DRAINAGE OF JEFFERSON COUNTY, TEXAS. 13 
values is considered the V^' ^^r ^l^*'^t unit. The vfiluo 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 basin 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 -yflK For land as level as some of the pumping 
districts the slopes of the water surfaces in the ditches must be used 
for computing V-^'- 
The value of M for any dramage 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 varymg conditions. 
Tlie value of C, since it can not be measured directly, is to bo com- 
puted by solving the formula for conditions where all the other 
factors, including B and Mj are known. A sufficient number of 
repetitions of this computation covering 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 
soil, 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 Hillebrant Bayou 0.66 was found 
for 1/ and 1.56 for G. This difference 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 the 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 
County w^hich can be wholly or partially drained by gravity consists 
in. (1) dividing that part mto its natural drainage units; (2) straight- 
ening and enlarging all the present watercourses that will become the 
main outlets or arteries for a complete drainage system; and (3) con- 
structing systems of parallel ditches, spaced one-haK mile apart and 
