Tidal range in the UK

[BarnetBlizzard]

Hi guys, 

My dissertation involves comparing the tidal range at coastal locations across eastern England and more specifically the Humber Estuary. Firstly, I picked this as a topic for my dissertation due to the lack of research and knowledge on the effects a changing tidal range, due to climate change, can have on our coasts.   I have produced some graphs for the tidal range at Immingham after retrieving data from the British Oceanographic Data Centre (BODC). These graphs show the average tidal range at Immingham for each year going back as far as 1956.  These graphs produced some results i did not expect. I expected a slight decline in tidal range at Immingham within the last 30-40 years after research however once the graphs were produced any decline was very hard to distinguish.  This was mainly due to repetitive periods of large mean tidal range and low tidal ranges. These occurred every 20 years or so. Initially I was slightly confused however after more research I discovered these repetitive periods could be due to something known as the Nodal cycle, which is a cycle related to the Moons nodes and its relation with the sun and moon (far more complicated then that). Furthermore, the Nodal Cycle occurs in cycles of 18.6 years, pretty similar to the periods I recorded in Immingham. 

However, the range between the peak and trough of my graph (highest mean tidal range and lowest mean tidal range -20 years apart) is 20cm.  I have found some sites which suggest the nodal cycle can only affect the tide by a range of 2 cm or so, far less than 20cm.  This is where my question is: is a difference of 20cm between the peak mean tidal range and the low mean tidal range far too large and not possible? I have looked for possible errors involving units however I can not see any thus far. 

Sorry for quite a difficult question, and there may not be anyone available to answer however I may as well try! 

Thanks,

BarnetBlizzard (excuse the name) 

[Interitus]

Seems statistical examination of tide gauges data does give a small nodal variation of up to 5cm but statistically significant for the northeast Atlantic area round the UK, and 3cm for perigee cycles (apsidal precession - 8.85 years but appears as a 4.4 yr quasi-cycle) - http://onlinelibrary.wiley.com/doi/10.1029/2009JC005997/full 

However computer modelling suggests that the 18.6 yr nodal range can be up to 50-80 cm and conversely from a UK perspective the perigean cycle can be 30-60 cm with the Irish Sea, Bristol Channel and English Channel affected at the top of this range - http://onlinelibrary.wiley.com/doi/10.1029/2010JC006645/full

Are these effects not seen in the tidal forecasts for Immingham, where the highest spring tides range from 7.51 to 7.99 metres between 2008 and 2026 - http://www.ntslf.org/tides/hilo?port=Immingham ?