Hydrology in Mountainous Regions. II - Artificial Reservoirs; Water and Slopes (Proceedings of two Lausanne Symposia, August 1990). IAHS Publ. no. 194, 1990.
Gully erosion resulting from anthropogenic-hydrological modifications: case of the Opi-Ugwogo-Abakpa Nike Road, Anambra State, Nigeria
C.O. OKOGBUE . & J . U. AGBO Depar tment of Geology, U n i v e r s i t y of N i g e r i a , Nsukka N i g e r i a
ABSTRACT This pape r d i s c u s s e s t h e g u l l y e r o s i o n a t t h e Opi end of t h e Opi-Ugwogo-Abakpa Nike Road in Anambra S t a t e , N i g e r i a . The g u l l y was i n i t i a t e d f o l l o w i n g t h e c o n s t r u c t i o n of t h i s road which c u t s a c r o s s an escarpment - t h e Udi Nsukka esca rpment . The s t u d y r e v e a l s t h a t t h e g u l l y e ros ion i s t h e r e s u l t of t h e b l o c k i n g of n a t u r a l d r a i n a g e c o u r s e s a l o n g t h e highway pa th d u r i n g c o n s t r u c t i o n , h e i g h t e n e d by t h e topography and v u l n e r a b l e g e o l o g i c format ion exposed i n t h e a r e a . P r o v i s i o n of a d e q u a t e and wel l c o n s t r u c t e d d r a i n a g e p a t h s may a m e l i o r a t e t h e s i t u a t i o n a l t h o u g h a complete r e l o c a t i o n / r e a l i g n m e n t of t he a f f e c t e d road s e c t i o n , s l i g h t l y to t h e sou th and away from t h e r i v e r f lood p l a i n , may be a b e t t e r c o s t - s a v i n g a l t e r n a t i v e .
INTRODUCTION
In t h e d e s i r e to b e t t e r l i f e , man h a s con t inued to i n t e r f e r e wi th n a t u r a l d r a i n a g e c o u r s e s . Such i n t e r f e r e n c e o f t en i s b e n e f i c i a l i n t h e l o n g r u n , e . g . b e n e f i t s from the c o n s t r u c t i o n of dams. However, t h e i n t e r f e r e n c e i s n o t a lways b e n e f i c i a l bu t may i n f a c t be v e r y d e t r i m e n t a l to t h e envi ronment . The i n d i s c r i m i n a t e dumping of r e f u s e a l o n g d r a i n a g e c o u r s e s (a form of r e f u s e d i s p o s a l i n many deve lop ing c o u n t r i e s ) has been known to cause floo.'ding due to d r a i n a g e b l o c k a g e . Opening up of new highways a l s o has caused t h e b l o c k i n g of s e v e r a l n a t u r a l d r a i n a g e ways t h u s l e a d i n g to f l o o d i n g and e r o s i o n . In a d d i t i o n t o e n g i n e e r i n g c o n s t r u c t i o n a g r i c u l t u r a l p r a c t i c e s a l s o expose t h e l a n d s c a p e to t he d e v a s t a t i n g e f f e c t s of e r o s i o n .
