Jurassic Coast - Chiswell3 scheme

Chiswell Case Study: The Scheme

Introduction

The View Today

Proc. Instn Civ. Engrs. Part 1, 1991, 90, Aug., 783-798

PAPER 9611 WATER BOARD

Chesil Sea Defence Scheme. Paper 1: concept, design and construction

B.J.Hook BSc, FICE and J.R.Kemble BSc, FICE

Abstract

The history of inundation by the sea of the eastern end of Chesil Beach at Portland, together with consequential flooding of the lower end of Chiswell and the Weymouth Road, the only road link between Portland and the mainland is discussed. Particular reference is made to the events in December 1978 and February 1979, which prompted the promotion of a Sea Defence Scheme to alleviate flooding. The investigations which led to the determination of solutions are described in the second joint Paper. [Not available on this web site].

The design and construction of the Sea Defence Scheme in three stages, following a preliminary contract for the construction of a trial length of gabion protection along the crest of the Beach, are described: stage 1 - improvements to the existing sea wall at the southern end of the Beach; stage 2 - the construction of an intercepting culvert and cut-off behind the properties in Chiswell, leading to an open channel that discharged into Portland Harbour, the system being designed to intercept sea water, both from percolation and over-topping events; stage 3 - the raising of the Weymouth Road to alleviate flooding and to maintain access to Portland during extreme events. The benefits of this Scheme are reviewed and the ways in which the sponsoring authorities' commitments to support the community of Chiswell, under General Improvement Area procedures, are explained.

Introduction

1. Chesil Beach is a natural shingle barrier, which extends westwards from Chiswell, on the Isle of Portland, for a distance of approximately 27 km. The initial length of 3 km from the cliffs at Portland comprises a natural strip of shingle which is about 200 m wide at its narrowest point, and accommodates the Weymouth Road, the only road link between Portland and the mainland.

2. Chesil Beach is a coastal feature of international significance and is a Site of Special Scientific Interest. The beach itself is one of the three major shingle beaches in Britain and is identified by the Institute of Oceanographic Science as the only one which is essentially a simple storm beach. The material of the Beach comprises mainly pebbles of chert and flint, with a natural grading, increasing in size from west to east; exposed pebbles in the region of 50-100 mm dia. occur at Chiswell.

3. There are many references to the scientific importance of the Beach, and the Department of the Environment's Inspector's report given after a Local Public Inquiry in February 1973 is of particular interest.

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		Chiswell Case Study: The Scheme
Introduction The View Today		Proc. Instn Civ. Engrs. Part 1, 1991, 90, Aug., 783-798 PAPER 9611 WATER BOARD Chesil Sea Defence Scheme. Paper 1: concept, design and construction B.J.Hook BSc, FICE and J.R.Kemble BSc, FICE Abstract The history of inundation by the sea of the eastern end of Chesil Beach at Portland, together with consequential flooding of the lower end of Chiswell and the Weymouth Road, the only road link between Portland and the mainland is discussed. Particular reference is made to the events in December 1978 and February 1979, which prompted the promotion of a Sea Defence Scheme to alleviate flooding. The investigations which led to the determination of solutions are described in the second joint Paper. [Not available on this web site]. The design and construction of the Sea Defence Scheme in three stages, following a preliminary contract for the construction of a trial length of gabion protection along the crest of the Beach, are described: stage 1 - improvements to the existing sea wall at the southern end of the Beach; stage 2 - the construction of an intercepting culvert and cut-off behind the properties in Chiswell, leading to an open channel that discharged into Portland Harbour, the system being designed to intercept sea water, both from percolation and over-topping events; stage 3 - the raising of the Weymouth Road to alleviate flooding and to maintain access to Portland during extreme events. The benefits of this Scheme are reviewed and the ways in which the sponsoring authorities' commitments to support the community of Chiswell, under General Improvement Area procedures, are explained. Introduction 1. Chesil Beach is a natural shingle barrier, which extends westwards from Chiswell, on the Isle of Portland, for a distance of approximately 27 km. The initial length of 3 km from the cliffs at Portland comprises a natural strip of shingle which is about 200 m wide at its narrowest point, and accommodates the Weymouth Road, the only road link between Portland and the mainland. 2. Chesil Beach is a coastal feature of international significance and is a Site of Special Scientific Interest. The beach itself is one of the three major shingle beaches in Britain and is identified by the Institute of Oceanographic Science as the only one which is essentially a simple storm beach. The material of the Beach comprises mainly pebbles of chert and flint, with a natural grading, increasing in size from west to east; exposed pebbles in the region of 50-100 mm dia. occur at Chiswell. 3. There are many references to the scientific importance of the Beach, and the Department of the Environment's Inspector's report given after a Local Public Inquiry in February 1973 is of particular interest.

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