1. Ripon, a City of Just Under 17,000 People, Is 54 Degrees N and 1.5 Degrees West and Sits Near to the Northern End of the Sout

RIPON CITY PLAN

Supporting Document: Sustainable Ripon

Appendix A.n: Ripon’s Geodiversity

Introduction

1. Ripon, a city of just under 17,000 people, is 54 degrees N and 1.5 degrees west and sits near to the northern end of the Southern Magnesian Limestone National Character Area (NCA 30) defined by the underlying Permian Zechstein Group, formerly known as the Magnesian Limestone. It creates a very long and thin area stretching from Thornborough in North Yorkshire to the outskirts of Nottingham. The limestones, marls and enclosed gypsum horizons create a ridge, or narrow belt of elevated land, running north–south and forming a prominent landscape feature. The geology has influenced the form of the landscape, the use of limestone for local building stone and mortar, the specialised limestone grasslands, and in Ripon’s case, and the villages to the north and south, produces unstable land and a particular topography that has defined its settlement history and has geographically limited development.

2. The key to the unique landscape and development of the settlement of Ripon is its geodiversity, defined as earth materials, landforms and processes that shape the Earth. The geodiversity is manifested as the rocks, minerals, soils, topography, rivers and landscape of the area. Ripon sits at 30m above sea level on fluvio-glacial and river terraces that form the higher land between the Ure, Laver and Skell valleys. In the recent past these rivers cut deeply into the underlying Permian deposits, especially to the west of the City, when the flow greatly increased with meltwater from the glaciers of the last Ice Age only 13,000 years ago. The Laver and Skell now sit as ‘misfit’ rivers in deep gorges best seen on the Laver at Bishopton on the footpath to Galphay Lane from the Riverside Caravan Park, and on the Skell at Fountains Abbey in the “valley of seven bridges”, and by High Cleugh on the City’s edge; the Ure gorge is seen at its best at Hackfall to the north of the City.

3. The rocks beneath the City consist of superficial fluvio-glacial and river deposits of boulder clay, sands and gravels overlying Permian limestones that have two distinct layers of gypsum and mudstones and marls. These dip gently eastwards and in turn are overlain in the east by the Sherwood Sandstone of Triassic age. The distribution of the solid geology is further complicated by small outliers of younger strata across the eastern half of the City due to localised foundering of strata caused by gypsum dissolution beneath. There are approximately 10m of gypsum in the Roxby Formation, and 35m in the Edlington Formation, and both beds rest on two limestones aquifers, the Cadeby Formation and the Brotherton Formation. The dip from west to east allows water to feed down the dip into the gypsum beds, then moves into the over-deepened valley of the River Ure buried by sands and gravels.

4. The valleys and higher ground were buried beneath the glacial and river deposits which also fill and cover many of the solution hollows and breccia pipes caused by dissolution of the gypsum making development patchy, and resulting in unstable land unique to Ripon and adjacent villages of Sharow, Littlethorpe and others within the strip of Permian strata that extends both north and south of the City.

Key Sherwood Sandstone Group Roxby Formation marl Roxby Formation gypsum Brotherton Formation limestone Edlington Formation marl Edlington Formation gypsum/anhydrite Cadeby Formation limestone Carboniferous

Fig. 1. Geological block diagram of the Ripon area. Geological boundaries as revised by A. H. Cooper (BGS) and A. Thompson (STM), 1996. Quatemary drift deposits not shown.

Visual sensitivity

5. The sequence of Permian and Triassic rocks decreases in age, and dips gently from west to east however the Permian limestones in the west, and the Triassic sandstones create ridges of higher ground separated by the wide flood plain of the River Ure. Consequently there is high visual sensitivity due to the prominent ridges and inter- visibility with the Ure valley flood plain. The views to and from the Magnesian Limestone and Triassic Sherwood Sandstone Ridges are sensitive to the introduction of tall vertical elements or large-scale development especially towards and from the Fountains Abbey World Heritage Site and the edge of the Nidderdale AONB.

Ecological sensitivity

5. There is also ecological sensitivity as a result of the presence of 2% of the UK’s Magnesian limestone grassland, and Geological SSSI at Quarry Moor, the Hell Wath Local Nature Reserve, and the green and blue ecological corridors of the Skell, Laver and Ure rivers that transect the City, especially on western edge, and link with the WHS, AONB, and the Local Plan Special Landscape Area.

Dissolution and subsidence

6. There are local artesian springs charged with sulphates, and dissolution gypsum cave systems have developed resulting in large surface collapses of up to 30m across and 20m deep. The subsidence occurs in a pattern related to the jointing in the rocks, and one significant subsidence event occurs every few years. These events show that some zones are more active than others with land either side of the Ure more prone to subsidence caused by the local escape of water from the caves into the buried Ure valley gravels. The superficial fluvioglacial and river deposits fill and cover many of the historic solution hollows and breccia pipes caused by dissolution of the gypsum.

