Proposed Development at Riverview, St Dogmaels Flood Consequence Assessment Report

Pembrokeshire County Council CONTENTS Pag e 1 Introduction 2

2 The Development 3

3 Flood Risk 5

4 Flood Proof Measures 6

5 Conclusions 6

Pembrokeshire County Council 1 INTRODUCTION It is proposed to erect a pontoon on the Teifi river connected via an access walkway to a base on the river wall at Riverview, St Dogmaels, Pembrokeshire. The site lies in Zone C2 as defined by the development advice maps (see Figure 1.1) This means that the site is considered to be at risk of flooding during at least the 1 in 1000 year event.

Figure 1.1 - Development Advice Flood Map

Considering the proximity of the site to the Teifi Estuary, it is possible that the development could be at risk of tidal inundation. This assessment is in accordance with the requirements of Section 7 and Appendix 1 of TAN 15.

Pembrokeshire County Council

Figure 1.2 – NRW Flood Risk Map 2 DEVELOPMENT The proposed site (coordinates 216373.246577) is situated north of the village of St Dogmaels, Pembrokeshire, a mile northwest of Cardigan, with access to the site off the B4546 (see Figure 2.1, 2.2, and Plate 2.1). The site lies adjacent to the Teifi Estuary, which flows into Cardigan Bay 2km north of the site.

Site

Figure 2.1 – Location Plan

Pembrokeshire County Council ==+ Figure 2.2 – Plan of Site Plate 2.1 – Aerial View of Site Proposed Development It is proposed to erect a base, on the east end of the garden, in the river wall to the east of the property Riverview, from which an access walkway will ran down to a floating pontoon. The pontoon will rise on the tide and settle on the riverbed at low tide.

Topography LIDAR data (Fig 2.3) shows the level of the location, where the proposed base for the access walkway pivot will be situated on the river wall and where the river wall protects the bank from the river, is 4.6m AOD (*7.04m ACD) (note the annotation-PENYWHELP HOUSE is incorrect and relates to the property south of Eriloe)

Pembrokeshire County Council

Figure 2.3 – Lidar Data of Site 6

3 FLOOD RISK Flooding can occur from a number of sources.

3.1 Fluvial Flooding from the Teifi at this location is considered by Natural Resources Wales to be tidally dominated, which is reasonable, considering the width of the Estuary at this point.

3.2 Tidal NRW now generally prefer the method promoted in the coastal flood boundary conditions (for UK mainland) joint research project between DEFRA and the Environment Agency to determine the T200 and T1000 high tide levels. This accounts for the joint probability of meteorological (surges etc) and astrological effects (the impact of the sun and the moon) on the tide. The levels for the adjacent coastline were obtained from NRW and have a 95% confidence level (see Table 3.1). As a precaution, the values can be increased by a recommended value of 0.2m for the T200 event and by 0.3m for the T1000 event.

In addition to this TAN 15 advises that 50 years of sea level rise should be added.

Table 3.1 - Water Surface Extreme Tide Elevation (m AOD)* T200 (0.5%) T1000 (0.1%) Level Confidence Level 3.659 +/- 0.2 3.92

Table 3.2 Tide Levels With Sea Level Rise 2008 Current 50 years of sea level Rise T200 3.59 3.615 4.005

T200 Precautionary 3.79 3.815 4.205

T1000 3.92 3.945 4.335

T1000 Precautionary 4.22 4.245 4.635

The wall at the end of the site is 4.6m AOD, which means it is likely to be barely overtopped during a T1000 event with 50 years of sea level rise even with a 0.3m precautionary increase.

3.3 Surface Water The site could be at risk from surface water flowing from the road should the highway drainage fail. However, on reaching the site, the water will drain into the Teifi Estuary. As suchPembrokeshire natural drainage will be sufficient to preventCounty the proposed Council development from flooding from this source.

The development will not result in a loss of permeable ground and will have no impact on water levels that could increase the risk of flooding to third parties.

3.4 Flooding From Wave Action The waves hitting the coastline of West Wales can have a fetch length of several thousand kilometres although this is reduced within Cardigan Bay. Cemaes Head and Cardigan Island along with Poppit Sands and Pen-yr-Ergyd protect the estuary itself.

6 7

These latter features create a natural harbour up river.

In addition the waves reaching the site 2.5km from the estuary are depth limited.

During the T1000 year event it is possible that waves of around 1.5m, from a north westerly direction could break on the cliffs below the site, assuming a MHWS of 2.26m AOD. Based on the shore protection manual, the maximum wave heights are likely to be closer to 0.5m assuming a fetch length of more than 1km from the inner estuary. Therefore, waves at current and future sea levels are not likely to adversely affect the development at 4.6m AOD.

4 FLOOD PROOF MEASURES Section 3 indicates that the development is expected to be flood free during both the T200 and T1000 events at 50 years and therefore no flood proof measures are required.

5 CONCLUSIONS With regard to the ground-based structures, LIDAR data indicate that the current ground level at the site of the proposed development is 4.6m AOD. With 50 years of sea level rise, a flood level of 4m AOD is expected during the T200 event and a level of 4.3m AOD is expected during a T1000 event. 4.6m AOD is only exceeded by a T1000 event, at the precautionary level, by 35mm (1.4 inches) Regarding the floating structures, the existing HAT (Highest Astronomical Tide), which the floating sections might be exposed to, is currently 5.4m ACD, equivalent to 2.96m AOD. Adverse wave effects are unlikely, and the configuration and nature of the design is capable of accommodating the anticipated sea level increases.

* Tidal levels are normally quoted relative to chart datum (CD), which is approximately the lowest tide level arising from astronomical effects and excluding meteorological effects. Ordnance datum is 2.44m above Chart datum for the Teifi Estuary (source- Bathymetry of the Teifi Estuary) .

Pembrokeshire County Council

7 8

Pembrokeshire County Council

8