Drainage-word.docx

DRAINAGE Drainage is the term applied to systems dealing with excess water. The three primary drainage tasks are (a) urban storm drainage, (b) land drainage, and (c) highway drainage. Often considered as minor problems for the hydraulic engineer and frequently designed as if the works were unimportant, these tasks actually involve substantially greater capital investment and probably consume more engineering time each year than all of the flood-mitigation activity undertaken. The primary distinction between drainage and flood mitigation is in the techniques employed to cope with excess water and in the fact that (with the exception of highway culverts and bridges) drainage deals with water before it has reached major stream channels. DEFINITION OF TERMS Crown – sometimes known as soffit, is the top inside of a pipe. (See Figure 1) Design storm – a storm whose magnitude, rate, and intensity do not exceed the design load for a storm drainage system or flood protection project. Intensity-duration-frequency curve – a graphical representation of the probability that a given average rainfall intensity will occur Return period – also known as “recurrence interval” is an estimate of the likelihood of an event such as flood to occur. It is a statistical measurement typically based on historic data denoting the average recurrence interval over an extended period of time. Runoff – that part of rain or other precipitation that runs off the surface of a drainage area and does not enter the soil or the sewer system as inflow. Stormwater – the excess water running off from the surface of a drainage area during and immediately after a period of rain. Subbasin – is a structural geologic feature where a basin forms within a larger basin. (See Figure 8.1) Topographic map – is a general-use map at medium scale that presents elevation (contour lines), hydrography, geographic place names, and a variety of cultural features. (See Figure 3 and samples can be seen in http://www.namria.gov.ph/topo50Index.aspx) Watershed – the region or land area that contributes to the drainage or catchment area above a specific point on the stream or river. (See Figure 8.2)

Figure 8.1

Figure 8.2

URBAN STORM DRAINAGE In cities, stormwater is usually collected in the streets and conveyed through inlets to buried conduits which carry it to a always feasible and pumping plants may be an important part of a city storm-drainage system. (See Figure 8.5 for sample storm drain) System components  Sewers Sewer pipes are available in a variety of materials. They can be made of cast and ductile iron, PVC (polyvinyl chloride), concrete, asbestos cement, HDPE (high density polyethylene), brick, and vitrified clay. Most new sewer pipe has a circular cross section; however, many older sewers, especially those made from brick, have different cross-sectional shapes.

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Vertical alignment Figure 8.3 illustrates how the vertical position of a sewer is defined by its invert level (IL). The invert of a pipe refers to the lowest point on the inside of the pipe. The invert level is the vertical distance of the invert above some fixed level or datum. Soffit level is the highest point on the inside of the pipe and the crown level is the highest point on the outside of the pipe. d a = invert level (IL) b = Soffit level c = Crown level d = Ground level D = internal diameter of pipe t = pipe wall thickness

c

b D

a t Figure 8.3

 Manholes Manholes are structures designed to provide access to a sewer. Access is required for testing, visual inspection of sewers, and placement and maintenance of flow or water quality monitoring instruments. Manholes are usually provided at heads of runs, at locations where there is changes in direction, changes in gradient; changes in size, at major junctions with other sewers and at every 90 to 200 meter intervals depending on the size of the sewer pipes. The diameter of the manhole will depend on the size of sewer and the orientation and number of inlets. Sample manhole design in Figure 8.4.

Figure 8.4

 Gully Inlets Gully inlets are inlets where surface water from roads and paved areas are entering the sewer system. Gullies consist of a grating and usually an underlying sump to collect heavy material in the flow. A water seal is incorporated to act as an odor trap for those gullies connected to combined sewers. Gullies are connected to the sewer by lateral pipes.

Combined inlet

Curbed inlet

Grated inlet

Gutter inlets

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 Ventilation It is important to have adequate air ventilation in all urban drainage systems, but particularly in foul and combined sewers. It is needed to ensure that aerobic conditions are maintained within the pipe, and to avoid the possibility of build-up of toxic or explosive gases.