Implications of dissolution

7. Subsidence caused by dissolution makes building construction difficult to impossible. The best approach with construction is good site investigation and hazard avoidance that can cope with any expected subsidence. Linear structures such as roads and bridges are very prone to subsidence damage, and the link below highlights Ripon’s problem and mitigation. www.bgs.ac.uk/science/landUseAndDevelopment/shallow_geohazards/developmentInRi pon.html

8. The presence of buried valleys beneath the Ure and Skell complicate the pattern of rocks at outcrop and influence gypsum dissolution, making it essential that there are no soakaways or broken water or sewerage mains in the City to provide water for rapid dissolution of the gypsum.

Planning policy

The Harrogate District Local Plan 2001 introduced a policy to manage construction proposals within the area susceptible to gypsum dissolution following research by British

geological survey and the then Department of the Environment. This area is known as Zone C where gypsum and/or anhydrite is likely to be present at relatively shallow depths and affected by ground water moving towards the buried valley of the River Ure. Accordingly, gypsum dissolution and associated subsidence are likely. Detailed investigation and mitigation will be required. Observation and experience of this area and the effectiveness of the original planning policy has shown the following problems.

Problems drilling boreholes and need for less invasive investigation

9. The policy introduced in the Local Plan recommended drilling boreholes through the gypsum to the underlying Cadeby Formation dolomite to prove or disprove the presence of voids, of gypsum and to characterise the site. Any holes drilled must be grouted with sulphate-proof grout and should conform to the current Environment Agency guidance on good practice for decommissioning boreholes and wells (Environment Agency 2012). The drilling of boreholes has been shown to open up hydrological pathways through the gypsum and could lead to increased dissolution and the release of artesian groundwater.

10. Depending on the techniques used, the drilling of boreholes commonly introduces high pressure air or water into the ground and can create vibration. Reactivation of existing subsidence features has occurred beneath or in close proximity to drilling sites. Ground stability and site safety should be considered before any invasive drilling in the Zone C area.

11. A few deep boreholes scattered across a site will not find all the cavities (if any) and ground conditions can change considerably in a metre or so. Consequently, the use of microgravity geophysics (Patterson et al. 1995), resistivity tomography geophysics, ground probing radar geophysics (Gutiérrez et al. 2008) and shallow boreholes are considered preferable to deep boreholes for proving the nature of rockhead and any soft ground associated with subsidence.

Sulphate-rich groundwater

12. In the lower parts of Ripon there are springs that emanate sulphate-rich groundwater, these are produced by the dissolution of the gypsum. Special concrete is required for construction in these areas. Springs and sulphate-rich groundwater are commonly associated with the areas of peat that in itself is also a geological hazard problem.

Avoidance of soakaways, water abstraction boreholes and deep ground source heat pumps in Ripon.

13. In areas of unstable ground and underground cavities the ingress of water into the ground and extraction of water from the ground are both proved to trigger subsidence events (Cooper and Calow 1998; Cooper et al. 2011). In addition the passage of enhanced quantities of groundwater can accentuate local gypsum dissolution causing subsidence (Cooper 1988). Consequently, there should be an avoidance of soakaways, water abstraction boreholes and open loop ground source heat pump installations. Furthermore, water supply and sewage pipes should be protected from subsidence and breakage, which can allow water ingress and aggravation of the subsidence situation.

Competence

14. The current policy adopted by HBC for development in Zone C is not working. At least one site developed in the recent past is now suffering subsidence. The problem appears to be the interpretation of the ‘competent person’ required to investigate, avoid and/or mitigate the problems. The ‘competent person’ term should now be replaced by a 'Registered Ground Engineering Specialist or Advisor’ listed in the RoGep register. In addition, the policy and form emphasise the requirement for the signing of the Ground Stability Declaration to be specifically included in the signatory’s professional indemnity insurance.

Ripon City Plan

15. The Ripon City Plan has been able through the experience of the past 19 years to revise the existing Local Plan policy. This appendix should be read alongside Appendix A.g: British Geological Survey submission and advice and Appendix A.k: Briefing report on the development control problems in Ripon.

References and sources of information The report by Thompson et al (1996) prepared for the Department of the Environment for the Ripon area describes the problems and gives the planning guidance that with minor changes forms the basis of this proposed policy – copies of this report are held by Harrogate Borough Council. A summary is also published by Thompson (Thompson et al. 1998).

Other publications specific to Ripon subsidence problems include (Cooper 1986, 1988, 1998, 2005, 2008; Cooper and Burgess 1993; Cooper and Calow 1998; Cooper and Saunders 2002; Powell et al. 1992). Many of these publications are deposited in Ripon Library and many can be downloaded as pre-print versions from the NERC Open Research Archive website – Author A H Cooper http://nora.nerc.ac.uk/view/author/3274.html The geological memoirs (Cooper and Burgess 1993; Powell et al. 1992) can be purchased from the British Geological Survey.