Erosion Control of Spillways & Flood Banks – technical document

Spillway at Loch Garry

Spillway at Loch Garry

Technical Note: Three Dimensional Composite-Turf Reinforcement Mats (C-TRM) versus Flat Three Grid Mats

Many manufacturers of erosion control products publish performance data, however few state the exact testing criteria that are used and the specific parameters of the testing.

Often, a product will be tested to one flume test for one hour and then these results used as general average performance values over all possible flow events.

Critical aspects of performance are often hidden or misrepresented, for example the duration of a flow event and the resultant reduction in performance as time increases.

One manufacturer claims to have a product that

can withstand flow velocities of 4.5 m/s before vegetation establishment.

A closer review of the original testing data shows the matting did withstand this flow velocity without physical damage to the mat but that there was also 0.5m of soil erosion from underneath this mat!

This highlights the issues for engineers and specifiers when comparing the claims of manufacturers on performance.

V-Max C350 and P550 have been independently tested at four research institutions and results are openly published from 1 hour to 50 hour flow durations.

The failure criteria are based on vegetation failure and soil loss, as these are the key factors that lead to a surface erosion protection failure.

VMax C350 with excellent grass establishment on spillway slopesShear Stress Turf spillway overtopping 2 months after installation with flows up to 6m per secShear Stress Turf spillway overtoppingAftermath Of Flood With TurfVMax C350 installed for spillway protection
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Shear Stress Turf spillway overtopping 2 months after installation with flows up to 6m per sec

Flat Erosion Control Mats

Flat One-Dimensional Mats have been shown to have high performance levels that are comparable to Three-Dimensional Mats, but only for very short time periods (less than one hour).

After this time the vegetation starts to be stripped out and failure occurs. Composite Turf Reinforcement Mats (C-TRM’s) have high performance levels that can sustain extreme high flow events for very long durations, as they provide a three-dimensional structure that slows flows velocities at the soil interface and physically supports the base of the grass stems.

Flat One-Dimensional Mats do not perform these functions.

Large Scale Channel Erosion Testing

During a 1-hour flow event, a flat Rolled Erosion Control Product ( RECP) with mature 1-year-old grass was stripped by 90% to leave only a 10% cover, whilst leaving 90% of the matting unprotected and unreinforced.

The cause of such a dramatic failure is due to the fact the mat is flat and has no three dimensional structure.

A 3-D structure is what allows grasses to withstand high flow velocities over long duration events (10 hours plus).

The C350 and P550 products are tested over very long duration flows of up to 50 hours. In addition, the C350 is tested without any coir fibre filter layer (i.e. just the 3-D grid layer).

This is a very important consideration as coir will biodegrade within 3 years and so the filtering benefit of this coir is likely to have been lost when a spillway actually overtops in the future.

Any testing on flat mats that contain a coir layer is done with this fibre layer still in place and gives completely artificial performance uplift.

Spillways are often designed to withstand long duration flows (10 hours or more) and changes in reinforced vegetation performance over time vary greatly depending upon the base-reinforcing product. Central to higher performance is a 3-D structure.

The C350 is 16mm thick with a corrugated central grid, which greatly reduces velocity and shear stress within the structure and therefore reduces erosive force around the soil surface and base of the mat where the stem/root interface is located.

Flat mats provide no 3-D structure and the root/stem interface is exposed to the full flow velocity and maximum bed shear stress. This is why performance drops dramatically with time as vegetation is stripped out of the matting.

A 90% loss of vegetation within 1 hour is catastrophic as this loss of vegetation cover exposes the underlying mat.

The performance of the mat is then reduced to its “un-vegetated” phase, which is usually below 2.5m/s flow over a 10-hour duration event. This will expose the underlying soils to significant erosion and rapid critical slope failure.

The photos and charts are taken from the National Transportation Product Evaluation Program: Large Scale Channel Erosion Testing independent report.

Image of Vmax C350 C-TRM showing 3D Composite Structure

Image of Vmax C350 C-TRM showing 3D Composite Structure

Image of Vmax P550 C-TRM showing Composite 3D Structure

Image of Vmax P550 C-TRM showing Composite 3D Structure

Product ECP3 Flat Mat 90% loss of vegetation after one hour

Product ECP3 Flat Mat 90% loss of vegetation after one hour

V-Max C350 23% loss of vegetation after one hour

V-Max C350 23% loss of vegetation after one hour

V-Max in Trials

In contrast the manufacturers of V-Max P550 and C350 publish both short and long duration flow performance and use vegetation loss and soil loss as clear failure thresholds.

Additionally, and critically, these V-Max products have been validated in thousands of applications including actual overtopping events of spillways in the UK.

As these can be rare events, the confidence and validation gained is a valuable reassurance that the large-scale flume testing and failure thresholds that are used by the manufacturer fully support and validate the published performance values.

V-Max P550 after a 5 hour over topping event with flow velocities modeled at 7m/s flow. There was no recorded damage to the vegetation layer and no soil lossNo damage to the grass after a major overtopping event at Conwy
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V-Max P550 after a 5 hour over topping event with flow velocities modeled at 7m/s flow.

V-Max Withstands High Flow Events

In conclusion, the V- Max C-TRM’s are a proven method of reinforcing vegetation so that it can withstand extremely high flow events, over a long duration and without significant damage to the vegetation or loss of underlying soils.

Flat, One-Dimensional copies do not have comparable performance and are shown to be subject to vegetation failure in very short duration flow events.

Published performance data of 1-D mats can be presented to show optimum performance, but without revealing these only apply over only very short duration events.

Such events do not reflect the actual conditions that will be experienced on an overtopping or flood event that will extend to several hours, in which time such systems will have failed.

Only a C-TRM (Composite-Turf Reinforcement Mat) has been proven to work in such high velocity and duration flow events.

Examples of our slipway and erosion control work include:

Prepared By: Technical Director, David Holland BSc, MSc, FRGS, MIEEM, MCIWEM, CEnv, C.WEM

References:

Benefits of Vmax3® TRM Design Compared to Flat Net “Permanent”
Lancaster, T. and D. Austin, 2003. Classifying Rolled Erosion Control Product: A Current Perspective. ECTC.
Federal Lands Highway Specifications National Transportation Product Evaluation Program: Large Scale Channel Erosion Testing independent report