How often does one get asked to map a sandbar?
PacificUAV recently received a request to map a sandbar in the Fraser River. The client wanted up-to-date accurate information on it’s extent, topography, and volume. In this short post we highlight some important considerations when mapping a sandbar.
Timing is Critical
The best time to survey a sandbar is when water level is at its lowest. This ensures that the greatest volume of sandbar is visible above the water line. For sandbars located in areas where tide influences water level it is important to find the best available information source on tides and schedule the survey to occur when the water level is at its lowest. The Fraser River is considered a tidal river over its lower sections which means ocean tides affect water level and current speed as it approaches the ocean. We obtained tide information from Fisheries and Oceans Canada and spoke with local river users to determine the best possible times for surveying the sandbar.
Some sandbars are connected to land and some are not. Sandbars not connected to land require special transportation arrangements. In this case we were lucky enough to have a client who knew the area very well and who could take to the sandbar by boat. Ensure that whoever is in charge of transportation is knowledgeable of the area, its local hazards, and can take you to where you need to go safely.
Beware of Tides
Sandbars generally have low profiles so even small changes in water level can significantly affect the amount of the sandbar that is visible about the water line. Tides at this particular site were surprisingly quick and we underestimated the time window available for the survey. As such we lost several ground control points to the water. For large sandbars we recommend conducting the survey is as short a time as possible. You can achieve this by using more than one drone to conduct the survey. This will also minimize temporal variability in the resulting imagery.
Sand, sand and more sand
Sandbars are generally long and linear, have low profiles, and are made out of sand (no kidding). These characteristics can present some challenges to the surveyor; it takes more effort and time to walk around on the sandbar, sand can get into your gear and equipment, and sandbars are exposed to the weather (sun and wind). So take the time to plan mitigation strategies for minimizing any negative affects these will have on your survey.