Figure 8.5

Information Needs and Design Criteria A condensed checklist of information needs for storm sewer design is presented below. There are many sources for this information, ranging from various local government agencies to federal agencies to pipe and pump manufacturers. The designer must also obtain future development information for areas surrounding the site of interest. Condensed Checklist of Information Needs for Storm Sewer Design (Source: Urbonas and Roesner, 1993)  Local storm drainage criteria and design standards  Maps, preferably topographic, of the subbasin in which the new system is to be located  Detailed topographic map of the design area  Location, sizes, and types of existing storm sewers and channels located upstream and downstream of design area  Locations, depths, and types of all existing and proposed utilities  Layout of design area including existing and planned street patterns and profiles, types of street cross-sections, street intersection elevations, grades of any irrigation and drainage ditches, and elevations of all other items that may post physical constraints to the new system  Soil borings, soil mechanical properties, and soil chemistry to help select appropriate pipe materials and strength classes  Seasonal water table levels  Intensity-duration-frequency and design storm data for the locally required design return periods  Pipe vendor information for the types of storm sewer pipe materials accepted by local jurisdiction Design criteria may vary from one city to another, but for the most part the following are a fairly standard set of assumptions and constraints used in the design of storm sewers (American Society of Civil Engineers, 1969, 1992). (a) For small systems, free-surface flow exists for the design discharges; that is, the sewer system is designed for “gravity-flow” so that pumping stations and pressurized sewers are not considered. (b) The sewers are commercially available circular pipes. (c) The design diameter is the smallest commercially available pipe that has flow capacity equal to or greater than the design discharge and satisfies all the appropriate constraints. (d) Storm sewers must be placed at a depth that will not be susceptible to frost, will drain basements, and will allow sufficient cushioning to prevent breakage due to ground surface loading. Therefore, minimum cover depths must be specified. (e) The sewers are joined at junctions such that the crown elevation of the upstream sewer is no lower than that of the downstream sewer. (f) To prevent or reduce excessive deposition of solid material in the sewers, a minimum permissible flow velocity at design discharge or at barely full-pipe gravity flow is specified. (g) To prevent the occurrence of scour and other undesirable effects of high-velocity flow, a maximum permissible flow velocity is also specified. Maximum velocities in sewers are important mainly because of the possibilities of excessive erosion on the sewer inverts. (h) At any junction or manhole, the downstream sewer cannot be smaller than any of the upstream sewers at that junction. (i) The sewer system is a dendritic network converging towards downstream without closed loops. Table 8.1 lists the more important typical technical items and limitations to consider. | Water Resources Engineering

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Table 8.1 Technical Items and Limitations to Consider in Storm Sewer Design (Source: Urbonas and Roesner, 1993) Velocity Minimum design velocity 2 – 3 ft/s (0.6 – 0.9 m/s) Maximum design velocity Rigid pipe 15 -21 ft/s (4.6 – 6.4 m/s) Flexible pipe 10 -15 ft/s (3.0 – 4.6 m/s) Maximum manhole spacing (function of pipe size) 400 – 600 ft (122 – 183 m) Maximum size of pipe 12 – 24 in. (0.3 – 0.6 m) Vertical alignment at manholes: Different size pipe Match crown of pipe or 80 – 85% depth lines Same size pipe Minimum of 0.1 – 0.2 ft (0.03 – 0.06 m) in invert drop Minimum depth of soil cover 12 – 24 in. (0.3 – 0.6 m) Final hydraulic design Check design for surcharge and junction losses by using backwater analysis Location of inlets In street where the allowable gutter flow capacity is exceeded

