Experiences of Groundwater Flooding, South Winterbourne Valley, Dorset Charles Bennett1, Brian Richards2 1Parsons Brinckerhoff, 2Dorset County Council

INTRODUCTION 100 124 90 Friar Waddon Rainfall (mm) 122

80 Kingston Russell Borehole 120 )

Groundwater Level (m AOD) D

118 O

70 A ) y m Heavy rainfall on the 6th and 7th July 2012 a 116 ( d

60 l /

SOUTH e v m 114 resulted in groundwater, fluvial and surface water WINTERBOURNE e m L

( 50 r l l 112 e t a f flooding in three villages in the South 40 a n i 110 w a d R n

Winterbourne valley in south west Dorset, UK. 30 u

108 o r G Rainfall records from nearby rainfall gauges 20 106 showed 112mm of rainfall in the 38 hour period 10 104 0 102 y y y y y y y y y y y y y y y y between1pmon6thJulyand3amon8thJuly. e e e e e e e e e e e e e e e l l l l l l l l l l l l l l l l n n n n n n n n n n n n n n n u u u u u u u u u u u u u u u u u u u u u u u u u u u u u u u J J J J J J J J J J J J J J J J J J J J J J J J J J J J J J J 1 3 5 7 9 1 3 5 7 9 1 3 5 7 9 1 1 3 5 7 9 1 3 5 7 9 1 3 5 7 9 0 0 0 0 0 1 1 1 1 1 2 2 2 2 2 3

The rainfall was assessed using the Flood 0 0 0 0 0 1 1 1 1 1 2 2 2 2 2 Figure 3. Project location. Estimation Handbook (FEH) methodology to have Figure 5. Groundwater (red line) response to an annual probability of 1.3% - 2.2% (return rainfall (blue) June – July 2012. period of 40 - 80 years). High flood levels caused INVESTIGATION the closure of the A35 trunk road through the Other contributing factors to flooding include restrictions in the capacity of the South valley and 42 properties in Winterbourne The factors contributing to the flood risk in South Steepleton, Winterbourne Abbas and Martinstown Winterbourne stream as it flows through the Winterbourne were assessed using eyewitness affected villages. The intense rainfall on saturated suffered internal property flooding. Water levels accounts, photographs and hydrological data, remained high for over two weeks following 6th ground also led to high volumes of surface water including rainfall records, flow data for the South runoff from agricultural land. July. Winterbourne stream and groundwater monitoring The high rainfall followed an extended period of boreholes. above average rain, with the total rainfall from The views of the local water supply and sewerage April – July 2012 close to twice the long-term undertaker, Wessex Water and the UK average for this four month period. Environment Agency were also obtained.

180 2012 Mean 1927 - 2012 CAUSES OF FLOODING 160

140 The primary factor contributing to the flooding risk 120 Figure 6. Example of restriction in capacity of )

m was assessed to be high groundwater levels, due 100 m

( South Winterbourne within the study area. l l to the underlying geology of the area. The South a f n

i 80 a Winterbourne takes its name from the ephemeral R OPTIONS FOR ALLEVIATION 60 chalk stream or ‘winterbourne’ that flows through

40 the valley. A high proportion of flows in winterbourne streams – and sometimes all – is of A range of measures were considered to alleviate 20 groundwater that emerges from the chalk bedrock the flooding in the affected villages: 0 in the lowest parts of the valleys. Steeply sided Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 2 Catchment wide approaches such as flood winterbourne valleys are particularly susceptible to relief culverts or diversion schemes not Figure 1. 2012 Dorset rainfall record against groundwater flooding. The fractured nature of the Dorset 1927-2012 long-term average. feasible for the relatively small number of chalk allows rapid infiltration to the water table, properties at risk of flooding; As the Lead Local Flood Authority for the area, resulting in rising groundwater levels, greater Large-scale groundwater pumping technically Dorset County Council led an investigation into the groundwater emergence and higher fluvial flows. 2 impracticable. flooding. Parsons Brinckerhoff was appointed to assess the flood risk in the catchment and identify 9 Household flood protection approaches in measures to improve protection and resilience coordination with improved management and against future flood events. maintenance of surface water infrastructure The catchment has suffered significant flooding in and improved awareness of groundwater flood the past, notably on 19th July 1955, when 280mm risk identified as the best approach for the of rainfall fell in 24 hours. This was a UK record management of future flood risk in the until 20091. catchment. The formation of a Community Flood Action Group Figure 4. Illustration of fractures within chalk was recommended to support this approach, geology (supplied by EA). creating a representative voice for flood concerns for the community. The Group would help to Analysis of groundwater records in the catchment coordinate local flood risk reduction and resilience over the period of the most recent flooding measures, such as maintenance of the indicated a sharp rise in groundwater level directly watercourse and the preparation of community following the 112mm of rain that fell on 6th - 8th flood plans to be implemented in the event of July 2012. Recorded groundwater levels rose forecasted periods of high flood risk. 8.78m between 7th and 8th July. Specific recommendations to help alleviate localised flood issues in each of the three villages were also made. These included removing Figure 2. Flooding in Martinstown, July 1955. obstructions to flow in the watercourse, such as low access bridges and the restoration of disused bifurcation channels.

References 1. Shaw et al, (2011). Hydrology in Practice. 3rd Edition. Oxford: Spon Press

International Conference on Flood Resilience Experiences in Asia and Europe 5-7 September 2013 Exeter United Kingdom