**5. Results and Discussion of the Analysis of the Main Impacts of Offshore Wind, Applied to Parc Tramuntana**

#### *5.1. Analysis of the Impact of the Project on Water Turbidity and Sedimentation*

The increase in water turbidity is one of the main impacts detected, and it happens both during the execution phase, where it is of greater importance, and during the operation phase. The effects that sediment resuspension can cause indirectly in the transport and deposition of suspended sediment were also analyzed.

The installation of anchors, chains, and cables on sedimentary substrate, and the consequent sediment resuspension, are likely to cause direct effects on water quality (turbidity, alteration of the trophic and chemical states), and in turn indirect effects on the biota that may be affected by the turbidity plume, particularly hard-bottom filtering and suspension-feeding species. It should be noted that no such seabed has been detected in the project area, the seabed affected being composed entirely of fine sands and muds.

In general, regarding the alteration in water turbidity, the main effects on marine fauna are a decrease in visibility (affecting the behavior of certain species in their ability, among other aspects, to capture prey or detect predators) or, if prolonged over time, affecting the feeding and breathing capacity of suspension-feeding and filter-feeding animals, including fish, whose gills may suffer physical damage due to the presence of abrasive particles in the water.

Reduced light penetration also affects the depth of the photic zone (the layer where sufficient light reaches for photosynthesis) and thus the primary production capacity of algae and phanerogams.

The subsequent deposition of suspended sediment on the seabed would have its greatest impact on sessile species, particularly on those species most sensitive to sedimentation.

Considering these effects, the area with the greatest sensitivity and potential impact due to turbidity is considered to be the *Cymodocea nodosa* meadow in the shallower area, at depths below 20 m, close to the exit of the route through the HDD. In the remaining area affected by the evacuation route and the area where the wind turbines will be installed, no significant presence of structuring sessile species was detected.

The increase in turbidity may also have an associated impact on water quality, when possible pollutants present in the sediment are mobilized. This potential impact is dismissed, given that the analysis of sediment samples taken in the area affected by the project shows no evidence of significant contamination.

In the Tramuntana project, during construction, the foreseeable increase in turbidity is mainly associated with the activities of drilling execution (HDD) in the maritime-terrestrial zone and export submarine cable burial by jetting for its protection. Both actions are temporary (with a permanence of the impact in the order of hours) and their effects on turbidity are reversible. In this phase, anchoring activities of anchor and mooring lines are short-time events, and have a lower capacity to generate turbidity, as well as the laying of inter-array cables, a part of whom can rest directly on the seabed, without the need for burial.

During operation of the wind farm, since most of the submarine cables are buried and immobile, the only contribution to increased turbidity on the seabed is that associated with the wind turbine anchoring systems, due to the resuspension of sediments from the seabed that may be produced by the movements of the mooring lines under the effect of waves and wind.

#### 5.1.1. Impact of HDD on Turbidity and Sedimentation (Construction Phase)

The HDD is a small diameter borehole that is drilled between the entrance pit, located on land behind the beach dunes, and the exit point in the sea 1770 m away from the previous one, at a depth of about 16 m. This is the technical solution proposed to allow the landing of the submarine cables without affecting the seabed surface and the biological communities present on it, some *Cymodocea nodosa* meadows of high ecological value.

These boreholes are drilled using drill heads operated from land and assisted by a jack-up structure (platform with legs) near the exit point (Figure 3).

**Figure 3.** HDD drilling scheme for land–sea transition. Source: Herrenknecht.

The drilling system requires the use of bentonite fluid (composed of water, bentonite, which is a natural clay, and a small proportion of polymeric additives, wetting and dispersing agents), to facilitate drilling and to extract the excavated material, which is constantly recirculated and recycled to an onshore separation plant. Only at the exit of the drilling head at the end of drilling path is the loss of a limited volume of this fluid foreseeable. This fluid would temporarily increase turbidity around the exit point.

To reduce the dispersion and sedimentation of these bentonite clays around the exit point, containment measures are planned to be implemented by temporarily casing the exit point, to allow collection and recycling of the clays.

