Proceedings of the 2nd International Conference on Transportation in Africa (ICTA2015) Majestic Five Hotel, Palapye, Botswana, 25-27 November 2015
Road Drainage Systems in Palapye: Suggestions for Adaptation to Storm Water Runoff and Floods Thabo M. Bafitlhile and Adewole S. Oladele Department of Civil and Environmental Engineering, College of Engineering and Technology, Botswana International University of Science & Technology, P/ Bag 16, Palapye. BOTSWANA.
[email protected];
[email protected] ABSTRACT Transport infrastructure systems play a major role in public mobility. Road drainage system is a very important tool when it comes to reclaiming and mitigating of problems caused by torrential rainfalls as proper design and maintenance should be in place for adequate drainage performance. Botswana has been affected by floods and is still experiencing this tragic event once it occurs. Flooding occurs mostly in the North-West, North-East, and parts of Central district due to heavy rainfalls experienced in these areas. Palapye is one area in the central district that has been experiencing floods ever since 1995 when its greatest flood on record occurred. The torrential rains destroyed homes, roads, flooded dams, fields and destroyed livestock and livelihoods. Heavy storms result in floods and inundation; this has been exacerbated by poor and absence of drainage structures. Furthermore floods and highway play major role in erosion and destruction of roads structures. Many culverts, trenches, and other drainage facilities lack the capacity to deal with current frequency for extreme flows. Future changes in the pattern of hydro climatic events will have implications for the design and maintenance costs of roads. Increase in rainfall and severe weather events can affect the demand for emergence responses. Therefore this paper attempts to propose a well-founded suggestion on adaptation of road drainage system to protect road infrastructure from destruction. KEYWORDS: Roads, Drainage, Runoff, Floods, High water flow
1.0 INTRODUCTION Transport infrastructure system plays a major role in public mobility (Kalantari, 2011). Therefore, proper management, responsiveness and high capital investment in the design and maintenance of such system are essential since is a critical component of society and civilization (Cao et al, 2006; Green Paper EU 2007). During the previous years, an increase in the frequency of extreme weather events such as heavy storms and floods has been reported in various areas of Botswana including Palapye. Due to the predicted increase in global average temperatures, the risk of extreme weather event will increase (Green Paper EU, 2007). During rainfall, part of the rainwater flows on surface and part of it infiltrates into the soil mass as gravitational water until it reaches ground water. Some water is retained in the pores of the soil mass and on the surface of the soil particles which cannot be drained by normal gravitational method. Surface water from the carriageway and shoulder should effectively be drained off without allowing it to percolate to subgrade. The surface water from the contiguous land should be prevented from entering the road way by designing side drains that have adequate capacity and longitudinal slopes to carry away the entire surface collected. There are many roads which are not having suitable drainage system in Palapye. Henceforth this cause failure of the roads due to many reasons like increase in moisture content, decrease in strength, stripping of bitumen, cutting of edges of pavement etc. Palapye as shown in Figure 1, there are less storm water drains and any other drainage facility that can meet the water flow. Moreover, the existing drains lack the capacity to deal with the current frequency
of extreme flow events. An upsurge in the occurrence of extreme weather events will inflict greater strain on the facilities for dewatering and drainage of roads (Kalantari, 2011). Scanty culverts, poorly maintained ditches and structures with limited capacity may lead to the serious damage to the entire road and transport system. (Gurjar, Agarwal, & Sharma, 2013) and (Carbone, Garofalo, Tomei, & Piro, 2014) (Rokade, Agarwal, & Shrivastava, 2012) stated that water in a pavement system is one of the principal cause of premature pavement failure.
Figure 1: Map showing the location of Palapye (study area) Most part of Palapye lies in a low lying topography as shown in figure 2 and due to this, during times of high rainfalls or even average rainfalls they result into flooding and inundation. One major problem in this area is storm water related flooding and accumulation of stagnant water (inundation), which is created by a low lying topography, increase in development and population, scantiness of proper infrastructure, poorly developed storm water management and poor maintenance of drainage systems.
