To avoid instability at the link location, the 1D and 2D models overlap over a distance of 3. The 1D model stretches from the M4 bridge on the west to the River Severn tidal limit on the eastern boundary Figure 1. The 1D model downstream boundary was set to water levels which were derived from the 2D model predictions at that location.
The upstream 1D model boundary was considered to be fresh water, therefore water quality indicator levels were set to zero at this boundary. The water quality indicators at the 1D model downstream boundary were derived from the 2D model. The flow and all the water quality indicators at the 2-D upstream boundary were obtained from the 1D model cross-section at the location of this boundary. Since this boundary was so far seawards of the region of interest, the concentrations of faecal indicator organisms were also set to zero along the downstream boundary.
The 1D model comprised 4 reaches and cross-section covering a length of approximately 80 km. A m grid size and SeaZone digital bathymetry SeaZone, were used in this study, while a grid dependency analysis showed no significant grid dependency Ahmadian et al.
The 1D model time step was set to 40 seconds and a time step of 10 seconds was used for the 2D model, resulting in an acceptable maximum Courant—Friedrichs—Lewy number of around 1.
Stapleton et al. There was a good correlation between the model predictions and validation data, with a statistical analysis of errors being provided in Table 1 and more information about the model validation given in Ahmadian et al. Table 1 Statistical errors between computed and observed data used for validation of the model Ahmadian et al. It was expected that the main source of uncertainty in the model predictions were due to the imprecise representation of the bathymetry and the roughness values.
The river currents were not significant in comparison with the tidal currents in the estuary and therefore they were not expected to be a significant source of error in the hydrodynamic predictions. However, they are considered to provide one of the major sources of bacterial and nutrient inputs to the estuary Ahmadian et al.
The other source of uncertainty in the water quality predictions is the ratio of water quality constituents associated with adsorption on the sediments, as compared with the corresponding values in the dissolved concentration phase. The barrage would include bulb turbines, each with a peak output of 40 MW.
There would be sluice gates located along the shallower parts of the estuary, i. Starting from high water, in ebb only generation mode, the water would be impounded in the basin at high tide high water holding phase , until the head difference between on either side of the barrage was high enough for realistic power generation.
Then the generation phase starts by opening the turbines and directing the flow through the turbines, for energy generation. The generation phase would then continue until the head difference reaches the minimum head difference for power generation, and the second holding phase starts. The filling phase starts by opening both sluice gates and turbines when the water level on the seaward side of the barrage is higher than the water level inside the barrage.
The filling phase would be followed by the holding phase when the water levels inside and outside of the barrage were at the same level, around the high tide. Starting from high water, in the 2-way generation scheme, the water would be held inside the barrage by keeping the sluice gates and turbines closed. The high water holding phase continues until the head difference across the barrage is large enough for power generation and the ebb-generation phase starts.
The generation continues until the head difference across the barrage is not sufficient to generate power. At this stage, the sluice gates are also opened to facilitate emptying of the basin, and until the water levels outside of the barrage are almost at the same level as the water levels inside. At this step, the second holding phase starts by closing both the turbines and the sluice gates.
This closure will continue until the water level outside the barrage is higher than the water level inside the barrage, for the start of power generation i. The sluice gates will be opened when the head difference across the barrage is insufficient to generate power as part of the filling phase.
The filling phase will be followed by a holding phase at high water. In this phase, the turbines and sluice gates are kept closed and water levels inside the basin are higher than the water levels outside of the barrage, where the water elevations will be falling due to the outgoing tide. The flow through the submerged sluice gates has been modelled using the orifice flow equation Ahmadian et al. For more information on the barrage modelling and operation see Baker , Xia and Ahmadian et al.
Modelling Results To investigate the hydro-environmental impacts of different barrage operation schemes, the model was set up for the existing conditions and the post-barrage scenario, using both ebb-only and two-way generation schemes. Figure 5 illustrates the water levels in the vicinity of the barrage, upstream and downstream of the barrage, as well as the power generated by the barrage.
Different variations of two-way generation have been used by various researchers Ahmadian et al. Figure 6 depicts the water levels close to the barrage both upstream and downstream of the barrage for two-way generation..
The results also show the power generated by this scheme, with a total power output of It is worth noting that due to a lack of detailed turbine characteristics, in this study the turbine power output was not changed in the secondary generation direction and the turbines efficiency was considered to be 1.
