Design of Roadside Channels with Flexible Linings
Hydraulic Engineering Circular Number 15, Third Edition
Chapter 1: Introduction
This manual addresses the design of small open channels called roadside channels that are constructed as part of a highway drainage system. Roadside channels play an important role in the highway drainage system as the initial conveyance for highway runoff. Roadside channels are often included as part of the typical roadway section. Therefore, the geometry of roadside channels depends on available right-of-way, flow capacity requirements, and the alignment and profile of the highway. The procedures in this manual may also be used for ancillary roadside drainage features such as rundowns.
Roadside channels capture sheet flow from the highway pavement and backslope and convey that runoff to larger channels or culverts within the drainage system. This initial concentration of runoff may create hydraulic conditions that are erosive to the soil that forms the channel boundary. To perform reliably, the roadside channel is often stabilized against erosion by placing a protective lining over the soil. This manual presents a class of channel linings called flexible linings that are well suited for construction of small roadside channels.
This manual is presented in dual units. The SI (metric) units precede the customary units (CU) when units are given. Design examples are provided in both systems of units.
1.1 Scope and Applicability
Channel lining materials fall into two classes: rigid or flexible channel linings. From an erosion control standpoint, the primary difference between rigid and flexible channel linings is their response to changes in channel shape (i.e. the width, depth and alignment). Flexible linings are able to adjust to some change in channel shape while rigid linings cannot. The ability to sustain some change in channel shape improves the overall integrity of the channel lining and reduces maintenance. Movement of a rigid lining at one location can result in a successive failure of the lining. Channel lining materials often experience forces such as frost heave, slumping or swelling of the underlying soils that can change the shape of the lining. These forces can displace rigid linings whereas flexible linings, if properly designed, will retain erosion-control capabilities.
Flexible linings also have several other advantages compared to rigid linings. They are generally less expensive, permit infiltration and exfiltration and can be vegetated to have a natural appearance. Flow in channels with flexible linings is similar to that found in natural small channels. More natural behavior offers better habitat opportunities for local flora and fauna. In many cases, flexible linings are designed to provide only transitional protection against erosion while vegetation establishes and becomes the permanent lining of the channel. Vegetative channel lining is also recognized as a best management practice for storm water quality design in highway drainage systems. The slower flow of a vegetated channel helps to deposit highway runoff contaminants (particularly suspended sediments) before they leave the highway right of way and enter streams.
Flexible linings have a limited hydraulic performance range (depth, grade, velocity and discharge). The magnitude of hydraulic force they can sustain without damage is limited by a number of factors including soil properties and roadway grading. Because of these limitations, flexible channel designs using the same lining material will vary from site to site and between regions of the country. Since the performance range for rigid channels is higher, such channels may be needed in cases where channel width is limited by right of way, but sufficient space exists for a high capacity channel.
Design procedures covered in this manual relate to flexible channel linings. Rigid linings are discussed only briefly so that the reader remains familiar with the full range of channel lining alternatives. The primary reference for the design of rigid channels is Hydraulic Design Series No. 4 " Introduction to Highway Hydraulics" (FHWA, 2001). For channels which require other protection measures, the design of energy dissipaters and grade-control structures can be found in Hydraulic Engineering Circular (HEC) No. 14 (FHWA, 1983).
Riprap design procedures covered in this manual are for prismatic channels typically having a maximum depth of 1.5 m (5 ft). However, the procedures for riprap design are not limited by depth with the exception of the limits cited on techniques for estimating Manning's roughness. The use of the procedures in Hydraulic Engineering Circular (HEC) No. 11 (FHWA, 1987) is recommended for nonprismatic channels.
The permissible tractive force and Manning's n values provided in this manual for grass-lined channels is based on the relative roughness theory, the biomechanical properties of grass (height, stiffness and density of the grass cover), and the properties of the underlying soil (particle size, density and plasticity). This method is comparable to methods used in agricultural channel design (USDA, 1987), but offers the highway designer more flexibility. This document provides a method of estimating grass properties for complex seed mix designs using a simple field test.