Figure 2: Showing satellite image for Palapye On account of this, many people have lost their property and road infrastructures are being ripped off because there is no preventive measure to reduce the possible effects of floods on road infrastructure. Therefore a robust, adequate and sustainable storm water management system must be implemented to reclaim road infrastructures from adverse effect of floods.
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1.1 Background Information Federation et al. (2013) recorded that between 16th to 23rd January, 2013 heavy rains caused extensive flooding in the central district of Botswana. At least 842 families (4210 persons) were affected. The torrential rains destroyed homes, roads, fields, livestock and livelihoods. Floods are not rare incident in Central part of Botswana especially in Palapye. Historical records show that Palapye has experienced many of catastrophic flood disasters. Figure 3 shows floods that hit Palapye in 1995 and ever since then, has been a major victim of floods recently. This has been exacerbated by poor and absence of drainage structures, as a result of flooding, hundreds of lives, property, physical structures are loss, damaged or destroyed especially the transportation and its infrastructure as are more frequently affected by floods. It is an undesirable circumstance for the residents since even average rainfalls results into inundation and flooding.
Figure 3: Floods in Palapye, 1995 (Source: Jacky, 2006) Development plan for Palapye stress that adequate storm water management is one of the areas to pay more attention and formulate strategies for extending and upgrading capacities of existing infrastructure services (sewage and storm water drainage system). This will inevitably help in achieving another goal of providing efficient, safe, convenient, cost effective traffic and circulation systems for Palapye planning area.
Figure 4: Satellite image showing roads in Palapye Figure 4 shows some of the roads in Palapye that require absolute attention as they gradually regress with time. Most of the roads in this area show the detachment of bituminous pavement layer due to water. Moreover running water along the slope also results in deterioration of the road due to erosion hence results in stripping of bitumen from aggregates. Formation of waves and corrugation due to poor drainage and water that gets collected on the road surface and penetrates into the road through crack. This causes the increase in moisture content of the 3
sub grade and hence the development of potholes and removal of aggregates. Excess moisture causes increase in weight and thus increases the stress causing the simultaneous reduction in strength of soil mass.
2.0
LITERATURE REVIEW
Sustainable Urban Drainage System (SUDS) is a philosophy used around the world to help reduce the excess of flow of storm water from spreading into unwanted areas (Button et al, 2010). Storm water management practice has evolved gradually over the last twenty years and it is focused foremost on flood and erosion control (Needhidasan & Nallanathel, 2013). Koetse and Rietvel (2009) stated that less has been done on the methods to reduce damage on the transport sector due to the impacts of climate change and extreme weather events. In addition Suarez et al, (2005) indicated that much less attention has been given to the potential impacts of climate change on urban transportation system. Little is known about how flooding events affect the performance of urban transportation networks as integrated systems (Suarez et al., 2005). The anticipated pattern of flooding occasion will influence the number of incident such as destruction of household, infrastructures, submerged and inundated transportation system and loss of life (Bizikova et al, 2008) There is a need for an effective flood resilience assessment process to identify possible risks to the transportation system (Prabath and Bandara, 2011). Information such as terrain, rainfall, drainage pattern that are relating to the factors affecting flood risk on transportation infrastructure is available in scattered manner. However, this information has not been analyzed in an integrated manner to identify the flood risk on transport infrastructure. Identification of flood risk of existing and proposed transport infrastructure will be very useful to minimize impacts due to any type of flood disaster. Development of robust road infrastructure capable of withstanding strains from a variety of weather calamities for a continued function of the transport system will be a very crucial aspect for Palapye since the area is prone to torrential rainfalls and floods along with climate change which contributes to severe storm due to increasing average annual temperatures and increasing precipitation. At the same time Palapye is growing at an alarming rate hence the increase in population and economic growth which are likely to increase the pressure to land development and will results in more impervious surface which will results in an increase in the runoff which must be handled by rivers, streams and storm water systems. Adverse effect of water can be reduced by averting water from entering the pavement, providing adequate drainage to remove infiltration, or building the pavement strong enough to resist the combined effect of load and water (Rokade et al., 2012). Pavement service life can be increased by 50% if infiltrated water can be taken away without any delay. Correspondingly, pavement systems incorporating good drainage can be expected to have a design life of two to three times than of the undrained pavement section. Rokade et al, (2012) “Excess moisture combine with traffic loads can have a negative effect on both material properties and overall performance of a pavement system. In view of the fact that damaging effects of excess moisture has long being recognized. Infiltration through the surface joints, cracks and other defects in the surface that provide an easy path for water is the most substantial source of excess in pavement. The damage can be mended by using surface and subsurface drainage system. Whereby surface drainage will remove and divert water from the roadway and adjoin land, generally inside drains and then the water is disposed of at the nearest stream or watercourse.
