Analytical model development of land use assessment impact on alluvial gravel bed rivers stability. (Gorgaanrood river)

Alluvial stream morphology and especially stable dynamic equilibrium (regime) in a River
is an important and fascinating issue in River engineering. Many laboratory and field
studies have been done in order to understand and find the relationships between variables
that affect the geometry of alluvial rivers. Most of the methods and solutions are empirical
and conceptual and the limited efforts have been done to achieve rational and analytical
methods for investigating the behavior of the river and the hydraulic geometry of the river
in stable condition.
This study evaluated changes in land use on the river banks in consider with hydraulic
geometry parameters changes in section of the river includes wide and depth. In this
regard, an analytical model nemed the rational regime model is used to achieve hydraulic
geometry parameters (width and depth) of river sustainable section by quantifying the
section control parameter and satisfy the hydraulic equations of the river. This model could
be used for the rivers with trapezoidal cross-section and vertical upper banks. This shape of
the cross section is consist with the sedimentological characteristics of the bank in which
the lower part of the banks is composed of the cohesionless material with a grain size
distribution similar to the bed material found in the channel, and the upper bank is
composed of the fine-grained materials reinforced with vegetation roots. Since width and
depth in the stable section of the river are obtained at the stability conditions of the banks,
the bank strength has been entered in the model as a major and important factor in the
formation of river stable geometry. The strength of the river bank depends on two
components: The frictional properties of the material (φ) and effective cohesion (C') that in
the model, the φ parameter have been used to analysis the stability of the lower bank and
the effective cohesion to analysis the stability of the upper bank. To develop the model, the
effect of vegetation cover in H parameters (maximum stable height of the bank) is
quantified accordingly.
In this study, the effects of land use change in the vicinity of Gorganroud River on its
sustainable section have been analyzed through the effect on the resistance of the edges.
The research results shows with changes in land use from pasture to residential, pasture to
agricultural and agricultural to pasture the water level of the river increase and river width
also extend. And also with changes in land use from agricultural to pasture, the results
shows decreases in water level of the river and reduce in the width. After calibration, the
model results shows good agreement with field observations.