Types of Culverts and Provision to Avoid Downstream Scour - Assignment Example

 Types of Culverts and Provision to Avoid Downstream Scour Culverts can be described as structures that are sized hydraulically and designed to allow water to flow under a highway, trail, railroad, and/or similar embankments. Generally, culverts comprise of structural materials around the perimeter and are distinguished from bridges by the fact that they are covered with embankments. There are various culvert shapes, materials and sizes from which culvert designers can choose. The selected design should integrate structural stability, hydraulic efficiency, serviceability, environmental conditions, economics, land use and traffic safety requirements. The type of culvert is described by the characteristics of size, shape, scale number, manning’s n value, scale number, slope (downstream and upstream inverts) and loss coefficient. The various types of culverts include: single or multiple pipe culverts, multiple or single pipe-arch culverts, multiple or single box culverts, bridge culverts and arch culverts. This paper discusses these types of culverts and also talks about the provision to prevent downstream scour. Types of Culverts 1. Single pipe Culvert (multiple identical barrels) (Single/Multiple Pipe Culvert n.d) These culverts are made of concrete materials, corrugated metal or smooth steel. Their main function is to deliver water from one side of the road or path to the other. They are also used as passageways by a variety of wildlife (Wyant 2002). The diameter of single pipe culverts usually range from one to six feet and are considered the least expensive. The ideal location for single piped culverts include: where draining water crosses under the road/path through its own gravity, where water springs and small streams flow across the path, and where it is easiest to disperse water from the path. 2. Multiple or single Pipe-arch Culvert (Single pipe-arch culvert n.d) These culverts have a flattened, but not completely flat bottom. The culverts comprise of a small radii at the corners of the base, a large radii at the base and a normal radii in the upper half of the culvert as shown above in the diagram. This type of culverts offer low clearance openings appropriate for big waterways. They are also more aesthetic and this makes them ideal for making the installation area more appealing. At low flows, these culverts may provide a better hydraulic advantage to fishes. In order to reduce the velocity of water, pipe-arch culverts are kept at a gradient below 1 percent. 3. Multiple or single Box culvert (Multiple Box Culverts n.d) These types of culverts are built on reinforced concrete boxes of rectangular or square opening with restricted spans of four metres (Osborn 2009). The top surface of the box culvert may be at the same level with the road or at a depth lower than the level of the road that is if the road is embanked. They are mainly used to convey water during periods of brief water runoffs. They can effectively be used by wildlife since they remain dry for the better part of the year. Generally, they provide more room for the passage of wildlife than the huge pipe culverts. 4. Arch culvert (Arch culvert n.d) Arch culverts are round culverts redesigned to maintain flow characteristics and at the same time allow for a lower profile. The materials used to build arch culverts include reinforced concrete, stone masonry and corrugated metal. They are most useful where the distance between the channel invert and the pavement surface is limited. These types of culverts are recommended for crossing rivers with migratory fish. They are most applicable where minimal obstruction to waterway is desired and where the foundation is acceptable for structural support. 5. Bridge culverts (Bridge Culvert n.d) These are large culverts that are designed for non-submergence in events such as flood. The major difference between bridge culverts and other types of culverts is that other culverts are designed to function with the inlet submerged incase such a condition occurs. Bridge culverts normally have a length of at least twenty feet along the roadway. They are normally designed to provide passage to vehicles and people over large water bodies. Unlike culverts, bridges do not have a floor and are rectangular, square, elliptical or even circular in size. Provisions for Downstream Scour on Culverts As mentioned earlier, a culvert's primary purpose is to permit water to flow under a railway, road or similar obstruction. It is, therefore, necessary to design the right size of culvert to allow for unrestricted water course. There are various measures that are taken while designing a culvert to help control scouring. One of the measures is that the design engineer is required to carry out an inspection on culverts across a watercourse and opt for the size that will work efficiently. For example, one should calculate the area of discharge of the waterway in spate conditions. The type of culvert to use always depends on the accessibility of the site, materials available and its purpose among other factors (Burns et al. 2010). A culvert can either be buried completely or a capping stone can be integrated into the path surface. The culverts may be placed in a straight section (for small burns), if possible where the water speed is probable to be low. Scour downstream can undermine culverts and, as a result, lower its stability consequentially leading to collapse. There have been numerous reported incidents of failure hence there is a need to evaluate all culverts for