Agger

The phænomenon of long continued high water or double high water in estuaries is, we believe, not uncommon. [...] The first high water appears to be considered by the Dutch, in common language, as the real high water, and the second is called the agger, but so little difference is there between them, that, in making the extensive series of simultaneous tide-observations proposed by Mr. Whewell, the agger was sometimes observed for high water.

A double tide or agger is a high tide consisting of two maxima of nearly the same height, separated by a relatively small depression; or a low tide consisting of two minima separated by a relatively small elevation.

[page 62:] As can be seen [...], once the water has reached the lowest level, it remains low for some time. After rising slightly, it then drops again and a second low tide follows. Only then does the water rise quickly and the flood sets in. A slight rise like this followed by a drop is called an agger or a double tide. […] [page 279:] […] these 'overtones' may cause the phenomena of agger or double high water […]

By the Augustan period, however, the old ashlar circuit with its earthen agger was beginning to be dismantled in some places.

But the Danes remained resolute in defence - largely thanks to a spirited display by captain Daniel Agger - and they went ahead with their first meaningful attack.