Bs Standard Drainage Structures Manual
CHAPTER 4 DRAINAGE DESIGN 4.1 General Considerations Roads will affect the natural surface and subsurface drainage pattern of a watershed or individual hillslope. Road drainage design has as its basic objective the reduction and/or elimination of energy generated by flowing water. The destructive power of flowing water, as stated in Section 3.2.2, increases exponentially as its velocity increases. Therefore, water must not be allowed to develop sufficient volume or velocity so as to cause excessive wear along ditches, below culverts, or along exposed running surfaces, cuts, or fills.
Provision for adequate drainage is of paramount importance in road design and cannot be overemphasized. The presence of excess water or moisture within the roadway will adversely affect the engineering properties of the materials with which it was constructed. Cut or fill failures, road surface erosion, and weakened subgrades followed by a mass failure are all products of inadequate or poorly designed drainage. As has been stated previously, many drainage problems can be avoided in the location and design of the road: Drainage design is most appropriately included in alignment and gradient planning. Hillslope geomorphology and hydrologic factors are important considerations in the location, design, and construction of a road. Slope morphology impacts road drainage and ultimately road stability. Important factors are slope shape (uniform, convex, concave), slope gradient, slope length, stream drainage characteristics (e.g., braided, dendritic), depth to bedrock, bedrock characteristics (e.g., fractured, hardness, bedding), and soil texture and permeability.
Slope shape (Figure 59) gives an indication of surface and subsurface water concentration or dispersion. Convex slopes (e.g., wide ridges) will tend to disperse water as it moves downhill. Straight slopes concentrate water on the lower slopes and contribute to the buildup of hydrostatic pressure. Concave slopes typically exhibit swales and draws. Water in these areas is concentrated at the lowest point on the slope and therefore represent the least desirable location for a road. Hydrologic factors to consider in locating roads are number of stream crossings, side slope, and moisture regime. For example, at the lowest point on the slope, only one or two stream crossings may be required.
Likewise, side slopes generally are not as steep, thereby reducing the amount of excavation. However, side cast fills and drainage requirements will need careful attention since water collected from upper positions on the slope will concentrate in the lower positions. In general, roads built on the upper one-third of a slope have better soil moisture conditions and, therefore, tend to be more stable than roads built on lower positions on the slope.
Natural drainage characteristics of a hillslope, as a rule, should not be changed. For example, a drainage network will expand during a storm to include the smallest depression and draw in order to collect and transport runoff. Therefore, a culvert should be placed in each draw so as not to impede the natural disposition of stormflow. Culverts should be placed at grade and in line with the centerline of the channel.
Failure to do this often results in excessive erosion of soils above and below the culvert. Also, debris cannot pass freely through the culvert causing plugging and oftentimes complete destruction of the road prism. Headwater streams are of particular concern (point A, Figure 60) since it is common to perceive that measurable flows cannot be generated from the moisture collection area above the crossings. However, little or no drainage on road crossings in these areas is notorious for causing major slide and debris torrents, especially if they are located on convex slope breaks. Increased risks of road failures are created at points A and B. At point A, water will pond above the road fill or flow downslope through the roadside ditch to point B. Ponding at A may cause weakening and/or erosion of the subgrade.
If the culvert on Stream 1 plugs, water and debris will flow to point A and from A to B. Hence, the culvert at B is handling discharge from all three streams. If designed to minimum specifications, it is unlikely that either the ditch or the culvert at B will be able to efficiently discharge flow and debris from all three streams resulting in overflow and possible failure of the road at point B. Slope shape and its impact on slope hydrology. Slope shape determines whether water is dispersed or concentrated. (US Forest Service, 1979).
A road drainage system must satisfy two main criteria if it is to be effective throughout its design life:. It must allow for a minimum of disturbance of the natural drainage pattern. It must drain surface and subsurface water away from the roadway and dissipate it in a way that prevents excessive collection of water in unstable areas and subsequent downstream erosion. The design of drainage structures is based on the sciences of hydrology and hydraulics-the former deals with the occurrence and form of water in the natural environment (precipitation, streamflow, soil moisture, etc.) while the latter deals with the engineering properties of fluids in motion. Culvert and road locations have modified drainage patterns of ephemeral streams 2 and 3. Locations A and B become potential failure sites. Stream 3 is forced to accept more water below B due to inadequate drainage at A.
