Lyme Regis

No Longer Slip-sliding Away

Elizabeth Johnson

Geology in Southwest England

Summer, 2006

(West, 2006)

Introduction

Lyme Regis is located in Dorset England, toward the western end of what is currently known as the “Jurassic Coast”. Two things set this town apart from other towns and villages we visited along the Jurassic Coast.

First, Lyme Regis is located on an exposed part of the shoreline, where there is no natural harbor and manmade structures are necessary to provide both a boat haven and protection from storms. The Cobb is the town’s jetty that extends into the sea, providing both harbor and defense. It dates at least to the 13^th century, has been destroyed by storms and rebuilt numerous times. Without The Cobb, there would be no town.

Second, Lyme Regis is built on a steep and actively eroding hillside, which has suffered major landslides in the past and is certain to experience more in the future. The geologic makeup of the hillside consists of a limestone base followed by layers of clay and topped with sandstone and sandy soil. This combination of sand over clay makes the cliffs inherently unstable. These clays are also loaded with fossils and have made the town internationally famous among scientists and fossil hunters. Unfortunately, clay does not make a good foundation for buildings. Lyme Regis’ treasure is also its curse.

In this paper, I plan to provide a brief history of Lyme Regis and The Cobb, and describe the geology that makes this a unique area. Then, I will discuss the current work being done to upgrade both the beaches and the town, which are intended to prevent or delay the erosion process and in the process improve the tourism industry for the town.

(West, 2006)

A Very Brief History

I imagine the shoreline of Lyme Regis in the Neolithic or Stone Age was quite different than today. Perhaps the river Lym was more significant than it is today; maybe there was a promontory or natural harbor that has long since eroded away. Evidence of human occupation in the area has been discovered in the form or stone tools, (Chapman & Clayton, 2006) so we know that prehistoric people at least passed through. If there were some natural estuary or protection from the elements, early people might have decided to stay a while. Although Roman occupation of the Southwest coast began in the first century Common Era (CE) no archaeological evidence has been found that they inhabited Lyme (Historical Appraisal, August 2005) the first written documentation of the town appears in 774 when Cynewulf, King of Wessex, gave land near the mouth of the Lym to the Abbot of Sherborne, who set up a salt works. After the Danes invaded for a second time in 840, Alfred provided ships to be harbored at Lyme. This is the first historical documentation of a harbor. (Roberts, 1834) What isn’t clear is whether the harbor was manmade or natural at this point in history.

Salt was the main industry of Lyme until1250, when Axmouth harbor was made useless by a major landslide at Haven Cliff. Lyme took advantage of the opportunity and developed their harbor for ships. Gradually, shipping took over as the main industry. Lyme added “Regis” to its name in 1284, when Edward I granted its charter.

The Cobb, a dominant and beloved feature of Lyme Regis, is actually a jetty extending into Lyme Bay that provides both protection from storms and a manmade harbor. There is little information on when the first Cobb was built but there is documentation that it was severely damaged by a storm in1313 and destroyed by another in 1377. The Cobb was rebuilt by driving two rows of oak piling into the sea floor, then placing large rocks between the rows. This was accomplished by tying the rocks to empty barrels, floating them into place, then releasing them to fall to the sea floor.

After suffering several more destructive storms, the southern arm was added in the 1690’s and replaced in 1793 after another storm destroyed it the previous year. In 1750, The Cobb was connected to the shore. In 1793, construction employed a new technology and made use of mortared stone. The Cobb was entirely rebuilt using Portland Stone and mortar in 1820. (Wikipedia, 2006) This is The Cobb you see today when you visit Lyme Regis. It is classified as a “Grade 1 listed building”. This listing puts The Cobb on the same level of historic significance as St. Paul’s Cathedral in London and prohibits alterations of any kind to the existing structure.

Lyme Regis enjoyed a long period as an important port, trading with France, Africa and the Americas and although small by today’s standards, was at one time estimated to be larger than Liverpool. However, that changed in the mid 17^th century as relations with France deteriorated and by 1700 trade had all but ceased (Historic Appraisal, 2005). Ships were being built larger, making the harbor unsuitable to any other than smugglers and fishermen. The harbor and port activities went into decline as the ships went elsewhere to unload their cargo.

English society had long been accustomed to traveling out of the cities during the summer months to escape the heat and diseases that bred there. France had been the destination of choice, with its sunny Mediterranean shores being the main attraction. But travel to France ceased when France and England went to war in the late 17^th century. Thus, the English stayed in England. English tourism, in the form of seaside holidays, has its roots in 1750 when a Dr. Richard Russell began advertising the health benefits of taking (swimming in) seawater (Chapman & Clayton, 1975). With its southern exposure and mild summers, Lyme Regis was in an ideal location to take advantage of this new fashion. The wealthy members of society began making pilgrimages to the sea and Lyme’s tourism industry was born. It is still the town’s main industry.

During the 19^th century, with the emerging science of geology and the radical idea of “deep time” (the notion that the earth is actually older than biblical accounts say it is), it became fashionable in elite society to collect fossils. This was an age of curiosity and scientific discovery, providing the people of Lyme Regis with yet one more source of income – fossil hunting. With the Blue Lias to the west and Black Ven to the east, Lyme Regis was and is a fossil-hunters Mecca.

When The Cobb was connected to the shore, the long-shore drift was cut off no longer supplied material to the beach. Then a seawall and promenade were built during the 19^th century, which had unintended consequences. The beach was isolated from the land and it lost an important source of sand and shingle from the constantly eroding hill above. The beach became depleted, continued to erode and, without sand and shingle to protect it, the seawall was being undermined by the waves. Tourists like nice, sandy beaches. They chose other locations for their holidays, and Lyme went into decline.

Landslides in Lyme

Erosion, landslips and landslides have been a feature of the Lyme Regis coastline since before humans inhabited the area and each event exposes more treasures: clues to earth’s ancient beginnings. Much of the original medieval town has been lost to erosion and landslides and, although efforts have been made in the past to slow or prevent them, they continue to plague the town. On Christmas Eve 1839 there was a tremendous landslide in the area known as the Undercliffs (Lyme Regis Museum, 2006). Other notable events in the past century include the erosion into the sea of a road between Charmouth and Lyme Regis in 1924 and a heavy landslip in the areas of Langmoor and Lister Gardens in 1962 (West, 2006). There have been more, with the most recent occurring in January 2006, between Lyme Regis and Charmouth (Chapman & Clayton, 2006).

The unique geology of this area makes Lyme Regis particularly prone to landslips. Lyme Regis is in a syncline with a Blue Lias base, followed by Shales with Beef and Black Ven topped with a bit of Greensand and chalk. Simply put, the town sits on stone, clay and then sand. Clay is impervious and slippery, and sand is porous. Since water cannot drain freely through clay and limsone, it is flows through cracks to the interfaces between the Blue Lias and clay. The combination of water and clay makes slurry and reduces the friction between the surfaces. The water fills the sand, making it heavier. Landslides are inevitable. When structures with impervious surfaces such as roads and car parks are added, the increased weight of the asphalt can worsen the situation.

The areas of Langmoor and Lister Gardens have historically been prone to landslips, with occurrences documented in the early 1900’s on (Historic Assessment, 2005). Lyme Regis was once a source of special limestone for cement that was valued for marine construction because of its durability and fast-setting qualities (West, 2006). Unfortunately, this quarrying undermined and weakened the slope. Some landslips occurred in the early 1900’s, and development in the area was avoided. However, with time all that was forgotten and in 1962 a developer started grading work in the area, with the intent of building 20 homes on the slope. He was unable to complete the project as the work triggered a huge landslide (West, 2006). The project was abandoned, the area was again re-graded, and Lister gardens were developed.

