is allowed to pass through the fence and kee[) the silt in suspension, but does not make 

 maneuvering into and out of slips difficult; a solid-diaphragm fence would result in 

 undesirable eddy currents and perhaps de[josition of silt in the berthing area. A floating log 

 camel along the outside of the fence may be needed to fend off exceptionally heav}- floating 

 debris. Bank protection may also be advisable, both for protection against wave and current 

 scour and to prevent damage by floating debris and ice. 



One method of protection along a waterfront is to excavate a shallow basin into the 

 riverbank (Fig. 35). The main current will still follow the old riverbed, but some residual 

 current may flow through the basin. A series of piles along the old bankline will help keep 

 debris out of the basin. They can normally be the guide piles at the outer end of floating 

 piers, thus serving a dual purpose. If debris flow is heavy, an overlapping mole may be left at 

 the upstream end of side basin, enclosing as much of it as necessary. The banks of the mole 

 may have to be revetted to prevent erosion. The ultimate step in such a modification is to 

 leave the mole long enough to enclose all of the basin but the entrance, so that it becomes 

 an off -river basin. 



_ River Flow 



Guide Piles Act As Debris Deflectors 



Q=^i=Q 



± ±J 



Figure 35. Shallow boat basin marginal to a river. 



Another version of the off-river basin is the utilization of one arm of a river bifurcation 

 around an island as a harbor. Usually, the smaller arm is selected, allowing the main current 

 to follow the larger channel. Debris deflectors may be required where the river bifucates to 

 prevent large pieces of debris from entering the boat basin, or the upstream end of the 

 smaller arm may be diked off as shown in Figure 36. Some widening of the river arm 

 selected for harbor use may be necessary to obtain enough water area for the desired 

 number of slips. If the river carries a heavy silt load, such enlargement should be kept to a 

 minimum to avoid siltation problems. 



78 



