WAVE ATTENUATION BY ARTIFICIAL SEAWEED 



by 

 John Ahrens 



I. INTRODUCTION 



This report discusses the wave tank testing of a low specific gravity 

 artificial seaweed field and its ability to attenuate wave action. Field 

 testing of the seaweed's potential to prevent scour or trap sand has pre- 

 viously been evaluated. Additional information on tests and applications 

 of artificial seaweed is found in Rankin and Cogan (1965), Wicker (1966), 

 Brashears and Bartnell (1967) , Nicolon of Holland (1972), and Bakker, 

 et al. (1973). 



II. TEST SETUP, CONDITIONS, AND PROCEDURES 



The artificial seaweed was tested at the Coastal Engineering Research 

 Center (CERC) in the large wave tank, 6.1 meters (20 feet) deep, 4.6 meters 

 (15 feet) wide, and 194 meters (635 feet) long (see Coastal Engineering 

 Research Center, 1971 for a description of the tank) . A riprapped wave 

 absorber slope occupied 46 meters (150 feet) of tank length during the 

 testing. Waves were generated by a piston-type wavemaker. 



Each seaweed unit (Fig. 1) was composed of a large number of slender 

 fronds made of stretched polypropylene foam strands (Fig. 2). The unit 

 was 2 meters (6.5 feet) wide, about 2.1 meters (7 feet) long, and bound 

 by horizontal stitching at 25-centimeter (10 inches) intervals. The fronds 

 had a specific gravity between 0.1 and 0.2, and were attached to a black 

 nylon bag which could be filled with weighting material to anchor the unit. 

 The seaweed unit was secured in the tank by running a heavy aluminum strap 

 through the nylon bag and bolting the strap to the floor. When the tank 

 was filled each unit formed an inverted curtain extending about 2.3 meters 

 (7.5 feet) above the tank floor. 



The artificial seaweed field was formed by seven rows of seaweed, each 

 row consisting of two seaweed units, spaced 3 meters (10 feet) apart along 

 the wave tank. Figures 3 and 4 show a cutaway view along the tank and a 

 cross-sectional view of the tank through the seaweed field, respectively. 



Gages were located on both sides of the seaweed field to measure wave 

 attenuation (Fig. 3). A 1.5-meter- long (5 feet) capacitance -type wave gage 

 with continuous resolution was located at the seaward tank station 522. A 

 3-meter-long (10 feet) step-resistance gage with sensitive elements 3 centi- 

 meters (0.1 foot) apart (Williams, 1969) was located at the landward tank 

 station 442. Output from the two gages was recorded on a dual-channel pen 

 and ink strip chart. The step-resistance wave gage is essentially a self- 

 calibrating gage. The capacitance gage was statically calibrated before each 

 data run and checked after each run to ensure that it maintained its cal- 

 ibration. 



