UYO URBAN DRAINAGE SYSTEM AND FLOOD VULNERABILITY MODELING WITH GIS

BY

SURV.O. ADENIRANPrincipal Lecturer,

Department of Geoinformatics,Federal School Of Surveying,

P.M.B. 1024, oyo, Nigeria,E-mail: woleadeniran2000@yahoo.com

AND

Mr. R. I. OLABANJOSnr. Project Surveyor,

Etteh Aro & Partners (Consulting Engineers),148, Oron Road, Uyo, Akwa Ibom State.

E-mail: robertolabanjo@yahoo.com

A PAPER PRESENTED AT THE 4TH ACADEMIC STAFF UNION OF POLYTECHNICS (ASUP) ANNUAL CONFERENCE AT

IKOT EKPENE, AKWA IBOM STATEFROM 23rd - 27th November, 2004

UYO URBAN DRAINAGE SYSTEM AND FLOOD VULNERABILITY MODELING WITH GIS

ABSTRACTMost hydrological and environment problems do have an obvious spatial dimension and within the domain of modeling, this is increasingly being addressed by partially distributed models in part made possible by the rapid development of computer technology. The ability of Geospatial Information System (GIS) to integrate spatial and attribute data from different sources with different formats, structures, projections, or levels of resolution is a powerful aid to spatially distributed models. This study is an attempt to demonstrate the usefulness of GIS technology in urban flood management. It involves integrating hydrologic and hydraulic data with other terrain data, subjecting them to spatial analysis in a GIS environment to form a comprehensive Drainage Information System and other Digital map Information of Uyo, the capital of Akwa Ibom State of Nigeria. Urban floods are experienced in most coastal or near-coastal cities of Nigeria due to their terrain characteristics and human impact on the environment. In such areas, Uyo being a good example, there is the need for monitoring and mapping of progression of flood vulnerability from root causes to unsafe conditions. Determining an urban environment’s flood risk vulnerability requires the application of GIS which is a system of hardware, software, and procedures design to support the capture, management, manipulation, analysis and display of spatially referenced data for solving complex planning and management problems. Appropriate geospatial data coupled with modeling application information in generating flood risk vulnerability map (areas of flood susceptibility) is also presented in the study. This serves as an early warning system (EWS) and also goes a long way in helping decision managers, Environmental Disaster Administrators and Politicians at Federal, State and Local levels in Introducing vulnerability reduction criteria in every activity in order to reduce flooding risk and promoting sustainable environmental management.

INTRODUCTION

Environmental issues, a good example of urban flood in many developing countries,

are among the most important facing decision makers. The issue of making optional

planning and policy decisions in flood management is one that has received continued

attention in recent gears and one main focus in attempts to deal with floods has been

to try to obtain a better understanding of the cause- effect components of the physical

process. (Simonovic, 1993).

The dynamics of hydrologic, hydraulic and human impacts (a consequence of human-

environmental interaction as depicted for example, by constructions (building) on

vulnerable areas of urban river flood plain, inadequate waste disposal management

practices, etc) imply that all environmental systems are tightly related dynamically

and spatially. Most hydrological and environmental problems do have an obvious

spatial dimension and within the domain of modeling, this is increasingly being

addressed by spatially distributed models. Elgy (1993) expressed that well established

drainage models are generally adequate for design and management of urban

drainage, but there is however a problem with the acquisition, manipulation and

storage of data for the models. Geospatial Information Systems (GIS) for managing

the data and associated analytical techniques for converting the data into information

are now vital tools in the assessment and management of healthy environment.

GIS is a system of hardware, software and procedure designed to support the capture,

management, manipulation, analysis and display of spatially referenced data for

solving complex planning and management problems. GIS has also been defined

according to Sowton (1991) as a system which allows the capture, updating and

display of a number of previously unconnected data sets, bringing them into a

common reference system for spatial analysis from which relationships can be

identified and decisions made. Goodchild (1993) listed the most frequently heard

justification among others for the use of GIS particularly in environmental modeling

and policy development as:

- The ability to preprocess data from large stores into a form suitable for

analysis including such operations as reformatting, change of projectors,

resembling and generalization

- Direct support for modeling such that forms of analysis, calibration of

models, foresting and prediction are all handled through instruction to the

GIS

- Post processing of results, including such operations as reformatting,

tabulation, report generation and mapping.

