Measuring stormwater pond volumes with a drone echo sounder

The challenge

The City of Hamilton looks after 130 stormwater management facilities, including 70 ponds and wetlands that must hold a set permanent-pool volume regulated by the Ministry of the Environment, Conservation and Parks. It surveys them on a three-year cycle to check sediment build-up and decide when to dredge. Dredging too often wastes money; too little risks flooding.

The existing method had clear limits:

• two city workers took a boat by truck to each pond and measured depth with a GNSS rod, which was slow and labour-intensive;
• some sites, such as ponds beside the Red Hill Valley Parkway, have little space next to a busy highway, with steep or slippery banks;
• only 10 points were measured per facility;
• thick underwater plants could make the bottom read too high, the rod's plate could sink into soft sediment, and wind pushed the boat off position.

The solution

The team planned a grid flight over each pond in UgCS and flew a DJI M300 RTK with SPH Engineering's system and an EchoLogger ECT 400S single-beam echo sounder. The sensor hung just below the water surface while the drone flew about 2 m above it, held steady by a radar altimeter. Each depth was positioned by the drone's RTK GNSS.

Hydromagic turned the sonar data into a 3D model of the pond bottom and calculated the water volume. A second drone captured a photogrammetric 3D model of the site above water. Dewatered ponds were surveyed with LiDAR, photogrammetry and traditional methods, so the new system could be checked against established ones.

The results

Calculated volumes differed very little between the drone echo sounder and the other methods. For facility SWMF 74 (Binbrook), at permanent-pool elevation:

• before dredging: 1,140 m³ by sonar, 1,230 m³ by photogrammetry and 1,360 m³ by LiDAR;
• after dredging: 1,690 m³ by sonar, 1,630 m³ by photogrammetry, 1,670 m³ by LiDAR and 1,590 m³ from the contractor.

Instead of judging capacity from ten readings, the city now gets a high-resolution map of each pond. That lets it time dredging to balance cost against the risk of non-compliance, avoid premature spending, disturb pond ecosystems less and keep workers away from the water.

Where this applies in India

Urban drainage and water bodies in India need the same capacity checks. The workflow suits:

• stormwater detention ponds in cities, smart-city projects and townships;
• urban lakes and tanks, to decide when desilting is needed and to estimate its volume;
• sediment ponds along highways and at industrial sites;
• universities and training institutes building drone survey programmes with local bodies.

SPH Engineering's bathymetry training is also available through HobiTech.

The equipment, available from HobiTech

Echo sounder	EchoLogger ECT 400S
Altimeter	Radar altimeter for True Terrain Following
Onboard computer	SPH Engineering's drone echo sounder system runs on SkyHub by SPH Engineering with the SkyHub echo sounder software
Flight planning	UgCS - Drone Flight Planning Software
Drone	DJI M300 RTK was used in this project. Ask us for current compatible drones, such as the DJI Matrice 350 RTK.
Processing	Hydromagic (used in the project)

Case study by SPH Engineering, published June 2024. This page summarises it in our own words; the work was carried out by Mohawk College with the City of Hamilton, not by HobiTech. Read the original case study on SPH Engineering ↗