CONSTRUCTION OF COAST DUNES 181 



The height of the artificial dunes in this region is usually 33 feet. Lafond 



says: 



"If higher, they cost much more to estabUsh and are more difficult to maintain, and 

 they give more of a lever to the winds, moreover, if they are built too high. Usually 

 a lower height is sufficient." 



From Point Arvert to Point Coubre (Charente Inferieure Department) 

 the height of the artificial dune is usually 23 feet. At Requin it is 

 extremely variable, being from 6.5 to 65 feet, and at Volcan 43 feet. From 

 the Tournegand Canal to Palmyre it is 16 feet in height and from there 

 on to the Grande-Cote it is but 13 feet. This shows how the height of the 

 artificial dune must vary with the local conditions. 



The destruction of these littoral dunes comes either from the wind or 

 from the sea. The wind is the commonest danger but the sea the most 

 difficult to combat. According to Lafond: 



" A littoral dune not too high and bordering a permanent beach is on the whole 

 easy to maintain. It is sufficient to keep the maram grass plantation (executed at the 

 time of construction) in good condition and to maintain its original density on the 

 different parts of the dune surface; the beach sand blown by the wind slides along the 

 dune where it is scattered as nourishment for the clumps of maram grass. The ex- 

 cess amount passes behind the dune and is scattered in the littoral hollows (ledes). 

 It does not cause any damage, however, because it is only a smaU amount and covers 

 the soil so slowly that the brush or shrubs can grow as fast as the sand covers the soil 

 and also keeps it fixed." 



It is not nearly so easy to maintain a high protective dune at Coubre. 

 Here the sand accumulates at certain points and forms hummocks which 

 must be fixed immediately. " The formation of hummocks is avoided 

 by not allowing the maram grass bunches to grow too thick and by 

 removing every obstacle on the dune." 



If hummocks of sand are formed their summit has to be broken so 

 as to make the sand mobile and allow the winds to blow it away. If 

 these irregularities can be avoided then, so far as the wind is concerned, 

 the artificial dune can maintain itself. 



Lafond says a dune can always be kept in good condition by means of 

 maram grass plantations judiciously placed so as to hold the sand in the 

 depressions and let it blow over the humps so as to have nothing but 

 regular slopes or long undulations. 



When the littoral dune is washed by the waves during storms it usually 

 suffices to build barriers to retain the sand in place and permit it to re- 

 sume its original shape. 



"If the breech is quite considerable, to smooth it over the sand is topped by means 

 of fagots planted in quincunx; often these quincunx are placed in two barrier lines, 

 the one completing the action of the other. Where the erosion by the water is caused 

 by dangerous currents then it is a very serious undertaking and masonry or expensive 

 cement work is often necessary." 



