A beach profile is the shore-normal profile of a beach. A beach profile records
the occurrence, location, and distribution of features defining the overall
beach morphology. In order to fully understand the processes affecting a beach
profiles should be made often, and should include the entire shore zone including
the shoreface. However, this is beyond the scope of the our field work. We will
be profiling the shore weekly.Although a beach
profile changes daily there is a general modal morphology that reflects the
conditions discussed below.
Waves and related currents
- Waves: Waves move sediment
either on or off shore
- Steep waves typically move
sand off shore and shallow waves move sand on shore
- Wave generated currents:
Although wave generated currents move sediment on and offshore, the net movement
they produce is typically along shore. The strength of the current is related
to the wave height and angle of wave approach.
- Shore normal currents
- wave current - mass
- swash and backwash
- rip currents
- Longshore current
- Tides determine the length
of time waves will act on a particular level of the beach
- Tides also influence sediment
grain size and variability
- The morphology of a particular
beach profile is largely governed by the amount and distribution of wave energy
and the ability of the available sediment to dissipate it. (This concept is
discussed in more detail later in Morphodynamics
Beach slope and gradient:
slope is the angle of the beach; gradient is h/l or tan(slope). The average
slope or gradient of the foreshore (MLW to MHW) is used to define whether
a beach is either steep (11° or .2) or shallow (.5° or .01).
Factors influencing beachface
Steepness of the beachface
is greatly influenced by the relative volume and strength of backwash and swash.
Backwash moves sediment offshore resulting in a flatter beachface. Swash on
the other hand carries sediment up the beach and will deposit it if allowed
to percolate into the sediments. However, backwash can more efficiently move
sediment because it works with gravity in the downslope direction. Swash on
the other hand must move sediment up slope against gravity.
QUESTION: What roles do swash
and backwash have in shaping the beach face? How do the above factors control
the relative volume of swash and backwash?
- Grain size
- The larger pores between
coarse clasts results in greater percolation and less backwash. Also
coarser material has a higher angle of repose. Therefore coarse beaches
tend to be steeper.
- Backwash volume is greater
along less permeable fine grain beaches resulting in flatter profiles.
- Well sorted beaches are
more permeable. Backwash is minimal and sediment is deposited.
- Saturation reduces permeability and increased
backwash and porewater pressure promoting a flatter beach.
Groundwater discharge and large short period waves increase saturation
and decrease percolation.
- Changes in permeability
are related to wave steepness (H/L). As wave steepness increases
beaches become flatter.
- Wave period (T), height
(H), and length (L) determine the volume of water reaching the
beach at any one time.
- When wave heights
increase , as during a storm the beach face becomes saturated
reducing the infliltration capacity of sediment. As a result
backwash increases and more sediment is carried seaward. The
beach becomes flatter.
- Location of water table
and volume of gw discharge at the beach face
- Groundwater adds to
backwash which carries sedment seaward.
of Geological Sciences, Salem State
College, Salem, MA