Question 1

What's a stepped spillway?

Accepted Answer

A stepped spillway is a chute spillway with a series of horizontal steps cut into the downstream face, instead of a smooth concrete chute. Steps create form drag and dissipate energy along the chute itself, reducing the size of the stilling basin needed at the toe. Roller-compacted-concrete (RCC) dams are usually built with stepped spillways because the lift surfaces double as steps.

Question 2

What's the difference between nappe flow and skimming flow?

Accepted Answer

At low discharge, water cascades down the steps as a series of free jets impacting each step — nappe flow. As discharge increases, the steps fill with recirculating eddies and the main flow skims over the pseudo-bottom formed by the step edges — skimming flow. Skimming flow has higher equivalent friction (f ≈ 0.2 vs ~0.05 for smooth concrete) but is more violent. The transition criterion is roughly d_c/h_step = 0.89 − 0.40·tan(θ) (Chamani & Rajaratnam, 1999) — this is the criterion the calculator applies. Boes & Hager (2003) give a different form, 0.91 − 0.14·tan(θ), which at a 51° chute is about 1.9× larger; the two disagree materially, so state which one a design used.

Question 3

Why use stepped spillways?

Accepted Answer

Steps dissipate roughly 50–80% of the maximum head along the chute (vs ~10% on a smooth chute), so the stilling basin at the toe can be much smaller — sometimes eliminated entirely. RCC and stepped masonry construction is cheaper than the formed concrete required for an ogee/smooth chute. Air entrainment from the step turbulence also reduces cavitation risk on the chute surface.

Question 4

What's the equilibrium friction depth?

Accepted Answer

In skimming flow, far enough down the chute, the chute slope's gravitational driving force balances the form-drag friction, and depth and velocity reach steady values d_e and V_e. Boes & Hager (2003) give d_e = (f·q²/(8·g·sinα))^(1/3) where f ≈ 0.2 for concrete steps. The equilibrium reach is the part of the chute past about 30·h_step from the crest where this assumption is valid.

Question 5

What is residual energy at the toe?

Accepted Answer

Residual energy H_res = d_e·cos(α) + V_e²/(2g) is the head remaining at the toe of the chute, after dissipation along the steps. It's compared to the maximum upstream energy H_max = H_dam + 1.5·d_c. The dissipation efficiency η = 1 − H_res/H_max is typically 60–95% for tall stepped chutes.

Question 6

When does cavitation become a concern on stepped spillways?

Accepted Answer

Boes & Hager (2003) recommend keeping unit discharge q ≤ 25–30 m²/s (270 cfs/ft) for steel-fiber-reinforced RCC steps; conventional RCC handles q ≤ 14 m²/s (150 cfs/ft). Above those limits, sub-atmospheric pressures behind the step risers can drive cavitation damage. Air entrainment helps, which is why most stepped spillways have natural surface aeration past the inception point.

Parameter	Recommended value	Notes
Step height h — steep RCC dams	0.6–1.0 m (2–3 ft)	Larger steps for higher unit discharge
Step height h — embankment overlays	0.3–0.6 m (1–2 ft)	Flatter slopes, smaller steps
Maximum unit discharge q_max	11.5·sin(α)·h^1/2	SI (m²/s, m); h = 0.6 m, α = 51° gives ≈7 m²/s
Friction factor f — smooth / non-aerated	0.18	Upstream of the inception point
Friction factor f — design value	0.20	Use this unless you have site data
Friction factor f — fully aerated	0.30	Fully developed aerated flow
Inception of air entrainment	≈30 step heights	Measured down-chute from the crest; surface aeration past this point sharply reduces cavitation risk
Skimming-flow threshold	d_c/h > 0.4	Below this the regime is nappe flow

Stepped Spillway Energy Dissipation

Flow regime transition

Boes & Hager design recommendations

Worked example

Example — RCC dam stepped spillway

Related tools