Tidal study, Somme Estuary, 24 August 1346
Establishment of the times of the tidal extrema in the Somme estuary for August 24th 1346; and additionally times of sunrise, sunset and other ephemera.
The traditional location of the crossing (gué de Blanquetaque) used by Edward III prior to Crécy is said to be at the approximate limit of the tidal Somme, perhaps 8000 metres above St Valéry, however it is known that England traded with Abbeville, and shipping must therefore have been able to navigate over the ford. Also, the chronicles all say that Edward III awaited the low tide before attempting to cross.
The modern canal, railway embankment, dyke etc have removed all traces of the original and natural landscape.
(i) The exact position of the gué de Blanquetaque is not known. For this study it is assumed to lie on the reach between Goui and Boimont on the left bank, and Grand-Port (Port-le-Grand) and Tombe de Paul on the right bank. References  and  mention Blanquetaque; the former stating: "... bourg de Port-le-Grand, à une petite distance au-dessus du gué de Blanquetaque...". Bellin,  planche 19, shows a number of shoal spots (1.5 to 2 pieds) approximately East of Saineville toward "G. Port". Despite an obvious scalar error in the 'toise'  (doubled to approximately 12.8 feet), we shall retain this general location for the ford.
(ii) What little credible evidence there is, would tend to demonstrate that silting has been in progress for several centuries. We shall therefore err on the side of considering that the volume of the estuary would, if anything, have been greater in 1346 than today (see caveat (a)).
(iii) It is also assumed that the gradient of the river bed was constant between Abbeville and the mouth of the Somme Estuary (here considered as Le Hourdel). Assuming, from the description above, that high water springs must have given a draft capability of 6 feet [13, 14] in Abbeville for access by traders, and that the channel depth at Le Hourdel at low water springs would also have been about this depth, then the tidal height at the presumed site of the gué would be some 19 feet above tidal datum. This statement is somewhat broad, but is not contradicted by any evidence, even anecdotal, that we have found.
(iv) That standard shoal water effects may be assumed, specifically Doodson  Progressive Waves, Chapter XVII, Standing Oscillations and Reflected Waves, Chapter XVIII and his conclusions Tides ... in Gulfs... Estuaries, Chapter XIX from which we retain the mathematical arguments of, inter alia, para 19.4
(v) That, as is expected from (ii), the tidal flow in the estuary of the Somme is stronger and appreciably shorter in time than the ebb; this is confirmed by local observation ; cf Forrester  "wave form distorted from its sinusoidal form ... the crest is found to propagate faster than the trough".
(vi) Modern time offsets  and  would indicate that Le Hourdel, at the mouth of the Somme but inside the offshore banks, would lag Dieppe by about 25 minutes (26 on HW neaps, 21 at HW springs), and St Valéry would lag a further 14 minutes neaps and 7 minutes at springs. By disregarding possible reflected wave effect, these offsets can be safely used for the mouth of the Somme estuary.
(vii) All astronomical algorithms are (first) based on recent resolutions of the International Astronomical Union; and use the new standard epoch J2000.0, the FK5 system, and new orbital theory and data from the Bureau des Longitudes in Paris. They can be safely assumed to be accurate to within a very few minutes for the period in question - and have been used for the almanac and tidal calculations.
(viii) We also assume that the "Old Style" (Julian Calendar) has been used for Thursday, Aug 24, 1346 (id est, Julian Day 2212919.5). We also will use GMT (Greenwich Mean Time) which for these purposes may be assumed to be the equivalent of TU (Universal Time).
(a) The Somme estuary is known to exhibit extensive movement of sand banks and mud flats; historic evidence suggests that this has always been a continuing process.
(b) The 16th century chart  is not helpful; the Bellin  chart (mid C18th) must be considered the earliest reliable work
(c) While there is no reason at all to doubt that today's harmonic constituents from the International Hydrographic Organization are relevant in this historic context (the geography and hydrology of the English Channel have been relatively constant), we do not have absolute proof of this assumption.
(A) Spring tides occurred on 19 Aug 1346 (34.3 feet), neaps on the 27th (11.9 feet) at Dieppe. The first tide on the 24th was of 21 feet, the last of 19 feet.
(B) High Water would have occurred at the entrance to the Somme Estuary (Le Hourdel) at 03:24 and 15:51 GMT.
(C) High Water would have occurred at the presumed ford some 41 minutes later, say 04:05 and 16:32 GMT
(D) Low Water would have occurred at the entrance to the Somme Estuary (Le Hourdel) at 9.56 and 22.34 GMT
(E) Disregarding riverbed gradient, theoretical Low Water would have occurred at the presumed ford some 77 minutes later, say 11:13 and 23:51 GMT
(F) However, from assumptions (iii) riverbed gradient, and (iv), reflected wave, we may deduce that the ebb tide would have receded below the presumed position of the gué at 07:21 and 19:44 GMT. The intermediary tidal flow would have reached the presumed position at 14.17.
