Assessment of the Hydraulic Performance of the Drainage System in Dire Dawa Town

The constructed drainage structure was expected to carry the design discharge which generated from the catchment area and dispose safely to the outlet points. The study has mainly focused to assessing current status of drainage system and evaluates the hydraulic performance of the drainage system. The rational methods have been used to calculate proposed discharge and HY-8 software used for culvert analysis. The current drainage status of the towns was poor, the road performance was reduced to deliver, the service life of the road reduced, and the cost for maintaining the road increased, the compression between the proposed and existed carrying capacity of drainage structures showed that the exist drainage structures are hydraulically insufficient. HY 8 result shows the existed culvert was hydraulically insufficient and 50-63.5% of the proposed peak discharge overtopped and flows on the road. other road drainage system, erosion and scouring prevention mechanisms and degradation


of the stream channel.
This thesis is limited to the assessment of the hydraulic performance of the drainage system in Dire Dawa Town and recommending some way of reducing or minimizing the problems which are found in this research assessment regarding the drainage system. This research gives high emphasis on water-related problems which lead to poor drainage. The work does not cover the structural analysis rather it simply identifies the shapes, size, and type of drainage required.

Research Methodology
The researcher uses both exploratory and descriptive way of expressions to reach a reasonable conclusion and achieving the research objectives.

Data sources
The researcher uses both primary and secondary data types. Under primary data sources the existed cross sectional area, width, elevations, side slope, bed slope, inlet and out let elevations, and roughness coefficient of different types of drainage systems in the town.
Under secondary data sources these researcher collects different data. From different organizations like, Metrological data, drainage master plan of the town, the land use land cover coverage's of the town, and different published and unpublished documents about drainages.

Data collection methods
For this study basically the following data collection methods have been employed: 1. Personal observations of the study area to assess the current status of the drainage system of the towns and to show the true state of the drainage system and the roads performance and Field survey of different site and collect the cross section, depths, widths slopes, elevations, etc to evaluate the hydraulic performance of the drainage system of the towns. 2. Direct interviews with the administrative body of the town sewerage and drainage system to answer the drainage coverage of the tows and to know the root cause for the occurrence of poor drainage in the towns.

Data analysis
The collected data were analyzed by using Microsoft Excel, Global Mapper, ARC-GIS 10.4, and HY-8 culvert analysis software. The ARC-GIS 10.4 and Global mapper program was used for the preparations of the study areas of sub-catchment areas, location map preparations of the study area, and land use land cover preparations. HY-8 program was used for the analysis of the existed culvert performance by iterating different discharge and shows how much of the water overtops from the culvert and directly flows on the road. Microsoft Excel program was used for the computations of proposed maximum discharges from each sub catchment areas by the rational formulae and different interviewed results has been analyzed by the excel sheets by simple mathematics. Data presentation Once the data has been analyzed the next step was presenting the data in a clear and concise manner, for that sick of matter the researcher uses figures which have been done by the global mapper and ARC-GIS 10.4 and Microsoft Excel, tables, photographs, and percentages.

The current condition of drainage system in the town
The current road coverage of the towns was 14.36% of the total area of the town (124.34 km 2 ) and most of the roads have been faced problems from the drainage system. Most of the problems which have been directly associated with the poor drainage system of the towns.
From the response of the engineers from drainage and road authority they have been formalized in the following points. The poor drainage affects the road performance every year by overspreading the floods on the road and creates inundation of the road and the nearby houses.
The poor drainage affects the road performance every year by overspreading the floods on the road and creates inundation of the road and the nearby houses.
For each rainy season the administrative body invests much money for maintenance and cleaning of ditches, but still the drainage problem reduced the service life of the road.
The impact of poor drainage condition on road pavement is very adverse. It causes pavement distresses and deterioration which affect the safety and riding quality on the pavement. The study investigated cases of pavement failures and damages due to poor drainage experienced during the rainy season Generally the effects of poor drainage system on the roads have been listed bellows:  Reducing the load carrying ability of the subgrade, sub base, asphalts and shoulder of the road.  Eroding the road side surface by washing away the top surface of the road.  Runoff the road and block a road by deposited waste material on the road and water leaved debris on the road and thaw weakening and stripping the asphalt in asphalt pavement road. The results from personal observation of the site have been figure out in pictures which existed in the figure ( fig.1 -fig.4) part bellows, and that shows the destruction of traffic flow by flooding the road, side deterioration and formation of gullies on the road sides, culverts has been filled with debris and flood overtopped on the roads etc.

