A Treatise On The Principles And Practice Of Harbour Engineering

ENTRÄNGE CHANNELS. 243 water. On ordinary occasions, however, the break is close to and on the bar ; and in what is considered a very heavy sea, the waves range between about 10 feet and 12 feet in height.” From whatever quarter they corne, the seas are broken on the shoal within the harbour and completely dis- sipated in the eastern wave basin. Entrance to Richmond River, N.S.W.1—The Richmond River is one of the most important rivers in Australia, serving a large agricultural and farming district. Owing, however, to the cultivation carried on along its bank, the flood waters are heavily charged with silt, and this naturally results in considerable deposits in the vicinity of the entrance. This dele- terious action is somewhat counteracted by occasional heavy floods, which scour the channel, although only a small percentage of the rainfall over the basin finds its way into the river, owing to the permeability of the soil and the natural reservoirs formed by swamps. The great drawback to the Richmond River has been the shifting character of its entrance, combined with a shallow bar and adjacent shoals. These evils were intensified by the conflict of the waters of North Creek with those of the main river at their point of meeting, between East and West Ballina. The position of the entrance has shifted through a distance of more than IL mile, and it has also been noted that floods have caused the channel to break through the Southern Spit on four occasions in thirty-five years. Thus, the navigation of the port has been dangerous at all times, and on many occa- sions impossible. In 1888, Sir John Coode was invited to report upon such means as were available for fixing the channel and regulating its width, so that the scour might be confined to a definite track of proper proportions; to neutralise the obstruction offered by certain rocks near the mouth of North Creek ; and to prevent the conflict of the waters from the North Creek with those from the main river. The remedial works executed, and in course of execution, are shown in fig. 218. .Some of these works have been carried out as designed; others have been somewhat modified in accordance with experience gained during the course of operations. The main features are sufficiently intelligible, and the only point calling for particular notice is the somewhat unusual addition of a middle training-wall. “The construction of the new middle training-wall,” says Mr Burrows, “ which reaches ordinary high-tide level for the greater part of its length, was determined upon by Mr Darley, then engineer-in-chief, at a time when the unfinished condition of other works in progress allowed a large sand-spit to form across the area between the mouth of North Creek and the south wall, and it was found necessary to train the tidal currents of the river at this place, so that the discharge at ebb-tide would tend to prevent the spit in- creasing the obstruction to navigation at the river entrance. Gaps or open- ings were left in the wall for the preservation of the old navigable channel 1 Burrows on Improvements at Entrance to Richmond River, Afin. Proc. Inst. C.E., vol. clx.