16 A TREATISE ON DREDGES AND DREDGING 



made from the field notes. Either on this map or a similar one 

 the required depths are marked, hence by comparison the total 

 depths of material to be removed are easily calculated. A series of 

 cross sections can be taken at some distance apart and thus the 

 volume of the excavation between the consecutive cross sections is 

 determined. The amount of cubic contents between the various 

 cross sections will give the total amount of material to be removed 

 in the proposed work. The manner of calculating the cubic content 

 between the consecutive cross sections is exactly the same as for 

 land work, namely by the mean end area or prismoidal formula. 

 Generally only the former method is used. 



Place Measurement. Engineers have two methods for estimating 

 the amount of the dredged materials by place measurement and 

 by scow measurement. 



When specifications state that the amount of the dredged material 

 will be measured in place, it means that it will be deduced from 

 two hydraulic surveys made, one before the work begins and the 

 second after the improvement has been completed. The difference 

 between these two surveys will represent the total amount of material 

 removed to obtain the required improvement. In practical work 

 a series of transverse cross sections are drawn on the plan of the 

 proposed improvement, extending all through the width of the 

 work. The area of the cross sections indicating the amount of 

 material removed is given by the difference of the two hydraulic 

 surveys. 



Scow Measurement. The second method of determining the 

 amount of dredged material is by scow measurement. This consists 

 in measuring the capacity of the scows or boats that carry away 

 the excavated materials and deduce the total amount of the dredged 

 material from the number of boats employed in transporting the 

 materials to the dumping place. This method can be compared 

 to the one sometimes used on land in which the amount of the 

 excavation is deduced by tallying the vehicles that arc carrying 

 away the excavated debris. 



The scows used in connection with dredging are of several different 

 types but chiefly the deck and dumping scows. When deck BCOW8 

 are used, the hold of the boat is correctly measured by the engineers 

 of the two parties and the cubic content of the scow is then 

 for measuring the material removed from the bottom. When 

 the dumping scows are used for the transportation of the debris, 



