(A) General. Channel stabilization is used to control the horizontal and/or vertical alignment of a watercourse, whether natural or constructed. The purpose of channel stabilization is to reduce flood hazards, erosion hazards and maintenance costs associated with the drainage of natural or urban runoff. Channel stabilization is generally required when flow velocities and soils conditions result in the potential for bank erosion. Guidance on permissible velocities for channels with erodible linings can be found in Design Charts for Open Channel Flow (1961), U.S. Department of Transportation. Some form of channel stabilization will generally be required in conjunction with new channel construction unless an analysis is performed, as described in §§ 152.80 - 152.83, that demonstrates an erosion hazard does not exist along the subject reach. A variety of stabilization methods are acceptable within their range of applicability. Stabilization methods, which have been found to be acceptable, include those described within divisions (B) - (H) of this section.
(B) Rock riprap. Rock riprap stabilization consists of either dumped rock, or rock held in place with wire mesh and rail piles. Rock may be placed on the banks only or on the bed and banks as the channel conditions dictate. Rock riprap is acceptable for use within the town if designed in accordance with the procedures presented within this chapter. Methods of riprap design other than the one presented herein may also be used, provided written approval is obtained from the Public Works Department.
(1) Riprap sizing.
(a) The chart provided on Figure 6.1 provides the median size of riprap, D50 for the average flow velocity with the riprap in place. A family of curves on Figure 6.1 is provided for channels with various curvatures. The angle of curvature specified on Figure 6.1 is defined as the angle made by the intersection of the centerline of the straight channel with a tangent to the outside bend (Figure 6.2). The chart provided on Figure 6.1 was developed under the assumption that the specific weight of the rock is 165 pounds per cubic foot. If rock of a substantially different specific weight is to be used, the D50 should be adjusted by use of Equation 6.1:
kr = 102.96k/(wr - 62.4) (Eqn. 6.1)
Where:
k = D50 from Figure 6.1, in feet.
kr= D50 for rock to be used, in feet.
wr= Unit weight of rock to be used, in pounds per cubic foot.
(b) Figure 6.1 is also based on the assumption that riprap will be placed on channel banks having side-slopes no steeper than 3:1. Dumped rock riprap is not permitted as a method of bank protection on side-slopes steeper than 3:1 unless an approved engineering analysis is performed which demonstrates the stability of the steeper slope. Generally, if side-slopes steeper than 3:1 are required for rock bank protection, then the rock must be held in place with wire mesh, gabion baskets or grout.
(2) Riprap gradation, thickness, and rock shape.
(a) The gradation of rock riprap should follow a smooth curve. The ratio of the largest size rock to D50 should be about two, and the ratio of D20 to D50 should be about one-half. The riprap blanket shall have a minimum thickness of 2.0D50. Table 6.1 provides a recommended design gradation for riprap.
(b) The shape of the riprap rock should be "blocky" rather than elongated. More nearly cubical rocks "nest" together, and are more resistant to movement. Also, rocks with clean, sharp edges and relatively flat faces will form a riprap mass having an angle of internal friction greater than round stones, and will be less susceptible to slope failure. The use of river-run rock may acceptable for riprap. However, the Town Engineer may require that river-run rock be grouted in place.
Table 6.1: Riprap Gradation Chart
(From USDOT, FHWA, HEC-11, 1989)
(From USDOT, FHWA, HEC-11, 1989)
Stone Size Range | Stone Weight Range | Percent of Gradation |
1.5 d
50 to 1.7 d50 | 3.0 W50 to 5.0 W50 | 100 |
1.2 d50 to 1.4 d50 | 2.0 W50 to 2.75 W50 | 85 |
1.0 d50 to 1.15 d50 | 1.0 W50 to 1.5 W50 | 50 |
0.4 d50 to 0.6 d50 | 0.1 W50 to 0.2 W50 | 15 |
(3) Riprap filters. Filters are generally required underneath rock riprap to prevent the material from being leached out through the riprap. Two types of filter materials are commonly used: gravel filters and fabric filters. Gravel filters consist of a layer of well-graded sands and gravel. Generally the thickness of a gravel filter should not be less than nine inches. Fabric filters are more commonly used, generally are very effective and easier to install than gravel filters, although care must be exercised in placing large rocks on the fabrics to prevent damage to the fabric. Filter fabric use shall be designed in conformance with the manufacturer's recommendations.