4.2 Estimating runoff Any drainage installation is sized according to the probability of occurrence of an expected peak discharge during the design life of the installation. This, of course, is related to the intensity and duration of rainfall events occurring not only in the direct vicinity of the structure, but also upstream of the structure.
In snow zones, peak discharge may be the result of an intense warming period causing rapid melting of the snowpack. In addition to considering intensity and duration of a peak rainfall event, the frequency, or how often the design maximum may be expected to occur, is also a consideration and is most often based on the life of the road, traffic, and consequences of failure. Primary highways often incorporate frequency periods of 50 to 100 years, secondary roads 25 years, and low volume forest roads 10 to 25 years. Of the water that reaches the ground in the form of rain, some will percolate into the soil to be stored until it is taken up by plants or transported through pores as subsurface flow, some will evaporate back into the atmosphere, and the rest will contribute to overland flow or runoff. Streamflow consists of stored soil moisture which is supplied to the stream at a more or less constant rate throughout the year in the form of subsurface or groundwater flow plus water which is contributed to the channel more rapidly as the drainage net expands into ephemeral channels to incorporate excess rainfall during a major storm event.
The proportion of rainfall that eventually becomes streamflow is dependent on the following factors:. The size of the drainage area. The larger the area, the greater the volume of runoff. An estimate of basin area is needed in order to use runoff formulas and charts. Topography. Runoff volume generally increases with steepness of slope.
Average slope, basin elevation, and aspect, although not often called for in most runoff formulas and charts, may provide helpful clues in refining a design. Runoff varies with soil characteristics, particularly permeability and infiltration capacity. The infiltration rate of a dry soil, by nature of its intrinsic permeability, will steadily decrease with time as it becomes wetted, given a constant rainfall rate. If the rainfall rate is greater than the final infiltration rate of the soil (infiltration capacity), that quantity of water which cannot be absorbed is stored in depressions in the ground or runs off the surface. Any condition which adversely affects the infiltration characteristics of the soil will increase the amount of runoff.
Such conditions may include hydrophobicity, compaction, and frozen earth. A number of different methods are available to predict peak flows.
Flood frequency analysis is the most accurate method employed when sufficient hydrologic data is available. For instance, the United States Geological Survey has published empirical equations providing estimates of peak discharges from streams in many parts of the United States based on regional data collected from 'gaged' streams. In northwest Oregon, frequency analysis has revealed that discharge for the flow event having a 25-year recurrence interval is Most closely correlated with drainage area and precipitation intensity for the 2-year, 24-hour storm event. This is, by far, the best means of estimating peak flows on an ungaged stream since the recurrence interval associated with any given flow event can be identified and used for evaluating the probability of failure. The probability of occurrence of peak flows exceeding the design capacity of a proposed stream crossing installation should be determined and used in the design procedure.
To incorporate this information into the design, the risk of failure over the design life must be specified. By identifying an acceptable level of risk, the land manager is formally stating the desired level of success (or failure) to be achieved with road drainage structures.
Table 25 lists flood recurrence intervals for installations in relation to their design life and probability of failure. Flood recurrence interval (years) in relation to design life and probability of failure. (Megahan, 1977). Design Life (years) Chance of Failure (%) 10 20 30 40 50 60 70 recurrence interval (years) 5 48 23 15 10 8 6 5 10 95 45 29 20 15 11 9 15 100+ 68 43 30 22 17 13 20 100+ 90 57 40 229 22 17 25 200+ 100+ 71 49 37 28 21 30 200+ 100+ 85 59 44 33 25 40 300+ 100+ 100+ 79 58 44 34 50 400+ 200+ 100+ 98 73 55 42. Based on formula P = 1 - (1 -1/T)n, where n = design life (years), T = peak flow recurrence interval (years), P = chance of failure (%). EXAMPLE: If a road culvert is to last 25 years with a 40% chance of failure during the design life, it should be designed for a 49-year peak flow event (i.e., 49-year recurrence interval). When streamflow records are not available, peak discharge can be estimated by the 'rational' method or formula and is recommended for use on channels draining less than 80 hectares (200 acres): Q = 0.278 C i A where: Q = peak discharge, (m3/s) i = rainfall intensity (mm/hr) for a critical time period A = drainage area (km²).
