06/03/2025
St Neots Common and Mill Lane flood attenuation proposal Overview
Mill Lane floods from two directions, the main river adjacent to the sluice, but also crucially due to indirect flood water flowing off St Neots Common, using the Victorian counter drain ( the main channel that can be seen running across the north end of the common ).
In the pre 'peak' events Mill Lane is flooded first due to the water from the common overwhelming the ditch adjacent to the road. It is not the size of the ditch that is the problem, but the fact that this route stops discharging once the river level rises, meaning it overflows across the road. Then once the river breaches adjacent to the sluice this adds to the quantity crossing carriageway. Crucially the direct breaching flow is usually only for a short period and recovers quickly, but it is the uncontrolled flow from the common that creates the long recovery of the route.
The plan is to allow the road to flood once the river is above circa 1.5m, but to attenuate it under this level. In this way the route will only close briefly for hours while the river 'peaks', but it would then reopen quickly and not suffer the slow extended recovery experienced at present while the common drains ( across Mill Lane ).
The Victorians created a diversion route for some of the River Great Ouse winter flow, to allow the Lamas Meadow to fill with river water for ice skating. The infrastructure used to control this has unfortunately been long forgotten and sits in a derelict condition. It is due to this that a large quantity of flood water is diverted across the common and floods Mill Lane.
Once the river reaches a certain level it floods across the meadow and adjacent common, and an unregulated flow discharges through the brick culvert in the north east corner ( beside the Mill Lane traffic lights ). This unregulated flow then overwhelms the ditch adjacent to Mill Lane which then breaches across the road. As the river continues to rise, the river then eventually breaches over the main river bank adjacent to the sluice.
The proposals are very simple.
We are intending to use the common as a proper flood plain, but in a controlled manner. A series of sluice gates will hold the water on the Common with the assistance of a bund at the north east corner. This will then discharge in a controlled manner along the ditch adjacent to Mill Lane after the main flood event has passed allowing the water on the common to drain over five or six days slowly. This ditch stops flowing during a flooding event, once the main river velocity and height reaches a certain point and therefore holding the water on the common creates a large attenuation volume easily in an area that creates no issues.
In parallel with the above, the main river bank is being reinstated and reinforced, where it has been eroded ( in the small field adjacent to the sluice ). If the river bank is returned to the correct height, the river will be contained ‘within channel’ until the river height reaches 1.5m.
The intent is to control the pre peak water and retain this on the common. The river will briefly flood ( if the river peaks over 1.5m ) breaching across Mill Lane for a number of hours. However once the river level drops below 1.5m again, Mill Lane will be usable as tge recovery will not be delayed while the common drains across Mill Lane. A partial trial was carried out during the winter floods in November, and Mill Lane was kept passable and only closed for a short timeframe. This trial only regulated the flow from the common, and did not include phase two ( ie the repairs to the river bank ).
