Riprap is a permanent layer of large, angular stone, cobbles, or boulders typically used to armor, stabilize, and protect the soil surface against erosion and scour in areas of concentrated flow or wave energy. Riprap is typically placed along graded ditch, channel, and shoreline banks over geotextile, which prevents erosional undercutting. It can also be used with other mixed size rock to construct retention berms for sediment traps and check dams protecting high volume/velocity culvert inlets. The Minnesota Department of Transportation classifies riprap by type as random riprap or hand-placed riprap, depending on the method of placement and the stone size specified.
Riprap is used to stabilize areas on a construction site with high erosive power by increasing surface roughness and slowing the velocity of runoff. Applicable areas on a site may include inlets and outlets of storm pipes and culverts, bridges, slopes drains, storm drains, and other areas where concentrated runoff may occur. Riprap is also effective for protecting and stabilizing slopes, channels, streambanks, and shorelines.
As discussed above, there are many applications where riprap is more desirable than other erosion control practices. Although riprap is not often considered aesthetically pleasing, it can be one of the most effective methods of erosion prevention and is particularly desirable in areas where conditions prohibit establishment of vegetation (for example, areas where velocities are too great for vegetation to withstand). Compared to other erosion control practices, riprap is relatively simple to install and maintain. However, riprap is typically more expensive to install compared to vegetation (i.e. due to equipment and handling costs) and does not provide some of the secondary benefits provided by vegetated practices (e.g., habitat enhancement).
Riprap is useful in areas in which the powers of erosion outweigh the stabilization capacity of other erosion control practices such as vegetative control and mulching. As noted above, riprap is especially useful for armoring channel and ditch banks, lake shorelines, and for sediment trap berms and high-volume/velocity check dams. Because of the “hard” look of riprap, its higher overall cost, the growing popularity of vegetated solutions using turf reinforcement matting and other products, and the difficulty in removing it after installation, contractors should ensure that the post-construction site design specifically includes riprap before using it during construction. Riprap may be unstable on very steep slopes. For slopes steeper than 2:1, consider using materials other than riprap for erosion protection such as turf reinforcement matting over seed, open-cell articulated concrete mats, or other slope protection geogrid products/matrices.
Riprap as a stabilization measure
Riprap is often used for stabilization of construction sites. Section 25.30 of the MPCA Construction Stormwater General Permit (2018) defines stabilization and specifically identifies riprap as an appropriate stabilization measure, stating: “’Stabilize’, ‘Stabilized’, ‘Stabilization’ means the exposed ground surface has been covered by appropriate materials such as mulch, staked sod, riprap, erosion control blanket, mats or other material that prevents erosion from occurring. Grass seeding, agricultural crop seeding or other seeding alone is not stabilization. Mulch materials must achieve approximately 90 percent ground coverage (typically 2 tons of mulch per acre).”
Because stabilization is required and essential for most construction activities, it is mentioned numerous times throughout the 2018 Permit. While “riprap” is not specifically mentioned in these instances, it is implicitly included due to its widespread acceptance as an effective stabilization practice. As noted above, before using riprap the site manager should check with the design engineer to ensure that the post-construction configuration includes riprap prior to its installation. In many cases it may be far less expensive to install the post-construction vegetation with whatever stabilization matrix is called for (e.g., turf reinforcement mat, articulated concrete mat, geogrid, etc.) during construction, rather than having to place and then later remove riprap. Several key locations in the permit that mention stabilization are noted below.
Riprap as an energy dissipation measure
In addition to stabilization, riprap is also used as an energy dissipation measure to reduce the erosive force of concentrated stormwater and prevent scour. Section 25.8 of the MPCA Construction Stormwater General Permit (2018) defines energy dissipation as “methods employed at pipe outlets to prevent erosion caused by the rapid discharge of water scouring soils.” Energy dissipation is mentioned at several locations in the Permit:
When properly designed and installed riprap can be an effective erosion control practice for armoring slopes and channel banks. When used as a containment berm (e.g., as a sediment trap), riprap can help to control sediment. The following table provides information on the expected performance benefits for riprap when used for erosion control.
Expected performance benefits for riprap.
