As an example , two prototype gates both 15 m wide and about 15.5 m high , for a head of about 15.2 m , weighed 93.8 and 117.7 t , respectively .sx The estimated weights given by the above formulas are about 102 and 107 t , satisfactorily splitting the two prototype weights .sx 5.6 Rising sector gate .sx This type of gate is illustrated in Fig. 5.2. The most famous application is for the gates of the Thames barrier , where the inherent stiffness of the design allowed a 60 m opening to be adopted for navigation , while any failure of a gate to close before a surge tide would not be catastrophic , the remaining gates providing enough obstruction to flows to prevent flooding upstream .sx This type of gate was not short-listed during the studies for the Severn Barrage Committee ( Ref .sx 1981(1) ) , largely because a good seal is required between the gate and the sill to prevent loss of water during the generating period , and this seal would be difficult to maintain .sx In addition , there was concern that the gap underneath the gate would trap sediments and lead to difficulties in opening the gate .sx These do not apply to the Thames barrier gates , where there is a 300 mm gap between the gates and their sills .sx Loss of water through this gap is of no economic consequence .sx A slightly modified version of this gate has been proposed for use where the sluice is over the turbine ( Ref .sx 1987(12)) .sx By making one of the longitudinal chambers watertight and buoyant , the forces needed to operate the gate would be reduced .sx In addition , because the gate is located well above the seabed , there would be little risk of sediment building up in the gap underneath the gate .sx 5.7 Gate operation .sx Lewin ( Ref .sx 1980(16) ) has estimated that the power required to open a 12 m square vertical-lift wheeled gate in 15 min is about 60 kW , i.e. about 15 kWh .sx A 20 m wide radial gate lifted 15 m will require about 100 kW .sx Assuming that similar power would be used to close the gates , which is pessimistic , these represent only around 0.02% of the output of a barrage .sx The simplest form of motive power for either vertical-lift wheeled gates or radial gates are chains or ropes passing over sprockets or drums .sx Each sprocket or drum is driven through a reduction gearbox by a single high speed shaft , itself driven by an hydraulic motor .sx The motive power for the hydraulic motors for a number of gates could be provided by a single power source .sx Alternatively , each gate could have its own power source , i.e. a hydraulic pump driven by an electric motor .sx The use of oil-hydraulic motors instead of direct electric drive has much to recommend it .sx Firstly , the hydraulic system is sealed against the corrosive effects of a marine atmosphere .sx Secondly , protection against overload , especially at the end of travel , can be provided simply and reliably by pressure relief valves instead of limit switches or other mechanisms which rely on electrical contacts which can corrode .sx Thirdly , safety features such as oil - immersed disc brakes , which cannot be released until the system oil pressure has reached a certain minimum value , can be readily incorporated into an oil - hydraulic system .sx An oil-hydraulic system will be more expensive than an electrical system , but , properly designed , will be far more reliable and have a much longer working life .sx Each of the radial gates of the Haringvleit sluices is raised and lowered by a pair of hydraulic rams , one at each end of the gate , operating through a mechanical linkage .sx This removes problems of corrosion of lifting chains or ropes ar the expense of requiring a sophisticated system to measure and adjust the travel of each ram so that the gate is not twisted .sx 5.8 Conclusions .sx The choice of type of sluice is , in practice , limited to the types discussed above , and will depend on site-specification factors as well as factors such as first cost .sx For sites with deep water , say 20 m or so at mean tide , the vertical lift wheeled gate appears to be the best choice .sx For shallower sites , especially those on the routes of migratory fish , the radial gated sluice is probably the best choice .sx Chapter 6 .sx Embankments and plain caissons .sx 6.1 Introduction .sx Only in the exceptional circumstances of a narrow , steep-sided site for a barrage would the 'working' components of a barrage , namely the turbines in their power house , the sluices and the ship lock , occupy the full width of the estuary .sx The remaining gaps would have to be closed by embankments or plain , i.e. non - working-caissons .sx This chapter considers both options .sx Except where access from one bank of the estuary has to be provided early in the construction programme , the non-working parts are the least cost items and should be least sensitive to tidal currents during construction .sx Thus they would normally be built when the expensive turbine caissons and sluices had been placed .sx This aspect , loosely referred to as the 'closure' of the estuary , is discussed in Chapter 8 .sx 6.2 Embankments .sx When considering embankments , the main questions to be addressed are :sx What has been the experience elsewhere ?sx What materials would be used ?sx How would these materials be transported and placed ?sx What are the costs ?sx Each of these is addressed in the following Sections .sx 6.2.1 Experience elsewhere .sx The tidal barrage at La Rance includes a short length of embankment , but this was built in the dry within the area enclosed by the main cofferdams and so was more akin to a normal dam than an embankment built in the sea .sx The 20 MW pilot plant at Annapolis Royal in Nova Scotia was built in an existing island and so no new embankments were required .sx Thus there is no direct experience of building an embankment for a tidal power scheme .sx However , there are several projects where relevant experience was gained .sx In Hong Kong in the mid 1960s a dam was built across the mouth of an inlet , Plover Cove , in order to form a fresh water reservoir .sx When