Section 4: Topography and GradingAnchor: #i1009135
The landform has a major influence on driver perception of the highway and the surrounding landscape. The alignment and profile of the roadway and right-of-way are set early in the design process. These early decisions actually determine the overall character of the subsequent landform.
Alignments that are set perpendicular to the natural lines of terrain will generally be the most problematic because they will require the greatest modification in grade. Typically radical changes result in step cut slopes and deep fills. Depending on the terrain and the type of substrate, these deeper changes in landform can be visually objectionable and difficult to maintain. (See Figures 4-8 and 4-9.)
Figure 4-8. Road alignments which minimize deep cuts offer a flowing, gentle appearance.
Figure 4-9. Deep cuts can be visually dominating, presenting a scarred appearance often difficult to re-vegetate.
During the design process considerations should be given to the landform that will result from any particular alignment. Primary considerations include:
- potential aesthetic qualities
- consideration of difficulties that may be encountered in stabilizing the resulting cut and fill slopes
Overall topics covered in this section are: landform manipulation for aesthetic purposes, designing for erosion prevention, limiting erosion potential, and ground surface protection for erosion control.Anchor: #i1009216
Landform Manipulation for Aesthetic Purposes
Manipulation of the landform is a very effective tool for modifying the visual scale of roadside elements. Landform manipulation will also add variety to the setting and reduce the sense of visual clutter common to urban centers and complex interchanges.
Design solutions that employ landform modification are most cost effective when undertaken in concert with the horizontal and vertical alignment of the roadway. Designers are encouraged to collaborate with landscape architects on projects where landform modifications can be used as part of the overall landscape and aesthetics design concept. Areas where landform modification should be considered as part of the overall solution include:Anchor: #i1009270
Large, Multi-Level Interchanges
Large interchanges are sometimes visually confusing as a result of numerous bridge supports. When approaching the interchange this may appear as a forest of concrete columns that have no pattern. These large highway interchanges frequently provide sufficient space to generate significant grade changes that provide a gentle contrast to a rigid architectural scene. Skillful manipulation of landform can help to reduce the visual scale and bring a better sense of visual order to the interchange.
Figure 4-10. Landform modifications in interchanges can reduce the apparent scale of the structure and make landscapes more visible to overhead lanes.Anchor: #i1009283
Rights-of-Way that Require Noise Walls
Noise walls, because of their length and height, may be visually overpowering in terms of relative scale. To be effective the walls often must be higher than the buildings they are designed to protect. Where possible, use landform modifications or plant material to gain this elevation difference. This technique minimizes the visual dominance of the walls and blends the wall into the landscape.Anchor: #i1009293
Where space and drainage patterns allow, berms may reduce or prevent median crossings and reduce headlight glare. Careful consideration should be given to the cross section to ensure driver recovery, mowing access, and drainage. No berms should be placed within the sight triangles at intersections or at crossovers on multi-lane divided highways.Anchor: #i1009305
Roadway embankments often form large physical landmarks so their aesthetic character can have a significant impact on the surrounding community. Excessively steep and abrupt embankment slopes do not blend well visually with adjoining landscapes, are difficult to maintain, may erode easily, and may limit or prevent the use of other landscape enhancements.
Many visual problems associated with embankments are related to the degree of difficulty in maintaining vegetation on steep slopes, near or between barrier devices, in inaccessible areas close to structures, signs, or luminaires. To reduce these problems, consider the following practices:
- employ measures to prevent vegetation growth within or between barrier devices (See Figures 4-11 through 4-14 for examples)
- provide paved mow-strips along the edges of walls or other structures (See Figures 4-15 and 4-16)
- reduce slopes where possible to increase maintainability and reduce the chance of erosion and slope failures
- combine signage and structures where possible to reduce obstacles to maintenance
Figure 4-11. Grass under guard fences is a continuous maintenance problem, requiring mechanical trimming or herbicide control.
Figure 4-12. Concrete paving under guard fences is effective at vegetation control but may be difficult to replace if damaged.
Figure 4-13. Open holes for posts allow replacement in case of damage but are susceptible to weed invasion.
Figure 4-14. Patterned concrete and color are options to add visual appeal under guard fences.
Figure 4-15. Concrete mow-strips are effective and recommended for reducing maintenance and improving appearance.