Link to this table
|Flow attenuation||Little or no design benefit|
|Runoff volume reduction||Little or no design benefit|
|Soil erosion||Primary design benefit|
|Sediment control||Little or no design benefit|
|Nutrient loading||Little or no design benefit|
|Total suspended solids||Little or no design benefit|
|Total phosphorus||Little or no design benefit|
|Heavy metals||Little or no design benefit|
|Floatables||Little or no design benefit|
|Oil and grease||Little or no design benefit|
Riprap is often required in places that are not easy to get to, yet it is often the really tough places to reach that experience riprap failure (MnDOT, 2006). The following considerations, which are discussed in greater detail in Design and Construction recommendations and specifications. are essential when planning riprap installation for a construction site.
Under-sizing riprap could greatly reduce its effectiveness. Follow MnDOT standards and specifications on riprap size and quality to ensure maximum effectiveness (see Construction recommendations and specifications). The following design recommendations apply to riprap.
Before laying riprap, filter material should be placed on a prepared surface in accordance with site plans unless otherwise required by the contract. The foundation surface should be relatively smooth and free of stones, sticks, or other debris. Filter material can be either granular filter or nonwoven geotextile filter, unless specified in the contract. The primary function of a filter material or filter fabric is to prevent soil from “piping” through the riprap stone.
If using granular material, spread the filter material to a minimum thickness of at least 6 inches over the prepared foundation. If using a geotextile, ensure that the fabric liner is pulled taut with no folds or creases before anchoring in place (if needed) with stables or anchor pins. To prevent water from flowing beneath fabric, overlap edges by at least a foot and a half in the downhill or downstream direction. Riprap should be placed over the geotextile fabric no later than seven (7) days after the fabric is laid. Avoid tearing, puncturing, or shifting the fabric and do not operate equipment on top of the geotextile or stones after placement. Geotextile filter material should not be used under hand placed or grouted riprap unless required by the contract.
If placing geotextile on slopes steeper than 3:1 (horizontal to vertical), anchor the geotextile in consecutive trenches on the contour at least every 15 feet. Geotextile should not be used on slopes exceeding 2:1 (horizontal to vertical).
Random riprap consists of various sizes of stone that produce a dense, uniform, stable layer of stone with no large voids . Large individual stones may measure up to 2 cubic ft and weigh 50 lbs or more. There is no minimum weight for smaller stones required for filling in between these individual stones. Use of the smaller rock prevents undercutting and erosion by flowing water, and helps to keep the larger rock in place.
Stone selected for riprap installations must meet MnDOT Specifications 3601 (Riprap Material), including the following:
Riprap sizing should also account for the expected velocity of water. The table below provides recommended sizing of riprap rock based on velocity.
Riprap size will increase with velocity. Maximum riprap rock size in diameter for different velocities. (Source: Adapted from New York State Standards and Specifications for Erosion and Sediment Control, 2016).
Riprap size will increase with velocity.
Link to this table
|Rock riprap maximum size based on velocity of flow|
Additional information on riprap sizing for energy dissipation can be found here: Sediment control practices - Outlet energy dissipation
When placing stones, care should be taken to not puncture the base fabric. Drop stones from a height not exceeding one (1) foot unless the base fabric is sufficiently covered with granular cushion (about 6 inches deep) in which case stones can be dropped up from a height of up to three (3) feet. Riprap should be placed starting at the lowest elevation and working upwards.
Random riprap should be position in such a way as to provide a uniform distribution of the various sizes of stone and to produce a dense, interlocked layer of stones. After placement, the surface of the riprapped area should be leveled to be flush with the surrounding ground to produce a reasonably uniform appearance and the thickness required by the contract.
For hand-placed riprap, embed the stones in the foundation material, with the axis of the stone that most nearly approximates the contract-required thickness of riprap laid perpendicular to the foundation slope. Lay stones with the least practicable space between them. Position the stones to stagger the joints up the slope. Place each stone to allow the foundation material and adjacent stones to carry its mass. Use selected stones set to line and grade to define the ends and edges of each riprap area. After laying the larger stones, fill the spaces between the stones with firmly seated, smaller stones to produce a uniform surface.