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The impact of floods require to be reduced by improving the status of drainage system through knowledge on storm water management system since floods are natural phenomenon that cannot be stopped (Sinha, 2011). Efficient drainage system is a beneficial tool for water resource administration for sustainable urban development. Rao & Rasmussen, (1987) emphasized that an adequate understanding of future storm water drainage pattern is required for planning of future storm water drainage systems in urban areas. Therefore the design of storm water drainage systems will be helpful to safeguard the transport system from being washed away or destroyed by floods and also reduce any destruction due to over flow of water. The overall aim of this paper is to propose a well-founded suggestion measure on adaptation of road drainage system to climate change that result in more frequent floods to protect roads infrastructure from destruction.
3.0 METHODOLOGY This chapter consists of four sub title and they are as follows. 3.1 Road Drainage Structures In Palapye, structures to drain water from roads are of three types: surface drainage systems (ditches) and sub-surface drainage systems (culverts). A few of these structures have been put in place, hence their presence is insignificant since heavy rainfalls always results into flooding and inundation. Moreover much more damage incurred due to water runoff, as they can’t even meet the hydraulic load. O’ Flaherty (2002) highlighted that proper design of drainage systems is an essential part of economic road design. The surface water is collected and disposed of. The water is first collected in the longitudinal drains, generally in the side drains and then it is disposed of at the nearest stream (figure 5). Cross drainage structures like culvert and small bridges may be necessary for the disposal of the surface water from the road side drains. Under this category surface water is intercepted and diverted to a natural channel or depression.
Figure 5: Surface drainage, (trapezoidal channel)
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Sub Surface drainage of highways its main role is to take of change in moisture content of sub grade caused by fluctuation in ground water table, seepage flow, percolation of rain water, movement of capillary water and even water vapor. The system is embedded and place filter or envelopes around the pipe to improve flow condition in the area (figure 6). However, only gravitational water is drained by usual drainage system.
Figure 6: Subsurface drainage
3.2 Common Causes of Road Damage and the Role of Drainage Water is one of the major causes of damage to road (Kalantari & Folkeson, 2013). The bearing capacity of the road is reduced by the water present in the road construction and this lead to more extensive road maintenance (Kalantari and Folkson, 2013). It is well known that the rate of road deterioration increases if the water content of the granular material increases (Abhijit, 2011). Inappropriate designs (size) of the drainage systems cause inefficient drainage. Due to under estimated size of storm water drains results into insufficient capacity to handle large water volume and cause deficient drainage and even flooding. Where this results in water logging of areas adjacent to a road and this may damage the road. Obstruction, blocking and clogging of drainage facility seem to be a common problem. Drainage structures are clogged or blocked by fine grade soils, and impermeable materials especially after heavy rainfalls and can lead to floods and even roads being washed away. All this give rise to maintenance needs since is very crucial to keep ditches of existing roads at sufficient depth. Therefore inadequate maintenance impose a negative impact on road drainage system hence malfunctioning of drainage system (Kalantari and Folkeson, 2013).