Figure 4-16. Weedy vegetation against structures tends to make the entire area appear untidy and trashy.Anchor: #i1009425
Designing for Erosion Prevention
Erosion of the ground surfaces in the roadway threatens the stability of the pavement and structures, increases costs for cleaning of drainage structures, and contributes to siltation and turbidity of nearby streams and lakes. The costs associated with the control and repair of erosive surfaces can be significant and sometimes directly impact driver safety. Many erosion problems can be prevented by fitting the roadway alignment to the terrain and keeping slopes as flat as possible. Desirable cross sectional properties of a highway are as follows:
- slopes as flat as economically practical (max. 3:1, 6:1 preferred)
- rounded top and bottom slopes
- stepped cut-slopes where suitable rock is encountered
- incorporation of streamlined cross-section design techniques to help reduce wind erosion (See the AASHTO web site at http://www.transportation.org)
The areas most susceptible to erosion are slopes (15:1 and greater) associated with embankments and drainage channels. Embankments often receive concentrated runoff from pavement surfaces that, depending on soil and slope conditions, may lead to significant erosion problems.Anchor: #i1009474
Limiting Erosion Potential
Sheet flows on slopes are the primary mechanism for transporting soil down the face of the slope. Erosion can be limited by intercepting any surface flow at the top of the slope and conveying it in a suitable channel to the base of the slope.
Concentrated flows should be intercepted at the top of the slope in diversion channels and conveyed in turf reinforced channels to the bottom of the slope. Turf reinforced channels use a permanent geosynthetic fabric to reinforce the channel and prevent scour of the channel surface. Paved gutters on the face of embankment slopes are not recommended. Paved channels interfere with mowing activities and frequently break up as embankments settle.
Designers can limit erosion potential through the use of approaches such as:
- vegetation maintenance on slopes
- drainage channels
- ponds and water quality structures
- ground surface protection for erosion control
- compost or mulch
- erosion control compost or soil retention blankets
- channel liners
- bioengineering techniques for erosion control and streambank stabilization
Vegetation Maintenance on Slopes
Many visual problems associated with embankments are related to the degree of difficulty in maintaining vegetation on steep slopes, near structures or between barrier devices, or in inaccessible areas close to structures, signs, or luminaires. The following practices help prevent potential problems in these areas.
- Employ measures to prevent vegetation growth within or between barrier devices.
Solid-surface paving materials such as concrete and asphalt have been successfully used in these areas. Asphalt paving should incorporate concrete edging in turf areas to protect it from deterioration. Textures and colors may be added to these materials to add variety and interest. These materials are preferred over loose aggregates and pavers since the latter are easily invaded by weeds. (See Figures 4-11 through 4-14.)
- Provide paved mow-strips along the edges of walls or other structures.
Concrete is the preferred material for use as mow-strips along beds and walls. Asphalt is not suited since its edges are easily invaded by weeds and grass and the edges are susceptible to damage by mowing equipment unless a concrete edge is included. (See Figures 4-15 and 4-16.)
- Reduce slopes where possible to reduce erosion and failures, and increase maintainability.
- Combine signage and structures where possible to reduce obstacles to maintenance.
Figure 4-17. Careful attention to sign/structure relationship can greatly improve maintainability and appearance.Anchor: #i1009641
Drainage channels are important components of the roadway and are detailed in the Hydraulic Design Manual (look for it to be available online in the near future). In addition to hydraulic design, channels should be designed to prevent long-term soil erosion. Design characteristics that will prevent or reduce erosion are as follows:
- maintain gentle well vegetated side slopes on borrow ditches and drainage ways
- utilize an approved channel lining material
Narrow, steep-sided channels are susceptible to erosion by high water-flow velocities making permanent vegetation difficult to establish. In addition, weedy growth in narrow channels is difficult to remove mechanically, requiring increased use of herbicides.Anchor: #i1009670
Ponds and Water Quality Structures
As highways and interchanges increase in size, larger land areas are incorporated within the right-of-way. Interchanges and intersections have become increasingly important as sites of water collection, detention, and/or metering as part of broader water quality management and storm water control. These areas help meet environmental requirements and can add to the aesthetic quality of the highway and the surrounding landscape.
Water detention basins and ponds are used to provide flood control and remove silt and contaminants from highway runoff to protect downstream water bodies. Properly designed ponds rely on the bio-filtration and particulate-capture capabilities of the soil and vegetation. The design of these structures begins with clearly defined goals for the hydrological needs for water control integrated with a thorough understanding of vegetation and ecosystem properties and their management. Design parameters that relate to the development of PS&E for wetlands, detention ponds, and water quality structures are as follows:
- provide setbacks and side slopes that allows driver recovery of errant vehicles (see Figure 4-18)
- protect the structure with curbs or other barrier devices where appropriate
- gentle side slopes to allow maintenance equipment access for mowing and other activities (6:1 max.)