Evaluating Hydraulic Performance of the Drainage System A. Comparison of proposed Peak design discharge and existed drainage structure capacity
From the table 1 which show that the proposed maximum discharge computations which indicated that column number 1 shows the sub-catchment name in the study area, the column number 2 shows that the area coverage of each sub-catchments measured by hectares, the column number 3 shows the weighted runoff coefficients of each sub-catchments, the column number 4 indicates that time of concentration from each sub-catchments, the column number 5 indicates that for each sub-catchments the intensity from ERA IDF curve measured by mm/hr for different return periods, and column number 6 indicates the computed proposed maximum discharge by rational methods(Q=0.00278CIA). From table 2 which show the existed working spaces of different types of drainage structures in different subcatchments of the study area measured using tap meters and also observing the roughness coefficients, and side and bed sloped of each drainage structures. Finally the computations of the carrying capacity of the drainage structures have been shown by manning formulae.  (1) Col (2) Col (3) Col (4) Col (5) Col (6) Catch. ID Area ( From table 3 which has shown below puts clearly the comparison between the existed drainage structures and the proposed maximum design discharge on each sub-catchment. From the table 3 column 1 indicates that the subcatchments name in the study area, the column number 2 indicates that the total discharge in each sub-catchment measured by m 3 /s, column number 3 indicates that existed drainage structures and their carrying capacity measured by m 3 /s, column number 4 indicates that the propped maximum design discharge for both the design and check periods measured by m 3 /s and finally column number 5 indicates the amount of flood which overtops from the existed drainage structures(col(4) for both design and check -col(3)) measured by m 3 /s.  finally at sub-catchment elven (S.CM_11), 80.69% for design and 84.32 % for checks has been overtopped directly on the surface of the roads. The over topped discharge directly affects the roads performance and creates an environmental problem.    Overtopping From the tables 5-table 8 the result shows that there is flooding or overtopping from the culverts at each maximum floods which have a magnitude of greater than 8.41 m 3 /s for the culverts one at station sub-catchment six(S.CM-6) and 7.21 m 3 /s for the culvert two at station sub-catchment eight (S.CM-8). 13.78 Depending on the observed difference between the proposed design discharge and the existed carrying capacity of the culverts the structure is hydraulically insufficient both for the design and check of proposed discharges. For culvert one the from the proposed design discharge, 35.8 % for the 25 year return period and 57.1% for the 50 years check the flood flows out off the culvert and directly flows on the road and sometimes causes flooding the nearby community houses. For culvert two depending on the proposed design discharge, 31.1% for the 25 year return period and 76.13% for the 50 year return period for check this much flood flows out of the culverts.

Conclusion
After careful assessments of the effects of the poor drainage system on the road performance, the following main results had observed and described as below.
From personal observation of the site the drainage system of the town was highly used the rectangular open channel without covering the top part of the channels this leads the channel to filled with wastes, commercial residues, and the community itself use the channel as a means of the trade area. Those activate create reducing the size of the drainage structure and immediately it creates an overtopping of channels and flow on the roads also some portion of the drainage blocked with waste crates water stagnation for long period of times.
From hydraulic performance assessment of the existed drainage structures almost all the drainage systems design is hydraulically insufficient means that the proposed design discharge for both the design and check is much greater than the existed drainages structure working space (hydraulic capacity), this happens due to less involvement of professionals ( hydraulic engineer, hydrologist), future projection of the design structures was not considered the land use land cover change highly affects the peak discharge but not highly considered in the design stage, the communities awareness about the drainage structures was not good they did not consider the drainage system as a means of disposing stormwater rather they think of that as solid waste material disposal means.
Still, areas without drainage channels construction exist and the flow from those places create an overflow on the road and deteriorates the side of the roads highly this happens due to poor drainage alignment system with the roads.
From HY 8 culvert analysis the existed culverts have encountered an overtopping problem and the active working space of the culvert was insufficient to carry and bypass the coming proposed peak discharges.