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The aim of the study was to investigate the morphometric characteristics of Obibia drainage basin which is wholly located within the Awka Capital territory of Anambra State, Nigeria. The basin was delineated with conventional method from topographic map (Udi SW) on the scale of 1:50,000 published by Federal Survey office. Field survey was conducted for 9 months between April and December 2015 and data were obtained on linear, areal and relief aspects of the basin to ascertain the morphometric characteristics of the basin. Principal Component Analysis (PCA) reduced our 19 variables into 5 while Principal Component Regression (PCR) was employed to reveal the relative contributions of morphometric variables. Result shows a bifurcation Ratio R (Rb) of 3.00 around the month of the river basin indicating higher risk of flooding. The relationship between mean stream length and stream order shows that order 2 drifts too far away from straight line on plotted graph suggesting some structural imbalances. The study recommends proper urban land use planning within the basin that would preserve the natural condition of the basin.

The problems posed by the morphometric characteristics of some urban river basins in Nigeria are widely recognized. In a comparative study of Ogunpa and Ogbere drainage basin in Ibadan, Ajibade, Ifebiyi, Iroye and Ogenteru [

Again Abua and Abua [

Furthermore, Uzor studied the Nyaba and Ekulu drainage basins in Enugu urban area and determined that high sediment yield of Nyaba basin has considerably raised the Nyaba river channel resulting in flash floods that occasionally destroy lives and property especially along the riparian environment.

The influence of high population, as well as, physical development of the urban area affects in various ways the morphometry of the urban drainage basin. However, despite these various studies of drainage basins located in urban areas, many more of such basins are yet to be studied to determine the influence of urban development on various morphometric parameters of such basins. One of such basins that have not been studied is the Obibia drainage basin despite the fact that it is located in the following Local Government Area (LGA) within the Awka Capital Territory: Anaocha, Njikoka and Awka South. However, over 80% of the basin is located in Awka South L.G.A.

The Obibia drainage basin is located between Latitudes 6˚05'N and 6˚13'N and Longitudes 7˚01'E and 7˚09'E. It has an area of 84 sq・km and covers parts of Anaocha, Njikoka and Awka South L.G.A (

The climate is hot wet equatorial with average maximum temperature of 28˚C and average minimum of 24˚C which depend on the season of the year. Rainfall is experienced for 8 months of the year from March to November, while dry season lasts from December to March. Total mean annual rainfall is 1800 mm,

while the vegetation is rainforest, but largely disturbed by urbanization and other human actions, thus creating a derived savanna as patches of outliers within the study area.

In terms of geology, the Obibia Basin lies mainly within Imo clay shale that runs from southern Igala in Kogi State. However, the Bende-Ameki formation which is sandy formation dominates the western and south-western parts of the area. This formation (Bende-Ameki) is a productive aquifer that is suitable for the exploitation of water though ground source. Some parts of the basin around Isiagu and Ezinator communities are dominated by Imo clay shale that is poor in the production of ground water. In the eastern parts of the basin, around Agu- Awka and Amansea, the major geological composition is the Ebenebe formation. The basin lies on Awka-Orlu upland and partly on the flood plain of Manu River. The plain area has an average elevation of 129.5 m while the upland area has a general elevation of 250 m to 500 m.

The basin, which is being located mainly in the urban area, has a high population density and many physical development activities which are likely to negatively affect the morphometry of the basin. The drainage pattern of the basin is essentially dendritic.

The basin was delineated from the UDI S.W topographical map on the scale of 1:50,000 published by the Federal survey office. The processes of geo-referencing and digitisation were subsequently employed (

Principal Component Analysis (PCA) and Principal Component Regression (PCR) were the major statistical techniques employed for analysis. PCA, reduced the 19 variables into 5 orthogonal components. After this, the principal component loadings were run from the five selected principal components. Subsequently,

S/N | Morphometric Parameters | Derivation Procedure | Reference |
---|---|---|---|

1. | Stream order (u) | Hierarchical order | Strahler, 1964 |

2. | Stream length (Lu) | Length of the stream | Horton, 1945 |

3. | Basin length (Lu) | This is the straight line from the mouth of the basin to the farthest point on the basin perimeter | Schumn, 1963 |

4. | Main stream length (km) | This is the length of the principal drainage | Schumn, 1963 |

5. | Main stream length (km) | Total stream lengths in the basin divided by total number of streams | Schumn, 1963 |

6. | Total stream length (km) | This is the total length of all the tributaries and the principal drainage | Schumn, 1963 |

7. | Basin perimeter (p) | This is the outer boundary of the drainage basin that encloses its area | Schumn, 1956 |

8. | Bifurcation ration (R_{b}) | R b = Nu / Nu + 1 , where u = total no of streams of order “u”, Nu + 1 = No of segment of the next higher order | Schumn, 1963 |