The ability of GIS to integrate spatial and attribute data from different sources with

different formats, structure, projections or levels of resolution is a powerful aid to

spatially distributed models. The usefulness of GIS technology in urban flood

management involves the integration of hydrologic, hydraulic with other terrain data

subjecting them to spatial analysis in a GIS environment to develop comprehensive

Drainage Information System (DIS): A good DIS serves as a decision Support for

urban flood management.

Urban infrastructural developments and other human impacts may lead to an increase

in vulnerability of urban river flood plains, and coastal cities with relatively flat

topography. There is the need for flood vulnerability assessment and monitoring

which could be achieved by applying GIS technology and this serves as early warning

process of hazard information production with prediction capacity being the main

objective but current visions and strategies are constantly evolving into risk

management concepts. Risk is inherent in development and it is the result of social

dynamics of human kind in action at local, national, regional and global scales. A

state attributable to a risk – or a series of risks – that has not been well-managed

results into a disaster. Development without considering risk management cannot be

sustainable and geospatial information is an important instrument for instable

developmental designating the years from 1990 to 2000 as International Decade for

National Disaster Reduction (IDNDR) defined the following three targets for all

countries to have in place:

(i) Assessment of risks posed by environmental hazards

(ii) Long-term preparedness and prevention plan and

(iii) Warning system

Awake! (July 22, 1995)

Uyo urban the area of study lies between Longitude 7o 471 E to 8o 021 east of

Greenwich meridian and between latitude 4o 581 and 5o 081 North of the Equator, is

the capital and commercial center of Akwa Ibom State of Nigeria. Uyo Urban is the

entire mass enclosed within from ring roads, that is ring roads 1, 2, 3 and the outer

ring road, and area approximately 19,000 hectares (Nya and Ekong, 2003)

The nature of Uyo topography is generally flat with isolated troughs that get flooded

whenever there is an appreciable amount of rainfall. Dumping of refuse in drains and

lack of maintenance (desilting, repairs etc) of these drains have resulted in most of the

drains being taken over by silt, debris refuse, etc. forcing inundated water to create

and find alternative channels. Increased physical developments (construction of

pavements roads and other infrastructures) have led to reduction of runoff percolating

into the ground and this attributes to the drainage problem. The non-usage of an

integrated terrain model, designing engineers who often packed heights in piece meal

also contribute to the drainage problem. This problem has persisted because there is

no relevant information about the drainage system (both natural and designed) of

Uyo. The present study is therefore necessary to develop Drainage Information

System and flood risk vulnerability information for Uyo metropolis which will go a

long way in maintenance and management of drainage system and prevention of

floods

MATERIALS AND METHOD

DATA REQUIREMENTS

Data could be grouped into primary and secondary sets. Primary data include

coordinated points obtained using Global positioning System (GPS), which were

subsequently used as ground control points to reference land use map and or the

photomap of Uyo. Also attributes were collected through social survey.

SECONDARY DATA SETS INCLUDE:

- Othophoto map of Uyo urban at a scale of 1:7,000 by Aerial sat (Nig0 Ltd,

Uyo

- Digital center map of Uyo at scale 1:7,000 by aerial Sat (Nig) Ltd, Uyo

- Soft copy of Uyo urban drainage and road map by Etteh Aro and partners,

Uyo (acquisition scale at 1:7000)

- Rainfall date of Uyo from university of Uyo metrological center

- Guide map of Uyo capital city by Akwa Ibon State by Ministry land and

housing, Uyo

- Run-off calculators for the drains from Etteh aro and partners, Uyo

HARDWARE AND SOFWARE REQUIREMENT

The hardware specifications used for the project are:

(i) A3 scanner

(ii) Pentium

(iii) 256 MB RAM space

(iv) PC

(v) 1.7 GHz clock speed

(vi) A3 printer (HP desk Jet 1180c)

(vii) 21’’ monitor

(viii) Keyboard

The following software are used in the execution of the project

(i) GEOCAL

(ii) Civil/Survey soft desk

(iii) AutoCAD Map R2

(iv) Arc view 3.2a

DATA BASE DESIGN AND CREATION

A database is the heart of GIS. It is an organized, integrate collection of non-

redundant data stored so as to be capable of use by relevant applications with data

being assessed by different logical paths. The process involved in integration of

databases of this study are summarized in figure below

Fig – Stages in Data Base Design (source: after Zeiler, 1999)

View of reality is the mental abstraction of the objects in the study area, as they

actually exist. The roads, the drainage route, building etc.,as they exist in real life.

