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?”
3.1.5 The water held in the reservoir in the high ground, which is both draining out and being topped up by further rainfall, maintains the level of the water table. The quantity of water draining away to rivers and water being extracted from the Upper Chalk by wells and pumps for use in homes, industry and agriculture deplete the water table. Recently rainfall has increased with heavier, more frequent and longer periods of rain. At the same time water use from the upper chalk layer has fallen, as domestic housing, heavy industry and agriculture are now taking their supplies from the deeper boreholes down to water bearing strata independent of the chalk above. Agriculture also requires less water as rainfall frequency and intensity increase. The combination of these is leaving more water in the upper chalk layers and the water table remaining at or near the surface, or rising.
3.2
Water Supply
3.2.1 Fowlmere along with other villages would have begun to be settled where a ready water supply was to be found at the surface emanating from natural springs. As the water table fluctuated and the some of the springs dried up wells would have been dug to follow the water down. On the Ordnance Survey second edition maps of 1903 over 50 water wells and lift pumps can be found in Fowlmere, nearly one per dwelling. These were all shallow, only deep enough to find the ground water in the chalk. In other villages where the ground water is deeper there are usually fewer wells or lift pumps, as they are costly to build and require more maintenance. In some villages only two or three would have been present and the villagers had to collect their water from a village pump.
3.2.2 Ground water level in the lower chalk layer fluctuates due to changing weather patterns and/or a changing amount of ground water being extracted for use by domestic dwellings, local industries and agriculture. When ground water drops some wells and pumps cease to provide a water supply. Some wells have become redundant; others have been deepened into the chalk to continue to produce a viable supply. Most recently boreholes have been put down to the water bearing strata of Lower Greensand, much deeper than and independent of the chalk layers. These deep boreholes are the ones generally supplying the water needs of Cambridgeshire today.
3.3` Surface Water Drainage
3.3.1 Surface water drainage in Fowlmere is not generally collected in a below ground pipe system for disposal. Rainwater is shed from roofs etc. either on to the ground or into soakaways. The water that is shed on to the ground runs over the ground to be carried away in channels and/or ditches, eventually finding its way into Wallington Brook. Water on the roads is collected in road gullies and is piped away to either ditches and into Wallington Brook, or into local soakaways.
3.3.2 The water put into the many soakaways percolates into the ground, finding its way down to the water table. Each soakaway is about 1.5 to 2.0 metres deep and has a volume sufficient to hold the estimated quantity of water that will run off the contributing area. The water then naturally soaks away by gravity into the ground over a period of time, usually before the next heavy rainfall event. Generally with a water table more than 1 metre below the surface the soakaways empty fairly quickly and no problems occur with water on the surface. With a water table nearer the surface soakaways will not always drain down before the next rainfall event, and water will remain on the surface for a short period until it eventually drains down. If at any time the water table is very close to the surface soakaways will not work at all, and will provide a ready path for ground water to come to the surface.
3.4.1 Anglian Water network drawings record that the sewer system covering the older areas of Fowlmere is foul sewage only. Though it is highly probable that some rainwater is also being put into this system, as over time owners make expedient connections to the sewer to clear rainwater from their property. The foul system drains into the pumping station behind Rectory Lane. The system is old and vulnerable to inundation through old pipe joints and cracks when the ground water level is high, along with manholes and vents etc. when water is lying on the paving surface. Therefore during periods of prolonged rainfall with the possibility of the water table being up to or above ground level the system may also be pumping large quantities of surface water additional to the usual foul sewage.
3.4.2 All of the later additions to Fowlmere are also on a foul sewage only system, with surface water going to local soakaways, ditches and the Round Moat. There are some properties remote from the sewer systems that are not connected to any Anglian Water drainage and are served by their own septic tanks or cesspits. The foul water collected in the new south area is collected in the pumping station between Ryecroft Lane and Chapel Lane, then pumped into the main pump station behind Rectory Lane.
