Water Retention Structure

Due to the increasingly noticeable effects of climate change — through extreme weather events such as heatwaves, heavy rainfall in short periods, extreme winds, and soil erosion — it has become necessary to adapt the land to these new weather conditions. Our region of Cova da Beira is also strongly affected by these changes. Since the region has a landscape with steep slopes and soil with low impermeability, whenever there is heavy rainfall, the water is channeled directly into the rivers at high speed, contributing to a sharp rise in river flow and, consequently, flooding.

Statistical data tells us that average rainfall in this region is 1082 mm per year. However, there is seasonal variation in both the probability and amount of monthly rainfall. As shown in Figure 1-a, the period during which the daily probability of rainfall is below 20% — more commonly known as the dry period — lasts approximately 4 months, from late May to late September. As shown in Figure 1-b, during this same period, the average accumulated rainfall is significantly below 50 mm per month.

a) Daily probability of precipitation in Covilhã.

b) Average precipitation (solid line) accumulated over the continuous 31-day period surrounding the day in question. Data between the 25th and 75th percentiles are shown in the dark grey band, while data between the 10th and 90th percentiles are shown in the light grey band. The thin dotted line represents the corresponding average snowfall.

Figure 1: a) and b) taken from ©WeatherSpark.com [2].

Therefore, in order to mitigate the consequences of drought periods and torrential rains, one of the first challenges to consider when starting work on any given piece of land is water management particularly how to promote infiltration and water conservation. Infiltration basin and swale techniques, widely used in permaculture, allow water to be retained in the soil and nutrients to be preserved, making it possible to create sustainable, productive, and biodiverse spaces that in turn support the flourishing of native fauna and flora [4].

Retention basins (ponds) are structures designed to hold rainwater runoff, helping to address both extreme flooding and drought while creating very rich micro-habitats (Figure 2). Swales, on the other hand, are built along contour lines, following the topographic shape of the land, in order to increase water retention and infiltration into the soil (Figure 3). Both ponds and swales are highly effective, relatively low-cost water management tools that make use of the site’s own natural elements. Implementing these two structures significantly reduces the speed of surface rainwater flow, which in turn brings the following benefits[4]:

water is retained within the system, feeding into the water table;
soil erosion is reduced;
nutrient levels and organic matter content in the topsoil increase, improving its fertility.

Figure 2: Retention basins under construction. Taken from [1].

Figure 3: Diagram of infiltration swales. Taken from [5].

On September 17th, 2021, I had the pleasure of taking part in a swale-building workshop organized by the EcoAtivo team in partnership with Ananda Valley. The chosen location was perfectly suited to this purpose: part of a rocky slope with sparse vegetation that was constantly exposed to rainwater erosion. The swales built typically took a half-moon shape and were reinforced with pruning offcuts from the farm. To make the swales even more resilient, native trees (oaks, cork oaks, etc.) were planted within them the following day. Figure 4 shows part of the result of our work.

Figure 4: Some of the infiltration swales (highlighted with orange ellipses) built as part of the workshop organized by EcoAtivo.

Written by Dmytro Ostapchuk

Dmytro studies Computational Engineering at the University of Aveiro, but is also an enthusiast of environmental regeneration.