Ch.6-7Science 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

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

What is Bathymetry?

• The measurement of depths of water in oceans, seas, and lakes. The topography of the ocean floor or underwater bottom

• In the same way that topographic maps represent the three-dimensional features (or relief) of overland terrain, bathymetric maps illustrate the land that lies underwater. Variations in sea-floor relief may be depicted by color and contour lines called depth contours or isobaths.

• In addition to the continental shelf, which is located off the US East Coast, NJ and NY also have the Hudson Shelf and Hudson Canyon

Bathymetry – East vs. West

Swell Growth

About Hudson Shelf • The geological coastline more than 20,000 years ago when ocean

levels were lower due to the ice age• In the middle of the Shelf is the Hudson Canyon, which extends

southeastward towards the end of the continental shelf • The Canyon is actually the ancient extension of the Hudson River

which starts about 100 miles southeast of New York City on the continental shelf and runs some 300 miles into very deep water.

• Comparable in size to the Grand Canyon, Hudson Canyon is the largest known ocean canyon on the U.S. East Coast and one of the largest underwater canyons in the world.

• The portion of the Canyon closest to the shore is call the Hudson Valley – this is the portion of the Canyon that will have the most effect on the surf for NY and NJ

Bathymetry

Map showing location of Hudson Canyon (red box) and relative water depths (80 meters is roughly 260 feet, 200 meters about 650 feet) in the area. (Credit: NOAA)

Bathymetric map with water depths in meters shows the Hudson Canyon' s steep slopes. (Credit: L. Macelloni, NIUST)) NOAA

A closer look at the bathymetric map, with the "craters" outlined with yellow circles.  A steep cliff is circled in red; soft coral communities were discovered in Oceanographer and Lydonia Canyons with similar terrain. The subtly irregular bottom features outlined in magenta may be hard coral communities or chemosynthetic communities featuring clams, mussels, and tubeworms. The scientists won't know what they are until they are photographed by an AUV during a cruise this summer. (Credit: L. Macelloni, NIUST)) NOAA

Bathymetry and Wave Period

• The longer period the swell, the deeper the energy will extend downward in the ocean

• Long period swell extends much deeper than short period swell and thus, given the same depth of ocean at an arbitrary point, the longer period swell will be more affected by the bottom than the shorter period swell

Bathymetry and Wave Period• As the long period swell approaches the shallower depths of the region,

the waves will begin to feel the bottom and refractive and attenuation processes will take place sooner than for a shorter period swell

• This does two things to the swell:– Change the direction of the initial incident angle of the swell (refractive

process)– Attenuation of swell energy due to the continental shelf

Thus various swells will behave differently in your region dependent upon the incident angle of the swell and the swell period

Impact of Swell Direction and Period along the NY/NJ coastline

Impact of Swell Direction and Period along the NY/NJ coastline

• Shorter wave periods, because they don’t ‘feel’ the bottom of the Hudson Valley finger, are not significantly refracted

• Longer period energy (14-20”) will be impacted by the Hudson Shelf and Canyon and this leads to refraction that may prevent the waves from approaching into the Valley finger– This system leads to smaller wave heights for spots in

Northern NJ and far western LI during long period SE swell events

Impact of Swell Direction and Period along the NY/NJ coastline

• In conclusion:

– Obviously a complicated issue along any coastline but especially here

– Changes in the bathymetry in the nearshore environment will also have an impact on what spots do best with longer period swell energy

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

Photo: Kyle Gronostajski

Photo: Greg Melega

Windswell vs Groundswell

• Surfline• Surfline – Sean Collins

Longest Wave in the World (Non-Tidal)

• Chicama - Peru