The Project
The operator of a reservoir engaged Trilogy to create a 3D model of the basin geometry and derive a precise relationship between water level and stored volume. The objective was practical: given a known water level reading, the operator needed to know exactly how much water the reservoir contained — and how much capacity remained.
The opportunity to answer it precisely arrived when the reservoir was emptied for scheduled maintenance. With a limited window before the basin was allowed to refill, the survey had to be planned, executed, and completed within that interval. Timing was the critical constraint.
Reservoir capacity is typically managed using a simple rated capacity figure from original design drawings, or a staff gauge lookup table from an early survey — both of which degrade in accuracy over time as basin geometry changes through sedimentation, erosion, or modification. Neither delivers volume at every water level increment. A 3D-model-derived table does.
Approach
Survey Window Coordination. The project required close coordination with the operator to align the aerial survey with the maintenance window. An empty reservoir is not a common condition — most operators drain for maintenance only occasionally, and the window between emptying and refilling is typically short. Trilogy confirmed survey scope, flight parameters, and marker installation plan in advance so that the field team could mobilise and complete the survey within the available time.
Water Level Markers. Before flying, physical water level markers were installed at the reservoir to provide fixed reference points within the basin. These serve two purposes: they provide ground control points for photogrammetric processing, improving absolute accuracy of the 3D model; and they provide visible indicators of water depth that can be read from subsequent aerial imagery if a future comparative survey is undertaken. Marker positions were recorded with RTK accuracy.
UAV Photogrammetric Survey. A drone survey was flown across the full reservoir extent using Real Time Kinematic (RTK) positioning, achieving horizontal and vertical accuracies of 1–2 cm. RTK positioning eliminates the need for ground control point grids across the basin floor — impractical to install uniformly across a large reservoir bed. The drone captured overlapping nadir imagery at consistent altitude calibrated to the basin dimensions.
3D Model and Volume Calculation. Survey imagery was processed to generate a dense point cloud and a georeferenced 3D surface model of the basin — capturing every contour, irregular depression, and sloped surface that determines how volume accumulates as water level rises. From this model, Trilogy calculated the volume at each 10 cm water depth increment from base to spillway level, integrating the enclosed volume at each level slice rather than interpolating between a small number of survey points.
The Deliverables
Two operational data tables were produced alongside the 3D model, each directly usable by the operator for day-to-day reservoir management:
- Table 1 — Volume Contained: for a given staff gauge reading or water level sensor output, the operator can look up the exact volume of water currently stored in the reservoir, at 10 cm resolution
- Table 2 — Remaining Capacity: for a given water level, the operator can look up the volume of additional water the reservoir can accept before reaching spillway level — critical for managing inflow events and scheduling releases
- 3D Model: a permanent georeferenced basin asset, queryable for future management and updatable by repeat survey to track sedimentation or geometry changes over time
The 10 cm increment resolution is the key operational difference from a traditional rated capacity estimate. At this resolution, an operator managing irrigation scheduling or a timed release can manage inflows and outflows to a known volume target — not just a level target — which is particularly valuable when the inflow rate is variable or the release volume needs to be metered accurately.
Why the Empty Basin Is the Opportunity
A reservoir's geometry can only be directly surveyed when it is empty. When full or partially full, the water surface obscures the basin floor and below-water geometry can only be inferred from bathymetric survey — a slower, more expensive process. Maintenance shutdowns are therefore the optimal moment to commission a capacity model. Planning the aerial survey as part of the maintenance program — rather than as a separate engagement — is the most efficient way to capture it.
The 3D model produced during the empty condition also serves as a baseline for monitoring: future surveys can be compared against the baseline to identify sedimentation accumulation, structural movement, or changes in usable capacity over time.
Outcomes
- Permanent, precise 3D model of basin geometry replacing estimated or historical capacity figures
- Volume lookup tables at 10 cm resolution from base to spillway — the operator can read a water level and know, not estimate, how much water the reservoir holds
- Remaining capacity table enabling volume-target management of inflows and scheduled releases
- Georeferenced baseline asset for future comparative monitoring of sedimentation and geometry change
- Survey completed within the maintenance window — no additional shutdown required