The effects of vegetation on riverbank stability in the Ayeyarwady River

Abstract

The Ayeyarwady River is the most important river in Myanmar, connecting the cities Mandalay and Yangon, making inland water transport economically attractive. The Ayeyarwady River has a dynamic character, caused by the significant discharge difference between the dry and rainy season, and the related suspended sediment transport. Point bar formation in inner bends causes flow deflection towards the opposite riverbank. This causes local instability with as a result the banks to retreat. Vegetation contributes to the strength of riverbanks, by increasing the shear strength of the soil through roots and increasing hydraulic resistance, reducing flow velocities near the bank. However, to what extent the vegetation on the Ayeyarwady riverbanks contributes to this stability is debatable. Therefore it was the objective of this thesis to obtain a better understanding of the effects of vegetation on riverbank stability. By means of a fieldwork photo material of the river reach between Mandalay and Pakokku was collected, which was used for the NDVI validation. NDVI is the Normalised Difference Vegetation Index and is a remote sensing indicator for the ‘greenness’ of vegetation on satellite imagery. More vegetation means a higher NDVI value, hence it provides information about the livelihood of the vegetation present on the subsoil. In combination with determined bank retreat rates from a yearly comparison of satellite images in Google Earth, it was examined whether higher NDVI values, obtained with the Google Earth Engine, resulted in reduced bank retreat rates and therefore if NDVI can be used as a bank stability parameter.
Nine regions of bank retreat were both altogether and separately examined, but the graphs showed no definite answer of retreat rates being dependent on NDVI. Therefore, the results of the riverbank retreat analysis were categorised based on location, erosion mechanism, the slope, and vegetation classes. The areas where fluvial entrainment was the primary erosion mechanism showed a clear trend. When NDVI was smaller than 0.2, maximum bank retreat rates appeared to be 200 meters per year. When NDVI was higher than 0.2, bank retreat rates did not exceed 80 meters per year. On satellite images, vegetation was not observed in these areas, so the influence of vegetation remained questionable. In areas where mass failure caused bank retreat, no reduction in bank retreat was found. The results showed considerable scatter, although much more vegetation was present on these banks. Water level variability played a crucial role in the evaluation of the net effects of vegetation on riverbank stability. During low water, vegetation cannot provide the positive impacts, especially on steep river banks.
It is not possible to identify vegetation types from NDVI records only. NDVI also does not show which erosion mechanism takes place. This makes riverbank stability difficult to predict by using NDVI only, and therefore, NDVI does not seem to be an appropriate estimator for the additional effects of vegetation on riverbank stability. However, it can be used in combination with other remote sensing techniques to identify healthy vegetation areas and to make roughness estimations in river planform analyses.