Stabilization and flood protection measures for the left bank slope of the Sant Pol stream, in the area of Can Roca

The Sant Pol stream is formed by the confluence of the d’en Mates and Vallalta streams. Both watercourses converge just downstream of the urban center of Sant Cebrià de Vallalta and flow into the sea 4 km downstream at Sant Pol de Mar.

Impact of Storms on the Sant Pol Stream

During the 2020 storms Gloria and Sant Jordi, the Sant Pol stream experienced significant flooding, causing severe erosion along its banks, including in the Can Roca area. Specifically, the streambed shifted toward the outer side of the meander, triggering erosion and compromising the stability of the left bank slope, which is approximately 10 meters high and 120 meters long.

Over the years, the Sant Pol stream has progressively narrowed and shifted toward the left bank in the Can Roca area, resulting in erosion at the slope’s base, affecting the Can Roca access road, and depositing materials in the streambed that may impact flow capacity within the urban area.

Objectives of the Proposed Measures

The primary goals of the project are to stabilize the affected slope and restore the Can Roca access road, along with additional measures to prevent future left-bank streambed shifts. The interventions aim to ensure environmental and landscape integration. The plan proposes to restore the channel’s original course before the storms caused bank collapse and includes realigning the channel toward the right bank.

Considering the high flow velocity (average values over 5.5 m/s), a water depth of over 4.0 meters for 500-year flood events, the significant height of the slope (over 9.5 meters at the most vulnerable point), and the presence of the Can Roca access road at the slope’s top, a solution combining riprap with bioengineering techniques was selected.

The slope protection includes a double layer of structural riprap weighing 1,000 to 3,000 kg, with a 3H:4V slope, reaching the water surface level for a 500-year return period flood (maximum height of 8.0 meters). The riprap was placed on a 20 cm thick gravel bed over a 200 g/m² geotextile layer, with the lower section integrated using willow stakes.

The section is further reinforced with cross spurs along the left bank approximately every 10 meters. These are constructed with three vertical and two horizontal rows of riprap weighing 300 to 1,000 kg, placed on a 20 cm thick gravel bed over a geotextile layer.

At the top of the slope, a fill was created up to the existing road, on which a 30 cm layer of organically enriched topsoil was spread and revegetated with hydroseeding of native species and the planting of plane trees. The lower terrace between the spurs was covered with a geomat made of an organic coconut mat reinforced by a polypropylene mat, capable of withstanding water flow speeds up to 6 m/s, and was revegetated with hydroseeding and white willow planting.