PROPOSED RE-GENERATION AT MAWGAN PORTH FLOOD RISK ASSESSMENT 22/12/2020 J-14070 www.nijhuisindustries.com/uk PROPOSED RE-GENERATION AT MAWGAN PORTH FLOOD RISK ASSESSMENT Report No. Issue Detail Originator Date Checked By Date 01 Cornish Fresh HG 22/12/2020 DH 22/12/2020 For: CJH Property Job No: J-14070 Date: December 20 Edition: 01 CONTENTS Item Content Page No. 1.0 Introduction 1 2.0 Site Location and Description 2 2.1 Site Location 2 2.2 Existing Usage 2 2.3 Proposed Usage 3 3.0 Existing Hydrology and Flood Alleviation Measures 4 3.1 Surface Water Hydrology 4 3.2 Mawgan Porth Stream 4 3.3 Catchment 4 3.4 Existing Flood Alleviation Schemes 4 4.0 Potential Mechanisms for Flooding and Associated Risks 5 4.1 Mechanisms for Flooding 5 4.2 Groundwater Flooding 5 4.3 Overland Sheet Flow 5 4.4 Fluvial (River) Flooding 6 4.5 Tidal Flooding 6 5.0 Fluvial (River) and TIdal Flooding 7 5.1 Method of Assessment 7 5.2 Flood Flows 7 5.3 Modelling Methodology 8 5.4 Sensitivity to Manning’s Number 10 5.5 Summary of Modelling Assumptions 11 5.6 Results 12 6.0 Flood Summary 15 7.0 Access and Egress 16 8.0 Surface Water Flooding 17 9.0 Proposals to Minimise Flood Risks 18 10.0 Effect on Adjacent Sites 19 11.0 Planning policy 20 11.1 Sequential Test 20 11.2 Exception Test 20 12.0 Conclusions and Recommendations 22 APPENDICES Appendix A Site Location Plan Appendix B Site Survey Appendix C TuFLOW Model Outputs Appendix D Hydraulic Calculations ___________________________________________________________________________________ J-14070 Mawgan Porth – Cornish Fresh Re-Development - FRA _________________________________________________________________________________ 1.0 INTRODUCTION The owners of Surfside Stores and the Cornish Fresh site are proposing to re-development and re-generate an existing site at Mawgan Porth, North Cornwall to provide a mixed-use development. During the pre-planning investigations it has become apparent that the site lies within the Environment Agency’s (EA) indicative flood plain for the area and therefore could be at risk of flooding. In view of this, it will be necessary for any application to develop the site to be accompanied by a Flood Risk Assessment in accordance with Planning Practice Guidance (PPG). In order to address this requirement, Cornish Fresh have commissioned Nijhuis Industries. The objective of this appointment is essentially to prepare a Flood Risk Assessment for the development generally in accordance with National Planning Policy Framework (NPPF) and Planning Practice Guidance (PPG) addressing the risks of flooding and providing proposals for minimising risks to an acceptable level if practicable. This report describes the findings of the Flood Risk Assessment. 1 P:\Job Files 14000-14099\J-14070 Mawgan Porth Cornish Fresh FRA\Technical\Report\J-14070 Mawgan Porth Cornish Fresh FRA.docx ___________________________________________________________________________________ J-14070 Mawgan Porth – Cornish Fresh Re-Development - FRA _________________________________________________________________________________ 2.0 SITE LOCATION AND DESCRIPTION 2.1 Site Location The proposed development site is located in the centre of Mawgan Porth, a small village on the North Cornwall coast. The site is currently occupied by a disused garage and some retail units. The River Menalhyl passes close to the site along its southern boundary. To the east, the site is bordered by open ground and a number of holiday units. The northern site boundary borders the main B 3276 road serving Mawgan Porth and a smaller road running up the valley to the east. To the west, the site has direct frontage onto the B3276 and sand dunes beyond. With regards to topography, the main site area is relatively level and set at a typical elevation of around 5.40m AOD. On plan the main site is roughly 40m wide by 60m long. In the wider context the site is situated in the lower reaches of the valley which extends to the east of Mawgan Porth for some distance. The main catchment originates around the village of St Columb Major some 3Km to the east. A site location plan is included in Appendix A and shown below. Site Location Figure 2.1 Site Location 2.2 Existing Usage Currently the site consists of a number of buildings of a range of construction types, age and condition with a variety of retail uses, generally associated with the sites beach front location, including two residential units. 2 P:\Job Files 14000-14099\J-14070 Mawgan Porth Cornish Fresh FRA\Technical\Report\J-14070 Mawgan Porth Cornish Fresh FRA.docx ___________________________________________________________________________________ J-14070 Mawgan Porth – Cornish Fresh Re-Development - FRA _________________________________________________________________________________ 2.3 Proposed Usage It is proposed to re-develop and re-generate the site to provide a range of A1, A3/A5 retail units on the ground floor together with residential apartments on the upper floors. Externally, there will be a public realm area to the front with car parking to the rear. A site design is shown in Appendix B. 3 P:\Job Files 14000-14099\J-14070 Mawgan Porth Cornish Fresh FRA\Technical\Report\J-14070 Mawgan Porth Cornish Fresh FRA.docx ___________________________________________________________________________________ J-14070 Mawgan Porth – Cornish Fresh Re-Development - FRA _________________________________________________________________________________ 3.0 EXISTING HYDROLOGY AND FLOOD ALLEVIATION MEASURES 3.1 Surface Water Hydrology The principal hydrological feature at the site is the River Menalhyl which is located close to the site to the south. 