Rational Method Design From an engineering viewpoint the design can be divided into two main aspects: runoff prediction and pipe sizing. The rational method, which can be traced back to the mid-nineteenth century, is still probably the most popular method used for the design of storm sewers. (Yen and Akan, 1999). Although criticisms have been raised of its adequacy, and several other more advanced methods have been proposed, the rational method, because of its simplicity, is still in continued use for sewer design when high accuracy of runoff rate is not essential. Using the rational method, the storm runoff peak is estimated by the rational formula 𝑸 = 𝑪𝒊𝑨 Where: Q = the peak runoff rate (ft3/s, m3/s) C = the runoff coefficient i = the average rainfall intensity (in/hr, mm/hr) A = the area of the tributary drainage area (acres, km 2)  Runoff Coefficient The runoff coefficient is a dimensionless value representing characteristics of the watershed that affect how much of the rain will become runoff. Coefficient selection is based on land use and soil conditions. The weighted C value to be based on a ratio of the drainage areas associated with each C value. The runoff coefficients for various types of surfaces (used by the City of Austin, Texas) are provided in Table 8.2. Runoff coefficients in the Philippines are shown in Table 8.3. 𝑾𝒆𝒊𝒈𝒉𝒕𝒆𝒅 𝑪 =

𝑨𝟏 𝑪𝟏 + 𝑨𝟐 𝑪𝟐 + … . . + 𝑨𝒏 𝑪𝒏 𝑨 𝟏 + 𝑨𝟐 + … . . + 𝑨𝒏

Table 8.2 Runoff Coefficients for Use in the Rational Method (Source: Chow, Maidment, and Mays, 1988) Character of Surface Developed Asphaltic Concrete/Roof Grass areas (lawns, parks, etc) Poor condition (grass cover < 50% of the area) Flat, 0 – 2% Average, 2 – 7% Steep, over 7% Fair condition (grass cover 50 – 75% of the area) Flat, 0 – 2% Average, 2 – 7% Steep, over 7% Good condition(grass cover larger than 75% of the area) Flat, 0 – 2% Average, 2 – 7% Steep, over 7%

Return Period (years) 10 25 50 100

2

5

500

0.73 0.75

0.77 0.80

0.81 0.83

0.86 0.88

0.90 0.92

0.95 0.97

1.00 1.00

0.32 0.37 0.40

0.34 0.40 0.43

0.37 0.43 0.45

0.40 0.46 0.49

0.44 0.49 0.52

0.47 0.53 0.55

0.58 0.61 0.62

0.25 0.33 0.37

0.28 0.36 0.40

0.30 0.38 0.42

0.34 0.42 0.46

0.37 0.45 0.49

0.41 0.49 0.53

0.53 0.58 0.60

0.21 0.29 0.34

0.23 0.32 0.37

0.25 0.35 0.40

0.29 0.39 0.44

0.32 0.42 0.47

0.36 0.46 0.51

0.49 0.56 0.58

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Table 8.2 Runoff Coefficients for Use in the Rational Method (Source: Chow, Maidment, and Mays, 1988)…..continued Character of Surface Undeveloped Cultivated land Flat, 0 – 2% Average, 2 – 7% Steep, over 7% Pasture/range Flat, 0 – 2% Average, 2 – 7% Steep, over 7% Forest/woodlands Flat, 0 – 2% Average, 2 – 7% Steep, over 7%

Return Period (years) 10 25 50 100

2

5

500

0.35 0.35 0.39

0.34 0.38 0.42

0.36 0.41 0.44

0.40 0.44 0.48

0.43 0.48 0.51

0.47 0.51 0.54

0.57 0.60 0.61

0.25 0.33 0.37

0.28 0.36 0.40

0.30 0.38 0.42

0.34 0.42 0.46

0.37 0.45 0.49

0.41 0.49 0.53

0.53 0.58 0.60

0.20 0.31 0.35

0.25 0.34 0.39

0.28 0.26 0.41

0.31 0.40 0.45

0.35 0.43 0.48

0.39 0.47 0.52

0.48 0.56 0.58

Table 8.3 Runoff Coefficients for Use in the Rational Method (Philippines, Source: Technical Standards and Guidelines for Planning and Design, DPWHMarch 2002)

Surface Characteristics Lawn, gardens meadows and cultivated lands Parks, open spaces including unpaved surfaces and vacant lots Suburban districts with few building Residential districts not densely built Residential districts densely built For watershed having steep gullies and not heavily timbered For watershed having moderate slope, cultivated and heavily timbered For suburban areas For agricultural areas