(In English units the formula is expressed as: Q = C i A where: Q = peak discharge (ft3/s) i = rainfall intensity (in/hr) for a critical time period, tc A = drainage area (acres). The runoff coefficient, C, expresses the ratio of rate of runoff to rate of rainfall and is shown below in Table 26. The variable tc is the time of concentration of the watershed (hours). Values of relative imperviousness for use in rational formula. (American Iron and Steel Institute, 1971). Type of Surface Factor C Sandy soil, flat, 2% 0.05-0.10 Sandy soil, average, 2-7% 0.10-0.15 Sandy soil, steep, 7 0.15-0.20 Heavy soil, flat, 2% 0.13-0.22 Heavy soil, average, 2-7% 0.18-0.22 Heavy soil, steep, 7% 0.25-0.35 Asphaltic pavements 0.80-0.95 Concrete pavements 0.70-0.95 Gravel or macadam pavements 0.35-0.70. Natural earth ft / sec meters / sec a.
Good Poor 1) Trowel finish 0.012 - 0.014 20 6.0 2) Float finish 0.013 - 0.015 20 6.0 3) Formed, no finish 0.014 - 0.016 20 6.0 b. Concrete bottom, float finished, w/sides of: 1) Dressed stone in mortar 0.015 - 0.017 18 - 20 5.4 - 6.0 2) Ramdom stone in mortar 0.017 - 0.020 17 - 19 5.1 - 5.7 3) Dressed stone or smooth concrete rubble (riprap) 0.020 - 0.025 15 4.5 4) Rubble or random stone (riprap) 0.025 - 0.030 15 4.5 c.
Gravel bottom, sides of: 1) Formed concrete 0.017 - 0.020 10 3.0 2) Random stone in mortar 0.020 - 0.023 8 - 10 2.4 - 3.0 3) Random stone or rubble (riprap) 0.023 - 0.033 8 - 10 2.4 - 3.0 d. Brick 0.014 - 0.017 10 3.0 3) Asphalt 0.013 - 0.016 18 - 20 5.4 - 6.0 Maximum recommended velocities Figure 86.
Ditch interception near stream to divert ditch water onto stable areas instead of into the stream. Environmental Protection Agency,1975). The procedure for calculating flow rates is the same as that discussed in Section 4.2.
The corresponding roughness factors (Manning's n) for open channels are given in Table 33. Ditches in highly erodible soils may require riprap, rock rubble lining, jute matting, or grass seeding. Riprap or rubble-lined ditches will tend to retard flow enough to allow water movement while retaining the sediment load at low flow periods. Lining ditches can reduce erosion by as much as 50 percent and may provide economical benefits by reducing the required number of lateral cross drains when materials can be obtained at low cost.
Ditch water should not be allowed to concentrate, nor should it be allowed to discharge directly into live streams. A cross drain such as a culvert should carry the ditch water across and onto a protected surface (Figure 81). Lindamood manual. Spacing of ditch relief culverts is discussed in Section 4.4.4 and 4.5. The ditch grade will normally follow the roadway grade.
However, the minimum grade for an unpaved ditch should be 1 percent. Runoff intensity or discharge values needed to calculate ditch size can be determined by calculations described below for culvert design. However, allowances should be made for sedimentation, plus at least 0.3 m between the bottom of the roadway subgrade and the full flow water surface.
The suggested minimum size of roadside ditches is shown in Figure 87. Minimum ditch dimensions. Velocity of the ditch water is a function of cross section, roughness and grade. For a typical triangular cross section the velocity can be calculated from Manning's equation: V = n-1. R2/3. S1/2 where V equals velocity in meters/second and the other values are as defined in Chapter 4.2.
For a triangular channel with sideslopes of 1:1 and 2:1, flowing 0.3 meters deep, the hydraulic radius, R, equals 0.12 m. Table 34 lists ditch velocities as a function of roughness coefficients and grade, and Figure 88 provides a nomograph for the solution of Manning's equation. In most cases ditch lines should be protected to withstand the erosion. For channels with grades steeper that 10 percent, a combination of cross section widening, surface protection and increased surface roughness may be required.
