Table 3.

RMEI relative importance (RF) and likelihood factors (LF)

Erosion vulnerability parameterRelative importance factor (RF)Likelihood factor (LF)
Very unlikelyUnlikelyLikelyHighly likelyAlmost certain
12345
F1: Kinematically viable mechanism for detachment*3Rock with three defects, basal defect sub-parallel to spillway floor, and no day-lighting basal release surface; or massive rock with effectively only two defect sets and no basal release surface.Rock with three or more defects, with: basal defect sub-parallel to spillway floor, joint 2 protruding from surface; or basal defect inclined upstream or downstream at >30° relative to spillway floor.Rock with three or more defects, with: persistent basal defect dip 10 to 30° upstream relative to the spillway floor; or persistent basal defect dip 10 to 30° downstream relative to the spillway floor.Rock with three or more defects, with: persistent basal defect dip ≤10° upstream relative to the spillway floor; or persistent basal defect dip ≤10° downstream relative to the spillway floor.Persistent basal defect sub-parallel to the spillway floor, day-lighting upstream or downstream; or persistent shear and/or closely jointed rock which erodes readily forming a release surface into the shear.
F2: Nature of the potentially eroding surface3Smooth water or glacier worn, with no protrusions of joint 2, no opening of defectsBedding surface with protrusions of joint 2 < 1 mm, and little or no opening of defectsRelatively small protrusions and defect openings (e.g. pre-split, or ripped and bulldozed)Irregular surface following defects, little opening of defects (e.g. blasted rock)Irregular surface following defects, extensive defect opening (e.g. heavily blasted rock)
F3: Nature of the defects2Very rough surfaces, e.g. JRC ≥12Rough surfaces, e.g. JRC 8–10Slightly rough surfaces e.g. JRC 4–8Smooth surfaces e.g. JRC <4Smooth or slickensided surfaces
No separationAperture <1 mmAperture 1–2 mmAperture 2–5 mmAperture >5 mm
UCS|| >50 MPaUCS 20–50 MPaUCS 5–20 MPaUCS 1–5 MPaUCS <1 MPa, or soft gouge >5 mm thick
F4: Spacing of basal defect1>3 m1–3 m0.3–1 m0.1–0.3 m<0.1 m
F5: Block shape§1≤0.50.5–11–22–5>5
  • * Defects include joints, bedding surfaces, shears, and foliation partings.

  • Select class which best fits the data taking into account the kinematically viable mechanism and which defects control the displacement of the block of rock from the spillway.

  • Joint 1 is basal defect of a block or region (bedding or joint).

  • § Block shape = joint 2 spacing/joint 1 spacing; joint 2 is sub-vertical defect normal to the flow in the spillway. JRC, Joint Roughness Coefficient.

  • || UCS, unconfined compressive strength of the intact rock in the vicinity of the defect surfaces.