Reducing flood transfer in highly drained agricultural watersheds: state of knowledge and research

Abstract

During most common rainfall events (one to five years return period), subsurface drainage increases the soil infiltration capacity. Consequently field-scale peakflow rates are generally lower than were surface runoff peakflow rates before drainage construction. However flood routing may be enhanced by arterial drainage ditches, often over-sized due to collector drain depths. This enhancement could be mitigated by letting the network overflow in its less critical zones, which could be achieved through reduced cross sections such as road crossovers. This solution applied to rivers is referred to as dynamic flood control. This paper presents some consequences of this principle applied to an arterial drainage network and brings design hints. In order to describe flow transfer through a small agricultural catchment (a few sq. km), a hydraulic model based on Saint-Venant equations was applied to an experimental catchment (Orgeval experimental catchment, Seine-et-Marne). The model was used to test additional storage places with a same mitigation objective. Adding reduced cross sections brought a gain in flood storage capacity until an upper number. Dispatched storage places appeared also more efficient than a single reservoir located downstream. Associated culverts, with two different diameters, also appear to control a larger range of events than a single reduced cross section. As a counterpart to these flood control measures, a complex modification is induced at subsurface drainage outlets. An additional peakflow rate mitigation within collector drains networks can be observed, but it means a reduced efficiency for field subsurface drainage. This paper presents a recently built physical model which begins to give results, and will help to explain this interaction, and to design flood mitigation measures with a better adequacy.