AREA PROPERTIES | |||

1. | Basin Area (A) | Area = map scale counted squares | Gregory and Walling, 1973 |

2. | Drainage Density (Dd) | EL/A where L = total length of all streams; A = Basin Area | Horton, 1945 |

3. | Stream frequency (Fs) | N/A where N = Total no of streams; A = Area of the basin | Horton, 1945 |

4. | Form factor | A/(Lb)^{2} where A = Basin Area, Lb = Basin length | Horton, 1932 |

5. | Circulatory Ration (Rc) | AπA/P^{2} where A = Basin Area, π = 3.14, Lb = Basin length | Horton, 1932 |

6. | Elongation Ration (Re) | 2 ( A / π ) / Lb where A = Basin Area, Lb = Basin length and π = 3.14 | Schumn, 1959 |

RELIEF PROPERTIES | |||

1. | Basin Relief (H) | Vertical distance between the lowest and highest points in the basin (max^{1} H-min. H) | Schumn, 1956 |

2. | Relief Ration (Rh) | H/Lb where H = basin relief, Lb = Basin length | Schumn, 1956 |

3. | Max Relief (maxH) | The highest point value of elevation in a drainage basin | Schumn, 1956 |

4. | Min Relief (minH) | This is the lowest elevation in a drainage basin. | Schumn, 1956 |

the Multiple Linear Regression (MLR) was then run on the selected loadings. In this analysis, the defining loadings known as the Component Defining Variable (CDV) were employed. Basin area was the dependent variable while other morphometric variables are independent variables. With this, a PCR equation was produced which was used to interpret the result of the analysis.

The result of the investigation into the linear properties of the basin including the bifurcation ratio, aerial and relief properties are presented in Tables 2-4.

In analysing the linear, areal and relief properties of the basin, PCA (CDV) was employed.

The relationship between streams numbers (N_{u}) and stream order (U) as well as the regression of the logarithm of stream numbers log N_{u} against stream order were plotted as shown

Stream order (u) | Stream number (Nu) | Total stream lengths | Mean stream lengths | Long Nu | Log Lu |
---|---|---|---|---|---|

1. | 64 (77.1%) | Lu (km) 56.5 | 0.88 | 1.806 | 1.752 |

2. | 15 (18.1%) | 15 | 1 | 1.176 | 1.176 |

3. | 3 (3.6%) | 20 | 6.67 | 0.447 | 1.031 |

4. | 1 (1.2%) | 10.75 | 10.75 | 0 | 1.031 |

Total | 83 | 102.25 | |||

Bifurcation ration (R_{b}) of the basin | |||||

1^{st} Order 2^{nd} Order | 2^{nd} Order/ 3^{rd} Order | 3^{rd} Order/ 4^{th} Order | Mean bifurcation Ration R_{b} | ||

4.27 | 5.00 | 3.00 | 4.09 |

S/N | Morphometric Properties | Calculated Values |
---|---|---|

1. | Basin Area (Km^{2}) | 84 |

2. | Perimeter (km) | 50.5 |

3. | Basin length (km) | 18.25 |

4. | Drainage Density | 1.22 |

5. | Stream Frequency | 0.99 |

6. | Elongation Ratio | 0.57 |

7. | Form Factor | 0.25 |

8. | Calculatory Ratio | 0.41 |

9. | Compaction Coefficient | 1.55 |

S/N | Morphometric Properties | Calculated Values |
---|---|---|

1. | Basin Relief | 0.6 |

2. | Relief Ration | 0.03 |

3. | Basin Slope | 1.03 |

4. | Max Basin Relief | 0.75 |

5. | Min Basin Relief | 0.15 |

S/N | Variable Label | Variable Code | Variable Description |
---|---|---|---|

1. | SOR | X_{1} | Stream Order |

2. | SLE | X_{2} | Stream Length |

3. | BLE | X_{3} | Basin Length |

4. | MSL | X_{4} | Main Stream Length |

5. | MEL | X_{5} | Main Stream Length |

6. | TSL | X_{6} | Total Stream Length |

7. | BAP | X_{7} | Basin Perimeter |

8. | BFR | X_{8} | Bifurcation Ratio |

9. | BAR | X_{9} | Basin Area |

10. | DRD | X_{10} | Drainage Density |

11. | SRF | X_{11} | Stream Frequency |

12. | FFA | X_{12} | Form Factor |

13. | CRA | X_{13} | Circulatory Ratio |

14. | ERA | X_{14} | Elongation Ration |

15. | BRE | X_{15} | Basin Relief |

16. | RRA | X_{16} | Relief Ratio |

17. | BSL | X_{17} | Basin Slop |

18. | MAR | X_{18 } | Max Relief |

19. | MIR | X_{19 } | Min Relief |

Furthermore, regression of logarithm of stream length Log (L_{u}) and stream order (U) as well as relationship between mean stream length and stream order (U) for the basin were plotted and shown in

