Living crib walls 

A brush mattress. Image by: Stream Corridor Restoration: Principles, Processes and Practices, 10/98, by the Federal Interagency Stream Restoration Working Group (FISRWG)

Living crib walls and brush mattresses are both structures that retain slopes and protect stream banks and stream beds, in order to stabilise soil and prevent erosion. Brush mattresses can be installed by a small team, and on large and small scales, while crib walls can be used on industrial scales, to provide permanent solutions. In both cases, the use of natural materials creates increased stabilisaiton as the root structures grow and settle. 

A living crib wall is a retaining structure made of on-site fill material, timbers and live branch cuttings, that provide slope stabiliation. They are generally used to stabilise steep banks but are also used in stream and river beds and banks (LaRiMiT, 2023).

A robust timber framework forms the main structure, then live plants are inserted as it is built. The roots provide additional reinforcement as they grow. As the plants grow, the roots take over, the timber weathers and the wall slips naturally into the landscape. This is especially advantageous where large amounts of retaining are needed. The living crib wall can create a large, natural wall of plants rather than a wall of impervious, hard material. 

Vegetated, or living crib walls often use horizontal anchor logs, angled at 10-15° towards the slope. The logs are anchored with interlocking elements (LaRiMiT, 2023). As with all retaining walls, the exact design depends on the volume of what is being retained and the size of the wall. They can be straight, curved or angled, and can be scaled up to significant heights. 

A brush mattress, also known as live brush mats or brush matting is a combination of live stakes, live fascines (a long bundle of organic material), and branch cuttings, installed to cover and stabilise stream banks (MassDEP, 2024). The ‘mattress’ is layers of living material laid in a criss-cross pattern.

The brush mattress forms a protective cover over a streambank that will become increasingly stabilised as the roots grow. They can be used to restore riparian vegetation and support native plants, reduce soil erosion and intercept sediment (MassDEP, 2024).

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Name of NbS

Living crib walls

Type of NbS

 Created or constructed living ecosystems

Location

living crib walls can be in urban and rural contexts, as well as streams and waterways. Brush mattresses are specifically for waterways

Case Study

 Belle Island Brush Mattressing

A vegetated crib wall system becoming part of the natural landscape. Image by Arborgreen

Relationship to Indigenous knowledge

Across Te Moananui Oceania, there is a need to utilise, restore and protect native vegetation. A living crib wall can use Indigenous knowledge of native or culturally appropriate species, how and where they grow best for example, to determine which species to plant. 


The design of a crib wall or brush structure can pay homage to weaving traditions, for example Aotearoa Māori’s rāranga (weaving) and other weaving practices across Te Moananui Oceania. The living members weave together and connect, forming a structure that supports native vegetation, a potential physical metaphor for Indigenous knowledge and relationships to nature. See also Uwhi. In other places in Te Moananui Oceania, such as Samoa (and indeed globally) there are traditions of planting and manipulating live stakes to form living fences or living

Climate change benefits
  • changes in rainfall
  • coastal erosion
  • coastal inundation and storm surge
  • Desertification
  • Flooding
  • reduced soil quality
  • soil erosion and landslides
  • urban heat island effect
  • Wind / storm damage

As the climate changes, Te Moananui Oceania is experiencing more frequent rain, which can cause widespread erosion and subsequent sediment washing into waterways (EPA, 2024). Increased sediment or a change in sediment distribution is damaging to fragile ecosystems, both in waterways and in the surrounding areas including coral reefs. Sediment also damages water quality, and contributes to freshwater scarcity issues in Oceania. 

Brush mattresses can be used on streambanks, where erosion has caused a need for protection from flooding, or wave-induced erosion. As the roots and plants grow into the streambank, they become an integral part of the landscape, connecting and supporting it. 

Living crib walls can provide similar solutions on larger scales; their construction method means they can be scaled up to industrial sizes, and have been used in Te Moananui Oceania and across the world along roads, parks, steep banks and other urban settings. 

A living crib wall creates an opportunity to introduce vegetated, nature-based solutions to large-scale retaining walls. Visually, they become walls of greenery in urban settings. Structurally, they address erosion, storm surge, coastal inundation and the urban heat island effect by using the natural growing and root processes of plants, and natural materials. 