Based on the dispersion studies of the most unfavorable bentonite fluid emission scenarios, carried out by means of the MOHID numerical model, the predicted impact, characterized by the exceeding of the reference turbidity level (defined in accordance with Additional Provision IX of Law 22/1988 of 28 July 1988 on Coasts as 1.5 times the normal average of suspended solids measured in pre-operational state, i.e., 6.75 mg/L), is temporary (lasting less than 2 h), of low intensity (with maximum values not exceeding 40 mg/L), and limited to the immediate surroundings (distances of approximately 30 m).

5.1.2. Impact of Jetting Operations on Turbidity (Construction Phase)

The jetting technique consists of opening a trench in the seabed by applying pressurized water jets that causes the soil beneath and around the cable to fluidize, allowing the cable to sink through the suspended sediments to the bottom of the trench, to the required burial depth (Figure 4).

**Figure 4.** Example of subsea cable burial by jetting. Source: CT Offshore.

This technique minimizes the impact footprint and sediment resuspension, being the cable installation and burial method that generates less spatial impact (reduced trench width) and less temporal impact (reduced execution time) of those analyzed in the project.

This burial technique would be applied from the drilling exit point (HDD) to the wind farm site, about 24 km away.

To assess the effect of this operation, the sediment dispersion generated per m of advance was simulated again by the MOHID numerical model, considering a conservative hypothetical situation, in which it is assumed that all the sediment ina2m × 1 m trench is resuspended. The results of suspended solids concentration in the water column under this assumption show that the time duration of significant concentrations (above 6.75 mg/L) near the bottom is, in all cases, low: 3 h in the shallowest areas and those closest to the phanerogam meadows (<20 m) and up to 5 h in the deepest ones (120 m).

The impacts are therefore temporary (no more than 3–5 h above the reference threshold of 6.75 mg/L), of low intensity (maximum values below 40 mg/L), and limited again to the immediate surroundings (distances of less than 30 m), and mostly in areas of soft, non-vegetated substrate, so that the effects of the temporary increase in turbidity in the benthic zone on the physical environment and biota are considered compatible, temporary, and of low magnitude. By way of comparison, it should be noted that these levels of suspended solids and their persistence are equivalent to those usually produced during sea storms around the meadows.

Sedimentation in the shallow zone is limited to the area near the trench, and at a distance of more than 30 m does not exceed 2 cm. In the 30 m closest to the trench, 3 cm of thickness can occasionally be reached, between 30 and 60 m distance, 2 cm are not exceeded, and at more than 60 m, 1 cm is not exceeded.

It should be emphasized that the numerical modeling carried out is very conservative, since it considers the suspension of all the material present in the trench. In real observations made in other projects for laying and burying submarine power cables [13,14], the volume of resuspended material has been less than 30% of the trench volume, and the mobilized material accumulates mostly in a short radius around the trench (at a distance of less than 10 m).

5.1.3. Turbidity Impact of the Movement of Turbine Mooring Lines (Operation Phase)

The anchoring systems for floating turbines are composed of three to four mooring lines, consisting of chains connecting the floating platforms with anchors buried more than 10 m below the seabed (Figure 5). Part of these catenaries rest on the seabed to counteract the movement of the turbines under the effect of wind and waves with its weight.

**Figure 5.** Schematic of the subsea mooring and cabling system of a floating turbine. Source: SENER.

A part of this resting section, approximately 330 m long, will be subject to certain movements that may cause the sediment to resuspend along its contact with the bottom, which may have indirect effects on the biota present in the area, in this case mostly benthic macrofauna that colonize the soft bottoms, as well as demersal species (e.g., hake).

It is estimated that the area affected by this effect is of the order of 1650 m2 for each catenary (along the resting section and with a maximum arc of movement of 4◦), or up to 6600 m<sup>2</sup> for each turbine (considering 4 mooring lines). Considering the area affected by the 35 planned turbines (0.23 km2) compared to the total area where the wind farm would be installed (about 95 km2), the area affected by this impact is about 0.24%. By contrast, the area currently subject to resuspension and turbidity on the seabed due to trawling activities in the Gulf of Roses (assuming a fleet of 21 vessels with an average operation of 180 days per year, with 5 h of fishing periods) is estimated at 950 km<sup>2</sup> (95,000 Ha). In addition, this activity generates turbidity values much higher than those associated with the mobility of the mooring lines of a wind farm [11].