3.4.3 All of the foul sewage in Fowlmere is collected into the Rectory Lane pump station, and is then pumped out of Fowlmere to Foxton Sewage Treatment Works (STW). When the incoming quantities of effluent exceed the rate that the pumps can discharge, the system becomes flooded, the pump station then needs to be emptied by road tankers for disposal elsewhere.
3.4.4 As well as pumped effluent from Fowlmere, Foxton STW also receives from: Thriplow, Foxton, Shepreth, Wendy, Croydon, Orwell and Barrington. All of these villages are similar to Fowlmere in having old sewer systems, which between them overwhelm the STW with surface water in times of heavy rainfall. The STW itself is located in the low-lying ground adjacent to The Cam between Foxton and Barrington and is vulnerable to flooding. When the STW cannot operate, through being flooded itself or it is overwhelmed by the incoming quantities of sewage, the supplying pump stations need to be emptied by road tankers for disposal elsewhere.
3.4.5 The domestic use of water in Fowlmere like the rest of the country has increased. More homes are now equipped with flushing toilets, baths, showers, washing machines and dish washers. All are being used more frequently than in the past. This exacerbates Anglian Water’s pumping problems as all of this used water is going back into the foul sewers for disposal and treatment.
3.4.6 If as intimated many property and landowners have partially blocked or removed surface water ditches, more water will be laying on the surface and getting into the sewers. When the water table is high septic tanks have no means to discharge their treated effluent into the ground, and along with soakaways provide a ready path for ground water to back flood through them to the surface along with their foul contents. Cesspits though should be self-contained with no outfall, therefore invulnerable to back flooding. But some owners wanting to save on emptying costs have put many to soakaways, and therefore they suffer the same fate as septic tanks.
4.0 Specific Problems in Fowlmere
4.1 From the given marked up drawing it appears that most areas which flooded during the winter and spring of 2000/01 are in the lower (below 25m) south east of the village and have been developed the most recently (1970-80s). Although there are some areas identified that flooded in older parts of the village:
4.1.2
Police House, High
Street
4.1.3 Wallington Brook runs between Bridge House and Police House before passing under the High Street. The sidewall of Bridge House forms one bank and a natural bank the other. The natural bank is about 750mm high and is at the lower end of a lawn sloping down from the Police House. During the floods of last winter and spring the brook overtopped the natural bank and inundated the lawn almost up to the building. Stormwater flowing down the High Street ran over the dropped kerb at the road edge, over the drive and garden and into the brook. For the past few years the brook has been mostly dry, with water only flowing after periods of heavy rainfall and soon abating. It was still flowing strongly on 30.10.01 not having rained since the 26/27th. This section of the brook had been dry and clear on 17.10.01 but was now about 100mm deep and flowing though partially blocked by vegetation that had been washed down from upstream. This could cause a restriction to the flow of water in the brook especially if caught in the downstream culverts.
4.2
Lynch Lane
4.2.1 This lane is much lower than the High Street. During last winter and spring a large amount of stormwater ran off of the High Street and down into Lynch Lane where the road and gardens were flooded, and remained so for many weeks. The Lane is also at the elevation natural springs occur and some of the water flooding the Lane and gardens could have risen here. When the downstream foul sewer in Rectory Lane could not cope with the floodwater and was full, upstream areas at Newton Road and Thriplow surcharged the foul sewer. Sewage contaminated water was then disgorged in Lynch Lane through the five unsealed manhole covers adding to the flood in the lane and gardens. Some properties had problems with their toilets not flushing away for a time. Anglian Water had to remove this contaminated water by tanker daily for many weeks, to the detriment of the road surface and inconvenience of the local inhabitants. Most properties in Lynch Lane put roof and surface water to soakaways, some owners thought that some properties are putting this rainwater into the foul sewer.
