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Difficulty: Very hardEnvironmental Hazards, Degradation, and Management

Southeastern Nigeria experiences severe environmental degradation due to catastrophic gully erosion triggered by human activities and physical vulnerability. Arrange the following geomorphic stages in the correct chronological sequence of gully morphogenesis from initial disturbance to advanced structural enlargement.

  1. 1Removal of protective vegetation canopy exposing friable surface soils to direct raindrop impact and aggregate breakdown (splash erosion).
  2. 2Concentration of unabsorbed surface runoff into localized paths of least resistance, generating shallow parallel channels (rill erosion).
  3. 3Deepening and coalescence of micro-channels as high-velocity flow causes vertical bed scouring and headward plunge-pool incision.
  4. 4Basal sapping by subsurface groundwater discharge combined with bank undercut, triggering massive sidewall slumping and lateral channel expansion.

Answer

The correct chronological sequence begins with vegetation clearance causing raindrop splash erosion, followed by surface runoff concentration into rills, then channel deepening through headward scouring into gullies, and culminates in groundwater sapping and sidewall mass slumping.
Gully erosion develops systematically from surface destabilization to deep structural collapse. The process begins with the removal of plant cover, exposing soil to splash erosion. Next, unabsorbed runoff concentrates into rills. As flow energy increases, rills deepen into gullies via headward erosion. Finally, when the gully floor reaches groundwater levels, seepage (basal sapping) and bank undercutting induce slope instability and mass slumping.

Step-by-Step Solution

1
Identify the initiating trigger of soil degradation.
Deforestation or land clearing exposes bare soil to splash erosion, breaking down soil aggregates.
Vegetation removal is the primary anthropogenic antecedent condition.
2
Trace the initial hydrological response of surface runoff.
Infiltrative capacity is exceeded, leading to sheet wash and small micro-channel (rill) incision.
Runoff gathers momentum and concentrates into discrete paths.
3
Determine the phase where rills transition into active gullies.
Concentrated flow scours deep into weak, un-consolidated subsoil strata, expanding rills through headward erosion.
Hydraulic force increases bed scouring depth beyond normal tillage or agricultural recovery.
4
Identify the mature stage dominated by subsurface hydrology and mass wasting.
Groundwater seepage (basal sapping) undermines sidewalls, causing structural mass slumping and rapid gully expansion.
Deep gullies intersect the local water table, introducing geotechnical failure mechanisms.

Key Concept

Morphogenesis of Gully Erosion and Mass Wasting Feedback Loops
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Topics

Environmental Hazards, Degradation, and Management

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