Hedgerows, Ditches and Canals

I was interested in Dr McDonald’s article regarding Kidlington Hedgerows (see here).  The item that attracted me was the ditch she described that runs along the canal (or what’s left of it) to the east of Kidlington and as she said forms its boundary. Ditches are of course located to remove water from open land in order to reduce flooding and increase crop yields.  However, this has done that and a good deal more!  It’s actually a navigation ditch and performs a very worthy engineering feat which is to prevent the build-up of flood water on the eastern side of the Oxford Canal, which could of course develop and flood the water course reducing the dynamics of the locking system deployed by the Oxford Canal as it follows its route to the city.  

So, first the geography: Kidlington was built on the flood plain of the river Cherwell that runs from north to south to feed the Thames at Oxford. But what is less obvious is that this plain tilts east to west meaning that surface water naturally flows over land to the many lakes and streams that feed into the Thames near the A40 at Eynsham. 

When James Brindley and Samuel Samcock designed and built the Oxford Canal, they knew they could ill afford to mess with the country’s natural drainage system; to the contrary, they wished to harness it.  Especially as in those days there were no storm drains like we have today.  Brindley’s basic principle of a contour canal is to keep the water between locks at the same level often meaning the canal meanders its way around the country as opposed to taking the most direct route. Where this was not possible there would be locks or it would be built slightly above ground level. In this case the canal near Kidlington is higher. The view at Stratfield Brake confirms this as the land runs down-hill to the navigation ditch which is clearly evident here some 20 feet from the canal, with the water levels having a difference of up to four feet depending on rainfall.  So, what prevents this natural flow of water from building up and flooding the area of the east bank?  Well. the answer is amazingly simple and Brindley used his engineering and surveying skills to resolve the issue. He instructed his Navies to build ditches alongside canals to collect unwanted water and protect the canal in areas that it might flood naturally. Using the natural contours of the land the navigation ditches collected this water into large ponds on either side of the canal at a specific low-level area adjacent to its course.

 And now the engineering bit: the ponds at the deepest points would be connected with a direct pipe or brick channel commonly called a culvert that ran under the canal, and as the water built up in the pond on the east bank its weight would cause it to flow via the culvert under the canal where it would appear on the western side at the same level and continue its natural course over ground to the Thames, using its own weight and flow.   This simple engineering trick known as a Syphon has been in operation for years and at no cost.  If you’re interested, the one at Stratfield Brake can be found at the furthest point south of the public area directly adjacent to the canal.

Nick Duval