Runoff coefficient 0.05-0.25 0.20-0.30 0.25-0.35 0.30-0.55 0.50-0.75 0.55-0.70 0.45-0.55 0.34-0.45 0.15-0.25

 Rainfall Intensity The rainfall intensity is the average rainfall rate considered for a particular drainage basin or subbasin. The intensity is selected on the basis of design rainfall duration and design frequency of occurrence. The design duration is equal to the time of concentration for the drainage area under consideration. The frequency of occurrence is a statistical variable that is established by design standards or chosen by the engineer as a design parameter.  Time of Concentration, tC The time of concentration is the period required for water to travel from the most hydraulically distant point of the watershed to the point of the storm drain system under consideration. The designer is usually concerned about two different times of concentration, one for inlet spacing and the other for piping sizing. There is a major difference between the two times.

(a) Inlet Spacing The time of concentration for inlet spacing is the time for water to flow from the hydraulically most distant point in the drainage to the inlet. Ususaly this is the timerequired for water to move across the pavement or overland back of the curb to the gutter, plus the time required for flow to move through the length gutter to the inlet. For pavement drainage, when the total time of concentration to the upstream inlet is less than seven minutes, a minimum tC of seven minutes should be used to estimate the intensity of rainfall. The time of concentration for the second downstream inlet and each succeeding inlet should be determined independently, the same as the first inlet. Runby travel time between inlets is not considered. In the case of a constant raodway grade, and relatively uniform contributing drainage area, the time of concentration for each succeeding inlet could also be constant. 𝒕𝑪 =

𝟒𝟏. 𝟎𝟐𝟓(𝟎. 𝟎𝟎𝟎𝟕𝒊 + 𝒄)𝑳𝟎.𝟑𝟑 𝑺𝟎.𝟑𝟑𝟑 𝒊𝟎.𝟔𝟔𝟕

Where: i = rainfall intensity (in/hr) c = retardance coefficient (see Table 8.4) L = length of flow path (ft) S = slope of flow path (ft)

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Table 8.4 Retardance coefficients (Source: Water Resources Engineering by Linsley and Franzini, 3rd edition) Surface Smooth asphalt surface Concrete pavement Tar and gravel pavement Closely clipped sod Dense bluegrass turf

Value of c 0.007 0.012 0.017 0.046 0.060

(b) Pipe Sizing The time of concentration for pipe sizing is the period required for water to travel from the most hydraulically distant point of the watershed to the point of the storm drain system under consideration. It generally consists of two components, the time to flow to the inlet which can consist of overland and channel or gutter flow, and the time to flow through the storm drain to the point under consideration. 𝒕𝑪 =

𝟏 𝑳 ∑ 𝟔𝟎 𝑽

Where: L = length of pipe along the flow path (ft) V = average velocity (ft/s) in the pipe  Area The area is the drainage surface measured in a horizontal plane. The area is usually measured from plans or maps using a planimeter. The area includes all land enclosed by the surrounding drainage divides. In highway drainage design, this area will frequently include upland properties beyond the highway right-of-way. STORMWATER DETENTION Urban stormwater management systems typically include detention and retention facilities to mitigate the negative impacts of urbanization on stormwater drainage. The effects of urbanization on stormwater runoff include increased total volumes of runoff and peak flow rates. In general, major changes in flow rates in urban watershed are the result of: 1. 2.

The increase in the volume of water available for runoff because of the increased impervious cover provided by parking lots, streets, and roofs, which reduce the amount of infiltration; Changes in hydraulic efficiency associated with artificial channels, curbing, gutters, and storm drainage collection systems, which increase the velocity of flow and the magnitude of flood peaks.

Types of Surface Detention (a) Extended detention basins (dry detention basin) Dry detention ponds empty after a storm. Water enters the basin, is impounded behind the embankment, and is slowly discharged through a perforated riser outlet. The coarse aggregate around the perforated riser minimizes clogging by debris. Typically once a required water-quality volume is filled, the remaining inflow is diverted around the basin or the pond overflows through a primary spillway. A large part of the sediment from the stormwater settles in the basin. Refer to Figures 8.6 and 8.7.

