The effects of Land reclamation along gravel-bed braided system: Mao River, Bhutan

Braided rivers are known as highly dynamic systems characterized by several channels (Williams, et al., 2013) divided by unsteady bars (Egozi and Ashmore, 2009). Training is often a necessary action in order to either control their lateral expansion (Carolina Rogeliz, et al., 2006) and undesirable bank erosion, or for land reclamation (Jurina, 2017). The latter is usually associated with local channel narrowing which leads to vast changes in river morphology. According to the equilibrium theory developed by Jansen et al. (1979), narrowing leads to channel incision, local decrease in longitudinal bed slope, upstream erosion, opposite bank pushing and decrease in the braided degree of the river (Duro et.al, 2016). Moreover, during high flow condition, local channel narrowing creates backwaters that increase the upstream water level and risk of flooding in the areas adjacent to the river. Thus, land reclamation might deeply change braided river systems and cause undesirable hydraulic and morphodynamic alterations. Therefore, efficiency of different river training alternatives regarding to land reclamation should be examined. Fracassi and Di Pietro (2018) presented some lateral land reclamation schemes that are floodable during high flows, which minimizes their hydraulic and morphological impacts. These interventions proved to be successful for farming along a few braided rivers of Bolivia. Agricultural land reclamation was obtained by constructing Gabions wall with openings to protect the land up to a specific flood and inundation level. In addition, the area was subdivided by screens perpendicular to the water flow to decrease the flow velocity during flood events and enhance deposition of fine material. Therefore, this type of interventions proved successful regrading to fertility rise in the new agricultural fields. However, there is no precise description of the river morphdynamic responses. This work aims at evaluating the degree of success of this land reclamation technique considering the morphological adaptation of the river. The Mao River is chosen as case study (Fig. 1). This is a gravel-bed braided tributary of the Brahmaputra. The study area is located in Bhutan, where the river channel has high potential of agricultural land reclamation. The degree of success is here considered in terms of water level raise during floods which depends on the morphological changes induced by the land reclamation scheme.