• Carter, 1988, Shoreline morphodynamics
• Carter and Orford, 1993
• Section 4.5 in EM 18100
• Short, 1979
• Wright and Short, 1984

Energy = the ability to perform work

work = force x d

Force = mass x acceleration

• Surf scaling factor (Guza and Inman, 1975): Epsilon = a * (2pi/T)²gtan²ß where a=H_b/2, T=period, ß (beta)=slope
  •  used initially to describe the conditions that define breaker type:
    • < 2.5 surging breakers
    • 2.5-33 plunging breakers
    • >33 spilling breakers
  •  Wright and Short related ssf to beach state
    • < 2.5 reflective
    • 2.5-20 transitional
    • >20 dissipative
• Dean's (1973) omega parameter: =H_b/(W_sT) where H_b =significant breaker height, T= significant breaker period, W_s = mean fall velocity
  • Related by Wright and Short to beach state
    • <1 reflective
    • 1-6 transitional
    • >6 dissipative
• Sunamura's (1988) dimensionless K* parameter: = H_b²/(gT²D) where D=particle diameter

  • 3.5-10 Reflective
  • 5-20 Intermediate
  • >20 Dissapative

• Energy is expended by performing work
  • moving sediment
    • the larger the sediment the greater the amount of energy expended
    • if the sediment is not large or abundant then the same amount of energy will transport it further
    • more energy is expended transporting sediment up a steep slope
    • If a narrow beach is efficient in dissipating energy then the area of surf may not change with increasing wave energy
    • However if a beach is not capable of dissipating an increase in energy then it will have to adjust to dissipate the energy over a broader area
  • breaking and pounding against headlands, seawalls, etc.
• Dispersal of wave mass
  • percolation
• reflection (how does this relate to breaker type?)
• friction and heat
• turbulence (how does this relate to breaker type?)
• transfer the energy to a secondary wave motion
  • formation of edge waves

• Normal incident waves: wind-generated gravity waves with periods between 1-30 seconds
•  shore-normal standing waves and progressive or trapped shore-parallel edge wave having periods longer that the incident waves:
  • Trapped edge wave are formed by reflection and refraction of incident wave as they reach the foreshore
  • progressive edge-wave are formed by refraction and reflection from headlands, jetties, etc. These wave generally die out at some distance from the headland as they travel along the beach face
  • Infragravity waves: waves having periods between 30 sec and 5 min; waves with periods >3 mins are attributed to surf beat
  • a. Many infragravity waves are the result of resonance. Resonance results when the addition of energy by a periodic force (e.g. incident waves) generates a periodic motion within a system (e.g. foreshore, trough, etc.) corresponding to that systems natural period of oscillation.
  • Subharmonic waves: infragravity waves that have twice the period of the incident wave
  • Leaky mode standing or edge waves are waves that radiate some energy back to sea. 
• Wave generated currents: Longshore currents and rip currents
• Non-wave generated currents: Tidal currents, currents produced by wind shear

• Characteristics
  • The beach is flat with a shoaling concave upward slope and no pronounced beach step
  • The surf zone is wide and one or more parallel bars occur in the nearshore.
  • Bars are separated from the beach face by a wide (100 m±) shallow (2-3 m) trough.
  • There is a large amount of subaqueous sediment (sand) storage.
  • Waves are typically spilling, breaking several times before reaching the shore.
  • There is little to no along-shore variation.
  • Onshore-offshore flow dominates. Infragravity waves in the form of leaky mode standing waves occur. These waves control the size of the trough and position of offshore bars.
  • Longshore currents are weak to absent.

Figure 2. Reflective Domain, extreme.

• Characteristics
  • Reflective beaches are steep and a high amount of energy is reflected or transferred to secondary wave motion. Therefore edgewaves are typically present; reflected in the presence of beach cusps.
  • The surf zone is narrow and nearshore bars are absent
  • There is a small amount of subaqueous sediment (sand) storage
  • longshore currents are commonly absent; vertically segregated on-shore and off-shore flow dominates; shoreward currents flow above and over seaward currents
  • no rip currents or runnels exist
  • Turbulent loss of energy occurs by breaking on the foreshore and by wave run-up (swash) on the beach face

• The intermediate states contain both dissipative and reflective elements
• These are profiles in various stages of recovery or erosion

Longshore bar-trough

Figure 3. Intermediate domain: longshore bar-trough

• Characteristics
  • Beach has a higher relief and steeper beach face than a dissipative profile.
  • Waves typically break on the bar and reform in the trough.

Figure 4. Intermediate domain: rhythmic bar and beach

• Characteristics

  • similar to longshore bar-trough, however both the bar and the foreshore contain rhythmic longshore undulations (spacing=100-300m)
  • cusp often present
  • weak to moderate rip circulation

  transverse bar and rip

  Figure 5. Intermediate domain: transverse bar and rip.

• Characteristics
  • development of transverse bars and troughs results in lateral reflective and dissipative domains
  • strong rip circulation
  ridge-runnel or low tide terrace

  Figure 6. Intermediate domain: ridge and runnel or low tide terrace.

• Characteristics
  • Steep beach face is fronted by a low and narrow bar on the LTT
  • rips are rare and weak
  • reflect and dissipated character changes with tidal phase (high tide=reflective; low tide dissipative)

• A beach may not achieve an equilibrium state if subject to constant extremes in wave energy
• High wave energy state may prevail because a longer time (lag time) is needed to obtain a low wave energy equilibrium
• Modal beach state may change laterally if there is a lateral gradation in sediment size and or wave energy
  • fine grain beaches subject to steep waves will have a modal beach state that is dissipative
  • coarse grained beaches may have a reflective modal beach state
• The formation of a particular beach state related to an event is dependent somewhat on the antecedent beach state 

Plum Island Field Trip

Your study

• Attempt to develop a beach state model for your beach
  • What is the sequence of changes that occurs?
  • What events or factors appear to drive the beach towards these changes
• Work within your limitations and make the most of what you can measure and observe
  • Limitations
    • Length of study
    • periodicity of measurements
    • cannot measure complex hydrodynamic behavior
    • measurements are restricted to the beach and the surf zone
  • What you can measure, observe or calculate
    • Wave parameters, such as Hb, T, breaker type, direction of wave approach
    • Swash phase, surf scaling factor
    • Longshore current: direction and velocity
    • presence of rip currents
    • infer the presence or absence of edge waves from cusps
    • beach morphology
    • sediment characteristics
• Define your model a precisely as possible but don't feel that you have to have a concrete explanation for everything
• Develop hypotheses and outline supporting evidence, analogs in found in research, pros and cons, what needs to be investigated further, etc. 

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Lindley Hanson/email /Gls214

Department of Geological Sciences, Salem State College, Salem, MA

 last modified 11/7/03