Question 1

What's the difference between Maynord and Isbash sizing?

Accepted Answer

Maynord (USACE EM 1110-2-1601) is the modern method for riprap in trapezoidal channels with quasi-uniform flow — typical channel and embankment-slope protection. It explicitly handles side slope, channel bend, blanket thickness, and a safety factor. Isbash (1936) is the simpler method for highly turbulent flow, used at outlet works, plunge pools, and sediment-control basin outfalls where flow is non-uniform. For most embankment-slope and channel applications, Maynord is the standard.

Question 2

What does K1 do in the Maynord equation?

Accepted Answer

K1 is the side-slope correction factor: K1 = √(1 − sin²θ/sin²φ) where θ is the side-slope angle from horizontal and φ is the angle of repose of the riprap (typically 40°). For a horizontal channel bottom, K1 = 1. For 1V:2H side slopes (θ = 26.6°), K1 ≈ 0.85. Steeper slopes need bigger riprap to prevent slope failure.

Question 3

When do I add the bend factor C_v?

Accepted Answer

At a channel bend, secondary currents and outer-bank velocity scour increase. Maynord uses C_v = 1.283 − 0.2·log10(R_c/W) for R_c/W ≤ 26, where R_c is centerline radius and W is water-surface width. For straight reaches, C_v = 1.0. Beyond R_c/W = 26 the bend effect dies away.

Question 4

How thick should the riprap layer be?

Accepted Answer

USACE EM 1110-2-1601 requires layer thickness T ≥ max(1.5·D50, D100). In practice this is usually 1.5·D100 to ensure full coverage of even the largest stone with at least one full layer of medium stones above. Below the riprap, place a filter layer (geotextile or graded granular filter) to prevent base-soil winnowing through the riprap voids.

Question 5

What specific gravity should I assume?

Accepted Answer

Limestone, granite, basalt: SG = 2.65 (typical default). Sandstone: SG = 2.50. Heavy slag or magnetite ballast: up to 3.0. Lightweight materials shouldn't be used for riprap. The riprap weight is what stabilizes it against drag and lift, so denser stone is smaller for the same protection — but density is rarely a free design choice; use what the local quarry produces.

Question 6

Is geotextile filter enough, or do I need a graded granular filter?

Accepted Answer

Both can work. Geotextile is faster to install but is vulnerable to puncture during placement and has finite life. Graded granular filter (sand + gravel layer per Sherard / NRCS NEH-633 filter criteria) is more durable and self-healing. For permanent dam-slope protection USACE recommends granular filter; for channel bank work, woven or non-woven geotextile is acceptable. Either way, the filter layer is required — riprap without filter will erode the base soil through its voids and progressively settle.

Side slope (V:H)	θ (°)	K₁
1:1.5	33.7	0.74
1:2 (typical embankment)	26.6	0.85
1:2.5	21.8	0.91
1:3	18.4	0.94
Horizontal (channel bottom)	0	1.00

Percent finer by weight	d/D₅₀ (typical)
D₁₀₀ (max)	1.5
D₈₅	1.30
D₅₀	1.00
D₃₀	0.85
D₁₅	0.70

Riprap Sizing Calculator

Side-slope correction factor K₁

Worked example

Example — channel bank at a bend

Recommended riprap gradation

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