An interesting development occurred in 1803 which, though not initiated by the Trustees of the Lea, their agreement, perhaps reluctantly, would have had to be obtained. Britain was at war with France and the success of Napoleon in Europe together with known preparations for invasion along the French coast had led to defence measures being taken along the south and east coasts by the erection of Martello Towers and by the construction of the Royal Military Canal in Kent and Sussex. The Supreme Command also considered the possibility of an attack being launched from the Low Countries on the flat lands of the Essex and Suffolk coasts followed by a drive westward against London. In this terrain there are no natural defence barriers until the Lea valley is reached. Even these are not great and in order to strengthen them the Supreme Command considered the feasibility of flooding the whole of the Lea valley.

On 15 July 1803, HRH the Duke of York, Commander-in-Chief, asked for a report as to the practicability of this scheme. John Rennie was instructed to carry out a survey. This was later referred to by Lord Cathcart as “A report … accordingly most expeditiously made by that very able engineer …” Coincidentally Rennie was also carrying out a survey of the navigation requested by the Trustees on 28 April 1803 but there is no suggestion that Rennie was selected by the Army because of his presence in the Lea valley or because of his knowledge of the area.

Rennie pointed out that the valley had a very gradual fall and although flooding did naturally occur from time to time it would be necessary to build dams at intervals along the valley to obtain flooding in normal weather conditions. The water would be deepest at each dam and the time of filling would be proportional to the distance the dams were apart. He therefore suggested it would be preferable to have lower dams and more of them. He went on:

“A consideration of material consequence, namely that as each dam must be defended against the enemy the more dams, of course, the more men will be wanted to defend them – on this point I must beg leave to decline to give an opinion, it being more in your Lordship’s province than mine.”

He then proposed 14 dams including one at Waltham Abbey which would have flooded the Powder Mills, one of the main sources of gunpowder for the forces, though this was not mentioned in the report. A further pertinent comment was:

“To get two Feet of water in the shallowest places will take 90 days or three months in ordinary times and less in flood, but the enemy could cut the banks and the whole would be drained in a short time … If therefore I may take the liberty to give an opinion I have my doubts whether it would be worth the attention of the Government to expend so large a sum of money (£20,000) as this will cost without any certain benefits to be derived from it towards the defence of the Metropolis.”

Despite Rennie’s reservations the Duke of York decided that measures should be taken and Samuel Wyatt contracted to do the work. Dams were constructed under the bridges on the Essex Road at Stratford and a dam with a floating barrier consisting of an old lighter acquired from the Navy was partially constructed in Bow Creek. By 28 October 1803 Rennie wrote to the Duke of York complaining that the works were not as forward as he would have liked but urgency was apparently not pressed because the work was still progressing four years later. By 1810 the military situation on the continent had radically changed while the limited amount of work done on the dams was already decaying and virtually useless. The whole scheme was then abandoned and no evidence survives today.