Episode 12 (of 18)
To my consternation it wasn’t going to be as easy as I’d thought – the tides did not just go in and out like on the Solway Firth – the books and papers in the reference library confirmed what I had heard – that in the Solent tidal flows were complicated and at springs there appeared to be two high tides, separated by a couple of hours.
At this point Svet interrupted me to ask what ‘springs’ were. I explained that there were two overall phases to tides around the British Isles in a period of a month. ‘Springs’ referred to the period when the gravitational pull of the moon and the sun worked together to drag a greater body of water around, thus creating very high tides – and very low ones. ‘Neaps,’ on the other hand, were not only Scottish root vegetables but also the period at the opposite end of the cycle when the pulls of the moon and the sun to some degree counterbalanced each other, resulting in a much smaller tidal range.
So, in the Solent spring tides are in two, apparently three tide cycles every 24 hours rather than the more normal two tide cycles. Neap tides conform much more to the normal.
Svet again interrupted for an overview of the Solent.
The Solent is a large area of tidal water off the south coast of Hampshire, bounded to the south by The Isle of Wight and to the north by Southampton Water, the estuary leading up to Southampton docks. It has two wide outlets to the open sea and the English Channel – one to the west at Hurst Point and one to the east at Portsmouth.
My studies had identified that the main cause of the second high tide (or ‘stand’ as us oceanographers call it) was not the particular characteristics of the Solent itself but the constriction in the English Channel between the Isle of Wight and the Cherbourg peninsular in France. This deforms the ‘Atlantic pulse’ which pushes and pulls the water back and fore up and down the channel, resulting in the daily tidal cycle. When it is high water at Dover it is low water at Land’s End and vice versa. Spring tides in The Channel in the area bounded by Portland, Cherbourg, Littlehampton and Le Harve experience four oscillations rather than the normal two. Within the shallow waters of The Solent a further thirty oscillations may occur and results in two high water stands.
I struggled to work out how exactly all this causes two high water stands in the Solent, but felt that in the scheme of things this was not unsurmountable for an experienced report writer like myself, and in any case felt that the local view that the two sea entrances were important factors that should be given more credence.
The western entrance, at Hurst Point, is closer to the mid length of The Channel and thus the tidal range is relatively low (imagine a point a foot out from the centre of a traditional see saw), whilst the range at the eastern entrance, at Portsmouth, is significantly larger (imagine a point at the far end of the same see saw). With a tide coming up The Channel the water at the eastern entrance has to rise much further than at the western end – but there is only one hour difference between high tide at the two points – so this extra rise has to be accomplished in a much shorter period of time and in fact overtakes it in height about an hour before overall high water. Thus, at the eastern end of the Solent, the tidal stream turns back westwards a couple of hours before the overall tide in The Channel reaches high water. I wondered whether this water, effectively going the wrong way, resulted in a first, earlier high water stand, followed by the ‘real’ high tide a couple of hours later.
These conditions are of particular interest to navigators, because in a typical tide cycle of 12.5 hours the flood (incoming tide) accounts for 9 hours of this, with the ebb (outgoing tide) running for only 3.5 hours, but with a high velocity of flow (up to 3 knots – which is strong!). The flood stalls for a couple of hours at about the half way point (called the Young Flood Stand) before accelerating again to a first High Water Stand, followed by a slight drop before peaking again a couple of hours later.
I finished my findings as we pulled out onto the spit at Calshot and I manoeuvred the van into a sheltered spot that nevertheless gave an excellent view across the Solent to The Isle of Wight and along both east and west channels out to sea. We got out to stand in a stiff breeze to observe the scene.
‘It’s so busy.’
‘Yup … there’s always something to see – container ships and luxury liners navigating around the Brambles Bank to move up Southampton Water; huge tankers bringing oil to the jetties at Fawley Refinery; and the whole area criss crossed by ferries linking the Isle of Wight to the mainland. On Summer days and particularly at weekends, big ship navigation is complicated by hundreds of private craft, motor launches and yachts, cruising the waters and in the latter case many racing in large fleets.’
‘What about the naval base?’
‘Oh that’s down at Portsmouth,’ I pointed, ‘you can’t see it from here …. and in many ways it’s not really part of the Solent, although you occasionally see naval craft passing through.’
‘Hmm ……’ she turned back to the van, ‘Well let’s get started on working up the structure of your report.’
‘Structure?’ I have to admit to being slightly taken aback and not a little put out, ‘I know how to write reports Svet!’
We climbed back into the van and I pulled some papers from a file on the bench seat. ‘Here’s my first draft – it’s not that complicated,’ I said rather huffily.
She took the three stapled sheets from me with an air of, it has to said, annoyance. She sat and read. The frown on her face growing deeper and more entrenched as she worked her way down the text.
‘Is that it Bo?’ she positively snapped at me and I felt those willies again, this time definitely in the plural.
‘Yes …’ I knew this wasn’t going to end very well but felt compelled to bat on regardless, ‘it covers the main points and comes to the obvious conclusion ….’
‘That tidal flows in the Solent, ‘ she read from the last paragraph, ‘although complex are the result of natural conditions, namely The Channel Pulse and the peculiar circumstances of the area itself, where the two entrances to the sea create counter flows during spring tides.’
She paused, shook her head and then glared at me, ‘and then this! – there is no evidence to suggest that the tidal flows are being manipulated by the Military?’
‘Well there isn’t! ….. any evidence I mean …… what on earth did you think I was going to find?’
‘Bo …… the KGB has invested heavily in you,’ she paused, ‘on the recommendation of Yuri and supported by me. Do you think that this,’ she waved my paper contemptuously at me ‘is going to be seen as a good return for all the money and time that has been lavished on you?’