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Planning Statement relating to basement tanking and flood risk
2.1. The building is a substantial load bearing masonry structure with internal cast iron load bearing columns supporting a steel truss frame roof. There is a basement under the entire building footprint containing masonry walls, masonry piers and steel beams carrying the suspended concrete ground floor slab. The building is dated by carved masonry on the external stonework as 1899. 2.2. The building sits alongside the North Quay which is subject to periodic tidal flooding.
3.1. The existing basement walls and floor do not include any recognisable tanking or water proofing system. Mean High Water tide level is above basement floor level and ingress of water through the existing basement walls currently occurs during high tides and floods the basement floor. The flood water recedes with the falling tide. 3.2. Drainage is present below the existing basement slab and it is likely that a pumped discharge was present historically, but it appears that any such pump discharge has not been is use for many years. 3.3. In order to bring back the basement to beneficial use, a proprietary basement waterproofing system is proposed to manage the water inflow and discharge the water to the existing drainage system. The system would be designed to achieve a Grade 3 usage as defined by BS 8102, ie "Ventilated residential and commercial areas, including offices, restaurants etc; leisure centres". A Type C (drained protection) waterproofing system is proposed, as defined by BS 8102, and would comprise cavity drain membranes, dry wall lining system, maintainable drainage channels, drainage sump and pump. A typical arrangement for the system is appended as Safeguard Europe Limited Drawing No. SBS4-001, Rev3. 3.4. During very high tide events there remains a risk of flooding of the basement, either as a result of direct inundation from external flooding to the perimeter of the building, or surcharging of the existing drainage system preventing effective discharge from the basement pump. In such circumstances, flood resilience measures, included as part of the proposed refurbishment, will prevent any serious damage to the building and allow effective cleaning up after the tidal flooding event.
4.1. The existing public footways around the perimeter of the building vary in level from 4.4 m AOD (south west corner) to 4.9 m AOD on the north elevation. The finished ground floor level of the existing building varies
Market Hall, Market Hill, Douglas slightly but is generally at 4.98m AOD. The basement floor level varies between 1.9m and 2.0m AOD generally.
4.2. The Mean High Water tide level is 2.50m AOD; the High Water Ordinary Spring Tide level is 3.22m AOD. The highest ever recorded tide was 4.58m AOD which occurred on 1st February 2002. The existing ground floor level of the building is therefore 400mm above the highest ever recorded tide.
4.3. The tidal flood mapping produced by the IOM Government for the Douglas Harbour area show that the area around building to be at risk of tidal flooding with a 0.5% (1 in 200) chance of happening in any year.
4.4. Former basement skylights are present to the perimeter of the building. Air brick are present in the external brickwork facing just above pavement level. The skylights have been bricked up internally and have had solid covers placed externally at pavement level. Even with various sealing works carried out as part of the refurbishment, these features present a potential route for tidal flood waters to enter the basement, overtopping any basement waterproofing measures as described in Section 3.
4.5. Precautions can be taken to reduce the risk tidal flood water entering the basement and the ground floor. Given the listed status of the building, more formalised flood barrier systems across door openings are not considered appropriate, but more traditional methods such as sand bagging are a viable option.
4.6. As noted in Section 3 above, the risk of damage from flooding to the building finishes and services will be mitigated by incorporating management and resilience measures in to the refurbishment works and the future running of the building. These measures include:-
BB Consulting Engineers Limited, 10 Myrtle Street, Douglas IM1 1ED Ref. 17-060; October 2017 Encs.
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