I n t h i s p r e s e n t paper t h e r o l e of man a s an a g e n t of e r o s i o n / g u l l y i n g a t t h e Opi-end of t h e Opi-Ugwogo-Abakpa Nike road s e c t i o n i n Anambra S t a t e , N i g e r i a i s d i s c u s s e d . This r o a d , s t i l l under c o n s t r u c t i o n f o r over ten y e a r s , i s f a c i n g s e r i o u s problems of abandonment due to e r o s i o n / g u l l y i n g a t t h e Opi end of t h e road s t r e t c h »
GENERAL CAUSES OF GULLY EROSION
The c a u s e s of s o i l / g u l l y e r o s i o n have o f ten been t r a c e d to s e v e r a l f a c t o r s l i k e g e o l o g y / s o i l t y p e , t opography , c l i m a t e , v e g e t a t i o n and human a c t i o n . In s o u t h e a s t e r n N i g e r i a , t he problem h a s been h i g h l i g h t e d by such workers a s Ofomata ( I 9 6 5 ) , F loyd ( 1 9 6 5 ) , Nwajide .&.. Hoque (1979)» Egboka 8c. Okpoko ( 1 9 8 ^ ) , Okagbue (1986) , Uma & Onuoha ( I 9 8 7 ) , Onuoha & Uma ( I988) and Okagbue &
407
C. O. Okogbue & J. U. Agbo 408
Ezechi (1988),, All workers seem to agree that soil/gully erosion is more severe in areas of rugged terrain underlain by friable and unconsolidated sandy bedrocko Onuoha & . Uma (I988), on the basis of intensity have, for example, sub-divided erosion menance in Nigeria into three zones namely, zone of incipient to slight erosion9 zone of s l i g h t to moderate e r o s i o n , and zone of i n t e n s e e ros ion w i th in which a r e found a r e a s of a c t i v e g u l l y i n g * This s u b - d i v i s i o n could be t r a c e d to two major causes - geology and topography* Areas of f l a t t e r r a i n show moderate t o low i n t e n s i t y e r o s i o n , whereas a r e a s of rugged t e r r a i n show p o t e n t i a l l y h igh e ros ion i n t e n s i t y » The p o t e n t i a l i s h i g h l y man i f e s t ed when t h e rugged t e r r a i n i s u n d e r l a i n by u n c o n s o l i d a t e d sand d e p o s i t s . This i s t y p i c a l of t he s c a r p s l o p e s of t h e two major c u e s t a s i n s o u t h e a s t e r n N i g e r i a - t h e 250km long n o r t h - s o u t h t r e n d i n g Arochukwu-Ohafia-Awgu-Udi-Nsukka cues t a and the 75km l o n g n o r t h w e s t - s o u t h e a s t t r e n d i n g Awka-urlu Uplands , which a r e d i s s e c t e d by numerous r i l l s , r a v i n e s and g u l l i e s of a l l s i z e s and s h a p e s ,
PHYSIOGRAPHY OF STUDY Arii A
FIG* 1 Map of N i g e r i a showing s o u t n e a s t e r n N i g e r i a and s tudy a rea»
The s t u d y a r e a ( F i g . 1 ) i s l o c a t e d w i t h i n t h e s c a r p s l o p e of t h e n o r t h e r n e x t e n s i o n of t h e 250km long n o r t h - s o u t h t r e n d i n g Arochu» kwu-Ohafia-Awgu-Udi-Nsukka cues ta» L o c a l l y t he c u e s t a i s winding c o n s i s t i n g of s e v e r a l r i d g e s and v a l l e y s , wi th t h e u n d e r l y i n g g e o l o g i c f o r m a t i o n s exposed a l o n g t h e s c a r p - f a c e t h a t e v e n t u a l l y f l a t t e n s eas twards inxo t h e Cross R ive r P l a i n . S e v e r a l r i v e r s t a k e t h e i r r i s e from t h e c u e s t a and flow a lmos t i n an e a s t e r l y
409 Gully erosion from anthropogenic-hydrological modifications
d i r e c t i o n through the Cross River Plain (F ig .2 ) . The Ora River of the study area (see Fig.2) i s one of such r i v e r s ; i t takes i t s r i s e from the cuesta and flows eastwards in to the Abonyi River which eventually empties in to tne Cross River.
f"< "[' Mllrt»
0 U = = T0
FIG. 2 Block diagram showing the Awgu-Nsukka cuesta. Note the pos i t ions of Opi, Ora River and Enugu. Abakpa Nike i s in ijiugu. (Modified from Ofomata, 1975).
The area i s charac te r i sed by heavy r a i n f a l l typ ica l of the r a i n f o r e s t regions of the t r o p i c s . The annual r a i n f a l l ranges from 1500 to 1800mm, accompanied by frequent storms of high i n t e n s i t y . There a r e two seasons, the rainy season spanning a per iod of about eight months (March to October) , and the dry season which l a s t s for four months (November to February) , with the c h a r a c t e r i s t i c dry winds of the hamattan period dpminating most of December and January. During the dry season average monthly maximum temperatures of up to 3^ c a r e recorded whereas the
C. O. Okogbue&J. U.Agbo 410
lowest average monthly minimum temperatures of 19 C a r e obta inab le espec ia l ly during the peak rainy season and the harmattan.
