M2Q3

Consider water flow over a natural river bed with a characteristic sediment size ds = 1 mm. Using the modified Shields diagram below, with water density
𝜌
= 1000 kg/m3 and viscosity
𝜈
= 10-6 m2/s, and without any other inputs, which of the following statements represents realistic conditions if the Shields parameter
𝜏
∗
= 0.75?

Question

M2Q3

Consider water flow over a natural river bed with a characteristic sediment size ds = 1 mm. Using the modified Shields diagram below, with water density 
𝜌
 = 1000 kg/m3 and viscosity 
𝜈
 = 10-6 m2/s, and without any other inputs, which of the following statements represents realistic conditions if the Shields parameter 
𝜏
∗
 = 0.75?

BlackTom AI · Accepted Answer

Question restatement: We are given water flow over a natural river bed with ds = 1 mm, ρ = 1000 kg/m^3, ν = 1e-6 m^2/s, and a Shields parameter τ* = 0.75. Using the modified Shields diagram provided, which statement best describes the sediment transport regime under these conditions?

Option A: There are not enough inputs to answer
Option B: The sediment moves as bed load only
Option C: The sediment moves predominantly as bed load, although some suspended load is possible above the bed load layer
Option D: The sediment moves as both bed load and suspended load

Analysis of each option:
- Option A (not enough inputs): The diagram is designed to map τ* versus particle size and flow conditions to infer transport regimes (no motion, bed load, or bed load plus suspended load). Since τ* = 0.75 and ds = 1 mm are provided, and the diagram is explicitly used to distinguish regimes, there is sufficient information to categorize the transport mode. Therefore this option is not correct because the given diagram, together with τ* and ds, is intended to yield a regime classification.
- Option B (bed load only): The modified Shields diagram typically shows a threshold behavior where increasing τ* leads from no motion to bed load, and eventually to bed load with suspended load as τ* or flow velocity increases. For a modest ds such as 1 mm, a τ* of 0.75 is often around or above the threshold for motion, and the dashed line indicating the onset of suspension suggests that some particles can be entrained into the suspension above the bed-load layer. If the point lies near the transition zone between pure bed load and mixed motion, claiming bed load-only would ignore possible vertical mixing and occasional suspension indicated by the diagram. Thus bed load only may be too restrictive for τ* = 0.75 at ds = 1 mm.
- Option C (predominantly bed load with some suspended load above the bed load layer): This option recognizes a regime where bed load is the dominant transport mode, with the possibility of occasional suspension above the bed-load layer. It aligns with a transition region where most grains move along the bed, but as τ* increases toward the threshold for suspension, some particles can begin to move in suspension. However, the diagram and the standard interpretation at τ* ≈ 0.75 for ds around 1 mm often place the system in a mixed regime rather than a dominant bed-load-only regime, though the wording here emphasizes predominance of bed load with some suspended load. This is a plausible description but may understate the extent of simultaneous bed load and suspension depending on the exact τ* positioning on the curve.
- Option D (both bed load and suspended load): The diagram explicitly shows a region labeled 'Bed-load and suspension motion,' with a dashed curve separating bed-load-dominated motion from regions where suspension becomes significant. Since τ* = 0.75 is reasonably high and ds = 1 mm falls in a size range where suspension becomes feasible at such Shields values, the most consistent reading of the diagram is that both bed load and suspended load occur. This option captures the coexistence of transport modes indicated by the diagram for these conditions.

In summary, the figure indicates a transition toward mixed transport (bed-load plus possible suspended motion) at the given τ* and ds. The most faithful option to this interpretation is the one stating that sediment moves as both bed load and suspended load, reflecting concurrent transport modes rather than a strictly single-mode regime.

Note on reasoning variety: I started with a high-level interpretation of the Shields diagram, then evaluated each option against how the diagram’s regions and thresholds map to τ* = 0.75 and ds = 1 mm, highlighting why a single-mode description fails and why the mixed regime best matches the visualization. This approach helps separate the intuitive belief (e.g., “bed load only”) from the diagram-driven inference (coexistence of bed load and suspension).

CVEN90051_2025_SM2 Exam: Civil Hydraulics (CVEN90051_2025_SM2)- Requires Respondus LockDown Browser

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