The impact of hydrogeomorphological characteristics on gullying processes in erosion-prone geological units in parts of southeast Nigeria

Figures & data

Figure 1. Geologic map of the study area showing the survey stations

Figure 2. Drainage map showing the surface water distribution

Table 1. The surface water distribution and measurements

S/no.	Surface water	Town	Estimated average width (m)	Estimated average depth (m)	Elevation (m)	Flow direction
1.	Atammiri Stream	Uli	8	2.0	38	Northwest
2.	Enyinja River	Uli	20	3.5	35	West
3.	Ulasi River	Okija-Ozubulu	22	3.0	32	West
4.	Okpu Stream	Okija-Ozubulu	10	1.5	26	West
5.	Okposi River	Okija-Ozubulu	20	2.0	28	West
6.	Nwocha River	Oraukwu	25	3.3	72	Southwest
7.	Oridike Stream	Oraukwu	15	2.0	78	West
8.	Mvomvo Stream	Oraukwu	13	2.5	76	Southwest
9.	Ezigbo River	Oraukwu	23	2.3	83	Southwest
10.	Iyi-Udo River	Oraukwu	23	1.7	82	West
11.	Mmaba Stream	Oraukwu	18	0.8	86	West
12.	Iyi-Ukwu Stream	Oraukwu	12	1.0	72	Southwest
13.	Iyeagu River	Nanka-Ekwulobia	25	4.0	160	West
14.	Agulu Lake	Nanka-Ekwulobia	55	8.0	134	Wester
15.	Atama Lake	Nanka-Ekwulobia	48	6.5	170	East

Table 2. Interpretation of the resistivity data (Based on Elkhedr, Mohamed, Ahmed, & Laust, 2004; Loke, 1999; Telford, Gilbert, & Sheriff, 1977)

VES name	Layers	Rho (Ωm)	Thickness (m)	Depth (m)	Elevation (m)	Water saturated?
Uli	1 2 3 4 5	896.81 937.54 12,388.00 1065.80 204.62	0.404 10.049 36.310 13.757 –	0.404 10.453 46.763 60.520 –	64.57 54.55 18.24 4.48 –	No No No Yes Yes
Okija-Ozubulu	1 2 3	88.66 5543.60 346.29	2.426 13.640 –	2.426 16.066 –	64.57 50.93 –	No No Yes
Oraukwu	1 2 3 4 5	197.97 4535.20 1326.70 4219.00 851.12	1.444 4.437 13.810 63.345 –	1.444 5.881 19.691 83.036 –	173.56 169.12 155.31 91.96 –	No No Yes No Yes
Nanka-Ekwulobia	1 2 3 4 5	50.722 1069.90 499.88 7289.70 81.383	0.554 2.000 7.046 89.588 –	0.554 2.555 9.601 99.188 –	313.45 311.45 304.40 214.80 –	No No Yes No Yes

Table 3. The ADMT values

S/no.	Depth (m)	Uli ADMT	Okija-Ozubulu ADMT	Oraukwu ADMT	Nanka-Ekwulobia ADMT
		mV
1.	5	0.055	3.464	32.615	0
2.	10	0.131	5.429	0	0.006
3.	15	0.068	14.785	0.015	0.684
4.	20	0.256	.294	0.018	0
5.	25	0.222	12.544	0	157.078
6.	30	0.190	6.335	3.605	98.257
7.	35	17.817	1.840	7.872	86.169
8.	40	0	7.815	13.377	187.414
9.	45	7.965	11.910	0	0.724
10.	50	5.679	2.896	156.333	1.089
11.	55	4.634	1.146	0.238	0.997
12.	60	5.026	4.457	0.366	1.192
13.	65	0.992	15.335	0	0
14.	70	6.055	16.930	0.227	0
15.	75	0	10.721	0.287	0
16.	80	0	17.862	0.321	0
17.	85	107.960	25.046	0.306	0
18.	90	0.380	13.632	6.729	0
19.	95	0.704	28.511	5.651	0
20.	100	0.405	2.654	41.214	0
21.	105	0.470	15.371	0.334	0
22.	110	10.482	8.776	31.649	–
23.	115	0.513	32.125	15.433	–
24.	120	0.827	19.359	33.323	–
25.	125	0.760	21.722	58.816	–
26.	130	0.679	37.205	0	–
27.	135	0.796	20.690	52.604	–
28.	140	0.547	0	0	–
29.	145	0.455	52.043	0.217	–
30.	150	0.350	16.861	0.232	–

Figure 3. The VES models for the four stations

Figure 4. The ADMT models for the stations

Table 4. Analyzed geotechnical properties of the soils

Parameter	Uli	Okija-Ozubulu	Oraukwu	Nanka
Fines content (%)	24.33	5.15	3.20	14.97
Sand content (%)	75.67	11.47	86.27	68.06
Gravel content (%)	0.00	83.38	10.53	16.97
Permeability (k, m/sec)	8.52 x 10^−5	2.53 x 10^−4	4.34 x 10^−4	8.52 x 10^−5
Moisture content (%)	2.00	2.00	2.00	3.00
Cohesion (kPa)	6	6	1	7
Angle of shear resistance (^o)	33	36	36	33
USCS*	SC	GP	SW	SC

*unified soil classification scheme.

Figure 5. Model showing the aquiferous layers

Table 5. The gully slope components

S/no.	Gully slope identity	Formation	Slope gradient (^o) range	Slope aspect (direction)	Number of gully slopes measured	Dominant erosion intensity
1	Uli gully	Benin	18–34	Various	8	Moderate
2	Okija gully	Ogwashi	20–33	Various	7	Moderate
3	Ozubulu gully	Ogwashi	20–50	Various	10	Moderate to high
4	Oraukwu gully	Nanka	30–70	Various	12	High
5	Nanka gully	Nanka	33–85	Various	24	Very high
6	Ekwulobia gully	Nanka	30–75	Various	15	High-Very high

Figure 6. 2D, 3D maps showing the topography of the area

Figure 7. Various slope outlooks at Nanka-Ekwulobia gullies. Red circles and arrows show building structures at the verge of collapse

Table 6. Gully attributes and intensity measurements

Gully	Geologic formation	Average elevation (m)	Depth of gully range (m)	Width of gully range (m)	Lateral extent (m)	Dominant erosion intensity
Uli	Benin	55	5–15	7–25	≥ 600	Moderate
Okija	Ogwashi	40	4–7	4–10	≥ 430	Moderate
Ozubulu	Ogwashi	50	15–25	30–75	800	Moderate to high
Oraukwu	Nanka	150	20–75	65–95	1000	High
Nanka	Nanka	300	75–10	350–750	1200	Very high
Ekwulobia	Nanka	270	40–62	75–110	≥ 750	High-Very high

Figure 8. Land use/land cover maps for (a) 1986 and (b) 2013 (modified after Ifeka & Akinbobola, 2015)

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