In analysing the morphometric data, they were first transformed into a matrix of standard scores after which the correlation analysis was performed on the transformed data (both the standardized data and Multiple correlation matrices were not shown). However, it was noticed that most of the properties were highly correlated. The problem of intercorrelation provoked us to subject the morphometric data to PCA in order to have a parsimonious number of clearly defined orthogonal (unrelated) factors that can explain the variations in the observed data matrix. When PCA was performed with varimax rotation and Kaiser normalization, four components were extracted. The four components shown in

The Multiple Regression Analysis was then performed on the component defining variables (CDVs) from each of the components. The reasons for employing the multiple regression on the CDVs (which are independent variables) is because the original explanatory variables (i.e. the raw data) are highly related

Variable Codes | Variable Label | Comp. I | Comp. II | Comp. III | Comp. IV |
---|---|---|---|---|---|

X_{1} | SOR | −006 | −0.785 | 0.061 | 0.779* |

X_{2} | SLE | 0.197 | 0.237 | 0.001 | 0.194 |

X_{3} | BLE | 0.887 | 0.197 | 0.009 | 0.003 |

X_{4} | MSL | 0.590 | 0.0112 | 0.326 | 0.128 |

X_{5} | MEL | 0.839 | 0.007 | 0.532 | 0.005 |

X_{6} | TSL | 0.398 | −0.295 | −0.000 | 0.240 |

X_{7} | BAP | 0.863 | −0.002 | 0.208 | 0.455 |

X_{8} | BFR | 0.974** | −0.172 | 0.898 | 0.006 |

X_{9} | BAR | 0.925 | 0.885 | 0.451 | 0.002 |

X_{10} | DRD | 0.006 | 0.003 | 0.409 | 0.005 |

X_{11} | SRF | 0.310 | 0.223 | 0.004 | 0.884** |

X_{12} | FFA | 0.909 | 0.240 | −0.002 | 0.006 |

X_{13} | CRA | 0.296 | 0.880 | 0.000 | 0.357 |

X_{14} | ERA | 0.023 | 0.937** | 0.556 | 0.001 |

X_{15} | BRE | 0.827 | −0.789 | 0.932** | −0.148 |

X_{16} | RRA | 0.005 | 0.784 | 0.167 | 0.367 |

X_{17} | BSL | 0.627 | 0.006 | 0.530 | 0.562 |

X_{18} | MAR | 0.661 | 0.699 | 0.281 | 0.250 |

X_{19} | MIR | 0.006 | 0.482 | 0.789* | 0.352 |

Eigenvalue | 7.35 | 5.54 | 3.88 | 1.69 | |

% variable explained | 36.77 | 27.72 | 23.41 | 2.80 | |

Cum % | 36.77 | 64.49 | 87.89 | 90.6 |

Significant loadings ± 0.70 and above ** component defining variables (CDV).

Statistic | Result |
---|---|

Multiple correlation | 0.952 |

Co-efficient of multiple determination (R^{2}) | 0.906 |

Standard Error of Estimates (SEE) | 0.100 |

Variable | Multiple R | R^{2} | R^{2} Change |
---|---|---|---|

X_{8} | 0.801 | 0.642 | 64.2 |

X_{14} | 0.870 | 0.757 | 11.5 |

X_{15} | 0.922 | 0.850 | 9.3 |

X_{11} | 0.952 | 0.906 | 5.6 |

which may cause inaccurate estimations of least square regression coefficients. Furthermore, the dimensionality of the regressions is reduced by taking only the CDVs for prediction. The intention of using PCR was to extract the underlying effects in the X data and to use these to predict the Y values. In this way, we could guarantee that only independent effects were used and local variance noise effects were excluded and this expectedly improves the quality of the model significantly. The dependent variable is the basin area with the loading of 0.925 in the first component. The PCR equation expressing the closest possible relationship between the basin area and the 4 specified variables is

Y ( X 9 ) = 1.24 × 0.46 X 8 + 0.26 X 14 + 0.32 X 15 + 0.20 X 11 (1)

The summary of the result is shown in

On the calculation of the relative importance of the variables, we computed successive values of the multiple correlation coefficient obtained by introducing successive independent variables at each computation [_{1}X_{2}, Ry,X_{1}X_{2}X_{3} and Ry.X_{1}X_{2}X_{3}X_{4}. The difference between the squared multiple correlation (R^{2}) is then regarded as the contribution of each variables (

Obibia river basin was found to be a 4^{th} order basin. The stream order was plotted against the stream numbers and logarithm of stream numbers against order are presented in _{b}) which is a dimensionless property, it shows only a small variation of different regions with different environment except where powerful geological control dominates [

