DEAIISrAGE OF JEFFEESON COUNTY, TEXAS. H 



FACTORS AFFECTING RUN-OFF. 



The amount of run-off depends upon the rate and amount of rain- 

 fall, modified by the size and form of the drainage area, the slopes 

 of the ground, the arrangement of the watercourses, the nature of 

 the soil, the transpiration of plants and evaporation, the natural 

 storage reservoirs, and the drainage improvements within the water- 

 shed area. In the following discussion the rate of run-off is expressed 

 in inches depth over the entire drainage area removed in 24 hours, 

 or in cubic feet per second, commonly abbreviated to "second-feet." 

 On the proper determination of the maximum rate of run-off that 

 the drainage works should remove will depend largely the economy 

 and efficiency of those improvements. Since the run-off is dependent 

 upon several factors of variable values, the determination of the 

 proper rate requires wide experience and mature judgment. 



RAINFALL. 



It is usual to determine first the intensity of the most severe storm 

 against which protection should be provided. The rainfall records 

 secured by the United States Weather Bureau in Jeft'erson County 

 and the vicinity show a very uneven distribution of the precipitation, 

 with storms of great intensity. More rainfalls and more heavy 

 storms occur from March to September than during the rest of the 

 3^ear, usually; therefore overflow is the more likely to occur during 

 the growing season. The average annual precipitation at Beaumont 

 during the past 15 years is about 42.6 inches, at Galveston 46.3 inches, 

 and at Lake Charles 53.9 inches. 



Some of the most notable storms recorded at Beaumont during 

 those years are as follows: October 14, 1902, 9.26 inches; November 4, 

 1902, 6.25 inches; and November 31, 1902, 6.87 inches. Lesser storms 

 during and immediately following these several precipitations must 

 have kept tlie ground saturated. June 25, 1905, 6.51 inches fell, 

 preceded and followed by heavy rains, and October 14, 1906, 5.12 

 inches of rain occurred. During May, 1907, 19.40 inches of rain 

 were recorded, 13.3 inches of which fell in two storms on the 23d 

 and 30th. April 14, 1908, 5.80 inches rainfall occurred, preceded 

 and followed by lesser storms, and during the first 10 days of July of 

 tlie same y(!ar 9 inches fell. It will bo noted that these excessive 

 storms are of infrequent occurrence, but the records from all the 

 stations show that from 1892 to 1912, between March and October 

 each yciiir ilu^n; occurred one or more storms of 3 to 4 indies at times 

 when tli(; gnjiind must liave \)VA)n W(ill saturated, and that heavier 

 storms were rather local and not general over the county. 



In vi(!W of the facts stated above, it seems wise to desigtv the drain- 

 age- ini|ti-ov<an<',nts for this county to care for the immi-oIF from a rain- 

 fall «jf t inches in 2 1 h(;urs. At vv-iy infrequent intervals the ditches 