the scour vulnerability. Scouring is the process by which soil particles or rock around the periphery of an abutment, pier or culvert along a water body, gets eroded and removed over a particular depth known as the scour depth. Scouring mainly takes place when the velocity of water flowing in a pipe increases or exceeds the limit value that the parent soil particles can handle. The soil around a culvert can be removed (siltation and seepages) and re-deposited in the occurrence of a flood without leaving any evidence that it has taken place. It is often initiated at the nose of the culverts or sharp bends, and it usually entails the structural integrity of the culvert and, as a result, causes failure of the culvert. There are a number of specifications or provisions that an engineer should consider incorporating when designing a culvert in order to avoid, or at least limit, the incidence of scouring. These specifications are described below: Gradients The pipes’ longitudinal run ought to be a minimum of 1:40 to avert silting (Ponnuswamy 2008). Where plausible, the pipes design should avoid gradients greater than 1:15; this will help prevent any potential scour problems at the culvert outlet. Construction The culvert bedding materials ought to be smooth and free of large stones that might perforate the pipe walls. The culverts should be laid on a bed of graded aggregate; care should be taken not to have the top level above the streambed level. The culvert should be backfilled with fine compacted graded aggregate; for example, those used for path base. The cover depth ought to be 50-100% of the culvert diameter, keeping in mind the expected loadings, with a minimal cover of 150mm. Concrete mattress and riprap at the downstream of grade to control structure are very effective in controlling scour. Sizing The pipes must never run full. Ideally, culverts should not exceed two thirds full at the worst-case scenario water flow. It is recommendable to use a pipe diameter that is twice that of the watercourse depth for deep and slow moving streams. In the case of ditch systems, culverts utilize several 300mm breadth water pipes set at regular intervals (usually at every 10-20 metres) depending on the quantity of water flowing into the ditches. Headwall A headwall should be constructed around both ends of the pipe (outlet and inlet); this will help retain backfill materials and pipe bedding and protect the ends of the pipe. These headwalls may be built as either mortared or dry. Moreover, their thickness should be about 150-450mm depending on the backfill depth (Jayaram 2004). All the stones should fit together and be tightly set into the ditch sides or streams. A big single mantle stone should be installed across the top of culvert ends taking care not to crush the inlet and outlet ends. Otherwise, stones should be arranged to give an arch around the pipe. Inlet and Outlet There should be flow splitters at the inlet of multiple culverts. Splitters are long thin stones fixed between pipes to govern water flow into culverts without washing out (scouring) pipe backfill materials. In addition, one should place a bed of even stones around the pipe inlet to prevent scouring around or under the pipe. For the outlet, "splash plate" should be installed. Splash plate is a sole flat stone directly below the culvert outlet that helps prevent undercutting. Additionally, there should be a broad bed of even stones, around the outfall; this will help prevent scour and erosion. Handrails These are constructed in a case where pipes present a falling risk to individuals that use a given path that the culvert is laid (Stewart et al. 2013). An engineer should consider designing a suitable handrail that depends on the types of users and the culvert depth. Conclusion The various types of culverts discussed above have their unique characteristics that make them only applicable in their unique environment. It is, therefore, important for the Engineer on site to use his knowledge and experience to identify the best culvert that caters for the environmental conditions of the area the culvert is being installed. One main advantage of culverts is their ability to install and uninstall with ease. They are also easily portable. Culverts however, may be restricted in certain areas due to environmental restrictions. In other cases, culvert operators have to seek technical assistance to rectify their sizes while installing. Wrong sizes or installations may cause harm to a stream. List of References Arch culvert n.d, photograph, viewed 10 November 2014, < www.aboutcivil.org> Bridge Culvert n.d, photograph, viewed 9 November 2014, < http://armcobarriers.com.au/Bridges%20%26%20Culverts.htm> Burns, S.E, Bhatia, S.K, Avila, C.M, & Hunt, B 2010, Scour and Erosion: Proceedings of the Fifth International Conference on Scour and Erosion, ASCE Publications, New York. Jayaram, M.A 2004, Design of Bridge Structures, PHI Learning Pvt. Ltd, Delhi. Multiple Box Culverts n.d, photograph, viewed 11 November 2014, < http://www.cretexnorth.com/> Osborn, L 2009, Trenchless Renewal of Culverts and Storm Sewers, ASCE Publications, New York. Ponnuswamy 2008, Bridge Engineering, Tata McGraw-Hill Education, Pennsylvania. Single Pipe-arch Culvert n.d, Photograph, viewed 10 November 2014, Single/multiple Pipe Culvert n.d, photograph, viewed 10 November 2014, Stewart, A.W, Gail, J, Sloane, C & Hoadley, G 2013, Clark’s Positioning in Radiography 13E, CRC Press, Boca Raton. Wyant, D 2002, Assessment and Rehabilitation of Existing Culverts, Transportation Research Board, Washington, D.C. Read More