The mean Rb of the basin is 4.09 which falls within the 3 - 5 range of basin with stable geologic structure which do not present distorted drainage pattern. However, the Rb of various orders vary indicating a level of distortion probably as a result of the on-going physical development taking place at various parts of the basin. For example, the highest Rb (5.00) is found between 2^{nd} order and 3^{rd} order indicating strong stability, while the Rb (3.00) is found between 3^{rd} order and 4^{th} order and this indicates a higher risk of flooding in that part of the basin. The basin area of our study area is 84 sq・km which is a relatively medium-sized basin likely to intercept high volume of rainfall and therefore higher risk of flooding. The drainage density of 1.22 indicates that it has a somewhat high permeability as would be seen from the Nanka and Ebenebe sands formation of the area, which easily infiltrate water. The elongation ratio of any basin ranges from 0.6 - 0.8. Higher values occur in areas of high relief and steep ground slope. These values are further categorized as circular (>0.9) and oval (0.9 - 8.0) and less elongated (<0.7). Flood flow in elongated basins is easier to manage than that of circular basins that are more prone to flash flood and higher discharge. The basin’s elongation ratio of 0.57 expresses elongation where flooding can be easily managed, what this means is that flood in the basin can be easier tackled. The circulatory ratio of natural basins range from 0.4 - 0.5 and incidentally this basin’s ratio of 0.41 falls within this range showing that the basin is very strongly elongated. This further shows that it has a reasonably high level of permeability as could also be seen from its medium to low relief which is between 100 m to 250 m above sea level.

The basin relief of the basin (600 m) goes to validate the medium to low relief found in the analysis of elongation ratio.

_{8} (BFR) of 0.974. This means that the bifurcation ratio is the strongest morphometric parameter in the basin. From the analysis of the bifurcation ratio, it was found that the Rb of the orders are not the same; 4.25 (between 1^{st} and 2^{nd} order), 5.00 (between 2^{nd} 3^{rd} order) and 3.00 (between 3^{rd} and 4^{th} order) and the mean Rb of the basin of 4.09, were calculated, generally the entire basin is between the high and low values indicating that some structural problems are already manifesting. The situation at the mouth of the Obibia basin with Rb of 3 reveals a predisposition to flood incidents. Most disturbing is that urban development is fast taking place in the entire basin although it is presently more pronounced at the middle and lower section of the basin where the present Awka town is located. Furthermore, X_{14} (ERA) (0.937) shows that the elongation ratio is a parameter to be closely observed. Although with its value of 0.57, it is slightly outside the range of 0.4 - 0.5 for natural basins. This, therefore, means that the flood condition is being created by the present physical development in many parts of the basin. Component III has X_{15} (BRE) (0.958) as the parameter with the highest loading, Basin relief was calculated to be 600 m, although that is higher than the general situation but goes to indicate that there are some areas of the basin that are high, as could be seen around Gama Hill east of the basin and in Agulu where the height relief is about 750 mm This situation are seen in patches in the basin. This elevation around Agulu is one of the reasons for erosion and excessive sedimentation which are the features of the area. Finally, component IV has XII (SFR) with highest loading of 0.884. Stream Frequency which is an indication of drainage pattern depends mainly on the lithology of the basin. It is an index of various stages of landscape evolution. The stream frequency of the area is being altered as vegetation is daily removed for various forms of construction. The high value of stream frequency and drainage density of the area strongly indicate possibility of flooding in the area especially at the built up area. The result of PCR on the four isolated morphometric properties shows that all of them operating together contributed 90.6% to the geomorphic and hydrologic problems within the area of the basin. On the calculation of the relative importance of the variables, it was seen that X_{8} (BFR) contributed as high as 64.2% of the 90.6% (which is 70.9%) of the contribution of the four parameters). The contributions of others include X_{14} (ERA)―11.5%, X_{15} (BRE)― 9.3% and X_{11} (SRF)―5.6%. This, therefore, means that greater attention will be contributed on the bifurcation ratio of the basin considering the on-going intensive construction of roads, houses, and increase in the area of paved surfaces and of course, destruction of the vegetation.

We have tried to examine the linear, areal and relief properties of the Obibia basin and related them to the on-going physical development of the area within the basin which covers a good part of the Awka capital territory. The revelation of four important morphometric factors that result in some geomorphic and hydrologic problems of the area is instructive. Out of these variables, bifurcation ratio contributes over 70% of all the contributions of the isolated factors. This means that this parameter should be more closely examined periodically. It has been concluded that adequate planning of the Awka capital territory environment may not achieve good result without considering these isolated factors.

Ezenwaji, E.E., Nwabineli, E.O. and Phil-Eze, P.O. (2018) Investigations into the Morphometric Characteristics of Obibia Drainage Basin, Awka Urban Area, Nigeria. Open Journal of Modern Hydrology, 8, 1-12. https://doi.org/10.4236/ojmh.2018.81001