Umhlanga Rocks coastal defense

The eThekwini coastline is a vulnerable coastline subject to chronic erosion and damage due to sea level rise. In 2007 a severe storm caused major physical and economic damage along the coastline, proving the need for action. Umhlanga Rocks is a densely populated premium holiday destination on the eThekwini coastline suffering from similar problems due to its narrow beaches and lack of dunes in certain places. Interference with the coast of Umhlanga can entice fierce resistance from different groups of stakeholders, which makes finding a suitable solution more difficult. The above leads to the following problem definition: Due to erosion and extreme weather conditions the coastline of Umhlanga Rocks is shifting on shore, causing narrow beaches, decrease of tourism and increased risk of failures of the coastal structures. The current situation requires a new long term safety strategy, taking into account the social, economic and environmental vitality of the Umhlanga Rocks area as well. To solve the problem definition the main question states: How can the eThekwini municipality create a sustainable Umhlanga coastline while adding value to the area? To answer the main question many different elements, varying from conventional coastal protection measures to experimental ideas that would increase local business, were formulated during a brainstorm session. These elements are ranked on their cost, added value and technological feasibility. From the highest ranked elements in each category 11 different alternatives are created. By performing a multi criteria analysis these 11 alternatives are narrowed down to three alternatives and an additional `do nothing’ option is included. The do nothing option assumes that the beach will be completely gone in 30-40 years. It is further assumed Umhlanga Rocks recreational businesses are coupled to the beach and property values will drop by 12% once the beach is gone. This gives a NPV of R.15,000,000,000. It is suspected that the equilibrium that normally exists in the cross shore sediment transport is disturbed and more sediment moves offshore than onshore. The sill is designed to prevent the sediment form moving too far offshore and thus to create a new equilibrium. The bar retaining sill consists of prefab concrete elements located just outside the surf zone. From the Delft3D model it followed that the beach stays roughly the same size after construction of the sill. The NPV bases the cost of the sill on reference projects. With the beach maintaining its current size the total NPV amounts to R.116,000,000,000. The nourishment option adds enough sand to the beach to compensate for erosion and add 15 meters of beach according to the Delft3D model. It is assumed the nearby sand depot can be used to perform the nourishment. The nourishment itself will be done using a dredger connected to a floating pipeline to pump the sand to the beach. Shovels will be used to divide the sand over the beach area. The cost of the nourishment is based on the cost of a similar project, with the benefit of the added beach area the total NPV is R.142,000,000,000. The submerged breakwater creates a calmer wave climate near the coast and will thereby reduce erosion and increase beach growth. The Delft3D model suggests an average beach growth of 20 meters. For the breakwater design different materials are considered. Geotextile systems look very promising and have several advantages over a rock or concrete breakwater. A design with geotextile bags and one with geotextile tubes is made for the Umhlanga Rocks area. The NPV of the breakwater amounts to R.131,000,000,000. A second multi criteria analysis is performed to determine the overall best option. According to this analysis the construction of a submerged breakwater made of geotextile tubes provides the best solution to the problem from social and technical preferences. Social preferences include perceptions of stakeholders involved like environmental groups and property owners, but take the construction and maintenance costs of the alternative into account as well. In this case the geotextile tube breakwater uses conforming materials, does not cause any visual horizon pollution and seems to deliver the best benefits for the price involved. At the same time the results from the 3D simulations have shown that the breakwater is able to perform well on the technical preferences including the breakage of waves offshore and increase of beach width.