CHAPTER 4 INTRODUCTION Amphibious  warfare  is  the  most  complex  operation in   modern   warfare.   The   safety   and   success   of amphibious landings are largely dependent upon known surf conditions, although several other environmental factors can also have a profound effect. Surf conditions are reported by various individuals, depending upon the specific   operation. Their  input  into  surf  forecasts (SURFCSTS)  are  key  to  major  decisions. In  this chapter, we will begin by discussing the causes of surf and surf zone characteristics in general. We will then focus  on  the  actual  surf  observation  (SUROB)  at  a beach, and describe the calculation of the modified surf index (MSI). Finally, we will discuss in detail how tides affect amphibious operations. SURF  ZONE  CHARACTERISTICS LEARNING   OBJECTIVES:   Identify   the manual that provides information on surf zone terminology  and  detailed  instruction  on  surf observation procedures. Identify the causes of surf. Recognize the effects of hydrography on surf conditions. Define the terms used in surf observations. Recognize   the   effects   of refraction on waves. On  occasion,  both  Navy  and  Marine  Corps  weather observers  may  be  called  upon  to  support  amphibious operations either in exercise conditions or in an actual beach  assault. When  the  beach  area  is  secure  (in friendly hands), you may be tasked with observing and reporting surf conditions from the beach. Under hostile conditions,  Navy  UDT/SEALs  or  Marine  Corps RECON personnel are normally tasked to conduct an on-scene,   covert   beach   survey,   which   includes observing   and   reporting   surf   conditions. Aerial reconnaissance photography and satellite imagery can also provide a good indication of surf conditions at a hostile beach-landing area. Analysis of surf conditions from  imagery  is  done  by  Navy  and  Marine  Corps analysts/forecasters  and  Navy  oceanographers  working with photographic analysts. SURF   OBSERVATIONS COMNAVSURFLANT/PAClNST    3840.1,    Joint Surf  Manual,  contains  comprehensive  information  on amphibious   operations,   including   surf   observation procedures and a listing of various tables used for surf index  calculations.  All  Navy  and  Marine  Corps  surf observers should review the Joint Surf Manual. CAUSES  OF  SURF Surf,  the  way  that  waves  break  near  a  beach,  is caused by either local onshore winds or by swell waves traveling from a distant fetch area. The term surf zone describes  the  area  between  the  shoreline  and  the outermost   limit   of   the   breakers. The  surf  zone encompasses the region between the first approaching breakers and the limit of wave uprush. Surf  created  by  local  winds  is  characterized  by breakers with irregular crests, short wave periods, and many whitecaps in the surf zone. Breakers produced by sea waves do not appreciably increase in height before they   break.   Surf   created   by   swell   waves   produces breakers  with  a  more  rounded  appearance  and  with  a more regular, but longer period. Swell waves offshore appear low and rounded, but just before breaking they rapidly increase in height and steepness. Although sea waves  and  swell  waves  usually  exists  simultaneously,  it is  the  swell  that  most  often  presents  a  problem  for amphibious operations. Surf forms as deep-water waves approach shallow water. Deep-water waves are waves moving over water that  has  a  depth  greater  than  one-half  the  average wavelength. Deep-water waves approaching the beach will  “feel  bottom”  when  the  depth  of  the  water  is approximately   one-half   the   deep-water   wavelength. When a wave feels bottom,  wave  speed  and  wavelength decrease while wave height increases. As this happens, the face of the wave becomes steeper. The steepness of a wave refers to the ratio of wave height to wavelength. Waves become breakers when the wave spills water down the face of the wave (the wave crest). Once the steepness of a wave becomes 1/7 of the wavelength, the wave  becomes  unstable  and  begins  to  break.  When approaching a beach, a wave will normally break when the water depth is 1.3 times the wave height, but may 4-1