01 August 2007
Modelling work and marine site investigation survey results announced
The beach modelling and marine site investigation, commissioned by Howard Holdings and undertaken by HR Wallingford and Fenton Holloway, has completed. The results show that possible effects of a marina can be managed.
The testing has explored the impacts of the marina proposals. As work commenced prior to the Draft Masterplan being agreed, the surveys are based on a marina with a capacity of 300 boats. A smaller marina, recommended in the final Masterplan, is likely to produce reduced impacts than those modelled and, therefore, there would be smaller impacts to manage. The engineering proposals remain the same.
The studies include the analysis of historical data as well as computer modelling and cover four key areas: flow and flushing, wave disturbance, sedimentation transport and beach impacts. Ground investigation has also been undertaken to assess the material in and around the proposed marina site.
Flow and Flushing
The results show a very small impact on tidal flows caused by the construction of the marina and confirm that flows inside the harbour and at the harbour entrance will be largely unaffected. The marina will also be flushed naturally by tidal movement.
A new marina will bring more boats to the area and, therefore, careful management and rapid deployment containment measures will be put in place to manage any risk of an accidental release of pollutants. The flow modelling shows that concentrations of pollutants on the beach as a result of accidental release in the marina or harbour will not be significantly different to that experienced at present. The Environmental Impact Assessment will examine this further.
Wave Disturbance
Wave disturbance modelling assesses the impact of the marina breakwaters on the nearby harbour areas and the beach and looks at wave conditions within the marina. It assesses these based on no marina and with the marina in place.
Currently, waves approaching from the East-North-East create the worst wave conditions in the outer harbour. A new marina will not change this significantly. Wave conditions approaching from the East-South-East will increase wave heights inside the outer harbour and the amount is subject to the location. However, the increase in significant wave height is typically no greater than 0.3m. This suggests that there may be more disturbance at certain times. Outer approaches to Weymouth Harbour are largely unchanged. The entrance of the marina will be well-protected.
To minimise the impacts, the south breakwater will have as low reflectivity as possible. This can be achieved by a perforated screen twin vertical wall breakwater or a rock armoured rubble embankment. Based on the poor ground conditions identified, the perforated screen construction is favoured. It is proposed that solutions be modelled extensively in all conditions to make sure they are effective.
Sediment Transport
The marina will have no detrimental effect on the silting of the harbour entrance. Although some silting of the marina entrance and, to a lesser extent, inside the marina will occur, it is anticipated that this can be easily managed by ploughing, which already takes place at the existing harbour entrance.
Beach impact studies
HR Wallingford carried out a scoping study, desk study and beach inspection to establish the beach evolution and processes, and the likely impact of the marina. This was followed by detailed studies and modelling.
The historic map study over 140 years shows steady increases in the volume and width at the south end of the beach. This is likely to be associated with the construction of the Portland breakwaters and the extension of the North Pier over that period.
Having examined the beach performance over 17 years (1986 to 2005), the beach is now in a state of equilibrium and will not significantly change in shape except for short-term fluctuations caused by the weather. Using the wave data available for the last 17 years and basing studies on the same wave conditions experienced during this period with no marina in place, a computer model was developed to project these same conditions into the next 17 years. This showed that the beach would remain in a state of equilibrium.
The proposed marina will extend the North Pier further and, therefore, create additional widening on the beach at the south end. According to the modelling results, the southern end of the beach could increase by up to 20 metres with the beach north of the Bandstand Pier decreasing by only a few metres.
However, by recharging the beach with 100,000 cubic metres of sand, there will be no net reduction in the beach width north of the Bandstand Pier and the southern end would increase by up to 40 metres. Once this has been done, the beach would once again be in an equilibrium state.
If suitable, the recharge material could come from the marina excavations. The marina could provide a total of 100,000 cubic metres, which is the maximum needed. Twenty samples of the material in the top 2 metres of the seabed in the marina area have been collected and these appear similar to the sand on the southern end of the beach. Laboratory testing will confirm this. The sand will only be used if it is proven beyond any doubt to be suitable.
Ground investigations
Between the surface sands and gravel & stiff clay on the seabed, layers of weak silts have been found. This means that piling will be required through to the clay level in the construction of the marina.
The excavated material arising from the building construction on the peninsula can be re-used as fill within a walled enclosure to form a hard standing area along the east wall of the peninsula. This is in line with Howard Holdings’ sustainability policy and its commitment to recycle and reuse materials where possible.
