River of Woe

see also: 2001 Flood

      and: Nailbourne Aquatic Society
 

The Nailbourne filling the floodplain in the valley floor
south of the village.

 

South Barham Road covered by the Nailbourne and no longer accessible to vehicles.   On the right is a path used by pedestrians to reach drier areas.
 

Looking the other way towards Railway Hill.

 

The Causeway with the Nailbourne running north from the Derringstone ford at the foot of Railway Hill.

 

A peak around the corner at the Valley Road bridge.   The bridge arch is not visible as the river is at full flood and it is unable to cope with the flow of water.
 

Finally a drier picture of the river course next to the Village Green taken in summer.

 

The Elham Nailbourne (often known locally as The Nailbourne) is one of three nailbournes, or intermittent streams, on the Chalk Downs of East Kent.   They can flow at a very high rate after times of heavy and continuous rainfall, but they may also lie dormant for long periods.

Nailbournes are sometimes known as woe-bournes.   One of their characteristics is that, along all or part of their length, they only flow for a few months at a time, in periods of high rainfall, particularly in the autumn and winters - and they do not necessarily flow every year.   To understand why this is so, one must look into the nature of the ground below the surface.

The Chalk is a mass of nearly pure limestone, laid down below the sea shore more than 100 million years ago.   Look at the face of the cliffs, or in a chalk quarry, and you will see that it is not one solid mass of rock, but is split up into layers, with numerous horizontal and vertical cracks or fissures, particularly near the top of the Chalk.   Also, in the upper bands, there are numerous beds of flints.   These cracks and the flint beds allow the passage of water through them, under the action of gravity.

As rain falls on the Downs, some is taken up by the vegetation, some evaporates and some soaks into the ground.   During the warmer period of spring and summer most rainwater is taken up by plants or evaporation but during the autumn and winter much soaks away in the ground.   It then continues downwards through the cracks and fissures in the chalk until it meets the clay beds underneath.   Here the water can no longer move downwards, the fissures at the bottom become saturated and the level of the top of the saturated zone (known as the water table) rises.   At the same time, under the force of gravity, the water attempts to move sideways and downwards towards nearby valleys, and eventually to the sea.

The result of this can be seen by the local water company noting the variation in the water level in the wells and boreholes in the chalk.   The level starts rising in the late autumn and continues until early spring when vegetation and evaporation take most of the rainfall and little is available to replenish the underground resources.   The water table in a typical chalk well rises and falls in level varying slightly, both in timing and range each year, depending on the rainfall.   Generally, only the rainfall between October and March will penetrate underground.

Over many centuries, the Nailbourne has carved a valley through the chalk hills, the valley floor sloping gradually northwards towards the coast - at first becoming the Little Stour, and later joining into the River Stour.   At a time of high rainfall, when the water table in the hills either side of the Elham Valley is above the valley bottom, water emerges in the valley as springs, creating a stream, or bourne.   On the other hand, in dry periods it is likely that the top of the saturated zone will be below the valley bottom, and there is therefore no feed into the stream – in other words – it is dry.

The Nailbourne is fed by springs at Etchinghill and Lyminge, and is almost invariably flowing at these uppermost extremities.   In a normal year, the stream flows through North Lyminge, and continues towards Elham.   If there has been a wet autumn or winter period, flow continues northwards through Barham and into the lake at Bourne Park.   Here, local springs are usually feeding the lake, and this is the normal source of the Little Stour.

The Nailbourne has slightly different characteristics to the other bournes in East Kent in that its bed is nearly impervious over much of its length.   This means that the stream does not have a direct connection with the cracks and fissures in the underlying chalk all the time.   Water enters the stream from springs at the top of the valley, and then flows in what is effectively an impervious channel, for some distance, until it meets another location where the bed is porous.   At this point, the water table may still be below the level of the bed of the stream, and the stream water therefore soaks away into the unsaturated chalk, with no further flow in the stream bed helping to raise the water level beneath.   If wet weather continues, the underground water table at this point will continue to rise, partly because of the continuing effect of the rain, until it reaches the surface.   At this stage, the water in the stream will no longer be able to disappear underground, and will continue above ground.

The Nailbourne features soakage points along its length, and these give the stream its unusual features.   In a wetter than average winter, the stream will flow with considerable strength past Railway Hill, Derringstone.   The next point of disappearance is adjacent to Valley Road close to Barham Village Hall.   Even with such a strong flow at Railway Hill, there may be no sign of water in the ditch adjacent to the Village Green.

The next soakage point is about 50 metres north of The Street. If the rains continue, the surface stream continues northwards.   If the stream starts flowing beyond this point, it will reach the Black Robin within two days, and continue to join up with the springs in Bourne Park within another few days - thus providing a continuously flowing stream from Etchinghill, through Elham and Barham, and onwards to the Little Stour.

When the wet weather continues, the underground water will continue to rise, even after the flow from upstream has continued along the stream bed, and this soakage area becomes an additional point of discharge into the stream.

Measurement of the Nailbourne under Black Robin bridge have shown a significant flow of water.   On one occasion, a surface velocity of 2 feet/second gave an approximate flow of 12 cubit feet per second, which is the equivalent of more than 6,000,000 gallons per day.   Other estimates have been as high as 20 million gallons per day.