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1.1. This statement has been prepared by BB Consulting Engineers in support of a planning application for the proposed 27 unit, residential development adjacent to Oatlands Road, Andreas, Isle of Man. This statement covers both the foul and surface water drainage. 1.2. This statement is to be read in conjunction with all BB Consulting Engineers information relating to this application.
2.1. The proposed development site is located to the north-east of Andreas Village, on the former Dale Nurseries site and within field 121338, adjacent to the Little Meddow residential development. 2.2. The proposed development site is both brownfield (former Dale Nurseries) and greenfield (field 121388). 2.3. The proposed development site is approximately 1.23 hectares. 2.4. The local topography has a general fall from north to south. 2.5. There is an existing public foul sewer located within the proposed development site. 2.6. There is a surface water pond and watercourse within the proposed development site. The watercourse flows in a south-westerly direction along the southern boundary of the site towards Andreas Village, where it connects to the existing public surface water drainage system.
3.1. The proposed foul drainage system is outlined for adoption by the Manx Utilities Authority (MUA) and is to be designed in accordance with the Manx Sewers for Adoption guidelines. 3.2. The proposed layout and topography of the development site permits the use of a gravity foul drainage system. 3.3. This system is to be sited within the proposed highways of the development site. 3.4. The proposed foul drainage system is to connect to the existing public foul sewer within the development site. The proposed connection is to be facilitated through the construction of a new manhole on the existing public foul sewer. 3.5. It has been confirmed by MUA that there is adequate capacity within the existing foul system and this connection has been agreed.
4.1. The proposed surface water system is outlined for adoption by the MUA and is to be designed in accordance with the Manx Sewers for Adoption guidelines. 4.2. The proposed layout and topography of the development site permits the use of a gravity surface water drainage system. 4.3. The proposed surface water drainage system is to utilise elements of Sustainable Drainage Systems (SuDS), replicating the existing drainage characteristics of the site where all surface water runoff enters the existing pond before discharging to the existing ditch along the southern boundary of the site. 4.4. The surface water flows from all impermeable areas are to flow to a detention basin located within the development's public open space. This is to be located in the vicinity of the existing pond. To provide the required attenuation storage, the outflow from the detention basin to the existing ditch is restricted to a rate of 7 litres/second/hectare.
4.5. The proposed detention basin is a landscaped depression, which will generally be dry except during, and immediately following, a rainfall event. It is to be designed and constructed in accordance with the guidelines of CIRIA Report C753, The SuDS Manual (2015). 4.6. The storage volume provided by the proposed detention basin is approximately $335 \mathrm{~m}^{3}$ and is in excess of the $260 \mathrm{~m}^{3}$ storage volume required to accommodate a 1 in 100 year storm event. 4.7. The proposed driveways to the dwellings are to utilise a permeable surfacing to reduce the volume of surface water runoff. An allowance of $20 \%$ of their surface area has been included in the impermeable area for the storage volume calculations to allow for any runoff which may occur.
Prepared by BB Consulting Engineers Limited 10 Myrtle Street, Douglas IM1 1ED April 2020 Ref 17-072
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