. . following a site visit at Orleans Gardens 10.07.13
It was good to meet with you and on the Riverside at Orleans House Gardens yesterday. This would certainly be an excellent location for Gloriana’s bargehouse. Clearly there are several issues that will have to be resolved but I do think these can all be worked out and, from a technical point of view, I believe the development of this project would be achievable. My summary thoughts at this stage are:
• It would be good to in some way reinstate the use of the former boathouse, but we will have to find out by site investigation what if anything remains underground from that structure.
• Some dredging could be needed adjacent to the existing river wall which might require some strengthening or underpinning, depending on the extent of deepening.
• Access to the site will require agreement with Hammerton’s to modify their pontoon berths and then approvals from the authorities. As we discussed I think this could provide a beneficial opportunity for all concerned.
• There is some linkage between the desired level of Gloriana while on display, the depth of excavation for the dock, access time requirements for transferring her between the river and bargehouse and how this manoeuvre is to be done. These all need to be considered jointly and in some detail.
• The proposal is for a wet dock accessible at higher tidal levels, although it has been mentioned that access at maintained water level would be preferable. Technically both would be feasible but for access at the lower, maintained, water level it would obviously be necessary to excavate more soil on land and in the river. This would be more costly not just because of the increased volume but it would also make the construction work slightly more difficult as it would be below the minimum water level, unless it is possible to carry out this construction during a period of drawdown. Maintenance costs including dredging and silt removal would also be greater for the deeper excavation.
• To facilitate Gloriana’s maintenance it would be desirable to dewater the dock, making it become a dry dock. When studying options for doing this, the relative merits of constructing a slipway rather than a dock, or even inside the dock, should also be considered.
• Some dredging will be required in the river and the creation of a new wet dock will cause river water to flow into the newly excavated space. Siltation will inevitably occur to some extent in both places. Mitigation measures can be put in place to minimise this effect.
• A guillotine gate can provide a simple and effective entrance to the dock and a walkway could be connected as an integral part so that gate and walkway lift out together. It can also be useful to help flush accumulated silt from the dock as in various examples we discussed. However this will have to be agreed and approved by the relevant authorities. Navigation in and out of the dock will inevitably require Gloriana to be aligned perpendicular to the river flow. At times the flow can be strong and this might require some delay until slacker water near to the turn of the tide. The entrance should be designed to allow some flexibility for this manoeuvre.
• I understand that Depressed River Mussels may be present in the area. These and other environmental issues will have to be taken into account and mitigation measures put in place if required. Obtaining consents from authorities for marine projects can take several months even for projects such as this that appear to have only limited impact. This needs to be taken into account in the programme.
I hope this is useful at this stage and I very much look forward to working with you to further develop this project towards its successful implementation.
Beckett Rankine Marine Consulting Engineers