American Journal of Geographic Information System 2014, 3(1): 1-9 DOI: 10.5923/j.ajgis.20140301.01 Delineation and Characterization of Sub-catchments of Owerri, South East Nigeria, Using GIS Akajiaku C. Chukwuocha1,*, Joel I. Igbokwe2 1Department of Surveying and Geoinformatics, Federal University of Technology, Owerri, Nigeria 2Department of Surveying and Geoinformatics, Nnamdi Azikiwe University Awka, Nigeria Abstract Catchment delineation and characterization are gaining increasing global attention as scientists seek better understanding of how runoff interacts with the landscape in the face of increasing flood devastations across the globe. All surface water flow systems occur in units of sub-catchments, the basic unit of landscape that drains its runoff through the same outlet to contribute to the main stream of the overall catchment. The delineation and characterization of sub-catchments would provide some basic data required for flood prediction, drainage design, water quality studies, erosion data, and sediment transport among others. In this study, Geographic Information Systems (GIS) were used to create a Digital Elevation Model (DEM) of Owerri, South East Nigeria. The DEM was validated using Global Navigational Satellite Systems (GNSS) surveys. The DEM was processed through a number of steps in GIS to determine drainage routes with a minimum accumulation threshold. All cells that contribute into each stream were dissolved into a single unit of sub-catchment polygon and delineated. Characteristics of the sub-catchments including the average slope, the longest flow distance, the area and the centroid coordinates required for input in the Storm Water Management Model of the Environmental Protection Agency of U.S.A. were determined. Keywords Digital Elevation Model (DEM), Sub-catchments, Drainage routes, Delineation, Characterization flooding has not abated. Delineating and characterizing the 1. Introduction sub-catchments of Owerri would be a very important step in Runoff prediction and control are gaining global attention understanding and managing flooding in the area. due to increasing flooding resulting from more tempestuous The reasons for delineating and characterizing the climate events owing to global warming. As atmospheric sub-catchments of an area include the following among temperatures nudge higher, the scientific world is responding others. One, delineating and characterizing sub-catchments with efforts at learning more of the way the morphology of provide the basis for predicting flooding, erosion and sub-catchments reacts with rain storm runoffs. sediment movements. Two, the critical characteristics of contributing area and slope are important to the Engineers Abdoulaye D. (2010) reporting for the United Nations who design the drainage systems of the urban areas. Lastly, Office for the Coordination of Humanitarian Affairs, the knowledge of flow direction at DEM cellular level is of Regional Office for West and Central Africa (UNOCHA/ great importance in environmental pollution studies when it ROWCA) writes, "Since 2007, the flood situation in West is considered that runoff flow direction and velocity are Africa is becoming more and more recurrent and the impact critical in understanding location of water borne diseases and on the population and infrastructures is becoming more pollution distribution. severe.” The aim of this research was to delineate the Owerri, capital city of Imo State, South East Nigeria has sub-catchments of Owerri urban area and to document the also had many cases of flooding in the past quarter of a important flow characteristics of each sub-catchment in a century, along with a good number of other Nigerian GIS environment. In order to achieve this, the drainage cities[2-3]. Vanguard Newspaper of Nigeria of Monday 27th routes were marked out in the GIS environment, and the June 2005 reported that, "about 3,500 families in Owerri DEM cells that contribute to each drainage route delineated municipality and its adjoining villages were at the weekend in the same polygon as a sub-catchment. The hydrological sacked by flood, which resulted from a heavy rainstorm that characteristics of the sub-catchments such as the average lasted for two days." The Newspaper put the cost of the slope, the longest flow distance, the area of each material loss at N200 Million. Over the years the perennial sub-catchment, and the centroid coordinates required as input data in the Storm Water Management Model (SWMM * Corresponding author: [email protected] (Akajiaku C. Chukwuocha) 5.0) of the Environmental Protection Agency of U.S.A., Published online at http://journal.sapub.org/ajgis were either automatically extracted or computed from the Copyright © 2014 Scientific & Academic Publishing. All