Figure 8.6

Figure 8.7

(b) Retention ponds (wet detention ponds) Retention ponds retain the water much longer above a permanent pool of water. A retention pond is basically a lake that can be designed to remove pollutants. Pollutants are removed by settling. Nutrients are removed by phytoplankton growth in the water column and by shallow marsh plants around the pond perimeter. Refer to Figures 8.8 and 8.9. | Water Resources Engineering

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Figure 8.8

Figure 8.9

Subsurface Disposal of Stormwater Swales are shallow vegetated trenches with nearly flat longitudinal slopes and mild side slopes. Filter strips are strips of land that stormwater must flow across before entering a conveyance system. These practices allow some of the runoff to infiltrate into the soil and filter the flow, removing some of the suspended solids and other pollutants attached to the solids. They also have the effect of reducing the directly connected impervious area and reducing the runoff velocity. They can be used for stormwater runoff from streets, parking lots, and roofs. Porous pavement and modular pavement (modular porous block pavement) can be used in parking areas to help reduce the amount of land needed for runoff quality control. Along the entire length of the trench, a perforated pipe allows distribution of stormwater. Perforated pipes allow the collection of sediment before it enters the aggregate backfill. Trenches are particularly suited for rights-of-way, parking lots, easements, and other areas with limited space. Their advantages are that they can be placed in narrow bands and in complex alignments. Prevention of excessive silt from entering the aggregate backfill and thus clogging the system is a major concern in design and construction. Sediment traps, filtration manholes, deep catch basins, synthetic fibercloths, and the installation of filter bags in catch basins has proven effective (American Iron and Steel Institute, 1995). Infiltration basins are retention facilities in which captured runoff is infiltrated into the ground. They are essentially depressions of varying sizes, either natural or excavated, into which stormwater is conveyed and allowed to infiltrate. Infiltration basins are typically used in parks and urban opens spaces, in highway rights-of-way, and in open spaces in freeway interchange loops. Infiltration basins are susceptible to clogging and sedimentation and can require large land areas. Standing water in these basins can create problems of security and insect breeding. Recharge wells can be used to dispose of stormwater directly into the subsurface. Recharge wells can be used to remove standing water in areas that are difficult to drain. They can also be used in conjunction with infiltration basins to penetrate impermeable strata. Another use is a bottomless catch basin in conventional minor system design. Typically, recharge wells are used for small areas and can be combined with catch basins. The use of a filter manhole in conjunction with a recharge well prevents excess silt entering the recharge well and causing clogging. Underground storage Underground storage can be effective where surface ponds are not permitted or feasible. These storage tanks can be either in-line, in which the storage is incorporated directly into the sewer system, or off-line, in which stormwater is collected before it enters the sewer system and then discharged to either the sewer system or an open water course at a controlled rate. When the capacity of an in-line system is exceeded, surcharging in the sewer can occur.

Swale

Infiltration basin

Filter strip

Underground storage

Porous pavement

Infiltration trench

Modular pavement

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LAND DRAINAGE During rain or irrigation, the fields become wet. The water infiltrates into the soil and is stored in its pores. When all the pores are filled with water, the soil is said to be saturated and no more water can be absorbed; when rain or irrigation continues, pools may form on the soil surface (Figure A).

Figure A During heavy rainfall the upper soil layers become saturated and pools may form. Water percolates to deeper layers and infiltrates from the pools.

