Clifton Drive Drainage Planning Statement

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Proposed Housing Scheme. Clifton Drive, Ramsey, Isle of Man For Haven Homes Planning Statement Relating to Site Drainage and Vehicle Tracking EXECUTIVE SUMMARY

• • The proposed schemes for both foul and surface water drainage will be fully compliant with current regulations.
• • The drainage proposals for the development at Clifton Drive will be designed to have no adverse impact on the existing drainage systems or the adjacent properties.
• • It has been confirmed by the Manx Utilities that there is adequate capacity within the existing foul system and existing pumping station subject to works undertaken as part of the development to reduce surface water infiltration in to the existing system to off set the increased foul flows from the proposed development.
• • Surface water attenuation has been introduced at the wet land area and development levels raised slightly above existing to militate against the risk of flooding.
• • Vehicle tracking demonstrates suitable access for the existing Ramsey Town Commissioners refuse vehicle.

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• 1. INTRODUCTION

• 1.1. This statement has been prepared by 58 Consulting Engineers in support of a Planning Application for a proposed 66 unit residential development to be sited on the predominantly green field site on the south east side of Clifton Drive and adjacent (on the south western boundary) to the existing Balleigh Park residential development.
• 1.2. Reference is made to 58 Consulting Engineers and the Hugh logan Architects drawings submitted within the Planning Application.

• 2. SITE DESCRIPTIONAND EXISTING DRAINAGE

• 2.1. The site is located on the south east side of Clifton Drive in Ramsey. Balleigh Park (an existing residential development) is situated on the south west boundary of the site. The south east and north east boundaries front on to open fields. An existing residential property, (Croftbank) at the northern corner boundary of the site will be redeveloped as part of the proposals.
• 2.2. An existing Manx Utilities' (MUI foul pumping station is located towards the western corner of the site on land owned by the Isle of Man Government. Existing public drainage runs across the south end of the site with associated easements for future repair and maintenance by MU.
• 2.3. The existing MU surface water system runs approximately west, exiting the site below Clifton Drive as a GOOmmdiameter pipe before discharging in to a field ditch beyond the existing residential development area.
• 2.4. The site is predominantly green field, with a general falls from north to south, and with a low lying area running east to west in the general area of the MU drainage easements.
• 2.5. Reference to the historic Ordnance Survey mapping (1866 to 1871) shows the site and surrounding areas as fields. Over the years many of the surrounding fields have been developed for the housing that can be seen today. A water course/ditch is shown on the historic mapping running south east to north west approximately on the line of the current MU surface water drainage.

2.G. Reference to the geological mapping indicates the underlying ground as drift of sand, silt and clay deposits. This mapping is supported by local knowledge and experience. Walkover surveys in October 2014, December 2014 and January 2015 indicated the north end of the site to be fairly steep and well draining ground with sand below the surface. The low lying south end of the site was wet underfoot and less well draining. MU have reported that the low lying ground containing their drainage is wet and has presented them with difficulties in the past when undertaking excavation works in this area. Trial excavations undertaken within the low lying area shows 400 to SOOmmdepth of black organic soils overlying silty sands

and gravels. Water seepage was noted in the sands at 800mm depth (January 2015)

indicating the level of the groundwater in winter.

• 2.7. Existing field ditches are present in the southern corner of the site and collect run~ off from the marshy ground just beyond the south east boundary of the site and the outer lying fields as far as the Jurby Road. The ditch discharges to a rudimentary soakaway and in to the underlying sandy soils. During the walkover surveys in October and December 2014, water was seen to be flowing in to the rudimentary soakaway. In January 2015 the water level in the ditch was higher (300mm below the adjacent ground level) and no discernable flow of water was apparent.
• 2.8. The central southern area of the site contains a 'low spot' (in the vicinity of Plots 16, 17 and 18) where ground levels drop from about 11m above Datum to 10.6m above Datum. From an examination of the site and the topographical survey it seems likely that this area may be subject to flooding; excess surface water will hold in the area before naturally dissipating to the underlying sandy soils, and the area will act as natural attenuation for surface water flows during heavy rainfall events. No flooding of this area has been noted during the three walkover surveys and the ground has not been sodden or boggy. Discussions with local residents and dog walkers' who use the land have advised that this area does occasionally flood and anecdotally this would appear to be very three years or so.

