Rip-rap

Category: NBS FOR EROSION CONTROL - Combined living/not living approach / NBS for erosjonsreduksjon - kombinerte levende/ikke levende materialer

Description

Rip-rap is a method typically used along stream banks or sea banks to protect soil from runoff erosion or wave erosion. It consists of stones with well graded weight and shape interlocked together to form a layer or protective mound (Figure 1). Stones used for rip-rap have an angular shape, are hard, durable and are usually dumped on a gravel bedding layer and/or a woven or nonwoven geotextile fabric (Theisen, 1992). Rip-rap structure has two functions: the stones absorb and deflect the energy of water waves, whereas the gaps between the rocks slow the flow of water by reducing its ability in eroding soil. These solutions are suitable for protecting slopes from severe wave actions were the use of vegetation alone does not work (Johnson 2003).

Advantages

  • Effective in protecting lake/stream/sea banks from severe wave actions were the use of vegetation alone does not work;

  • reduced ability of water in eroding soil;

  • providing a habitat and cover for water fauna of the channel/river/sea;

  • effective for steep slopes;

Disadvantages

  • the soil must be stable and tightly compacted before placing rip-rap;

  • high demand of labour and cost;

  • for long big slopes, equipment as crane or dump truck are needed to move and place the rocks;

  • high impact on the landscape.

Figure1. Rock rip-rap system along a river bank (USDA - NRCS EFH Chapter-16 , 1996).

 


Design methods

(from NY – Department of Environmental Conservation)

The slope where to implement rip-rap measure must be stable with a maximum steepness of 50%. Before laying the stones the soil must be compacted. The stones typically used for rip-rap are limestones or granite chunks from 30 to 45 cm long, having at least two fractured faces. Round shaped rocks are not suitable because they can easily slide down the slope. The rocks should be interlocked each other forming a layer. A first "filter layer" is always needed at the base of the rip-rap, with the function of protecting the substrate of soil from erosion and to prevent undercutting of the rock from the waves. This is made of stones not larger than 8 cm in diameter. A filter fabric (geotextile) is usually placed underneath  the "filter layer" in contact with the ground soil. The rocks are then dumped (manually or with the help of a crane or a dump truck) on the "filter layer" in order to form other layers. The top layer is also called "armor layer" because it takes the initial impact from the waves. If there are live trees or native vegetation along the slope, the rip-rap should laid around them without disturbing or damaging the existing vegetation barrier (Figure 2).

 

Figure 2. Scheme of rip-rap measurement along a stream bank, with existing vegetation (USDA - NRCS EFH Chapter-16, 1996)

 

Vegetated rip-rap

Rip-rap can be used in combination with other erosion protection measurements, such as live staking (Figure 3). This is preferred when the soil underneath the rip-rap has to be reinforced with roots action. In this way the rip-rap prevents wave action from eroding the shore, while the plant roots bind the soil below. Leaves and shoots of plants provide shade on the rocks by increasing wildlife and fish well-being along the rip-rap. Moreover, with the presence of vegetation, the impact of the rip-rap on the surrounding landscape is reduced.

Figure 3. Scheme of rip-rap with live stakes (USDA - NRCS EFH Chapter-16 , 1996)

Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 2 Only suitable for erosion control along stream/river/sea banks. It can provide protection against back erosion.

Living vegetal material must be used with rocks.
Topple 2
Slide 5
Spread 3
Flow 5

Material type

Descriptor Rating Notes
Earth 5 Applicable irrespective of material type.

Living vegetal material must be used with rocks.
Debris 5
Rock 3

Depth of movement

Descriptor Rating Notes
Surficial (< 0.5 m) 7 Highly effective for surficial soil and in some cases for shallow soils (vegetated rip-rap can provide a reinforce also to the soil underneath the rock layers because of roots)
Shallow (0.5 to 3 m) 6
Medium (3 to 8 m) 3
Deep (8 to 15 m) 2
Very deep (> 15 m) 2

Rate of movement

Descriptor Rating Notes
Moderate to fast 5 The measure can protect also from high speed waves erosion.
Slow 5
Very slow 6
Extremely slow 9

Ground water conditions

Descriptor Rating Notes
Artesian 4 Applicable irrespective of groundwater conditions, but mostly used along stream/river banks or coastal shorelines (from low to high water level).

Use of adequate vegetal species to water abundance are recommended.
High 5
Low 3
Absent 2

Surface water

Descriptor Rating Notes
Rain 6 Applicable irrespective of surface water conditions, this measure is typically used for protecting slopes from river/sea/torrent water waves, it can also provide protection against rain/snowmelt erosion
Snowmelt 6
Localized 7
Stream 9
Torrent 9
River 9

Reliability and feasibility criteria

Criteria Rating Notes
Reliability 8 Reliable according with a correct installation and correct material involved.
Feasibility and Manageability 10 Simple installation with the proper equipment, maintenance is needed frequently

Urgency and consequence suitability

Criteria Rating Notes
Timeliness of implementation 8 Fast installation and immediate effectiveness
Environmental suitability 6 It can give negative impacts to the surrounding landscape. Sometimes is combined with live stakes technique to reduce the possible negative impact to the waterbodies.
Economic suitability (cost) 6 Demanding cost if the area interested is wide.

References

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