complete , the water behind the dam was pumped out and the reservoir filled with fresh water .sx The dam was built mainly of dredged sand on a foundation of relatively soft seabed deposits .sx Fig. 6.1 shows the completed dam , which bears quite a good resemblance to an artist's impression of the proposed embankment for the Severn barrage as seen from the Welsh shore at low water ( Fig. ) .sx Ref .sx 1965(3 ) describes the project in detail .sx figure&caption .sx In the mid 1970s , an even more imaginative project was built in Hong Kong to help meet the increasing demand for water .sx This was the High Island reservoir and comprised the building of a large dam at each end of High Island to link the island to the mainland .sx The area enclosed was then converted to a fresh water reservoir with a top water level some 20 m above sea level .sx What made this project particularly interesting was the building of the cofferdam at the eastern end , in water about 30 m deep , in a location exposed to typhoon waves from the South China Sea .sx Fig. 6.3 shows the seaward face of the cofferdam , which formed a permanent part of the complete structure .sx In the Netherlands , the Delta project has recently been completed .sx Apart from the Osterschelde storm surge barrier , discussed in Chapter 4 , and the Haringvleit sluices ( Chapter 5 ) , this project has comprised a succession of large embankments built to close off estuaries or embayments from the sea in order to prevent a recurrence of the disastrous flooding which occurred on 31 January 1953 as a result of an exceptionally high surge tide .sx These embankments have generally been based on an initial closure structure which has allowed control to be achieved over the tidal flows ( Refs .sx 1965(1 ) , 1972(1)) .sx After closure , the embankments have been completed by covering the initial structure with large quantities of dredged sand with shallow slopes suitably protected against wave attack .sx The experience gained has left the Dutch dredging contractors pre - eminent in their field .sx figure&caption .sx A variety of closure methods was used in the Delta project , including :sx floating in a series of conrete caissons ; a series of caissons with temporary openings which were opened as soon as each caisson had been floated into position , and then all closed at the same time to achieve total closure ; and the dropping of thousands of 2 t concrete blocks by a specially built cable car system , thus building up a long weir which eventually appeared above the sea and allowed dredging to begin .sx A similar system was erected to enable the Osterschelde to be closed from the sea , but this was removed when the environmental lobby persuaded the government that an open barrier with gates should be built instead .sx In Germany , the river Eider was closed by a clever system which started with a series of steel frames across the gap .sx Perforated steel piles were then installed progressively by sliding them down slots in the steel frames so that they spanned 'on edge' , thus presenting a moderate obstruction to the tidal flows ( Fig. ) .sx Dredged sand was pumped into position between the frames and settled out between the piles , some being carried through the perforations .sx Additional piles were inserted as required to prevent undue loss of sand , but at the same time to avoid large loads on the frames .sx The only materials to be brought to the site were the steel frames and piles .sx All other methods have involved large quantities of rock , concrete blocks or concrete caissons .sx The projects described above are important for tidal power in that they demonstrate a variety of engineering skills that could be used to construct embankments for a tidal power sheme .sx However , they have all been built in locations where the tidal range does not exceed 3.5 m , one third of the typical spring tide range of a good tidal power site .sx Consequently , the differential heads across the structures , and the velocities through remaining gaps , would have been much less than those that could occur during the final stages of building a tidal barrage .sx Other problems follow .sx For example , temporary ( and permanent ) protection against waves during construction has to be much more extensive in area if the still water level can rise and fall over 10 m instead of 3 m. In 1975 , towards the end of a major study of the feasibility of storing fresh water in a reservoir formed by enclosing a large area of the intertidal foreshore of the Wash bay on the east coast of England , by an embankemnt built mainly of dredged sand , a trial embankment was built 4.5 km offshore ( Fig. ) .sx This trial , costing pounds 2.4 million , was necessary to evaluate the practicalities of handling and placing large quantities of sand , gravel and rock in a spring tide range of over 6 m , i.e. about double the range experienced elsewhere .sx Ref .sx 1976(5 ) is a detailed report .sx This trial highlighted the importance of providing suitable protection of the dredged sand against scour by tidal currents and waves if large losses of materials or double handling were to be avoided .sx It also demonstrated the feasibility of transporting slope protection materials by sea , and of handling and placing them using floating plant .sx Fig. 6.6 shows a 2000 t capacity coaster being unloaded by floating cranes onto 500 t capacity barges , this operation being necessary because the water depth at the site was insufficient for the coasters .sx Fig. 6.7 shows the placing of filter material on the sand .sx The sand was excavated at a rate exceeding 1 t/s by a modest-sized cutter suction dredger from a nearby borrow area and pumped directly to the site via a 700 mm diameter pipeline laid on the seabed .sx There are many other marine engineering projects which have been successully built and where the experience is of some relevance to the design and construction of embankments for a tidal barrage .sx To take one example , islands have been built of dredged sand in the Beaufort Sea , north of Canada , for oil exploration .sx