Fluvial watercourses

ACTION B3

Recovery and rehabilitation the old floodplains of Limia River

PARTNERS RESPONSIBLE FOR THE ACTIONS

ACTIVITIES

The environmental recovery of modified river bed, and the regeneration and reconnection of lagoon systems, are viable options that contribute to reducing the presence of nutrients in the water and eutrophication, while recovering habitats of special importance in spaces included in the Natura 2000 Network.

The action will seek to favour the connectivity of the Limia River, currently being canalized, by recovering a section of the old channel. The place of action will be the Veigas de Ponteliñares, in the municipality of Rairiz de Veiga.

In times of the drying up of the old Antela Lagoon, the Limia River was channeled and rectified, giving rise to its current rectilinear layout. This caused water to circulate through its old meanders, which collected only the rainwater from the surrounding plots. This resulted in a limitation of the function as a fluvial ecosystem of the river itself, isolating from these processes the old meander, which even became a depressed zone where water accumulation phenomena occurred, creating humid areas without connection. It is necessary to add that part of this meander was filling in the old works of canalization, with the surplus material of the creation of the new channelled Limia River.

The area of the area included in the action is 8.5 hectares; the length of the open meander is approximately 900 meters. It is included in the Special Protection Area (SPA) of “A Limia”, close to the Site of Community Importance (SCI) “Veiga de Ponteliñares”; also forming part of the Important Area for Birds “A Limia” and the Allariz Biosphere Reserve, characteristics that give it great environmental and ecological importance.

The existing channeling of the contemporary channel of the Limia River to its alluvial plain will be permeabilized and the generation of new wetland areas and the opening of a secondary branch or meander, by the old route of the mentioned river.

The objective in the future is to expand this step to more river stretches apart from the once already recovered.

Plot contributed by the San Lourenzo Neighbouring Mountain Community.

RESULTS

The main recovery actions of the old meander consisted of:

Improvement of existing river bank vegetation, through forestry treatments consisting of pruning, elimination of dead specimens, mainly alders, birches and willows with danger of falling on the river bed and elimination of channel obstructions for dead vegetation.
Rehabilitation of the old channel, through the reopening of the same, with a suitable longitudinal slope to allow the circulation of the water, excavating the river bed and extracting a lot of sediments and vegetation deposited. During the excavation, in the areas where it was possible, slopes will be created with the objective of naturalizing the channel as much as possible and thus create zones of rapid and slow water passage. Likewise, the new channel will be connected to the small natural and artificial lagoons existing near the new flood plains, as well as to the new biotopes that existed in the area, in order to cause the water to remain as long as possible. Veigas and thus enhance the purification function of the vegetation.

The connection of the meander recovered with the Limia’s river bed, disconnected by the service road that accompanies the current channel, was carried out with the construction of two works of passage, one of entry and another exit, made with prefabricated frames and stone fins of irregular breakwater. The interior of the frames was covered with stone bed in order to integrate them landscape and favour sedimentation. The slopes were revegetated with coconut blanket, sowing of herbaceous plants and willow cuttings, so that the vegetation itself will be responsible for environmentally integrating the works carried out.

Mainly at the beginning of the meander, 1,130 autochthonous macrophytic plants, previously collected from the nearby Limia river, will be planted manually. With this plantation, the naturalization of the new fluvial habitat will be ensured.
Two biotopes have been created associated with hygrosophical media to house species of flora and fauna (birds, reptiles, amphibians and fish) priority for the species included in any of the categories of the Galician Catalog of Threatened Species.

Once the recovery of the meander was completed, monitoring work began on contaminant reduction rates after passing through the new channel, in comparison with the rectified Limia River, as well as monitoring the evolution of the different biotopes generated. To do this, both partners, CHMS and DXCN, developed a system to monitor the different physical-chemical-biological parameters of water quality and biological indicators regarding birdlife, flora and herpetofauna.

Views of the recovered meander:

Pollution reduction rates are being monitored through the evolution of water quality, both physical and chemical parameters, as well as a monitoring of the evolution of ecosystems taking into account birds, flora and fauna. herpetofauna, the results detected so far are shown below:

Environmental restoration of banks and riverside forest

The works carried out for the restoration of the area are indicated below:
5,054.36 m2 of selective manual clearing of tree and shrub vegetation on the banks of the old canal of the River Limia.
1,130 units of autochthonous planted vexetais species, coming from near the action zone (
350 m of trenching with manual and mechanical means, in all types of terrain and with variable depths for the interconnection of the water channels with the existing natural lagoons within the action area as well as with the new biotopes to be built, with transport of excess materials to other areas around the action.
Construction of dous biotopes associated with hygrophilic environments to house species of flora and fauna (birds, reptiles, amphibians and fish) with priority for species included in any of the categories of the Galician Catalogue of threatened species.

Evolution of water quality

For the monitoring of the actions, periodic samples are taken in the recovered meander and in the Limia River where the water normally flows, in order to compare the nutrient reduction rates between both.
So far it has been proven that through the circulation of water through the meander there is a reduction of nutrients (phosphates, phosphorus, nitrates and nitrites) higher than what occurs in the river, in the same section.
The organic matter is more favourable in the recovered meander than in the section of rectified river.

Meander reduction difference recovered against river

Less BOD5

Less phosphates

Less phosphorus

Less nitrates

Less nitrites

In terms of improving water quality through the analysis of the biodiversity of species that inhabit the recovered meander, it is observed that macroinvertebrate species are colonizing the meander, which assures us that the animal communities are being implanted in this ecosystem. On the other hand, it is verified that the indicator of diatoms is also positive and that the communities of macrophytes have a good representation. Given that the latter are the ones that have a greater participation in the processes of natural purification, they ensure this essential function that the project pursues.
The results are encouraging and would allow demonstrating that part of the project's objectives consisting of favouring the retention of nutrients in a natural way are being achieved.

Evolution of the generated biotypes

The expansion of planted hygrophilous species has slowed its growth due to the irregularity of rainfall in 2018. The same has occurred with the colonization by avifauna, herpetofauna and ichthyofauna for the colonization of inland wetlands.
Although it is beginning to be verified that species linked to the fluvial environment, such as the chestnut sandpiper, use it as a river section to feed them.

Results

Video