A GIS tool for the design and assessment of road drain spacing to minimize stream pollution: RoadCAT

Unsealed roads are an important source of runoff and sediment which can affect the hydrology and water quality of streams. The Road Connectivity Assessment Tool (RoadCAT) is being developed based on the conceptual framework of volume-to-breakthrough and hydrological connectivity between roads and streams in managed forest environments that allows identification of the different types of delivery pathways and estimation of the runoff volumes delivered through them. RoadCAT is built in ArcGIS's model builder using existing and customised toolboxes. RoadCAT uses rainfall event intensity data, a digital elevation model, roads vector layer and drain points layer (if assessing existing drains) to model drains likely to cause gully erosion at drain outlet, drains connecting with adjacent streams, volume of runoff likely to connect and quantity of suspended sediment potentially transported to the stream based on empirical event-based models. This paper reports on the development of the RoadCAT tool and its application to a South Australian Water Corporation reserve in which new roads are being constructed for reserve management and fire fighting access, and the opportunity presented to model an unconstrained drain spacing design to prevent gully erosion at drain outlets and minimise road runoff connectivity to reduce stream pollution with suspended sediment.