Development of VFDM: A Riparian Vegetated Filter Dimensioning Model

Abstract

Riparian buffer zones are considered as one of the best beneficial management practices (BMPs) to improve surface water quality. In most cases they are assumed to be responsible for large reductions in sediment and agricultural chemical losses from cropland to water bodies. Filtration, deposition, infiltration, adsorption, absorption, decomposition, and volatilization are thought to be the biophysical processes governing the reduction of agricultural contaminants transported by surface runoff. Vegetation at the downstream edge of disturbed areas may effectively reduce runoff volume and velocity; initially because of increasing hydraulic roughness, and subsequently by enhancing water infiltration. Decreasing flow volume and velocity lead to sediment deposition in the vegetative filter (VF) as a result of reducing the transport capacity of runoff. Dimensioning and positioning these structural BMPs represents a real life challenge for soil conservation engineers, managers, planners and policymakers. Different factors, such as trapping efficiency; implementation, management, and opportunity costs (resulting from cropland surface reduction); and government policies and regulations need to be weighed to meet this challenge. Trapping efficiency depends on many parameters, including: (i) VF characteristics such as width and slope, vegetation height, vegetation density, species composition; (ii) flow characteristics such as runoff velocity, discharge volume, water height; and (iii) sediment characteristics such as particle size, aggregation, and concentration. Government policies and regulations may include dimension and location of VFs and/or a cropland percentage that needs to be converted into VF areas. The main objective of this paper is to describe the development of a mathematical model to determine the optimal dimensions of riparian vegetated filter strips (RVFSs) in agricultural watersheds. The model calculates the optimal width with respect to vegetation, topographical, hydrological and sedimentological characteristics. Future work will involve linking the model to an economics model in order to evaluate the effectiveness of RVFSs with respect to both water quality and economic objectives. SILVIO J. GUMIERE1,2 and ALAIN N. ROUSSEAU1