CFLAG have at last been able to meet with the County Council Highways section regarding the two most important bridges which affect the flooding of the City. The Council are investigating the following matters and their responses will either be reassuring or setting the agenda of how we get the flood risk authorities and government to urgently include solutions within their plans.
Botcherby Bridge
· In regard to the AiP (approval in principle) check by the authority on what the flood design level of protection used by the Environment Agency was is the parapet strengthening works to see if this is fit for current purpose?
· The Councils view of the effects of the planned further raising the level of flood defence against the parapet (or indeed above it) which appears to have no integral reinforcement only one light reinforcement mesh recently retrospectively applied as part of the post 2005 works considering much of the mortar has been washed out during Desmond (was this upgraded/reviewed following 2015 prior to application?) reference residual risk classification of risk to life being high.
· Base construction type – concrete box base? Potential for river bed gravel accumulation to be regularly cleared in order to maintain a maximum bridge aperture for the river.
· With the design level flood peak being potentially 500mm higher than Desmond (as the 2million cu mx that flooded the Warwick Rd area would still be in the river Petteril) is there risk of the structure floating if anchorage was not part of its original construction - either with flood levels being equal on either side or with significant differential in a Petteril biased flood event? Cross section in these extreme conditions indicates the bridge would be displacing a considerable volume of water below the peak level, ie a semi-submerged structure as we know that concrete ships do float!
Eden Bridge
· What are the risks involved to the bridge with a 500mm higher flood peak to Desmond given that it is agreed there was a significant differential in levels during Desmond clearly confirming the bridge was putting a break on conveyance velocity and backing up?
· If the current bridge does not have sufficient conveyance capacity for a Desmond + 500mmm flood peak how are the volumes to be mitigated? Vis a vie the point made in our ”Tale of Two Bridges” presentation where the 1812 conveyance design provided 10 arches with 5 of these now blocked off and subsumed under Hardwicke Circus, the Civic Centre and other post Victorian development.
· It is understood that the original stone bridge (down stream side), whilst being assessed of dense stone construction has no binding concrete saddle, content or significant ties to the 1930’s bridge widening. What is the potential for afflux to de-stabilise the stone arches and rip the older bridge away downstream?
We look forward to returning with responses to the above.