- use natural, freeform design of the edges of the ponds (see Figure 4-19)
- select vegetation types adapted to the hydraulic and soil regime of the region and zone of the structure
- use proper botanical names for all plants
- prepare a detailed grading plan to establish shape and grades of the structures
- prepare appropriate cross sections of weirs, dams, outlet structures, subdrainage, and spillways
- prepare specifications for filter sands, soil additives, clay or fabric liners, and temporary erosion control
Figure 4-18. Allow adequate recovery areas around ponds and basins.
Figure 4-19. Use native vegetation at basin edges to provide a natural appearance.Anchor: #i1009769
Ground Surface Protection for Erosion Control
The primary material used for erosion control on the roadside and in drainage channels is vegetation. However, prior to establishing permanent vegetative cover it is usually necessary to protect the soil surface from immediate erosion. There are a variety of natural and synthetic materials available for temporary erosion control.
There are two primary considerations in selecting an appropriate temporary erosion control measure: soil type and slope. Research shows that soils with greater than 17 percent fine sand or silt particles are highly erosive. Soils with less than 17 percent sand or clay particles can be considered cohesive soils or clays.
As the degree of slope increases, the potential for sheet erosion increases. Sandy soils can be protected with hydraulically applied mulch or straw or compost on slopes up to 7 percent (14:1). Above 7 percent an approved soil retention blanket or erosion control compost should be considered. Cohesive clays can be protected with mulches on slopes up to 20 percent (5:1). Above 20 percent, riprap should be considered.Anchor: #i1009791
Compost or Mulch
The primary function of mulch is to maintain moisture in the soil and foster seed germination and plant development. There are three types of mulch used for erosion control:
- straw or hay mulch
- cellulose fiber mulch
Straw and Hay Mulch. Straw and hay mulch are used as surface covers in conjunction with some type of tacking agent (tackifier). The tackifier helps form a uniform mat and prevents migration of the material down the hill. Straw can also be crimped to help prevent material loss or migration.
Cellulose Fiber Mulches. Cellulose fiber mulches are hydraulically applied. Cellulose mulches may be applied with the seed mix and a tacking agent to form a more consolidated mat on the surface. TxDOT maintains a list of approved cellulose mulch materials. Depending on the soil type, hydraulically applied mulches can be used on slopes up to 3:1. If the soil is very sandy cellulose mulches would not be recommended on slopes over 6:1.
Compost. Compost may be blended to make compost manufactured topsoil in flat areas. Slopes up to 3:1 should utilize Erosion Control Compost.Anchor: #i1009821
Erosion Control Compost or Soil Retention Blankets
TxDOT maintains an Approved Materials List for Soil Retention Blankets at http://www.dot.state.tx.us/services/maintenance/erosion_control.htm. This list describes the conditions for application and provides a list of materials approved for each group of uses.
Erosion Control Compost. Erosion Control Compost may be pneumatically applied on slopes 3:1 or flatter or spread with a blade up to 3:1., (refer to the current Compost Specification). This material not only provides short term erosion control with mulch chips but also works to improve the existing soil.Anchor: #i1009838
TxDOT maintains an Approved Materials List for channel liners at http://www.dot.state.tx.us/services/maintenance/erosion_control.htm. This list describes the conditions for application and provides a list of materials approved based on its resistance to tensile stress.
There are two broad types of channel lining material: organic/organic composites and synthetics. In general, organic and composite lining materials are used for vegetation establishment and offer no long-term channel protection. The synthetic materials are longer lived and work with the root matrix of the vegetation to provide long term protection of the channel. Synthetic materials are preferred in ditches and channels that will have frequent flows at depths greater than 8 in (200 mm).Anchor: #i1009855
Bioengineering Techniques for Erosion Control and Streambank Stabilization
Bioengineering is the use of live plant materials or plant materials with geosynthetics to accomplish slope and streambank stabilization needs and erosion control. TxDOT is developing technical data for the selection, design and specification of bioengineering technologies appropriate to Texas. Information from this project will be included here at a later date.Anchor: #i1009865
Plant materials are an integral part of bio-technical or bioengineering erosion control methods. Success of any technique relies on the proper selection and handling of plant materials.
Herbaceous plants. Herbaceous plants do not develop any woody tissue and will generally follow a seasonal cycle of growth, reproduction and die back. Grasses and forbs are the most common types of herbaceous vegetation used in erosion control. Native grasses used for erosion control must be selected for their ability to persist in specific environmental conditions and should have physical properties suited to erosion control.
Woody Plants. Woody plants used in bioengineering are those materials that will root easily from woody stock. The most common species are the willows (Salix) and the poplars (Populus).
Live cuttings. Research is being conducted on handling live cuttings for this application. Information from research will be included here at a later date.Anchor: #i1009890
Other Bioengineering Techniques
Wattle fence and wattles, living cribs, reed plantings, live fascines, and other bioengineering techniques are being investigated for this application. Research findings will be included here when they become available.