RN-49 : Overland flow in mountainous areas

Abstract

Runoff in mountainous regions results from rainfall, snowmelt and glacier-melt. The different components of runoff are generally considered to be surface run off, sub-surface runoff and ground water runoff. Surface runoff consists of overland flow and channel flow. Overland flow is that part of surface runoff which flows directly over the land surface towards the stream channel.

Overland flow is known to occur as thin sheet flow, before surface irregularities cause a gathering of runoff into discrete stream channels. The primary distinguishing characteristics of overland flow is its shallow depth relative to roughness elements. The overland flow is an unsteady free surface flow and most dynamic part of response of watershed due to excess precipitation.

Overland flow is important from the point of view of the quantity of water transported, but more so from the point of view of its interaction with land surface and consequent contribution to the total surface runoff. It is also an important factor from the view of land-use practices as large scale erosion takes place because of the overland flow.

The overland flow from a mountainous watershed is recognised as a non-linear process. In general, there are two non-linear approaches which are used in analysing watershed response; system approach and hydrodynamic approach. System approach develops input-output relationships without making any explicit assumptions regarding the internal structure of the system. The approach requires the assumption that certain general laws of physics hold and further requires a geometrical abstraction of the real-world phenomenon. It is, therefore, the hydrodynamic approach which has been mostly used for overland flow modelling by several investigators.

The various steps in applying the models based on hydrodynamic approach are:(i) to decide upon the model geometry wherein the catchment may be represented by simple geometric elements such as combination of planes and channels or linearly converging and diverging sections. (ii) to decide upon the form of resistance law and infiltration equation, and (iii) to solve the hydrodynamic equations.

Non-linear behaviour of overland flow models poses the difficulty in solving the hydrodynamic equations. Therefore, two simplified approaches namely Horton-Izzard approach and kinematic wave approximation are used to solve the hydrodynamic equations.

The review indicated that the kinematic wave approximation to the hydraulics of overland flow is better for rough and steep slopes. Several investigators emphasised that the approximation is valid for almost all cases of overland flow.

Various investigators developed both analytical and numerical solutions to kinematic wave equation. Analytical solutions to the kinematic wave equations provide answers for a simplified class of problems while problem of a more general type are handled with numerical solutions through attendant discritisation of the solution domain. The assumption leading to the analytical solutions are restrictive and as such their practical utility is greatly diminished, when compared to numerical solutions of kinematic wave equations. The numerical solutions are also far from being elementary, as the stability and convergence criteria need to be respected. Furthermore, numerical schemes may result in ''kinematic shocks' which need to be modelled properly.

Future research needs to be oriented towards finding more accurate estimate of rainfall excess and soil-data, which is important in calculating infiltration rate in a watershed. The quantification of errors due to geometric simplification and the effect of different physiographic features on resulting overland flow hydrograph may help generalise the extent of geometric simplification of a watershed.