Editing Jetty (section)

===At the mouth of tidal rivers===
Where a river is narrow near its mouth, has a generally feeble discharge and a small tidal range, the sea is liable on an exposed coast to block up its outlet during severe storms. The river is thus forced to seek another exit at a weak spot of the beach, which along a low coast may be at some distance off; and this new outlet in its turn may be blocked up, so that the river from time to time shifts the position of its mouth. This inconvenient cycle of changes may be stopped by fixing the outlet of the river at a suitable site, by carrying a jetty on each side of this outlet across the beach, thereby concentrating its discharge in a definite channel and protecting the mouth from being blocked up by littoral drift. This system was long ago applied to the shifting outlet of the [[river Yare]] to the south of [[Great Yarmouth|Yarmouth]], and has also been successfully employed for fixing the wandering mouth of the Adur near [[Shoreham-by-Sea|Shoreham]], and of the Adour flowing into the [[Bay of Biscay]] below Bayonne.

When a new channel was cut across the [[Hook of Holland]] to provide a straighter and deeper outlet channel for the river [[Meuse (river)|Meuse]], forming the approach channel to [[Rotterdam]], low, broad, parallel jetties, composed of [[Fascine mattress|fascine mattresses]] weighted with stone, were carried across the foreshore into the sea on either side of the new mouth of the river, to protect the jetty channel from littoral drift, and cause the discharge of the river to maintain it out to deep water. The channel, also, beyond the outlet of the river Nervion into the [[Bay of Biscay]] has been regulated by jetties; and by extending the south-west jetty out for nearly {{convert|0.5|mi}} with a curve concave towards the channel the outlet has not only been protected to some extent from the easterly drift, but the bar in front has been lowered by the scour produced by the discharge of the river following the concave bend of the southwest jetty. As the outer portion of this jetty was exposed to westerly storms from the Bay of Biscay before the outer harbour was constructed, it has been given the form and strength of a breakwater situated in shallow water.<ref name="EB1911" />