Tunisia’s coastal erosion lesson
After I returned from a long and demanding scientific expedition in the coastal areas of North Africa, my colleague took me for a walk on Corona del Mar State Beach. A few steps in, he started recalling 15-year-old memories of the beach and how it has changed since. The beach seemed to be disappearing because of sea level rise, he said.
Many people make that observation, but from research I helped conduct off the coast of Tunisia, I knew the reason was probably more complicated than that.
With colleagues from the National Institute of Marine Sciences and Technologies in Tunisia, I studied how the shorelines of North Africa have moved inland over the span of a little more than a century. We saw evidence of sea level rise, but also realized that something else was responsible for amplifying and accelerating the seawater’s inland movement with unprecedented magnitude.
Our research showed that about 70% of the sandy beaches in the Gulf of Hammamet, a popular tourist area off the coast of Tunisia, persistently eroded at an average rate of about 1.6 feet a year between 1887 and 2018. This alarming rate is about seven times greater than the estimated average of 2.75 inches a year suggested by global-scale modeling for sandy beach evolution.
At a well-visited beach in a tourist zone established in 1880 in the northern region of the gulf, erosion reached 12.5 feet a year. That is equivalent to the sea advancing inland the length of about five football fields over more than 130 years, the period of time we studied.
We found that sea level rise, global warming and melting polar ice caps were not the only reasons coastal cities in arid areas are at risk of flooding. The rapid housing development in coastal areas close to the beach plays a major role.
The Southern California coastal climate from Los Angeles to San Diego mirrors that of the Tunisian coasts in North Africa — and the same mistakes that led to coastal flooding there are being made here. Over time, the massive urban development along Southern California beaches has contributed to erosion of its coasts.
Our research in North Africa shows that fast, urban development in coastal areas — especially those with semiarid environments — is causing abnormal shoreline retreat that is often confused with sea level rise. It also impacts the environment in several ways, our study found.
Urban areas close to the coast physically block sediment that would normally arrive at the beach via runoff. The sediment generated by even sparse rain would normally replenish the sand on our beaches and compensate for the natural erosion.
But as we endure more droughts and more houses are built, our coasts are getting fewer sediment deposits and are deteriorating. The shoreline is intruding inland, making it appear like the sea level is rising.
The coastal erosion also triggers arid land conditions that travel a few miles inland because of increased seawater in the soil, which accelerates moisture evaporation when illuminated by the sun.
As seawater advances inland, it allows for an increasing infiltration of saltwater inward into the groundwater, drying up the surrounding soil and vegetation. This, in turn, increases the risk of fires. This cycle also forces increased water usage to maintain soil moisture for growing vegetation.
Our study in Tunisia suggests that saltwater from the coast penetrates underground aquifers up to four miles inland, causing soil degradation that reduces the vegetation cover by almost 20%. Such an intrusion of salt in Southern California can accelerate corrosion that is capable of severely damaging house foundations.
In semiarid areas like Southern California, we need to stop developing land in close proximity to the ocean and work on developing solutions to end erosion. For instance, a mechanism could be developed to artificially replenish sand on the eroding beaches, funded perhaps by an environmental tax paid by those who live in houses built along the coast.
When development by the coast is allowed, strict limits should be placed on density. The fewer houses there are, the easier it is for erosion to be kept in check. The larger, faster and denser the housing we develop, the less sustainable our coastal cities become. It’s something to think about the next time you take a stroll along a local beach.