RULES AND PRACTICES 35 



The tabulations in the Bridge Book list vertical clearances referred to low water and 

 high water. However, the vertical clearance notes at the front of each part of the book give 

 the following information : 



Tidal Waters — The cleaxance shown refers to Mean High Water and Mean Low Water, 

 except in regions of diurnal inequality where the Mean Lower Low Water clearance may 

 be given. 



Nontidal waters— The clearance shown at high water refers to the stage at which navigation 

 generally ceases. 



The latter category shall be treated individually, such as the Columbia River where charted 

 clearances are referred to Columbia River Datum (MLLW during lowest river stages) . Chart 

 6151 clearances are referred to MLLW. 



Clearances above dams are referred to the datum of soundings which is ordinarily the 

 Normal Pool Elevation. 



Normal Water Surface elevation on certain rivers is, for aU practical purposes, to be 

 considered MHW. 



Notes regarding clearances at bridges, etc., must conform to the following standards: 

 (6 pt. No. 3 L.C.G.C. Caps) 



OVERHEAD CABLE CROSSINGS (Fig. 1) (H^) 



Overhead cables shall be indicated by a dashed line with clearance referred to MHW*. 

 The clearance authorized by the Corps of Engineers shall be used for charting rather than the 

 actual or "as-built" clearance. Cables known to carry high voltages are to be labeled, thus: 



OVERHEAD POWER CABLE 

 AUTHORIZED CLEARANCE 40 FEET 



♦Clearances on Columbia River Charts are referred to Columbia River Datum (MLLW during lowest 

 river stages). Chart 6151 clearances are referred to MLLW. 



For reference to datum other than MHW, see above Bridge Clearance notes. 



A card file (Form C&GS-961) of cable clearances is maintained in the same manner as for 

 bridge clearances. The fUes are compiled from information furnished by the Corps of Engineers 

 in the form of construction permits. 



The appUcation of overhead cable data is the same as that previously described for bridge 

 clearances. 



