Plants and soil bioengineering
In our rivers and wetlands, we find herbaceous plants with very special properties due to their good adaptation to a highly dynamic area: floods, droughts, sediment transport, etc. River trees and shrubs (willows, alders, etc.) have been traditionally used to stabilise river margins. However, for these to be effective, they need minimal humidity levels which are difficult to reach in the Mediterranean area.
We are talking about stabilisation techniques which use living materials or naturalistic engineering, all based on the use of living materials: stakes, planting, lattice, faggots, etc. In addition to moisture, an obstacle to these techniques in areas of high demographic pressure is that the development of trees in river beds which have been artificially reduced can be problematic in the case of floods.
Hence soil bioengineering, without discarding the use of trees and shrubs in certain cases and circumstances, opts for the use of herbaceous plants as the fastest stabilisation method.
Once we have stabilised our restoration setting, we may consider plantations according to the characteristics of the environment, the project and the viability of their maintenance.
Soil and water bioengineering applied to rivers has used herbaceous plants since the end of the 20th century. The most commonly used plants include yellow lily (Iris pseudacorus), reed (Phragmites australis), bulrush (Scirpus holoschoenus), which is resistant to both droughts and floods, spiny rush (Juncus acutus), which is saline-resistant, Carex vulpina, Carex pendula, Claudium mariscum, Typha sp, etc. There are more than 30 species in the Iberian Peninsula with interesting properties which are useful to environmental and structural problems. There are still many species to research we may be able to work with in the future.
These plants have properties that make them ideal for river bioengineering work. In the case of the lily, for instance, it literally sticks to the ground with a pivotal root reaching over two meters in depth, and folds completely in floods to avoid pulling out. It is a very interesting system because the strategic introduction of this species can be more profitable, in many ways, than a breakwater. In addition, this plant is not only able to withstand floods without any impact, but also serves as the structuring agent at the basis of a river bank, preventing water from ripping other carpeting species. For example, the introduction of this species at the front of wattle marshland in Besòs river (Barcelona) has prevented many rips. Furthermore, in thriving also in eutrophic water, this species can produce leaves of almost two meters, which also protect the soil when bending.
Another example is the reed, Phragmites australis. This is another interesting herbaceous plant which acts as a vegetable mat combining a reasonably good resistance along with a great indirect capacity for water depuration. The amalgamation of bacteria acting in organic decomposition, in symbiosis with their own rhizomes, results in a very effective natural system. The same bacteria used in conventional water plants for biological treatment, which is known as a bacterial soup, are also associated with this type of reed. This species protects microorganisms from environmental changes that can be lethal and creates a stable structure.
In the tertiary treatment ponds of Can Cabanyes in Granollers (near Barcelona), the effects of reed and bulrush only lessen ammonia nitrogen from 38.46 to less than 1 mgN l , and BOD5 from 43.66 to 7.75 mgO l, between exit and entrance water. On the other hand, it is also worth mentioning that this species can withstand both an intense low flow in summer and life submerged half a meter deep.