22 BULLETIN 193^ U. S. DEPARTMENT OF AGEICULTTJEE. 



bottom widths of the ditches by 8 feet for 14 to 25 foot widths, by 

 9 feet for 30-foot widths, and by 10 feet for 35 to 50 foot widths. All 

 bridges should be placed on concrete abutments. 



RAILWAY CROSSINGS. 



Culverts of 36-inch and 48-inch pipe are recommended where rail- 

 ways cross drainage ditches that will need to carry not more than 

 about one-fourth or one-third, respectively, of the computed capacities 

 of ditches with 2-foot bottom widths, as planned for highway cross- 

 ings. All other railway crossings are estimated as half- through plate 

 girder bridges on concrete abutments, with spans about equal to 

 one-haK the sum of the top and bottom widths of the drainage ditch. 



IRRIGATION CANAL CROSSINGS. 



As all irrigation canals consist of two levees between which the 

 water flows over the natural ground surface, these must be carried 

 over the drainage ditches at the crossings. It is impossible to deter- 

 mine the exact number of such crossings there will be, as many of the 

 present irrigation canals may not be in use when the drainage ditches 

 are constructed, and the data are not at hand to show where the 

 proposed ditches cross all the irrigation laterals. The cost has been 

 estimated for each crossing of a drainage ditch with a main irrigation 

 canal in use at the time of the survey, and the total for each district 

 has been included in the total cost estimates given on later pages. 

 Culverts of 36-inch and 48-inch pipe should be used for the small 

 ditches under the same conditions as explained for highway crossings. 

 Where it is expected that the drainage ditch wiU carry one-half the 

 computed capacity of the smallest ditch recommended, or more, the 

 irrigation canal is to cross the dramage ditch in a wood flume sup- 

 ported on timber bents or piles. Each flume should be about 10 feet 

 longer than the top width of the drainage ditch, thus allowing about 

 5 feet at each end for a firm bond with the bottom of the irrigation 

 canal to prevent leakage. 



PINE ISLA>fD BAYOU IMPROVEMENT. 



All trees and brush should be cleared from the old channel of Pine 

 Island Bayou for a strip 100 feet wide, which w^ill give the present 

 channel a capacity of 1,600 cubic feet per second, equivalent to about 

 0.6 inch run-off per 24 hours from the drainage area above the 

 Jefferson-Liberty County line. From the county line to Voth, such 

 a strip would contain 270 acres; clearing this would cost, at $40 per 

 acre, $10,800. Divided along districts Nos. 1, 2, and 4 in proportion 

 to their areas, this cost would amount to $1,605, $3,870, and $5,325, 

 respectively, or 27.7 cents per acre. 



TAYLORS BAYOU IMPROVEMENT. 



The drainage areas and computed maximum run-off for Taylors 

 Bayou at several points have been determined as f oUows : 



