The geographic location of Germany accounts for the regional distinc- 
tion necessitated by dissimilar conditions on the Baltic Sea (a tideless 
inland sea), and the North Sea (a marginal sea characterized by its high 
and low tides). In the Baltic Sea the coastal currents generally are 
unimportant, while the tidal currents of the North Sea create a periodic 
movement of water masses. In addition, the currents there are augmented, 
when the filling and emptying of tidal flat areas (Wattenraum) occurs 
through submarine canyons (Strom gaten), tidal flat streams and river 
outlets. 
Distinctions exist also between beaches with strong tidal or drift 
currents parallel to the coast with or without surge and those beaches with 
surge but without strong tide or drift currents. The surge itself often 
produces "a surge current" (Brandungstromung) parallel to the coast and 
of an intensity exceeding considerably that of the tide or drift current. 
Prior to 1860 only a few reporters were writing on the construction 
and effect of sea groins because not many such structures were in existence 
then, and consequently only a few observations were made and data collected. 
Gothilf Hagen was the first German coastal engineer to write on his 
experimentations with the sea groins on the Baltic Sea (1863). He traveled 
extensively in Holland, Belgium, England and other countries and presented 
his observations and the results of his investigations and experiences in 
a manual on coastal structures. A substantial part of his manual is valid 
up to this day. 
At the XVth International Navigation Congress of 1931 in Venice, 
considerable attention was given to the defense against the sea on shores 
with and without major sediment movement. German papers, by Coen Cagli, 
R. Schmidt and H. Heiser, presented there, played an important role during 
the discussion. German authors found then that a thorough and systematic 
investigation of the coastal zone is paramount for coastal protection and 
coastal structural planning. They found that up to this day no generally 
applied rule for sea groin construction is apparent, indicating the diffi- 
culties associated with the clarification of this complex problem. 
Often the most informative documents were simple published statements 
describing how the groin or group of groins were built, the construction 
material used, the dimensions selected and the experience gained from the 
construction. Occasionally we find that these data indicate the purpose 
and reason for the construction and also give information on the effective- 
ness of the installation. Of course, data on the effectiveness of groins 
are of limited value if collected immediately or soon after the completion 
of the structure. Usually a minimum period of 10 to 20 years is required 
for an objective evaluation of the groin effectiveness. Valuable informa- 
tion and experience was accumulated by manufacturers of building material; 
the bituminous material industry in particular. Recommendation regarding 
groin effectiveness made without a sufficient period of observation should 
be considered with some reservation. 
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