Accuracy: For the above conclusions (A) may be considered accurate as to date, and to within one foot for amplitude; (B) and (C) may be considered accurate to within 30 minutes (98% probability) and more pragmatically to within fifteen minutes; (D), (E) and (F) accurate to within 75 minutes (98% probability) and more pragmatically to within half an hour.
Daily Almanac : all times Local = GMT + 0h 0m
Thursday, Aug 24, 1346 at 0h 0m 0s. Julian Day: 2212919.50000
At 50°09.000'N 1°44.000'E. Height 0 ft Equ of Time - 0h 0m 30s
|SUN : Rise = 05h06m
MOON : Rise = 13h07m
|Transit = 11h52m
Transit = 17h52m
|Set = 18h38m
Set = 22h33m
New Moon : Aug 17, 1346 at 15h52m
First Quarter: Aug 24, 1346 at 13h03m
Full Moon : Sep 01, 1346 at 16h49m
Last Quarter : Sep 09, 1346 at 09h24m
VENUS : Rise = 06h06m
MARS : Rise = 08h01m
JUPITER : Rise = 19h19m
SATURN : Rise = 18h24m
Transit = 12h36m
Transit = 13h48m
Transit = 01h20m
Transit = 23h22m
Set = 19h04m
Set = 19h33m
Set = 07h18m
Set = 04h24m
Monthly Tidal Predictions: astronomical data for Tidal Harmonic predictions for Aug 24, 1346 at 0h 0m 0s:
Mean Longitude of the Moon (L') = 234.1°
Mean Ecliptic Longitude Sun = 158.1°
Longitude Mean Ascending Node Moon (N = -N') = 198.7°
Longitude Mean Perigee = 139.2°
Longitude Mean Perihelion = 271.7°
|DIEPPE: Lat:49°56'N Long:001°05'E|
Month of August, 1346 G.M.T. +0.00
Notes and bibliography:
JANSZOON, LUCAS, Spieghel der Zeevaert, Leiden, 1584, 1585
BELLIN, Atlas Maritime des Costes de France, 1764, Tome V, planches 18 et 19
RECLUS, ELYSEE, Nouvelle Gégraphie Universelle, Tome II, Hachette, 1885
DOODSON, A.T., Admiralty Manual of Tides, HMSO, 1941
PROUDMAN, J, Halley's Tidal Chart, Journal Royal Geographical Soc. Vol 100, 1942
Editions Cartographiques Maritimes, Le Perreux-sur-Marne; based on SHOM, Service des Phares et Balises, Services Maritimes des Ponts et Chaussées.
BEAUCHAMP J. 1994, Rôle des facteurs dynamiques dans le colmatage de la Baie de Somme. Ann. Soc. Géol. du Nord
Admiralty Tidal Stream Atlas, English Channel, NP250 3rd ed., Hyrographer of the Navy, 1973
Annales Maritimes et Coloniales, Nos 9 et 10, Paris, 1826
The toise equals six pieds - feet of different lengths were used in France, but based on the 18th century Paris pied the toise equals 6.39 (English) feet or 1.95 meters (Spiers and Surennes, 1912; OED 1933, traces this back to 1598)
FUIDGE, JH, The Historical Map of Europe, George Gill, London, 1917
FORRESTER, W.D., Canadian Tidal Manual, Ottawa, 1983
KEBLE CHATTERTON, E., Sailing Ships, the story of their development from earlist times to the present day, London, Sidgwick and Jackson, 1909
BRINDLEY, H.H.; CALLENDAR, GEOFFREY; et al, Mediaeval Ships, the Gold Noble of Edward III, etc; Mariner's Mirror, Volume 2, 1912
Other documents consulted:
CRONE, G.R., The Mariners Mirrour 1588, Geographical Journal, Vol 119, 1953, Royal Geographic Society
KENNISH M.J., Practical handbook of Marine Science, CRC Press, 1989
PROUDMAN, Prof. J., Halley's Tidal Chart, Geographical Journal, Vol 100, 1942, Royal Geographic Society
SVERDRUP H.U., JOHNSON M. et FLEMING R.H., The Oceans, Their Physics, Chemistry, and General Biology, Prentice Hall, 1942
Offsets used (various sources):
Le Treport: 50°04'N 1°22'E
Type: Tidal substation
13.5NM at 53° from Dieppe
High tide time: +0:06 neaps, -0:02 springs
Low tide time: +0:06 neaps, +0:11 springs
Cayeux s/ Mer: 50°11'N 1°29'E
Type: Tidal substation
21.8NM at 45° from Dieppe
High tide time: +0:10 neaps, +0:07 springs
Low tide time: -0:08 neaps, +0:13 springs
Le Hourdel: 50°13'N 1°34'E
Type: Tidal substation
25.2NM at 47° from Dieppe
High tide time: +0:26 neaps, +0:21 springs
Low tide time: +0:05 neaps, +0:18 springs
Type: Tidal reference
51.6NM at 21° from Dieppe
Despite being reference, offset from Dieppe
High tide time: +0:12 neaps, +0:22 springs
Low tide time: +0:34 neaps, +0:44 springs