| Background: | ||||
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Colliery shale and sandy soils have one thing in common; both are highly susceptible to erosion by flowing water. Both soil types are present at Calverton Colliery Tip Restoration Project near Nottingham. Over 1km of drainage ditch had been severely eroded before any vegetation had chance to establish. Initial options included rock riprap lining, but the client and consultants preferred a more natural vegetated option. |
Calverton Colliery Tip, Nottingham |
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Analysis
of the channels revealed that even if vegetation could be established
the un-cohesive sandy soils would not remain stable as the predicted
flow velocities would be too extreme for a grass cover alone to
withstand. |
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| Design: | ||||
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Laying of NAG C350 at Calverton Tip |
Salix
were approached to provide an initial design analysis and review of
methods, which would enable the establishment of a vegetative cover
both in the short and long term. Using the “shear stress” approach the
level of erosive force was first assessed using the North American
Green ECMDS software (Erosion Control Materials Design Software). This
information was then compared to know design values for unreinforced
and reinforced vegetative linings on sandy soil types. |
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Critically, the erosion protection needed to be able to withstand potentially high flow velocities (3.5m/s) immediately after installation and prior to vegetation becoming established. In an unvegetated state open three-dimensional erosion control mats fail at very low velocities (0.5m/s), as they only actually cover 5-10% of the soil surface. However, the vegetative stem and root reinforcement provided by a three- dimensional mat was needed to reinforce the vegetation to enable the grass to withstand the higher velocity flow events. |
North American Green C350 at Calverton Colliery Tip, Nottingham. |
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The
only product that would meet both the short term erosion control of the
bare recently seeded soil whilst also giving three dimensional
reinforcement to the vegetation once established was North American
Green’s (www.nagreen.com) V-Max
range of products. The ECMDS analysis showed that the C350 Composite
Turf Reinforcement Mat (C-TRM) would provide the short term erosion
control function and also reinforce the vegetation to withstand the
storm flow events. |
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The eroded channels were re-profiled and shaped in March 2004 and seeded and protected with the C350 within a two-week period. The seed and fertilizer were protected from washout by flows in the channel from day one. An added benefit was the rate at which vegetation began to establish due to the mulching effect of the coir fibre composite layer in the 3-D net of the C350. |
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| Results: | ||||
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The
channels were tested early on after completion with numerous storm
events occurring in the first few months. The high level of erosion
control performance provided by the C350 prevented any erosion of fines
or loss of seed. Vegetation is well on its way with 60 – 80% cover in
the first year and the project is deemed a success by all those
involved. |
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Calverton Colliery Tip before (above left), during (above right) and after (below). |
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