What is soil and water bioengineering

Soil and water bioengineering is a scientific-technical discipline that uses the biogenic properties of some plant species, based on the practical merging of knowledge from the science of biology and engineering itself, and the diverse use of living plants as well as the materials derived from them, while making the most of topography, soil and microclimate in each case.

We refer to soil and water bioengineering applied to landscape in particular because we aim to implement all soil and water bioengineering techniques applied to the environment, which have a structural and landscape purpose in the environment. With regards to the materials used, bioengineering can be naturalistic (a set of techniques that use living and raw materials such as logs, stones, soil…) or may use biophysical engineering (a set of techniques that use living materials as well as made products such as blankets, geonets, geosynthetics, etc.).

When these techniques are in use, the living beings responsible for the work are plants; therefore, while achieving structural goals we can also bring about positive environmental change.

The main rule is that we always work with native species to solve problems and improve good ecological status of the spaces in which we act.

With regards to soil and water bioengineering techniques applied to rivers to improve the quality of water, we work with hydrophytes or macrophytes to provide a suitable habitat for the bacteria in charge of the decomposition of organic matter.

An effective tool

Soil and water bioengineering is the technical discipline and an effective tool which allows for the application of scientific knowledge in the use of matter and energy sources, through useful inventions and constructions based on living matter. Although we need moisture for the development of living materials, good results may also be obtained on dry slopes.

The most sophisticated technique is the Krainer wall, developed in the north of Italy, which is built using wood trunks stitched to other added materials. Therefore, it represents an improvement on more traditional techniques, obtaining higher resilience, but it still needs a climate with constant humidity.

In the 1980s in Germany, new techniques appeared which combined inert materials, such as coir fiber, with living materials. These living materials are usually herbaceous plants, and their great variety allows for many possibilities. They are riparian herbaceous species adapted to stress conditions, such as a lack of water, water impact, poor soil, etc.

In some stretches of a river, the decline in a section makes it impossible to plant trees, which can have an impact on the hydraulic capacity. In these stretches, which are mainly urban, the use of herbaceous plants, specifically helophytes, can prove very suitable. Many of these species are specifically adapted to floods. When water flows, they fold on the ground and are very elastic, so that they have a minimal hydraulic resistance and protect the soil.

As in previous examples, the technique is based on using native species and the preparation of a suitable substrate. At the same time, this must have some physical characteristics allowing for good hydraulic resistance.

One of the most commonly used techniques is pre-vegetated fiber-structured roll. It is a coir fiber cylinder of 30cm in diameter, packed in non-biodegradable netting. This inert material, coir fiber, is one of the slowest-degrading natural fiber materials and is totally harmless. In fact, coir fiber is obtained from the fruit shell, which is widely used in food and the pharmaceutical industry, and is known as copra, a by-product which until now was difficult to sell. This material, which has a uniform structure inside the fiber roll, is appropriately pressed so that there is a balance between the degradation of the fiber and the use of this space by plant roots. In this manner, its structure does not experience damage over time despite being entirely overfilled by vegetation. To accelerate the process as much as possible and promote resistance to drying and to other environmental variables, they are pre-vegetated at the nursery.

It is recommended to collect plant materials from the area of intervention: planting it, inserting it into the structure, growing it at the nursery and finally placing it in the environment.

There is, in fact, a wide range of products and techniques that aim to obtain more mature plants while ensuring their structural role. They are also based on coir fiber: structured-fiber plant units, different formats of pre-vegetated mats…

In the early 1990s, a type of flexible gabion was developed which, through a high-resistance network structure, allowed us to use a smaller filling stone with the same resistance while allowing plant colonization. Recently, many new products have also been developed, such as geonets and organic blankets, which are highly-resistant plant materials.

Soil and water bioengineering applied to restoration of rivers

Soil and water bioengineering requires extensive prior studies on the ecosystem and river dynamics so that the plants are placed where they should be and in the correct way, while ensuring their survival and development.

These techniques can also contribute to structural goals, replacing other techniques used until now in pipelines and protection works, such as breakwaters, and concrete on many occasions. The latter can destroy the fluvial dynamics and have a strong impact on aquatic ecosystems, banks and landscape.

Soil and water bioengineering techniques are increasingly known and used in Spain. They are also well-developed in other European countries such as the United Kingdom, Germany, Italy and Sweden. It is expected that they will be used more and more frequently, replacing the current piping projects based on breakwaters and concrete.

At Naturalea, we have been clearly committed to the future development of these techniques for a long time now, and we offer an opportunity to incorporate this new way of understanding into the restoration and improvement of our rivers.

Plants work

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 stabilize 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 stabilization 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 stabilization method.

Once we have stabilized 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 vulpinaCarex pendulaClaudium mariscumTypha 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.

A well-structured plant provides immediate effectiveness

In soil and water bioengineering, the technical solution used is very important, and in recent decades quite a lot of research has been done in this field. A German engineer, Lothar Bestmann, more than forty years ago, began to look for technical solutions for plant establishment. After some research, his first finding was that helophytic herbaceous species do not need soil but they can thrive with water, nutrients and a substrate in which to develop.

At substrate level, all kinds of materials were tested: straw, branches, fibers, etc., until an excellent material was found, which was also a by-product at the same time: coir fiber. The first idea, therefore, was to grow plants in pots with coir fiber. The result was a plant unit structured in fiber, which also possessed a vital amount of rhizomes, so it had stock reserves for growth, for withstanding low flows and floods, etc.

Given the demands of modern soil bioengineering, which aims to perform the most effective interventions with nearly immediate results (offering guarantees when there is uncertainty and risks involved in river dynamics) and to operate like conventional engineering solutions, research continued and more complex forms of presentation were developed.

The goal is, thus, to create more developed units in order to accelerate establishment and improve resistance. New materials consisting of grassland structured in fiber and reinforced with a mesh were produced. This provides tangential resistance thanks to a double-mesh made using coir fiber.

This product consists of well-structured herbaceous blankets with a highly-developed root system, which will be prepared in shallow pools. Several technical names have been coined: grassland structured in fiber, plant pallet, plant carpet, coir pallet, etc. These vegetated carpets allow structural stability in water with ordinary speeds of less than 1.5 m/s and their establishment in permanently flooded areas.

However, in some cases plant structure was still not developed enough and therefore research carried on until the true brick of biological engineering was created: cylindrical structures of coconut fiber, pressed and packed in a permanent polypropylene mesh. The key to this finding is that fiber degradation is slower than root growth, so that roots take over the tubular structure entirely and stick to the ground. Currently, in cold and non-eutrophic waters, polypropylene mesh is replaced with coconut mesh.

This product is currently being used all over the world and has different names: fiber roll (the original name), bio roll, coir logs, etc. The key element, we must not forget, is the vegetation, since the structure serves only as a system for the effective establishment of plants in especially dynamic areas. Since the central element is still the plant, the fitted structure is pre-vegetated, and the plant is well-developed. Also, whenever possible, it is necessary to work with varieties typical of the area.