Diversion headworks crest setting in canal design — the crest level primarily depends on multiple hydraulic and operational criteria; identify the most comprehensive choice.

Introduction / Context:
The crest level of a diversion headworks (weir or barrage) is a keystone decision in canal engineering. It governs how much hydraulic head is available at the canal regulator, the sediment-exclusion efficiency, and the safe passage of floods. Examinations often ask whether crest level should be tied to a single factor or a combination of interdependent design inputs.

Given Data / Assumptions:

• A canal offtakes from a river via a diversion headworks comprising a crest (weir/barrage), undersluices, and a head regulator.
• Full Supply Level (F.S.L.) of the canal is preselected from command requirements.
• Pond level (upstream water level) must reliably supply the canal even at low river flow.
• Design discharge and control needs determine regulator openings and approach conditions.

Concept / Approach:

The crest level must be high enough to create a dependable pond level above the canal F.S.L., while accommodating ecological flow, sediment management, and flood routing. It is therefore set by simultaneously meeting: (i) canal F.S.L. and head across regulator, (ii) discharge-capacity/control requirements, and (iii) the desired pond level envelope for multiple flow regimes.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Carry out a backwater/gradually varied flow analysis to verify that for critical low-flow conditions the pond level still exceeds the canal F.S.L. by the target margin, while flood routing keeps upstream afflux within allowable limits.

Why Other Options Are Wrong:

Choosing only F.S.L., only discharge parameters, or only pond level neglects key coupled constraints. “None of these” is incorrect because all listed factors do govern crest fixing.

Common Pitfalls (misconceptions, mistakes):

Designing for a single criterion (e.g., just canal F.S.L.) and ignoring flood afflux or sediment approach; confusing pond level with long-term storage (it is operational pondage, not reservoir storage).

Final Answer:

all the above