Sustainable Measures of Gully Erosion Management: application to the Nigerian Situation

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Introduction

Erosion and the gully type is the greatest geohazard in Nigeria affecting all the states in the Southeast and some states in the other regions of Nigeria. In Southeast particularly the state of Anambra is the most hit having more than 1000 active gullies with notorious gully erosion site as the Nanka gully erosion site and the Ekwulobia gully erosion site.

Many human and economic losses have been recorded due to this menace. This prompted the Federal government of Nigeria to partner with World Bank to create the Nigeria Erosion and Watershed Management Project (NEWMAP), which has been actively engaged in many activities aimed at controlling gully erosion in about 22 states in Nigeria currently as of July 2021. The majority of these states come from the southern part of Nigeria which is most affected by the gully erosion menace. The measures employed by NEWMAP on this project include the use of concrete structures, rip rap, and even vegetation.

In spite of all these means, new gully erosion sites are silently developing and cutting off roads and arable lands. It is unarguable that soil properties and many bad human practices contribute to this menace. Let us look at the suggestion from some experts in developed countries of the world who are also experienced in the management of this problem.

Types of Erosion

Mr Amir Shahkolahi identified 4 different types of erosion which are:

1. Raindrop (splash) Erosion: This type of erosion results when the force of raindrops falling on bare or sparsely vegetated soil detaches soil particles.

2. Sheet Erosion: This type of erosion occurs when these soil particles are easily transported in a thin layer, or sheets, by flowing water.

3. Rill and Gully Erosion: This type of erosion occurs when sheet erosion is allowed to concentrate and gain velocity. It detaches more soils thus cutting rills and gullies. It is often seen as the terminal phase of a four-stage erosion process associated with rainfall.

4. Stream and channel erosion: This type of erosion is associated with natural water bodies.

Some of these types of erosion are illustrated diagrammatically in Figures 4 and 5.

Soil Erosion and Design Applications by Prof Jean-Louis Briaud

Causes of Gully Erosion

Prof Briaud, identified three inputs to gully erosion namely:

1. Soil (Erodibility): This is defined as the relationship between the erosion rate and velocity of the water near the soil-water interface or the relationship between the erosion rate and the shear stress at the soil-water interface, diagrammatically put as Ż = f (τ).

Erodibility depends on soil water content, soil unit weight, soil plasticity index, soil undrained shear strength, soil void ratio, soil swell, soil mean grain size, soil percent passing #200 (0.075 mm), soil clay minerals, soil dispersion ratio, soil cation exchange capacity, soil sodium absorption rate, soil pH, soil temperature, water temperature, water salinity, water pH.

2. Water (Velocity) and

3. Geometry (Dimensions)

Sustainable Measures of Erosion Control

Mr Amir identified seven (7) effective erosion control principles according to Rivas (2006) namely:

1. Reducing erosive forces
2. Increasing resistance forces
3. Applying good erosion control techniques
4. Modifying the topography
5. limiting exposure
6. Keeping runoff velocities low
7. Inspections and maintenance treatments

He identified two erosion control best management practices. These are:

A. Source Control Measures: These involve processes that are used to protect the surface of the soil from the erosive energy of rain splash and runoff and to prevent the initial dislodgement of soil particles. Systems under are also called offensive systems.

B. Runoff Control Measures: This includes modification of slope surfaces and reduction of slope gradient, controlling flow velocity, diverting flow around affected areas, and providing upstream storage for runoff.

Physical systems used to control these erosion whether by source control measures or runoff control measures fall into two systems. These are Rigid systems and Flexible systems.

Rigid Systems are non-erodible, permanent, and long duration, but are susceptible to failure due to foundation instability. Examples are cast-in-place concrete, asphaltic concrete, stone masonry and interlocking modular block, fabric foundation systems for concrete, or partially grouted riprap.

Flexible Systems include vegetative linings, cobbles, rock riprap, gabions and rock mattresses, cellular confinement systems, and permanent non-biodegradable Rolled Erosion Control Products (RECPs).

Among all these systems, some are temporary while some are permanent (see Figure 10). The question is which of these systems is most suitable?

The choice of the most suitable system would depend on:

1. Source/Type of erosion
2. Cost
3. Permanent or temporary solution
4. Safety (during installation, operation, and maintenance)
5. Environmental considerations
6. Hydraulic conditions (e.g. flow rate and duration)
7. Durability and long-term performance

Erosion control systems in Nigeria largely involve the use of what is termed hard armouring systems such as rock, gabions, and concrete/concrete mats (See Figure 11).

