Water Quality Implication of a severn Barrage

Transcription

1 Water Quality Implication of a severn Barrage by Reza Ahmadian, Bettina Bockelmann-Evans and Roger A. Falconer Hydro-environmental Research Centre School of Engineering, Cardiff University 1

2 General Challenges Growing world-wide increase in demand for energy - particularly in India and China Tidal energy generation has advantage over wind and waves - tides are predictable UK target of 15% of energy from renewables by 2020 about 35% of electrical energy Wales 2025 target for marine renewables is 14 TWh/yr Barrage could generate 9 TWh Why Severn Estuary and Bristol Channel are ideal for tidal energy? 2

3 Wales Severn Estuary England 3

4 Potential for Tidal Energy Provision From tidal range:- Power A H 2 H = level difference across barrage / lagoon A = wetted surface area upstream of barrage From tidal stream:- Power V 3 V = mean free-stream tidal current 4

5 Mean Spring Tidal Stream Resource Source DTI Atlas of Marine Renewable Energy Resources 5

6 Mean Spring Tidal Range Resource Source DTI Atlas of Marine Renewable Energy Resources 6

7 Current Short Listed Proposals 7

8 Severn Barrage Proposal Site Slides - courtesy of STPG / David Kerr 8 Some key facts: 3 rd highest spring tidal range 14 m Brean Down to Lavernorck Point Length about 16 km Generate 5% of U.K. electricity Total cost 20 bn Save > 6.8 million tonnes carbon pa

9 Barrage Layout (1989 Report) Key facts: 216 turbines each 40 MW 17 TWh pa 166 sluices Ship locks Fish pass included Public road? 9

10 Barrage Effect on Tides Barrage Tide Enters Severn Estuary Flow through turbines 10

11 Proposed Operation - Ebb Generation 11

12 Existing Estuarine Environment Tide Range - 14 m on springs, 7 m on neaps High tidal currents and large inter-tidal areas 30 Mt sediment suspended on springs, 4 Mt neaps Little sunlight penetration through water column Reduced saturation dissolved oxygen levels Ecology Harsh estuarine regime with high currents Limited aquatic life in water column / bed Bird numbers per km 2 are relatively small 12

13 Changing Natural Environment Climate Change Temperature rise will affect ecology, birds etc Sea level rise will lead to increased flood risk Water Quality Cleaner effluent discharges with EU WFD Nutrient reduction will affect aquatic life Legislation Long term projects (>120 yr) require assessment against future not just current - environment 13

14 Bird Numbers - Changing Citation category Internationally important populations of Annex 1 species Internationally important populations of migratory birds Nationally important bird populations Source - RSPB Species numbers between Bewick s Swan Shelduck - 2,892 Dunlin - 41,683 Redshank - 2,013 European Goose Wigeon - 3,977 Pochard - 1,686 Ringed Plover Curlew - 3,096 Whimbrel Spotted Redshank - 3 Species numbers between (Red - Less, Blue - More) Bewick s Swan Shelduck - 3,272 Dunlin - 23,312 Redshank - 2,566 European Goose Wigeon - 8,062 Pochard Ringed Plover Curlew - 2,545 Whimbrel Spotted Redshank

15 Main Effects of Barrage Spring tide range reduced from 14 m to 7 m Significant loss of upstream inter-tidal habitats Reduced tidal currents upstream of barrage Reduced turbidity / suspended sediment levels Increased light penetration through water column Increased primary productivity and changed bio-diversity of benthic fauna and flora Upstream tidal range of 7m is still relatively large compared to most deltas world-wide 15

16 Severn Estuary Hydraulic Model 16

17 Severn Barrage - Grid Configuration Cardiff Inner Barrage 17

18 Grid Refinement Around Barrage Cardiff Embankment 12 Sluices Shipping locks 80 Sluices Flat Holm Sub-stations 168 Turbines Sub-stations Steep Holm 48 Turbines 74 Sluices Embankment Weston 18

19 (a) Velocity Field Around Barrage Flood Frame Apr 2008 Hydrodynamic Results in Nodes 2 m/s water level(m) Ebb 2 m/s water level(m)

20 Maximum Water Levels Reduced flood risk Without Barrage Spring Tide 20 With Barrage

21 Swansea Minehead Newport Avonmouth Beachley(M48) Sharpness Newnham Epney High Water Level (m) (Datum relative to Avonmouth) Peak Water Levels Existing Cardiff-Weston Barrage Fleming Lagoon Shoots Barrage Cardiff Distance from Gloucester (km) 21

22 Maximum Tidal Currents Reduced tidal currents Without Barrage With Barrage 22

23 High Suspended Sediment Levels Dynamic region of high turbidity 23

24 Suspended Sediment Levels Mean Flood Reduced sediment levels & clearer water Without Barrage With Barrage Mean Flood - Spring Tide 24

25 Diffuse Pollution Effects? Creature Faecal production (g per day) E. coli per g faeces E. coli load (per day) Human x x10 9 Cow x x10 9 Hog x x10 9 Sheep x x10 10 Ducks x x10 10 Turkeys x x10 8 Chickens x x10 8 Gulls x x10 9 Wales Population: Humans m Sheep m 25

26 Enteric Bacteria Transport water column Wastewater outfalls Catchment runoff Water birds Sediment re-suspension Enteric bacteria Advection Diffusion/dispersio n input Overall reduction Decay Adsorption Deposition output 26

27 Riverine and WwTW Sources Rivers WwTWs 27

28 Experiments to Measure Decay Samples taken from 5 sites along estuary Dark and irradiated microcosms tested 4 times for each site Cellulose diacetate bandpass filter Artificial light source calibrated to provide average radiation conditions during July and August Chiller/heate r Mixing unit Matt black lining 28

29 Relationship with Turbidity/SS Empirical relationships developed between turbidity, suspended solids and T 90 values Real-time T 90 in numerical model - varying with time, location, predicted SS level and radiation patterns 29

30 Experiments to Measure Adsorption Two beakers incubated at 15 C one mixed and one allowed to settle - two sites tested Mixed beaker bacteria levels remained constant Settled beaker bacteria levels fell as finer particles settled 30

31 Net Bacteria Source Term Net source term in bacterial transport model extended to include following processes: Dynamic bacteria decay Deposition reduction Entrainment from bed Discharge from outfalls Q N dep kb t n s kc s C 0.1* QeroCbo e Cn S n 1 An H Q 31

32 Bacteria Levels Mean Ebb Reduction in bacteria levels Without Barrage With Barrage Mean Ebb - Rivers in Flood 32

33 Summarising Severn Barrage would have a lasting impact on a unique UK macro-tidal estuary: Provide 5% of UK s electricity from renewables Reduce intertidal habitats by about 14,000 ha Reduce flood risk - upstream and downstream Reduce tidal currents and suspended sediments - increasing light penetration and water clarity Change ecology and benthic flora and fauna Enhance opportunities for tourism & recreation 33

34 Thank You 34