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]

2.7    EFFECTS OF BLOCKAGE
2.7.1     Overview
These are the effect blocked drainage, canal and culvert causes to man and his physical environment. Once a structure blocks, flood peak discharges in the downstream channel will generally decrease, although the effect may be minor in many cases, and in some cases can lead to a slight increase. When the culvert blocks, water levels upstream of the structure rise. If the embankment level does not greatly exceed the channel bed level, floodwater will flow over the embankment, which acts as a weir. The reduced throttling effect of the weir, relative to the smaller opening of the culvert, together with the small storage volumes in the upstream channel, will give only a small reduction in peak discharge. If the embankment level is considerably higher than the channel bed, upstream flood levels will rise but may not overtop the embankment.
In most cases the bank elevations of the upstream channel are lower than the road or rail embankment. Thus culvert blockage will cause diversion of flows out of the channel to other points on the catchment. If the water is diverted into an adjacent stream, flood peaks in the immediately downstream channel can be significantly reduced, but at the expense of increased flooding problems in other parts of the catchments. Alternatively, the diverted floodwater may rejoin the same stream at some point lower in the catchment, increasing flood damage en-route. In both cases, there is considerable potential for damage to property from flooding in overland flow paths particularly if such flow is unexpected.
2.7.2     Consequences of blockages on flooding
Most instances of blockage occur where a stream is carried under a road or rail embankment by a culvert or a bridge opening. Blockage of the structure has four consequences for flooding in the catchment:
• Flood levels upstream of the structure will always increase.
• Flood peak discharges in the channel downstream of the structure may change, due to the flood routing effect of the water stored upstream of the structure.
• The increased upstream flood levels may cause some of the floodwater to be diverted out of the stream to some other part of the catchment.
• Scouring of the road or rail embankment by the overtopping flows may cause it to collapse, releasing a surge of water into the downstream channel.
2.8    C0NTR0L MEASURE
*Removal of illegal structure along the drainage channel.
* Prohibition of dumping refuses along drainage channel.
* Provision and maintenance of adequate capacity of dumping site.
*Environmental enforcement programme.
*Construction of embankment to raise river bank.
*Recycling