Flood Risk Assessment In support of Commercial Development at 40 St Thomas Street, Oxford OX1 1JL Jamie Finch FCIHT 3rd March 2021 Revision [0] Flood Risk Assessment at 40 St Thomas Street, Oxford Contents Amendments ........................................................................................................................................... 2 References .............................................................................................................................................. 2 Introduction ............................................................................................................................................ 3 Development Description and Location .................................................................................................. 3 Development Classification ..................................................................................................................... 3 Sequential Test ........................................................................................................................................ 3 Definition of the Flood Hazard ................................................................................................................ 6 Probability ............................................................................................................................................. 10 Climate Change ..................................................................................................................................... 10 Detailed Development Proposals .......................................................................................................... 10 Flood Risk Management Measures ....................................................................................................... 11 Off Site Impacts ..................................................................................................................................... 11 Residual Risks ........................................................................................................................................ 12 Flood Risk Conclusion ............................................................................................................................ 12 Appendices ............................................................................................................................................ 14 1 Flood Risk Assessment at 40 St Thomas Street, Oxford Amendments Revision Date Description 0 03/03/2021 First Issue References Reference Title A CIRIA Publication 753 – The SUDS Manual (2015) B Sewers for Adoption 7th Edition (Wrc 2012) C Part H of the Building Regulations (2015) D Technical Guidance to the NPPF (2012) E Planning Practice Guidance (2018) 2 Flood Risk Assessment at 40 St Thomas Street, Oxford Introduction This Flood Risk Assessment has been developed in support of a new planning application for the construction of a new replacement Commercial Building located at 40 St Thomas Street, Oxford, OX1 1JL. This document aims to identify any flood risk areas, offering suggested mitigation where appropriate. A Location Plan and layout of the proposed scheme and be found in the appendices of this document. The site currently operates as a Pre-School, which will be demolished and replaced with a new building as part of the proposals. The area of development is under 1 Hectare, however part of the site is shown to be situated within Environment Agency Flood Zone 2. Therefore, the site is subject to a formal Flood Risk Assessment (FRA), which is to be submitted and approved by the Local Planning Authority in consultation with the Statutory Consultees. Development Description and Location The development proposal is for the construction of a new commercial building with associated parking. This proposed site is located within the City of Oxford. The site external levels are currently set at around 57.41mAOD. The location of this proposed application site is shown in the location plans in the appendices. Development Classification The flood risk vulnerability classification for this type of development in this location is considered as Less Vulnerable which is defined in Table 2 of the Technical Guidance to the National Planning Policy Framework. Sequential Test As set out in the National Planning Policy Framework, the aim of the Sequential Test is to direct new development to areas with the lowest probability of flooding. 3 Flood Risk Assessment at 40 St Thomas Street, Oxford The starting point to the Sequential Test is the Flood Zones, which can be seen below and also in the appendices. The map clearly shows Zones 2 and 3, whereas Zone 1 is the remainder of the land not hatched. This confirms that the site is situated marginally within flood zone 2. These flood zones refer to the probability of river and coastal flooding, ignoring the presence of any existing flood defences. The Strategic Flood Risk Assessment for Oxford District Council refines the information on the probability of flooding and also takes into account other sources of flooding which have been assessed in more detail further in this flood risk assessment. This proposed development is classified as Less Vulnerable in accordance with Table 2 of the Technical Guidance to the National Planning Policy Framework. 4 Flood Risk Assessment at 40 St Thomas Street, Oxford Following an assessment of Table 3 of the Technical Guidance to the National Planning Policy Framework, it states that development in this situation with a site classified as Less Vulnerable and in Flood Zone 2 and 3, this proposal is classed as Appropriate Development: If the application of the Sequential Test is not possible to comply with, a site-specific flood risk assessment must demonstrate that the development will be safe for its lifetime taking account of the vulnerability of its users, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall. 5 Flood Risk Assessment at 40 St Thomas Street, Oxford Definition of the Flood Hazard For the preparation of this Flood Risk Assessment it is necessary to consider all of the potential sources of flooding and how this may affect the site. A checklist has been provided below to help identify these areas which can be discussed in greater detail in this section. Possible Source of Flooding Applicable to this Site? Fluvial Flooding from nearby rivers or streams Coastal Flooding Surface Water Runoff from the Site, Adjacent or Nearby Land Flooding from Surface Water Sewers Flooding from Foul Water Sewers Flooding from Reservoirs Flooding from Groundwater Flooding from other Sources Flood mapping confirms there is a marginal flood risk to the proposed site, as it is situated just within Flood Zone 2. Full details and mapping are provided in the appendices in the EA Product 4 information and the SFRA mapping. The Product 4 information confirms that the site sits within an area that has been modelled and provides information for the 1% AEP + 35% Climate Change is 57.29mAOD and the 0.1% AEP predicted flood level is 57.31mAOD. The existing external ground levels surrounding the existing building are at about 57.41mAOD at the proposed site. Finished floor levels in any case should be a minimum 150mm above external levels, therefore It is proposed that the internal levels of the new building are set at 57.56mAOD. The risk of coastal flooding can be ignored in this case, as the tidally influenced waters cease at Teddington, London and there are no other coastal waters that would be a risk to the proposed development. 6 Flood Risk Assessment at 40 St Thomas Street, Oxford When checking the flood maps for surface water flooding, there are no risks to the application site. This can be seen below and in the appendices. There are no expected issues of surface water or foul water drainage flooding at the application site. Although there are public surface and foul water sewers in the vicinity of the site, the existing and proposed development are well above the level of the highway, therefore any potential surcharge incidents in the public sewer network would not affect the new building. 7 Flood Risk Assessment at 40 St Thomas Street, Oxford The flood maps have been checked against the risk of reservoir flooding and there is a risk shown in the Environment Agency maps, although the spread of water in this instance seems to be more sporadic than constant. If in the unlikely event there was a reservoir failure, the risk of inundation to the proposed building is very unlikely. The flood risk map can be seen below and also in the appendices. 8 Flood Risk Assessment at 40 St Thomas Street, Oxford The application site lies upon a bedrock geology comprising of the Oxford Clay Formation, overlain by Alluvium, locally known as the Northmoor Sand and Gravel Member. In areas rich in alluvium, water can pass through this sandy gravel and rise quickly, flooding low lying land, however as the proposed building will sit comfortably above the level of the road and surrounding land, groundwater flooding is not considered an issue. There are no other sources on or off site that are thought to be a flood risk at this site. 9 Flood Risk Assessment at 40 St Thomas Street, Oxford Probability The proposed development is situated marginally within Zone 2 which can be seen in the Flood Map for Planning, contained in the appendices. The District Council for this area have prepared a Strategic Flood Risk Assessment which refines the