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gavias retain dams rehabilitates fuerteventura water hundreds

Fuerteventura rehabilitates hundreds of gavias, dams to retain water
#1
Diario:

Fuerteventura rehabilitates hundreds of gavias, dams to retain water.

The objective of the Island Hydrological Plan with this type of works is to increase the infiltration of rainfall into the aquifer and improve the quality of the groundwater masses

The rains arrived on the island in the final stretch of 2023, however, the feeling of the Majorero population was that most of the rainwater "went to the sea", despite the fact that some ponds and gavias were flooded. From Diario de Fuerteventura we analyze this phenomenon and we see that it is not as true as it seems.

Certainly, it has been observed that, after the rains, a lot of water is directed towards the sea as in the rest of the continent, but we can see that in Fuerteventura firm steps are being taken to alleviate desertification and retain rainwater, in ponds and rainfed dams that have a persistent drought.

The Insular Water Council reports that the Insular Hydrological Plan has contemplated, for several years, works on rainfed dams of different sizes, given that there are more than a thousand on the island. Most of them have a capacity of between 75,000 and 10,000 cubic meters, although there are dams of up to one million cubic meters. The Council has to comply with the environmental objectives set out in the Management Plan in accordance with the Water Framework Directive (WFD).

For years, Fuerteventura has been pursuing not only that surface water, when it rains, stays in the territory, in greater quantity and for as long as possible through systems of gavias and ponds, but also a second consequence derived from this action: increasing infiltration into the aquifer. "If water is injected, it contributes to improving the quality of groundwater bodies," sums up the manager of the Insular Water Council, Domingo Montañez.

The Fuerteventura Hydrological Plan reflects that groundwater bodies have "poor quality", among other reasons due to the risk of nitrate contamination and high salinity. "What we try to do is to keep rainwater on the ground for as long as possible so that it can infiltrate, take advantage of it through ponds or reservoirs and prevent it from going into the sea." This phenomenon is linked to another problem, such as the loss of territory due to soil erosion.

"The priority is to act in those areas that intercept the natural drainage network, that is, in those dry dams of loose materials that are intercepting public or private channels of great magnitude to recover them because some have a life of more than 40 years and have become terrified, reducing their storage capacity and desired function," sums up the manager of the Insular Water Council.

For this reason, the Cabildo undertakes works to clean the ponds, but they were carried out in the past without following the technical criteria. There is now a regulation that states that essential issues, such as safety and drainage capacity, must be addressed. Many of these dam slopes do not meet the technical conditions and are also being reinforced. In terms of their drainage capacity, many dams do not have the flood flow required by law. The objectives are to recover the initial reservoir capacity, improve its safety and drainage conditions in order to enhance its functionality.

The Hydrological Plan proposes rationing and sustainable management of water resources. "We carry out studies and act according to the flow rate so that the investment is in line with reality," says the manager. The Insular Water Council also acts in the feeding of the system of traditional gavias that allow the accumulation of more than 300 cubic meters of liquid element.

Steps are being taken to mitigate desertification and retain rainwater

The latest works carried out by the Insular Water Council have been carried out in dry dams in Tarajalejo and another two in the Barranco de Cardón, with a capacity of about 50,000 cubic metres, which have been cleaned for reuse. Another one that has overflowed in recent years after the rains and that has recently been renovated is located in the Barranco de la Muley, within the municipality of Puerto del Rosario.

Another objective of these actions is the use of rainwater for the agricultural sector. To receive supply from these dams, a permit from the Insular Water Council is needed because they are works in the public hydraulic domain and to be able to use it authorization for irrigation is required, but the reservoir water could be captured directly through a pipe. However, the indirect benefit of improving groundwater quality has been observed. "Curiously, when it rains, the water table levels of the wells (shallow, between nine and 30 meters) that are in the vicinity of these rainfed dams rise, since the wells are fed by the filtering dams and recharged from the aquifer in the area."

Island Hydrological Plan

For his part, the island councillor for Waters of Fuerteventura, Adargoma Hernández, announces that an attempt will be made to definitively approve the third cycle of the Fuerteventura Hydrological Plan at the end of the first quarter of 2024. An essential document for the use and conservation of water in Fuerteventura, which includes a series of actions that will increase the flow for the activity of the primary sector and recover the cultivation area that existed in Fuerteventura. "It includes actions to act on riverbeds, reservoirs and other exploitation systems."

