CHANNEL PLAN FORM CHANGES ALONG THE ALLUVIAL SECTION OF RIVER OSUN, NIGERIA

Abstract

Alluvial channels form a morphological continuum defined as straight, meandering and or branching, which may be stable or change over time and space. Earlier researches have focused on single plan form, to the neglect of the possible transformation from single to multiple forms within a single river channel. This study, was designed to analyse the plan form changes and their associated factors within the alluvial section of Osun River, with single (meandering) and multiple (anabranching) channel reaches.

Based on the concept of downstream hydraulic geometry, the alluvial section of Osun River was divided into meandering and anabranching reaches. Data on the river morphological variables were collected from 44 cross-sectional points from the meandering (34) and anabranching (10) reaches using standard field procedures. River pattern was quantified using sinuosity index and average number of channels across valley. Cross-sectional form was quantified using width, depth, area and width/depth ratio (WDR). Process-form relationship was characterised using channel discharge (Q), percentage bank clay (Bc), bank shear (Bs), median bed sediment grain size (D50), valley slope (S) and valley width (Vw). Aerial photographs of 1963 and Landsat imageries of 1972, 1984, 2005 and 2012 were analysed for channel plan form changes. Pairs of images were overlayed and geo-processed to obtain width, area, sinuosity and lateral migration patterns. Data were analysed using descriptive statistics, correlation, linear and multiple regression at α.0.05.

The number of channels across valley was 1.03±0.17 and 4.5±1.08, while channel sinuosity index was 1.5 and 1.2 for meandering and anabranching reaches, respectively. The Q (282±27.91 m3/s; 482±26.82 m3/s), Bc (10.88±2.99%; 29.22±7.69%), Bs (10.72±2.44 kPa; 27.93±5.00 kPa) and log of Vw (5.04±0.36 m; 6.54±0.28 m) were lower in the meandering than anabranching reaches, respectively. Channel pattern was directly related to Q (r=0.86), Bc (r=0.79), Bs (r=0.87), Vw (r=0.81) and inversely related to D50 (r=-0.78) and S (r=0.78). Cross-sectional form was directly related to Q (r=0.72), D50 (r=0.79), S (r=0.79) and inversely related Bc (r=-0.75), Bs (r=-0.87). The equation predicting the relationship between WDR (y) and downstream distance (x) was y = 22.9 - 0.39x. Furthermore, the equations predicting the relationships between plan form control variables and channel pattern as well as cross-sectional form was y = 1.18 + 0.21Q + 0.3Bc + 0.60D50 – 0.12S – 0.21Vw + e (where e is stochastic error) and y = 8.01 + 0.34Q - 0.06Bc -0.91Bs + 0.29D50 + 0.31S + 0.31Vw + e. There were changes from single channel to multiple (anabranching) channel and from meandering to sinuous river pattern as predicted by the decrease in WDR with increasing distance downstream.

The transition from meandering to anabranching plan form within the alluvial section of Osun River was observed. This was due to increased discharge, bank strength and reduced valley slope of the river channel.

Keywords: River Osun, Alluvial channel, River meandering and anabranching, Hydraulic geometry.

Word count: 493 words.