Measurement and modeling of concentrated runoff in grassed waterways

Abstract

Grassed waterways (GWWs) are a common measure to drain surface runoff from fields without gullying along the drainageway (thalweg). Moreover, they have a great potential to reduce runoff volume and peak discharge rate, especially if they are located in relatively small watersheds typical for many small patterned landscapes in Europe. Due to the flow characteristics in a GWW, an area of shallow sheet flow on the side-slopes and another of concentrated flow along the thalweg can be identified. The runoff control on the side-slopes is comparable to that of vegetative filter strips, which was intensively investigated in many studies. Our objectives were to use experimental field data, modeling and a sensitivity analysis to evaluate the parameters (morphology, soil, vegetation, water input) dominating the concentrated runoff along the thalweg of a GWW, and thus to optimize GWW layout. The experimental data were derived by pumping concentrated inflow to the upstream end of two GWWs (290 and 370 m long). The used model is simulating infiltration according to the Philip's [Adv. Hydrosci. 5 (1969) 215] equation and routing the runoff with a kinematic wave approximation. The experiment showed a great difference in runoff control between the two tested GWWs, e.g. one reduced runoff volume by 90% the other by 49%. The model agreed well with the experimental data. From the sensitivity analysis of the model parameters, it appeared that the main reason for the difference in runoff characteristics was the flat-bottomed compared to more or less v-shaped cross-section of the thalweg. Differences in hydraulic roughness between the tested GWWs were small, but in general hydraulic roughness is a sensitive parameter in runoff control of a GWW, because in case of grass submergence or high runoff velocities grass is bent to the ground, and hence the hydraulic roughness drops drastically. Soil conditions are only prominent if higher saturated conductivities were assumed than measured at the test site. For colluvial soils typically found where GWWs are established similar efficiencies in runoff control can be expected for similar inflow hydrographs. In general, a high potential of GWWs to reduce runoff volume and peak discharge could be verified within the examined scope of site conditions.