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The Nailbourne


The Nailbourne filling the floodplain


South Barham Road covered by the Nailbourne


Looking towards Railway Hill.


The Causeway

The Elham Valley Nailbourne (more often known simply as "The Nailbourne") is one of three bourns, 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 (sometimes years).

 

The bourns are sometimes known as "woe-bourns" with association to times of national problems.   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.

 

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 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 bourns 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.

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.