Ditch velocities for various n and grades. Triangular ditch with side slope ratio of 1:1 and 2:1, flowing 0.30 meters deep; hydraulic radius R = 0.12. Slope (%) n 0.02 0.03 0.04 meters/sec 2 1.7 1.2 0.9 4 2.5 1.6 1.2 6 3.0 2.0 1.5 8 3.5 2.3 1.7 10 3.9 2.6 1.9 12 4.3 2.9 2.1 15 4.8 3.2 2.4 18 5.3 3.5 2.6 Figure 88. Nomograph for solution of Mannin's equation (U.S. Of Commerce, 1965). EXAMPLE: Determine whether the water velocity for a road ditch will be below critical levels for erosion.
If velocities are too high, make and evaluate changes (see also U.S. Forest Service, 1980). Ditch dimension is a symmetrical, triangular channel, 0.39 m deep with 2.5:1 slopes with sandy banks (SW) and a slope of 0.003 m/m. The hydraulic radius, R, is equal to area divided by wetted perimeter. R = 0.38 m² / 2.1 m = 0.18 m Converting to english units, divide meters by 0.3 m/ft. R = 0.60 ft 2.
Obtain roughness coefficient from Table 32 (n = 0.020). Obtain maximum allowable velocity 0.46 to 0.76 m/sec (Table 31).
Convert to english units by dividing by 0.3 m/ft. Vmax = 1.5 to 2.5 ft/sec 4. From Figure 88, find the velocity for the specified ditch (2.9 ft/sec). Convert to metric by multiplying by 0.3 m/ft.
Vditch = 0.87 m/sec 5. Compare the calculated ditch velocity with the maximum recommended velocity for sandy channels: Specified ditch maximum velocity 0.87 m/sec 0.46 - 0.76 m/sec The ditch has too great a velocity given the conditions stated above. Therefore, measures must be taken that will reduce the water velocity.
Water velocity in ditches can be reduced by protecting the channel with vegetation, rock, or by changing the channel shape. (With vegetative protection, the friction factor (n) becomes 0.030 - 0.050 and the maximum recommended velocity becomes 0.9 - 1.2 m/sec.) 6. Obtain velocity for specified ditch with vegetative protection by referring to Figure 88 (1.9 feet per second). Compare the calculated ditch velocity with the maximum recommended velocity for vegetation protected channels (average turf) with easily eroded soils: Specified ditch maximum velocity 0.57 m/sec 0.9 - 1.2 m/sec 8. If the specified ditch has a lower velocity than the recommended maximum velocities, it should be stable as long as the vegetation remains intact. Berms can be constructed of native material containing sufficient fines to make the berm impervious and to allow it to be shaped and compacted to about 90 percent maximum density. Berm dimensions are illustrated in Figure 89.
Minimum berm dimensions. 4.4.4 Ditch Relief Culverts Water collected in the cutslope ditch line has to be drained across the road prism for discharge at regular intervals.
Cross drains should be installed at a frequency that does not allow the ditch flow to approach maximum design water velocities. Intercepting dips or open top culverts (Chapter 4.4.2) perform adequately up to a certain point. However, these techniques are not adequate or appropriate when the following conditions are present either in combination or alone: - high traffic volumes or loads and characteristic rutting. steep side slopes. large volumes of ditch water from rainfall,snowfall, springs, or seepage. Ditch relief culverts do not impact or impede traffic as dips and open-top culverts do. Intercepting dips may become a safety hazard on steep slopes as well as being difficult to construct.
Bs Standard Drainage Structures Manual Transmission
It is also undesirable to have large amounts of water running across the road surface because of sediment generation and seepage into the subgrade. The frequency, location and installation method of ditch relief culverts is much more important than determining their capacity or size. Ditch relief culverts should be designed so that the half-full velocities are 0.7 to 1.0 m/sec in order to transport sediment through the culvert and should be at least 45 cm (18 inches) in diameter depending on debris problems. Larger culverts are more easily cleaned out than narrow ones. Every subsequent relief culvert should be one size larger than the one immediately upstream from it. This way, an added safety factor is built in should one culvert become blocked.
As with dips, open top culverts, and water bars, ditch relief and lateral drain culverts should cross the roadway at an angle greater than or equal to 30° downgrade. This helps insure that water is diverted from the roadside ditch and that sediment will not accumulate at the inlet. Accelerated ditch erosion may (1) erode the road prism making it unstable and unusable, and (2) cause culverts to plug or fail, thereby degrading water quality. Selection of proper location is as important as spacing. Spacing recommendations should be used as a guide in determining the frequency of cross drain spacing. Final location is dictated by topographic and hydrologic considerations. Considerations discussed for for cross drain locations are also valid for culverts (see Figure 85).