These systems have evidence of past performance history but they have some disadvantages such as high initial installation cost and installation difficulty, high maintenance cost, high carbon footprint, and low benefit water quality. Thus, they are not sustainable.

Recently there are many sustainable means of erosion control largely grouped into Rolled Erosion Control Products (RECPs). These are prefabricated products manufactured from wood excelsior, straw, jute, coir, polyolefins, PVC, or nylon (Shahkolai, 2021).

RECPs are divided into two namely:

Temporary RECPs

These are temporary solutions until vegetation grows, e.g. mulch control netting (MCN) and erosion control blanket (ECB) (see Figure 12).

Vegetation is good to control erosion but has limitations beyond certain high erosive forces. In this situation, the permanent RECPs come into play.

Permanent RECPs

Permanent RECPs include:

Geosynthetic Turf Reinforcement Mats (TRMs) (Figure 13)

Turf Reinforcement Mats (TRMs) –

                           (see Figure 14)

High-Performance Turf Reinforcement Mats (HPTRMs)

                            (see Figure 14)

These products can be used for both vegetated and unvegetated surfaces and can be applicable to channel production, bank/pond remediation, levees and dam production, slope stabilization, slope erosion control, shallow plan slope failure, etc

Advantages of TRMs and HPTRMs

1. Sustainability

2. Cost Saving

3. Environmental-friendly

4. Traffic Reduction

5. Emission Reduction

Note: Table 1 shows the important properties of RECPs 

Table 1: Important properties of permanent RECPs

Design of TRMs/HPTRMs for the Stormwater Channel

This design can be carried out using HEC-15 (Hydraulic Engineering Circular No-15) approach

Design Criteria

τ_d = maximum shear stress on the channel bed; τ_d = γ_dS_o

τ_p = permissible shear stress of the channel bed; τ_p ≥ SFτ_d (SF is normally 1.3)

Permissible Shear Stress

1. Bare Soil (no vegetation)

For fine-grained, non-cohesive soils (D75 ˂ 1.3 mm) is relatively constant and estimated at 1.0 N/m²

For coarse-grained, non-cohesive soils (1.3 mm ˂ D75 ˂ 50 mm), the following equation applies:

τ_p,soil = αD75 (α is the unit conversion constant = 0.75).

For cohesive soils, the base equation for permissible shear on the soil (without vegetation) is:

τ_p,soil = (c₁PI² + c₂PI + c₃) (c₄ + c₅e)²c₆

where PI is the plasticity index, e is the void ratio, and c₁ to c₅ are coefficients depending on the soil type and PI (Kilgore and Cotton, 2005).

2. Vegetated Soil (No reinforcement/TRM)

Permissible stress, τ_p = τ_p,soil (1 – C_f)^-1(n/ns)²

where C_f is the grass factor, n_s is the soil grain roughness and n is the overall lining roughness

3. Reinforced Vegetated Soil (with reinforcement/TRM)

Permissible shear stress, τ_p = τ_p,soil (1 – C_f,TRM)^-1(n/n_s)²

where C_f,TRM is the TRM cover factor, n_s is the soil grain roughness and n is the overall lining roughness

Recommendation

Owing to the numerous advantages associated with the use of TRMs/HPTRMs, I recommend that erosion control agencies in Nigeria should study how these sustainable measures can be integrated into our gully erosion control strategies.

References

Briaud, J.L. (2021): Soil erosion and design application. 2^nd Edition Lecture Series on Advancement in Geotechnical Engineering from Research to Practice (AGERP). Distinguished Professor at Texas A&M University and President of ASCE.

Mezie, E.O and Nwajuaku, A.I. (2020): Investigation into the causes of gully erosion in parts of Anambra state. Journal of Science and Technology Research 2(1) pp. 132-142

Oseni, B. (2012). Factors that can expose soil to erosion (causes of soil erosion), [online], www.nigeriaenvironment.blogspot.com.ng, Retrieved on 25/01/2016.

Shahkolahi, A. (2021): Erosion control techniques for drainage channels. 2^nd Edition Lecture Series on Advancement in Geotechnical Engineering from Research to Practice (AGERP). Technical Manager, Global Synthetics, Australia.