The use of echo-location in determining shapes, distances or movement of objects is a technique used in nature by animals, such as bats and dolphins, to navigate and hunt. The technological form of echo-location, known as sonar (SOund NAvigation Ranging), has been around for more than 100 years. The same principle of sending a signal from a remote location and measuring how long the echo or reflection takes to return is applied using other types of signals as well, often with even greater speed and accuracy. Radar (RAdio Detection And Ranging) uses radio waves for this purpose while LiDAR (Light Detection And Ranging) uses light from lasers.
An accurate picture or “model” of the earth’s surface is important to better understand the world or, in this case, the watershed in which we live. Within the last few years, two separate, but related LiDAR projects have been undertaken in the Grand River watershed using lasers and measuring systems mounted in aircraft.
The first was completed as a joint project between the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) and the Ministry of Natural Resources and Forestry (MNRF). This project involved the collection of ground surface information outside of water bodies, referred to as topographic LiDAR. Within the Grand River watershed, this project collected very precise elevation data at a density of approximately 10 points per square metre. With a watershed area of 6,800 km2, this translates into roughly 68 billion elevation data points.
The second project was completed by the GRCA with funding support from the federal government’s National Disaster Mitigation Program. Its primary focus was the collection of below-water ground surface data, or bathymetry.
The use of LiDAR technology to study bathymetric surfaces in shoreline environments is not new, with the first application taking place at Lake Ontario in 1968. Until recently, however, there were limitations to its use due to the weight and power requirements of the equipment itself. These limitations restricted it to military applications and larger aircraft capable of handling the loads. Bathymetric LiDAR technology has advanced, resulting in smaller equipment that requires less power making it much easier to mount advanced systems in smaller aircraft, providing access to previously unreachable areas. The GRCA’s project is groundbreaking in that it is, to our knowledge, the first time it has been used in a riverine environment in Canada.
Beyond equipment size and site accessibility, there are many other constraints to the application of bathymetric LiDAR that had to be considered in planning the project. The technology relies on light to not only reach its target (i.e., the riverbed), but to do so with enough intensity that the reflected portion has enough energy to bounce back to sensors in the plane. This is less of an issue with topographic LiDAR, assuming the work is completed in cloud-free conditions. Water makes the collection of LiDAR data much more challenging. Unlike some shoreline environments, rivers and creeks pose their own challenge where elements such as cloudy water due to silt and sand, ice coverage or underwater vegetation growth can all combine with water depth to create conditions that greatly reduce visibility. In short, if light cannot penetrate it, the technology will not be effective.
After assessing the various constraints, the GRCA felt that the likelihood for successful application of bathymetric LiDAR technology across large portions of the watershed was reasonable and undertook the project, with all fieldwork collected in late fall 2018. While still being processed, preliminary review indicates the data obtained is of high quality and accuracy at many locations across the watershed. As expected, in areas where deep waters and/or turbid conditions exist, the collection of data was less successful.
Data from the two projects is being combined to create a topo-bathymetric elevation model, which is a three- dimensional digital picture of ground surface elevations above and below the water line. This type of elevation model can be used to answer many questions related to surface hydrology mapping, flood risk management, water supply and quality, infrastructure and construction management, agriculture and land management, forest resources management, as well as land-use and urban development among other applications. The initial focus for GRCA staff is on watershed hydrology and river hydraulics, and their relation to riverine flooding. The elevation model forms the basis for subsequent analysis that help predict, for example, how runoff from rainfall and/or snowmelt events flows across the surface of the watershed and in developing a better understanding of how deep and fast the water becomes once it enters the creek and river system. The product created from this analysis is called flood hazard mapping. This information is critical to guiding key GRCA activities such as land use planning and regulation, the design and operation of flood control infrastructure and, during flood events, forecasting and warning activities.
All of these activities are directly tied to the GRCA’s strategic objective of protecting life and minimization of property damage from flooding and erosion.
The ability to obtain comprehensive and accurate elevation data is by no means new. Good old-fashioned, boots-on-the-ground hand surveying is an activity that continues to be an invaluable resource of elevation information. However, this approach, particularly when undertaken at a large-scale is time-consuming, expensive, and occasionally dangerous (especially in water) and can be prone to human error. Boat- mounted sonar represents another option, but is also relatively slow and has physical limitations.
Advancements in airborne LiDAR data acquisition capabilities offer the potential for data to be cost-effectively collected at a quantity, accuracy and speed not attainable using other technologies. Advances in data management / processing capabilities, and mathematical modeling and mapping mean that we are better able to use this data in many types of studies. While the GRCA’s initial efforts are directed toward improving our understanding of flood hazards and the reduction of associated risks, the data itself will be made publicly available on the GRCA website once a thorough review has been completed, and will be of potential benefit to a wide variety of applications. The LiDAR data will significantly enhance the types of analysis that can be completed and how that work is undertaken.