Figure 6.1: Riprap Sizing Chart
Figure 6.2
(C) Concrete. Concrete lined channels, reinforced with rebar or wire mesh, are often used when flow velocities are high, or when there is limited width for the construction of a channel. Concrete lining of the entire channel (i.e. banks and bottom) is usually required for very high flow velocity and steep channel gradients. However, an earthen bottom and concrete-lined banks, with bank toe-downs and, in some instances, periodic grade-control structures are also practical in certain locations. Toe-downs refer to the vertical distance that the bank protection extends below the invert of the channel. Concrete or "shotcrete" channel lining without some form of rebar or mesh reinforcement is not permitted by the town. The minimum thickness for concrete lined channels is six inches, measured perpendicular to the face of the lining.
(D) Gabion baskets and mattresses. Gabion baskets and mattresses are specially designed wire mesh containers for rock riprap stabilization. Gabions are generally used when adequate rock size or gradation is unavailable for ordinary dumped riprap. Additionally, gabion structures can be constructed on steeper slopes than dumped riprap, and will therefore require less right-of-way. In general, the minimum thickness of a gabion basket should equal two-thirds of the D50 rock size determined from Figure 6.1. Additionally, an adequate gravel or fabric filter should always be installed with gabions. The maximum allowable side slope for construction of gabions is 2H:1V.
(E) Grouted rock. Grouted rock provides another bank stabilization option when adequate rock size or gradation is unavailable for dumped riprap. Grouted rock may be placed on slopes as steep as 1:1 provided the underlying soils have adequate strength. During construction of grouted rock bank protection, it is important that the grout is applied in a manner that ensures grout penetration to the bottom of the riprap blanket. The minimum thickness of grouted-rock bank stabilization shall be equal to two-thirds of the D50 rock size determined from Figure 6.1. The rock shall be free of fines that prevent full penetration of grout.
(F) Articulated revetment units. Articulated revetment units (ARU's) are a stabilization material that is composed of a system of interlocking concrete blocks that may be used to line drainageways. ARU's have limited application in this area, and are used primarily on small watercourses that have very flat side-slopes and very low velocities of flow. Certain manufacturers are producing ARU's that are suitable for larger watercourses. These particular products have the advantage of allowing vegetation to grow through the ARU mat. However, in many cases, cost is a limiting factor in utilizing these products. The design engineer should consult the manufacturer's design specifications in order to evaluate the suitability of using a particular ARU on a project.
(G) Soil cement. Soil cement, also termed Cement Stabilized Alluvium (CSA), is primarily used as channel bank stabilization on large alluvial watercourses in Arizona and elsewhere. It may also be used to line channel bottoms, as well as for use in the construction of larger scale grade-control structures, collector dikes, and spillways. Soil-cement bank stabilization is normally placed on 1:1 slopes, and consists of 6 to 8-inch vertical lifts, 8 to 10 feet in width, placed horizontally in a stair-step manner in order to attain the desired height of channel bank. Soil cement can also be placed on 3:1 (or flatter) slopes, at a minimum thickness of 8 to 12 inches where a lesser level of protection is acceptable. This latter technique is often termed soil-cement "slope paving". Soil cement design applications shall consider the effects of freeze-thaw cycles.
(H) Erosion control mats and vegetation lining. Erosion control mats may be used for bank and bed stabilization where appropriate. Biodegradable matting shall be used to establish grass-lined channels that do not require permanent mats. Permanent erosion control mats are allowable where design conditions warrant their use, and provided the manufacturers design guidelines are followed.
(Res. 1637, passed 2-28-02; Am. Res. 2871, passed 9-17-15; Am. Ord. 872, passed 9-17-15)