Thursday, 14 November 2013

Bar Hill Parish Council Minutes of Environment Committee Meeting 06-NOV-2013

Here are the draft minutes (subject to approval at the next meeting) from the meeting of the 6th November. As always any questions let me know!

To approve minutes of the previous meeting 
The minutes of the meeting held on 6th September 2013 were read and approved as a true record. Proposed by SJ and seconded by MP. All in favour.

Matters arising for discussion and decisions to be made from the previous minutes Litter Picking - BW asked whether the employment of the litter picker has taken place. Noted that there are a couple of areas for him to litter pick. AS confirmed that several people turned up for the Litter Picking Day and several bags of litter were returned to the PC office.

Play Area and Skate Park - BW read a letter received from FOOTPATH which requested help from the Parish Council to fill in the grant forms as the Parish Council owns the land where the play area and skate park is. There was discussion regarding the skate park and its viability due to the holes in the ramps and consequently dangerous to use. It was suggested that signs be placed stating that it was not to be used temporarily whilst repairs are being carried out.

There was no information regarding a quotation for the reduction of the trees which front the Skate Park. This was discussed at the October Parish Council meeting.

Village Stream and Nature Reserve 

A presentation/update was given by Rob Mongovan. Please see below the letter from Bob Bray, this was discussed with Rob and he gave us further information on how this work would take place over the next few months.

The Committee agreed that funding would be paid by the Parish Council, as agreed at previous Parish Council meetings, to ensure that this vital work should take place due to flood risk. Rob is also contributing funding to this work. Rob is also contacting Rachel Hobbs at Cambridgeshire County Council to complete a form – ‘Consent to do works on Watercourses’ on which he will enter the information but it needs to be signed off by Parish Council.

There is to be a Working Party date on 9th February 2014 for all village residents to participate working at the Nature Reserve, the Cambridge Conservator Volunteers will be helping on that day also.

This review of the Flood Storage Area (FSA) and associated watercourses is based on a walkover of the site on 16 August 2013 in the company of Rob Mongovan, Ecology Officer for South Cambridgeshire District Council.

The purpose of the site visit was firstly to gain an understanding of the hydraulic character of the stream delivering runoff to the flood storage area, the flood storage area itself and the nature of the stream below the flood storage area until it leaves the locality of the housing affected by flooding.

The review considers the issues and possible practical mitigation measures that might be considered as a small intervention to reduce the impact of flooding whilst minimizing any effects on the biodiversity in the FSA wetland,

The character of the Bar Hill Stream upstream of the Flood Storage area The Bar Hill stream rises only a short distance upstream from the FSA and the settlement of Bar Hill. However the land through which it passes is arable farmland with reasonable slopes down to the stream for this part of the country. The rate of runoff from surrounding agricultural areas is therefore likely to be quite rapid and although there are narrow buffer strips along the sides of the stream there is little to prevent runoff entering the stream unhindered.

The stream bed is deeply incised with the effect that flows are concentrated in the channel and flow quickly to the inlet into the FSA

This situation is different to a natural stream catchment where natural vegetation, including trees and permanent vegetation, intercept much of the initial flow and the stream profile would be much wider allowing the runoff to be held within the stream bed itself. However this situation is the common experience in lowland Britain particularly in arable areas like Cambridgeshire.

It has proved quite difficult to obtain information on the hydraulic characteristics of the stream as the modeling packages that provide the information on stream flows are not held by all drainage specialists. The following information is provided to give an indication of flows and volumes generated by the stream and the storage possibilities within the FSA but would require detailed modeling to guarantee a fully accurate picture of the stream hydraulics.

The storage potential of a 600mm high flow control at the outlet of the wetland

This chart gives an indication of the assumed storage available, 1980 sq M, and the volume generated at a number of return periods. The return period is a measure of the likelihood of any storm occurring in a particular period of years. Therefore in the greatest storm event likely to occur each year, the 1 in 1 year return period, 11% of the storm would be stored whilst in the greatest storm considered, the 1 in 100 year return period (based on a critical storm duration measuring both intensity and duration of the storm) only 2% of the storm would be stored.

However this is raw date and importantly does not take into account the time taken for the runoff to flow through the wetland. It is likely that a much greater reduction in the flow would be observed because of this effect.

The wetland is reducing the peak flow by delaying the rate that water passes into the stream below the FSA.

