Bank erosion and protection in the Niger delta

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					Hydrological Sciences -Journal- des Sciences Hydrologiques,iS,i, 6/1993

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Bank erosion and protection in the Niger delta
T. K. S. ABAM
Department of Geotechnical Surveying Technology, Rivers State Polytechnic, PMB 20, Bori, Nigeria

Abstract A field study of erosion processes and local responses was carried out at representative sites of major soil groups in the Niger delta. The soils at these sites varied widely from plastic silty clays to silty sands of varying relative densities. Information on soil type, stratification and state of compaction together with tidal velocity distribution and pool level variation in river channels was used to establish predominant mechanics of erosion. The influence of vegetation on bank stability is dependent on the stage of growth of the vegetation, the relative location of the vegetation in the bank area, the bank steepness and its height. Although bamboo trees and raphia palms on lower parts of channel banks impede flow, thus aiding sedimentation, they may not be effective in checking bank recession. The difficulty of achieving an effective bank protection with raphia palms can be overcome by an improvement in bank protection design layout along the line of a filter which preferentially allows water to escape from the mesh. An upper bank protection may not be effective if low bank areas are scoured as this results in overhangs which readily fail. Consequently, bank protection measures must cover the entire bank profile.

Erosion littorale et la protection des rives dans le delta du Niger
Résumé Une étude de terrain des processus érosifs et de leurs effets locaux a été menée en divers sites du delta du Niger, représentatifs de différents types de sols, allant des argiles plastiques aux sables vaseux de densités variées. Les principaux processus d'érosion ont été identifiés grâce aux données concernant les types de sols, leur stratification et leur état de compaction, la distribution des vitesses des courants de marée et la variation des niveaux dans les différents biefs. L'influence de la végétation sur la stabilité des berges dépend du niveau de développement de cette végétation, de sa localisation sur les rives, de la pente et de la hauteur de celles ci. Bien que les bambous et les raphias situés à l'aval des biefs ralentissent le courant, favorisant ainsi la sédimentation, ils peuvent s'avérer incapable de prévenir le recul des berges. La protection des berges pourrait être améliorée grâce à un filtre permettant le passage préférentiel de l'eau à travers ses mailles. Des mesures visant à protéger le sommet des berges peuvent se révéler inefficaces si leur base est affouillée, provoquant des surplombs instables. Les mesures de protection des berges doivent donc concerner celles ci dans leur totalité.

INTRODUCTION The Niger delta is criss-crossed by a network of rivers (Fig. 1) which inundate the intervening land mass periodically. Two types of inundation occur, namely
Open for discussion until I December 1993

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T. K. S. Abam

semi-diurnal and annual. Semi-diurnal inundation is associated with tidal activity and its effects are most pronounced in the southern part of the delta. Tidal influences on inundation diminish inland and reduce to zero along the line joining Obete to Amassoma (Fig. 1). A typical semi-diurnal tidal inundation for the area is shown in Fig. 2. The spring tidal range varies across the coastline with an average of 1.8 m.

SC«LE(milei)

Fig. 1 Niger river delta showing drainage pattern and limit of tidal influence.
Water Level With Reference To MSL (m)

7-2-80

17-2-80 12 0 0 NOON 1 2 0 0 p.m Ig-2°- 8 O 12 OONOON

Fig. 2 Variation in tidal water level at Opobo town (data from NEDECO, 1980).

Bank erosion and protection in the Niger delta

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Annual inundation is the direct consequence of the seasonal rains which result in peakfloodsbetween October and November every year. At peak flood, the water level rises by 5 to 8.5 m in some areas (Fig. 3), submerging a large part of the land mass. Rapid flood recession is usually accompanied by bank failures. The total land loss by bank erosion in a year is estimated to be 4.2 x 106 m2. Locally, however, the rate and extent of land loss is determined by a combination of factors such as hydrology, meteorology, soil type, vegetation, river bank geometry and relative location along a river system (Hagerty et al., 1981). This paper appraises the role of these and similar factors in causing bank erosion and describes local responses as well as remedial measures.
90-

80 WATER LEVEL(m) 70-

50

40

30

20

10

Au

<J

Sept

Oct

Nov

Dec

Jan

MONTH

Fig. 3 Monthly variation of water levelfor a selection of locations along the river Niger and its distributaries (datafromIFERT, 1988).