• 3. FOUL DRAINAGE

• 3.1. The proposed foul drainage system is outlined for adoption by MU and is designed in accordance with the Manx Sewers for Adoption guidelines. A 3m easement will be provided to both sides of any drain within private land.
• 3.2. The proposed layout and topography of the site permits the use of a gravity foul drainage system.
• 3.3. The proposed foul drainage system is outlined to connect into the public sewer running across the south end of the site at two locations.
• 3.4. It has been confirmed by MU that there is adequate capacity within the existing foul system and pumping station. However the existing foul system connecting to the pumping station is suffering from surface water infiltration and during heavy rain the pumping station struggles to cope with the increased flows. As part of the development, and in agreement with MU, works will be undertaken to address some of the infiltration to offset the additional foul flow from the development.
• 3.5. The existing rising main running across the south end of the site is reported by MU as shallow with limited depth of cover. Should the minimum cover of 1200mm to the proposed road surface not be achievable, the rising main may need to be re.laid as part of the development and with the agreement of MU.
• 3.6. MU require that any habitable room is positioned at least 15m from the foul pumping station wet welt and this exclusion zone is shown on the drawings.

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• 4. SURFACEWATER DRAINAGE

• 4.1. The proposed surface water system is outlined for adoption by MU and is designed in accordance with the Manx Sewers for Adoption guidelines, A 3m easement will be provided to both sides of any drain within private land.
• 4.2. The proposed layout and topography of the site permits the use of a gravity surface water drainage system.
• 4.3. tn accordance with Sustainable Urban Drainage principles surface water flow from property roofs and hard standings will be discharged to localised soakaways where ground conditions and layout considerations permit. Individual soakaways will be designed in accordance with BREDigest 365 and will include provision for future inspection and maintenance.
• 4.4. Surface water flows which it is not possible to return to the ground locally via soakaways will be attenuated before discharging in to the exiting public surface water system.
• 4.5. Surface water flows in to the public surface water system will be attenuated for design rainfall events with a lin 50 year return period as prescribed in Manx Sewers for Adoption. Attenuation storage will be provided at a low lying wet land area located in the southern end of the site within the Public Open Space area.
• 4.6. The outflow from the attenuated wetland will be at a rate equivalent to a green field runoff of 7l/s/ha as required by MU.
• 4.7. The existing field ditch on the southern boundary which currently naturally attenuates existing field run-off flows from the adjacent land will be cleared and linked to the proposed low lying wetland area. The combined arrangement of existing field ditches and wetland will be designed to provide attenuation for the increased surface water run-off from the development and the existing run-of from the adjacent fields. The final size of the wetland attenuation volume required will be determined once the extent of individual plot soakaways is known and the runoff areas are confirmed. An initial assessment of 300m3 has been calculated.
• 4.8. The attenuated flows are to discharge in to the existing surface water sewer running westward in the southern end of the site. MU has reported that this 600mm diameter outfall sewer has recently been cleaned and the downstream outfall field ditches cleared.

S. FLOODING RISK

• 5.1. The site is not outlined as at risk on the Isle of Man Flood Maps issued by MU. The majority of the site is fairly steeply sloping and presents minimal risk of flooding to individual houses. Where driveways slope down from the highway toward the house adequate drainage and appropriate flood path provision will reduce the risk of localised flooding of individual houses.

• 5.2. All adoptable surface water drainage will be designed in accordance with Manx Sewers for Adoption guidelines.
• 5.3. MU has advised that flooding has occurred previously on Clifton Drive at the location of the two existing road gullies positioned at the low spot on the road (lO.94m above Datum). Conversations with local residents have confirmed this and advised that the water has in the past extended across the full width of Clifton Drive towards the garage of property on the other side of the road and then following the natural topography westwards along Romney Wynd. As noted above MU have undertaken cleaning works in the last 12 months to the local surface water system in response to these flooding incidents.
• 5.4. As noted in 2.8 above there is low lying area of the site prone to flooding and which provides a natural attenuation of flows to the underlying sandy soils. The ground levels will be raised in these areas as part of the development. The 'lost' natural attenuation area will be replaced by the proposed wetland area.
• 5.5. The proposed development levels create a new road (Country Lane) which is set slightly above the level of the proposed wetland. Road crossfalls and positive gradients of driveways up to the proposed dwellings immediately north east of Country Lane provide for finished floor levels of Plots adjacent Country Lane at 11Am above Datum. These properties would be the lowest lying on the development and would be 400mm above the proposed top of bank of the proposed wet land / existing ditch, and at least 400mm above any flooding level experienced by local residents. In an extreme flooding event (outside usual design parameters) surface water would sit along the southern channel of Country Lane and therefore not present a risk of flooding to the properties.

• 6. VEHICLETRACKING 6.1. At the request of the Ramsey Town Commissioners, vehicle tracking has been

undertaken for a Dennis Eagle Elite 3 axle RCVfront steering only refuse vehicle. The results of the tracking are shown on the enclosed BBCE drawing and demonstrate that the refuse vehicle can service the proposed development.

• 7. DRAWING SCHEDULE 14.112/05 Vehicle Tracking - 3 axle Refuse Wagon 14-112/10 B Proposed Site Drainage

BS Consulting Engineers Limited 27 Duke Street, Doug/as/M1 2AZ February 2015

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