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]

Estimating the impacts of blockages in a design event requires consideration of the following issues on a site-by-site basis for different storm probabilities or average recurrence intervals:
(a) The quantity and type of debris that would reach the structure in the design event.
(b)  The type, location, porosity and timing of a blockage at the structure, and the likely extent and coincidence of such blockages across the catchment.
(c) The impacts of likely blockage mechanisms on the hydraulic behavior of the structure, particularly in regard to changes in flood levels and discharges both at the structure and in adjacent water courses if blockage could cause or modify flow diversions.
 (d) The impacts of likely blockage on the catchment, community assets, and public safety.
(e) The environmental impact of blockage, such as interference to fish passage.
When considering the likely impacts of debris blockage on major waterway structures such as bridges, canal and larger culverts, appropriate analysis should also be made of the following matters:
(i) The consequences of blockages in excess of the adopted level.
(ii) The consequences of flow in excess of the adopted design discharge.
(iii) The likelihood and consequences of damage to the structure as a result of blockage.
2.6.2      Hydraulic Impacts
Waterway structures are designed to convey water at both high and low flows and the blockage of these structures can significantly reduce their capacity. The most common impact of this reduced capacity is an increase in flood levels upstream of the structure, which may cause anything from nuisance flooding to the inundation of public and private structures. If the blockage affects low flows, the increase in water level may be relatively small, but water will tend to pond for long periods, causing environmental concerns. At high flows, a partial blockage of the structure can cause increased flow velocities leading to increased channel erosion and/or damage to the structure or adjacent infrastructure. Such conditions can also increase the hazard to pedestrians and traffic as a result of increased overtopping flow depths and velocities.
2.6.3     Economic Impacts
Financial impacts are usually very important and can usually be measured more accurately than many of the other impacts. These impacts include direct physical damages as well as indirect costs such as those associated with traffic delays. Even if there is no clear physical damage, the indirect costs associated with traffic inconveniences or nuisance flooding may be significant.
2.6.4     Social Impacts
It is noted that whilst floods may not be the most costly in terms of financial loss, they are the most costly in terms of loss of life and injury. There are also social impacts, including the stress suffered by residents and businesses affected by debris-induced flooding. The blockage of storm water inlets causes pounding on roads potentially resulting in traffic delays as well as inconvenience to pedestrians.
2.6.5      Environmental Impacts
Blockages and resulting changes in flow patterns can affect the environment in one of several ways. Blockages may cause pounding along flow paths close to the inlets of drainage systems or culverts. This pounding may persist for long periods causing health concerns such as those arising from breeding of mosquito and biting insects.