Talk Wetlands and management of runoff upstream of roadways and railways

Road and rail networks are linear transport infrastructure on the ground (ILTS). The ILTS can widely collect the runoff by means of drainage ditches that run along the upstream of ILTS, and then are directed to underground passages to evacuate the runoff downstream. In the aim to not create flash floods in the downstream of a ILTS, a retention device can be installed to complement the upstream drainage device. For security issues, they are placed just downstream of ILTS.

The RIZHU project aims to analyze the opportunities of creation or reactivation of wetlands in hillslopes and rivers that are located upstream of a ILTS. The objective is to slowdown the runoff before it reaches the ILTS's drainage system.

The RIZHU method aims to reveal, where in a landscape, a humid zone exists, was drained or could take place. This opportunity analysis must be completed with a safety study.

On an operational level, RIZHU method is a new approach of the hydraulic context beyond the simple runoff contribution to a ILTS. By knowing which part of a watershed generates the runoff and which are the runoff pathways that feed a storage area where a humid zone takes place or can develop, it seems possible:

  • to look in a before-project stage, for a planned ILTS, where existing and potential humid zones are located. This, in the aim to reduce the environmental impact of a new ILTS, while seeking to promote the humid zone functioning and as well provide ecosystem service and flood control service. This, to protect the ILTS but also the more downstream issues;
  • to diagnose, for an existing ILTS, the effects of land use change towards more runoff. This, for considering solutions for maintaining the security-level of the ILTS by integrating the development of humid zones in the land use change;
  • to reconnect the runoff feeding of humid zones for which the runoff production area has been cut by the ILTS.

This strategy of spatial analysis aims to maintain or rebuild the network mesh of humid zones that was interrupted by the ILTS construction. A max distance of 500m between humid zones is required to ensure its ecological functioning.

The RIZHU method is based on hydrological analysis using GIS layers. The topography, soil and land use are combined. In this work, it is first introduced the method of selection of GIS indicators. It is then evaluated the ability of the method to identify humid zones by comparing the predictions with existing wetland. An application on two test sites located along a high-speed train line, provides an initial operational evaluation of the method.

First results indicate the method is able to find existing humid zones in a region which was not used to calibrate it. Second, a field trip revealed that the method can fail due to missing information in the present method. Third, another field trip revealed that the method was able to detect a small humid zone isolated in a drained agricultural plain.

wetlands, biodiversity, runoff control, railways, roadways