Figure B Before heavy rainfall

Figure C After heavy rainfall

Part of the water present in the saturated upper soil layers flows downward into deeper layers and is replaced by water infiltrating from the surface pools. When there is no more water left on the soil surface, the downward flow continues for a while and air re-enters in the pores of the soil. This soil is not saturated anymore. However, saturation may have lasted too long for the plants' health. Plant roots require air as well as water and most plants cannot withstand saturated soil for long periods (rice is an exception). Besides damage to the crop, a very wet soil makes the use of machinery difficult, if not impossible. The water flowing from the saturated soil downward to deeper layers, feeds the groundwater reservoir. As a result, the groundwater level (often called groundwater table or simply water table) rises. Following heavy rainfall or continuous overirrigation, the groundwater table may even reach and saturate part of the rootzone (Figures B and C). Again, if this situation lasts too long, the plants may suffer. Measures to control the rise of the water table are thus necessary. The removal of excess water either from the ground surface or from the rootzone, is called drainage. Excess water may be caused by rainfall or by using too much irrigation water, but may also have other origins such as canal seepage or floods. In very dry areas there is often accumulation of salts in the soil. Most crops do not grow well on salty soil. Salts can be washed out by percolating irrigation water through the rootzone of the crops. To achieve sufficient percolation, farmers will apply more water to the field than the crops need. But the salty percolation water will cause the water table to rise. Drainage to control the water table, therefore, also serves to control the salinity of the soil. Drainage can be either natural or artificial. Many areas have some natural drainage; this means that excess water flows from the farmers' fields to swamps or to lakes and rivers. Natural drainage, however, is often inadequate and artificial or man-made drainage is required. Types of artificial drainage Surface drainage Surface drainage is the removal of excess water from the surface of the land. This is normally accomplished by shallow ditches, also called open drains. The shallow ditches discharge into larger and deeper collector drains. In order to facilitate the flow of excess water toward the drains, the field is given an artificial slope by means of land grading (Figure D).

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Figure D The field is given artificial slope to facilitate drainage

Subsurface drainage Subsurface drainage is the removal of water from the rootzone. It is accomplished by deep open drains or buried pipe drains. i. Deep open drains The excess water from the rootzone flows into the open drains (Figure E). The disadvantage of this type of subsurface drainage is that it makes the use of machinery difficult. Figure E Control of the groundwater table by means of deep open drains

ii. Pipe drains Pipe drains are buried pipes with openings through which the soil water can enter. The pipes convey the water to a collector drain (Figure F). Drain pipes are made of clay, concrete or plastic. They are usually placed in trenches by machines. In clay and concrete pipes (usually 30 cm long and 5 - 10 cm in diameter) drainage water enters the pipes through the joints. Flexible plastic drains are much longer (up to 200 m) and the water enters through perforations distributed over the entire length of the pipe. Figure F Control of the groundwater table by means of buried pipes

iii. Deep open drains versus pipe drains Open drains use land that otherwise could be used for crops. They restrict the use of machines. They also require a large number of bridges and culverts for road crossings and access to the fields. Open drains require frequent maintenance (weed control, repairs, etc.). In contrast to open drains, buried pipes cause no loss of cultivable land and maintenance requirements are very limited. The installation costs, however, of pipe drains may be higher due to the materials, the equipment and the skilled manpower involved. | Water Resources Engineering

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iv. Tile drainage The purpose of subsurface drainage is to lower the water table in the soil. The water table is the level at which the soil is entirely saturated with water. The excess water must be removed to a level below the ground surface where it will not interfere with plant root growth and development. Root growth requires air to be present in the soil. Both water and air need to be present in the spaces between the soil particles, often in equal proportions. If water fills all of these spaces (saturated), there is no room for air. Tile drainage should be designed so the water table between tile lines can be lowered within 24 hours after a rain to a level that will not cause crop injury. Generally, most field crops are not injured if the water table is lowered to at least six inches below the ground surface in the first 24 hours after a rain. During the second day after a rain the water table should be lowered to approximately one foot and on the third day to 1.5 feet below the ground surface. The soil types in an area to be drained greatly influence the type of system that will be installed and indicate if special problems should be anticipated. Tile drains are placed at uniform depths where possible. The topography of the land influences the grades available, and it is often possible to orient the drains within the field to obtain a desirable grade. The grades should be sufficient to result in a non-silting velocity yet be flat enough that the maximum allowable velocity rate is not exceeded and the drain is not subjected to excessive pressure flow. Too much flow will cause erosion around the drain. Some arrangements of tile drains (Figure G) Figure G Some arrangements of tile drains Parallel tile drain

Herringbone tile drain

The herringbone system (b) consists of parallel tile laterals that enter the main at an angle, usually from both sides. This system is used for long, relatively narrow wet areas such as those next to flat drainage ways. The parallel or gridiron system (a) is similar to the herringbone system except that the laterals enter the main from only one side. This system is used on flat, regularly shaped fields with uniform soil types. The double-main system (c) is a modification of the gridiron and herringbone systems. It is used where a depression, which is frequently a natural watercourse, divides the field. A random system (d) is used where the topography is undulating or rolling and contains isolated wet areas.

