RESEARCH PROJECT – High Borrowdale, Cumbria

In 2018, Prof Lois Mansfield began work exploring the landslide issues we faced on our land at High Borrowdale. This initial investigation outlined why our land at High Borrowdale was particularly susceptible and described a plan for a pilot research project to stabilise slopes using natural geotextiles. In this summary of her final report, Lois reports back on the outcomes of the three-year project and what has been learnt.

Read the Full Final Report here:
A report for Friends of the Lake District, United Utilities & the East Africa Sisal Company,
by Lois Mansfield, CNPPA, University of Cumbria
& Charles Fox, Photomonitoring Surveyor, FLD volunteer

Soil Stabilisation Research Outcomes - A Summary
By Prof Lois Mansfield, University of Cumbria

Natural geotextiles exist in three mat-like forms: artificial (eg plastic); natural (eg coir) or biopolymers, which are blend of the first two. They are draped over the exposed eroded terrain and pinned down. The theory is that their net-like design allows vegetation to grow through and re-stabilise the slope. For this slope stabilisation trial, we focused on the use of natural geotextiles to be in-keeping with the local environment, keep costs to a minimum and ensure we did not introduce unwanted pollution at the end of the process. We chose to test three types of material; sisal (Agarve sisalana), an un-tried product on slopes in the UK alongside the better known products of jute (Corchorus capsularis) and coir (formed from waste product of Cocos nucifera processing) [see Figure 1].

Fig 1: Samples of the textiles – Sisal Jute, Coir

The study had two main aims: to test the efficacy of sisal as a tool for slope stabilisation in comparison to other geotextiles and to re-stabilise the debris flow scars at High Borrowdale in order to prevent further slope destabilisation. In order to answer these, three exposed scars were selected in one of the gulleys. Working with volunteers from Friends of the Lake District, the textiles were stitched into large sheets, spread over the bare soil and pegged down using giant metal tent pegs (see Figure 2].

Fig 2: Metal tent pegs

Every month for three years (except during lockdown) a diligent volunteer went out and photo-monitored each of the geotextile plots and a control one, which had no textile on it and was left to the elements. This involved standing in the same place in front of each plot and taking a photograph of how well the vegetation was growing through the textile.

Pictured: Stitching sisal into large sheets on-site

Pictured: Pegging down sisal on steep gillside

Pictured: Carrying sisal to gillside

The photographs were then analysed using a piece of free software from the internet called ‘Image- J’. This software allowed Prof. Mansfield to calculate the area of each mat that was still bare or had vegetation cover; thus plotting the change in cover over time, the results of which are shown on Figure 3.

Fig 3: Performance Comparison between three Geotextiles & Control with regard to Vegetation Cover June 2018 to July 2021. Vertical axis denotes % vegetation cover

From the field trial results, coir outperforms jute and sisal with regard to re-vegetation rates. Jute and sisal perform to roughly the same effect for re-vegetation, despite their very different weave densities. The control plot recovered the least of all trials. Overall, the sisal retained more water than the other textiles. Jute is the cheapest product with the greatest open space, easiest to manipulate on steep slopes and lightest. Coir and sisal after 36 months still had integrity, the jute was at the point of disintegration.

It is evident from the field trials and data analysis that coir is the most effective geotextile for the re-vegetation of slopes and stabilisation. These results contradict other research into geotextiles where sisal was much more effective than coir or jute on slopes suffering from soil erosion. There could be several explanations for this; first, the wetter climatic conditions between research areas, second angle of slope was twice as steep at High Borrowdale (38o compared to 17o) and third the sisal weave density was much tighter than other research trials (see Figure 1).

In relation to weave density, the three textiles used were ‘off-the-shelf’ deliberately to use what is generally available to all land managers. It is evident from this trial that weave density could have a role to play in relation to textile efficacy and makes logical sense; larger gaps allow light to penetrate to encourage seedling regrowth. This was particularly evident in relation to the sisal, which was acting more as a suppressant than support re-growth, and in turn demonstrates that its current weave density is more appropriate to water retention uses such as in peat dams. 

Regional modelling of climate change does suggest we could see drier summers (-23.82%) and wetter winters (+26.20%) by 2050 in the Lake District.  In fact, the contemporary experience of increased debris flows (our particular form of landslide at High Borrowdale) in and around the Lake District as seen in 2015 (Storm Desmond) could be indicative of our climate crossing a threshold, whereby these sorts of mass movement become more typical. Second, the site angles in this study were excessively steep ranging from 38 to 46o, which is not untypical of many glaciated valleys in the Lake District.

Together, climate change and slope angle are two of the main triggers causing debris flows to which we can add other factors of ‘availability of material’ (glacial till from the valley sides) and land use. Consequently, there needs to be a better understanding as to the extent of these types of factors across the region to aid in proactive slope stabilisation management rather than reacting to catastrophic slope failure.  A proactive management tool to address this was tested in North-East Wales by some University of Chester researchers in 2012. They constructed a landslide susceptibility map to divide the area into low, moderate and high classes. Whilst land use was not included in their tool, it did show its feasibility to be proactive for the whole of the Lake District.

For High Borrowdale, performance for the three textiles showed any use of geotextile was better than nothing at all (the control). Coir performed best with regards to re-vegetation, but not so well regarding its inherent physical properties. Sisal performed the worst both in terms of re-vegetation, cost and its manual handling potential. Interestingly, jute came out the best in terms of summative performance, with only its integrity the worst after 36 months. Such a set of results suggests that there are different appropriate contexts for the use of geotextiles. Coir is best used for 30o+ slopes in need of 3+ year’s work or sites needing quick regrowth; jute is best for 30o+ slopes under 3 years and sisal, where water retention is of primary concern.

Having said this, manoeuvrability on steep slopes such as those at High Borrowdale needs to be factored in. Both the coir and sisal were heavy and difficult to manually move especially on slopes with little traction or integrity, whereas the jute was much easier. The other characteristic to consider is the inability of coir to absorb water, which means it is possible coir work could be ‘washed out’ by similar high level precipitation events like those experienced in 2009 and 2015, with the water getting under the matting.

In conclusion, whilst coir may seem the most efficacious for re-vegetation, jute may be a better solution for volunteer teams or land managers interested in keeping costs down. Further work remains to explore what happens when the weave density of sisal is changed along with some form of landscape-scale ‘landslide’ susceptibility mapping for the Lake District. It would be interesting to explore these ideas further.

Read the Full Final Report here:
A report for Friends of the Lake District, United Utilities & the East Africa Sisal Company,
by Lois Mansfield, CNPPA, University of Cumbria
& Charles Fox, Photomonitoring Surveyor, FLD volunteer