Later, when a paved car park was built above Langmoor Gardens, the need for adequate drainage was apparently overlooked and the landslide system was again reactivated. (Combes, 2006) The water that was originally dispersed throughout the entire area of the car park was now forced between the interface of the limestone and clay. This slippery interface combined with the weight of the asphalt acts like icing on a tilted wedding cake and follows the force of gravity. The car park was threatening to slide down to the sea, taking roads and homes with it. Fortunately the problem was detected early, and emergency work was done to head off disaster. (Another car park on Charmouth Road is experiencing similar problems today).

Charmouth Road Car Park, August 2006

Please use this link for related maps, photos and diagrams:

http://www.dorsetforyou.com/media/pdf/0/f/Lyme_Regis_Leaflet6ppfinal_-web.pdf

The Lyme Regis Strategy Plan

In the early 1990’s the West Dorset District Council, Engineering Division began the process of developing a Strategy Plan for Lyme Regis (2002).

This plan begins with a detailed study of four areas: Cobb Gate and the Harbor (Phase II), Monmouth Beach and Ware Cliffs (Phase III), East Cliff (Phase IV) and The Cobb (Phase V). (Phase I, completed in 1995, was a new promenade.) The Strategy Plan describes problems and predicts consequences, makes recommendations for remediation and explores alternative philosophies.

In Section 3.2, the plan addresses the Recommended Generic Objectives:

Lyme Regis is faced with a formidable array of coast protection challenges. In order to fulfill the recommendation of ‘hold the line’ in the Shoreline Management Plan, it is considered that there is no other realistic alternative in the long term other than to implement new coast protection schemes….

As the probability of damage due to marine erosion and landsliding are both relatively high, there would be little benefit in implementing foreshore protection works without also carrying out slope stabilization works, and vice versa.

In other words, the recommended strategy recognizes the need to treat the upland and shoreline as a system, rather than treating them as separate, non-related problems. With this in mind, Section 3.3 addresses the alternatives.

The alternatives include ‘do nothing’, ‘do the minimum’, ‘react to failures once they have occurred’, ‘early warning systems’ and ‘coast protection works’. The first three philosophies are dismissed because they are “Discordant with Preferred Strategic Options of the Shoreline Management Plan”. The study suggests that ‘do the minimum’ is acceptable only in non-urgent cases, and in the short-term. This philosophy is not considered sustainable in the long term. The ‘react to failures’ philosophy is what has historically happened and is also undesirable, as “remedial works to treat a failure once it had caused the damage would be significantly greater in cost than those required to prevent the failure in the first place.”

In the end, the Strategy Plan recommends implementing a limited early warning system in high-risk areas and comprehensive coast protection works:

Coast protection works: major strengthening works comprising improvement to the coast protection walls, the provision of beach renourishment and control structures, slope stabilization and drainage works, is considered to be the only realistic philosophy which will meet the objectives of the Shoreline Management Plan.

The only place where the ‘do nothing’ plan was adopted was Phase III: Monmouth Beach and Ware Cliffs. They felt this decision was justified because there would be no economic benefit for making improvements in this area. (I think there are economic and environmental benefits in leaving this section of beach unimproved.)

This report was precluded by extensive research performed by the West Dorset District Council, along with consulting engineers High-Point Rendel that included in-depth studies of the coastal system, from the sub-tidal zone to the top of the coastal slope (Engineering Division Preliminary Studies, 2000).

It’s difficult to list all the areas that were studied. They researched the history of landslides and development of coastal structures, performed external and geophysical surveys of the coast protection structures, did bathymetric surveys, side scan sonar surveys, shallow sub-bottom profiling, grab sampling, marine seismic profiling and seabed videoing. They took aerial photographic surveys of beaches and landslides, and did geological mapping of the area. That was just the beginning. They analyzed of the coastline evolution, beach changes, landslides, water, rainfall, and cored boreholes to gain subsurface data. They performed slope stability analyses and investigated existing drainage systems. As a result of these studies, they were able to put together the most comprehensive picture of Lyme Regis’ geological and coastal condition ever.