GEOLOGY
The study area f a l l s within the southeastern Nigeria sedimentary bas in . I t i s underlain by rocks of the Mamu, Ajali and Nsukka Formations which form the north-south trending cuesta a l ready mentioned (see Figs 2, 3 and k).
FIG. 3 Surface geology of the Opi area and environs .
Capping the cuesta to the west i s the i ron- s t a ined Nsukka Formation which l oca l l y forms l a t e r i t i c r e s i s t a n t caps over the ridges* This Formation general ly cons i s t s of sha l e s , coa l s , and f ine to coarse grained sandstone un i t s which a re ferrugenized and l a t e r i t i z e d l o c a l l y . The Ajali Sandstone Formation conformably under l i es the Nsukka Formation and cons i s t s of f r i a b l e , poorly consol idated, whi t ish , medium to coarse grained sandstones. The upper pa r t i s l oca l l y i ron-s ta ined due to l a t e r i z a t i o n . I t i s very well exposed along the scarp face of the cuesta and i s the so le formation underlying the erosion s i t e s a s well a s the va l l eys thai, adjoin the r idges of the cuesta . I t i s a p r o l i f i c aqui fer y i e l d ing l a r g e q u a n t i t i e s of water to wells as well as sus ta in ing many
411 Gully erosion from anthropogenic-hydrological modifications
p e r e n i a l s t r e a m s e . g . t h e Ora R ive r of t h e s tudy a r e a , d u r i n g t h e d ry s e a s o n . Under ly ing the A j a l i Format ion i s t h e Mamu F o r m a t i o n , a p . a r a l i c sequence c o n s i s t i n g of c o a l s , s h a l e s and s a n d s t o n e u n i t s . The upper p a r t of t h e Format ion c o n s i s t s of s a n d s t o n e u n i t s . The Format ion i s exposed f a r t h e r down t h e escarpment b e f o r e t h e s l o p e f l a t t e n s i n t o t h a t p o r t i o n of t h e Cross R ive r P l a i n ( s e e F i g . 2 ) u n d e r l a i n by t h e Nkporo/Asata S h a l e F o r m a t i o n , a l a t e r a l e q u i v a l e n t of t h e Enugu S h a l e . F i g . 3 i s a s k e t c h of t h e s u r f a c e g e o l o g i c l a y o u t of t h e Opi a r e a , whi le F i g . h i s a s e c t i o n th rough the s u r f a c e geo logy .
FIG. k Cross s e c t i o n through F i g . 3 showing d i s p o s i t i o n of g e o l o g i c u n i t s .
GENESIS OF PRESENT PROBLEMS
I n 1975 a road l i n k between Opi and Abakpa Nike through Ugwogo was P l a n n e d . This road was t o cu t a c c r o s s t h e 250km long n o r t h - s o u t h t r e n d i n g Arochukwu-Ohafia«Awgu~Udi-Nsukka c u e s t a , n e a r Opi . There were f i l l i n g s of d e p r e s s i o n s and c u t s of e l e v a t e d a r e a s . The a l i g n m e n t of t h e road n e a r Opi was ve ry c l o s e to t h e Ora River and i n t h e same d i r e c t i o n a s t h e main r i v e r f low. A s e c t i o n of t h e road which i s now bad ly g u l l i e d ( F i g . 5 ) a c t u a l l y l i e s a lmos t w i t h i n t h e f lood p l a i n s of t h e r i v e r .
Because of t h e rugged topography , s e v e r a l smal l t r i b u t a r i e s , e s p e c i a l l y i n t e r m i t t e n t ones t h a t a r e ve ry a c t i v e on ly du r ing s t o r m s , empty i n t o t h e Ora R i v e r . Such t r i b u t a r i e s a s would be expec ted do n o t n e c e s s a r i l y d r a i n in t h e same d i r e c t i o n a s t h e main r i v e r . Many of t h e i r d r a i n a g e p a t h s were t h e r e f o r e i n t e r f e r e d w i t h , some comple t e ly b locked d u r i n g t h e road c o n s t r u c t i o n .
