Impact of Dredging the Okpoka River on Coastal Infrastructure: A Case Study of the Akpajo Bridge

Abstract

The Akpajo Bridge is deteriorating by the day and steadily awaiting eventual collapse due to extensive sand excavations in river beds proximal to the bridge. Sand mining has deepened the river channel, altered the hydrodynamics of the channel flow, increased river discharge beneath the bridge, accelerated shoreline erosion, scoured the piers thereby reducing the axial pile capacities supporting the bridge and ultimately compromising the safety of the bridge. Large scale sand mining from river beds is continuing on a scale never seen before in the Niger delta, due to the necessity of reclaiming land for development purposes and to meet construction needs in the region. Regulations are weak, monitoring of sand mining is non-existent. There is also a general lack of understanding of the risks to coastal infrastructure involved with sand mining in river beds. This paper investigates the threat to the stability and safety of the Akpajo bridge caused by the extensive mining of sand in the area. It establishes through computational analysis that a minimum distance of 94m (for sand river beds) from a bridge should be observed to guaranty the safety of bridge foundation. For clay riverbeds, slightly shorter minimum distances can be considered safe. The study further shows that the capacity of sand borrowing in river channels to generate bank instability is dependent on the composition and stratigraphy beneath the river bed.