Blandford Ditch is an overflow channel for flood waters on the Basingstoke Canal in Hampshire. The ditch begins at the downstream end of a concrete culvert and chute that leads from the Basingstoke Canal off-take structure.
The culvert discharges onto a steep concrete chute and down onto a historically armoured bed. Over many years, floodwaters have scoured out the armoured bed, leading to a heavily incised channel, with dangerously undercut banks and overhanging trees offering minimal bio-diversity.
The Clients key drivers for the project were: reducing the erosive force of waters entering the channel; creating a shallower river gradient; and ultimately creating a self-regulating, diverse ditch able to operate in periods of both high and low lows from the Basingstoke Canal.
FiveRivers was awarded a Design & Build contract after a successful tender process. An outline options appraisal was undertaken before the preferred solution was taken through detail design, consenting, to successful delivery.
In its previous state, the ditch was both a H&S risk to potential users of the area, unsustainable with poor diversity. The ditch had badly eroded banks which were liable to collapse. The client needed to carry out works to ensure that the Suitable Alternative Natural Greenspace (SANG) was appropriately enhanced. Grainger PLC came to FiveRivers to design and construct a nature based solution, for the ditch, with the objective of securing a self-regulating, bio-diverse ditch to take low and high flows of water from the canal.
- High level options appraisal to identify how the channel could be enhanced.
- Final detailed design and consent for the preferred solution.
- 275m of bed raising, in-channel berms and backwater creation to enhance the bio-diversity of the new channel.
- Temporary dewatering of the channel at the inlet structure from the canal.
- An energy attenuator was installed on the concrete chute to dissipate energy, with a 25m rock ramp, raising the channel bed to mitigate the historical incision.
- Selective tree removal and pollarding to enable bank regrading, daylighting and removing all dangerous trees.
- Tree root ground protection was installed to provide mechanical access.
- Flex-MSE retaining walls to reinforce the bank close to the public right of way.
- 74 trees were pollarded or removed to increase sunlight reaching the new banks and make the area safe for residents. A majority of the wood was chipped for biomass with 30 large limbs saved for future large woody material (LWM) projects. Trees were retained wherever possible, especially Category A and B specimens.
In total, 300m of the Blandford Ditch were successfully restored over 4 weeks.