3.3 Effects of Poor Drainage on Roads Excessive moisture within a pavement structure can adversely affect pavement performance. A pavement can be stable at given moisture content, but may become unstable if the materials become saturated. High water pressure can develop in saturated soils when subjected to dynamic loading (Gurjar et al., 2013). Stated adverse effects related to excess water: reduction of shear strength of unbound materials, differential swelling in expansive sub grade soils, stripping of asphalt in flexible pavements, movement of fine particles into base/sub base course materials resulting in a reduction of the hydraulic conductivity considerably (Diefenderfer et al, 2001).
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3.4 Methods to Adapt to Protect Roads against Floods On the basis of literature, approaches to minimizing road damage and mitigating the consequences of high flows and other types of strains are as follows. A functioning drainage system is important to reduce damage and maintenance costs.
3.4.1 Prevent moisture from percolating the pavement system. This is the best method to reduce the impairment effect of moisture on pavement. An effective course to minimize surface infiltration is by advancing on surface drainage system. The road should be adequately sloped to drain the water away from the travel lanes and shoulders and directed to drainage channels in the system (Mukherjee, 2014). The channel should be sized based on the probable maximum precipitation to accommodate the anticipated storm water flow. In addition adequate cross slopes and longitudinal slopes should be employed to drain water from the pavement surface quickly. However, it is very difficult to completely prevent moisture from entering the pavement course since it enters from different sources. Nonetheless, a suitable design can reduce the amount of moisture entering the pavement system (Rokade et al., 2012).
3.4.2 Inlets These are part of drainage system that receives runoff and permit the water to flow downward into underground storm drains as shown in figure 6 below.
Figure 7: Inlets Drainage inlets should be provided as needed to prevent ponding and limit the spread of water into lanes.
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3.4.3 Storm Sewers These are underground pipes shown in figure 8 for collecting precipitation runoff from a road side inlet for conveyance and discharge it into local rivers, stream and drains.
Figure 8: Pipe draining runoff Storm sewers usually sized using Manning’s formula taking into account anticipated runoff to determine the pipe size.
3.4.5 Surface water Channels: Surface water channel is suitable for area where there is limited land for open drains and also in areas where there is footpath. Also used on the edge of the road to prevent erosion as shown in figure 9.
Figure 9: Surface water channels Significant benefits may be possible to locate channel outlet at appreciable spacing and possibly coincident with watercourse.
3.4.5 Provide moisture – insensitive materials This is another way of preventing moisture accelerated damage by using moisture insensitive or nonerodible base materials that are less affected by the detrimental effects of moisture. Materials like cement treated base are used to prevent pumping and loss of fines from beneath the treated base in areas with adverse site conditions.
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3.4.6 Subsurface drainage Sub surface drainage attempts to keep the variation of moisture in sub grade soil to a minimum as shown in figure 10.
Figure 10: Subsurface drainage This can be achieved by lowering the water table, control of seepage flow and control of capillary rise. However, only gravitational water is drained by usual drainage system.
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DISCUSSION
Palapye is one area that is prone to flooding and inundation, and the condition of the road worsen with time and the situation gets worse during raining periods. Inadequate drainage system as measures to control water flow exacerbate the conditions as road infrastructure is easily being washed away. Consequently, more frequent intensive rains can increase the number of incidents such as landslides, road closure and bridge support being submerged or inundated. Poor drainage systems lead to leaky system which could raise the hydrostatic pressure gradient, and eventually lead to internal erosion (Kalantari & Folkeson et al, 2013).
5 CONCLUSION Drainage is key element in the design of pavement system. Maintenance of adequate water content in granular road materials is very helpful, but if the water content is higher than the optimum level will result in the emergence of negative effect like instability of the base because of settlement which may lead to serious problems. A major objective in pavement design should be to prevent excessive water or moisture from entering the pavement system that is the base, sub base, sub grade and other susceptible paving materials. When designing road system the designer should consider providing all three type of drainage system as they share a symbiotic relationship and should be considered in the overall drainage deign of a project. Therefore, a serious study is required on the effective of drainage quality on pavement performance in Palapye and quantifies the benefit of the good drained system with respect to undrained or poor drainage system.
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