Hydrologic analysis and hydraulics design of surface drainage for stormwater: A neglect of vital aspect

The importance of constructing drainage structures for the conveyance of direct runoff to suitable locations has long been identified by man. However, it has been observed that most constructed drainages did not serve the intended purpose during the design period despite being structurally sound with proper maintenance culture. This has been attributed to inadequate or lack of hydrologic analysis and hydraulics design before construction, as most design only consider the structural aspect. Hence, this work considered the hydrologic and hydraulics design of the drainage system of a proposed paved/tarred road, in line with engineering standards. This was achieved by incorporating environmental parameters such as digital elevation model (DEM) and land use-land cover map as well as hydrologic parameters such as composite runoff coefficient, rainfall intensity-duration-frequency (IDF) curves, concentration time and drainage area, in obtaining the expected surface runoff from the Rational Formula. Thereafter, the hydraulics design was done using the well-known Manning’s Equation in accordance with most or best economic section. It was revealed that the catchment area capable of draining direct runoff into the drainage channels is 0.0874km2 while the composite runoff coefficient and concentration time were 0.57 and approximately 120minutes respectively. It was also revealed that the 2.9km drainage on either side of the road will have a channel width of 510mm, flow depth of 255mm in addition to a freeboard space of 51mm and a bed slope of 0.01. The hydraulics designs were checked and it was observed that the design permits a flow velocity of 1.69m/s, which is sufficient for self-cleansing velocity without scouring of the bed and side walls. The engineering drawings of the designed channels were also provided and necessary recommendations were made.