390 REPORT OF OFFICE OF EXPERIMENT STATIONS. 



INTERNAL DRAINAGE. 



After the embankment is completed a system of internal drainage 

 must be constructed to collect the water and lead it to the tide gate 

 or pumping plant. On most tracts there are old channels that can 

 be used to advantage in planning a system of internal drainage that 

 will lessen the cost of excavation. Since the natm'al drainage is 

 intermittent, effective only at low tide, it must be supplementexl 

 by a large storage capacity. Hence the ditches should be made 

 wider than would otherwise be necessary. The removal of one-half 

 inch of rainfall in twenty-four hours from a comparatively level tract 

 of land, when the rainfall does not exceed 60 inches per annum, has 

 •proved to be adequate for the successful culture of field crops. If 

 the tide gates are made of ample capacity to discharge all the water 

 from the storage basin at each low tide (twice in twenty-four hours), 

 ditches having a capacity for storing 0.25 inch of rainfall from the 

 entire watershed without raising the water in the ditches more than 

 1 foot will be sufficient to properly drain the land for either meadow 

 or cultivation. Since most of the salt marshes are practically level, 

 the ditches vdW have but little grade and will depend upon the hych-au- 

 lic head to induce a current. As the water nearest the gate will be 

 discharged in the shortest time, it is expedient that the greater part 

 of the storage capacity be as near the outlet as practicable. When 

 the gate opens, this portion will be discharged at once, and should 

 it close before the ditches are entuely empty the water in the more 

 remote laterals will flow into the'basin just formed, thus holding the 

 water down to the lowest level to be obtained without pumping. 



In planning a system of ditches it should be the aim to locate them 

 in such a way as to leave the land in the best possible shape for culti- 

 vation. Where there is high land that drains into the marsh, an 

 intercepting ditch should be cut along the foot of the slope to lead the 

 water into the storage basin without overflowing: the low land. The 

 depth of the ditches required and their distance ajmrt depend largely 

 upon the character of the soil. The main ditch shoidd be as deep as 

 the sill of the tide gate, while the laterals should range from 2h feet 

 deep at the upper end to 3 or 3^ feet at the lower end, according to the 

 depth of the outlet channel. In most places ditches in marsh soil will 

 stand with almost ])erpendicular banks, owing to the fibrous roots in 

 the material, so to increase the storage ca])acity they should have 

 wide bottoms, with bank slopes one-half horizontal to 1 perjien- 

 dicular, imless jilaces should be found where the material requires a 

 greater slo])e. It is not jiracticable to give sj)ecifi(' (Ui-ections for 

 laying out a system of internal drainage, as each marsh must be viewed 

 and treated according to its shape and peculiar location. 