It can be seen that the maximum water levels upstream of the barrage decreased considerably for both modes of operation. The water levels downstream of the barrage were reduced in the vicinity of the barrage and then increased further downstream in the Bristol Channel.
The extent of the reduction in water level upstream was found to be more significant for two-way generation, which impacts also on the levels downstream. The reduction in the maximum current speed downstream of the barrage is predicted to be more substantial with the two-way generation scheme over 0. The maximum current speed upstream of the barrage was reduced more significantly for the ebb- only generation scheme compared to the two-way generation mode, with the reduction in the maximum current speed being generally found to be just over 0.
Figure 9 depicts the predicted suspended sediment levels across the Severn Estuary and Bristol Channel for the pre- and post-barrage scenarios, at high water spring tide at Barry black dot in Figure 9.
Similar to the previous velocity predictions, it can be seen that the suspended sediment levels are reduced both upstream and downstream of the barrage both for ebb-only and two- way generation modes. It is also worth noting that the operation of the barrage will cause a time lag and a shift in the mean ebb and flood velocities due to the holding phase and a comparison of the suspended sediment and bacteria concentrations levels must ideally be carried out over a number of tidal cycles.
Studying the concentration of the suspended sediment over a number of tidal cycles for ebb-only and two-way generation modes shows that the reduction in the concentration of the suspended sediment is less significant for two-way generation. This is not clearly seen in Figure 9 due to changes in the tidal conditions upstream of the barrage, and as a result of the operation of the barrage as shown in Figure 5 and 6.
In other words, the condition of the tide will change upstream of the barrage in relation to the reference point of the figures, namely the Barry Site - located downstream of the barrage, as a result of the barrage operation.
This reduction in suspended sediment levels is consistent with the reduction in the maximum velocities as shown in Figure 8. The simulation results show that the faecal bacteria levels are reduced both upstream and downstream of the barrage, for both ebb-only and two-way generation modes. The reduction in faecal bacteria levels is linked to a reduction in the transport of bacteria, due to reduced currents and reduced suspended sediment levels.
The latter contributes to a decline in the faecal bacteria levels by a reduced bacteria level attached to the reduced suspended sediments and a corresponding increase in the bacterial mortality levels, due to reduced turbidity and elevated light penetration as described in section 2. As for the suspended sediment levels, investigating the concentration of the bacteria levels throughout a number of tidal cycles under ebb-only and two-way generation confirms that the reduction in the bacteria concentration was by and large less significant for two-way generation.
Conclusions This study investigates and compares the hydro-environmental impacts of the STPG Severn Barrage using two significant operation schemes, namely ebb-only and two- way generation. The model was validated and modified to simulate the hydro-environmental impacts of the STPG Severn Barrage using ebb- only and two-way generation schemes.
The domain of this study was extended from the outer Bristol Channel to the limit of the tidal River Severn, close to Gloucester. This study confirms the results of recent findings, in that two-way generation would generate almost the same amount of power as an ebb-only generation scheme. Using an average spring tide, the predictions show that there will be a significant reduction in the maximum water levels and an increase in the minimum water levels upstream of the barrage.
This reduction in the maximum water levels is more significant when two-way generation is used, while the increase in the minimum water levels is more significant when implementing ebb-only generation. These highly coloured expectations have given place to a more sober and reflective outlook. You can also search for this author in PubMed Google Scholar. Reprints and Permissions.
Nature , — Download citation. Issue Date : 01 April Anyone you share the following link with will be able to read this content:. Sorry, a shareable link is not currently available for this article. Provided by the Springer Nature SharedIt content-sharing initiative. Advanced search. The seaward boundary was specified using a water elevation boundary condition.
Other studies have included the discharge from the River Severn, either as a flow boundary or by linking a 1D river model.
However, the SEPM does not take this discharge into account. Hence the upstream extent of the model was treated as a land boundary. All the land boundaries were defined using a no-slip condition.
A time step of 0. Other parameters were set based on calibration with laboratory data, and recommendations from the user manual [19].
The main feature that can be observed in this figure is the tidal range amplification, which occurred due to the natural shape of the estuary. This increased with distance upstream; between Points H and A the amplitude increased by approximately 1.
This reduced amplification is a result of doubling the time period to 40 s, as this affected the advective processes in the model. This limitation of the physical model therefore results in an under prediction of the tidal energy resource.
