SANTA CRUZ DE TENERIFE, June 19 (EUROPA PRESS) –
The Demarcation of Santa Cruz de Tenerife of the College of Civil Engineers, Canals and Ports has drawn up a report that analyzes the technical feasibility of ash from the Cumbre Vieja volcano (La Palma) for the regeneration of ‘black sand’ beaches .
To do this, samples of ash from Cumbre Vieja have been compared with other native black sand from two beaches in the north of Tenerife, San Marcos in Icod de los Vinos and El Socorro in Los Realejos. The report further discusses the leaching of volcanic ash.
The report begins by commenting on the stability of a beach both in plan and in profile. The stability of the plan is conditioned by the directions of the waves that affect it and modified by the rocky outcroppings or poles of the beach environment.
Profile stability is characterized by the slopes of the submerged beach and the ‘stram’, that is, by the section of the beach profile between the depth at which the waves can no longer move the sediment or the Significant Transport Limit (LTS) to the dry beach or area of the profile that is out of reach of the waves.
The report highlights that the profile stability of a beach is dynamic, the sand moves between a minimum slope (with large storms) and a maximum (with ordinary waves) with two stability conditions: that the beach maintains its volume of sand without significant losses and that the beach is capable of recovering from the minimum to the maximum slope after each storm. The line sought (curved or straight) is the average stability profile of the beach over an average calendar year.
Many models have been developed looking for a mathematical equation that represents the line of that average profile. The fundamental conclusion is that this equation essentially depends on two parameters: the size of the sand, represented by its median diameter (D50) and its density (?s), although other parameters may also intervene, but are not related to the two quoted or not relevant.
Therefore, the basic rule for choosing borrowed sand to regenerate a beach is that it be of the same or greater granulometry (D50) and density (?s) than the native sand. Somewhat higher values of these two parameters would increase the stability of the beach to be regenerated.
If the borrowed material is finer than the native one, a part of the fines will escape in suspension through off-shore transport due to waves and if it is very thick it will be more stable but less comfortable for the user.
MATERIAL ANALYSIS.
For these reasons, the report has analyzed the granulometry and density of both the native material from the beaches of San Marcos and El Socorro, as well as various samples of volcanic ash from the Cumbre Vieja volcano.
On the one hand, sand samples from San Marcos beach have been analyzed by different consultants and professionals. The average values of the two fundamental parameters are: D50= 0.34 mm and Average density= 2.84 t/m3
Sand samples from El Socorro beach have also been analyzed by different consultants and professionals. The average values of the two fundamental parameters are: D50= 0.37 mm and Average density= 2.97 t/m3
Finally, numerous ash samples from the Cumbre Vieja volcano have been analysed, both at the ULL School of Civil Engineering and at the Canary Islands Government’s Public Works laboratory. The values of the two fundamental parameters of the samples have varied: between 0.305mm and 2.039mm the median size (D50); and between 2,632 t/m3 and 3,061 t/m3 the densities, indicating that there is a wide range of ash to choose from.
Thus, the report concludes that the ashes from the Cumbre Vieja volcano are valid for the regeneration of black sand beaches such as those indicated, although it is necessary to choose ashes whose size and density have values equal to or greater than the native material of the beaches.
It is also possible to choose ashes with values somewhat lower than those of the native material, but in this case, the volume of contribution must be increased, in the proportions usually used in the regeneration of beaches, to compensate for the loss of fines.
LEACH ANALYSIS
Additionally, a leaching analysis has been carried out to determine the concentration of heavy metals soluble in water, which are released when the ashes come into contact with it.
These concentrations must comply with reference parameters, which, depending on the values presented in the analysis of the ash samples, would qualify them as: inert, non-hazardous or dangerous.
The analysis of the leaching has been carried out taking 5 ash samples from different places around the Cumbre Vieja volcano, all the samples analyzed presented a concentration of heavy metals soluble in water lower than the limits established for inert material in the reference standards, so we must classify volcanic ash as inert.
