Transmissivity and groundwater flow exert a strong influence on drainage density

The extent to which groundwater flow affects drainage density and erosion has long been debated, but is still uncertain. Here, I present a new hybrid analytical and numerical model that simulates groundwater flow, overland flow, hillslope erosion and stream incision. The model is used to explore the relation between groundwater flow and the incision and persistence of streams for a set of parameters that represent average humid climate conditions. The results show that transmissivityand groundwater flow exert a strong control on drainage density. High transmissivity results in low drainage density and high incision rates and vice versa, with drainage density varying roughly linearly with transmissivity. The model evolves by a process that is defined here as groundwater capture, whereby streams with a higher rate of incision draw the watertable below neighbouring streams, which subsequently run dry and stop incising. This process is less efficient in models with low transmissivity due to the association of low transmissivity and high watertable gradients. A comparison of different parameters shows that drainage density is the most sensitive to transmissivity, followed by parameters that govern initial slope and stream erosion. These results imply that permeability and transmissivity exert a strong control on drainage density, stream incision and landscape evolution and that models of landscape evolution may need to explicitly include groundwater flow.

Modelling the effects of permeability, groundwater flow and water table depth on landscape evolution

<p>The role of groundwater flow in determining overland flow, drainage density and landscape evolution has long been debated. Landscape models often only address groundwater as a simplified storage term and do not explicitly include lateral groundwater flow, although recently some model codes have started to include lateral flow. However, the role of groundwater flow on landscape evolution has not been explored systematically to my knowledge. Here I present a new numerical and analytical model that combines groundwater flow, saturation overland flow, hillslope diffusion and stream erosion. A number of model experiments were run with different values of transmissivity and groundwater recharge. The model results demonstrate that transmissivity, groundwater flow and the depth of the watertable strongly govern overland flow, the incision of stream channels and erosion rates. The results imply that the permeability and transmissivity of the subsurface are important parameters for explaining and modelling landscape evolution.  </p>

Experimental Study of Flow Structure in Converging–Diverging Axisymmetric Geometry

Control valves, nozzles, and other mechanical flow constricting constructions often introduce highly turbulent flow downstream of vena contracta. When solid particles are present in the fluid stream erosion can occur due to the high fluid velocity and turbulence, both in the converging and diverging section of a constriction. This paper presents an experimental study of such flow structure in a converging–diverging valve geometry. The purpose of the study is to obtain velocity profiles downstream of vena contracta by the use of particle image velocimetry measurements. Profiles and flow structure, with and without introduced swirl component, is investigated and discussed with respect to erosion in such specific geometries.

A Model for Sustainable Water Supply in Rural Communities: The Case of Ekondo-Titi, Cameroon

<p><em>One would have expected that Ekondo-Titi which has the rare luxury of benefiting from the radial drainage pattern of the Rumpi Hills (which serves as the main catchment in Ndian Division) would be self-sufficient in terms of water quality and quantity. This community is rather greeted with water scarcity which is partly blamed on the failure to initiate and sustain water supply schemes. Forest destruction in favor of oil palm plantations by Pamol and small holder schemes have contributed to degrade the major water catchments thereby increasing stream erosion and siltation. This has altered the flow characteristics of streams. This paper proposes a model for sustainable water supply by indicating the opportunities that exist for internal and external actors to galvanise their resources to ensure adequate water supply in this fast developing rural community. It was observed that a major setback in the attainment of this goal remains the insufficient resources (material and financial) and the absence of a synergy among the key actors of Ekondo-Titi. The model recommends the strengthening of stakeholder synergy and where necessary, further support from external actors in the process of initiating and sustaining water development projects.</em></p>