The current performance information for manufactured channel linings is based on industry testing and design recommendations. Product testing is routinely conducted by major manufacturers using either their own hydraulic laboratories (Clopper, Cabalka, Johnson, 1998) or using facilities at university labs. Industry protocols have been developed for large scale testing (ASTM D 6460) that provides a consistent test method for flexible channel lining materials. Small-scale tests (i.e. bench tests) have been developed that are intended for qualitative comparison of products and product quality verification. Data from bench testing is not sufficient to characterize the hydraulic performance of manufactured linings. While there is a qualitative understanding about manufactured-lining performance, large-scale testing is currently needed to determine performance properties.
Considerable development and research has been performed on rigid and flexible channel linings. Prior to the late 1960's, natural materials were predominantly used to stabilize channels. Typical materials included rock riprap, stone masonry, concrete, and vegetation. Since that time a wide variety of manufactured and synthetic channel linings applicable to both permanent and transitional channel stabilization have been introduced. Since the publication of the 1988 edition of HEC No. 15, erosion control material manufacturers have developed protocols for testing flexible linings in hydraulic laboratory flumes under controlled conditions.
The market for flexible channel lining products has expanded and there are a large number of channel stabilization materials currently available. Channel stabilization materials can be broadly classified based on their type and duration of installation. Two basic types of lining classes are defined: rigid and flexible. Rigid lining systems are permanent, long-duration installations. Flexible linings systems can either be long-term, transitional, or temporary installations. The following are examples of lining materials in each classification.
- Rigid Linings
- Cast-in-place concrete or asphaltic concrete
- Stone masonry and interlocking modular block
- Soil cement and roller compacted concrete
- Fabric form-work systems for concrete
- Partially grouted riprap
- Flexible linings
- Vegetative (typically grass species)
- Rock Riprap
- Wire-enclosed riprap (gabions)
- Turf reinforcement (non-degradable)
- Bare soil
- Vegetative (annual grasses)
- Gravel mulch
- Open-weave textile (degradable)
- Erosion control blankets (degradable)
- Turf reinforcement (non-degradable)
- Bare soil
- Vegetative (annual grasses)
- Gravel mulch
- Open-weave textile (degradable)
- Erosion control blankets (degradable)
Sprayed on mulch is a common application for erosion control on hill slopes. Mulch is combined with a glue or tackifier to form slurry that is pumped at high pressure onto the hill slope. The only channel lining tested in this class is fiberglass roving (McWhorter, Carpenter and Clark, 1968). This lining is not in use because during maintenance operations, mowers can rip up large sections of the roving. Also, although some tackifiers have been reported to encourage growth, asphalt tackifier usually inhibits vegetation establishment and growth.
An emerging product in this class is a form of sprayed on composting. Used both for hill slopes and for channels, the objective of the product is to accelerate vegetative establishment. As such, composting does not represent a lining product class, but is a strategy to shorten transition periods. Other new products may emerge in this class, but until full scale testing is conducted (in accordance with ASTM D 6460) they will not be covered in this manual. Products that address only hill slope or embankment erosion control and not channel applications are also not included in this manual.
1.3 Rigid Linings
Rigid linings (Figure 1.1) are useful in flow zones where high shear stress or non-uniform flow conditions exist, such as at transitions in channel shape or at an energy dissipation structure. They can be designed to include an impermeable membrane for channels where loss of water from seepage is undesirable. Since rigid linings are non-erodible the designer can use any channel shape that is necessary to convey the flow and provide adequate freeboard. This may be necessary where right-of-way constrains the channel width.
