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On the sign:
[Sign 1] THIS WALL WAS ERECTED AND THE PILES FIXED ANNO DOMINI 1817 WILLIAM SMITH. WARDEN
[Sign 2] VARIABLE HIGH TIDES MARCH 30 1874 JOSEPH GILES, WARDEN
[Sign 3] EXTRAORDINARY HIGH TIDE JANUARY 7TH 1928 WHEN 75FT OF THIS WALL WERE DEMOLISHED LEONARD COLLYER, WARDEN.
Apparently the wall which is part of the Thames Path acts as a barrier against the tides of the River Thames. In any case, the signs indicate the abnormal tide levels. The names of the individuals mentioned on the signs appear to be inspectors on behalf of Trinity House - Britains lighthouse network