231 



William H. McAnally 



USAE Waterways Experiment Station 



P. 0. Box 631 



Vicksburg, MS 39180-0631 



SECT. 6. INTERACTION WITH NATURAL FEATURES AND CONSTRUCTED WORKS 

 Text Discussion 



The text correctly notes that "The major . . . question ... is whether 

 to include projected long-term estimates or not." Design of many 

 constructed works is based on economic justification of incremental 

 benefits vs incremental costs. Present practice usually includes a 

 straight- line extrapolation of present rates of apparent sea level rise 

 (sea level rise plus subsidence) to account for future changes. The costs 

 of building additional structure freeboard to account for accelerated 

 apparent sea level rise can be high; they are justifiable only when either 

 the projected rate is known with some confidence or if the potential risks 

 of not building it are too great to bear. 



Unfortunately, raising structures some time after their construction 

 can be extremely difficult. Berm costs include not only construction 

 costs, but also those to purchase the land for extra base width. The 

 latter can be politically and socially unsatisfactory when the areas 

 adjacent to the structures have become developed. Other measures, such as 

 advance purchase of easements and innovative construction techniques, may 

 be needed. 



Remarks about groin flanking problems also apply to transverse 

 training structures in tidal rivers (which usually work better than beach 

 groins). Flanking of these structures is a significant problem that sea 

 level rise may exacerbate, but overtopping of dikes may not be a problem. 

 Sometimes training dikes work better when overtopped. 



Roy Beard (1987) has noted that the present established (by law and 

 precedent) practice of designing for specific flood levels (i.e., a design 

 storm) is not consistent with the goal of flood protection for the common 

 good. A more rational approach is design to protect against a specific 

 level of economic or social damage at given risk levels. This applies 