The r e s u l t of t h e i n t e r f e r e n c e was t h a t d u r i n g heavy r a i n » s t o r m s c h a r a c t e r i s t i c of t h e a r e a , t h e runof f , t h a t would h i t h e r t o have been n a t u r a l l y c h a n n e l l e d i n t o t h e r i v e r through t h e d r a i n a g e c o u r s e s , sought a l t e r n a t i v e ways of d i s c h a r g e . And s i n c e no a r t i f i c i a l d r a i n a g e s were p rov ided d u r i n g t h e road c o n s t r u c t i o n , t h e u n c h a n n e l l e d runof f carved d i s c h a r g e channe l s f o r i t s e l f . E r o s i o n was t h u s i n i t i a t e d .
C. O. Okogbue&J. U.Agbo 412
FIG® 5 Photograph showing road s t r e t ch being se r ious ly at tacked by runoff, Man standing i s used to show depth of erosion*
The erosion was most severe near the toes of the cut slopes which unfor tunate ly a re underlain by the f r i a b l e and eas i ly g u l l ied (Okagbue & Ezechi, 1988) Ajal i Sandstone Formation, Because the Formation a l so forms the grade on which the road was construc-ted9 the erosion led to not only cut slope undermining but a lso to the undermining of the pavement (see F ig . 6 ) . Such undermin-
FIG. 6 Photograph showing undermining of cu t -s lope as well a s pavement. Note the slump fea tures on the cut slope as well a s the col lapsing of the pavement.
413 Gully erosion from anthropogenic-hydrological modifications
in.g' led to lots'»of slum-ping/sliding along the cut slopes as well a s the col lapse of the undermined sec t ions of the pavement. In these s ec t i ons , the e n t i r e pavement has been completely washed away (See Figs 7 and 8) such tha t the road has now become a drainage way during storms.
The f i l l sec t ions of the road have been s imi la r ly a t tacked a s the embankment toes become undermined by runoff. In these s e c t i o n s , g u l l i e s as deep as the height of the cut slopes have been generated. This i nves t iga t ion a l so revealed tha t the grading or l e v e l l i n g of the ground created l ands l ide suscept ib le conditionso In the f i l l s ec t ions , for example, f a i l u r e planes develop on the former ground surface where vegetation was not proper ly cleared p r io r to regrading. During the rainy season, increased s o i l sa tura t ion r e s u l t s where surface and subsurface drainage was destroyed by regrading. The f i l l s , though compact ed , l o s e s t rength during these rainy periods and become subject to mass movement.
I ••••". •:-•.• '
FIG. 7 Photograph showing a s e c t i o n of pavement t h a t c o l l a p s e d r e c e n t l y . The d e b r i s a r e y e t to be c a r r i e d away by runo f f .
SUGGESTIONS FOR MITIGATION
The e r o s i o n menace in t h e s t u d y a r e a i s q u i t e a l a r m i n g t h a t u r g e n t a t t e n t i o n i s needed , i f t h e road c o n s t r u c t i o n must c o n t i n u e . The menance i s so complex t h a t on a f ace v a l u e i t would a p p e a r t h e p rob lem i s i n s o l v a b l e . However, i f t h i s road i s to s t i l l r e t a i n i t s o r i g i n a l a l i gnmen t then a l o t of work h a s t o be done .
C. O. Okogbue & J. U. Agbo 414
FIG0 8 Photograph showing a comple te ly washed away s e c t i o n of pavement .
I t i s conceived t h a t i f wide r e - i n f o r c e d c o n c r e t e d r a i n a g e s a r e p r o v i d e d on both s i d e s of t h e road from Opi town, s t r a i g h t down i n t o t h e banks of t h e Ora R i v e r , and i f a t t h e same t ime t h e s l o p e s of t h e c u t s and embankments a r e c o n s i d e r a b l y f l a t t e n e d , t h e menance could be b rought under c o n t r o l . T h i s , however i s l i k e l y to be ve ry expens ive a s t h e p r e s e n t g u l l i e s have to be f i r s t f i l l e d b e f o r e any c o n s t r u c t i o n s can be u n d e r t a k e n .
