Dynamic Flood Routing in Sediment-Affected Reservoirs Using Satellite- Derived Area-Elevation Curves: A Case Study of Golestan Dam, Iran

Dams are among humanity's most ambitious attempts to tame the river, acting as massive sponges that soak up sudden surges of water to protect towns and farms downstream. To do this safely, engineers rely on a fundamental understanding of the dam's shape: how much space is available to hold water at any given height. This relationship, known as the area-elevation curve, is usually mapped out when a dam is first built, based on the clean, empty valley it was designed to fill. For decades, the standard practice in flood safety has been to assume this shape never changes. The logic is simple: if you know how much water fits in a bucket, you can predict exactly how much will spill over the top when a storm hits. However, this assumption ignores a slow, relentless force that reshapes every reservoir over time: sediment. Rivers carry sand, silt, and mud, and when they slow down upon entering a reservoir, this material settles on the bottom. Over years and decades, this accumulation raises the floor of the reservoir, effectively shrinking the bucket and altering its shape, often in ways that are difficult to see from the surface.

In a recent study focused on the Golestan Dam in northern Iran, researchers set out to test what happens when we stop assuming the bucket stays the same size. The team, led by engineers from Gorgan University of Agricultural Sciences and Natural Resources and Sultan Qaboos University, investigated how the gradual filling of the reservoir with sediment changes the way floodwaters behave. They chose the Golestan Dam because it sits in a region prone to intense rainfall and flash floods, making accurate flood prediction a matter of public safety. The dam, completed in 1999, was originally designed to hold a specific volume of water, but by 2015, hydrographic surveys—essentially underwater maps of the reservoir floor—revealed a dramatic change. The sediment had raised the bottom of the reservoir by more than five meters and reduced the total storage capacity at the spillway level by nearly 40 percent. This meant the reservoir was significantly shallower and held less water than the original blueprints suggested.

To understand how this physical change affects flood safety, the researchers turned to a modern tool that bypasses the need for expensive and infrequent underwater surveys: satellite imagery. Using the Google Earth Engine, a powerful cloud-based system that processes vast amounts of data, the team analyzed more than 100 images captured by the Sentinel-2 satellites between 2015 and 2025. These satellites take pictures of the Earth every few days with a resolution sharp enough to see details as small as a single car. By applying a specific color-analysis technique that distinguishes water from land, the researchers were able to trace the exact outline of the reservoir's surface at different water levels. This allowed them to reconstruct the current shape of the reservoir, creating a new, updated map of how much water fits at every height, effectively updating the bucket's dimensions based on what the satellites actually saw rather than what was drawn on paper twenty-five years ago.

The team then fed these two different versions of the reservoir's shape into a computer model designed to simulate flood routing, which is the process of tracking how a flood wave moves through and out of a dam. They ran simulations for four different types of storms, ranging from a common 25-year event to a rare, catastrophic 200-year flood. In each scenario, they tested two conditions: one where the reservoir was empty and ready to catch water, and another where it was already full. The results revealed a stark difference between the old, static view of the dam and the new, dynamic reality. When the researchers used the updated, sediment-filled shape of the reservoir, the simulations showed that the dam would release significantly more water during a flood than previously calculated. For a massive 200-year flood, the updated model predicted that the dam would release about 12 million cubic meters more water than the old model suggested. This is a volume of water roughly equivalent to filling four thousand Olympic-sized swimming pools.

The implications of this extra water are profound for anyone living downstream. The study found that the loss of storage space due to sediment means the reservoir cannot hold back as much of the flood peak as engineers once believed. In the simulations, the peak flow of water rushing out of the dam was higher, and the total volume of the flood wave was larger when the sediment was accounted for. This effect was most dramatic when the reservoir started out empty, a condition where one might expect the dam to perform at its best. Even in this ideal scenario, the sediment-choked reservoir had less room to absorb the shock of the flood, leading to a quicker and more forceful release of water. The researchers also observed that the water level inside the reservoir rose higher than expected in the updated models, bringing the water closer to the top of the dam wall. For an earth-fill dam like Golestan, where the structure is made of compacted soil and rock, having the water level rise too close to the crest can pose a serious safety risk.

The study also highlighted a subtle but important flaw in how we traditionally measure reservoirs. When the researchers compared their satellite-derived maps with the 2015 underwater survey, they found that the two methods agreed well in the middle range of water levels but diverged significantly at the extremes. At very low water levels, the satellites detected small pockets of water in shallow depressions that the underwater survey had missed, likely because the survey boats could not navigate those shallow, sediment-filled areas. At the very top, near the spillway, the underwater survey appeared to rely on straight-line estimates that did not match the complex, curved reality of the reservoir's shape. This suggests that traditional surveys, while valuable, can sometimes miss the fine details of a reservoir's true geometry, especially as it changes over time. The satellite approach, by contrast, captures the actual water surface as it exists on a given day, offering a more realistic picture of the reservoir's current capacity.

Ultimately, the research demonstrates that relying on the original design of a dam is no longer sufficient for managing flood risks in a changing environment. The sediment that accumulates in reservoirs is not just a nuisance that reduces water storage; it fundamentally alters the hydraulic behavior of the entire system. By ignoring these changes, engineers and safety officials may be underestimating the danger of a flood, believing the dam can hold back more water than it actually can. The study proposes that using satellite imagery to regularly update the shape of reservoirs offers a practical, cost-effective way to keep flood models accurate. Instead of waiting decades for a new underwater survey, authorities can use the constant stream of satellite data to see how the reservoir is evolving and adjust their safety plans accordingly. For the Golestan Dam and countless others around the world, this shift from a static view of the past to a dynamic view of the present could mean the difference between a managed flood and a disaster.