Figure 1.1. Rigid Concrete Channel Lining
Despite the non-erodible nature of rigid linings, they are susceptible to failure from foundation instability. The major cause of failure is undermining that can occur in a number of ways. Inadequate erosion protection at the outfall, at the channel edges, and on bends can initiate undermining by allowing water to carry away the foundation material and leaving the channel to break apart. Rigid linings may also break up and deteriorate due to conditions such as a high water table or swelling soils that exert an uplift pressure on the lining. Once a rigid lining is locally broken and displaced upward, the lining continues to move due to dynamic uplift and drag forces. The broken lining typically forms large, flat slabs that are particularly susceptible to these forces. Freeze thaw cycles may also stress rigid channels. The repeated cycling of these forces can cause fine particles to migrate within the underlying soil causing filter layers and weep holes to clog and further increase uplift pressure on the lining.
Rigid linings are particularly vulnerable to a seasonal rise in water table that can cause a static uplift pressure on the lining. If a rigid lining is needed in such conditions, a reliable system of under drains and weep holes should be a part of the channel design. The migration of soil fines into filter layers should be evaluated to ensure that the ground water is discharged without filter clogging or collapse of the underlying soil. A related case is the build up of soil pore pressure behind the lining when the flow depth in the channel drops quickly. Use of watertight joints and backflow preventers on weep holes can help to reduce the build up of water behind the lining.
Construction of rigid linings requires specialized equipment and costly materials. As a result, the cost of rigid channel linings is typically higher than an equivalent flexible channel lining. Prefabricated linings can be a less expensive alternative if shipping distances are not excessive. Many highway construction projects include paving materials (concrete and asphaltic concrete) that are also used in rigid channel linings. This may provide an economy of scale when similar materials are used for both paving and channel construction.
1.4 Flexible Linings
Flexible linings can meet a variety of design objectives and serve a variety of roles in the construction of a project where prismatic channels are required for conveying stormwater runoff. Flexible channel linings are best suited to conditions of uniform flow and moderate shear stresses. Channel reaches with accelerating or decelerating flow (expansions, contractions, drops and backwater) and waves (transitions, flows near critical depth, and shorelines) will require special analysis and may not be suitable for flexible channel linings.
Several terms are used to describe the longevity of flexible linings - permanent, long-term, transitional, temporary, and short-term - to name a few. Recognizing that nothing is permanent, long-term is defined as serving the desired purpose throughout the lifetime of the drainage channel given appropriate maintenance. The other terms imply that changes must occur either in the removal of the channel or replacement of one lining type with another. However, the designer should keep in mind not only the manufacturer's claims of longevity, but also site-specific maintenance practices and climate or geographic location in selecting a lining type for a given transitional or temporary application.
1.4.1 Long-term Flexible Linings
Long-term flexible linings are used where roadside channels require protection against erosion for the service life of the channel.
Vegetative linings consist of seeded or sodded grasses placed in and along the channel (Figure 1.2). Grasses are seeded and fertilized according to the requirements of that particular variety or mixture. Sod is laid with the longest side parallel to the flow direction and should be secured with pins or staples.
Figure 1.2. Vegetative Channel Lining
Vegetation is one of the most common long-term channel linings. Most roadside channels capture only initial highway runoff and so remain dry most of the time. For these conditions, upland species of vegetation (typically grass) provide a good lining. However, upland species of vegetation are not suited to sustained flow conditions or long periods of submergence. Common design practice for vegetative channels with sustained low flow and intermittent high flows is to provide a composite lining with riprap or concrete providing a low flow section. There are plant species that are adapted to wet low land conditions that can be used for the low flow channel in cases that warrant the additional design and construction effort (wetland replacement for example).
Where vegetation provides the long-term channel lining, there is a transition period between seeding and vegetation establishment. The initial unvegetated condition of the lining is followed by a period of vegetation establishment that can take several growing seasons. The channel is vulnerable to erosion during the transition. Transitional flexible linings provide erosion protection during the vegetation establishment period. These linings are typically degradable and do not provide ongoing stabilization of the channel after vegetation is established. Non-degradable linings have an expected life of several years beyond vegetation establishment, which enhances the performance of the vegetation. At this time it is not known how long an installation of non-degradable flexible linings will last together with vegetation.
