FLOODS — BROOKS AND THIESSEN 347 



THE SOUTHERN APPALACHIAN AND SOUTHEASTERN FLOODS OF 



JULY 1916 



Two slowly moving hurricanes went to pieces in the southern Appa- 

 lachians July 10 and 15. The combination of exceedingly moist 

 tropical air converging at hurricane velocities for several days over 

 the southeastern United States produced rainfalls generally in excess 

 of 8 inches, and over considerable areas in excess of 20. Altapass, 

 N. C, where topography augmented normal hurricane processes in 

 forcing air upward, recorded a fall of 22.22 inches in 24 hours. 38 

 Excessive though this was, it is far exceeded by the 24-hour fall of 

 45.99 inches, with which an 88-inch four-day rainstorm began, as a 

 severe typhoon reached Baguio, in the highlands of Luzon, P. I., 

 July 14-15, 191 1. 37 The maximum 5- and 10-day rainfalls in 1916 

 were 24.45 and 31.07 inches. 



CONCLUSION 



Great floods may occur in the eastern United States at any time of 

 the year, for there is always an extensive warm-water surface near 

 by, from which great volumes of vapor may be transported, while, 

 not far distant, throughout the year there are cold surfaces to furnish 

 moving wedges of cold air to elevate the tropical air. The polar air 

 masses engage the tropical masses normally every few days, and rains 

 fall. The great floods occur, however, only when a persistent high 

 over the western Atlantic sends the tropical air inland for days in 

 succession, and then only when the polar masses meet and elevate the 

 tropical air over the same area or region for one to several days in 

 succession or on repeated occasions. Topography plays an important 

 role in determining where slowly moving fronts will stall: sometimes 

 even minor divides are effective. Snow cover seldom contributes 

 much to great floods, though by helping to keep cold air masses cold 

 it may contribute to stalling and to the maintenance of the tempera- 

 ture contrast between the cold and the warm air masses. 



What can man do about great floods? 38 He cannot change the 

 amount of rain that will fall; but he can change the rate at which it 

 will run off. He cannot change the amount of snow that will fall; but 

 by dense evergreen foliage he can hinder it from reaching the ground 

 and, once on the ground, from being converted into rapid run-off. 

 Once the water is running off, he can hold back some of it by numerous 

 dams on small streams and by larger dams below; and, by adequate 



«« Henry, A. J., Floods in the East Gulf and South Atlantic States, July 1916. Monthly Weather Rev., 

 vol. 44, pp. 466-476, 1916. 



37 Cf. Co-Ching Chu, Distribution of precipitation in China during the typhoons of the summer of 1911. 

 Monthly Weather Rev., vol. 44, p. 446. 1916. 



• 8 A symposium on recent eastern floods and the national aspects of flood control held in Pittsburgh Octo- 

 ber 13 and 14, 1936, is summarized in Civil Engineering, vol. 7, pp. 25-32, 1937, and published in full in the 

 March issue of Proceedings of the American Society of Civil Engineers. 



