Energy and Climate Change CommitteeSupplementary written evidence submitted by The Bristol Port Company

Introduction

1. Hafren Power (HP) has submitted a proposal to the Energy and Climate Change Committee (ECCC) to construct a power generating barrage from Cardiff to Weston.1 In both their written and oral evidence, HP have made a number of claims about the effects that their proposed barrage will have on the tidal levels upstream of the construction. HP have not yet made available any detailed plans of their barrage proposal which support accurate analysis of their claims. However, the broad dimensions of the barrage can be estimated, its approximate route identified, whilst the size and number of turbines has been stated at 9.0m diameter and 1026; these factors can be used to underpin some basic analysis of the effects of the barrage on the tidal regime in the Estuary. The Bristol Port Company (TBPC) has commissioned HR Wallingford (HRW) to comment on HP’s claims, and to answer some specific questions on the way the barrage is likely to affect tidal levels at Avonmouth.

Hafren Power Claims

2. HP’s claims are as follows:

(a)Claim 1: “Reduction in tidal range: for spring tides, the range would reduce from around 14m to around 10m”.73

(b)Claim 2: “The barrage is a complete control mechanism. The thousand-odd turbines can obviously be used in ebb and flood. They can be turned to—I say—freewheel, but feather the blades because the pitch can be changed. So they can let the water flow through, which could actually replicate the whole tide”.2

(c)Claim 3: “You can raise the tide, and if you operate the turbines in a pumping mode you can do it as well because you can raise the tidal height”.74

(d)Claim 4: “We can actually improve their [TBPC’s] access to their existing docks by raising the height of the tide at particular times to suit them”.74

TBPC/HRW Analysis—Claim 1—Tidal Range Effects

3. Extensive analysis exists on the ebb-only generating barrage considered under the Severn Tidal Power (STP) Feasibility Study,3 as well as an assessment of a “conventional turbine technology” flood-ebb generating scheme. Whilst the HP proposal seeks to use very low head bi-directional turbines with high reversible efficiency and which would allow both ebb and flood generating windows, the earlier STP work nonetheless provides a reference point.

4. Further modelling of UK tidal power resources have been undertaken by HRW for the Energy Technology Institute (ETI). The Severn Barrage was studied, including flood-ebb generation schemes.4 However the particulars of turbine characteristics remain under development and so these results not used for the present review.

5. Professors Roger Falconer and Binliang Lin at Cardiff University have produced a model used widely by Hafren which indicates that that new proposals for ebb and flood generation would reduce high water levels by around 1.4m greater than an ebb-only barrage.5 This is reported to verify the results of Hafren’s numerical model used to support the business case for their proposed Severn barrage. This modelling suggests that the reduction in high water levels upstream of a barrage could be in the order of 2m for spring tides.73

6. HRW state in a response sought by TBPC to the tidal level claim:6 “Upstream of the barrage, the tide would have a reduced tidal range compared with now (a lower mean water level with a higher low water and a lower high water) and the tide would probably have this sort of shape (with pumping, the lows and highs would become more pointed)”. “Probably” because in reality the form of the upstream water level variation would depend upon the detailed equations describing flows through the turbine ducts (and any sluices) when not generating power and when generating power (and usually different equations at different power levels), etc:

7. For example, when HRW simulated a flood/ebb scheme during the DECC STP studies,7 upstream of the flood/ebb generating barrage at Avonmouth a >4m reduction in high water and a >2m rise in low water was predicted. Downstream of the barrage (near field) the tide would appear similar in shape to now, but would have a reduced high water and a raised low water. The flood/ebb barrage HR Wallingford tested led to a near field reduction in high water of <0.5m and an increase in low water by 1.3m just downstream of the barrage. Further downstream, in the Bristol Channel at Ilfracombe there was a small reduction in high water and ~0.5m rise in low water.

8. The HP scheme is different in that it uses future technology and has many more turbines and a lower starting head for generating electricity than modelled in the DECC STP studies. Rolls Royce and Atkins have considered the technical feasibility of future technology (ie very low head bi-directional turbines8). They report that 0D modelling for the 1000 turbine case suggests a 2–3m reduction in spring tide high water levels in the upstream basin and a 2m rise in low water, both changes reduced however by a factor of two with pumping. Although 0D models are useful for concept design, 2D/3D models (rather than 0D) would usually be required for a detailed assessment of impacts (eg on tide levels) associated with a scheme. Roger Falconer’s team does not appear to present results for the 1000 turbine case. Additional evidence presenting the effect of the Hafren scheme on tidal levels should be sought.