188.8.131.52 Cobble Lining
Cobble lining consists of stone in the size range of small cobbles, 64 to 130 mm (2.5 to 5 inches), and tends to have a uniform gradation. The cobble layer is placed on engineering fabric on a prepared grade (Figure 1.3). The cobble material is composed of uniformly graded, durable stone that is free of organic matter. Cobbles are typically alluvial in origin and smooth and rounded in appearance.
Cobble linings are often used when a decorative channel design is needed. Cobble linings are composed of smooth stones that do not interlock, so they are not suitable for placement on steep grades or on channel side slopes that are steep. As with riprap and gabion linings, a filter material is required between the stone and the underlying soil.
Figure 1.3. Cobble Channel Lining
184.108.40.206 Rock Riprap
Rock riprap is placed on a filter blanket or prepared slope to form a well-graded mass with a minimum of voids (Figure 1.4). Rocks should be hard, durable, preferably angular in shape, and free from overburden, shale, and organic material. The rock should be durable and resistance to disintegration from chemical and physical weathering. The performance of riprap should be determined from service records for a quarry or pit, or from specified field and laboratory tests.
Riprap and gabion linings can perform in the initial range of hydraulic conditions where rigid linings are used. Stones used for riprap and gabion installations preferably have an angular shape that allows stones to interlock. These linings usually require a filter material between the stone and the underlying soil to prevent soil washout. In most cases, an engineering fabric is used as the filter. Care should be taken to provide adequate permeability in the filter to prevent uplift pressures on the lining.
Figure 1.4. Riprap Channel Lining
220.127.116.11 Wire-Enclosed Riprap
Wire-enclosed riprap (gabions) is a wire container or enclosure structure that binds units of the riprap lining together. The wire enclosure normally consists of a rectangular container made of steel wire woven in a uniform pattern, and reinforced on corners and edges with heavier wire (Figure 1.5 and Figure 1.6). The containers are filled with stone, connected together, and anchored to the channel side slope. Stones must be well graded and durable. The forms of wire-enclosed riprap vary from thin mattresses to box-like gabions. Wire-enclosed riprap is typically used when rock riprap is either not available or not large enough to be stable. Although flexible, gabion movement is restricted by the wire mesh.
Figure 1.5. Wire-Enclosed Riprap
Figure 1.6. Installed Wire-Enclosed Riprap
18.104.22.168 Turf Reinforcement
Depending on the application, materials, and method of installation, turf reinforcement may serve a transitional or long-term function. The concept of turf reinforcement is to provide a structure to the soil/vegetation matrix that will both assist in the establishment of vegetation and provide support to mature vegetation. Two types of turf reinforcement are commonly available: gravel/soil methods and turf reinforcement mats (TRMs)
Soil/gravel turf reinforcement is to mix gravel mulch (see Section 22.214.171.124) into on-site soils and to seed the soil-gravel layer. The rock products industry provides a variety of uniformly graded gravels for use as mulch and soil stabilization. A gravel/soil mixture provides a non-degradable lining that is created as part of the soil preparation and is followed by seeding.
A TRM is a non-degradable RECP composed of UV stabilized synthetic fibers, filaments, netting and/or wire mesh processed into a three-dimensional matrix. TRMs provide sufficient thickness, strength and void space to permit soil filling and establishment of grass roots within the matrix. The mat, shown in Figure 1.7 and Figure 1.8, is laid parallel to the direction of flow. TRM is stiffer, thicker (minimum of 6 mm (0.25 in)) and denser than an erosion control blanket (ECB). These material properties improve erosion resistance. The TRM is secured with staples and anchored into cutoff trenches at intervals along the channel. Two methods of seeding can be used with TRM. One choice is to seed before placement of the TRM, which allows the plant stems to grow through the mat. The second choice is to first place the TRM then cover the mat with soil and then seed. This method allows the plant roots to grow within the mat.
