Bathymetric Survey Case Study: Measuring Silt Loss in a 30-Year-Old Reservoir

The client needed a number that irrigation departments rarely have on hand: exactly how much water storage capacity a reservoir had actually lost since it was built. Design records showed the original capacity, but thirty years of monsoon runoff had carried silt into the basin, and nobody had measured how much. Without that number, the irrigation planning for the coming season was based on a figure that no longer reflected reality.

The Challenge

The reservoir spans 1.8 square kilometres at full storage level, with depths ranging from 2 metres near the inlet to over 18 metres near the dam wall. No underwater survey had been done since the original impoundment. The irrigation department needed a bed profile accurate enough to calculate actual live storage capacity, not the design capacity from decades-old drawings.

Survey boat running SBES transects across the reservoir at full pool.

What We Delivered

An underwater bathymetric survey using a single beam echo sounder mounted on a survey boat mapped the entire bed in parallel transects 20 metres apart, tighter near the dam wall where depth changes fastest. GPS-tagged depth readings were tied to the shoreline topography collected by drone photogrammetry above the waterline, giving one continuous 3D surface from dry bank to reservoir bed.

The Result

Depth contour output highlighting the silt accumulation zone near the main inlet channel.

What This Changes

A 22 percent capacity loss is not a rounding error, it is the difference between a reservoir that can meet a full irrigation season and one that runs short in a dry year. With quantity takeoff data now in hand, the department has a defensible basis to plan desilting operations where the silt has actually accumulated, rather than guessing. This is the kind of number that only comes from measuring the bed directly, and it is the reason bathymetric surveys pay for themselves within a single planning cycle.