The value of a desk study for building a national river obstacle inventory

This study evaluates two desk-based approaches for building an inventory of man-made river obstacles. The creation of a river obstacle inventory is a vital first step in developing a prioritization process for obstacle removal and/or modification. In this study, a desktop geographical information system analysis of two rivers and their tributary network was undertaken, using two different approaches. The first involved analysing historical maps, satellite imagery, and Ordnance Survey Ireland Discovery Series maps and producing a geo-referenced layer of all the potential river obstacles. The second involved developing a geo-referenced layer of potential river obstacles based on the intersections between elements of the transport network (roads and railways) and river systems. To determine the effectiveness of the desk studies, the located obstacles were cross-referenced with actual obstacles verified through a field survey. The desk studies identified several thousand potential obstacles. The study utilizing a range of maps consistently located a greater number of actual obstacles than the desk study based on intersections between the transport and river networks. The results indicate that desk-based research offers an efficient and effective method for locating river obstacles and can guide subsequent field surveys aimed at confirming the presence of obstacles. This is particularly useful for eliminating from study large stretches of rivers that would otherwise need to be walked to confirm the presence, or otherwise, of potential river obstacles. In this regard, desk-based exercises can offer opportunities to save on both time and cost in larger river assessments.