4. Tank Width and Wave Reflection . 



Irregular waves must be generated by running continuously for rela- 

 tively long time periods to simulate coastal wave conditions and to obtain 

 a satisfactory sample of the extreme values of runup. Generating waves 

 continuously for 5 to 10 minutes may be desirable. The influence of wave 

 reflection in a narrow wave tank without an absorber beach or filter will 

 be difficult to evaluate under continuous running conditions. Reflection 

 is affected by the wave period, wave height, water depth, and the slope 

 and roughness of the runup board. All of these factors can be expected to 

 influence the modification of the generated waves and consequently the 

 runup . 



Reflection shifts the energy distribution in the wave spectrum toward 

 lower frequencies since long waves reflect better than short waves. Long 

 waves runup more efficiently than short waves; therefore, it is probable 

 that reflection has considerable influence on the extreme values of the 

 runup. Since the influence of reflection is inherently difficult to 

 delineate, it is advisable in future experiments to reduce the influence 

 of reflection as much as possible. It is recommended that future investi- 

 gations of runup and overtopping caused by irregular waves be conducted 

 in a relatively wide tank, using a generous part of the width for an 

 absorber beach. Consideration should also be given to using a wave filter 

 in the tank between the wave generator and the runup slope. 



The recommendation for the use of a wide tank is not intended to pre- 

 clude all use of narrow tanks. A narrow tank could be used to gain 

 experience in using a data acquisition system, developing instrumentation 

 and test procedures, and learning how to simulate coastal wave conditions. 

 All of these elements are important to a comprehensive study of wave runup 

 and overtopping. 



5. Secondary Waves . 



Because of the influence of secondary waves on runup noted by Multer 

 (1973) , an attempt should be made to detect their influence on irregular 

 waves. It is recommended that some tests be conducted, which are other- 

 wise replicates, using different distances between the slope and the wave 

 blade. 



6. Data Analysis and Theoretical Development. 



Some high runup values are caused by high crests instead of high wave 

 heights. Since, from a theoretical point of view it may be easier to work 

 with crest elevations than wave heights (Cartwright and Longuet-Higgins, 

 1956), an investigation as to how well this incident wave parameter 

 correlates to extreme values of runup would be useful. 



The loss of runup crests and visual observations of runup both in the 

 laboratory and in the field indicate that a large runup is often produced 

 through the complicated interaction of two or three waves. Through the 



23 