At present runoff is being held back by the vegetation and the time it takes to flow through the wetland is governed by this physical resistance. In times of high flow it is clear there are flow channels conveying water directly to the outfall, which is a large pipe with little effect on the flow, located at the apex of the triangular wetland where it rejoins the stream.

The character of the Barr Hill stream below the Flood Storage Area

Once the stream re-enters the original channel it again demonstrates a deeply incised bed with little opportunity to store water. As the stream makes its way through the wood and then the housing it appears that at some time in the past the bank sides have been raised. This action was common in the past when the response to flood risk was to keep the flow in channel until it was past a critical flooding location. Unfortunately as development has accelerated this has only moved the problem to another place.

The consequence of these actions in the past is to restrict the capacity of the stream to respond to increasing runoff by reducing the natural flood plain of the stream itself. The development around the stream appears to discharge runoff from roofs and roads directly into the watercourse. This not only reduces the capacity of the stream to deal with water coming from upstream but delivers polluted runoff directly into the stream which cannot be treated naturally as there is no place left for this to occur. This kind of situation is one of the drivers for Sustainable Drainage Systems or SuDS now being enacted as part of the Flood and Water Management Act 2010.

In the present context it is difficult to see how any affordable intervention can be provided within the housing area and attention therefore returns to the possibility of enhancing the performance of the FSA.

Enhancing the performance of the Flood Storage Area whilst protecting biodiversity.

The causeway at the bottom of the wetland is 2-3M high and could conceivably be modified to store greater volumes of water by controlling the flow into the outlet pipe.

However the wetland habitat is home to a variety of plants and animals that would be affected by this intermittent rise in water levels. The outline calculations demonstrate that there are significant flows entering the wetland and that even in the 1 in 1 year return period event a 600mm storage would only store 11% of the runoff generated upstream, although this could be a conservative estimate.

An assessment of the impact on biodiversity was made on site with Rob Mongovan and concluded that a discreet area near the outlet could be flooded to 600mm which would allow Water Voles in particular to migrate a relatively short distance to safety. This assessment also concluded that the other animals that inhabit the wetland would also be able to adapt to this predictable area of inundation.

Although the volume appears small in the calculations it would have the effect of delaying the response time of the runoff flowing into the stream and allow the instant response further down to flow onwards, particularly during intense summer storms when flash flooding is most common.

The suggested mechanism for this attenuation is to create a reverse V control structure 600mm high that protrudes backwards into the wetland. The open slot is thereby protected from immediate blockage and can be easily cleared by simple action from the bank. The slot could be sized at a general ‘greenfield rate of runoff’ of 5L/sec/hectare depending on the practicality of construction to maximize the effect of peak flow reduction. Once the storage volume is reached water would flow over the weir and into the pipe as it does at present. In the event of blockage the flow would continue to behave as it does now until the blockage is cleared.

This intervention is simple and cost effective and can be constructed to give the opportunity of modification should either the storage level be changed or the slot be modified. I hope this preliminary review offers an understanding of how a simple intervention can have a significant benefit on flood management with minimum impact on existing biodiversity.

It is very difficult to accurately model these natural systems but this study gives an indication of the benefits that could be provided by a simple flow control device and 600mm storage within the wetland.

This review is a strategic overview of the condition found at Barr Hill based on a single day visit and a simple modeling exercise. In the event that the Council decides to proceed with an action of this kind I would recommend a detailed evaluation of the circumstances by a fully qualified Engineer as the structural requirements of the weir and control would be significant in this location.

Please contact me if you have any questions regarding the strategy that has been developed to meet both the requirements of flood mitigation and the conservation of biodiversity at the Barr Hill site.

