Introduction

Protecting riverbanks and coasts aims to decrease the overall water velocity and reduce (soil) erosion. It stabilizes the slopes by covering unconstrained and angular rocks or stones along channels, rivers, or waterbodies, often called “riprap”. Riprap can also be installed at slopes exposed to weathering and where it is not possible to plant vegetation. They can be built with natural materials (e.g., stones) or artificial (e.g., concrete blocks) and can be either graded or uniform. The first includes stones of mixed size, while uniform riprap uses only one stone size. Graded riprap is usually preferred to uniform stones because it is easier and less costly to install. Bank protections can also be achieved with other construction methods, such as gabion walls (see Measure 05).

Benefits & Risks

The benefits of riprap include its simple installation and maintenance. While well designed riprap allows shrubs to grow, larger vegetation such as trees should be removed because they may cause the riprap to collapse. Generally, if the riprap is not placed properly or along too steep slopes, there is a risk of stone movement. Moreover, riprap comes with the risk of creating scour in the lower parts of the installation. Compared to using vegetation for erosion reduction, riprap is more expensive and provides fewer habitats for other species. However, snakes tend to use the riprap as a habitat which needs to be communicated to the residents of the refugee settlements.

Environmental Impact

Although riprap uses non-engineered materials (rocks), it is not considered as environmentally friendly because it requires high transport emissions for placing the rocks at the chosen location. Riprap also traps heat between the rocks which may affect the water temperature of the adjacent waterbody. The increasing water temperature can result in thermal pollution and changing aquatic ecosystems. Chemicals to avoid weed on the riprap can also harm the environment.

Good Practice

Vegetated Riprap

Design Example 01, Vegetated riprap.

Vegetated riprap incorporates a combination of rock and native vegetation in the form of live cuttings. It provides shade, cover, and input of small organic debris to the stream. At the same time, it improves the fish habitat and supports bank protection through root mass development. An additional benefit of vegetated riprap is a potential fodder supply for local animal populations. Note that well-graded riprap will form a dense and flexible cover that can adapt well, even on uneven surfaces, better than uniform riprap. Even if riprap is ranked as engineered works, when vegetated, it may become similar to the natural banks. Rriparian vegetation fosters the slowing down of the flow. Overall, the risk mitigation of vegetated riprap has a positive effect. However, in some specific cases, you may want to accelerate the flow (which induces a lowering of the flow depth).

Design Example 02, Artificial riprap for flash flood mitigation.

When the soil and the embankment are loose, one could consider using artificial concrete elements for "armoring" the riverbed. These elements form a flexible layer, mimicking a large boulder. Their shape allows them to interlock and avoid large erosion.

References

Dale’s Marine Construction Inc.    (2021):  How to install riprap and have a good defining line between yard and riprap

Massachusetts Clean Water Toolkit    (2023):  Riprap

Flexamat    (2022):  Negative Impacts of Riprap on Lakeshores

Score Card

Criteria

Scale of Intervention

Shelter-Plot-Block Settlement Supra-settlement

---

Type of Intervention

Engineered Nature-based Hybrid Non-structural

---

Targeted Natural Hazard

Pluvial Flood Coastal/Riverine Flood

---

Strategy Type

Relocate Reduce Hazard Magnitude Reduce Asset Vulnerability Reduce Casualties

---

Implementation Time

Short (1 day ‐ 1 month) Medium (1 month ‐ 1 year) Long (> 1 year)

---

Effect Duration

Short‐term ( <1 year ) Medium‐term (1 year to 10 years) Long‐term (>10 years)

---

Targeted Vulnerable Assets

Buildings Transport Technical Infrastructure Land Cover

---

Investment Costs

Low Medium High

---

Maintenance Costs (yearly)

Low (<10% investment costs) Medium (10-50%) High (>50%)

  Annual check-ups, or after major weather events, concerning damages, obstructions, or woody vegetation (which needs to be removed) are necessary in terms of riprap maintenance.

Materials

Unconstrained/angular rocks, filter material (e.g., sand, gravel, crushed stone or filter fabric), sometimes concrete for artificial ripraps.

Download Factsheet

   PDF