4.3
Rectory Lane
4.3.1 This lane falls from both ends to a low point at Mere Cottage. There are only three gullies and a narrow grass verge to remove surface water, none of the fourteen foul manholes have sealed covers. During periods of heavy rainfall water floods the road and can find its way down into the foul sewer through any of the manhole covers in the lane. If the lower especially, or any of the three gullies is blocked or has a restricted outfall due to the water level in the brook being high the lane will flood at Mere Cottage. Moss Cottage on Long Lane, The Old Maltings (which in the past would have required a copious clean water supply) and the north end of Rectory Lane to Mere Cottage are all at about the 23 metre elevation the natural horizon for springs to occur, as is the existing pond behind ‘Pennwood’. Any water rising from the ground here will also collect at the low point by Mere Cottage.
4.4 Chapel
Lane No. 20
4.4.1 A large low area at the rear has been dry for many years (thought to be 26years) and the grass mown regularly up until recently, to the rear of this area is a concreted deeper depression used as a fishpond. Over the period of one very wet weekend a spring re-emerged and filled the pond to a depth of about three feet, the pond now fills an area of similar size and shape as that shown on the 1903 Ordnance Survey maps. There is a 12-inch outlet pipe under the patio and garage to a manhole at the front of the building. This manhole also collects from a road gully and discharges through one of two 9-inch pipes across the road into a ditch, another road gully also discharges into this ditch.
4.5 Chapel Lane Nos. 22 to 28, 46 & 48
4.5.1 The rear gardens of these properties all slope down away from the buildings into a large depression of varying depth. This area flooded almost up to the back door steps of some of the houses. Older residents of the village remember that as children before the houses were built they used to play in the water, as this area was then generally flooded.
4.5.2 Surface water or ground water from the natural spring that has reappeared fills the depression across the back of these gardens. It is only when the water level in the pond at No. 20 drops does the water run-off from the other gardens. The water table may stay at a similar level with the pond remaining in the garden of No. 20 for a period of time.
4.6 Chapel Lane No. 52 to 72
4.6.1 The gardens across the back of Nos. 52 to 68 have been extended across another similar depression. The old boundary line may have also been a ditch that drained any water into the brook across the back gardens of 70 and 72. A ditch at this location would have shed any water running down the hill into the brook acting as a cut-off protecting the original cottages in Chapel Lane.
4.7 Round Moat
4.7.1 The ditch forming the moat around this historic area is of varying depths with standing water left after outflow has ceased. It also has incoming surface water drains from the surrounding estate roads that need to be drained away. The only outlet to this moat is the link drain across to the Wallington Brook, and this appears to be above the level of the moat. The link ditch and the brook are both overgrown.
4.8 New Barn Conversion at Lower Farm
4.8.1 This barn has had flooding in the grain store at the rear every winter. A spring fed pond higher up the slope at the back drained by ditch across the back of the grain store and down to the road towards the brook. Whilst the spring has been inactive the pond has been filled and the ditch that used to run across the back of the building has been removed. The spring has now re-emerged and is flowing over and through the ground into a new sump where it is being pumped out to the road. The main barn conversion has been subject to serious flooding by the ground water coming up through the floor of the building, not in the door.
4.9 Shepreth Road at North Farm
4.9.1 Culverts under the Shepreth Road are blocked or partially blocked preventing or restricting flow from the uphill ditches across the road and down to the brook. The ditch alongside North Farm is overgrown at the top end and almost non-existent at the lower end where it joins the brook. The brook here has been well cleared and is flowing well.
4.9.2 All culverts and ditches are to be cleared out to promote a good flow to the brook, they are then to be routinely inspected and maintained on at least an annual basis.
4.10 Fowlmere Nature Reserve
4.10.1 The two ditches on the west side of the reserve are flowing well, but the one on the east side by the car park is only just showing movement in the water. There are two foot bridges over the ditch that have been constructed by infilling the ditch with a culvert pipe to carry the flow, they appear to be blocked or partially blocked restricting flow from the ditches to drain the land upstream. There are large areas of the surrounding fields under water, which are not parts of the wetlands of the RSPB Nature Reserve. With water held on the fields here it is possible that groundwater is not being allowed to freely drain from areas uphill in Fowlmere. There may be other restrictions further downstream.