To minimize c o s t and p o s s i b l y t h e r e - g e n e r a t i o n of t h e g u l l i e s , a comple te r e - a l i g n m e n t / r e l o c a t i o n of t h i s s e c t i o n of t h e r o a d , s l i g h t l y t o t h e s o u t h , away from the f l ood p l a i n s of t h e Ora R ive r might be a b e t t e r a l t e r n a t i v e s o l u t i o n . T h i s , however s would s t i l l r e q u i r e t h e p r o v i s i o n of d r a i n a g e s a s t h e cues t a would s t i l l be cu t a c c r o s s , b u t t h i s t ime n o t ve ry c l o s e t o t h e r i v e r »
CONCLUSIONS
The f o l l o w i n g c o n c l u s i o n s can be drawn from t h e above s tudys
1 . S e r i o u s g u l l y i n g can be g e n e r a t e d by human a c t i v i t i e s , a l b e i t i n a d v e r t e n t l y . The g u l l y e r o s i o n d i s cus sed h e r e i n i s l i n k e d to e n g i n e e r i n g c o n s t r u c t i o n .
2* Adequate g e o l o g i c a l and h y d r o l o g i c a l i n v e s t i g a t i o n s must p r e c e d e road c o n s t r u c t i o n s i f man-made h a z a r d s must be minimised o r a v e r t e d » I t i s most l i k e l y t h a t such i n v e s t i g a t i o n s were n o t done p r i o r to t h e c o n s t r u c t i o n of t h e Opi-Ugwogo-Abakpa Nike Roade
3 . Because of t h e topography and t h e n a t u r e of t h e g e o l o g i c a l f o r m a t i o n s exposed in t h e s tudy a r e a , a complete r e - a l i g n m e n t / r e l o c a t i o n of t h e road s e c t i o n s e r i o u s l y a f f e c t e d by g u l l y i n g i s a d v o c a t e d , coupled wi th t h e p r o v i s i o n of a d e q u a t e d r a i n a g e s on bo th s i d e s of t he r o a d .
415 Gully erosion from anthropogenic-hydrological modifications
REFERENCES
Egboka, B . C E . , & Okpoko, E . I . (198*0 Gully e ros ion in t h e Agulu-Nanka r eg ion of Anambra S t a t e , N i g e r i a . I n : C h a l l e n g e s in Afr ican Hydrology and Water R e s o u r c e s , ( P r o c . Hara re Symp0 , August 1984) 335-344 . I n t e r n a t i o n a l A s s o c i a t i o n of H y d r o l o g i c a l S c i e n c e s P u b l . n o . 144.
F l o y d , B0 (1965) S o i l e r o s i o n and d e t e r i o r a t i o n i n E a s t e r n N i g e r i a , N i g e r i a n Geogr. J . 8_, 3 3 - 4 3 .
Nwaj ide , O.S. & Hoque, M. (1979) Gu l ly ing p r o c e s s e s i n E a s t e r n N i g e r i a . The N i g e r i a n F i e l d . 5f£(2), 64-74
Ofomata, G.B.K. (1965) F a c t o r s of s o i l e r o s i o n i n Enugu a r e a of N i g e r i a . N ige r i an Geogr J . <8 45-59
Ofomata, G.E.K. (1975) N i g e r i a in Maps; E a s t e r n S t a t e s . E t h i o p e P u b l i s h i n g House, Benin , N i g e r i a .
Okagbue, C.O. ( 1 9 8 6 ) . Gul ly development and advance in a r a i n f o r e s t zone of N i g e r i a . P r o c . 5 t h I n t . Congr. I n t . ASSOC. Bngng. Geol . Buenes A i r e s , A r g e n t i n a , 1990-2010.
Okagbue, C.O. Se E z e c h i , J . I . ( I988) Geo techn i ca l c h a r a c t e r i s t i c s of s o i l s s u s c e p t i b l e to s e v e r e g u l l y i n g i n E a s t e r n N i g e r i a . B u l l . I n t . Assoc . Engng. Geol . 3 8 , 111-419 .
Onuoha, K.M.5 & Uma, K.O. (1988) An a p p r a i s a l of r e c e n t g e o l o g i c and h y d r o l o g i e h a z a r d s in N i g e r i a . I n ; N a t u r a l and man made Haza rds , Kluwer Academic P u b l i s h e r s , The N e t h e r l a n d s .
Uma, K.O. & Onuoha, K.M. (1987) Groundwater f l u x e s and g u l l y deve lopment in s o u t h e a s t e r n N i g e r i a . I n ; Groundwater and M i n e r a l Resources of N i g e r i a , E a r t h Evo lu t i on S c i e n c e Monograph S e r i e s , F r i e d r . Vieweg & John Wiesbaden.
