20 BULLETIN 71, U. S. DEPARTMENT OF AGRICULTURE. 



blow for any considerable lengtb of time directly on shore may cause a rise of 

 several feet in the water. Such rises are commonly called storm tides. Their 

 effect is so great that often they reverse the ordinary tide, and the maximum 

 height of water may be reached at the usual time of low tide. Storms of this 

 character usually occur during the months of August, September, and October, 

 and are known as tropical hurricanes. Those that are 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 ex- 

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

 most characteristic as well as one of the most severe storms ever experienced 

 in this section occurred September 20, 1909. 1 It produced high tides all along 

 the coast and gave the maximum tide at the mouth of Bayou Terrebonne, about 

 40 miles west of New Orleans. 



As it was apparent that this storm was very general in character and caused 

 high tides all along the coast, Mr. A. M. Shaw, who at that time was carrying on 

 investigations in this section for the Department of Agriculture, made special 

 efforts to ascertain the maximum height reached by the tide, and for this pur- 

 pose visited points all along the coast. The height of the tide was ascertained 

 by the combined testimony of all reliable parties who were on the ground and 

 by high-water marks on buildings. Only information from reliable sources was 

 accepted. To quote from Mr. Shaw's report : 



The high-water marks obtained were fairly consistent, and were quite gen- 

 erally above any authentic high-water mark in the same locality. 



Figure 1, page 2, shows the section under discussion, with the maximum 

 heights of water above mean tide. With few exceptions this was the highest 

 tide ever experienced on the coast, although more violent storms of limited 

 extent have been recorded. Commenting on these high-water marks Mr. Sbaw 



These may be considered authentic, excepting that the heavy seas that swept 

 over the site of the camp at Sea Breeze made it impossible to get an accurate 

 record at this point. The record shown of 15 feet may be in error 2 or 3 feet. 



In regard to the extreme height at this point, he further says : 



An examination of the accompanying map will show that the mouth of Bayou 

 Terrebonne lies between two large bays, and it is possible that this unusual 

 rise may be accounted for, at least in part, by the sudden veering of the wind, 

 thus concentrating the extreme tides of both bays on the central point. The 

 foregoing report of the progress of the storm at New Orleans shows that such 

 a sudden change in the direction of the wind did occur at that point and a 

 similar decided change was described by a number of persons on both Bayous 

 Terrebonne and La Fourche, all of the accounts agreeing that this change took 

 place while the storm was at its worst. 



An examination of the conditions at this point on the coast shows further 

 reason for this extreme height of tide. The watejr in both Timbalier and Terre- 

 bonne Bays is comparatively shallow, ranging in depth from 4 to 12 feet. It 

 has been the observation of the United States Coast and Geodetic Survey that 

 where the water is shallow for a great distance offshore, heavy winds cause a 

 flow of the entire depth of water, thus allowing no undertow. This action would 

 give the water a tendency to pile up wherever it encountered an obstacle. This 

 explanation is further supported by the fact that on more exposed portions 

 of the coast, but where tbe water was deeper, the tide was not nearly so high. 



1 For a minute description of this storm at New Orleans, see U. S. Dept. Agr., Monthly 

 Weather Review, 1909, p. 623. 