5.0 Conclusions
5.1 Most of the surface water problems that have occurred recently in Fowlmere appear to be due to two causes:
5.2 One - the rise of the water table causing long dormant natural springs to become active again. Some ditches that drained these springs have been filled and are not now available to clear the newly flowing water.
5.3 Two - the period of prolonged heavy rainfall at the time, along with the evident lack of maintenance of most of the ditches, culverts beneath roads and the Wallington Brook led to the flooding of Fowlmere when the water could not drain away quickly enough.
5.4 The above affected Anglian Water’s foul sewer system where the manholes by not having sealed covers left them vulnerable to water on the surface flooding in, surcharging the system causing contaminated water to be discharged through similar manhole covers at lower elevations. Water entry appears to have happened in Rectory Lane and High Street, along with ground water above the level of the sewer entering through cracks and open joints in the pipes, surcharging the sewer in Lynch Lane where it was discharged through the manhole covers.
5.5 Though Fowlmere Nature Reserve is remote from the village itself, it is below the level of and downstream of Fowlmere. Any floodwater remaining on and in the ground here is preventing surface and ground water draining out of the north and west of the village.
6.0 summary
6.1 The problems experienced last winter are due to an accumulation of detrimental changes to the natural and historic drainage of the area over the years combined with a particularly wet winter period causing high ground water levels. To prevent a reoccurrence of the flooding of last year will require prolonged action on a number of fronts. No single action will solve the problems that Fowlmere experienced last winter. What can be said with certainty is that the drainage system has to be improved to cope with greater volumes of water than it has ever had to deal with. This is due to the effects of climate changes, inappropriate use of soakaways and if Anglia Water manage to reduce the inflow of surface water to the foul system then this will have to be drained as well.
6.2 The solution, if put
simply, is to collect more effectively surface water and ground water and drain
it swiftly downstream of Fowlmere. In
times of heavy rain the receiving River Cam is also flooding, but there is
sufficient level difference between Fowlmere and the Cam to temporarily store
the water on the fields downstream of Fowlmere.
7.0 Recommendations for Remedial Works
7.1
District/County
Council
7.1.1 Improve the flow characteristics of the Wallington Brook downstream from the gardens to the rear of 66, 68 & 70 Chapel Lane towards the River Cam as far as possible. Provide additional culvert or widen the culvert beneath High Street and the channel beyond to cope with high flows.
7.1.2 To drain adequately the complete ditch system from the newly reformed spring pond at the rear of no. 20 Chapel Lane needs to be improved. The ditch to Ryecroft Lane into the moat, the moat, the link to the brook and any road culverts, are all to have a continuous fall to the invert level of the brook at the junction.
7.1.3 Wallington Brook is to have a new sidewall constructed along the length adjacent to the Police House up to a height the same as the ground floor of the building. The land between this new wall and the house garden is to be made up to the same level as the garden.
7.1.4 Replace the dropped kerb across the drive of the Police House and the entrance to Lynch Lane with a channel drain, draining into the brook to prevent surface water flowing off of the High Street.
7.1.5 Road gullies draining surface water from the roads are to be checked so that the outlet pipes are clear not partially blocked and all freely drain. Clean out if necessary.
7.1.6 Check that all surface water ditches are flowing freely into the Wallington Brook. Clean out if necessary.
7.1.7 Check that all surface water culverts beneath roads are free flowing and draining the water from ditches quickly. Clean out if necessary.
7.2
Anglian Water
7.2.1 Replace old sewers or re-line existing sewers, to prevent joints and cracks admitting ground or surface water.
7.2.2 Seal manhole covers with new covers and frames or internal sealing plates, to prevent ingress of ground and surface water into sewer pipes and the over loading of the foul pump station.
