✍ Scribed by Ana Matias; Óscar Ferreira; João Alveirinho Dias; Ana Vila-Concejo
No coin nor oath required. For personal study only.
Two effectiveness indices are proposed in this paper for evaluating dune recovery techniques: nourishment effectiveness (N e ) and sand fence effectiveness ( F e ). Both indices were designed to be applied during monitoring programmes and were tested for Cacela Peninsula, Portugal. On this peninsula, a total volume of 480,000 m 3 of sediments were dredged from the inner backbarrier channel and used for replenishment during the winter of 1996/1997. A 32% loss of material to the sea occurred due to superficial drainage during the spreading process. During the subsequent 25 months, 33% of the deposited material was eroded (50% due to wave action, 45% related to inlet displacement and 5% to lagoon processes).
The determined nourishment life-time, based on monitoring data, was about 11 to 15 years, as compared to the designed lifetime of 15 to 20 years. These values could be used as a guide for other re-nourishment projects. The value for the nourishment effectiveness index (N e ) obtained was about 75% to 76%, which corresponds to a reasonably effective protective scheme for this coastal dune.
A set of fences was also installed and were effective sand traps in areas where the seaward nourishment bluff was small or absent. The sand fence effectiveness index ( F e ) gave values increasing eastwards from 6% to 77%, corresponding to the longshore accumulation gradient related to the bluff height. This demonstrates that sand trap efficiency is very small when they are erected on top of obstacles (natural or artificial) to aeolian transport.
For adequate dune recovery, in which the morphological, sedimentary and ecological characteristics are maintained, a combination of dune and upper beach nourishment and sand fencing is recommended. The effectiveness indices proposed here have the potential to be useful management tools but their use must be validated with the analysis of other monitored dune recovery cases.
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