STUDY METHODOLOGY Based on a modified engineering soil map (Akpokodje, 1987) the Niger delta was subdivided into four zones. These zones correspond to the soil groups differentiated in Fig. 4. Typical sites were selected within each zone for

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T. K. S. Abam

•g \

B r o w n Sandy C l o y

( BS C - 2 )

;•_ ' L i g h t Grey To Dork S l i g h t l y O r g a n i c Fine Sand And Silty .' 3 ; Clay ( L - G F S C - 3 ) '/}-' ' Dark To Dark Brownish Organic A n d P»c1y Clay Of '- - .i-l H « l h Plasticity ( D O P C - 4 1

Fig. 4 Modified distribution of major soil groups in the Niger delta (modifiedfromAkpokodje, 1987). detailed observation and measurement. The sites included Ndoni, Akinima, Agbere and Port Harcourt from each zone respectively. The measurements carried out included soil properties (cohesion, angle of internal friction, bulk unit weight), recession rates of river banks and flow velocity. The physical process of bank erosion along a 250 m stretch of river bank at each site was carefully observed. The monitoring of the recession was based on indicator pegs installed exactly 10 m from the edge of the bank at each of the sites. The pegs were wooden stakes (50 x 50 x 300 mm) driven into the ground. A similar approach was adopted by Hooke (1979, 1980), Thorne & Tovey (1981) and Hagerty et al. (1983). The approach enabled some quantitative measure of material removal or deposition at a site to be readily determined. In presenting the results of this study, a case history type treatment requiring subdivision into zones has been adopted in order that the distinctive characteristics of each major soil group may be duly reflected. RESULTS AND DISCUSSIONS Zone 1 The soil type exposed on the river bank in this zone consisted of whitish to dark greyish clayey silty sand, overlain by a thin reddish brown sandy clay

Bank erosion and protection in the Niger delta

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loam measuring about 0.5 m in thickness. The sandy soil was predominantly medium dense with average unit weight 19 kN m"3. Soil cohesion was very low (less than 5 kN m"2) with an angle of internal friction averaging 30°. The drop in channel water level was rapid (0.3 m day"1) compared to the soil permeability (1.2 x 10"8 m s"1) thus resulting in undissipated pore pressure in the soil. Average moisture content measured from soil samples obtained from the river bank in May 1988 was about 21 %. However, due to increased precipitation, the moisture content rose to 28% in September 1988. The increased water content increased soil unit weight, which in turn, increased the motive force causing bank failure. At the same time, the average pore pressure increased from 10 kN m2 to 25 kN m"2 thus reducing the soil's resistance to mass failure. Flow in the channel is unidirectional. In October 1988, the average flow velocity reached 1.5 m s"1 at the river bends. The mean velocity was determined from measurements made at several points on verticals located at distances of 2.5 m from the channel bank. Average flow velocity at low water level in straight sections was 0.45 m s"1. The concave bends were the main points of attack at low water. As the water level increased, the point of attack migrated downstream. The relatively low cohesion in the soil and high (all season) flow velocity ensured that erosion by fluvial action was continuous throughout the year. Consequently the bank failure rate was high and accounted for a significant part of the observed bank recession. The banks were generally steep with angles ranging from 65 to 90°. Bank heights varied from 4.5 m to 15 m with roughly 80% of the height exposed above channel water level at low water stage. Because the banks were steep, with high bank failure rate, vegetation was barely sustained on the bank face. However, close to the crest of banks, the trees which have large biomass acted as surcharge and readily induced bank failure. An average bank recession rate of 4.75 m per annum was measured for the zone.

