ENV 100 Chap.5 Waves, Sediment, and Coastal Change
Waves, Sediment, and Coastal Change
This chapter follows wave energy from deep water to breaking at the shore, then tracks sediment through swash, backwash, rip currents, longshore currents, and bed return flow. Beach change is interpreted as movement within a coastal cell. Muriwai evidence connects modern measurements with volcanic and sedimentary history.
What this chapter covers
- 01
Wind waves and swell
- 02
Deep and shallow water
- 03
Refraction and diffraction
- 04
Nearshore circulation
- 05
Longshore and cross-shore transport
- 06
Beach profiles and erosion
Worked example · free
Interpret repeated beach-level readings
- +1Calculate 7 cm of initial surface lowering.
- +1Calculate 4 cm of later accumulation from the minimum.
- +1Compare the final reading with the baseline.
- +1Conclude partial recovery with 3 cm net lowering.
Key terms
- Wave base
- Wave base is the approximate depth below which orbital wave motion becomes negligible.
- Refraction
- Refraction is the turning of wave crests as different parts change speed in shallow water.
- Rip current
- A rip current is a concentrated seaward flow within nearshore circulation.
- Longshore drift
- Longshore drift is net sediment movement caused by oblique swash and downslope backwash.
- Progradation
- Progradation is seaward growth of a shoreline through net sediment accumulation.
Waves, Sediment, and Coastal Change FAQ
When does a wave feel the seabed?
As water depth becomes small relative to wavelength, orbital motion interacts with the bed. The wave slows, wavelength shortens, height may grow, and breaking becomes likely.
Is beach erosion always permanent loss?
Sediment may shift offshore, alongshore, or into temporary storage and later return. A conclusion needs repeat profiles, sediment budgets, wave conditions, and the boundaries of the coastal cell.
How do humans alter coastal erosion?
Development can occupy the active zone, structures can interrupt sediment transport, and altered catchments can change supply. Effects should be compared updrift, downdrift, cross-shore, and through time.
Why must a beach reading use a baseline?
A change has meaning only relative to the original reference and the instrument geometry. Successive readings can show erosion followed by accumulation, yet the final surface may still remain below its starting level. Reporting every interval prevents partial recovery from being mistaken for full recovery.
What evidence supports longshore drift?
Oblique wave approach, angled swash, downslope backwash, alongshore currents, and mapped sediment changes can support the interpretation. A single arrow on a sketch is not enough; direction should be consistent with wave conditions and with measured gains and losses along the connected coast.
Assessment move
Begin with a cross-shore wave sketch that follows energy from deep water to the breaker zone. Add orbital motion, changing depth, shorter wavelength, increased steepness, breaking, and the resulting nearshore flows. On a second plan-view drawing, show wave crests approaching obliquely, swash moving up the beach at an angle, backwash returning downslope, and net longshore sediment movement.
Use distinct arrows for water motion and sediment transport so they are not mistaken for the same quantity. Create a coastal-cell budget with sources, temporary stores, pathways, and losses. This prevents the word erosion from being treated as though sediment has vanished from the system. For beach-profile data, always write the instrument geometry before calculating.
More exposed rod means a lower sediment surface only if the rod remained fixed. Calculate change over each interval and then compare the final reading with the original baseline. State both the magnitude and direction. Add an alternative explanation such as local scour, inconsistent placement, or a shifted reference point, and name the repeat observation that would test it.
Field revision should pair an observation with an inference: grain size, clast shape, bedding, slope, and exposed length belong on one side; transport direction, depositional environment, and erosion episode belong on the other. Practise writing captions that state the relationship, not merely the object shown. Alternate calm and storm scenarios and predict cross-shore as well as alongshore change.
For human effects, identify whether a structure changes sediment supply, blocks a pathway, occupies the active zone, or transfers an effect downdrift. Avoid moral labels until the process is clear. Before the field trip or test, rehearse a five-minute evidence paragraph with reference point, interval, measured change, mechanism, uncertainty, and next measurement.
Finish by checking scale: one rod describes its immediate location, a profile samples a transect, and a coastal-cell conclusion requires evidence across connected stores and pathways.
