Why are groynes not being replaced?

Timber groynes have been used for well over a century to help protect our coastline and are they are now a familiar part of the landscape. In recent years there has been a move away from this method of defence. This article attempts to explain why this is the case and how alternative strategies are being used.

By the 19th century shingle beaches that had built up in the 10,000 years since the last Ice Age had started to erode. In 1500AD The Crumbles extended 2km further seaward than it does today, consisting of a mass of shingle that had been driven ashore by rising tides. At first its erosion fed beaches to the east, but eventually this source was exhausted so that all areas of the coast started to erode.

150 years ago there was no real alternative but to use timber groynes in an attempt to protect communities that had developed along the coast during the Industrial Revolution. In recent decades advances in machine technology has meant that there are now different techniques that can be used, and as we become more aware of man’s impact on our natural environmental, sustainability too is a major consideration.

So, let’s look at some of the issues;

1. Sustainability. Tropical hardwoods typically used for groyne construction take about 100years to grow and then probably only last 30-40 years. They grow in small groves and hence a lot of jungle is cleared when only a few trees are harvested. These days planning consent is unlikely to be granted for a timber groyne scheme, and there has been a move towards rock groynes, as at seen at Bulverhythe and Shoreham.

   Major timber schemes now unlikely to be approved Greenheart – dense, resists insect attack, lasts longer Only 1-2 trees/acre of rainforest Eastbourne used 4,000 trees Only grows in northern South America – mostly Guyana 25% of total has been harvested since 1967

2. Expense. A well-constructed timber groyne is not cheap. For instance those at Eastbourne constructed 10 years ago cost £100,000 each – probably £120,000 today. Pevensey had 150 groynes and we would have had to spend half the original £30m budget replacing them – a budget required to last 25 years.
3. They don’t work quite as well as people think. If you build a groyne, it will fill with shingle, and retain that material most of the time. However, the influence of a groyne probably only stretches 60-70m updrift. Any further and it has no benefit. Whilst it is filling, it prevents beach moving down drift, thereby starving these beaches and causing further problems. Once a groyne is full, beach continues past at the same rate as it did before the groyne was built. So whilst it will hold a certain amount of beach in place, in the longer term it has minimal impact on littoral drift.
4. Today the Environment Agency is trying to address coastal issues based on how best to manage our coasts for the benefit of the UK as a whole. Pevensey currently has a net loss of beach of around 25,000cu.m per year as prevailing wind and waves move shingle from west to east. Even with a fully-functioning groyne field, we would still need an annual shingle top-up costing at least £500,000 per year. On the other hand, once that beach has been provided, it can be used to the benefit of many. Our contract requires that we allow most of it to drift onto Bexhill, from where it will move to Bulverhythe, Hastings, Pett & Rye. So for the cost of four groynes each year we are adding benefit to a far wider area.
5. Modern plant. Once sediments have been added to the beach they can be managed as they drift west to east. Unfortunately they rarely drift at a constant rate, plus some stretches of coast are being managed on an unnatural alignment, because a lack of planning constraint in the past allowed properties to be built in inappropriate places. Dump trucks used today carry 40tonnes at a time (about 23cu.m) so in a single day each can shift significant quantities of beach from accreting to eroding areas. If undertaken regularly, keeping haul distances low, beach can be moved for less than £2 per cubic metre. Spending money on recycling shingle rather than groyne building allows resources to be mobilised as soon as there is a problem. In this way protection of “at risk” areas can be achieved more effectively, whilst retaining the flexibility necessary to target any and every area that needs repair.

Rock Groynes

Use of rock groynes and rock revetments is currently seen as best practice, and undoubtedly in many places they represent the best way of protecting coasts if we are to ‘Hold the Line’ on current beach alignments. They do have advantages over using timber;

• They are re-using a waste product as, for instance, most granite is mined for work tops and other flat surfaces.
• Rock is durable and lasts a lot longer than timber.
• If situations change they can be dismantled and used elsewhere.

On the other hand;

• Rock groynes tend to be even more expensive than timber groynes
• They significantly change the character of a beach
• They can introduce health & safety risks if the public – especially children – start climbing on and in them.
• Rock groynes don’t stop shingle movement and indeed sediments pass through rock structures much more than they do through timber ones.