American Journal of Engineering Research (AJER) 2015 American Journal of Engineering Research (AJER) e-ISSN: 2320-0847 p-ISSN : 2320-0936 Volume-4, Issue-7, pp-103-111 www.ajer.org Research Paper Open Access www.ajer.org Page 103 Establishing the Driving Forces and Modeling of flooding in the Lafa River Basin, Accra, Ghana. Anthony Ewusi 1 , Jamel Seidu 1 , Asare Asante-Annor 1 ,Emmanuel Acquah 2 1 Geological Engineering Department, University of Mines and Technology, Ghana 2 Ghana Irrigation Development Authority, Accra, Ghana ABSTRACT : The Lafa River Basin (LRB) experiences increased frequency and magnitude of floods, causing increases in areal and depth of inundation. This translates into huge economic losses and loss of human lives. The effective management of flood disaster of the LRB is hinged on the identification of the factors responsible for the floods, modelling of the flood situations and a quantitative assessment of the hazard zones. Hydrologic Engineering Centre-Hydrologic Modeling System (HEC-HMS) and Hydrologic Engineering Centre-River Analysis System (HEC-RAS) models have been integrated with Geographical Information System (GIS) to identify the driving forces behind the floods, and assess the hazard zones of the Basin. Physiographical characteristics are responsible for the floods and analysis indicate that the LRB is a small urbanised elongated drainage basin of area 27.945 km2 with predominantly steep slopes, and impervious surface soils. The Peak flows produced by 2, 15, 25, 50, and 100 year return period storms have been found to inundate 233.88 ha, 292.92 ha, 298.36 ha, 305.00 ha, and 311.24 ha respectively. 1.68 %, 5.54 %, 5.91 %, 6.41 %, and 6.63 % respectively of total inundation areas were above inundated depth of 2 m and are located around culvert locations on the Lafa stream. Keywords-Basin, Flood plain, Hydrological analysis, Modelling, Peak flows I. INTRODUCTION Urban flooding is generally induced by torrential rains that generate excess precipitation beyond the capacity of river channels and other drainage systems. As the water rises to flood stages, areas along the middle and lower reaches of rivers are flooded resulting in negative impacts on social and economic developments. The Lafa River Basin (LRB) has consistently been affected by floods during rainy seasons (June - July and September - October). The Basin in the past 20 years has been experiencing increased frequency and magnitude of floods causing increases in areal and depth of inundation which translates to huge economic losses to the inhabitants. The Hydrological Services Department together with the Ga West Municipal Assembly had put in several mitigation measures including desilting the river channel biannually, and pulling down unauthorised buildings within the floodplains. Various portions of the river reaches have also been modified to reduce meandering to ease water flow in the river channel. However, these measures have not yielded the expected mitigation of flooding in the Basin. Increased urbanisation has also led to increasing impervious coverage increasing the flood hazard levels in the Basin. Devastating floods as a result of torrential rainstorms in October 2011 displaced four thousand, four hundred and thirteen (4,413) people from their homes, three (3) corpses were recovered, and properties worth thousands of Ghana cedis were destroyed (Anon., 2011). The spatial integration of hydrological and hydraulic models into a GIS, enables flood visualisation, animation, and flood hazard levels assessment. Maps of flood situations showing hazards levels are widely used for planning, designing, and forecasting, so that appropriate structural and non-structural measures could be established for efficient flood mitigation and control. It is also used by emergency response personnel to identify safe areas during emergency evacuation exercises. This paper therefore seeks to identify the driving forces behind the floods in the LRB using GIS technique, model the flood situations, and to assess the flood hazard zones in the Basin. These models will form the basis on which stakeholders would identify the appropriate mitigation measures for effective management of floods in the Basin.