TIDAL, POWER TURNBULL 539 



As to the best type of dam to build at Hopewell, I feel that I 

 should offer no opinion, for the question of dam design is an engi- 

 neering specialty, and only an expert in this particular branch of en- 

 gineering could decide the best type to adopt, and he only after a 

 systematic line of borings were obtained along a proposed site. It 

 has been suggested to me that a dam composed of hollow sections of 

 concrete is a satisfactory and cheap type to build, the sections being- 

 built in a dry dock, floated into position, and then sunk by filling the 

 interior with rock and gravel. However, I doubt if this is a well- 

 proven system, and, as I say, the question of best type should Joe 

 decided by an expert of long experience. Such estimates of cost that 

 I have been able to make have been based on the cyclopean concrete 

 type of dam. In considering the best type to build at Hopewell the 

 question of the tidal currents would have to be considered, and while 

 these currents are not very swift, when the great height of the tide 

 is considered, nevertheless they should receive attention. From 

 Figure 5 you will note some current measurements that I made by 

 using a ship's log attached to an anchored boat. When the tidal 

 range was 38 feet the channel current reached a maximum of 4.2 

 knots, and you will note that although the flood current soon dies 

 out the ebb persists at near its maximum until almost the time for 

 the next flood tide to begin. Out of the channel the current runs 

 swiftly for much shorter periods, and the tidal current makes*shore- 

 ward as the shores are approached until we often have currents at 

 i ight angles to the main stream. The dotted curve shows some meas- 

 urements taken rather off the channel, with a tidal range of 28 feet, 

 and the maximum under these conditions was 2.6 knots. 



SPECIAL PROBLEMS 



Before leaving the engineering problems that are presented by 

 this novel plant, I will mention three other items that should be 

 considered. 



The question of subnormal neap tides requires especial attention, 

 for while my calculations are based on the least range, viz, 32 feet 

 of ordinary neap tides, nevertheless there are certain tides which 

 occur sometimes three days a month, sometimes five days a month, 

 and sometimes not at all in a month, which have a lower range than 

 32 feet, and may sometimes have a range as low as 25 feet. 



Now, if our plant were built and running at full capacity, with 

 a range of 32 feet, there would be an impairment of the regular 

 capacity whenever these subnormal neaps occurred, which is about 

 15 per cent of the time, and I think some provision should be made 

 to deal with them adequately. One method might be to keep the 

 turbines and generators well ahead of the normal demand and use 

 these extra ones only during the subnormal neaps; another method 

 would be to keep the Memramcook shoveled out, as already described, 