For fully grouted riprap, use clean stone and ensure grout fills the spaces between stones throughout the entire thickness of the riprap. Stones should be thoroughly wetted immediately before placing the grout, and the surface should be swept with a stiff broom after pouring grout to finish. [See https://www.dot.state.mn.us/bridge/hydraulics/resources/matrixriprapsummarysheet.pdf for further information] on grouted riprap.
After placing riprap, ensure that each riprapped area has a minimum thickness of at least eighty (80) percent of the thickness per the contract requirements, and an average of at least ninety-five (95) percent of the thickness when measured at right angles to the face.
The Construction recommendations and specifications discussedin the next section elaborates these design recommendations.
The MnDOT Standard Specifications for Construction (2018 Edition) include several specifications related to riprap.
MnDOT Specification 2511 is dedicated entirely to riprap; specifically, (2511.1) “providing and placing stone riprap, with or without grouting, as a protective covering on earth slopes, piers, abutments, walls, or other structures, where the soil is susceptible to erosion.” Further, “The Department classifies riprap by type as random riprap or handplaced riprap, depending on the method of placement and the stone size specified.”
MnDOT Specification 2511 Part A includes guidance for placement of riprap, stating: “Provide and place stone riprap as shown on the plans or as directed by the Engineer. Excavate and shape the foundation for the riprap, with or without filter material, to the cross-sections as shown on the plans, unless otherwise directed by the Engineer. Compact loose foundation material before placing the riprap or filter material. If the contract requires, place a layer of riprap at least a 1 ft thick on a filter material, unless otherwise required by the contract or directed by the Engineer. Grout riprap as required by the contract or as directed by the Engineer. Place the riprap on a filter layer consisting of granular material or geotextile. Fully grouted riprap is not allowed in public waters.”
MnDOT Specification 2511 Part B discusses placement of filter material under riprap, stating: “Place filter material under the riprap unless otherwise required by the contract. The Contractor may choose the type of filter material, except as required by the contract.” Per Part B, filter material may either be granular or geotextile material. Parts B.1 (Granular Filter) and B.2 (Geotextile Filter) provide detailed guidance on proper placement, dimensions, and applicable site conditions for the use of granular filter or geotextile filter materials in conjunction with riprap. In all cases, ensure that the geotextile underlayer is properly staked to the slope in accordance with manufacturer’s recommendations to ensure it remains in place as the stone is installed.
MnDOT Specification 2511 Part C provides detailed guidance for placement of riprap stone, including Random Riprap (Part C.1) and Hand-Placed Riprap (Part C.2). Further, Part D describes proper procedures for grouting riprap, and Part E provides guidance on required thickness when placing riprap.
MnDOT Specification 2511.4 prescribes “method of measurement” related to riprap. Part A (Riprap) states: “If measuring riprap of each type and class by volume, the Engineer will calculate the volume based on the actual surface dimensions as staked and the thickness shown on the plans or specified in the special provisions. If measuring riprap of each type and class by mass, the Engineer will calculate the mass based on scale tickets of materials delivered and placed within the staked areas.” Further, Part B (Filter Materials) states: “If measuring filter materials by weight, the Engineer will calculate the weight based on scale tickets of material delivered and placed within the staked areas. If measuring filter materials by volume, the Engineer will calculate the volume based on the actual surface dimensions as staked and the thickness as shown on the plans. The Engineer will measure geotextile filter material by area based on the actual surface dimensions as staked, with no allowance for overlaps or seams.”
MnDOT Specification 2511.5 prescribes “basis of payment” related to riprap, stating: “The contract unit price for riprap of each type and class includes the cost of providing the materials, excavating and preparing the foundations, and placing the riprap stone, grouting, and filter materials as required by the contract. The Department will pay for filter materials of the type specified, if included in the contract.” Further, 2511.5 states that payment by the Department for riprap and filter material will be based on the following schedule:
The relevant specifications noted above in 2511.4 (Method of Measurement) and 2511.5 (Basis of Payment) are important when considering cost implications of these practices, discussed further below.
