Question 1: A proper coverage by deep-rooted plants are the main factor towards a properly functioning riparian area. |
The relationship between deep-rooted plants and riparian status, independently on the cause, evidences the importance of having larger plants protecting banks and the rest of riparian area.
As easily observed in the field, the coverage loss is not necessarily an exposed soil, it can be grass planted by contractors after filling eroded banks, or rocks in rip-rap works. Both grass and rip-rap are common treatments for bank failures and usually present higher costs, especially to the environment in long-term, such as erosion and stream sedimentation. |
Question 2: Where conventional treatment is not a necessary evil.
Grass after land fill
The grass, currently used in conventional treatments to protect streambanks, is not the proper species because it does not have deep roots to stabilize the soil. That probably explains why there are problems with superficial erosion in infrastructure works involving run-off and water bodies.
Vegetation in civil engineering works has an aesthetical function so far. That is why grass has been used as a "green" finish. Besides, it is cheap and grows relatively fast. [7][8]
Depending on the situation, grass can be used combined to larger plants, which are meant to perform the soil reinforcement.
Vegetation in civil engineering works has an aesthetical function so far. That is why grass has been used as a "green" finish. Besides, it is cheap and grows relatively fast. [7][8]
Depending on the situation, grass can be used combined to larger plants, which are meant to perform the soil reinforcement.
Rip-rap
Rip-rap is another common treatment to stabilize river banks after failures. The environmental concern about this technique is the aquatic habitat loss and the lack of biodiversity. Those factors, besides the absence of deep-rooted plants, easily explains why a riparian area with this treatment cannot be considered properly functioning. Regarding flooding protection, rip-rap is able to keep a bank stable, however, the hydrodynamic effects are questionable when compared to a healthy riparian area. Dense shrub plantation can dissipate much more energy from water than rocks in rip-raps.[4]
Pseudo-Hybrid: Conventional + plants
As shown in the Introduction, vegetated rip-rap is another type of treatment for streambank rehabilitation. It is mostly a conventional rip-rap with plants in an density that should not disturb the rip-rap effect. According to the results, a rehabilitated area should have a coverage of at least 60% with deep-rooted plants which means 1 plant each 2 square meters approximately. .
This necessity of coverage by plants casts doubt on the effectiveness of rip-rap after this modification, which means whether the retaining function would be jeopardized as mentioned by the U.S. Army Corps of Engineers in the Introduction.
This necessity of coverage by plants casts doubt on the effectiveness of rip-rap after this modification, which means whether the retaining function would be jeopardized as mentioned by the U.S. Army Corps of Engineers in the Introduction.
Question 3: Bioengineering is a potential way to upgrade the treatments for stream bank and slope failures.
The fact that banks with low coverage of deep-rooted plants are more likely to have a not properly functioning riparian area encourages the use of bioengineering in rehabilitation works even if still based on experience or belief. According to the results, the coverage with willow stakes, for example, should cover more than approximately 60% of the area.
Moreover, this provides support for more research to fill the gap, mentioned in the introduction, regarding geotechnical and hydrological information on bioengineering to guide managers and project designers decide whether this technique is applicable or not.
It is important to notice that bioengineering does not replace conventional engineering in any case. There are cases where bioengineering is not applicable and conventional engineering is needed.
Moreover, this provides support for more research to fill the gap, mentioned in the introduction, regarding geotechnical and hydrological information on bioengineering to guide managers and project designers decide whether this technique is applicable or not.
It is important to notice that bioengineering does not replace conventional engineering in any case. There are cases where bioengineering is not applicable and conventional engineering is needed.
Human activities
Anthropic damages on a riparian area is another important factor that influences the functionality of a riparian zone. It is related to the loss of deep-rooted plants because those plants can be completely eliminated where there are intensive human activities.
According to the results, there are more the anthropic damages in a 'not properly functioning' riparian area.
Human activity alone such as recreation does not appear as a risk factor due to the overall low impact. However, watershed managers should be aware that the intensive use of the land even with only one factor, leads to damages of riparian area and consequently lower water quality [3].
According to the results, there are more the anthropic damages in a 'not properly functioning' riparian area.
Human activity alone such as recreation does not appear as a risk factor due to the overall low impact. However, watershed managers should be aware that the intensive use of the land even with only one factor, leads to damages of riparian area and consequently lower water quality [3].
Final considerations
The lack of a proper soil coverage with deep-rooted plants and the intensive use of the land overcome the risk presented by natural attributes of a site, for example, geology or topography.
Moreover, human activities, such as grazing, agriculture, recreation as well as the conventional treatments on streambanks, affect the riparian area and its functionality.
Conventional treatment in a bank represents only a static feature which physically retains a bank without positive interaction with the surroundings. Due to that, those treatments are subjected to new erosive processes and cannot provide all the benefits of a riparian zone which is composed mostly of deep-rooted plants.
Therefore, from the riparian functionality perspective, conventional treatment can be seen as an anthropic damage rather than a proper treatment for streambank failures due to the lack of deep-rooted plants as a component.
Plants or bioengineering does not replace conventional treatment when they are required because there are stress limitations for plants when compared to steel or rocks. However, due to liability issues, managers and contractors prefer conventional treatments over bioengineering even when not necessary. This makes conventional treatment, which causes environmental losses, not a necessary evil, but a convenient one.
Finally, it is still unusual and technically difficult to acknowledge plants as construction material such as concrete, steel or rocks. Due to that, there is a fair reluctance to accept larger plants as a functional material in current project designs, which are required to be stamped and, thus, liable. Therefore, public policies and treatment for streambank failures need to keep on being upgraded by promoting more riparian planning and, especially, start acknowledge plants as construction material.
Moreover, human activities, such as grazing, agriculture, recreation as well as the conventional treatments on streambanks, affect the riparian area and its functionality.
Conventional treatment in a bank represents only a static feature which physically retains a bank without positive interaction with the surroundings. Due to that, those treatments are subjected to new erosive processes and cannot provide all the benefits of a riparian zone which is composed mostly of deep-rooted plants.
Therefore, from the riparian functionality perspective, conventional treatment can be seen as an anthropic damage rather than a proper treatment for streambank failures due to the lack of deep-rooted plants as a component.
Plants or bioengineering does not replace conventional treatment when they are required because there are stress limitations for plants when compared to steel or rocks. However, due to liability issues, managers and contractors prefer conventional treatments over bioengineering even when not necessary. This makes conventional treatment, which causes environmental losses, not a necessary evil, but a convenient one.
Finally, it is still unusual and technically difficult to acknowledge plants as construction material such as concrete, steel or rocks. Due to that, there is a fair reluctance to accept larger plants as a functional material in current project designs, which are required to be stamped and, thus, liable. Therefore, public policies and treatment for streambank failures need to keep on being upgraded by promoting more riparian planning and, especially, start acknowledge plants as construction material.