L'O BULLETIN 652, U. S. DEPARTMENT OF AGRICULTURE. 



of Timballier and Terrebonne Bays and at the southwest end of Lake Pont- 

 chartrain. Low, barren strips of land like thai near Port Eads, and islands 

 like Timballier, which allow the water to pass on across without much obstruc- 

 tion, are not likely to experience as high tides as land which is higher or 

 covered with trees, or as that portion of the mainland which is covered with 

 a heavy coat of tall grass. 



The rising water Hows inland with considerable velocity and piles up on any 

 obstruction, such as a railway embankment or a strip of heavy timber. Such 

 an obstruction will delay the progress of the water until it rises sufficiently 

 to pass over or around the obstacle. Usually sufficient time is not allowed for 

 the water to assume anything near a level stage over any considerable area, 

 and great differences of level are to be expected within a short distance, 

 especially near the limit of overflow or where the water is obstructed. This is 

 shown by the heights observed south of Houma, on Bayou Terrebonne, and 

 between Michaud and Lee Station, on the Louisville & Nashville Railway. Quite 

 often there occurs a shifting or even a reversal in direction of the wind as the 

 water reaches its highest stage. This will cause great local differences in 

 stage. Points a long distance from bodies of open water do not experience 

 overflow from tides, as the water does not have sufficient time to travel inland 

 before a change in the wind occurs, and heavy strips of timber retard the flow 

 to a remarkable degree. 



No method of forecasting the maximum height of water to be expected at a 

 given point seems, possible. However, as the highest stages recorded in the 

 last three storms are the highest yet experienced in those comparatively limited 

 areas, it is not likely that they will be exceeded soon. In the areas removed 

 from the centers of these storms higher tides may be caused by storms striking 

 the coast at other points, and future storms undoubtedly will cause fides 

 which show a different variation, as between the various localities, from those 

 already experienced. It is suggested that in estimating the height of tide to 

 be expected at a given point an area in the region of maximum recorded tides 

 which is similar in all features to the one in question be selected for compari- 

 son. As future storms may exceed those of the past in both intensity and 

 duration, a margin should be allowed for safety. 



From an examination of the heights reached by storm tides, as shown in 

 figure 1, it would appear that reclamation districts on many portions of the 

 coast will be compelled to build levees not only to keep out the waters of the 

 surrounding swamps, but also to prevent tidal overflow in times of storm. 

 The heights of tide indicated on this map are the highest experienced since 

 the country has been settled, and they should govern the heights of levees in 

 the various localities. Areas well inland from large bodies of water connect- 

 ing directly with the Gulf need not expect to feel the effect of storm tides. 



In connection with the generaj problem of protection from tidal overflow! 

 the plan of providing a protection levee for the whole coast line, rather than 

 that of constructing individual levees for each district, has been considered by 

 local engineers. The feasibility and cost of such a plan could be determined 

 only after a complete survey of the district had been made and a compre- 

 hensive plan had been carefully worked out. However, some general features 

 can be stated to give an idea of the nature of the problem. The larger the 

 lover- district, other things being equal, the less the cost per acre for levee; 

 for the length of the levee per unit of area, for districts which are roughly 

 square, varies as one divided by the square root of the area. Moreover, doing 

 work along broad lines and handling earthwork in large quantities wijl reduce 

 1 he unit costs of construction. In fixing the outlines of such large areas advan- 

 tage can be taken of many natural features of the land to be protected, which 



