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]

(b) Drainage system inlets and pipes:  This includes components of urban drainage systems located within road reserves and urban overland flow paths. Frequently blockage in this type of system is generally less likely to cause the same extent of damage associated with blockage of bridges and culverts, but the consequences can still be serious from a traffic and safety perspective, and can cause serious inconvenience and nuisance. However in certain circumstances, in densely developed urban areas, pit blockage can cause significant monetary damage due to flooding of buildings upstream.
(c) Canal and waterways:  Blockage of natural and constructed waterways can occur at any location, typically as a result of large debris snagged against bank vegetation, or debris passing slowly down the channel. The consequences of such blockage are increased flood levels, diversion of surface flows and the possible relocation of the waterway channel as a result of severe bank erosion.
2.5    CAUSES OF BLOCKAGE
2.5.1     Overview
 Illegal refuse dumping in the world at large has become a huge problem and a menace in the society today. International, National and Local waste management agencies have invested great effort in combating illegal refuse dumping in the society, but very little or no result have been achieved. Illegal dumping of refuse is found among all kind of environment, both in urban and rural areas. Its consequences cannot be over emphasized. Land, abandoned buildings and gullies have been converted into refuse dumps sites by dwellers. These activities have lead to environmental degradation, air pollution, land pollution and poor economic and social environment. People living close to illegal dump sites suffer as a result of these problems. With regards to all the above mentioned, the researcher was inspired to carry out an investigation into the cause of blocked drainage, canal and culvert in Ilorin West Local Government Area, Kwara State.
2.5.2     Modes of blockage
Four modes of blockage were identified:
(a) Progressive build up of sediment, scoured from upstream bed and banks, in the barrel of the culvert. This blockage typically developed from the bottom up, and in cases of partial blockage, only the lower part of the culvert was affected.
(b) Initial blockage by large items of floating vegetation, such as trees or parts of trees. This material then provides support for smaller vegetation, such as shrubs and grasses. Vegetation debris came from collapsing banks or adjacent overbank areas, and typically blocked the culvert from the top down.
(c) More abrupt blockage by urban materials, including refuse, building materials, fences and sheds, which are swept into streams by overland flows and by streams breaking their banks.
(d) Less common circumstances where a large item, such as a motor vehicle or shipping container suddenly and totally blocks the culvert opening. Larger items such as the shipping container are even able to block the larger openings of bridges.
2.6     IMPACTS OF BLOCKAGE
2.6.1     Overview
 Blockages cause changes to water level, flow direction and velocity so therefore cause damage to the affected hydraulic structure, but more importantly, blockages can often cause damage to separate but nearby public and private assets, as well as increased safety risks to both the public and emergency and maintenance personnel.