LABORATORY EFFECTS IN BEACH STUDIES 



iVolume II. Movable- Bed Experiments with 

 H^/L^ = 0.021 (1970) 



by 

 Charles B. Chesnutt and Robert P. Stafford 



I . INTRODUCTION 



1 . Background . 



Wave heights in movable-bed, coastal engineering laboratory experiments 

 and models vary both in space and time (Fig. 1). Such variability is com- 

 mon over the constant depth section of wave tanks with movable beds 

 (Savage, 1962; Faircnild, 1970a, 1970b; Galvin and Stafford, 1970). Because 

 wave height enters many coastal engineering formulas to the second or third 

 power, small variations in laboratory wave height can cause large errors 

 when extrapolated to prototype conditions. Wave height variability is one 

 of several laboratory effects, separate from scale effects, which can 

 hinder attempts to solve coastal engineering problems in the laboratory. 



The Laboratory Effects in Beach Studies (LEBS) project was initiated 

 in 1966 to investigate the causes of variation in wave heights observed 

 in longshore transport experiments in the Shore Processes Test Basin (SPTB) 

 at the Coastal Engineering Research Center (CERC). Originally, wave height 

 variability was thought to be caused by either resonance, generator mal- 

 function, or inaccuracies in the wave gages. However, preliminary experi- 

 ments indicated that wave reflection from the movable-bed profile and 

 variation in the reflection as the profile adjusted to wave attack were 

 the major causes of wave height variability. Since reflection occurs 

 naturally when waves travel onto a beach, reflection cannot be eliminated. 

 Therefore, the emphasis of the LEBS experiments has been to learn how 

 reflection is affected by naturally occurring changes on the beach in order 

 that coastal engineering laboratory studies can be better interpreted. 



A total of 10 detailed LEBS experiments were conducted, and this report 

 (Vol. II) discusses the first 2 experiments completed in 1970. The other 

 eight experiments are discussed in five data reports (Vols. Ill to VII) 

 as part of a series of eight reports on LEBS. Volume I of the series 

 (Stafford and Chesnutt, 1977) discusses the contents and primary purposes 

 of the reports. 



The two experiments covered in this study have also been discussed in 

 part in earlier reports. Chesnutt, et al. (1972) discussed the develop- 

 ment of the profiles in four LEBS experiments, including the two in this 

 study. Chesnutt and Galvin (1974) analyzed the relationship between 

 reflection variability and profile development in the same four experiments 

 discussed by Chesnutt, et al. (1972). Chesnutt (1975) analyzed other lab- 

 oratory effects observed in three LEBS experiments, including one of the 

 two in this volume. 



