Fast flowing water upstream to the Gelvert Bridge (2 Jan 2014)
Ed. This post was written to cover some of the effects of the storm of 23/24 December 2013 on Fleet Pond with photos taken on 26 December and 2 January.
If you are unfamiliar with any of the locations mentioned, please see the map here (click it to enlarge).
Colin Gray writes:
A powerful storm and very heavy rain over the night of 23rd / 24th December was a significant test of the diversion channels installed, but not yet fully functional, in the Gelvert Stream at Fleet Pond. The aim of these channels was to divert up to two thirds of silt laden water away from the main stream and into the widened channel of Coldstream Ditch.
The extension of the Ditch by a long channel would carry the water along the eastern edge of the marshes complex where the water flow would be slowed, allowing silt to be deposited before it reaches the main pond. Deposited silt could then be removed by land-based machinery, which would avoid the high cost of heavy machinery floated on pontoons to dredge the main pond.
The planned control sluices have not yet been installed but the fact that water can be carried by the new channels was proved to dramatic effect at the northern channel, which was completely full on the morning after the storm.
The southern channel did accept a high flow but demonstrated that the gap built in the sluice structure was inadequate; water spilled over the channel bank overwhelming the narrow gap where the sluice would have been installed. The bridge over the Gelvert Stream still presents an obstacle to a free flow of water at high volumes. Water built up against the bridge supports, overflowed onto the adjacent footpaths and covered the bridge platform with about 8cm of water.
The Coldstream Culvert (2 Jan)
The new, much wider, culvert of the Coldstream Ditch, where it passes under the footpath, coped very well. There was some overflow into the adjacent woodland and across the footpath at Coldstream Glade but not of significant depth.
Sandy Bay on 26 December 2013
Sandy Bay on 2 January 2014
At Sandy Bay the new line of islands along the eastern marsh edge (to the right of the Bay) were completely submerged. This is not necessarily a bad thing as eventually we plan to plant these with reeds and reed would not be averse to submersion in water. It does mean that diverting the Gelvert flow into the main pond at Sandy Bay along the rear of these islands would obviously not work at such high water volume input.
Example of the Gelvert braiding (2 Jan)
The shallow channels cut into the reedbed to the west (left side) of the Gelvert at Sandy Bay, called “braiding”, took some of the flow, which would help to filter out some silt before the water reached the main pond.
Fleet fortunately did not suffer the disasters inflicted on other towns in Hampshire, for which we must be grateful. As a test of our silt defence measures at Fleet Pond it was a useful if rather excessive test.