Performance Evaluation of Drainage System In Shire Endaslasse Town, Northern Ethiopia

Storm water drainage problem is one of the major challenges facing in Shire Endaslasse town, Ethiopia. In a town, Street flooding and over topping drainage system problems are occurring at the rainy season. This causes ponding which poses difficulties in ease of transportation and it hinders the day to day activity of the people. So, the study focuses on the performance of storm water drainage system in Shire Endaslasse town using Arc GIS and SWMM5.1. For this study, the primary data were collected by field survey and interview with the council body. Simulation results for storm events show that in some of the drainages systems in different regions of Shire Endaslasse town have flooded. During the field observation, the drainage structures are filled with solid wastes, in adequate inlet and outlet structures and some of the top element of the manhole has been broken this may causes a problem of aesthetic and healthy at large it may increase flood risk. The flooding risk in the drainage systems is very high due to the drainage system is undersized to cope with the current rainfall rates, but also is very limited to face the upcoming predicted rainfall.


Introduction.
Across the world, flood is amongst the most devastating natural disaster, which caused loss of human lives and threats to infrastructure, the environment, and public services [1][2][3].The hydrological balance is affected by factors as the rapid growth of settlements and irregular urbanization on population increases and industrialization in developing countries, particularly in Ethiopia [4,5].The evolution of land use is much related to urban development and increment of flood derived from it due to increasing proportion of the total land areas becomes covered with impermeable surface and decreasing the infiltration of water to the ground. Urbanization covers a large part of the ground with roofs, roads, and pavements and increases the rate of runoff. As a result of rapid urbanization and population growth, many towns are becoming hotspots for risk and disaster in Ethiopia [6].In the world, particularly in Ethiopia, the flood management is based on the concept of risk analysis with the likelihood of flooding assessed and consequences assessed regarding to damage and the magnitude of consequences [7].Urbanization enhances the frequency of highly flowing discharge of precipitation via conveyance systems that disregard soil moisture replenishment and ground water recharge [8].The runoff amount in urbanization is highly increased due to increase impermeable, resulting in overwhelming of designed capacity of the storm water drainage systems [9,10].This is because all these phenomena has been happened flooding even small amount of rainfall from altered catchment of urban areas. Because of increasing in population density and development of urban infrastructure without paying due consideration to drainage aspects and increased in paved surface is the main causes of urban flooding [11,12].As a result ,floods have sever on the storm drains, deterioration of roads and blockages of storm drain due to solid waste, which increases the flood peaks and flood volumes [13,14].To encounter these adverse urban water problems has habitually consisted of urban drainage infrastructures to minimize runoff accumulation. The urban drainage system is responsible for quickly removing storm water runoff through the drainage networks formed of a series of pipe and manholes connected to each other to the outlet point. However, these drainage networks do not have sufficient capacity to properly dispose the inflows, which may result flooding and surcharges along the network [8,15].To evaluate floods of urban areas and checking performance of urban drainage systems, Engineers have been developed Hydrologic-Hydraulic models used to rainfall-runoff simulation process, and flood routing in the storm water network.
Modeling of the urban flood of the drainage systems can be adopted by various software like SWMM, MIKE URBAN, HEC-HMS, HEC-RAS and RAFTS (runoff analysis and flow training simulation [12,13,16]. EP-SWMM is one of the most complete and widely used models throughout the world for planning, analysis, and design related to drainage systems with in urban areas. SWMM is more benefits than other urban watershed models because it used in large urban catchment and it demonstrates on drainage systems for storm water runoff and waste water management, considering both combined and sanitary sewer design and performance [8,14,[17][18][19][20].Most models produces gross errors in simulating complex hydraulic and hydrologic systems due to range of practical issues. Mathematical models are a combination of runoff generation model and runoff model. The runoff produced software changes rainfall to surface runoff and translates in to the catchment outlet [21][22][23]. Due to availability and wider adopted used in the world to model the urban flood, SWMM is used to check the performance of the drainage systems for this investigation. Shire Endaslasse town is similar to many Ethiopian towns; rapidly expansion on the surface has not been accompanied by suitable expansion and upgrades of the storm urban drainage networks. As the town has expanded, the current network has become unsuitable, leading to more frequency surface water flooding generated from rainfall. Therefore, Shire Endaslasse town usually suffer from urban storm water drainage management problem such as overtopping and flooding the drainage network as shown in figure 1 and the existed urban storm water drainage infrastructure has not been constructed and provided at a city wide network basis and the inlet as well as outlet structures have constructed with no due consideration to the existing drainage system. Hence, there is a need of studying the performance evaluation of drainage system in Shire Endaslasse town, northern Ethiopia.

