Hydrography, Sediments and Sedimentation in a Low-Energy Embayment, Knebel Vig, Denmark

The energy inputs, waves, tides and currents in the low-energy area, Knebel Vig, are examined. In the main part of Knebel Vig the available energy leads to deposition. In stormy periods this can amount up to 8.19 g m-2 day-'. Three distinctive sediment provinces can be recognized. The criteria involved are bottom sediment grain size, organic content and textural type. Introduction Water circulation in coastal embayments with a very small tidal range, such as in the inner Danish waters is principally related to the interaction of the coastal configuration, the bathymetry, the tidal current system and the offshore regional current system between the North Sea and the Baltic. The resultant current pattern is important in terms of sediment supply, transport and deposition. The aim of the present investigation in Knebel Vig is three-fold: (1) Determining the current patterns induced by interaction between coastal configuration, tidal currents and regional current, (2) investigating the interrelationships between observed bathymetry, sediment distribution and the hydrodynamics, and (3) studying the sedimentation processes in terms of the supply of clay, silt and sand. The relation between hydrodynamics and sedimentation in Danish waters has never received much attention. Most of the existing papers have dealt with the Danish Wadden Sea on the exposed and strongly tidal North Sea coast (Gry 1942, Hansen 1951, Hansen 1956, Olsen 1958, Jacobsen 1962, and Bartholdy 1979). The present study deals with a protected micro-tidal embayment on the Kattegat coast. Methods of study The detailed bathymetric data presented in Fig. 1 were derived by the authors from original fair charts produced by the Royal Danish Hydrographic Office. Water levels were registered at station K (Fig. 1) on a self-recording gauge. Wind velocity and direction were registered at station W (Fig. 1) at a height of 4 m with a self-recording Lambrecht Windmeter. The position of the vessel during sediment sampling and current measurements was plotted by means of Decca. The accuracy of the plots can be estimated to be within + 10 m. Information on currents was collected by means of direct-reading current meters: OTT type Muldedag and Braystroke type MK II. Current direction and velocity were also observed by following drifting logships on radar. Salinity and temperature were determined in situ with a salinometer, type Eil MC 5. Wave refraction diagrams are based on the method of the U.S. Army Coastal Engineering Research Center (1975). Stoke wave theory (Oviatt and Nixon, 1975) was used to compute the wave-induced velocities. Samples of bottom material were collected by the research vessel 'Genetica' from 11 locations in Knebel Vig (Fig. 1), using a Petersen grab. Additional samples were collected from 16 locations in shallow water using a Van Veen grab. Suspended material was collected in sediment traps modified after Oviatt and Nixon (1975). Each trap was placed I m off the bottom at three stations (Fig. 1) and provided four replicate samples. Core samples were taken at the same three stations using the Kyholm Corer (N0rnberg and Christiansen, 1980). Grain size analysis was carried out using standard sieving and pipette analyses. The textural types were identified using Folk and Ward's classification (1957). The 1 mean and Geografiska Annaler 63 A (1981) 1-2 95 This content downloaded from 40.77.167.44 on Thu, 23 Jun 2016 06:41:20 UTC All use subject to http://about.jstor.org/terms CHRISTIAN CHRISTIANSEN, HENNING CHRISTOFFERSEN and KNUD E. SCHULTZ