Flooding Incidence and Drainage Network Analysis in Bonny Island, Nigeria

The incidence of occasional flooding of an estate, a tank farm in Bonny Island, Niger Delta was
investigated. The study was carried out to identify the remote causes of flooding and in turn proffer a
solution. Detail field investigation involved identification of thirty one road side drains of rectangular
cross-section; measurement of drains inverts (spot heights) at selected locations yielded estimates of
longitudinal slopes (0.000416 – 0.0074 m/m), a case of very mild slopes. The invert profiles of 15 road
side drains indicated a case of inconsistent slopes, a mix of positive and negative slopes over short
intervals, the observation accounts for siltation and ponding in the drains. The redesigns of the
existing drains were actualized via the use of MODRAIN code, based on the principle of best
hydraulic section with input data options for rectangular or trapezoidal channels; constant or variable
bottom slopes and runoff coefficient(s). A comparison of the existing and newly designed drains
with respect to cross-sectional areas confirmed that 80% of the existing drains are oversized, in
what is captioned “Bigger existing drains”. Apparently, the issue of occasional flooding of the Estate
cannot be all blamed on inadequate drain size but on existing bottom slopes (very mild slopes) of the
drains.