Zone 2 The soils in this zone were mainly brownish to greyish medium plastic sandy clay and bank heights varied from 3.5 m to 8.7 m. Although the banks were lower, the soil's shear strength was higher than in Zone 1 (25 kN m"2 to 150 kN m"2). The permeability averaged 2.5 x 10"10 m s"1. In the flooded areas, the high water mark was level with the land surface giving the soil a saturated unit weight of about 19.8 kN m"2 while the majority of the banks were steep with angles averaging 65°. The river banks were well vegetated by bamboo and raphia palm shrubs of medium maturity. These shielded the face of the bank from precipitation, served as current breakers and enhanced the strength of the bank materials (Gray, 1974). Flow velocities as high as 0.8 m s"1 were slowed down to about

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0.2 m s"1 near the bank. Sedimentation of silt and clay thus occurred. Banks in this zone were reasonably stable in spite of their steep angles and height. Locally, a net sedimentation of 30 mm per annum was recorded between December 1988 and January 1991. Zone 3 This zone was within the meander belt sediments consisting mainly of medium plastic silty clays underlain by silty sand. Bank height and steepness averaged 7.5 m and 78° respectively. The soil's strength was subject to strong seasonal influence with dry season values generally twice as high as those of the wet season (85 kN m~2 for the dry season and 34 kN m"2 for the wet season). Bank failures were prevalent and arose mainly from undermining of the bank. Many bank failures were associated with deep tension cracks (up to 1.5 m). Rotational failures were commonly observed in the area. Large trees, typical of the rain forest, were also common and some of these (especially those with broad fibrous roots less than 3 m in depth) have been directly involved in bank instability processes. Bank recession rates varied from 2 m to 5 m per annum. The composite bank stratigraphy exposed soils of differing erodibilities to fluvial erosive forces. Simons & Li (1982) classified these forces into two broad categories: (a) those which have their major impact at the water surface; and (b) those which act with greatest intensity nearer the base of the submerged banks. Banks that are exposed to surface processes gradually adjust by developing a berm wide enough to dissipate the forces causing erosion. Charlton (1982) noted that the growth of vegetation on such berms strengthened the soil and curtailed further significant upper bank erosion. During high flow velocities, high tractive stresses scoured depths below the water surface. In response, the whole bank profile usually experienced severe erosion. After each erosional event, the bank developed a new geometry with a net inland shift in the bank line. Since these categories of erosion processes operated at different locations on the bank, an upper bank protection measure designed to prevent erosion by wave-related forces would not prevent lower bank erosion. Therefore, for effective control of bank erosion, it is necessary to recommend the protection of the whole bank from both types of erosion, wherever the forces are sufficient to cause bank erosion. A local erosion protection effort involving the use of wooden piles was executed within this zone in 1987 (Ebietoma & Jaja, 1988). The groynes consisted of two sets of wooden piles 230 mm and 90 mm thick respectively (Fig. 5). The smaller piles were installed nearer the banks while the larger piles were installed farther into the channel to withstand the stronger hydraulic influence of the flow. A depth of pile embedment of 3 m was considered

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Fig. S Sketch showing a groyne or row ofpiles. adequate. This gave a factor of safety of 1.1 with respect to depth, i.e. the depth of embedment was 1.1 times me minimum deptii required for groyne stability. The distance between successive piles was 1 m. Seven rows of groynes aligned normal to me bank were erected at 10 m intervals. Each groyne or row of piles extended a distance of 10 m into the river. The selection of this distance was based on a balance between economical application of the groyne and the groyne's ability to perform efficiently its primary function of guiding the flow within the river. The desired degree of permeability of each groyne or row of piles was achieved by tying nets of woven canes between adjacent pairs of piles. In order to overcome anticipated problems offlotation,the cane nets were anchored to the river bed with bagged shells mixed with coarse sand materials. The groynes failed after one hydrological season. The failure of the groynes was subsequently investigated and was found to be due to insufficient pile depth (Abam, 1993). Groyne failures such as those observed in this case can be avoided by adopting a liberal factor of safety (not less than 2.0) in calculating the required foundation depth. In this way, unanticipated factors which could lead to increased disturbances can be conveniently accommodated. For example, the build-up of suspended and bed load sediments during floods results in increased average fluid density and incident momentum on the upstream side of groyne. A greater foundation deptii is thus required in order to provide adequate passive resistance for groyne stability.