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HIGHWAY DRAINAGE Highways occupy long, narrow strips of land and pose two types of drainage problems. Highways cross many natural drainage channels, and the water carried by these channels must be conveyed across the right of way without obstructing the flow in the channel upstream of the road and causing damage to property outside of the right of way. Longitudinal Drainage Unless stormwater resulting from rainfall on the highway right of way and tributary slopes is properly controlled, severe erosion of the shoulder and cut slopes may create a costly maintenance problem and dangerous driving conditions. In humid regions erosion can often be controlled by establishing vegetative cover on slopes of cut and fill and along shoulders. In this case the water should be spread out as much as possible since erosion increases with a power of velocity which in turn increases with depth. A good grass cover can withstand velocities up to about 2.5 m/sec. Bermuda grass is typical of turf-forming grasses offering good protection. Grass will not withstand immersion for long periods, and slopes of at least 0.005 should be provided so that water will drain off rapidly. In arid regions where natural rainfall is insufficient to maintain a vegetal cover or in humid regions where runoff is high, other means of protection are necessary. Slopes of cuts may be protected by an intercepting dike or ditch at the top of the slope. In long cuts the dike should approximately follow a contour and, if possible, should divert the water into a natural drainage way without allowing it to reach the roadway. If it is necessary to direct the water to the roadway ditch before the end of the cut, a paved spillway section down the cut slope will be necessary. Contour furrows at intervals above the top of the cut slope will retard overland flow, and the landowner may benefit from the additional moisture retained on the land. Roadside ditches are usually constructed in a shallow V-shape since this section is easily maintained with graders, is less hazardous, and permits the shallow flow necessary to avoid erosion. The ditch should be large enough to carry the design flow with a freeboard of 3 to 6 in. (8 to 15 cm). The flow line of the ditch is sometimes placed low enough so that underdrainage from the highway base course may enter the ditch, although where the base course and underlying soil are of low permeability, very little underdrainage can be expected. Bare earth will withstand velocities between 1 and 4 ft/sec (0.3 to 1.2 m/sec), depending upon the soil types. If the computed velocity for the design flow is too high for the soil, protection may be provided by lining the ditch with asphalt, concrete, dry rubble, or sod. An alternative to lining the ditch throughout its entire length is drop structures at intervals which permit the remainder of the ditch to be maintained on a grade sufficiently low to prevent excessive velocities. Drop structures may prove to be a traffic hazard. A third alternative is to divert the flow to a natural drainage way by use of a paved chute across the side of the fill before the accumulated runoff in the ditch is sufficient to cause harmful velocities. Drop inlets discharging the flow into a pipe which carries it to the downstream side of the fill are preferred for intercepting the flow in the uphill ditch unless the intercepted water can be discharged directly into a culvert on the upstream side of the roadway. If interception is necessary in long cuts, a drop inlet discharging into a buried pipe running parallel to the highway is required until a point is reached where the flow can be directed into a natural drainage way. Depressed highways and underpasses sometimes create a difficult drainage problem. Occasionally the flow which collects at the bottom of an underpass can be carried away by gravity to a storm drain or natural channel. Often, however, a small automatic pumping station set in a sump beneath the roadway is required to lift the water to a point where it can be carried away by gravity flow. Cross-drainage Since highways cross many natural drainage channels, provision for carrying this water across the right of way is necessary. It should be noted that a considerable saving is often possible by locating a highway along ridge lines and eliminating channel crossings. Since an average of one-fourth of highway construction costs is for drainage structures, some saving may be realized even though the ridge route involves greater length and less satisfactory grades and alignment. Cross drainage is accomplished by culverts, bridges, and dips. Culverts and bridges carry the roadways over a stream, and the distinction between the two types of structures is mainly on the basis of size. Frequently structures with a span in excess of 20 ft (6 m) are classed as bridges, while structures of shorter span are called culverts. This is arbitrary, and from the hydraulic viewpoint it might be more proper to include with culverts all structures designed to flow with submerged inlet. A dip is a depression of the roadway so that flow from a cross channel may pass over the road.  Culverts The essential features of a culvert are the barrel which passes under the fill; the headwalls and wingwalls at the entrance and endwalls or other devices at exit to improve flow conditions and prevent embankment scour; and in some cases debris protection to prevent entrance of debris which might clog the culvert barrel. Culvert barrels are made of a variety of materials, the main basis of selection being the cost of the installed culverts. Small culverts may be made from precast concrete, vitrified clay, cast iron, or corrugated steel pipe. | Water Resources Engineering