3.2 Mawgan Porth Stream The River Menalhyl is conveyed past the site in a manmade channel. The bed of the river is made up of earth/gravel with gabion basket lined banks. Above the channel bed the banks are made up of grass giving way to roads and parking areas in the overbank areas. Close to the site the stream passes under a small pedestrian bridge, before passing under the main arched bridge carrying the B3276 road. The pedestrian bridge has 4 rectangular openings for flow conveyance. 3.3 Catchment The catchment area of the River Menalhyl at the subject site is some 44km2 with the catchment extending to the east of Mawgan Porth. Much of the land beyond Mawgan Porth is rural resulting in a catchment described as Essentially Rural as defined in the Flood Estimation Handbook. The catchment takes in the villages of St Mawgan and St Columb Major. 3.4 Existing Flood Alleviation Schemes Reference to the Environment Agency indicative flood map shows that flood defences are present through this stretch of the river. Inspection on site shows manmade banks constructed of gabion basket with raised banks. 4 P:\Job Files 14000-14099\J-14070 Mawgan Porth Cornish Fresh FRA\Technical\Report\J-14070 Mawgan Porth Cornish Fresh FRA.docx ___________________________________________________________________________________ J-14070 Mawgan Porth – Cornish Fresh Re-Development - FRA _________________________________________________________________________________ 4.0 POTENTIAL MECHANISMS FOR FLOODING AND ASSOCIATED RISKS 4.1 Mechanisms for Flooding A number of possible flooding mechanisms have been considered at the site, these are discussed further below and where necessary are covered in more detail in Section 5 of the report. 4.2 Groundwater Flooding The site is clearly located close to the River Menalhyl so the ground water level beneath the site is likely to be largely controlled by the watercourse; the river channel would act as a sump to drain water in the ground away from the site. The typical river level is about 1.4m below the ground level providing a good hydraulic gradient between the site and the river for the drainage of groundwater. Ground levels on the site are such that the river bank is the lowest part of the surrounding ground, so if groundwater was issued to the surface then it could drain overland to the river channel at shallow depth. In consideration of these facts, it is highly unlikely that the site will be subject to significant flooding from groundwater and this mechanism is discounted from further consideration. 4.3 Overland Sheet Flow The land surrounding the site to the south is steeply sloping and as such could represent a flood risk. The EA have produced mapping that indicates the risk of surface flooding to an area. The extract for the site is reproduced below and shows that the site is deemed to be at risk from surface water flooding. It is noted that the watercourse to the south would likely intercept any surface water flooding from the south, the two highways to the west and north would also intercept surface water flooding. Figure 4.1 Environment Agency Surface Water Flooding Map 5 P:\Job Files 14000-14099\J-14070 Mawgan Porth Cornish Fresh FRA\Technical\Report\J-14070 Mawgan Porth Cornish Fresh FRA.docx ___________________________________________________________________________________ J-14070 Mawgan Porth – Cornish Fresh Re-Development - FRA _________________________________________________________________________________ Given the nature of the site, and surrounding topography, and the more dominant flooding mechanisms from river and tidal sources, overland sheet flooding has not been considered further within this report. It is anticipated that any mitigation measures against fluvial or tidal flooding would protect the re-development from surface water flooding. 4.4 Fluvial (River) Flooding The site is situated close to the River Menalhyl. The difference in elevation between the ground level on the site and the typical water level in the river is about 1.4m. As such, direct fluvial flooding from the stream is a potential flood mechanism. Therefore, the potential risk of fluvial flooding from the River Menalhyl is sufficient to warrant further detailed consideration. This is covered in Section 5 of this report 4.5 Tidal Flooding The site could be subject to tidal flooding due to its proximity to the coast. Therefore, the risk of tidal flooding is considered further in Section 5 of the report.