Zone 4 This zone corresponded to the dark to dark brownish organic peaty clay soil described by Akpokodje (1987) and comprises mangrove swamps with signi-

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T. K. S. Abam

ficant occurrences of sands mainly in the island bodies within the zone. The sand was loose to medium dense around the shoreline, possibly due to the relief of lateral pressure by erosion. The bank materials were fairly uniform in particle size distribution and showed relatively high permeabilities (1.05 x 10~5 m s"1). Some areas of this zone had an irregular surface. The uneven topography appeared to have been due to differential areal erosion associated with the drainage of surface water. Continued differential areal erosion has frequently resulted in rapid gully development. The gullies acted as conduits for flood water into the body of the land mass. The gully walls weakened through interaction with water and this sometimes resulted in gully wall deterioration and failure, leading ultimately to widening of the gullies. These gullies propagated inland faster than the shoreline receded through a cut back mechanism. By this process, accelerated coastline recession was introduced. About 12 m of the shoreline was lost between 1962 and 1990, an average of 0.43 m year"1, whereas the gullies were estimated to propagate at rates averaging 1 m year"1. Peak tidal velocities over the river section at spring in this zone reached 1.0 m s"1 (Fig. 6) at mid, high and low water. At this time, a significant part of the bank may have been exposed above the channel water level. Near the river bed the velocities were reduced to about 0.2 m s"1 (Fig. 7) which was generally sufficient to entrain fine sand and silt particles (NEDECO, 1980). Thus bank overhangs were preferentially developed in this zone (Okagbue & Abam, 1986). Irregular protrusions on the bank surface helped to generate eddies which further accentuated the erosion processes. Waves were also active

Fig. 6 Variation in average tidal velocities in Imo River and Strongface Creek (both near Opobo Town and within the mangrove swamp transitional zone) (data from NEDECO, 1980).

Bank erosion and protection in the Niger delta
Con cove Bank Conve * Ban k Vertical flow I Profile

239
A v e r a q e Velocity D i s t r i b u t i o n Over Cross S e c t i o n

\

1

;

Velocity

Tv-l ~*—"
rodinq Area \

^Zy

^])

-^

-"^ Deposition Area
Distance

Fig. 7 Flow pattern at a typical bend. eroding agents due to the fairly large fetch area. Waves were generated by wind and by boat traffic which plies these routes regularly. The waves break at the shoreline and erode the sandy bank materials in the process. The water table in this area was low, rising to only about 0.5 m above sea level. The water table was strongly influenced by tidal fluctuations. Seepage erosion was usually experienced immediately following the recession of the flood tide especially during the spring tide when a higher intertidal range occurred. The shoreline area was vegetated predominantly by red mangrove trees, with occasional occurrence of raphia palm. A few large trees occurred either at the toe of the bank or farther away from the crest of the bank. In these locations large trees did not cause any serious mass instability problems. The raphia palms were generally short and had fairly broad stem and fibrous root systems that firmly held the soil fabric in place, thus reducing the chances of surface erosion. These broad root systems enhanced the structural strength of the soil. When these were favourably aligned to the shoreline they served as wave and current breakers. In this way they reduced the hydrodynamic energy and induced siltation. However, when soil masses were separated by large tension cracks, the raphia palms were no longer able to hold the soil peds together. The erosion protective value of raphia palms has been recognized in this zone and some raphia palm seedlings have actually been planted at the water front of concave river bends. However, due to the hydrodynamics of flow in meandering channels and the sequence of bank erosion and deposition, these raphia palm seedlings were moved to the point bar side of the channels where they grew and stabilized the soil. The probable mechanism for the transportation of these seedlings can be deduced by considering the flow and velocity distribution in river bends (Fig. 7). The helical flow resulted in the erosion of the concave bend and the deposition of the eroded material at the convex part of the river channel (Ippen & Drinker, 1962; Hooke 1979, 1980). It was possible, however, to prevent this hydrodynamic dispersal of the raphia seedlings with some kind of permeable fencing as illustrated in Fig. 8. The fencing ran parallel to the shoreline and comprised wooden stakes with average mesh width of 0.06 m. Permeable barriers also comprising wooden stakes were installed at intervals of 2 m to delimit horizontal migration and

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T. K. S. Abam

ensure even distribution of the seedlings. Such fencing needed to allow the free flow of water but prevent the passage of the seedlings. Apart from planting seedlings, individual efforts at shore protection by the building of concrete wall structures and wooden fencing across the shoreward side of houses are common. Such structures usually serve two purposes, namely: (a) to protect houses from the incursion of the sea; and (b) to allow gradual reclamation by refuse dumping to proceed uninterrupted by erosion.
Permeable B a r r i e r s To Delimit Horizontal M i g r a t i o n

Delimit Movement

Fig. 8 Design of permeable fences to check migration of raphia palm seedlings.

