THE INFLUENCE OF WAVES ON THE HYDRAULICS OF SEA OUTFALLS

Most of the larger cities in Denmark are located near the sea. About 60% of the sewage is therefore discharged through sea outfalls. Most of these sea outfalls were built in the seventies. In most cases a plastic pipeline were used, and the pipeline was ballasted with armoured concrete blocks and burried in a trench 1 - 2 m underneath the seabottom. To give the necessary initial dilution a diffusor with one or several contracted, horizontal outlets ends the pipeline. The Danish coasts are in general shallow and exposed to waves. The littoral sanddrift is considerably and the sea outfalls need to cross one or more bars. The diffusors are often placed outside the bars, but nevertheless sediment transport can occur around the diffusor. Figure 1 gives some typical data for Danish sea outfalls.