CHAPTER V. 



HYDROGRAPHIC SURVEYS. 



HYDROGRAPHIC SURVEYING comprises that part of river surveying which has to 

 do with the water itself. It may be treated under the following heads: 



(1) Soundings. 



(2) Discharge. 



(3) Slope. 



These comprise such matters as depth, velocity, sectional area, material held in 

 suspension, location of obstructions, etc., and such surveys are generally made in con- 

 nection with a topographical survey of the banks. It is, however, necessary on some 

 large streams with shifting channels to make frequent hydrographic surveys in certain 

 localities purely for navigation purposes. 



(l) SOUNDINGS. 



Cross-section. The discharge which a stream has at a certain height of water is 

 usually found by taking a cross-section at right angles to the direction of the current 

 and multiplying the submerged surface or wetted perimeter of this profile by the mean 

 velocity of the current, according to formula. The section should be taken at a point 

 where the river is of uniform width and of regular form for some distance above and 

 below, so that the directions of the currents may be practically parallel, thus avoiding 

 oblique velocities. As sections of a river-bed are quite irregular and would not be the 

 same at points but little separated, it is usually best to take several sections at estab- 

 lished intervals, and thus secure a mean cross-section. The stage of water at the com- 

 mencement of the work, together with changes which take place during its progress, 

 should be known and recorded. 



When the soundings have been completed, and levels taken for that portion of 

 the river-bed which lies above the water at the time of sounding, the whole can be plotted 

 and the area determined for any stage proposed. The points at which soundings are 

 taken along the range-line should be accurately located, so that each day's notes may 

 be referred to the same ground. 



Methods. By a Cord Across the Stream. Where the river is not too wide, a strong 

 cord, which has been well stretched and wet for some time, or a wire, may be stretched 



from shore to shore. The latter is much to be preferred, however, as a cord will stretch 



25 