CONCLUSIONS In all the Niger delta, highflowvelocity has been shown to be a major cause of erosion. Observed differences in erosion rate within each soil group were accounted for mainly by differences in flow velocity. In Zones 1 and 3, for example, bank erosion continued all through the year because of the very high flow velocities there coupled with the predominantly erodible soil composition and composite stratigraphy. The rapid drop in water levels compared to the soil permeability has also been recognized to be a major contributor to bank erosion. Gullies connected to the coastline frequently resulted in accelerated recession of the coastline. The soil type and stratigraphy of the bank are critical factors and determined the predominant mechanism of bank recession. The extent of the intertidal range defined the erodible range of a bank face and thus constituted vital information for planning and designing economic protective structures. An upper bank protection design will not be effective if high flow velocities occur the lower bank. For this reason, it is necessary to implement an upper bank erosion protection measure which is also able to withstand the forces to which it is exposed at the lower bank area. Bank erosion protection using locally available and cultivable bamboo trees and raphia palms proved effective with a proper design of layout.

Bank erosion and protection in the Niger delta

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REFERENCES
Abam, T. K. S. (1993) On the control of erosion using permeable groynes. J. Environ. Geol. Wat. Sci. (accepted for publication). Akpokodje, E. G. (1987) The engineering geological characteristics and classification of the major superficial soils of the Niger delta. Eng. Geol. 23, 193-204. Charlton, F. G. (1982) River stabilization and training in gravel-bed rivers. In: Gravel-BedRivers ed. R. D. Hey, J. C. Bathurst&C. R. Thorne, John Wiley & Sons, Chichester, UK, 635-657. Ebietoma, J. & Jaja, E. (1988) Erosion control pilot scheme using open laced groynes (IFERT Internal Report). Rivers State University of Science and Technology, Port Harcourt, Nigeria. Gray, D. H. (1974) Reinforcement and stabilization of soil by vegetation. /. Geotech. Engng Div. ASCE100, (GTC) 695-699. Hagerty, D. J., Spoor, M. F. &Unrich, C. R. (1981) Bank failure and erosion on the Ohio river. Eng. Geol. if, 141-158. Hagerty, D. J., Sharifounassab, M. & Spoor, M. F. (1983) River bank erosion - A case study. Bull. Assoc. Eng. Geol. 20(4), 411-437. Hooke,J.M. (1979) Analysis ofthe processes of riverbank erosion./. Hydrol. 42,39-62. Hooke, J. M. (1980) Magnitude and distribution of rates of riverbank erosion. Earth Surf. Processes 5, 143-157. IFERT (1983) Data bank institute of flood, erosion, reclamation and transportation. Rivers State University of Science and Technology, Port Harcourt, Nigeria. Ippen, A. T. & Drinker, P. A. (1962) Boundary sheer stress in curved trapezoidal channels. /. Hydraul. Div. ^5C£88(HY5), 143-179. NEDECO (1980) Report of an investigation of flood and erosion control at Opobo. Prepared for the Federal Government of Nigeria. Okagbue, C. O. & Abam, T. K. £ (1986) An analysis of stratigraphie control on riverbank failure. Engng Geol. 22,231-245. Simons, D. B. & Li, R. M. (1982) Bank erosion on regulated rivers. In: Gravel-Bed Rivers éd. R. D. Hey, J. C. Bathurst& C. R. Thorne etal. John Wiley & Sons Ltd., Chichester, UK, 717-754. Thorne, C. R. & Tovey, N. K. (1981) Stability of composite riverbanks. Earth Surf. Processes Landforms, 6, 469-484. Received 8 May 1992; accepted 2 March 1993