Figure 1.7. TRM Profile with Vegetation/Soil/TRM Matrix (Source: ECTC)
Figure 1.8. Installed TRM Lining Before Vegetation (Source: ECTC)
1.4.2 Transitional and Temporary Flexible Linings
Transitional linings are intended and designed to facilitate establishment of the long-term flexible lining. Commonly the long-term lining would be vegetation. Temporary channel linings are used without vegetation to line channels that are part of construction site erosion control systems and other short-term channels. In some climates, rapidly growing annual grass species establish quickly enough to be used as a temporary channel lining.
Many of the transitional and temporary linings are described as degradable. Functionally, this means that the structural matrix of the lining breaks down as a result of biological processes and/or UV light exposure. In the case of organic materials, the lining becomes a natural part of the underlying soil matrix. In the case of degradable plastics, many products lose their structural integrity and degrade to a powder that remains in the soil. The long-term environmental effects of widespread use of such products are unknown and require study.
126.96.36.199 Bare Soil
The properties of site soils are important in the design of all flexible linings because erosion of the underlying soil is one of the main performance factors in lining design. The erodibility of soil is a function of texture, plasticity and density. Bare soil alone can be a sufficient lining in climates where vegetation establishes quickly and the interim risk of soil erosion is small. Bare-soil channels may have a low risk of erosion if grades are mild, flow depths are shallow, and soils have a high permissible shear stress resistance (high plasticity cohesive soils or gravelly non-cohesive soils).
188.8.131.52 Gravel Mulch
Gravel mulch is a non-degradable erosion control product that is composed of coarse to very coarse gravel, 16 mm to 64 mm (0.6 to 2.5 inch), similar to an AASHTO No. 3 coarse aggregate. Placement of gravel is usually done immediately after seeding operations. Gravel mulch is particularly useful on windy sites or where it is desirable to augment the soil with coarse particles. Application of gravel can reduce wheel rutting on shoulders and in ditches. It can also be used to provide a transition between riprap and soil. Unlike riprap and other stone linings, gravel mulch should be placed directly on the soil surface without an underlying filter fabric. Constructing intermediate cutoff trenches that are filled with gravel enhances stability of the lining.
184.108.40.206 Vegetation (Annual Grass)
If the construction phasing permits and the climate is suitable, annual grasses can be seeded in time to establish a transitional vegetative lining. Seed mixes typically include rapidly growing annual grasses. Soil amendments including the application of fertilizer and compost improve grass establishment. To be effective, these annual grasses need to be well established though the transition period and at a sufficient density to provide erosion control. Sodding is another rapid method of vegetation establishment for ditches. The sod needs to be staked to the ditch perimeter where flow is expected to prevent wash out.
220.127.116.11 Open-weave Textile (OWT)
Open-weave textiles are a degradable rolled erosion control product that is composed of natural or polymer yarns woven into a matrix. OWT can be used together with straw mulch to retain soil moisture and to increase the density and thickness of the lining. OWT is more flexible, thinner and less dense compared to erosion control blankets (ECB). The OWT (Figure 1.9 and Figure 1.10) is loosely laid in the channel parallel to the direction of flow. OWT is secured with staples and by placement of the fabric into cutoff trenches at intervals along the channel. Placement of OWT is usually done immediately after seeding operations.
Figure 1.9. Open Weave Textile Lining
Figure 1.10. Installed Open Weave Textile Channel Lining
18.104.22.168 Erosion control blanket (ECB)
Erosion control blanket is a degradable rolled erosion control product that is composed of an even distribution of natural or polymer fibers that are mechanically, structurally or chemically bound together to form a continuous mat (Figure 1.11). ECB is stiffer, thicker and denser than an open-weave textile (OWT). These material properties improve erosion resistance. The ECB is placed in the channel parallel to the direction of the flow and secured with staples and by placement of the blanket into cutoff trenches. When ECBs are used and ultimately degrade, the long-term erosion protection is provided by the established vegetation.
Figure 1.11. Erosion Control Blanket (ECB) Lining (Source: ECTC)