DEAINAGE OF JEFFERSON COUNTY, TEXAS. 13 



values is considered tJie V^ foi' that unit. The value of -yjF for 

 the whole drainage basin is the mean of the values for the separate 

 units, each weighted according to the area of the unit. Special care 

 is required in dividing the drainage units for a basui consisting of 

 lands rising quickly from a main channel of small slope, that the 

 large slope of the lands near the outlet may not have undue weight 

 in the final value of -^[W. For land as level as some of the pumping 

 districts the slopes of the water surfaces in the ditches must be used 

 for computing -ypF'. 



The value of If for any drainage area is affected by several factors, 

 as noted on page 12. This ratio has been determined by stream 

 measurements for certain areas outside Jefferson County, and the 

 values for the various districts in this county were calculated by 

 comparison with known data, estimating the combined effect of 

 evaporation, seepage, and percolation as 0.1 inch per day and making 

 due allowance for other varying conditions. 



The value of C, since it can not be measured directly, is to be com- 

 puted by solving the formula for conditions where all the other 

 factors, including D and M, are known. A sufficient number of 

 repetitions of this computation covermg several drainage areas 

 having similar soil characteristics should determine reliable values 

 for C if the various modifying factors are correctly involved in the 

 formula. Quite a number of such computations have been made, 

 which have given results quite uniform for each type of soil considered. 



The value for C, determined in the manner just indicated, for clay 

 soU, such as most of that in Jefferson County, is approximately 1.50. 

 Using 0.87 for M, found in the manner described, for Taylors Bayou 

 at the points where the run-off measurements were made, C is com- 

 puted as 1.49. For the gaging of HHlebrant Bayou 0.66 was found 

 for Jlf and 1.56 for C. This difTorence in these two values for J/ is 

 due principally to the great difference in the size of the areas drained. 

 The value of C computed from tlie gaging of Brays Bayou near 

 Houston agreed well with the values given above, so 1.50 was used 

 for C in the computations for the Jefferson County ditches. The 

 maximum rate of run-off in 24 hours of course would not exceed the 



rainfall. 



PROPOSED PLAN OF DRAINAGE. 



The general plan proposed for the drainage of that part of Jefferson 

 Connty which (;an be wholly or |)urtiiUly (Iniiiiod by gravity consists 

 in (1) dividing that part into i(,s iialural (haiiiage unils; (2) straight- 

 ening and enlarging all tlie present watercourses that will become the 

 main oiiflcts or arteries for a complete drainage systeMi; and (.'{) con- 

 structing sysl(!riLs of j)urall(^l ditches, spaced one-hull" mile a|)arl/ and 