Along with being able to identify the extent and types of the landslide systems, the study recognized that where there was once a substantial beach along the Lyme Regis frontage it had since disappeared. The beach has declined due to a number of reasons, both natural and man-made. First, there is no longer a natural supply of beach-forming shingle along the coastline. This is “part of a natural process of fragmentation of the beaches along the West Dorset coastline” (Engineering Division Preliminary Studies, 2000). Second, when The Cobb was connected to the shoreline in the 1750’s the west to east long shore drift was interrupted, resulting in substantial build-up of Monmouth beach at its eastern side, unfortunately these last two were at the expense of the western end of the Lyme Regis beach. Third, the seawall cut off supply from the upland slope. At the time of the study, the beach was depleted and lacked the protection offered by sand and shingle. The seawall was being eroded away from underneath, had been breached in several places and was at risk of collapse.

Coastal Protection

The next step in the process was to design protection for the shore and slope. Coastal engineering begins with calculations of wave height, length and energy, current, beach slope etc. It also depends upon proving the calculations by testing the design in a wave tank before actually building the structure. As part of these studies, preliminary engineering was begun on Phase II. Part of the process involved 1:500 scale model tests of the shoreline system. This testing was performed at the HR Wallingford testing facility in Oxfordshire, England. After testing and input from the public, a design was completed and work was ready to begin. All they needed was money. With a £20 million price tag, this project would have been impossible to undertake for a town with only about 3,700 residents, (Dorset County Council, 2004) but fortunately they were able to apply for and were granted funds from the Department for the Environment, Food and Rural Affairs (DEFRA).

The entire Lyme Regis harbor, from The Cobb to Cobb Gate, has undergone major rehabilitation. Beacon Rocks, at the end of The Cobb, were realigned and extended by 110 meters. The North Wall Rockery was realigned. New jetties were built below Lister Gardens (73 meters) and at Cobb Gate (57 meters). Rock armor was added on the east side of the Cobb Gate jetty. The alignment of the North Wall Rockery, Lister Gardens and Cobb Gate Jetties was designed to prevent replenished beach material from washing away too quickly. Old and deteriorating groins were removed to provide an uninterrupted beach from Lister Gardens to Cobb Gate. A new seawall and promenade were built, extending from the Lister Gardens jetty to Cobb Square.

Finally, sand and shingle (pebbles of about 50 – 75 mm diameter) were placed on the beach, raising it by 2 meters. While the entire beach was initially to be covered with shingle, public opinion and consideration of the tourism industry that supports the town swayed the design and portion of the beach has been replenished with sand. Thus, the most protected section of beach, between the North Wall Rockery and Lister Gardens, has sand and the more exposed area, from Lister Gardens to Cobb Gate, has shingle. It seems tourists are drawn to sand. In fact, businesses at the sandy end of the beach have fared much better than at the shingle end. If you visit Lyme Regis on any fine day during the holiday season, you’ll find the sandy beach packed shoulder to elbow while the shingle beach is nearly vacant.

The reason the entire beach isn’t covered in sand is because sand is too lightweight. The wave testing proved what the engineers suspected – sand would not provide sufficient energy absorption to protect the seawall from the waves. In addition, it would wash away too quickly from less protected areas of the shoreline. It is anticipated that the sand beach will need to be replenished every 5 years.

Lyme Regis sand beach, August 2006

Every stone size, from sand to armor rock, was carefully selected for optimum durability, size, weight and aesthetics.

Although to you and me it might look like a pile of rocks, the actual design and construction of the Beacon Rocks extension is highly technical (Combes, 2006). This extension is designed to protect Lyme Regis from the most severe storm waves. First, the sea floor was dredged to a suitable and stable base. Next, core rocks ranging in size between 2 and 5 tonnes each were placed. Then comes the tricky part. Armor rocks sized from 9 to 18 tonnes were placed using Global Positioning System (GPS). Beacon Rocks is constructed of 11,000 tonnes of rocks in the core and covered by 26,500 tonnes of armor rocks. These armor rocks range in size from 9 to 18 tonnes. Using GPS technology, each rock had a predetermined location and as was placed, its location was recorded on drawings to be kept for future reference or study.

