What Is Riprap?
Riprap refers to a layer of large, irregular stones placed along a shoreline slope to absorb and deflect wave energy. The term is also applied to quarried rock used to armour embankments, streambanks, and lake shores. The stones are selected based on size, weight, gradation, and resistance to abrasion—characteristics that determine how well the material stays in place under repeated wave action.
A revetment is the broader engineered structure that incorporates riprap: a sloped facing, typically placed directly against an erodible shoreline bank or bluff, designed to prevent undercutting and slumping. Revetments differ from seawalls in that they conform to the existing slope rather than being vertical barriers.
How Riprap Revetments Function
When waves strike a riprap slope, energy is dissipated through the voids between stones and through the turbulence created as water moves through the irregular surface. A well-graded riprap layer—meaning it contains a range of stone sizes—performs better than uniform-sized stone because the smaller material fills gaps and reduces the "rocking" of individual pieces.
Below the rock layer, a filter layer or geotextile fabric is typically installed. This separates the underlying soil from the rock, preventing fine particles from washing out through the voids—a process called internal erosion or piping. Without this filter, a revetment can settle and fail even when the rock itself appears intact.
Riprap Applications in Canada
Great Lakes Shorelines
Ontario's Lake Erie and Lake Huron shores have seen extensive riprap installation on private and municipal properties. During high-water periods in 2017–2020, when Lake Ontario and Lake Erie reached near-record elevations, bluff erosion accelerated markedly along the north shore of Lake Erie in Norfolk and Haldimand counties. Many landowners installed riprap revetments as an immediate response. The Province of Ontario's shoreline engineering guidelines note that riprap is appropriate where wave energy is moderate to high and where the underlying soils are susceptible to rapid erosion.
Atlantic Canada
In Prince Edward Island, the Provincial Shoreline Management Policy acknowledges riprap as one acceptable form of shore protection for properties meeting specific setback and risk criteria. The red sandstone bluffs of PEI erode at highly variable rates depending on storm frequency and frost action. Riprap has been applied at a number of municipal infrastructure sites, including road embankments approaching coastal bridges.
In New Brunswick and Nova Scotia, riprap is commonly used to protect highway embankments near tidal estuaries and exposed headlands. The Nova Scotia Department of Transportation and Infrastructure Renewal has standard specifications for riprap gradation on provincial road corridors.
British Columbia
On the BC coast, riprap is used extensively to stabilise ferry terminal approaches, port infrastructure, and sections of the Sea-to-Sky Highway near Howe Sound. The combination of high wave energy, vessel wake from commercial shipping, and rainfall-driven slope instability creates conditions where hard armoring is often the first choice for critical infrastructure protection.
Limitations and Documented Problems
Riprap revetments address erosion at the protected site, but can shift erosion to adjacent unprotected areas. This effect—often called terminal scour or flanking—occurs when the edges of a structure redirect wave energy toward the adjacent shore. In PEI, this dynamic has been documented at several sites where neighbouring landowners installed revetments in sequence, accelerating erosion at gaps between structures.
Riprap does not restore sediment supply. Natural sandy beaches are maintained by a continuous input of sediment from eroding bluffs and transport from updrift sources. Once a revetment is in place, the protected bluff no longer contributes sediment. Over time, beaches fronting riprap structures can narrow and eventually disappear—a process that affects both recreational access and the natural wave dissipation that an intact beach provides.
Maintenance requirements are ongoing. Stones shift over time, filter fabric can tear, and the toe of a revetment can be undercut if scour exceeds the depth of the stone layer. Periodic inspection and repair are necessary for any riprap installation to remain effective over decades.
Regulatory Context in Canada
In Canada, shoreline works that alter the bed or banks of navigable waters require approval under the federal Navigation Protection Act. Projects affecting fish habitat trigger assessment under the Fisheries Act managed by Fisheries and Oceans Canada (DFO). Provincial and municipal permits are also required in most jurisdictions, and setback requirements vary considerably. In PEI, the Planning Act and associated shoreline regulations prescribe minimum setbacks and require professional engineering reports for hard armoring structures above a certain scale.
Alternatives to Hard Armoring
Where site conditions allow, living shorelines and vegetation-based buffers can provide erosion control without the sediment-supply disruption associated with hard armoring. In sheltered embayments and low-to-moderate energy environments, saltmarsh restoration or beach nourishment may be more appropriate than rock placement. The choice between hard and soft engineering approaches depends on wave exposure, available space, sediment characteristics, and the nature of what is being protected.