Physical Agents of Land Loss: Landslides & Cliff retreat
Some land losses along sea cliffs in parts of California and Alaska are related to faulting and earthquakes, but most of the land losses along cliffs are intermittent and caused by combined marine and subaerial erosion. High storm waves attack and erode the base of the cliff, which causes over-steepening. This results in collapse of the cliff and accumulation of debris (talus) at the cliff base. When marine erosion is not active, surface processes (gullying and slumping) may add to the rock debris deposit that protects the base of the cliff. Eventually marine erosion removes the protective covering of rock debris, undermines the toe of the slope, and causes renewed slumping (Emery and Kuhn, 1982; Komar and Shih, 1991). Sunamura (1983) summarized the relationship between cliff erosion and cliff resistance.
Weather patterns, sea-level fluctuations, composition of the cliff, and structural dip of the cliff strata control land loss along sea cliffs. Emery and Kuhn (1982), among others, have noted that cliff retreat occurs frequently in the winter in association with storms and high rainfall. Human activities such as watering lawns and draining septic tanks can artificially increase ground-water levels and either initiate or aggravate slumping at the face of the cliff.
Cliff retreat is not necessarily catastrophic and there are many physical and chemical processes that can cause sediment to gradually accumulate in rock debris slopes. An understanding of these processes is important to avoid conducting any activity that would increase slope instability or be ineffective in controlling landslides. For example, seawalls are commonly constructed on the beach at the bases of sea cliffs to stop slumping when the processes causing the slumping are acting at the top or within the cliff. Most states with actively retreating sea cliffs have issued maps and reports that identify the hazard prone areas and describe the probability of future slope failure.
Land loss caused by slumping can also occur at scales much smaller than sea cliff retreat. For example, erosion along some tidal creeks and dredged channels is caused by bank failure that forms small slump blocks (Ginsburg and Perillo, 1990). The channels widen as the slump blocks are removed by tidal currents or by ice flowing through the channels.