9. The analysis reviewed suggests that the reduction in tidal range at Avonmouth could be as much as 6.0m, or as little as 2–3m with pumping taking place. However, there can be no certainty of the scale of the impacts on water levels both upstream and downstream of the barrage, without first seeing the turbine technology and associated sluices further developed and tested as prototypes, and then appropriately detailed hydrodynamic modelling undertaken which can be thoroughly peer reviewed and verified.

TBPC/HRW Analysis—Claim 2—The Barrage is a Complete Control Mechanism which can let the Water Flow through to Replicate the Whole Tide

10. The flow rate through the barrage would be the sum of flows through the numerous turbine ducts (with turbines either generating power or freewheeling), sluices and any other openings. There would unavoidably be some head loss across the structure from the interaction of the flow with the various elements of the structure which would cause the tide range upstream to reduce to some extent. Pumping can be used to mitigate the effect on tide range.

11. The proposed (future) technology is a very low head bi-directional turbine with high reversible efficiency. This is considered by Rolls Royce to be technically feasible in their Report.79 (Annex A of this report suggests that 1000 turbines are feasible because of the “pushed out” plan shape of the barrage). The blades are variable pitch blades. Section 3.2.6 of the same report states “With a pumping mode of operation, it is possible to more closely replicate the natural tide”. The results presented by Rolls Royce suggest that with pumping the whole tide can be replicated during neap tides but not for a large spring tide.

12. Pumping at around the time of high and low water brings more water through the barrage at a time when the head difference is not large enough for power generation. It takes energy to do the pumping (cost) but the benefit is a reduction in the time before power generation may resume, an increase in the power that may be generated during the next generating phase, and a slight reduction (improvement) in high and low water level changes compared with present day levels.

13. From the reviewed information it appears that the pumping capacity would be unlikely to be able to fully replicate large spring tides and so reduced operability may be an impact for larger vessels entering Portbury Dock (for larger vessels DECC STP navigation studies considered 130,000dwt vessels with 14.5m draught). To comment fully TBPC would need to see more detail as to how this commitment might be met for the larger vessels.

TBPC/HRW Analysis—Claim 3 and 4—Pumping to Raise Tide

14. When HRW modelled a flood/ebb Cardiff-Weston barrage as part of the DECC STP studies,79 (note this was a smaller barrage with a smaller basin area than the Hafren proposal), a discharge rate of 200,000m³/s into the area upstream of the barrage gave a tidal level rise of 4m in 3.5 hours or 1.15m per hour. So roughly 600,000,000m³ water would need to be pumped to raise water levels upstream of a barrage by 1m (for a spring tide). Inside the basin the tide level peaked at 8–8.5m OD.

15. In order to calculate the pumping requirement to move this volume of water in a practical time frame during which no power will be generated (say a single tide of 6 hours), the 1026 turbines are assumed to be capable of working in a pumping mode. Each pump would need to be capable of pumping close to 600,000 tonnes of water in a six hour period, or 100,000 tonnes per hour in order to raise the depth at Avonmouth by one metre. This is clearly a significant technical challenge, which will also require a considerable consumption of power. It seems highly questionable that a power generator would find it tolerable to support the loss of generating capacity and the cost of pumping 600,000,000 tonnes in order to restore, let alone improve, depths of water for Bristol shipping.

Conclusions

16. It is regrettable that the paucity of detail available from HP does not allow more meaningful analysis of their claims for the effects on the tidal regime upstream of their barrage proposal. Analysis of such detail that is available show clearly that their claims are unrealistic and based upon many assumptions.

17. The loss of HW at Avonmouth will remove TBPC’s unique access to deep water and the associated deep-draught trades which use it. Any notion that allowing free-flow through the turbines to replicate the existing tidal regime, let alone pumping to improve the water levels available today, simply does not stand up to the most basic of scrutiny.

March 2013