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CAMBRIDGESHIRE COUNTY COUNCIL

     

 

FOWLMERE FLOODING

WINTER/SPRING 2000/01

 

STUDY OF THINGS WET

     

 

NOVEMBER 2001

 

 

 

                                                                                                                                    Prepared by:

 

 

                                                                                                                                    .............................................................

 

                                                                                                                                    K. R. Stapleton

                                                                                                                                         

 

 

                                                                                                                                    Checked by:

 

 

                                                                                                                                    .............................................................

 

                                                                                                                                    C. C. Cumberland

                                                                                                                                    BEng CEng MICE

 

 

                                                                                                                                    Authorised by:

 

 

                                                                                                                                    .............................................................

 

                                                                                                                                    M. Miller

                                                                                                                                    BSc (Hon) CEng. MIStructE.

 

                                                                                                                              WS Atkins – East Anglia

                                                                                                                              Wellbrook Court

                                                                                                                              Girton Road

                                                                                                                              Cambridge   CB3 0NA

                                                                                                                              Tel: (01223) 276002

                                                                                                                                    Fax: (01223) 277529

 

 

 

 

 

 

 

 

Fowlmere Report

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

JOB NUMBER :  BC3291

DOCUMENT REF :  BC3291Report

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Originated

Checked

Reviewed

Authorised

Date

Revision

Purpose

Description

WS ATKINS – EAST ANGLIA

CONTENTS

 

Page No

 

1.0       Introduction                                                                                                         1

2.0       Background Information                                                                             2

3.0       Fowlmere’s Surface Water Problem                                                      4

4.0       Specific Problems in Fowlmere                                                                 9

5.0       Conclusions                                                                                                          13

6.0       summary                                                                                                                 14

7.0       Recommendations for Remedial Works                                            15

 

 

Appendices

 

Appendix 1      List Ordnance Survey sheets Second Edition

 

Appendix 2      Ordnance Survey Fowlmere 1903

 

Appendix 3      Ordnance Survey Fowlmere 2000

 

Appendix 4      Areas That Flooded Winter Spring 2000/2001

1.0     Introduction

 

1.1       A study to ascertain the reasons why Fowlmere has been having problems with foul water sewage and surface water flooding over the winter/spring period 2000/01, requested by Cambridgeshire County Council Highway Maintenance Division.  Current Ordnance Survey maps and those of 1903, British Geological Survey maps, Council minutes and reports and Anglian Water sewer plans have all been studied.  Local residents and ` Parish Council representative have been interviewed as to what happened and where, and the village has been walked through several times, studying the lie of the ground and the state of the sewers, ditches, brook and culverts. 


2.0     Background Information

 

2.1              Topography

 

Fowlmere has been built on the north-facing slope down from the Barkway ridge, the crest of which is some 8 Km to the south.  The old village lies on a slight rise forming a causeway across the broad shallow valley of the Wallington Brook and is mainly to the west of the brook.  The causeway across the shallow valley forms a basin upstream to a depth up to 2 metres; the only waterpath through is the culvert for the brook.

 

2.2              Geology

 

The Barkway ridge consists of a top layer of Chalky Boulder Clay overlaying Upper Chalk, a Horizon of Chalk Rock, and Middle Chalk.  At and around Fowlmere the Middle Chalk changes to Lower Chalk with a Horizon of Melbourn Rock between. The rise that Old Fowlmere has been built upon is Lower Chalk surrounded by the outcrop of Melbourn Rock. 

 

2.3              Hydrology

 

The Upper and Middle Chalk rock layers are pervious to water that has fallen on the surface.  The Melbourn Rock is a denser and less pervious form of chalk that the water is retained upon, building a head of water in the chalk layers above. The Middle Chalk interface with the Melbourn Rock outcrop is the region at which the spring line forms, issuing water held in the Middle Rock.  This spring line follows the outcrop of Melbourn Rock through the villages of Ashwell, Melbourn, Fowlmere, Thriplow, Whittlesford and Babraham mostly at about 23 metres elevation.  There are several springs around and within Fowlmere and some within the RSPB reserve 1Km to the west.  There are about 12 other small streams along 13 Km of this north-facing slope either side of Fowlmere, all issuing from a line of springs where the strata of Melbourn Rock outcrops between the layers of Lower and Middle Chalk.

 

When the water table is low, as has been the case for many years some natural springs have become dormant, and possibly built over. Recent years have been getting wetter causing a gradual rise in ground water in the surrounding hills.  Some old springs have re-emerged with the resultant water seeking to follow historic watercourses/ditches down towards the main Wallington brook, some of these may have long since been built over and lost. 

 

The Wallington Brook originates on the top of Barkway ridge in Newsells Park at about 130 metres elevation.  It has a catchment area of approximately 5,000 Hectares (19.3 sq. miles) and drains northwards down via Flint Cross and Fowlmere eventually into the River Cam (this branch also called Rhee) at Shepreth.  There is one other watercourse draining north off of this ridge from Chrishall to Ickleton 7 Km east of Fowlmere, into the River Cam (branch also called Granta).

 

2.4              Fowlmere

 

The older parts of the village have generally been built on the rise of Lower Chalk within the Melbourn Rock outcrop, on the west side of the Wallington Brook, along a causeway the High Street and across the low ground towards Thriplow.  All above 24 metres and with St Mary’s Church on a mound at 26 metres beside the High Street and above the spring line. 

 

As Fowlmere has expanded it has spread out from the higher ground into the lower lying areas.  The most recent additions (1970-80s) being built south of the High Street on the Middle Chalk outside the Melbourn Rock outcrop in the area lower than 25 metres and beside the natural water path down to The Cam.  This area is a shallow basin with approximate elevation 23 to 23½ metres, surrounded by ground rising to above 30 metres to the east, west and much higher to the south.  This area is also at the 23 metre elevation that natural springs occur, and currently contains the Moat at ‘Crow’s Parlour’ and the ‘Round Moat’ fed by water from Brook Farm Pond, a natural spring when active. 


3.0     Fowlmere’s Surface Water Problem

 

3.1       Answering the question;

 

“If the surface water in Fowlmere is the result of a rise in level of the water table, why does the floodwater fall and then rise again even after 8 days of no rain?”

 

3.1.1    During periods of heavy rain the ground initially soaks up most of the rainfall with some running off through ditches etc.  Eventually the ground becomes saturated when no further water can soak in or it cannot soak away quick enough, therefore it remains on the surface.  If the ground is flat, ponds form, otherwise the water flows across the surface down to the ditches, or to the lowest areas where large lakes form. 

 

3.1.2    Once it stops raining the water lying on the surface either continues to drain away over the surface to the lower areas and the brook, or gradually soaks into the ground leaving the surface dry.  Thus the first flood abates and was not caused by a rise of the water table, but by the temporary saturation of the subsoil.  All of this water soaking into the ground is contributing to the water table gradually causing it to rise. 

 

3.1.3    Water falling onto higher ground up to 8Km south on the Barkway Ridge is also contributing to the problems to come at Fowlmere.  The natural shape of the land creates a catchment area of 5000 Hectares (50 sq. km, 19.31 sq. miles, or 12360 acres) for rainfall that eventually finds its way into Wallington Brook upstream of Fowlmere on its way to the Cam.  Water