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Minimum diameter for culverts should be about 18 to 24 in. (45 to 60 cm), although 12-in. (30-cm) pipe might be used for small flows carrying no debris or trash. It is common to place the culvert axis normal to the highway center line even though this requires some changes in the natural channel. The alternative is a skew culvert, which will be longer than the normal culvert and will rewuire more complex construction of headwalls and endwalls. As a general rule, it is best that culvert alignment conform to the natural stream alignment. Culvert parts

Box culvert

Arch Culvert

Culvert parts A = barrel B = headwall C = wingwall D = apron

Debris barrier

 Bridge Waterways

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The waterway opening of a bridge must be large enough to pass the design flood without creating excessive backwater. If stream velocities are low or there are only a few piers in the stream, the backwater* problem may be of little importance. Massive piers or long-approach embankments encroaching on the waterway of the stream may result in considerable damage from high stages above the bridge and may increase the likelihood of the bridge being overtopped by floodwater. *A backwater is a part of a river in which there is little or no current. It refers either to a branch of a main river, which lies alongside it and then rejoins it, or to a body of water in a main river, backed up by the tide or by an obstruction. Backwater profile

Economic design requires the determination of the minimum clear length of span which will not cause intolerable backwater conditions. The first step in the study is a determination of the permissible backwater heights, based on field investigation of lands and structures along the stream which might be harmed by an increase in stage. The second step is the determination of the stage in the channel downstream from the bridge site at design flow. Bridge waterway

Bridge waterway

 Dips Dips sometimes offer an economical solution to the cross-drainage problem in arid areas where streamflow is infrequent and of short duration, provided the channel to be crossed is shallow enough to permit construction of the dip without excessive grading of the approaches. The upstream edge of the roadbed at the dip should be even with the bottom of the channel to avoid scour which might undermine the road. The downstream edge of the road should be protected with a cutoff wall, paving, or rock riprap for the same purpose. The profile of the dip should be as close to the shape of the stream cross section as possible, to eliminate interference with streamflow. If the stream transports heavy debris, the road surface should be made especially heavy. Posts should be set to indicate the edges of the road, and suitable warnings should be posted against crossing the dip when flow is occurring. To avoid continuous overflow on perennial streams one or more small culverts may be provided under the roadway to accommodate low flows.

A rolling grade use undulating crests and dips to divert water off the tread

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Design of insloped dips for forest roads. A to C, slope about 10 to 15 cm to assure lateral flow; B, no material accumulated at this point - may require surfacing to prevent cutting; D, provide rock rip-rap to prevent erosion; E, berm to prevent overflow; F, culvert to carry water beneath road; G, widen for ditch and pipe inlet (Megahan, 1977).

Design of outsloped dips for forest roads. A to C, slope about 10 to 15 cm to assure lateral flow; B, no material accumulated at this point - may require surfacing to prevent cutting; D, provide rock rip-rap to prevent erosion; E, berm to confine outflow to 0.5 m wide spillway. (Megahan, 1977).

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