This finding is further evident in Figure 5, in which predictions are given of the tidal velocities in the SEPM. Points A and F showed the least agreement between the physical and numerical model predictions, which was thought to be due to the lower location of the ADV probe, and the interaction between the probe and the relatively shallow body of water in that area.
Both models showed faster currents predicted on the ebb tide, as expected, and this can be clearly seen in Figure 6, which gives the velocity predictions across the domain for a flood and ebb tide, respectively. The modelling results showed that for a flood tide the peak velocities are in the region of 0.
Using the appropriate scaling factor, which was modified to account for the time period scaling issue see Ellis [16] , these velocities correspond to 1. In general these agree well with the estimates of tidal velocities in the Severn Estuary, however, they are lower than expected, which is a result of the scaling issue already discussed.
The minimum water levels upstream were increased significantly, from Downstream a small reduction was observed throughout the estuary as the natural frequency of the estuary was changed due to the presence of a barrage structure. These conclusions are the principal findings from previous studies, and the increase in the minimum water levels inside the basin would result in large areas of intertidal habitat being permanently flooded, a finding that has raised many environmental concerns.
A reduction in the velocities throughout the estuary was observed, except for local increases around the barrage site due to the filling and emptying processes of the barrage. The magnitude of these local increases would not be expected to be transposable to the prototype scale, as they are dependant on the exact geometric details of the barrage structure, and modelling such fine details was not feasible at such a small scale.
This reduction was significant on the ebb tide. Starting heads of 0, 3, 4, 5 cm were tested by manually opening and closing the shutter at different intervals.
It was found that with 0cm head the tidal regime behaved much like the natural conditions, with a small decrease in elevations and velocities throughout the estuary. As the starting head was increased this affected the increase and decrease respectively in the maximum and minimum water levels, upstream of the barrage, as well as throughout the estuary.
A similar behaviour was also observed with respect to the tidal velocities. The reader is referred to Ellis [16] for the full results of the different starting heads. This equates to a 2. This would result in a smaller loss of intertidal habitat compared to the STPG scheme, as well as providing further flood protection. For all of the barrage schemes tested the velocities in the estuary were reduced, however, scaling issues were observed with larger starting heads, as opening and closing the shutter in a short time period led to a water hammer effect, resulting in instabilities in the free surface and velocities.
It can be seen also, however, that further downstream at Point H these oscillations had dissipated. Finally, contour plots of the velocities at mid-flood and ebb tide from the DIVAST model are given in Figure 12, which further illustrates the reduction in velocities compared to the natural condition.
Attempting to model power take-off is impractical at this scale and would also require further details of turbine performance and operating conditions. Hence, whilst it is acknowledged that the calculated energy values bear little resemblance to the prototype scale, comparisons can still be made between different schemes with regards to the total energy yield through the barrage structures. This was calculated using equation 3. The significant difference between the two schemes is that whilst with the STPG scheme the power generated occurs once per tidal cycle, whereas with the Hafren Power barrage this energy will be produced more evenly throughout the tidal cycle.
This is advantageous in aspects such as connectivity to the electricity grid. Two schemes were investigated, namely the ebb generation STPG scheme, and the two-way generation Hafren Power proposal. The hydro-environmental model DIVAST was used to replicate the physical models, and modifications were made to include the operation of barrage structures.
The results confirm the conclusions made from previous studies with regards to the STPG scheme, with the main points being that upstream of the barrage minimum water levels will increase dramatically, permanently flooding areas of intertidal habitat, and downstream of the barrage a slight reduction in water levels was observed. Finally, the mean velocities throughout the estuary will decrease.
As exact details of the Hafren Power scheme are not in the public domain, conclusions are made more generally about the impact of a two-way generation scheme. The starting head dramatically affected the hydrodynamics of the estuary. With no starting head tidal elevations and velocities are similar to the natural state, although there is a reduction in both throughout the estuary.
It was found that a 3cm head would increase these minimum levels by 2cm a 2. The numerical model predictions showed very good agreement with the obtained physical model data, although some numerical instability was observed due to opening and closing the barrage over a short time period.
It has been demonstrated that the modelling methodology is suitable to the assessment of the hydrodynamic impact of a barrage. Modelling at a prototype scale would enable further assessments to be made with regards to the hydro-environmental impact and energy yield from a barrage. The authors would also like to thank Mr Paul Leach for manufacturing the barrage models.
Kadiri, R. Ahmadian, B. Bockelmann-Evans, W.
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