Based on the studies carried out, analyzing the possible movements in the catenaries, low speed movements are expected and the effects are not likely to reach a distance greater than 5 m at both sides of the chain, but will be limited to the furrow of maximum amplitude of the footprint considered for the catenary. The level of turbidity generated would therefore be close to that observed due to natural variability, and lower than that usually produced during episodes of strong currents or anthropogenic activity (e.g., if compared to the effect on turbidity produced by trawling, which turbidity plumes can exceed 100 m of thickness and reach suspended solids concentration of 200 mg L−<sup>1</sup> close to the sea bottom [15]).

The temporal effects of turbidity, although significant, will therefore be of low magnitude and low periodicity occurrence, considering the slowed movement of the chains. These effects on the physical environment are considered compatible, considering the adaptive capacity of the species that colonize the affected sedimentary bottoms.

### 5.1.4. Overall Assessment of the Impact on Turbidity and Sedimentation

During the construction phase, the impact on the increase in turbidity and sedimentation is considered significant in the shallow area, considering that it happens in waters of protected natural areas (RN2000) and the presence of the marine phanerogam *Cymodocea nodosa* meadow in the landing area, despite the fact that the effect is temporary (hours) and the rate and spatial extent of sedimentation is limited.

By assessing the different descriptors of each impact (turbidity and sedimentation), according to the described methodology, a Moderate impact rating is obtained for both during the construction phase, as shown in Table 2:


**Table 2.** Overall assessment of the impact on turbidity and sedimentation during the construction phase.

In terms of the impact on this vector during the operation phase, the expected impacts on turbidity are very limited, restricted to less than 1% of the project area, which in relation to the current impact of trawling in the area occupied by the park where this activity would cease (about 600 ha, equivalent to 6% of the park's surface), represents, both in terms of the extent and magnitude of the impact, a relative improvement in terms of turbidity conditions and recoverability of the seabed from sediment resuspension. From the modeling carried out, it is concluded that the levels of suspended solids in the water column derived from the movement of the chains are scarcely significant (less than 2 mg/L at distances of less than 25 m) at all times, being considered of low intensity and persistence (levels of less than 1 mg/L in less than 2 h).

The assessment of these impacts in the operation phase is summarized in Table 3:


**Table 3.** Overall assessment of the impact on turbidity and sedimentation during the operation phase.

#### *5.2. Analysis of the Project's Impact on Underwater Noise*

The project has the potential to alter the acoustic environment of the area, both during the construction and operation phases.

In the study area, background underwater noise (reference situation), which is related to natural sources (e.g., wind) and artificial sources (e.g., maritime traffic, professional fishing), was measured by means of PAM (Passive Acoustic Monitoring) equipment. The recorded noise values reach peaks (SPL) of 156–159 dB re 1 μPa and cumulative averages (SEL) of 134–139 dB re 1 μPa at the wind farm proposed location. The greatest contribution is from maritime traffic, highlighting the important fishing activity linked to the port of Roses.

Underwater noise has the potential to alter the acoustic environment of the area and affect several species, especially marine mammals, with special attention to cetaceans present in the study area (mainly bottlenose dolphins), and chelonians such as loggerhead turtles (with less presence in the area).

The acoustic impact will depend on the distance to the source of the receptor. Considering the distribution of cetaceans in the study area, observed during the monitoring campaigns carried out during the last year, the main type of detected cetaceans are dolphins (bottlenose and striped dolphins), associated with a medium frequency hearing range (150–160 kHz), and to a lesser extent species with a low frequency hearing range (7–35 kHz), such as the fin whale.

In relation to the potential effect on turtles (loggerhead turtle), a reference level RMS of 166–175 dB re 1 μP [16] is considered to be the range for behavioral change effects, based on experimental measurements with species in captivity.

According to *MAGRAMA* [17], the reference levels for the definition of exclusion zones are 160 and 180 dB rms, corresponding to thresholds for which behavioral changes and physiological damage are detected in cetaceans, respectively.
