Gully erosion episodes in the São Pedro valley at São Pedro de Moel, Portugal

In the surroundings of São Pedro valley, in the village of São Pedro de Moel, several episodes of gully erosion of dune sands have occurred in recent decades, with relevant occurrences in 2002, 2016, 2017 and 2021. 

The São Pedro valley has an unusual geomorphology caused by the presence of a hidden karst spring creating a short water line with a length of only 475m until it reaches the sea at the São Pedro de Moel beach. The upstream zone of the São Pedro valley presents a singular shape similar to an elongated horseshoe, due to the soil erosion, by the action of the permanent karst spring.

The inclination of the valley slopes varies between around 30°, exceptionally reaching 50º in the most abrupt areas when the vegetation supports the dune sands, and a minimum of about 10° in the flattest areas of the slopes.  

The stratigraphic sequence of the São Pedro valley consists of Toarcian limestone at the base, overlapped by undifferentiated Plio-Pleistocene soil, covered by dunes and dune sands, locally overlapped by road fills essentially built with dune sands. 

The gentle slope of the ground to the west, towards the sea, in the surroundings of the valley, together with the low permeability streets paving, favors the surface runoff to the west, until reaching the alignment of the São Pedro valley. At this point the runoff waters, are redirected and strat to flow north, according to the alignment of the initial section of the São Pedro valley. 

The generality of the gully erosions mentioned before resulted from the occurrence of episodes of intense precipitation that concentrate quantities of water above the normal. By analyzing the last erosive process, it was confirmed that the beginning of the gully erosion occurs when the water overturns the crest of the slope, flowing along the higher inclination, perpendicular to the face of the slope, until the bottom of the São Pedro valley. The erosive process develops very quickly, and last a few tens of minutes. The absence of storm drainage systems in the streets at the crest of the São Pedro valley aggravate the erosive phenomenon, allowing the concentration of surface runoff water, increasing the erosion processes.  

To solve this recurrent problem, punctual stabilization works were initially carried out, but proved to be inefficient. Recently, stabilization work more expensive, more elaborated but more effective were used, using a surface protection of the slope with a Reno mattress, complemented with the construction of surface drainage systems in the streets, with capacity to drain the storm waters and properly route those exceptional flows. 

This case study allows to verify that to obtain good urban performance, geomorphology, geology, hydrology, hydrogeology and engineering must be considered in a complementary approach.