Effects of breaking waves Breaking waves often have foamy or aerated water When bubbles or cavities collapse, water moves rapidly across cavity & generates locally very high pressure Collapsing cavities & bubbles blast the substrate to bits Breaking wave & its backwash move water with rock detritus, & so abrade the substrate Effects of breaking waves concentrated in narrow range of depths, creating a wave-cut terrace At landward end of terrace, may have notch in sea cliff; erode of sea cliff by undercutting & collapse of cliff Debris, both large & small, on beach, is rounded

Erosional coastlines Wave refraction focuses wave energy at headlands, so get more erosion occurs there –At headlands may see evolutionary sequence: sea caves, sea arches, sea stacks, & tombolos Suspended detritus is transported, but also is deposited when currents slow Wave refraction tends to focus wave energy away from intervening pocket beaches, so more deposition occurs there After a long time, erosion & deposition may generate a smooth beach line perpendicular to the dominant wave propagation direction

Features on depositional coastlines Sandy shoreline where waves affect or disturb sediment = littoral zone Divide littoral zone into –Foreshore = water’s edge –Shoreface = region seaward of foreshore, where waves break –Backshore = region landward of foreshore –Longshore bar = a submerged, asymmetric pile of sediment over which waves will first break –Surf zone = extends from where waves first break to foreshore –Swash zone = edge of water, where water swashes in and backwashes out –Beach face = dipping, relatively planar surface on which waves break –Berm = crest of beach face

Sediment sorting Swash, the current generated by incoming wave, is strong but of short duration –It moves coarse debris toward shore Backwash, the current generated by outgoing wave, is relatively weak & of longer duration –It move finer material but leaves coarse detritus behind A beach will have a limited range of sizes of waves that strike it, & so swash zone will be composed of detritus of limited size range Beaches are usually well sorted

Beach profiles Swash is stronger than back wash Asymmetry of wave-generated currents may lead to seasonal changes in beach profile –Stronger waves in winter move detritus from beach face to longshore bars & create steeper beach face –Weaker waves in summer move detritus onto beach, creating a gentler beach face

Two important features of surf zones As waves break on beach, can get local piles of water that dissipate by draining perpendicular to shore in localized rip currents Waves often strike beaches obliquely, so swash goes obliquely up shore face, & backwash goes down beach; zig-zag motion causes longshore transport or littoral drift - movement of sand along the shoreline

Ocean tides Tidal variation = regular, periodic, & predictable changes in mean sea level Tidal range = difference between water height at high tide & water height at low tide Tidal currents = movement of water generated by or associated with the change in mean sea level –Flood tide - movement of water toward shore or into a restricted body of water –Ebb tide - movement of water away from shore or out of a restricted body of water

Tidal variation Tides related to the moon & sun, but do not occur equally at all locations on earth –Different locations on earth have different tidal ranges –Frequency of tides at a particular location may vary with a daily or yearly cycle –Tide ranges vary with an astronomical cycle that is 18.6 years long Examine & classify tides by determining the tidal curve at a location Tidal curve = plot still water height vs. time

Tidal periods No two places have identical tidal curves, but tides everywhere have a fundamental period of 24 hours 50 minutes, the length of time between successive passes of the moon overhead Tidal day = 24 hours 50 minutes Tidal period = length of time between successive high or low tides Tidal periods at all locations on earth are either 24 hours 50 minutes or 12 hours 25 minutes

Types of tides Classify tides using –Number of high tides per day –Relative heights of successive high tides –Length of time between equivalent high or low tides Distinguish three types of tides Diurnal or daily tides Semidiurnal or semidaily tides Mixed tides

Diurnal vs. semi-diurnal tides Diurnal or daily tides - have a single high tide & a single low tide per tidal day Semidiurnal or semidaily tides - have two high tides & two low tides of approximately equal height each tidal day –Semidiurnal tides may have daily inequity, where successive high tides have different heights –Semidiurnal tides often easy to predict because high (or low) tides occur a consistent length of time after the moon has passed overhead

Mixed tides When heights of two successive high tides or two low tides are markedly different, we have a mixed tide –Identify higher high water tide (HHW), lower high water tide (LHW), higher low water tide (HLW), & lower low water tide (LLW) –Times of high stands & low stands are not simply related to passage of moon overhead