Ch.6-8Science of Forecasting Waves

GNM 1136

Ch.6 Refraction

Challenge• What causes waves to break?• What conditions are best for NJ?• Describe the NJ coastline.

Refraction

• What is refraction?• “Refraction is one of the most important

phenomena in the study of all waves, not just water waves.”

Refraction

• Refraction is the bending of a wave as it propagates over different depths. (fig.6.1)

• Where do you prefer to surf? Shallow or deep reefs/beach breaks

Focusing and Defocusing

• Concave Refraction or focusing =

Focusing and Defocusing

• Convex refraction or defocusing =

Characteristics – Fig. 6.2 & 6.3

More Refraction

Bathymetry – East vs. West

How to use detraction to determine type of break

• Point-break = Rincon, CA

• Beach-break = Atlantic City, NJ

• Reef-break = Sunset, HI

Figure it out

• What direction will the waves break in LBI, NJ in a North swell event?

• What about a Sothern Hemi swell in San Diego, CA?

NJ North Swell

Southern Hemi Swell, CA

Figure 1.9. Wave refraction. Wave crests approaching a typical North Carolina shoreline bend or refract, causing the waves to strike the shoreline almost head-on. As

the wave breaks, a portion of the energy flows in the direction show by the arrows, forming the longshore current. The longshore current transports sediment. Structures sucn as the one shown here, interrupt sediment transport, causing downdrift erosion.

Drawing: Charles Pilkey.

Rincon, CA

Ch.7 Wave-Breaking

• The “Breaking profile” depends on bottom topography, wind, swell period, etc…

• Airy wave theory vs Stokes Theory-wave is tripped up..toppling head first-The top of the wave is faster-Fig.7.1

Breaking Waves

• There are 3 basic types of breaking waves, depending on the type of shoreline they’re hitting. Spilling breakers occur on gently sloping coasts where the waves break slowly and over a long distance, with the crest spilling gently down the front of the wave. That’s what we have here. If the coast is steeper, the waves slow down more quickly and so the crest curls way over the front of the wave and plunges down towards the base---in other words it curls. This is a plunging breaker and is a good surfing wave like you’d have in Hawaii. In some cases, where the coastline if very steep, the wave builds up very suddenly and breaks right onto the beach. These are surging breakers.

Breaking Waves

• As waves enter shallow water they slow down and they change shape, increasing in height.

• Once they reach a water depth of approximately 1.3 times their height they start to break.

• How rapidly this happens is affected by the local sea bed, on a gently sloping beach with light winds the wave will gradually increase in height eventually the top starts to spill gently forward.

• These waves break slowly, they’re definitely surfable and can form the staple part of many surfers diets but are often slower to ride.

• Video Clip

Breaking Waves• Alternatively the wave encounters shallow water

more abruptly, a rock reef or a steep beach with well defined sand bars, in this situation the wave rapidly changes shape and the lip plunges forward in the most extreme case producing a hollow barreling wave.

• This sort of wave is the stuff of magazine covers, surf videos and a challenge for confident and competent surfers.

Beach-Breaks

• Closeouts• Onshore winds

Offshore Winds

Windswell vs Groundswell

• Surfline• Surfline – Sean Collins

Longest Wave in the World (Non-Tidal)

• Chicama - Peru

Ch.8 How Waves Move Sediments

Waves, Beaches, and CoastsWater waves are another agent of erosion, transportation, and deposition of sediment. Along the shores of oceans and lakes, waves break against the land, building it up in some places and tearing down in others.

• The energy of the waves comes from the wind. This energy is used to a large extent in eroding and transporting sediment along the shoreline. Understanding how waves travel and move sediment can help you see how easily the balance of supply, transportation, and deposition of beach sediment can be disturbed. Such disturbances can be natural or human-made, and the changes that result often destroy beachfront homes and block harbors with sand.

• Beaches have been called "rivers of sand" because breaking waves, as they sort and transport sediment, tend to move sand parallel to the shoreline. In this chapter we look at how beaches are formed and also examine the influence of wave action on such coastal features as sea cliffs, barrier islands, and terraces.

Coastal Geomorphology

Winter vs. Summer Profile (Fig. 8.3)

Views of UCSB Beach at three different times ofyear. The upper left is during the fall, when the sand is high. The upper right is after the first winter storm, and the figure to the left is in January. During this quarter, and while you are at UCSB, be sure to notice the effect of storms on our beach.

Sand Spit – Sandy Hook, NJ

Figure 178. Maps showing the progressive shoreline changes at Sandy Hook (after NPS Spermaceti Cove Visitor Center display).

Sandy HookFigure 179. Sedimentation features on aerial photographs of Sandy Hook: A. Plum Island represents the remnants of a washover fan. The sawtooth pattern along the Atlantic Ocean side reveals the buildup of sand adjacent to stone groins transported northward by longshore drift. B. an accretionary sandy buildup on the northern tip of the spit. The large dark area is a newly-formed freshwater pond. The dotted straight line is the 9 Gun Batter completed in 1902.

Jetty or Groin?

Rip Currents

Barnegat InletFigure 7. Inlet channel conditions at Barnegat Inlet, New Jersey, August 1944.