Robert Bray for Robert Bray Associates

Date 3 November 2013

Works outstanding and approval given

064/12 - Hedging of hawthorn and sloe needs to be cut back between 21-43 Brambles. MWP quoted £499.87. Ferguson quoted £980. MWP accepted on PO 1137

003/13 - Dead trees/thick brambles to be removed from the rear of 10 Almond Grove to the rear there is a little green area, where it has become a dumping ground. CGM quoted £840. Ferguson quoted £420. Fergusons accepted on PO 1122 @ £378

012/13 - Scrubland area behind 186/187 Spinney to be cleared of all dead overgrowth and dumped waste and possibly have this area. CGM quote £1,680. Ferguson quote £1817. CGM accepted on PO 1116

027/13 - Trees to the rear of 69 Foxhollow need to be trimmed back away from garden. MWP quoted £746. Ferguson quoted £470. Ferguson accepted on PO 1139

030/13 - Large bushes to the side of 11 Acorn need to be taken out. S Lawlor quoted £50. Accepted on 1125

033/13 - Tree to rear 100 Field View (on perimeter road) to be cut back. Ferguson to quote

038/13 - 2 dead trees by sub-station close to 54 Acorn Ave. Also some dead bushes to be removed. CGM quoted £240. Accepted on PO 1130

046/13 - Hedge at back of property, 67 Little Meadow, is getting tall and overgrown and needs cutting back. CGM quoted £180. Accepted on PO 1131

048/13 - Back of 30 Watermead, the brambles by the brook are growing over her fence. Need to be cut back. CGM quoted £240. Accepted on PO1138

052/13 - Trees to side of her property, 174 The Spinney, are getting close to her house and need cutting back. CGM quoted £150. Accepted on PO 1132 

053/13 - Grass to back of 14 Partridge Drive is overgrown and needs cutting back. S Lawlor to quote

054/13 - Can the overgrown path between Bar Hill and Dry Drayton be cut back. Ferguson to quote

059/13 - Self-setting trees need to be removed as they are very close to fencing of 14 Spinney CGM quoted £120. Accepted on PO 1141

061/13 - Street light in play area to be repaired. K&M lighting to quote

062/13 - Road outside Bar Hill Motors to be repaired. Sean Gardiner to quote

063/13 - Hedging to the rear of garden of 1 Chestnut Rise (on perimeter road) to be cut back. MWP quoted £199. Accepted on PO 1135

063a/13 - Section 2 & 3 of above. MWP quoted £180. Accepted on PO 1136

066/13 - Tree located at front of 55 Acorn Ave to be cut back as berries are dropping onto car. Also tree to the far side of garage to be taken back away from guttering. MWP quoted £251 for 2 trees. Accepted on PO 1142

067/13 - Rear of 178 The Spinney. Trees to be cut back away from fencing. MWP quoted £250. Accepted on PO 1143

068/13 - Rear of 16 Acorn Ave. Trees to be generally looked at to ensure they are not encroaching on to property. Ferguson to quote after Councillor has inspected

069/13 - Trees along spine path (to village hall) need to have lower limbs removed. CGM to quote

073/13 - Footpath outside PC office leading to school very cracked (wheelchair went over). Clerk contacted County Council

074/13 - Area outside 39 Brambles needs to be tidied up with self-setting tree to be removed. S Lawler to quote

075/13 - Trees to front of 105 Hollytrees to be cut back. CGM to quote

076/13 - Hedge to side of 32 Spinney to be cut back. Fergusons to quote

077/13 - Ash tree to rear of 137 Appletrees to be trimmed. MWP to quote
078/13 - Hedging to side of 43 Little Meadow to be cut back. Fergusons to quote

079/13 - Trees to the back of 141/142 Hillcrest needs to be looked at. Fergusons to quote

080/13 - Brambles to the side of 98 Pheasant Rise to be removed. CGM to quote

It was queried whether there could be a tender document sent out to three companies which do tree work and ask them to quote for perhaps two days a week to do a day’s work cutting back, removing dead trees, clearing etc., which could be more cost effective than giving individual jobs as on the sheet. As an example the amounts given are approximately £7653. This would then save the Clerk time in having to send out details of all the areas requiring these actions.

It was also queried whether all the items on the list belong to the PC or to the house owner. BW/SJ will check.

054/13 - 14 Partridge Drive please can this read 16 Partridge Drive – Bramble overgrowth to the side of the property.

Items for information

The donation given by the late Tom Flanagan’s estate was discussed and several suggestions made:
An Orchard at the front of the village where the old basketball court is;
Trees and bench at the corner of the Church & Hanover area
Fruit trees at the Nature Reserve, this was the favourite choice of the committee.
AP will approach Garden Centres to discuss trees to plant.

Village Green
The committee were advised that the aeration of the Village Green, so far, is successful and it is suggested that aeration takes place again in March 2014; this has a cost of £600. To be confirmed at the next PC meeting.