Section 2: Soil Erosion Control ConsiderationsAnchor: #i1000968
Understanding erosion is necessary as a basis for adequate control measures. Erosion is caused by rainfall, which displaces soil particles on inadequately protected areas, and by water running over soil, carrying some soil particles away in the process. The rate of soil particle removal is proportional to the intensity and duration of the rainfall and to the volume and characteristics of the water flow and soil properties. Deposition of water-borne sediment occurs when the velocity decreases and the transport capacity of the flowing water becomes insufficient to carry its entire sediment load.
Schematically, Figure 13-1 illustrates the typical forces involved in soil erosion.
Figure 13-1. Typical Forces in Soil Erosion
It is usually not practical for the department to reduce erosion generated upstream of the highway. If possible, locations with high erosion potential should be avoided. In areas of considerable natural erosion and accelerated erosion, the quantity of sediment that reaches a stream before highway construction begins should be documented in a descriptive or qualitative way.
Damage that can occur on highway projects is not limited to the construction site. Sedimentation or degraded water quality may occur far downstream from the point where erosion occurs. The potential for damage exists because highways pass through watersheds, disrupting the natural drainage pattern. In addition, highway construction requires the removal of existing vegetation and the introduction of cuts and fills. This exposes large areas of disturbed soil, which increases the erosion hazard.
The potential for erosion is minimized by the following measures:
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- flat side slopes, rounded and blended with natural terrain Anchor: #FIHELILH
- drainage channels designed with due regard to width, depth, slopes, alignment, and protective treatment Anchor: #MMMFFJGE
- protection at culvert outlets Anchor: #FGNGIGFH
- proper facilities for ground water interception Anchor: #MQGKKFIM
- dikes, berms, and other protective devices Anchor: #KPKEMEMI
- protective ground covers and plantings.
Erosion is a natural process that human activities often accelerate. Erosion and sedimentation are usually undesirable from an environmental standpoint. Technical competency in evaluating the severity of erosion problems and in planning and designing preventive and corrective measures is essential toward the goal of obtaining economical and environmentally satisfactory methods for erosion control.
Individuals involved in the process of controlling erosion and sedimentation include planners, designers, construction engineers, project inspectors, and contractors.
Effective and practical measures are available to minimize the erosion hazards and prevent sediment from reaching streams. Preventive measures taken during construction are more effective and economical than corrective measures. Erosion control involves the prevention of soil movement while sediment control deals with the interception of sediment-laden runoff and separation of soil particles already in motion or suspension. Erosion control at the source is the first consideration with sediment control the backup or last resort. Contact the DES-HYD for detailed information.
To deal adequately with the erosion and sediment problem, the erosion and sedimentation processes must be understood, erosion and sediment control plans must be developed, construction operations for erosion and sediment control must be scheduled, specific erosion and sediment control measures (when, where, and how) must be constructed, and water quality must be monitored and maintained.
The following general guidelines are considered BMPs:
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- Select a route where erosion will not be a serious problem. Anchor: #LJLNKKNN
- Design slopes to be flatter than with soil limitations. Anchor: #LKNGFKEJ
- Reduce the area of unprotected soil exposure. Anchor: #HPNHIKHE
- Reduce the duration of unprotected soil exposure. Anchor: #FPGLKJKK
- Protect soil with vegetative cover, mulch, or erosion resistant material. Anchor: #NOKFMJEF
- Retard runoff with planned engineering works. Anchor: #JTKHFJGK
- Trap sediment using temporary or permanent barriers, basins, or other measures. Anchor: #HSNLHIKG
- Maintain erosion control work, both during and after construction. Anchor: #KONNMKIM
- Obtain easements for legal control, where necessary.
Natural Drainage Patterns
The natural drainage pattern, including subsurface flow, must be examined for the alternate routes considered. The drainage pattern beyond the vicinity of the proposed highway location must also be studied either to minimize and avoid damage to adjacent property or streams, or to anticipate expensive preventive or corrective measures. In consideration of design work on existing roadways, the established patterns of drainage (as contrasted to natural patterns) must be examined.Anchor: #i1001134
Crossings should be made as nearly as practical at a right angle to the direction of flow. Emphasis should be given to the direction of the flood flow where it is different from that of the low water. The direction, rate, and volume of flood flow at various stages in the location of bridge openings should always be considered. A highway built on the neck of a horseshoe bend that is subject to overflow is poorly located because the correct location of relief bridges sometimes varies with the flood stage.
Whenever practical, stream crossings should be at stable reaches of a stream. Meanders in the stream that are subject to shifting should be avoided. Meandering streams have inherent problems of having no stable place to cross because the sinusoidal pattern of the stream naturally tends to progress in a downstream direction.
The number of stream crossings and the disturbance of streambeds should be minimized. Crossing and then re-crossing the same stream should be avoided. Undue scour and erosion that might result in a complete change in the river channel should be avoided.
See Chapter 9, Planning and Location Considerations, for more details on planning and location.Anchor: #i1001154
Encroachments on Streams
If a proposed highway alignment will encroach upon a stream, consider moving the highway away from the stream to avoid erosion and sedimentation problems. For an existing roadway that already encroaches on or near a stream, improvements or rehabilitation work should be planned to minimize further encroachment. If the stream impinges and encroaches on the highway, the highway itself may need to be protected.Anchor: #i1001169
Public and Industrial Water Supplies and Watershed Areas
If possible, crossing of a catchment area of a water supply should be avoided. Such crossings could entail building costly temporary facilities for the water supply. Some industries require higher quality water than is required for drinking water, so problems with industrial water supplies may be as great as those with a public water supply. When crossing a water supply catchment area cannot be avoided, any corrective measures and their costs should be determined before making the choice of the route.Anchor: #i1001179
Geology and Soils
Knowledge of the area’s geology allows the highway designer to detect potential problem areas and anticipate subsidence, landslides, and erosion problems. Terrain features are the result of past geologic and climatic processes. Erosion and deposition by running water are major geologic processes in shaping the terrain. A study of the terrain and the character of natural and accelerated erosion can aid in judging the complexity of the erosion and in estimating what erosion control measures may be required.
