Peculiar rain – and more runoff

In keeping with the requirements of the National Planning Policy Framework (NPPF), Dorset Council’s Flood Risk Management Officer states: .”Regardless of prevailing risk, development, through the introduction of impermeable areas, has the potential to exacerbate or create flood risk“. North Dairy Farm is in a unique location – shaped by Dorset’s record rainfall – and the area floods!

Late October on the ‘Povert canal’ – or the road to Hazelbury Bryan as outsiders call it!

Three times as much rain

The lowland coastal areas are the sunniest parts of Dorset. While Bournemouth records well below the national average in annual rainfall (483 mm) the North Dorset Downs experience lower temperatures and more precipitation than the coast. Approximately three times as much rain (1500 mm pa) falls on the high ground and escarpments above Hardy’s Vale overlooking North Dairy Farm.

Increasing precipitation

Southern England is one of the more sheltered parts of the UK, the windiest areas being in western and northern Britain, closer to the Atlantic. The Met Office predicts that within 40 years the average warmest summer day in the southwest will increase by 3 °C to 31 °C. It predicts that the region will have one of the highest annual temperatures in the United Kingdom and there will be an estimated 53 millimetres (2.1 in) increase in winter precipitation.

On the high ground!

Altitude affects rainfall. Moist air ascending the Dorset Downs may be cooled below the dew point to produce clouds and rain. A map of average annual rainfall, therefore, looks similar to the topographic map below.     

The wettest areas in Southern England are the higher parts of Dorset and the South Downs, with an average of over 950 mm per year. While Dorset’s annual average precipitation is 483 mm, areas of the North Dorset Downs and escarpments can receive over 1400 mm. 

Saturated for six months

The periods of prolonged rainfall frequently lead to widespread flooding, especially in winter and early spring. The Applicants soil survey notes that the North Dairy Farm ground is saturated for six months a year.

(Fig. below, annual rainfall is shown in shades of purple) The 42 square kilometre catchments focus their flows around and over the farm site. They combine a few meters north of it, eventually, joining the River Stour.

Dorset’s intense precipitation events

Whereas the Spetisbury solar development is on dry freely draining chalk land, the North Dairy Farm proposed site is mainly covered with impervious clay, and unlike Spetisbury, almost surrounded to the south by the high downs and escarpments.

Averages don’t tell the story!

The threefold increase in the rainfall averages between the coastal areas and the ground above the Blackmore Vale is significant and has created and shaped the ‘Vale of the little dairies’. Rain can fall in very destructive ‘rapid discharge’ events that give the Vale its distinctive farming landscape and often dictate the position of its field boundaries. It is remarkable how little the field shapes around the farm have changed since the post-middle ages, thanks, in part, to the area’s multitude of natural waterways.

The Applicant suggests that significant changes to the fields have taken place since 1888 and that the fields “generally have lost their historic hedge field boundaries”. This is misleading and inaccurate – see 1888 map (left) and 2022 (right). They are almost unchanged.

Dorset holds the UK record!

High-intensity rainfall also results in flooding. The thunderstorms that broke out during the afternoon and evening of 18 July 1955 resulted in a remarkable 279.4 mm rainfall at Martinstownthe highest daily rainfall ever recorded in the UK.

Increasing risk

Government organisations predict Dorset and the south will experience a rise in temperature and become the hottest region in the United Kingdom. This will increase the frequency of summer thunderstorms and intense rainfall events. The biggest increases are predicted to occur over Dorset and northwest England.

Peculiar and unique

North Dairy Farm is hydrologically unique. The Hydro-GIS Review and the SHV representations point out that the Cook and McCuen findings cannot be relied on where the land has very low rates of infiltration, is saturated for six months a year and is at the focus of two catchments that have exceptionally high levels of intense rainfall, and ground subject to rapid discharge flash flooding.

The image of another BSR solar development shows very poor grass cover on the worked-on area. The lower right clearly shows the ground scouring caused by rivulet run-off from the lower edges of the solar panels. The kinetic energy of the water draining from the solar panel could be as much as 10 times greater than that of rainfall. (Cook and McCuan 2013)

Energy ten times greater than rainfall

Cook and McCuen determined that the kinetic energy of the water draining from the solar panels could be as much as 10 times greater than the rainfall. Factor in the predicted increased rainfall intensity due to climate change (more rain falling in a shorter time – more often) to the ‘high energy’ water draining from the panels, and it is possible (as Natural England and the Council’s Flood Risk Management Team suggest) that soil below the base of the solar panels could erode. According to the Cook and McCuen modelling, this would result in high rates of, possibly channelised surface runoff, flowing over saturated ground that has a very low infiltration rate. Most importantly, this would result in shorter times to peak flow, and importantly, as Cook and McCuen’s results imply, increased downstream flooding.