Rights Reserved GIS into the attribute tables of the sub-catchments. Study area boundary <all other values> Aboh-Mbaise Ahazu Mbaise Ehime Mbano Ezinihitte Mbaise Ideato North Ideato South Ikeduru Isiala Mbano Isu Mbaitolu Mjaba Ngor-Okpala Nkwere Nwangele Obowo Oguta Ohaji/Egbema Okigwe Orlu Orsu Oru East Oru West Owerri Municipal Owerri North Owerri West Unuimo <all other values> 320.05350896 360.276453488 461.573081563 466.236801963 487.301701449 495.270605097 495.736435724 615.032625947 628.232693178 631.90240374 652.461431266 653.824198211 683.164517103 713.981928217 725.350858253 744.662830032 869.883640897 886.316975499 1157.54464801 1158.68691022 1251.65930107 1477.02156835 1942.42028488 Legend imo state LGAS Subcatchments Area . 216000 207000 198000 189000 180000 171000 162000 153000 144000 135000 .000000 .000000 .000000 .000000 .000000 .000000 .000000 .000000 .000000 .000000 .000000 .000000 558000 558000 .000000 .000000 546000 546000 Obowo Okigwe Kilometers EhimeMbano Ezinihitte Mbaise 30 .000000 .000000 Ahazu Mbaise Aboh-Mbaise 534000 534000 Unuimo IsialaMbano .000000 .000000 Ikeduru Ideato North Ngor-Okpala IdeatoSouth 522000 522000 Nwangele Nkwere Owerri North Isu Orlu .000000 .000000 0 Text 510000 510000 Mbaitolu Owerri Municipal Mjaba 1:250,000 2 Orsu Owerri West Scale: .000000 .000000 OruEast 498000 498000 15 OruWest 1.1. Study Area .000000 .000000 Ohaji/Egbema 486000 486000 Oguta 30 .000000 .000000 474000 474000 .000000 .000000 462000 462000 .000000 .000000 Akajiaku C. Akajiaku Chukwuocha C. 450000 450000 .000000 .000000 .000000 .000000 .000000 .000000 .000000 .000000 .000000 .000000 171000 216000 207000 162000 153000 144000 135000 198000 189000 180000 of Owerri, South East of East Using GIS Owerri,Nigeria, South et al. : Delineation and Characterization of Sub of and Delineation Characterization Figure 1a. Map of Nigeria with LGAs, showing Imo State South East Nigeria Figure 1b. Map of Imo State with LGAs, showing the project area - catchments catchments Figure 1c. Location Map of the Project Area in Owerri, South East Nigeria American Journal of Geographic Information System 2014, 3(1): 1-9 3 The study area is the urban area of Owerri, Imo State, the boundaries between different catchments can be South East Nigeria and its environs. Owerri is the capital city inferred.” of Imo State, South East Nigeria. Owerri with a population of Lo, C. P. and Yeung, A.K.W. (2002) writes, "The about 150,000 situates between 50 20'N, 60 55'E in the delineation of linear topographic features (e.g. river channels south-western corner and 50 34'N, 70 08'E in the and ridges) is a much researched topic area in digital terrain north-eastern corner. The old city of Owerri is bordered on modeling... At present, the most commonly used method its south by Otamiri River and on its west by Nworie stream. appears to be the so-called "hydrological approach" However with the development of the New Owerri City over proposed by Mark, D.M. (1984). In this method, the side Nwaorie stream, on the western side, the two water "drainage area" of each DEM elevation cells (i.e., the bodies now transverse the town. number of elevation cells that drain into the cell) is first determined by climbing recursively through the DEM. This 1.2. Sub-Catchment Delineation and Characterization process results in a matrix, called the "drainage area Using GIS transform”, which contains the drainage areas for all the Geographic Information Systems (GIS) are increasingly grids in the DEM. The information in the drainage area being recognized for their use in storing and analyzing the transform is then used to trace the "channel pixels," as large volumes of data necessary for managing the identified by those cells with large drainage areas. Channels environment. Paracchini, M. L., et al (undated) notes the are recursively followed upstream until there is no more need of GIS in handling the large datasets involved in point that exceeds a minimum threshold. Once the drainage monitoring the state of environment and adds that the use of network has been delineated, ridges may be delineated either GIS for handling environmental data in the European Union by gray-scale thinning of all non-channel pixels or by tracing has assumed a formal role as Member States are explicitly the boundaries of the catchment area. asked to provide data in GIS compatible format. Bertolo F. (2000) writes, "The last years saw a general 1.3. Sub-Catchment Characterization Parameters recognition of the catchment or the drainage basin as the most A sub-catchment is characterized when vital parameters of significant surface unit in environmental studies. Traditionally its morphology are determined and recorded. Sub-catchment catchment boundaries have been manually derived from