Some soil types are known to be more erosive than others, and their identification is a valuable aid in route selection and erosion control. The U.S. Department of Agriculture classification of soils is helpful. Soil survey maps, prepared by the Natural Resources Conservation Service (NRCS), show this classification as well as the engineering classification of soils. Local NRCS offices can give much assistance in both soil identification and erosion control measures applicable to the local area.
Problems in route selection for a new roadway can sometimes be avoided. For an existing roadway, however, problems must be recognized and precautions taken in the design.Anchor: #i1001208
Coordination with Other Agencies
Plans or projects of other agencies, such as the USACE, NRCS, and TCEQ, might affect or be affected by the location of a proposed highway, or by improvements or changes to an existing roadway. These agencies should be contacted to learn of their plans for controlling bank erosion, protective works, and stream grade control structures or channel modifications.Anchor: #i1001218
Independent roadway grade lines that fit the terrain with a minimum of cuts and fills reduce exposed areas subject to erosion. Alignment and grade, consistent with highway safety criteria, must be blended or fit to the natural landscape to minimize cut and fill sections and reduce erosion and costly maintenance. Slopes of the roadway cross section should consider soil stability, climatic exposure, geology, proposed landscape treatment, and maintenance procedures.
Depressed roadways and underpasses require careful consideration of drainage to avoid deposition of sediment and debris on the highway and in drainage facilities. Both ground and surface water can do the following:
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- pass through the highway right-of-way Anchor: #GLQHHKGH
- be intercepted with minimum disturbance to streams Anchor: #LILHINGJ
- be intercepted without causing serious erosion problems.
The cross section can be varied, if necessary, to minimize erosion and to facilitate safety and drainage. Generally, good landscaping and drainage design are compatible with both erosion control and safety to vehicles. Right-of-way constraints often prohibit extreme flattening of embankment slopes, but they should be an important consideration to the designer in their effect on erosion.Anchor: #i1001253
Severe Erosion Prevention in Earth Slopes
A concentration of storm water flowing from the area at the top of cut or fill slopes causes severe erosion of earth slopes. The concentration of storm water at the top of cuts should be avoided. These guidelines should be followed in areas of severe erosion prevention in earth slopes:
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- Dike or berm construction – During project construction and immediately thereafter, construct a dike or berm at the top of the cut to prevent water from running down the slope. The dike or berm should be borrow material to avoid disturbing the natural ground, in conjunction with a grassed channel or paved ditch. Anchor: #NGGKHKLG
- Outlet protection – Water can be spread over the natural slope or carried to lower elevations in chutes or closed pipes. Protect outlets for such high velocity chutes from scour. Streams in cut sections require special attention. Anchor: #HMLKINLJ
- Serrated slopes – In some areas of Texas, serrated cut slopes help establish vegetative cover on decomposed rock or shale slopes. Serrate any material that is rippable or that will hold a vertical face for a few weeks until vegetation becomes established. Anchor: #NKKNNFKM
- Shoulder drains -- Where vegetation cannot be established or where flow down the fill slope is objectionable, collect the runoff at the shoulder edge and direct it to an adequate inlet and chute.
Channel and Chute Design
Surface channels, natural or man-made, are usually the most economical means of collecting and disposing of runoff in highway construction if concentration of flows cannot be avoided. A well-designed channel carries storm water without erosion or hazard to traffic and with the lowest overall cost, including maintenance. To minimize erosion and avoid a safety hazard, channels should have mild side slopes and wide rounded bottoms. Such channels can be protected from erosion by lining them with materials such as grass, rock, or concrete.
Chutes generally are applied to steep slopes and carry water at high velocities. Pipe chutes are preferable to open chutes because the water cannot jump out of the chute and erode the slope. Dissipation of energy along the chute or at the outlet is usually necessary. In highly erosive soil, watertight joints may need to be provided to prevent failure of the facility.
Variations in channel alignment should be gradual, particularly if the channel carries flow at high velocity. Whenever practical, changes in alignment should be located on the flatter gradients to prevent erosion caused by the overtopping of the channel walls and the associated erosion. Although rectangular channel sections are usually more expensive, they are preferred on bends of paved channels to give a more positive control of the flow.
If the bank and bed material will erode at the prevailing velocities, channel lining should be considered. Protective linings for channels and streams can be very expensive. A special effort should be made to develop the most cost-effective erosion protection, including maintenance, for the particular location.
Several applications are effective for both channel and bank protection, including spur dikes, permeable spur jetties, gabions and revetment mattresses, and sheet piling. For many of these protective appurtenances, no rigorous design is available, and experience or intuition is the best guides for their consideration and application. See Chapter 7, Channel Linings, for more information.
Culverts and bridges generally constrict the floodway and increase velocities, thus developing higher erosion potential. In many instances, erosion and scour at these locations damage the highway embankment, the structure itself, or the downstream channel. The energy of high velocity flow should be dissipated at the outlet of culverts and chutes where necessary, or the area protected by riprap or other types of protection. Some velocity control devices and methodologies are illustrated in Chapter 8, Velocity Protection and Control Devices.