Considerations given for stream culvert installation, inlet and outlet protection, should also be used for ditch relief culverts. Culvert outlets with no outlet protection are very often the cause of later road failures.
Bs Standard Drainage Structures Manualidades
Normally, culvert outlets should extend approximately 30 - 50 cm beyond the toe of the fill. Minimal protection is required below the outlet for shallow fills. However, on larger fill slopes where the outlet may be a considerable distance above the toe of the fill, a downspout anchored to the fill slope should be used (Figure 90). Culvert outlets should be placed such that at least 50 meters is maintained between it and any live stream. If this is not possible, the rock lining of the outlet should be extended to 6 meters to increase its sediment trapping capacity (Figure 91). Coarse slash should be placed near the outlet to act as a sediment barrier.
Where fills consist entirely of heavy rock fragments, it is safe to allow culverts to discharge on to the slop. The size and weight of fragments must be sufficient to withstand the expected velocity of the design discharge.
Rock aprons (Figure 92) are the least costly and easiest to install. A guide for selecting rock for use as riprap is illustrated in Figure 93. Ditch relief culvert installation showing the use of headwall, downspout and a splash barrier/energy dissipator at the outlet. Minimum culvert grade is 3 to 5 percent. Exit velocities should be checked. Environmental Protection Agency, 1975).
Ditch relief culvert in close proximity to live stream showing rock dike to diffuse ditch water and sediment before it reaches the stream. Environmental Protection Agency, 1975). Energy dissipators. (Darrach, et al., 1981).
Size of stone that will resist displacement by water for various velocities and ditch side slopes. 1 ft.= 30 cm (U.S. Of Commerce, 1965).
The determination of culvert spacing for lateral drainage across the roadway is based on soil type, road grade, and rainfall characteristics. These variables have been incorporated into a maximum spacing guide for lateral drainage culverts developed by the Forest Soils Committee of the Douglas-fir Region in 1957. The spacing estimates are designed for sections of road 20 feet wide and include average cut bank and ditch one foot deep. Table 2 (Chapter 1.4.1) groups soils by standard soil textural classes into ten erosion classes having erodibility indices from 10 to 100, respectively. (Class I contains the most erodible soils and Class X the least erodible soils.) In order to arrive at an erosion class for a particular soil mixture, multiply the estimated content of the various components by their respective erosion index and add the results. Example: Name of component% Content Erosion index Total Erosion Index rock 20 100 20 Fine Gravel 50 90 45 Silt Loam 30 70 21 86 86 = Erosion Class VIII The spacing of lateral-drainage culverts can then be obtained from Table 34. The summary equation used to calculate values in Table 34, expressed in metric units, is: Y = (1,376 e0.0156X )(G R)-1 where: Y = lateral drain spacing (meters) e = base of natural logarithms (2.7183) X = erosion index G = road grade (%) R = 25-year, 15-minute rainfall intensity (centimeters/hour) Values in Table 34 are based on rainfall intensities of 2.5 to 5 cm per hour (1 to 2 in/hr) falling in a fifteen minute period with an expected recurrence interval of 25 years.
For areas having greater rainfall intensities for the 25 year storm, divide the values in the table by the following factors: Rainfall intensity Factor less than 2.5 cm/hr (1 in/hr) Whatever the intensity (0.75, 0.85, etc.) 5 to 7.5 cm/hr (2 to 3 in/hr) 1.50 7.5 to 10 cm/hr (3 to 4 in/hr) 1.75 10 to 12.5 cm/hr (4 to 5 in/hr) 2.00 Roads having grades less than 2 percent have a need for water removal to prevent water from soaking the subgrade or from overrunning the road surface. Thus, spacing for roads with 0.5 percent grades is closer than for roads with 2 percent grades. Usually, local experience will determine the spacing needed for road grades at these levels. Guide for maximum spacing (in feet) of lateral drainage culverts by soil erosion classes and road grade (2% to 18%).
(Forest Soils Comm., Douglas Fir Reg., PNW, 1957).