The Tidal Regime of the Severn Estuary: The Practicalities of Monitoring Tidal Flow

Paul Evans, Research Assistant & PhD Researcher – LCRI MarineSchool of Earth and Ocean Sciences, Cardiff University

Overview

• Tidal Regime of the Severn Estuary

• Tidal Energy in Wales

• Case Study 1: Tidal flow Monitoring in the Irish Sea

• Case Study 2: Tidal flow Monitoring in the Severn Estuary

• Further Work

Severn Estuary Tidal Regime

• Partially enclosed body of tidal waters – 250km

• Hypertidal (MTR>6m); ca. 7m mouth of Channel, ca. 12m upper reaches

• Semi-Diurnal

Uncles (2010)

Severn Estuary Tidal Regime

• Fast tidal currents = Strong vertical and horizontal mixing, and large bed stresses = Horizontal dispersion, near vertical homogeneity in salinity, temperature and phytoplankton

• Max. mean spring and neap tide in the vicinity of Flat Holm = 3.0 ms-1 and 1.0ms-1

Uncles (2010) Uncles (2010)

Severn Estuary Tidal Regime

• Hydrostatic head at the mouth and the landward extent of the estuary during flood and ebb conditions create a tidal flow – peaks lag behind the high and low tides by ca. 3 hours

• Newport Deep (NE flood)

• Bristol Deep – SW ebb –greater tidal flow (narrow and sinuous) Cotter (1975)

Flat HolmSteep

Holm

Bristol

Deep

The

Wolves

Tidal Range vs. Tidal Stream

• Tidal range – impounding high tide level water behind a barrage or in a lagoon can create a head above the lower tidal water outside, which can be run through turbines when releasing the impounded water. This technology is likened to damming a reservoir once or twice daily.

• Tidal stream involves extracting kinetic energy directly by use of turbines from the horizontal tidal current generated by the movement of the tidal bulge as the earth rotates.

Tidal Range Technologies

Xia et al. (2010)

Natural Condition

Xia et al. (2010)

Cardiff to Weston Barrage

• Cardiff to Weston Barrage –

Installed capacity = 8640 MW; Annual

electricity supply = 17 TWh;

Generation of 5% of UK’s electricity

Xia et al. (2010)

Tidal Range Resource

Xia et al. (2010)

Fleming Lagoon

Xia et al. (2010)

Shoots Barrage

• Fleming Lagoon – Installed capacity =

1500 MW; Annual electricity supply = 3.2

TWh

• Shoots Barrage – Installed capacity =

1050 MW

Tidal Stream Resource

Ebb tide – HW+3 (Milford Haven)Flood tide – HW-3 (Milford Haven)

Welsh Tidal Stream Resource

Potential Areas:

– Anglesey

– Pembrokeshire

– Bristol Channel

Source: Renewables Energy Atlas

Marine Renewable Energy in Wales

Welsh Government Policy:– 40GW marine renewable energy

potential

– Capture 10% of wave and tidal stream energy off the Welsh Coastline by 2025 (equates to 4GW potential)

LCRI Marine:– Cutting edge research to support industrial development

– Tidal resource assessment

Marine Current Turbines – ‘SeaGen’

• Skerries Tidal Stream Array

• 9 SeaGen devices array with a total capacity of up to 10MW

Tidal Energy Limited – ‘DeltaStream’• Demonstration project in Ramsey Sound, Pembrokeshire – capacity to provide electricity for up to 1000 homes

• LCRI Marine survey data used to inform site selection

Tidal Stream Devices in Wales

http://www.tidalenergyltd.com/?page_id=640

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Case Study 1: Irish Sea

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The Bishops and Clerks – a historical hazard to navigation

“The Bishop and these his clerkes preache deadly doctrine to their winter audience" George Owen, 1552-1613.

.

RNLI Lifeboat Joseph Soar – south end of Ramsey IslandRNLI Lifeboat Swn-Y-Mor alongside World Concord, 27th

November 1954

Case Study 1: Irish Sea

• Rock Outcrops

• Small Islands

• Underwater pinnacles

• Deep channel

• Large island

http://www.ramseyisland.co.uk/ramsey-and-the-outer-islands/ramsey-island/

Case Study 1: Irish Sea

The Bitches

Horse RockWhitesand Bay

Case Study 1: Irish Sea

• Fast spatial and temporal varying flow

• Varying location of shear line between eddies

• Up- and down-welling

Case Study 1: Irish Sea

Case Study 1: Irish Sea

Case Study 1: Irish Sea

Slack

Slack+1

Slack+2

Slack+3

Slack+4

Slack+5

Case Study 1: Irish Sea

HWHW+0.5

Case Study 1: Irish Sea

Case Study 2: Severn Estuary

Survey Area

Flat Holm Island

The Wolves

Case Study 2: Severn Estuary

Case Study 2: Severn Estuary

Case Study 2: Severn Estuary

Further Work

• Analysis and interpretation of flow data

• Laboratory experiments – wake studies downstream of cone

• Numerical model validation

• Further surveys in vicinity of Flat Holm

• http://www.lcrimarine.org.uk/

Thank Youfor

Listening

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