Planning Statement Drainage Flood Risk

Castletown Commissioners, Redevelopment of School Hill & Westhill Avenue, Castletown, Isle of Man

19/00961

Planning Statement Relating to Drainage and Flood Risk

1. Introduction

1.1. This statement has been prepared by BB Consulting Engineers in support of a planning application for the proposed redevelopment of the existing public housing estate of School Hill Avenue and Westhill Avenue in Castletown, Isle of Man. This statement covers the foul and surface water drainage.

1.2. This statement is to be read in conjunction with all BB Consulting Engineers drawings relating to this application.

2. Site Description

2.1. The site is located to the north of Castletown, at the junction of the A5 (to the north) and Malew Street (to the east). The site is brownfield, and is currently occupied by the existing public housing estate, comprising of 91 housing units.

2.2. The proposed redevelopment scheme for the site will create 133 housing units, based on the principle of retaining the approximate layout of the existing road network.

2.3. The local topography has a general fall from west to east.

2.4. The existing residential estate is served by a combination of public drainage systems. There is a foul/combined system which discharges to a combined sewer within Malew Street at manhole 3805.

2.5. It is understood from discussions with the Manx Utilities Authority (MUA) that the majority of surface water runoff from the impermeable areas of the estate discharge to the combined system, however there is a surface water/highway drain system that serves some areas of the estate. The surface water/highway drain system discharges to the public surface water sewer in Malew Street at manhole 3809.

3. Foul Drainage

3.1. The proposed layout and topography of the site permits the use of a gravity foul drainage system. 3.2. The proposed foul drainage system is outlined to connect into the existing public combined sewer located to the east of the development site, within Malew Street at the existing MUA manhole 3805. 3.3. The foul drainage system is outlined for adoption by the MUA and will be designed in line with Manx Sewers for Adoption guidelines. 3.4. The connection in to the existing public combined system has been agreed with the MUA.

4. Surface Water Drainage

4.1. The proposed layout and topography of the site permits the use of a gravity surface water drainage system. 4.2. The surface water flows will be attenuated within attenuation tanks located beneath the highway of the development site. 4.3. The outflow from the attenuation tank will be at a rate of $25 \mathrm{l} / \mathrm{s}$. This discharge rate is based on a greenfield runoff rate of $71 / \mathrm{s} / \mathrm{ha}$. 4.4. The attenuated flows are to discharge in to the existing surface water sewer located to the east of the development site, within Malew Street at MUA manhole 3809 within the site boundary. 4.5. The surface water drainage system is outlined for adoption by the MUA and will be designed in line with Manx Sewers for Adoption guidelines.

5. Flood Risk Assessment

5.1. The site is not outlined as at risk on the Isle of Man Flood Maps, issued by the MUA.

Prepared by BB Consulting Engineers Limited 10 Myrtle Street, Douglas IM1 1ED August 2019 Ref 19-013