Flood Event Assessment - Lessons Learnt from the September 2012 flood event
One of the key impacts of predicted climate change is an increase in severe weather events, with an associated risk of exacerbated flooding and erosion issues.
In September 2012, the flood resilience projects undertaken to date by Cheviot Futures met their biggest test with a large flood event affecting all sites.
Some sites fared better than others, but all offered some lessons to be learned and experience to be gained. The value of Cheviot Futures demonstration works lies in the acceptance that some work will prove to be less viable than alternative options, and the process of determining what works and what does not.
At Clifton-on-Bowmont the bank protection ELJ survived intact, as did the newly constructed bar apex ELJ features (although one is now within the Bowmont due to channel change!). The willow spiling work and the Filtrexx bank stabilisation both suffered minor damage to the downstream extent, highlighting the need to effective integration to stable ground and consideration of the effects on flow of features such as mature trees. The timber palisade revetment site suffered significant damage for the second time (a section was removed in May 2012 and additional work carried out by the farm business to reinstate and extend the work), suggesting that this approach was not the most appropriate for the site.
At Swindon Haugh the newly completed bank protection works, utilising a variation on the ELJ approach was removed in its entirety by the high flows. This has been put down to the requirement to alter the specification during works to take account of the concrete-like nature of the eroded bank face which prevented vertical timbers being driven backwards into the bank itself. This evidences that a series of logjams as bank protection needs to be secured into the bank itself, as per the original specification.
Of the 21 bar apex logjam features erected, two were lost in the flood event and a further two suffered damage. The remaining features have provided valuable early evidence of their actual behaviour, to compare to the theoretical concept.
At Venchen the floodplain specification fencing work demonstrated its value. Approximately 50m of fence suffered minor damage during the flood event, but was re-erected at minimal repair cost within a matter of days. The fence broke close to an intended break point, meaning that a distinct section lay flat once waters receded, without significant negative impact on the rest of the fence-line. It would be worth installing an additional break-point at this location in future, should this prove to be a regular vulnerable point. This is something which can only be fully determined following flood events and by working closely as we did with the landowner.
At Kelsocleugh the recently completed logjam works fared reasonably well, reflecting their headwater location and resultant comparative reduced vulnerability. The bar apex logjams all survived intact, as did the structure of both the bank protection and grade control logjam, although some additional erosion of the banks at the grade control site, and some of the backfill material from the bank protection site was evident. We are reasonably conifdent in our conclusion that the latter was largely due to the flood event occurring so soon after works were completed (approx. 5 weeks), meaning that backfill had had insufficient time to settle and consolidate.
At Elilaw the flood event was in advance of the final element of works being completed on site. However, there was evidence that the farm pond feature did fill in excess of its regular fill level, and then discharge to the watercourse following the main flood peak, and also that some water was spilled from the ditch bund to the floodplain storage area. There were still issues of water running down the road in the village of Netherton downstream, partly attributable to debris blocking the culvert feature. It is hoped that the fully completed scheme of works will assist in holding additional flow back in future events, maximising the potential for this feature to cope with high flows.
At Kirknewton the bank protection works based on a vertical timber palisade concept was only in place for approximately 4 weeks in advance of the large flood event at the end of September. Of the 200m completed, approximately 30m was removed by the action of the water, with flow undermining some of the timbers. This offers valuable lessons in terms of the suitability of this approach and whilst some damage was sustained, it is considered that the timbers did offer some protection to the flood bank behind. The work has now been superceded by sediment management efforts on the site, undertaken by Northumberland County Council in relation to the West Newton Bridge structure.