Mapping Culverts at QUBS. A GIS Adventure.

By Aaron Sneep (Summer Work Experience Program – SWEP – intern 2017)

Mapping Culverts at QUBS By Aaron Sneep (Summer Work Experience Program intern 2017) Queen’s University Biological Station (QUBS) strives to be one of the best field stations in North America. To provide more resources for researchers at the station, QUBS wishes to develop a detailed, fine-scale hydrological map not only its land holdings but also adjacent lands. This summer’s field and data management technicians, Dayna Zunder and myself, have been hard at work collecting data on the locations of culverts in the region to contribute to this hydrological map (Figure 1).

Dayna at culvert
Figure 1. Field and Data Management SWEP intern Dayna Zunder posing alongside a culvert being mapped using a handheld GPS.

The culvert data will be used to complement and correct computer generated terrain models of the region produced from LIDAR (Light Detection and Ranging) data (see Figure 2). The reason we require the locations of culverts to supplement the LIDAR data is rooted in the process of LIDAR data collection. During LIDAR data collection, laser light is emitted from an aircraft flying overhead and each data point collected during the process represents a surface that reflected a laser back to the receiver. LIDAR data is often referred to as a point cloud due to the appearance of the data collected. Figure two shows an example of a LIDAR point cloud.

Figure 2. Side view of a Lidar point cloud for a cluster of trees. Each point in the figure represents a surface that reflected a point of laser light back to the receiver on board a plane flying at low altitudes overhead. (Sneep unpubl. 2017)

Using the point clouds, we can generate terrain models, often referred to as DEMs (Digital elevation model), or DTMs (Digital Terrain Model). With these models, we can simulate how water flows through the region (i.e. imagining how water flows from a high point in the landscape to lower elevations). However, there are concerns and caveats with this approach. The laser used during LIDAR collection cannot penetrate through solid objects such as a bridge or roadway overlying a culvert. In addition, due to limitations in spatial resolution it can be challenging to distinguish the locations of culvert openings using LIDAR data alone. Knowing the locations of culverts and bridges allows us to indicate their locations in the terrain models, and change our flow models that would otherwise appear as impediments to indicate that water is able to flow through these points in the landscape. Therefore, collecting spatial data for bridges and culverts in the region allows us to more accurately represent the flow of water through the watershed. Figure 3 shows a comparison of a DSM with and without culverts indicating where water flow can occur.

Figure 3. Left, digital elevation models (DEM) taken from a section of the Rideau trail running through the Massasauga tract of QUBS property, and on the right the same DEM supplemented with culverts and an outline of the Rideau trail. By looking at the DEM alone, it appears that the Rideau trail is a barrier to water flow. However, by knowing the locations of culverts along the trail, we know that water is able to flow under the trail and allows connectivity between the two water bodies on both sides of the trail. With this information, the DEM could be altered to more accurately model the flow of water in the region.

These models of water flow can be useful in biological research because of the important role connectivity can play in biological systems. For example, connectivity between different areas within the watershed can have implications for nutrient transport, species interactions, and dispersal and gene flow. As such, understanding hydrological connectivity within this watershed we hope will be widely applicable and encourage additional research at QUBS. There is still much work left to complete the hydrological model for this watershed, but future researchers at QUBS can look forward to using an accurate, fine-scale hydrological map to illuminate their research questions.