Riprap Apron & Basin

Riprap apron

D₅₀ = 0.2 D (Q / (g^1/2 D^2.5))^4/3 (D/TW) (HEC-14 Eq. 10.4, recommended for circular culverts). Tailwater is limited to 0.4 D to 1.0 D, and taken as 0.4 D when unknown. Rock is adjusted inversely with specific gravity. With supercritical flow in the culvert, D′ = (D + y_n)/2 replaces D in the equation and the tailwater limits.

Class and dimensions

The class with a D₅₀ at or above the required size, from HEC-14 Table 10.1, sets the length and the rock depth. The width starts at 3D and flares 1:3 on each side to 3D + 2L/3.

Riprap basin

For a trial D₅₀: h_s/y_e = 0.86 (D₅₀/y_e)^−0.55 V_o/(g y_e)^1/2 − C_o (Eq. 10.1), with the lengths L_s = 10h_s, L_A = 5h_s, L_B = 15h_s and their minimums, and the velocity at critical depth at the exit (Eq. 7.14).

The calculator is tested against the cases below every time its code changes.

Compared with the customary-unit examples of FHWA HEC-14 Sections 10.1 and 10.2 to the precision they print. Other cases are hand calculations of the published equations. No reference program has been run for this page.

• Riprap aprons: HEC-14 uses them primarily for culverts of 60 in or smaller, and notes they do not dissipate much energy: they spread the flow, and if too short they move the erosion downstream.
• Tailwater above 1.0 D needs HEC-14 Figure 10.3, a chart this page does not compute. The same chart sets more riprap downstream of a basin with high tailwater. Table 10.1 stops at a 22-in D₅₀.
• HEC-14 does not say whether D′ also applies to the tailwater limits, the length or the width. The page uses it for the tailwater limits, which gives the larger rock, and the actual D for the length and width. Box culverts are not covered by Eq. 10.4 as printed; the page takes a diameter.
• A filter under the rock is called for (HEC-11) and is not sized here. Stilling basins and other dissipators in HEC-14 are not computed.

Which velocity do I use for a basin?

The outlet velocity for the flow type: full barrel, or the outlet depth from the Culvert calculator.

What is the equivalent brink depth?

For a rectangular outlet, the outlet depth. For a pipe, (A/2)^1/2 with A the flow area at the brink (HEC-14 p. 10-2).

Can I attach the calculation sheet to a report?

You can print it and include it as a supporting calculation that you have checked. It is a calculation aid, not a sealed engineering document: the engineer of record reviews and is responsible for what goes into a report.