Study area Description
The town of Shire Endaslasse is located in northern part of Ethiopia. Its administrative location is Tigray National Regional State, North western zone. While, its absolute geographic coordinates are 422795m E and 1559778m N, the average elevation of the town is approximately 1923 m.a.s.l. The town is found about 1106kms faraway from Addis Ababa via Mekelle-Adigrat through Amhara region way and about 300kms from Mekelle, regional capital, to Northwest of the region. The town occupies a total area of 1512.5 hectare's.  Temperature data were obtained from the nearby Shire Endaslasse station that is located at an average distance of 5 km from the area. The mean minimum monthly temperature is expressed between 13.1℃ in September and 18.7℃ in May and the mean maximum monthly temperature range from 19℃ to 32 ℃ in May. Mean annual rainfall of the town is estimated to be 600mm that about 80% of the rainfall is received from mid-June to mid-September with little rain from December to January. The highest rainfall is usually received in July to August where it goes on decreasing to the month of September.

Surveying Data collection
In this study, a general field survey of the existing condition of the drainage structures was employed. For the hydraulic analysis of the drainage system of shire Endaslasse town, have collected survey data of the junction or nodes such as Invert elevation of for each junction, coordinates and segment of the drainage was taking as shown figure 4.

Rainfall Data
Rainfall data were obtained from Ethiopian Meteorological Services Agency which is nearby Shire Endaslasse station located at an average distance of 5 km from the area. Available rainfall data on this station has been collected and analyzed in order to prepare the necessary depth or intensity input data for peak discharges computation. For this study, 38 year historic observation data is used to calculate the peak drainage storm water of the Shire Endaslasse town. Due to shortage of recorded rainfall data, it is difficult to develop the IDF curve for this study.
Therefore, the Ethiopian road authority developed IDF is adopted for this study.

Urban storm water drainage lines
There are two types of urban drainage lines in Shire Endaslasse town. Namely: concrete and masonry drains, which are constructed by the municipality and Ethiopian Roads Authority in order to safely discharge the flood generated within the town in the process of urbanization. But, as observed during data collection through field survey with the help of road net-work, these drains becomes causes of flooding by collecting rain water from various parts of the

Land use And Land cover
The land use land cover is extracted from the master plan of Shire Endaslasse town and categorized of different land use with ArcGIS analysis and where a land use layer displaying commercial, residential, green, service, road, administrative and industry areas considered calculating the maximum flood. Land use have a direct impact on the flood amount, speed and potential to create damage that is why the study gives notice for land use and land cover of the catchment.

Sub catchment.
Based on urban drainage, the sub-catchment was identified. The study area was divided into sub catchment based on Topographic map, building blocks, direction of flow in drainage.61 sub catchment were determined in figure (6).  Different case scenarios have been considered in this study to obtain a fully understanding of the system performance under multiple working conditions. Firstly, the model had been run with the continuous rainfall events to analyze the current performance, as shown in Table 1. Moreover, different local improvements in the system nearby the properties which have been flooded in previous years were modeled and analyzed. The above table 1 showed the calculation of peak runoff for the sub catchments which are the most runoff producers. Those subcatchments are the main source of the runoff to become the drainage network flooded.

Network simulation
The drainage systems modeled to cope with a 25years return period rainfall in terms of water level below the surface in drainage systems. This implies that the flooding risk must be verified in the nodes (manholes) in the systems, whereas water level at each manhole is checked independently with a longitudinal profile in drainage systems which are connected. The general network performance is determined by maximum water flow production. The water elevation profile in the manhole was over flooded as shown in the figure 8, and 9.   As is seen in the above figure 10, this area had a severe water gathering in the street and it flooded to nearby houses. In this area, 30 commercial buildings and 10 residential houses suffered from flooding. When concluded that, the flooding is much due to undersized of the drainage pipe and slopes of the consecutive nodes and due to unmanageable entrance of solid waste materials to the manhole. The water profile plot and tabulated is obtained for the junction in between J17 and J149 to outlet 10 as shown in the above table 3. The simulation status shows that sections between these junctions are the most surcharged (flooded).

Conclusions
This study evaluated the performance of storm water drainage system of Shire Endaslasse town and peak runoff in all sub catchments and outfalls. The hydrology was simulated using EPA SWMM5 for all sub catchments of the drainage system of Shire Endaslasse town. According to simulation some of the modeled drainage systems on the drainages system are flooding. And also total average flow 29.406 m 3 /s and total maximum peak flow to outfall 49.76m 3 /s were occurred from all 61 sub catchment .The flooding risk in the drainage systems is very high due to the drainage system is undersized to cope with the current rainfall rates, but also is very limited to face the upcoming predicted rainfall. The main road is a commercial land use type and produces more solid waste materials to each near manholes. The cause for flooding problem at the study area are construction and/or design problems and drainage poor management.

Declarations Authors Contribution Statement
HGG. Conceptualized the research, performed the formal analysis, and wrote the paper under the supervision of AM. All authors revised the paper and agreed on the final version of the paper.

Funding Statement
Funding was provided by Aksum University.