8 BULLETIN 114, U. S. DEPARTMENT OF AGRICULTURE. 



RUN-OFF. 



The "run-off" from a natural drainage basin is generally . under- 

 stood to be that water which flows from the area through the surface 

 outlet channel. In designing drainage improvements, consideration 

 must be given to the maximum rate of run-off for which provision 

 should be made rather than to the total quantity to be removed. The 

 principal factors affecting this rate in the Black and Boggy Swamps 

 drainage district are rainfall, soil, and topography. 



RAINFALL. 



There are no stations of the United States Weather Bureau within 

 this district, although it is well surrounded by stations at which con- 

 tinuous precipitation records have been kept for years . From a s tudy 

 of the records secured at Allendale, Walterboro, and Yemassee, S. C, 

 and Statesboro, Ga. (see fig. 1), the rainfall in the Black and Boggy 

 Swamps district can be learned with sufficient accuracy. These 

 daily records for the years 1903 to 1912, inclusive, have been carefully 

 compared with each other and with the daily records secured at 

 Whiteville, N. C, for comparing this district with the Chadbourn and 

 Lyon Swamp districts as explained in the following paragraphs. 



PROBABLE RATE OF RUN-OFF. 



Drainage districts where conditions are similar to those in the 

 Black and Boggy Swamps drainage district have been completed in 

 southeastern North Carolina and have been in operation for sometime, 

 but no measurements of flood run-off from these ditches have been 

 made. These districts, however, afford the best guide in designing 

 the ditches for this district. At Chadbourn, N. C, the rainfall, topog- 

 raphy, evaporation and transpiration of plants, climate and seasons, 

 the natural reservoir and storage capacities of the streams, and the na- 

 ture of the vegetable growth are very nearly the same as in the district 

 under discussion. Several ditches which drain areas varying in size 

 from 400 to 2,150 acres have been constructed in that locality and 

 have been in operation for about two years. The principal difference 

 between conditions at Chadbourn and in Black and Boggy Swamps 

 district is that the soil at Chadbourn is a sandy loam 3 to 8 inches deep 

 underlain by a clay or clayey sand that is quite permeable and which 

 will absorb water very readily; this, as compared with the soil in the 

 district under discussion, will give a low run-off. The ditches at 

 Chadbourn were designed to remove 1 inch in depth of water from 

 the entire watershed in 24 hours when running bank full. Where 

 those ditches were constructed as designed they are working satis- 

 factorily. 



In Bladen and Pender Counties, N. C, the Lyon Swamp Canal, 

 which drains an area of about 18,000 acres, has been completed for a 