7.2.3 Remove Thriplow effluent from the Fowlmere gravity system, by pumping to Rectory Lane pump station or direct to Foxton STW, removing some of the surcharge in Lynch Lane.
7.3.1 Replace footpath causeways across ditches in the RSPB Nature Reserve with open span bridges, to open up the flow.
7.4.1 Divert any surface water presently entering into the foul sewer system towards a dedicated surface water system, to ease the burden on the foul pump station. This could be in surface swales/ditches/channels and/or buried pipes discharging into the Wallington Brook.
7.4.2 No 20 Chapel Lane the ditch outfall to the Round Moat appears to flow when full but needs to be dug deeper and to a fall to enable the pond water level to be lowered. At present a short length from the pipe outlets is ponding. The existing outfall from this pond may need improving, either by a new larger pipe outfall or a new ditch dug around the end of the building to cope with any increased flows. If the pond is to be retained as a feature a weir can be built before the outlet, and with the downstream pipe and ditch improved any high flows over the weir will be easily cleared.
7.4.5 The spring and ditch at Lower Farm should be re-instated at a defined location with culverts if required allowing the spring water to drain into the original ditch system that still exists.
7.5.2 Identify all original natural spring locations. Provide new ditches and reinstate as many of the old ditch systems as can be identified.
7.5.3 If ground water level continues to remain high soakaways will not clear surface water efficiently if at all and will have to be replaced by surface ditches and channels draining into Wallington brook. A pumped system discharging downstream of the High Street may be required to assist a gravity drain in times heavy rainfall.
7.5.4 Design for additional culvert or widening of the existing culvert beneath High Street and the channel beyond to cope with higher flows.
|
OrdnanceSurvey Sheet No. |
Cambs. County Sheet No. |
Area Covered
|
Revision. Date |
|
LIII. 7. |
80.3 |
Barrington West |
1938 |
|
LIII. 8. |
80.4 |
Barrington East to Harston Mill |
1937 |
|
LIII. 11. |
89.1 |
Shepreth to Malton Farm |
1903 |
|
LIII. 12. |
89.2 |
Foxton |
1938 |
|
LIII. 15. |
89.3 |
Meldreth and Bury End Farm to Shepreth |
|
|
LIII. 16. |
89.4 |
Fowlmere Northwest to Green Man Shepreth |
1903 |
|
LIV. 13. |
90.3 |
Fowlmere Northeast |
|
|
LVIII. 3. |
98.1 |
Melbourn Northeast |
|
|
LVIII. 4. |
98.2 |
Fowlmere Southwest, Watercress Beds and RSPB |
1903 |
|
LIX. I. |
99.1 |
Fowlmere Southeast |
1903 |
Ordnance
Survey Second Edition - Sheet Layout of 1:2500 Plans
|
LIII. 1 79.1 |
LIII. 2 79.2 |
LIII. 3 80.1 |
LIII. 4 80.2 |
LIV. 1 81.1 |
LIV. 2 81.2 |
|
LIII. 5 79.3 |
LIII. 6 79.4 |
LIII. 7 80.3 ü |
LIII. 8 80.4 ü |
LIV. 5 81.3 |
LIV. 6 81.4 |
|
LIII. 9 88.1 |
LIII. 10 88.2 |
LIII. 11 89.1 ü |
LIII. 12 89.2 ü |
LIV. 9 90.1 |
LIV. 10 90.2 |
|
LIII. 13 88.3 |
LIII. 14 88.4 |
LIII. 15 89.3 |
LIII. 16 89.4 ü |
LIV. 13 90.3 |
LIV. 14 90.4 |
|
LVIII. 1 97.1 |
LVIII. 2 97.2 |
LVIII. 3 98.1 |
LVIII. 4 98.2 |
LIX. 1 99.1 ü |
LIX. 2 99.2 |
|
LVIII. 5 97.3 |
LVIII. 6 97.4 |
LVIII. 7 98.3 |
LVIII. 8 98.4 |
LIX. 5 99.3 |
LIX. 6 99.4 |
|
 |