Restoring a ford as a stock and vehicular crossing point
The ford was built as part of comprehensive restoration of the river bed after deepening by dredging in the 1960s (See 5.3 for full details and a location plan). The ford forms one of a series of fixed points that stabilise the bed at its restored level. Old O.S maps indicated a ford had existed prior to dredging but it had been removed during these works; see photo of conditions pre-restoration. The farmer was keen for the ford to be restored as a stock and vehicular crossing point.
DESIGN
The practicalities of sustaining a ford at this location demanded an understanding of the hydraulic and sediment patterns that would exist after the river bed had been raised by about 1m as part of the river bed restoration works. The river conditions at the approach and exit would be important factors.
The length of the submerged part of the ford needed to be at least 20m i.e. twice the normal width of the river, in order to ensure that normal water depths were ‘fordable’, typically 30cm or less. Approach ramps on both sides needed to be flatly graded at about 1 in 10 to suit vehicles and should blend with natural bank profiles rather than be severely cut into them. The overall length of the ford, between bank tops, needed to be 40m to meet these requirements. This compared with just 15m between bank tops for the natural channel.
Study of the old OS maps indicated that the original ford was broadly of the dimensions that were needed but it was still necessary to form a view on why it was sustained by the river and did not narrow through sediment deposition making it unusable. It was well known that many fords constructed at inappropriate sites become unusable due to rapid siltation.
The ford is located between two opposing bends in the river alignment such that shoals of gravel naturally accumulate on the inside of each. The two shoals would typically be joined by an underwater bar of gravel aligned diagonally across the channel. The natural cross-section of the river, drawn across this diagonal bar and up the flat shoal profiles each side, would roughly match the ford profile needed.
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