93 129 24 14 103 110 111 59 49 5 50 26 45 32 71 Oct 97 34 134 121 66 66 57 136 75 173 168 167 82 177 129 145 36 109 Nov 59 151 63 95 133 154 187 87 47 167 44 220 125 25 81 153 115 112 Dec 34 121 152 130 91 30 108 101 175 164 26 139 74 61 89 84 96 97 Total 801 903 973 968 868 728 846 959 915 1131 852 1121 701 834 697 830 1062 893 (Data derived from that supplied by Environmental Agency for Rivers House, Blandford.)

Note: The 2 month delay between rainfall and watertable maxima. Flood season covers Nov-Mar, 6 of 11 floods occurd Nov 26-Dec 15. The high July watertable average, caused by a very wet '07 and '08.

Watertable at Bear Mead, Dorset Average cms above datum '03 '04 '05 '06 '07 '08 '09 '10 Average Jan - 172 129 114 202 185 159 160 Feb - 140 110 116 179 163 181 148 Mar - 133 86 114 164 152 145 132 Apr - 134 100 107 108 123 92 111 May - 115 89 104 117 133 96 109 Jun - 93 92 107 116 143 97 108 Jul - 95 84 97 162 150 95 114 Aug 55 82 71 72 119 107 104 87 Sep 49 77 51 59 88 102 72 71 Oct 48 130 65 86 81 108 68 84 Nov 104 121 123 142 114 181 176 137 Dec 158 125 144 194 175 158 211 166 (Data measured by author at Bear Mead)

River Flow rate at Bear Mead

Water speed measured by timing a floating branch to travel the drift line (about 167 metres). (This method does not work in summer with river <200 because of weed). Drift length and river width measured using Google Earth. Depth measured using a kayaker with a pole with scale to sound the river in several places. Centre of picture is at O.S. ref: SY 989-994 North shore 164.64 metres, Centre line 167.08, South shore 170.76, Average width 21.62, Average depth 2.70 at River=290 (measured by Warren in a red kayak) Volume 167.08 x 21.62 x 2.70= 9,753 cu metres for river=290. Floods when river height=350 when flow rate=80 cu m/sec=80 tonnes/sec

River Flow readings Estimated Flow rate in cubic metres/second for different river heights. Flow rate=[167.08x21.62x (height-0.2)]/time. River Drift Water Flow Date Height Time Speed Rate 2008 cms secs m/sec cu m/sec 4 Jun 290 180 0.93 58 5 Jun 224 260 0.64 31 6 Jun 201 360 0.46 20 10 Jul 278 202 0.83 50 11 Jul 310 180 0.93 62 12 Jul 250 240 0.70 38 13 Jul 213 316 0.53 24

MASS TRANSPORT

On 4-5-6 February 2008, 32 mms of rain fell on the 1,300 sq km Stour catchment area in 48 hours, a total of 42 million tonnes. About 20% became surface runoff, causing an increase in the river flow rate (highlighted), estimated at transporting an extra 8.4 million tonnes in 6 days. A peak of 3.4 million tonnes was reached on the third day after the rain. About 80% soaked into the ground, causing the groundwater level to rise by an average of 2.56 cms. This stored rainwater would return to the river by seepage over a period of months or years, or be reduced by evaporation or transpiration during the remaining summer. In the floodplains, the groundwater rose by 55 cms before seepage caused it to return to its previous value in about 7 days.

On 9th and 10th July 2008, 47 mms of rain fell on the 1,300 sq km Stour catchment area in 24 hours, a total of 61 million tonnes. About 17% became surface runoff, causing an increase in the river flow rate (highlighted), estimated at transporting an extra 10 million tonnes in 5 days. A peak of 4 million tonnes was reached on the second day after the rain. About 83% soaked into the ground, causing the groundwater level to rise by an average of 3.9 cms. This stored rainwater would return to the river by seepage over a period of months or years, or be reduced by evaporation or transpiration during the remaining summer. In the floodplains, the groundwater rose by 80 cms before seepage caused it to return to its previous value in about 7 days.

Comments on pH Pure, distilled water has a pH value of 7.0. Stream water usually ranges from pH 6.5 (slightly acidic) to a pH of 8.5, an optimal range for most organisms. Rain water by contrast is naturally acidic at about 5.6. The pH of a stream affects the organisms living there. The optimum for most is between 7 and 8 and aquatic insects 4-5. Acid rain, from car exhaust or coal-fired power stations, causes a drop in the pH of water. Pollution from accidental spills, agricultural runoff and sewer overflows can also change the pH. Buffering capacity is water's ability to resist changes in pH, and is critical to the survival of aquatic life. Limestone soils act to neutralize acidity and often result in a more basic pH. While young fish and insect larvae are sensitive to a low pH (acid), extreme values on either end of the scale can be lethal to most organisms. The type of geology and soil that the water is flowing over affects pH, e.g. chalk soil makes the result slightly alkaline and clay soil slightly acidic. A river flowing through chalky soil results in the pH level being about pH 8, very slightly alkaline pH is also affected by photosynthesis of algae and other plants in summer. pH declines gradually in response to higher rainfall and increased total annual discharge. Rivers flowing through agricultural areas carry increased sediment loads, have elevated temperatures, pH and PO4 concentrations, and lower oxygen. Macroinvertebrate communities vary according to land use and are most severely impacted downstream on towns. THE pH SCALE Higher numbers indicate alkalis, lower values signify acidic liquids: 13 - bleach 10 - soap 8.2 - pre-1750 oceans (average) 8.1 - current oceans (average) 7.8 - oceans in 2100 (projected average) 7 - pure water 3 - vinegar 0 - battery acid (Source: NMEA) Response time of sources to rain is: River 33 hours, Well 10 hours, Millstream 3 hours, and Scrape 0 hours pH Readings Rain Date Buffer River Outflow Well MStream Scrape Tapwater Rainwater 0 17 Jun - - - - - - 7.13 - 0 18 Jun - 8.29 - 7.84 - - - - 2 19 Jun - - - - - - - - 1 20 Jun 6.87 8.29 - 8.12 8.52 - - - 7 21 Jun - - - - - - - 5.35 0 22 Jun 6.88 8.26 - 8.35 7.88 7.34 - - 0 23 Jun 6.88 8.41 8.43 8.09 7.83 7.24 - - 0 26 Jun 6.85 8.59 8.50 8.29 8.31 7.15 - - 4 03 Aug 6.87 8.21 - 7.51 7.04 7.18 - 5.34

Level Calibration near Flood

Main 12 Nov 2008 1250 hrs Lev=344 0 Min=110

Repeater 12 Nov 2008 1250 hrs Lev=338 -6 Min=

Mill Stream 12 Nov 2008 1300 hrs Lev=332 -12 Min=245

Eyebridge 12 Nov 2008 1322 hrs Lev=342 -2 Min=275

Pamphill Green viewpoint, 12 Nov 2008, 1325 hrs, Lev=344, 30% flooded

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