Early learnings from the Taw bridge sensors

Our Taw Bridge sensor has only been running for one week, but it is already providing insights on the river hydrology. The sensor is attached to one of the bridge pillars, and rests in a pool of water at the base of the pillar. This siting was important, as some of the individual sensors need to be kept wet at all times.

The water level of the lagoon varies with the tide and the volume of water flowing down the river. The incoming tide overtops the sandbank and the lagoon disappears under several metres of water. At low tide, river water rushes out of the estuary and the bridge pillar is isolated again in its pool of water. We were not exactly sure on the balance of fresh and saltwater within the lagoon, but the sensor is giving us some interesting data.

The orange line in the above graph shows the variation in tide height within the estuary. The first few peaks are spring tides, and the daily height then decreases each day as part of the natural lunar cycle. The blue line shows how salinity varies at the bridge pillar. It peaks shortly after high tide, with a large volume of seawater surrounding the sensor; as the tide drops, river water pours out of the estuary and inundates the lagoon. Salinity drops to near zero and the sensor is measuring river water until the next incoming tide. It is also noticeable that tides of less than 2.5m barely increase the salinity at the sensor. An additional factor not considered here is the impact of rainfall on the volume of freshwater flowing downstream.

We will continue to monitor salinity as an important indicator as to the source of the water being measured. We will also carry out visual checks to establish when the lagoon is isolated from the rest of the river. These hydrological impacts are key to understanding pollution within the estuary.