Image 2 - Shows the beach at various stages. Scenario 1 shows the impact with the new marina and Scenario 2 shows the impact after the dredging of the sand and the recharge of the beach
Modelling work and marine site investigation survey results announced
The beach modelling and marine site investigation, commissioned by Howard Holdings and undertaken by HR Wallingford and Fenton Holloway, has completed. The results show that possible effects of a marina can be managed.
The testing has explored the impacts of the marina proposals. As work commenced prior to the Draft Masterplan being agreed, the surveys are based on a marina with a capacity of 300 boats. A smaller marina, recommended in the final Masterplan, is likely to produce reduced impacts than those modelled and, therefore, there would be smaller impacts to manage. The engineering proposals remain the same.
The studies include the analysis of historical data as well as computer modelling and cover four key areas: flow and flushing, wave disturbance, sedimentation transport and beach impacts. Ground investigation has also been undertaken to assess the material in and around the proposed marina site.
Flow and Flushing
The results show a very small impact on tidal flows caused by the construction of the marina and confirm that flows inside the harbour and at the harbour entrance will be largely unaffected. The marina will also be flushed naturally by tidal movement.
A new marina will bring more boats to the area and, therefore, careful management and rapid deployment containment measures will be put in place to manage any risk of an accidental release of pollutants. The flow modelling shows that concentrations of pollutants on the beach as a result of accidental release in the marina or harbour will not be significantly different to that experienced at present. The Environmental Impact Assessment will examine this further.
Wave Disturbance
Wave disturbance modelling assesses the impact of the marina breakwaters on the nearby harbour areas and the beach and looks at wave conditions within the marina. It assesses these based on no marina and with the marina in place.
Currently, waves approaching from the East-North-East create the worst wave conditions in the outer harbour. A new marina will not change this significantly. Wave conditions approaching from the East-South-East will increase wave heights inside the outer harbour and the amount is subject to the location. However, the increase in significant wave height is typically no greater than 0.3m. This suggests that there may be more disturbance at certain times. Outer approaches to Weymouth Harbour are largely unchanged. The entrance of the marina will be well-protected.
To minimise the impacts, the south breakwater will have as low reflectivity as possible. This can be achieved by a perforated screen twin vertical wall breakwater or a rock armoured rubble embankment. Based on the poor ground conditions identified, the perforated screen construction is favoured. It is proposed that solutions be modelled extensively in all conditions to make sure they are effective.
Sediment Transport
The marina will have no detrimental effect on the silting of the harbour entrance. Although some silting of the marina entrance and, to a lesser extent, inside the marina will occur, it is anticipated that this can be easily managed by ploughing, which already takes place at the existing harbour entrance.
Beach impact studies
HR Wallingford carried out a scoping study, desk study and beach inspection to establish the beach evolution and processes, and the likely impact of the marina. This was followed by detailed studies and modelling.
The historic map study over 140 years shows steady increases in the volume and width at the south end of the beach. This is likely to be associated with the construction of the Portland breakwaters and the extension of the North Pier over that period.
Having examined the beach performance over 17 years (1986 to 2005), the beach is now in a state of equilibrium and will not significantly change in shape except for short-term fluctuations caused by the weather. Using the wave data available for the last 17 years and basing studies on the same wave conditions experienced during this period with no marina in place, a computer model was developed to project these same conditions into the next 17 years. This showed that the beach would remain in a state of equilibrium.
The proposed marina will extend the North Pier further and, therefore, create additional widening on the beach at the south end. According to the modelling results, the southern end of the beach could increase by up to 20 metres with the beach north of the Bandstand Pier decreasing by only a few metres.
However, by recharging the beach with 100,000 cubic metres of sand, there will be no net reduction in the beach width north of the Bandstand Pier and the southern end would increase by up to 40 metres. Once this has been done, the beach would once again be in an equilibrium state.
If suitable, the recharge material could come from the marina excavations. The marina could provide a total of 100,000 cubic metres, which is the maximum needed. Twenty samples of the material in the top 2 metres of the seabed in the marina area have been collected and these appear similar to the sand on the southern end of the beach. Laboratory testing will confirm this. The sand will only be used if it is proven beyond any doubt to be suitable.
Ground investigations
Between the surface sands and gravel & stiff clay on the seabed, layers of weak silts have been found. This means that piling will be required through to the clay level in the construction of the marina.
The excavated material arising from the building construction on the peninsula can be re-used as fill within a walled enclosure to form a hard standing area along the east wall of the peninsula. This is in line with Howard Holdings’ sustainability policy and its commitment to recycle and reuse materials where possible.
Image1 - Shows the beach without the marina and with the proposed marina
Image 2 - Shows the beach at various stages. Scenario 1 shows the impact with the new marina and Scenario 2 shows the impact after the dredging of the sand and the recharge of the beach
ENDS
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