DUNE BUILDING AND STABILIZATION WITH VEGETATION 



by 

 W.W. Woodhousej Jr. 



I . INTRODUCTION 



This report describes the state-of-the-art of utilizing natural 

 processes in the growth and protection of coastal dunes. These dunes 

 may appear in the form of barrier dunes or dune fields near the shore or 

 as dune ridges and dune fields of various kinds farther inland. This 

 report deals primarily with the barrier-type dunes that form in contin- 

 uous ridges, usually parallel to the beach, and tend to act as barriers 

 to storm tides and waves (Fig. 1). 



1. Formation . 



Barrier dunes are usually formed through the trapping of sand by 

 diine vegetation. This vegetation has the special adaptation that enables 

 the dunes to establish, grow, and trap sand in a harsh environment. In 

 the absence of such vegetation, the sand tends to drift into migrating 

 or "live" dunes and dune fields that move back and forth with the wind. 

 Where onshore winds dominate, the dunes coalesce into large sand masses 

 and migrate inland, engulfing everything in their paths (Fig. 2). This 

 also occurs when the vegetation on stabilized dunes is seriously damaged 

 or destroyed. Extensive areas of migrating coastal dunes have developed 

 on various coasts because of fire, lumbering, grazing, and foot and 

 vehicular traffic. Migrating dunes were a concern of the colonists on 

 Cape Cod as early as 1714 (Westgate, 1904). 



Barrier dune formation by vegetation usually begins with the entrap- 

 ment of seeds or plant material in debris deposited back of the beach 

 berm by storms or very high tides. Initially, these may develop as a 

 series of small dunelets formed around individual seedlings or clumps of 

 seedlings, and enlarge and eventually grow together into a continuous 

 dune (Fig. 3). The irregular front is later cut back to form a dune 

 "line" by storm surge and waves. In more favorable circxamstances, large 

 nifflibers of seedlings establish in debris along a continuous drift line, 

 and grow a continuous dune from the start. 



In either case, the natural development of a substantial barrier 

 dune depends on a fortuitous sequence of events and circumstances: the 

 deposition of debris plus sand at the right time and place, followed by 

 suitable seeds or plants and the continued deposition of sand, together 

 with a year or two of favorable growing conditions without excessively 

 severe storms. However, the complete sequence does not occur on most 

 coasts every year and is very rare on some coasts. The early phases 

 frequently develop on many coasts, but the embryo dunes are usually 

 destroyed by storm activity before gaining sufficient mass and eleva- 

 tion to survive normal weather events. However, it is a process which 

 can be simulated, managed, and utilized as an engineering tool (Fig. 4). 



