Blench curves application — for a given channel discharge, Blench curves relate the head loss (HL) across a fall to which specific energy quantity?

Introduction / Context:
Falls and transitions on canals are designed to dissipate surplus energy safely. Empirical aids like Blench curves are used to estimate head loss (HL) and downstream conditions for given discharges and approach states, assisting in economical stilling arrangements and protection works.

Given Data / Assumptions:

• Channel with a fall/energy dissipator handling a known discharge.
• Objective: estimate HL and downstream specific energy for design.
• Blench curves provide empirical relationships between HL and energy terms.

Concept / Approach:

The performance of a fall is governed by conditions in the hydraulic jump or turbulent roller downstream. Accordingly, the energy state immediately downstream (specific energy downstream) is central to evaluating head loss and ensuring non-scouring velocities. Blench curves capture this relationship for typical canal geometries and flows.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Compare curve-based HL with calculations from momentum-based hydraulic jump theory using downstream tailwater rating; reconcile for design safety factors.

Why Other Options Are Wrong:

Upstream specific energy is less indicative of dissipation demand; critical depth/depth alone ignores velocity head; “none” is incorrect because downstream energy is the key correlate.

Common Pitfalls (misconceptions, mistakes):

Using approach depth instead of downstream energy; ignoring tailwater variability; applying curves outside their empirical range.

Final Answer:

specific energy downstream of the structure