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Societal / socio-cultural benefits
  • disaster risk reduction and resilience

A brush structure can provide immediate protection on the soil banks of a waterway, to protect against floods, higher levels of water velocity, and storm surges. Natural materials capture sediment and rubbish, can be quickly installed, and simply maintained. 

Living crib walls are structurally sound, made more so by interconnecting roots as they grow. They can be used on slopes that need retaining and protection from sea level rise, coastal erosion and flooding, and can be considered a long-lasting solution. 

Aesthetically, both living crib walls and brush mattresses may be preferable to hard infrastructure solutions because they blend into the natural landscape.

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Ecological and biodiversity benefits
  • Habitat provision
  • Species maintenance

The living structure of a brush mattress improves fish habitat by shading the stream, lowering water temperatures and offering protection from predators. It can also be a supporting habitat for birds, insects and other living organisms. Living crib walls can create places for birds and insects to nest for example. 

By intercepting sediment and rubbish, the brush mattress improves the general health of the waterway without introducing foreign, unnatural materials.

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A vegetated crib wall. Photo by LaRiMiT
Living fences, Samoa. Photo by Pedersen Zari, 2023.

Technical requirements

A living crib wall: depending on the size of the retaining needed, the wall may require specific engineering. Different soil and earth types will require different combinations of timber and fixings, and different climates will require careful consideration of suitable plant species.

Brush mattresses: the stream bank will need to be graded to a slope not exceeding 2:1, then a trench formed and the branches laid perpendicular to the stream’s flow (MassDEP, 2024). The branches are grouped and placed based on their size and where the live sprout buds are. Substrate conditions need to be considered as they influence root penetration and growth of the sprouting plants.

Cuttings should be installed in the non-growing season, and maintenance mist be factored in until the vegetation becomes well-established. A toe protection is needed in most cases to protect the brush mattress. A living crib wall is simple to construct and can use local, sustainable resources like timber and native plant species. The structure can also recycle large timber members and logs in its frame.

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Issues and Barriers

A living crib wall of any significant size may need to be engineered. However, small-scale living crib walls can be installed by anyone with the right education and training. 

Brush mattresses also require a specialist to ensure the system is designed and installed correctly, and to ensure the correct vegetation is used, to ensure the growth of the root systems is maximised. Considerations should be made to ensure the brush mattress is the correct strategy in relation to the magnitude of erosion, the hydrology and the velocity of the stream or river or coastal condition (Allen & Fischenich, 2001). Brush structures are usually most effective in sunny conditions, and their efficacy is dependent largely on the site conditions and the material selection. 

Brush mattressing being implemented by a community in Detroit. Photo by: J. McCullah
The same brush mattress after 18 months. Photo by J. McCullah

Opportunities

As Oceanic nations continue to battle climate change impacts, and gather strategies to adress challenges such as coastal erosion, flooding, increased temperatures and increased rainfall, both brush structures and living crib walls can be included in strategic planning for vulnerable places. 

On smaller scales, locals can be trained in how to create brush structures, and they can be implemented reasonably quickly, both as a means for increasing disaster resilience when planning for extreme weather events and in remediating areas of known or predicted increased erosion stress. 

On larger scales, living crib walls can be implemented as a means to deal with erosion. They may take more time to implement, but can be seen as a solid, permanent solution to retaining eroding slopes, in a way that supports local biodiversity. The wall can become a habitat for birds and insects, supports fish health, and can be planted with native plants.

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Financial case

The cost of a living crib wall completely depends on its size and complexity. However, the fact that it is possible to use recycled materials keeps costs low, and the positive impact on the solid and water environment is an additional financial consideration.  A brush structure is likely to cost less than an alternative hard-infrastructure solution. The materials have little cost – they are foraged / grown locally. Allen & Fischenich (2001) estimate that one to two people can create a 1m2 brush mattress, including plant harvest, collection, transport to the project site, fabrication, and installation.

Plan view of stream bank covered by brush mattresses. Image source: Darby 1991.
References

Further resources: