Therefore, a 3× more resistant layer could correspond to a less
coherent cliff (e.g. silt) with a more resistant layer of intermediate coherence
(e.g. shale), while a 5× more resistant layer may correspond
to a silt cliff with a more coherent layer (e.g. sandstone), respectively.
The effect of a resistant layer is evident in the shore profile shape for
all degrees of resistance and is marked by the raising and steepening of
the profile in this area as indicated on Fig. 4a. The trend of increasing
shore profile slope with increased material strength is consistent with
previous model results and field observations for homogenous cliffs
that the platform gradient increases with strength (Trenhaile and
Layzell, 1981;Walkden and Hall, 2005). The shore platform slopes gently
from the upper boundary of the resistant layer to the cliff toe, whose
position is (after 5 ka) substantially unaffected by the presence of a
more resistant layer, as demonstrated by Fig. 4b which shows the
non-dimensional rate of cliff toe retreat rate (heterogeneous cliff toe
retreat as a fraction of the rate for a homogenous cliff).
When the simulation is run for a longer period of time (Fig. 5a) the
influence of the hard layer on the softer platform above can be seen to
propagate upwards through time. After 5 ka the effect has reached an
elevation of around 0 m relative to MSL (from −1 m), after 10 ka the
effect has reached the cliff toe, and from then on the cliff toe retreat
rate begins to fall. The long period of time between the resistant layer
being encountered and the response of the cliff retreat may seem counter intuitive:
at a real site we might well expect that local amplification
of wave breaking caused by a resistant layer of bedrock should influence
the rate of cliff retreat more quickly. This effect is discussed in
Section 5.1