Investigation Into The Causes Of Blocked Drainage, Canal And Culvert In Ilorin-west Local Government Area
[A CASE STUDIES OF AGBO-OBA, OJA-TUNTUN AND SAWMILL]

CHAPTER TWO
2.0      LITERATURE REVIEW   
2.1     INTRODUTION
Flooding is one of the most costly of natural disasters in Nigeria, as our population growth continues, and as we in turn urbanize and modify catchments, we have a need to create engineering structures in our catchments, flood plains and watercourses. These structures include pipes and open channel drainage networks, flood control systems and waterway crossings.
The city Engineers lamented that blockage in drainage; canal and culvert remain major problem in some communities in Ilorin west local government, because of indiscipline on the part of residents. This includes the blockage of roads, canals and waterways by building materials and general lack of appreciation of the harm that these materials can cause to the drainage system.
Drainage, canal and culvert involves providing facilities that collect, transport and remove water from the road. If blocked become harmful to man and his physical environment, thus. The design must also consider the water reaching the roadway embankment through natural stream flow or manmade ditches (Shahin etal, 1984).
    Proper road drainage, canal and culvert are absolutely critical if w e expect roads to stand up to the damaging effects of weather and traffic. For long term non deteriorating, roads cannot be built without providing good drainage. However not all water can be termed to be bad for the road (UNH, 2009).
         On 17th August, 2005 a severe storm in the city of Ilorin caused extensive flooding, with considerable change to property, disruption of services and the loss of life. Inspection of the flooded waterways in the days following the storm revealed that a very large number of culverts and bridges have been blocked by debris. As a result of these blockages ,flood levels upstream of many roads and rail crossing increased and flood waters were  diverted out of the normal stream channels into overland flow path, increasing the extent of flood damage.
              The experience from this flood indicates that there is a high probability that drainage, canal and culvert openings wall will block during large storm events. This has significant implications for urban storm water management practice, as a result of this flood. Ilorin west local government council has revised its hydraulic structure design code to include the possibility of structure blockage by debris (Durotoye J.O., 2005).
2.2    BLOCKAGE ISSUE
Data on blockages is very difficult to obtain and there are several reasons for this. Blockage, even at a single location, can vary greatly from event to event. Blockage may result from the accumulation of floating and/or non-floating debris and this debris can collect across the inlet, within the barrel or at the outlet (if screened). During a single event, the degree of blockage may vary from a single piece of floating debris jammed across the inlet of a structure to a fully blocked structure. Even when apparently fully blocked, the debris may be relatively porous providing some residual flow capacity. The timing of the blockage can also be highly variable, with debris arriving progressively or rapidly in a pulse. Blockages involving floating debris may arrive or build up as a floating raft that rises with the floodwater and is only deposited over the inlet as floodwater recedes. Blockages involving deposition of large quantities of non-floating natural material such as sediment will typically transport most debris at the peak of the flow, but deposition will typically be greatest on the flood recession as velocities drop. Irrespective of the debris type, it is difficult to infer the extent and impact of blockage during the flood or at the flood peak from what remains and is observed after the event. When the structure affected by blockage is owned by government agency or other large organisation, there is often a significant separation between the designer, asset management and maintenance personnel. This means that designers may not be aware of practical problems related to blockage or other issues that may arise during operation or an asset. This is a particular issue for government agencies which own significant assets. However it also occurs with privately developed infrastructure, such as culverts in a large subdivision. Councils, which are ultimately responsible for the asset, are strict in the assessment of design assumptions. The sensitivity of the hydraulic capacity of structures to variations in the blockage mechanism (location, type, timing, porosity) was explored by Rigby and Barthelmess (2011) in a theoretical assessment using hydraulic modeling. This work demonstrated that major changes in flood levels and discharges can occur at a structure, depending on the blockage mechanism triggered.