Perceived reality is for example the causes and extent of flood, the worth of

destruction etc.

The conceptual design is a concise description of data requirements of the users of the

users and includes detail description of data types, relationships and constructions;

these are expressed using the concepts provided by the high-level data model

The Logical design is the process by which conceptual design is transformed from

high level data model into implemental data model and its result is a data base scheme

in the complementation data model of DRMS

Physical design is the representation of internal storage structures and file

organizations for the database in the format of the implementation software for the study

SPATIAL ANALYTICAL OPERATIONS

- DATABASE DRAINAGE NETWORK

The drainage and the natural drainage network database were created and

spatial queries carried out using Arc view 3.2a

- TOPOGRAPHICAL INFORMATION

The topographic information for landscape knowledge is essential database

to be included in the structure of a drainage information system. Using the

3D analyst extension of in Arc view 3.2a, the 5m contour map was used to

develop a Triangulated Irregular Networks (TIN) model of the study area.

A TIN model is preferred than a raster DEM because its representation is

particularly suitable for depicting fluvial landscapes. Sinks were identified;

slope and aspect maps were created from the TIN model. The slope affects

the overall rate of movement down slope, while aspect defines the

direction of flow.

Hydrological modeling involves the use of TIN to generate flow direction

there, flow length theme, and flow accumulation theme. The following

hydrological characteristics used in modeling Uyo drainage information

system were developed:

(i) Flow direction determined by finding the direction of steepest

descent from each cell using the slope map and is calculated as:

drop = change in z value / distance x 100

Flow accumulation calculates the accumulated flow, or a number of up-

slope cells based on a flow direction grid

Flow length calculates the length of flow path for each cell to the pour

point for each water shape

Vulenerability maps were produced by combining thematic maps

showing human impacts, e.g. waste disposal sites, pavements, etc. with

flow accumulation maps to produce hazards maps

Some hydrologic characteristics were employed in calculation of run-off

for various catchments (see the table below) using appropriate equations.

Average coefficient of run-off 0.53

Concentration time for pavements

and land

I9.26

Average rainfall intensity (ms) 3.4

(Source: Etteh Aro and Partners, Uyo)

The information about run-off obtained for various catchments and

hydrologic model (flow accumulation, flow length, flow direction) were

used in locating their appropriate drainage structures which composed of

sub-surface conduit channels, surface channels, catch pits, sand pills and

out falls.

DICUSSION OF RESULTS

The drainage database created and other topographic, hydrologic,

hydraulic information and flood risk vulnerability maps all form the Uyo

urban drainage Information System.

The TIN developed is a common basis for finite element solution of water

and transport problems. The hydrologic and hydraulic information

obtained go around a long way in understanding the basic process in an

open – channel and sub-surface-channel flow

CONCLUSION

The management of Uyo Urban flood being a spatial problem, is complex,

but not without solution if timely, adequate and reliable geo-spatial

information is provided

An over view of development of Uyo Urban Drainage Information System

has been made in this paper. This involves inventories and creation of a

database of the designed and natural drainage network systems from which

queries could be carried out in Arc view 3.2a. Topographic, hydrologic

and Hydraulic analyses where carried out and flood hazard maps, all which

resulted in information forming part of the Uyo Urban Drainage

Information System which could serve as a Decision Support System

(DSS) for planning Uyo Urban flood management procedures.

ACKNOWLEDGEMENT

The authors acknowledge the data contributions from Ette Aro & Partners,

Consulting engineering firm on construction of Uyo Urban Drainage

network.

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