MnDOT Specifications 3601 provides materials specifications for “riprap material,” including (3601.1) “stone and filter layer material for use in random or hand-placed riprap, gabion, and revet mattress construction.” Specification 3601.2 Part A.1 provides specifications related to quality of riprap stone, stating: “Provide stone of the quality approved by the Department and meeting the following requirements:
To determine suitable quality of stone, [MnDOT] may consider the results of laboratory tests, the performance of the stone under natural exposure conditions, the performance of the riprap from the same or similar geological formations or deposits, or other tests or criteria.” MnDOT Specification 3601 Part A.2.a provides guidance for random riprap by referring the user to Table 3601-1, which outlines gradation requirements for random riprap.
MnDOT standard riprap gradations
Link to this table
|Approximate Percent of Total Weight Smaller than Given Weight|
MnDOT Specification 3601 Part A.2.b describes requirements for hand-placed riprap, stating: “Provide individual stones with a weight of at least 50 lb. The Department will not require a minimum weight for smaller stones required for filling in the narrow openings between individual stones (chinking).”
MnDOT Specification 3601 Part B provides guidance for filter material to be used in conjunction with riprap. Per Part B.1.a, granular filter material to be used under Class I random riprap shall meet the Class 5 gradation requirements of Table 3138-3, and granular filter material to be used under other riprap shall meet the requirements of Table 3601-2.
MnDOT specifications for granular filter material under Class I random riprap
Link to this table
|Granular Filter Material Under Class I Random Riprap|
(Source: adapted from MnDOT (2018) Table 3138-3)
MnDOT specifications for granular filter material under all other riprap
Link to this table
|Granular Filter Material Under All Other Riprap|
(Source: adapted from MnDOT (2018) Table 3601-2)
Per MnDOT Specification 3601 Part B.2, geotextile filter to be used with riprap shall meet the requirements of Specification 3733. MnDOT Specification 3733.1 notes the following regarding use of geotextiles under riprap:
Geotextile Types 3 and 4 must meet the following specifications, per 3733.2.B:
MnDOT specifications for geotextile under riprap.
Link to this table
(a) Minimum Average Roll Values (MARV) based on average of at least three tests per swatch. (b) Needle-punched nonwoven. Do not use thermally bonded (heat-set) fabric. (c) Adhere to this requirement if the contract requires or allows seams. Strength specifications apply to factory and field seams. Use thread for sewing that has strength of at least 25 lb . Sew seams with a Federal Type 401 stitch using a two-spool sewing machine, and install seams facing upward. For seaming with adhesives, see the Approved/Qualified Products List available at the Department’s website. (d) For U.S. sieve sizes, the AOS Number must be equal to or greater than the number specified. (e) Permittivity: P = K/L, where K = fabric permeability and L = fabric thickness. (Source: adapted from MnDOT (2018) Table 3733-1)
In general, riprap should be inspected after high flows to ensure deficiencies are identified and corrected early. Recommended inspection and maintenance activities for riprap include the following:
Riprap costs are typically much higher than other forms of erosion control. The following table summarizes estimated BMP costs based on MnDOT data summarizing average bid prices for awarded projects in 2017.
Unit costs related to riprap
Link to this table
|Bid Item||Item Description||Units||Average Price|
|2511501/00010||Random riprap Class Special||cubic yard||$39.00|
|2511501/00011||Random riprap Class I||cubic yard||$64.10|
|2511501/00012||Random riprap Class II||cubic yard||$74.82|
|2511501/00013||Random riprap Class III||cubic yard||$66.12|
|2511501/00014||Random riprap Class IV||cubic yard||$65.27|
|2511501/00015||Random riprap Class V||cubic yard||$71.64|
|2511505/00010||Hand-placed riprap||cubic yard||$750.00|
|2511604/00200||Articular concrete riprap||square yard||$75.51|
|2511607/00011||Random riprap special||cubic yard||$47.33|
|2511607/00012||Random riprap (matrix)||cubic yard||$465.25|
|2511607/00120||Install Random riprap||cubic yard||$47.00|
(Source: MnDOT 2017 Average Bid Prices)
Except where more stringent requirements are presented in this guidance, BMPs shall comply with MnDOT and other state requirements. Primary design references include:
The following is a list of additional resources that are not specific to Minnesota: