WET LANDS OF SOUTHERN LOUISIANA. 19 



are all dependent on this Levee system, arc among the oldest and most uniformly 

 successful in the country. Engineers who have made a detailed study of the 

 levee system are certain that with its completion this entire area will be pro- 

 tected from overflow of. the Mississippi River. 



The above remarks apply only to the alluvial section of the State, as very 

 little land west of the Atchafalaya River is affected by Mississippi overflow. 

 Some of the larger streams, such as the Calcasieu and the Sabine, flood the 

 alluvial flats immediately along their banks to a depth of perhaps 4 or 5 fool, 

 hut as a whole the wet prairie lands of the western portion of the coast are 

 froo from river overflow. 



Tidal Overflow. 



The daily range of tide along this portion of the Gulf coast is small, the 

 average being from 0.5 to 1.5 feet. However, as is true of all low, flat coasts 

 bordering on wide areas of comparatively shallow water, heavy winds blow- 

 ing for any considerable time directly on-shore may cause a rise of several feet 

 in the water. Such rises commonly are called storm tides. Their effect is 

 s<> great that often they reverse the ordinary tide, and the maximum height 

 of water may be reached at the nsual time of low tide. Storms of this character 

 usually are confined to the months of August, September, and October, and 

 are known as tropical hurricanes. Those severe enough to cause large rises 

 in the tide occur at comparatively long and irregular intervals. Sometimes 

 they will affect only a comparatively small portion of the coast line, while at 

 other times a general rise of several feet will be recorded all along the coast 

 line, with a limited region where the storm center strikes the coast experiencing 

 a tide of perhaps twice the height of the general rise. 



Within the period covered by the investigations three characteristic storms 

 caused abnormal tides along the Louisiana coast. These occurred, respectively, 

 on September 20, 1909; August 16-17, 1915; and September 29, 1915. 1 H*gh- 

 water records for these storms were collected and are shown on figure 1, the 

 heights of water caused by the different storms being given in distinctive 

 symbols. Where only one height is given, it is much greater than that for 

 either of the other storms. While the centers of these storms did not follow 

 the same path, the effects of all three were felt in varying degree on the en- 

 tire coast. It is difficult to draw any general conclusions in regard to tides 

 caused by tropical hurricanes. However, it is believed that it will be of value 

 to summarize the facts that were observed in the very detailed examinations 

 that were made. 2 



The highest tides and the greatest damage seem always to be east of the 

 track of the storm center, and the effect of the disturbance usually extends 

 farther to the east than to the west, although differences in character and 

 shape of coast line sometimes will make the opposite true. Regions fairly 

 close to the track, whether to the east or west, will suffer. The height of the 

 tide depends on the intensity, duration, and direction of the wind and on 

 the shape and exposure of the coast line. The offshore depth of water also 

 will have some influence and the nature and amount of vegetation on the 

 land will have a great effect on the heights at points not immediately on the 

 shore line. Funnel-shaped bays facing directly toward the hardest wind are 

 likely to have the highest tides, as shown by the records at the upper end 



1 For detailed descriptions of these storms see U. S. Dept. Agr., Monthly Weather 

 Review for the months in which the storms occurred. 



2 For a full report of the investigations on storm tides along the central Gulf Coast, 

 see special report of C. W. Okey, of September, 1916. 



