Storm overflow design: In combined sewerage, which arrangement is most effective for diverting excess storm water into a natural drainage while allowing dry-weather flow to continue toward the treatment works?

Introduction / Context:
Combined sewer overflows (CSOs) protect treatment plants and trunk sewers during heavy storms by spilling the excess to natural drains or watercourses. The hydraulic arrangement must reliably separate dry-weather flow (DWF) from storm surges with minimal maintenance and clogging risk.

Given Data / Assumptions:

• Combined sewer carrying DWF and storm runoff.
• Need to pass DWF onward while diverting excess to a drain.
• Simplicity and self-cleansing preferred.

Concept / Approach:
The leaping weir uses hydraulic momentum of higher storm flows to leap into an overflow channel, whereas low DWF hugs the invert and continues to the outfall works. The configuration has proven effective for automatically apportioning flows without movable parts.

Step-by-Step Solution:
At low flow: stream does not leap; DWF continues to interceptor.At high flow: increased velocity and depth cause the jet to leap into the overflow, diverting excess.Therefore, among listed options, the leaping weir provides the intended selective diversion.

Verification / Alternative check:
Design manuals describe leaping weirs as standard CSO structures due to hydraulic reliability and low headloss for DWF.

Why Other Options Are Wrong:
Broad-crested/side weirs may work but are less selective and prone to setting issues for DWF control.Siphon spillways are for dams/reservoirs and not typical for urban CSOs.None of these: incorrect since a known effective arrangement exists.

Common Pitfalls:

• Confusing dam spillway devices with urban sewer overflow structures.
• Ignoring DWF bypass requirement in favour of storm-only diversion.

Final Answer:
Leaping weir overflow