He then reiterates that "the Cabildo and the Insular Water Council carry out work for the arrangement of the systems for the use of surface waters on the island". Hernández points out that "in Fuerteventura there are more than 1,100 catchments between ponds, marshes and dams, covering an area of more than 3.2 million square metres, which translates into more than 330 hectares".

Hernández details that these tasks are carried out in cooperation with the area of Works and Machinery of the Cabildo, for the cleaning and optimization of the different systems that are distributed throughout the island territory, "giving preference to those that are located on land of public hydraulic domain or that intercept the territorial drainage network". In this way, the aim is to "recover its reservoir capacity, both for rainwater harvesting and for aquifer recharge," he insists.

Las Peñitas Dam

One of the historical works of the State is the Las Peñitas dam, built during the Franco era. A very characteristic reservoir in Fuerteventura for its materials, its trachyte masonry wall, in the central ravine of the island.

The reform project proposed by the Island Water Council includes an intermediate action between the maintenance of the ecosystem of flora and fauna and capturing groundwater with a work of emptying part of the sludge that has completely covered the dam, which is abandoned and currently unused. An initial project of more than three million euros in three phases, which is at a standstill. To put the dam into operation, the channel would be narrowed, the reeds and invasive flora would be eliminated and part of the protected tamarisk trees would be transplanted to another area. An unknown work that has many detractors.

Los Molinos Dam

For its part, the Los Molinos dam always has water and has never overflowed. With a capacity for two million cubic meters of water, it must be cleaned because it has a lot of water in the reservoir, it has springs that drain into the dam and are saline so they spoil the quality of the water in the dam. It also has a small volume of water, it is also open and what it collects evaporates quickly, but it is a very attractive and visited wetland.

The dam once fed the rural town of Las Parcelas, which was built with a network of water pipes surrounding the houses, built between gavias that were cultivated in the 1960s, and generated a flourishing agricultural activity in the area.
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#2
Diario:

"Before rehabilitating the Las Peñitas dam, it is necessary to analyze whether it is worth it"

The young Majorera Gara Torres Lillo prepared her Final Degree Project in Environmental Engineering on the Las Peñitas dam and its possible rehabilitation, since it is filled with sediments in 94 percent of its initial capacity.

To this end, it presented a proposal for the management of the reservoir based on studies carried out over the last 25 years on the rate of erosion of soils, saline springs and plant communities present in the area.

In this way, in his analysis he highlights that "the main problems that the dam currently presents are: the clogging of the reservoir by sediments, which causes a reduction in its capacity, and the salinization of the scarce reservoir water, which causes a reduction in its quality".

Likewise, according to their study, the experimental results of the erosion rate indicate that 32,484 cubic meters of soil are eroded annually in the hydrographic basin, of which 4,278 cubic meters of sediment have accumulated in the dam per year since its construction.

However, the measures proposed by the public administration have been aimed at extracting the sediment from the reservoir and depositing it in its surroundings, while increasing the wall of the dam by one metre. This, according to Gara, "is a proposal not without impacts and with little medium-term viability due to the persistence of the causes of the problem, since the 134,000 cubic meters of capacity of the dam that are intended to be increased would be filled in a period of approximately 31 years."

In addition, it should be taken into account that the contribution of saline water in rainy years, through the Vega de Río Palmas spring (4.3 grams of salts per liter), produces a rapid salinization of the reservoir water and that the clay sediments removed from the dam have a low agronomic value due also to their high salinity.

For all these reasons, Gara Torres considers that, if the objective is the use of water for agricultural use, it would be appropriate "to increase the height of the wall of at least six meters with a capacity of 372,000 cubic meters and a clogging period of 86 years, so that the effects of sediment deposition and salinization of water are less evident. although not free of impacts on vegetation."

It also highlights that it is not possible to act only on the dam without considering the hydrographic basin in which it is located, so it is essential to carry out actions such as the restoration of the traditional systems of gavias and ponds, the revegetation of the mountains and the limitation of grazing.

In conclusion, "it is necessary to thoroughly evaluate whether it is worthwhile to carry out any intervention in the dam from an economic, technical and environmental point of view," says student Gara Torres.

From an economic point of view, it is necessary to determine whether the investment to be made in the dam to obtain water for agricultural use is more profitable than obtaining it through current desalination systems with renewable energies. From a technical point of view, it is necessary to study the possibility of increasing the dam wall while preserving the original wall of lime mortar and syenite stones, which has great cultural and historical value. And finally, says the student, "from an environmental point of view, it is important to decide on the most appropriate strategy regarding the conditions of the watershed, which has saline springs, scarce vegetation and a high rate of erosion."

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