QUANTIFYING AND INTERPRETING LONG-TERM EROSION RATES ALONG HIGH RELIEF COASTLINES

As societal demands to live near coastlines continue to put pressure on coastal resources, areas that were once sparsely populated due to remoteness or rugged terrain are being developed at an accelerating rate. While numerous techniques have been developed and applied to measuring coastal erosion along sandy beach coasts and low-relief cliffed coastlines, little research has focused on the measurement and interpretation of erosion rates along high-relief coasts such as Big Sur, CA.

High-relief coasts are commonly found along tectonically active continental margins where rapid uplift of the terrain has produced mountain ranges immediately adjacent to the ocean. Along such coastlines, beaches are uncommon and generally confined to small pocket beaches formed where streams discharge. Marine terraces with well-defined cliff-edges may not be consistent or continuous along the coast. Erosion processes along high-relief coasts range from surficial mass movements such as slumps and blockfalls near the base of the slope to deep-seated rotational landslides that may extend from the base of the slope to the ridge top.

Along the Big Sur coastline, landslides regularly damage or block California Coastal Highway 1, the only paved transportation route along a 120 km stretch of the coast. The California Department of Transportation recently determined the need for long-term cliff retreat rates in order to develop a coastal management plan. In addition to defining the “cliff erosion feature” along the coast cliff retreat rates for nine approximately 5-km stretches of coast were calculated and compared to previously quantified slope retreat rates to assess which rate was more valid for identifying coastal hazard or vulnerability.

The retreat rates of the cliff erosion feature vary along coast and appear to be related primarily to the underlying geology. In all areas, the cliff retreat rates exceed the slope retreat rates, suggesting the entire coastline is becoming steeper and therefore is unstable and susceptible to continued landsliding. We also found that areas with the highest slope retreat rates are not the same as areas of highest cliff retreat. The highest cliff retreat rates are those with moderate to low slope retreat, suggesting that these areas are apparently experiencing faster steepening and are thus high-hazard areas.