The sand and shingle were brought in on barges and transferred to smaller barges, which were able to enter the harbor at high tide and dump their loads near the shore. Then, at low tide, heavy equipment was used to push the sand and shingle up onto the beach and spread them in the desired locations.

The jetties are constructed of a precast concrete core installed on a poured foundation, then faced with cut limestone.

Finishing up the Cobb Gate Jetty, August 2006

Construction and The Benefits of a Global Market

There is a truly global element to the shoreline construction materials (Combes, 2006). Armor rocks were quarried in Norway of igneous rock. Interestingly, these rocks are not specifically quarried for use as armor rocks, but are actually a byproduct of another industry: granite countertops. They were selected for this project based on their size, mass and durability, and also for their economic benefit. The sand (37,000 tonnes) was imported from northern France and the shingle (72,000 tonnes) came all the way from the Isle of Wight. It’s the jetties that win the prize for materials transported the farthest distance. The limestone facing these structures is not from England, France or even Norway. It was quarried and cut into predetermined sizes in China before being shipped to Lyme Regis.

The sand, shingle and limestone were selected for size, weight and durability, and also for appearance. The coastal protection work was completed in August 2006.

Upland Stabilization

As previously noted, this project also involves slope stabilization. Work on Langmoor and Lister Gardens began in April 2005 with completion anticipated in April 2007. As with the coastal protection scheme, the design actually began during the research phase, with core holes drilled to determine the underlying strata and a study of water flow patterns on the slope. A model study of the slope was also performed (Combes, 2006).

The entire slope stabilization project was a delicate process, and the contractors used extreme care to avoid triggering another landslide during construction. Equipment size was very limited and only increased as more slope stability was achieved. A total of 1150 piles were auger-cast (drilled into the ground and filled with concrete) rather than driven to prevent excessive vibration of the slope. The 300 mm diameter piles were drilled through the soil and clay and into the limestone bedrock in the Langmoor and Lister Gardens and just behind the seawall. These piles act as dowels to hold the land in place. Soil nails (sort of like piles, but smaller diameter and horizontal) were also drilled into the slope. Counterforts (large granular buttresses) and drilled drains were installed and a large “French Drain” was constructed just below the car park to provide controlled drainage of the slope.

To monitor slope stability, strategic piles and ground locations are fitted with inclinometers (Childs, 2006). The inclinometers are constructed of a plastic pipe or casing with 4 grooves equally spaced around the inside surface. The casings for this project are up to 29.5 meters long. An inclinometer probe is a stainless steel cylindrical instrument, about 25 mm in diameter and 800 mm long. It is fitted with spring-loaded wheels near each end that are designed to run along the grooves in the casing. This probe is inserted into the bottom of the casing and pulled through, taking measurements at 1/2-meter increments to determine deflection and thus ground movement. Measurements are recorded at initial installation and predetermined intervals thereafter. Construction equipment sizes are only increased after inclinometers indicate sufficient stabilization. For this project, the inclinometers are checked daily or weekly through the construction process, depending upon the level of activity and will continue to be monitored at less frequent intervals after construction is complete.

After the stabilization work is complete, 7,500 square meters of gardens will be re-planted for the enjoyment of both residents and tourists alike.

Conclusion

Lyme Regis is a beautiful town. It has a rich history and fossil content, with the advantage of a mild climate (except for when there are storms) and incredible views. This is a place worth preserving.

It’s impossible to eliminate slope failures and coastal erosion, but damage to property and loss of life can be minimized with a comprehensive and proactive plan. Previous attempts at addressing these issues were done on a piecemeal basis and did not consider the area as a system but looked only at the local and immediate problem.

The measures taken by the Dorset County Council and Lyme Regis are comprehensive in scope, consider both shore and slope as a system and go a long way to slowing processes of erosion and landslides while at the same time provides the tools needed to respond proactively in preventing future landslides. The Council worked with engineers and citizens to develop a plan that meets the needs of the people of Lyme Regis. Tourists have again returned to the beaches.

This project returns Lyme Regis to its rightful place as “The Pearl of Dorset”.