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NOTES APPERTAINING TO DIAGRAMMATIC DRAWING OF GOTE ROAD FLOOD PROTECTION
Geographically the Gote area is probably the lowest lying area in Cockermouth, adjacent to the rivers Cocker and Derwent and has been prone to flooding for many years. Detailed maps of the area dated 1860 show many watercourses in the area of the present allotments, with a structure known as Blackfriars Bridge crossing these streams in a position where the green is presently visible outside 61 Gote Road, other stories tell of a large pond in this area where drovers use to let the cattle drink before entering Cockermouth market, as it is the lowest downstream point on the River Derwent it is not surprising that all water discharges aim for this location, with one fundamental flaw, when the river is in flood many of the discharge pipes cease to work.
It was fairly obvious from the outset that the river was probably the major cause of flooding, (ignoring November 2009 as a one off) for many years the river has come over at this point but in the last 6 years we have had 3 floods, so why all of a sudden is this occurring more frequently, is it?
- Global warning
- Poor river management
- Alterations in pipework associated with the UU pumping station
- Blocked storm water drains
- Bigger demand on the drainage system
- Increase development in the Cockermouth area.
- Poor upper level river management.
All these questions needed addressing before deciding on an action plan, it did however become quickly apparent there were three sources of water creating the problems;
- River flooding
- Surface water flooding
- Rising ground water flooding
HOW TO ADDRESS THESE PROBLEMS
RIVER FLOODING; various ideas were looked at including digging a swale through Broomlands thereby taking the pressure off the Pump House corner of the river, but this would not give a 1:100 year defence; heavy rock armour defence on the river embankment, but this brought up all sorts of environmental and aesthetic issues, so it was narrowed down to what is called a retracted linear defence (an earth embankment and concrete wall in parts) from Gote Bridge to Waterside Cottage, giving a 1.100 year defence with a 300mm freeboard, this is calculated by using previous flood height records and then adding 300mm as a safety buffer.
SURFACE WATER FLOODING; early on in the appraisals it was becoming apparent that the sole issue of flooding was not down to the river alone, to better understand the highway drainage layout drawings were requested from Cumbria County Council, we were informed none were in existence, but Andidrain were in a position to conduct a complete and detailed survey and 3-4 days would be allocated for this project, nearly 2 weeks later the survey was completed, highlighting many totally blocked main drains and much of the trunk sewers network collapsed or broken through old age.
Also highway inspectors thought that the said sewers ran into the adjacent United Utilities pumping chamber and then pumped away, this theory was flawed and it was discovered that the storm water sewers ran directly into the river, that is when the river is low enough to accept the water; in a flood situation the water backs up the pipes and eventually floods the Gote area.
GROUNDWATER, no matter how water enters your house, flooding is flooding after all, residents told stories of water bubbling up through the floors when the river was high. So, we started taking daily readings from Peizometers left in the ground by the Environment Agency, a Peizometer is a small diameter tube set into ground in some cases 10 -15 meters deep to establish the geology and hydrology of the ground, over a period of 6 months it was clearly established that the readings taken exactly matched that of river flow heights, varying from ground level to 1.8 metre below ground level, so therefore when the river was in flood the water was at ground level and started percolating through the floor slab and into the houses. How do we deal with that? the only ground engineering solution was to place French drains (gravel filled trenches with perforated pipes) around the most prone areas i.e to the rear of the defence and in the allotments. The drain down the centre of the road is already acting as a massive land drain, but designs are being worked upon to place a more permanent scheme in Gote Road itself when funds permit.
To add to all the problems each source of water was handled by different organisation; RIVER water by the Environment Agency, SURFACE WATER by Cumbria County Council Highways and GROUNDWATER by a combination of Cumbria County Council and Allerdale Borough Council.
And to add further complications, a new law was passed in the spring called the Flood and Water Management act 2010, which defined who was responsible for what and how they should go about identifying responsibilities, this threw some of the authorities as they had not fully prepared themselves for the introduction of this act.
To construct a substantial earth bund complete with compacted clay core to a predetermined level and stone faced concrete retaining wall within the grounds of Waterside Cottage.
The drawing indicates the various techniques used to overcome all of these problems, the most complex being the construction of a major pumping station, working on the principles that under normal rainfall and river levels the entire Gote Road drainage scheme will work under gravity, only when the river rises to a flood situation and the flap valves situated on the end of each pipeline automatically close, will the piped water build up, fill the overflow chamber which in turn discharges into the pumping chamber; at a pre determined level the twin pumps will activate and discharge the water over the embankment and back into the river.
Many people have said yes but what if there is a major flood in Gote Road, all the water will be trapped by the embankment, not so, as a major draw down structure has been constructed, complete with Penstock Valve one end (a penstock valve in this case is a large gate raised by winding a spindle similar to that found on canal lock gate) at the other end is a large flap valve restricting the river water from entering Gote Road.
ARE WE STILL AT RISK?
Hopefully not, the works have been designed on a 1:100 flood which means the residents should be able to get insurance cover, but once again let’s make it quite clear no defences in the world would have stopped the November 2009 flood, even if the National Grid or local sub station fails, we still have the capacity to couple up a generator and keep the pumps working.
In my opinion the works have been well thought through and have covered all eventualities, with further proposed works scheduled for the town this summer, together with improved highway drainage in Gote Road, the Gote area will no longer be at risk.
Mike Apperley (Civil Engineering Advisor to the Group) tel 01900 825889 dated January 2011