Predicting the implications of tidal energy barrages: the use of analogues in exploring environmental issues Roger Morris Bright Angel Coastal Consultants
Structure of this presentation Introduction to the UK tidal energy debate The Severn Estuary Generic impacts of tidal power barrages Setting parameters for analogues What different structures do? Interpretation and aplication
Severn barrage proposals
The Severn Estuary Characteristics (inter-alia) Third largest tidal range in the World (after the Bay of Fundy and Ungava Bay in Canada). Exceptional suspended sediment concentrations that change in response to springneap cycle. Relatively low primary productivity. Soft muddy banks.
Tidal power known impacts Barrages are known to: Reduce tidal propagation; Reduce flushing; Change the halocline; and Impact upon fish movement and populations. They will also have more farreaching knock-on effects.
Modelling - some basic thoughts Models can only be constructed on the basis of what we know (analogues). Some of the principles are simple e.g. if you impede water movement there will be a reduction in the energy available to move both water and sediment. Modelling in multiple dimensions and involving sediments gets complicated and expensive.
Applying analogues Analogues can be used to identify the relationship between cause and effect. They provide a first-order analysis. This follows into the development of conceptual models. Conceptual models form the basis of the algorithms required to construct mathematical models.
Setting the parameters for analogues Finding suitable analogues depends upon understanding what a particular structure will do to coastal/estuarine geometry. In the case of a barrage the basic changes will be: Reduced cross-sectional area; Creation of a headpond with fluctuating water levels; and Impediments to movements by mobile animals.
Issues to consider when investigating analogues The types of changes in tidal propagation. Bank responses to wind-driven wave action. Sedimentation patterns. Sediment behaviour related to tidal energy. Plants and animals in responses to salinity regimes. Responses of migratory birds to food availability. Responses of fish populations to loss of adults. Responses of phyto- and zoo- plankton to changing nutrient loads and water clarity.
A wealth of information Man has been modifying the coast for hundred; perhaps thousands of years. There are numerous examples of changes such as: Sea walls Channel deepening Bridges Storm surge barrages Causeways
We can even use history to develop analogues
Finding suitable analogues - requisite parameters We must look for examples of reductions in tidal range. We need to understand how estuaries with high sediment loads respond to reduced tidal energy and why they do so? We must explore tidal propagation impacts on fine sandy or weakly cohesive sediments. Sediment sources need to be comparable. The major change in form involves the development of a temporary reservoir for several hours each day. There is a need to find analogues where the changes are of sufficient magnitude for a response to have been detected.
Barrages and causeways Eastern Schelde Storm Surge Barrage Ems Estuary
Analogues of reduced tidal propagation Impact Location Notes Reduced tidal propagation upstream Eastern Schelde Overall tidal range reduced by around 0.5m Wansbeck estuary Tidal influences only reach the upper section on spring tides. Creation of sediment exporting conditions Alde/Ore Estuary Blyth Estuary Several UK examples of extreme narrowing of long sections of estuaries. Reduced tidal propagation immediately downstream Modelled for Severn Estuary Not proven empirically from monitoring of historic sites Increased tide heights far-field effects Modelled for Severn Estuary and Bay of Fundy Not proven empirically from monitoring of historic sites
Increased accommodation space Impact Location Notes Reduced tidal propagation Alkborough, Humber Estuary UK Effect modelled and identified as effective flood risk management tool.
Channel deepening
Analogues of Increase in sub-tidal cross-sectional area Impact Location Notes Increased tidal propagation Elbe, Ems (Germany) Seine (France) This is a well-established issue where there has been channel deepening to facilitate access to inland ports. Increased suspended sediment load Ems (Germany) This is an exceptional case on the basis of a major change to SSC.
Reduced Suspended Sediment Concentrations Annapolis Royal Eastern Schelde
Analogues of Reduced Suspended Sediment Concentrations Impact Location Notes Annapolis Royal Bank erosion and sub-tidal deposition. A temporary phenomenon that is dependent upon the size of the estuary. Eastern Schelde Bank erosion and sub-tidal deposition.
Temporary or permanent lake formation Impact Location Notes Annapolis Royal Bank erosion Two stage changes make this a useful analogue. Cardiff Bay Bank erosion where undefended Issue has only recently become significant because it is a long way upstream. Wansbeck Estuary Bank erosion Significant issue because erosion is mobilising industrial waste.
Interpretation and application It is important to remember that analogues do NOT give refined answers. Their strength lies in providing basic explanations to decision-makers that can be backed up by examples. If a project is taken to the construction phase, much more detailed analysis will be necessary, including the use of models.
Interpretation 1 We know that changing tidal propagation will change sediment mobilisation patterns, especially sub-tidally. This creates a sink. We also know that reducing the tidal range reduces the area of foreshore that is affected by wind driven waves. This combination of factors is the key to understanding the main physical responses.
Interpretation 2 Not everybody will understand the analysis many think that coastal evolution only involves currents determining where sediments are deposited. Relatively few people will grasp the significance of the changed relationship between tidal (current) energy and wave energy.
Interpretation 3 Annapolis Royal is critical
A wider canvas? Experience from around the world can be used to explore many of the responses that may occur as a consequence of creating barrages. Biological responses are intimately tied to physical changes, so there is a need to think about the physical changes first. Once a physical change has been identified, then a relevant biological analogue can be sought.
Summary Analogues are a tool but not the total solution to problem-solving. There is a wide range of possible structures that can be used as analogues. It is not what the structure is designed to do that matters; it is the changes to physical processes that result that matter. Biological and physical responses are linked, so the key is understanding the phyical responses and applying them to the biology.
Thank you for inviting me and for listening