100% increase in runoff

The Cook and McCuen modelling also assumes situations with healthy grass beneath the panels, and bare ground in the spacer section, which: “would simulate the condition of unmaintained grass and soil compaction resulting from regular maintenance vehicles driving over the spacer section. In these conditions, the peak discharge increased by 100%, which reflected both the increases in volume and a decrease in timing.” This condition is illustrated in the British Solar Renewables image above.

Evidence of flooding ignored

The North Dairy Farm Solar Flood Risk Assessment was undertaken for the Applicant by RMA Environmental in 2021. It noted: there is not sufficient data of historic flooding to make a fully informed judgement as to whether this area would be at a significantly increased risk from runoff from the solar park”. This statement is misleading. Representations to Dorset Council from one (among others) downstream riparian owner stated: “Old Boywood Farm has been owned by our family since the early 1900s. We have data and photographic evidence available spanning the last 58 years.” It appears the Applicants have ignored the offers of flooding evidence.

Infiltration will not work

Following the SHV representations about flooding and surface runoff, and the comments by the Council’s Flood Risk Management Team, the Applicant finally dropped the “no drainage needed” claim and proposed an infiltration-based system incorporating swales. However, they appear to have ignored the clear warning in their RMA ‘Flood Risk Assessment’ (FRA), paragraph 4.33, that: “The reported hydrological characteristics of the Site suggest that infiltration may not be feasible”.

Creating flood risk

The Council’s Flood Risk Management Officer (Chris Osborne) noted:

  • Regardless of prevailing risk, development, through the introduction of impermeable areas, has the potential to exacerbate or create flood risk
  • In keeping with the requirements of the National Planning Policy Framework (NPPF), all major development proposals must take due consideration of stormwater management and should offer a drainage strategy that does not create or exacerbate off-site worsening and should mitigate flood risk to the site.”
  • Whilst Dorset Council Flood Risk Management have some expertise with respect to hydrology, we are not hydrologists and I have not undertaken an academic literature search on this topic“.
  • PV panels “can, however, cause erosion, hence Sustainable Drainage Systems can be useful for storing flow to prevent turbid runoff from discharging into the natural environment.

Natural England also notes: “Concentrated runoff from the panels is likely to lead to erosion of the ground surface below, contributing significantly to water quality issues downstream/downslope.

From ‘not needed’ to essential!

The Applicant suggested from the beginning, that perfect grass would avoid the need to provide Sustainable Drainage Systems (SuDS) for the PV-panelled areas. They also said that undertaking soil infiltration tests would be unnecessary. It is now widely recognised that the intense rainfall events onto the saturated or flooded ground and the high kinetic energy flows from 55 acres (approximately) of impervious PV panels “has the potential to exacerbate or create flood risk.”


As mentioned, the Flood Risk Management Team have stated that they are “not hydrologists, and have not undertaken an academic literature search on this topic“. The team has also withdrawn from providing comment as a consultee on solar developments stating: “FRM actually have withdrawn from providing comment on such schemes, since January 2022, on a risk-based approach” and need to focus limited available resources elsewhere.

Not “fair, reasonable and practicable

In the light of the Applicant’s record of significantly underestimating the flood risks, the Councill’s lack of hydrological expertise and resource constraints, we believe it would not be safe, or meet the Government Guidance tests of being “clearly seen to be fair, reasonable and practicable”, to leave the critical matter of site run-off to be dealt with later (after a Grant of Approval) by the attachment of pre-commencement conditions.

Infiltration tests are imperative

Dorset Council’s Strategic Flood Risk Assessment states that it is “imperative that site-specific infiltration tests are conducted early on as part of the design of the development, to confirm whether the water table is low enough to allow for Sustainable Drainage System techniques designed to encourage infiltration” . Without the results of these tests, it is impossible for the Applicant to demonstrate (justify) that the “outlined” drainage system would:

  • Comply with the guidance, to avoid, reduce, delay and manage surface water flows
  •  Mimic the existing greenfield surface runoff volumes, and critically
  •  Reduce downstream flooding at the time of peak flow.

No such tests have been undertaken, therefore, granting approval would be unreasonable.

The Applicant has not ‘demonstrated’ that the existing runoff from the site will not be exceeded. Therefore, the Application is incomplete and inadequate, and granting approval would again be unreasonable.

For these, and the other reasons stated in the SHV Letters of Representation, we ask that the Application is refused.

Offsite worsening

The Flood Risk Management Team letter of December 3rd makes reference to this matter, as does the Essex Council Solar Guidance, and our reference to the Mynydd Y Gwrhyd Solar Farm Drainage Strategy. and: Save Hardy’s Vale – Letters to the Planning Officers

The ‘lake’ among the green grass in this image was the result of heavy rain on a day in October 2021. The flooded area also happens to be the place chosen by the developer to be the ‘Temporary Works Compound’ where all the heavy equipment needed to construct the solar generating plant is to be unloaded and stored.

North Dairy Farm is in a unique location – shaped by Dorset’s record rainfall.

© 2022

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