Flood-Control Reservoir types and Operational Challenges

 Lecture 12


FLOOD-CONTROL RESERVOIRS

Flood-control reservoirs, also known as flood-mitigation reservoirs, are artificial or natural water storage systems designed to manage and reduce the impact of flooding. 

A flood-control reservoir is designed to store a portion of floodwater in order to reduce the peak flow of the flood at a specific location that needs protection. In an ideal scenario, the reservoir is located just upstream of the area that needs protection.

The reservoir operates by "cutting off" the flood peak. This is done by releasing all the water entering the reservoir until the outflow reaches the maximum safe capacity that the downstream channel can handle. Any excess water above this safe capacity is stored in the reservoir. Once the inflow of water into the reservoir decreases and falls below the safe capacity of the downstream channel, the stored water is gradually released. This process helps to free up storage space in the reservoir, preparing it for the next flood event.

When the reservoir is located immediately upstream of the protected area, the flood hydrograph (a graph showing the rate of flow of water over time) at the protected point is the same as the hydrograph released from the dam. In this case, the peak flow is reduced by an amount labeled as "AB" in Figure below.

 

However, if there is some distance between the reservoir and the protected area, but no additional water (local inflow) enters the river between these two points, the operation of the reservoir remains quite similar. The only difference is that the natural flood hydrograph is further reduced as the water travels downstream from the reservoir. As a result, the reduction in peak flow that was "AB" at the dam becomes a smaller reduction, labeled as "CD," at the control point (the protected area).

In cases where there is significant local inflow (additional water entering the river) between the dam and the protected area, the reservoir must be operated differently. The goal is to minimize the peak flow at the protected area, rather than just at the dam. Typically, local inflow causes the flood peak to occur earlier than the peak from the upstream flow. To manage this, the reservoir operation strategy involves releasing less water early in the flood event and then increasing the releases later, timed so that the higher releases arrive after the peak of the local inflow. This helps to ensure that the combined flow from the reservoir and the local inflow does not create a dangerously high peak at the protected area.

Hence, the flood-control reservoir plays a crucial role in managing floodwaters by storing excess water and releasing it in a controlled manner. The operation strategy depends on the location of the reservoir relative to the protected area and the presence of local inflows. The ultimate goal is to reduce the flood peak at the protected area, ensuring safety and minimizing damage.

Types of Flood-control Reservoirs

There are two basic types of flood-control reservoirs

·        Detention reservoirs (or storage reservoirs, or detention basins) and

·        Retarding reservoirs (or retarding basins)

Detention Reservoirs

A detention reservoir is a type of water storage facility designed to manage and control floodwaters. It is equipped with outlets and spillways that are regulated using gates and valves. These gates and valves are operated based on the decisions and expertise of the project engineer, who determines when and how much water should be released to prevent flooding downstream.

The primary purpose of a detention reservoir is to temporarily hold excess water during periods of heavy rainfall or flooding, and then release it in a controlled manner to avoid overwhelming rivers or other water bodies. This is different from a conservation reservoir, which is mainly used to store water for long-term purposes such as drinking water supply, irrigation, or hydroelectric power generation.

One key difference between detention reservoirs and conservation reservoirs is the design of their sluiceways (channels or passages for water release). Detention reservoirs require larger sluiceway capacities because they need to quickly release large volumes of water, either before a flood to create storage space or after a flood to rapidly lower the water level. This rapid drawdown capability is essential for effective flood control, as it helps to minimize the risk of overflow and damage to surrounding areas.

Retarding Reservoirs

A retarding reservoir is designed with permanent, non-adjustable outlets that control the release of water based on the amount of water stored in the reservoir. These outlets operate automatically, meaning they do not require manual intervention or gates to regulate the flow. Typically, the outlet system includes a large spillway without gates or one or more sluiceways (channels or passages for water) that are also ungated.

In simpler terms, as the water level in the reservoir rises or falls, the outlets naturally adjust the amount of water being released. For example, when the reservoir is full, more water flows out through the spillway or sluiceways. Conversely, when the water level is low, less water is released. This automatic regulation helps manage water levels efficiently and prevents overflow or excessive drainage without the need for human control or mechanical gates. This design is particularly useful in managing floodwaters or maintaining stable water levels in the reservoir.

Operational Challenges in Flood-Control Reservoirs

Flood-control reservoirs are essential for managing and mitigating flood risks, but their operation is fraught with several challenges. Below is a detailed explanation of the key problems encountered during the operation of these reservoirs, simplified for better understanding:

1. Excessive Reservoir Inflow Floods

  • Idealized Operation vs. Real-World Scenarios: In an ideal situation, the operation of a flood-control reservoir is determined by the maximum capacity of the downstream channel (how much water it can safely handle). However, this ideal scenario becomes complicated when the volume of floodwater entering the reservoir is so large that it approaches or even exceeds the reservoir's storage capacity.
  • Need for Accurate Forecasts: To manage such situations effectively, accurate predictions of the incoming flood volume are crucial. Without precise forecasts, it becomes difficult to adjust reservoir operations (like releasing water) to prevent overtopping or dam failure. If the flood volume is underestimated, the reservoir may not be able to handle the excess water, leading to potential disasters downstream.

2. Forecasting Local Inflow Between the Dam and Control Point

  • Local Inflow Challenges: In many cases, there are smaller streams or tributaries that contribute water (local inflow) between the dam and the control point (the location where flood management is monitored). This local inflow can significantly impact the overall flood situation downstream.
  • Importance of Stream Flow Forecasts: To plan reservoir operations effectively, it is essential to predict these local inflows accurately. Forecasts are typically made using data from rainfall and river gauges spread across the basin. This data helps in applying rainfall-runoff relationships, unit hydrographs, and flood routing techniques to estimate future inflows.
  • Limitations of Forecasts: While these forecasts can be quite accurate (within ±10% under favorable conditions), they are not foolproof. For example, if unexpected rainfall occurs after a forecast is made, the actual inflow could be much higher than predicted, leading to errors in reservoir operations.

3. Recurring Floods

  • Reservoir Capacity and Flood Reduction: A flood-control reservoir is most effective when it is empty, as it can store the maximum amount of floodwater. However, after a flood event, a portion of the reservoir's storage capacity is occupied by the collected floodwater, which must be gradually released to make space for future floods.
  • Risk of a Second Flood: If a second flood occurs before the reservoir has been fully emptied, the remaining storage capacity may not be sufficient to handle the new flood. This means that reservoir operators often need to reserve a portion of the storage capacity as a precaution against a second flood. As a result, the full capacity of the reservoir cannot be used to control any single flood.
  • Potential for Increased Flood Risk: In some cases, if a second flood occurs while the reservoir is still full, the reservoir may inadvertently worsen the flood situation downstream by releasing excess water. This dual challenge—uncertainty about future inflows and the need to reserve storage—means that flood-control reservoirs cannot be fully effective in all scenarios. Their flood reduction potential is often less than what might be calculated under ideal conditions, except for smaller floods that require only a small portion of the reservoir's capacity.

4. Synchronization of Flood Flows

  • Downstream Flood Amplification: A significant operational problem arises when water released from a reservoir (which may exceed natural flow levels) combines downstream with floodwaters from a tributary. This synchronization can result in a combined flow that is much larger than the natural flood flow would have been.
  • Mitigation Through Advanced Weather Forecasts: The only way to minimize this risk is through accurate weather forecasts several days or even weeks in advance. Such forecasts allow reservoir operators to plan releases in a way that avoids synchronization with tributary floods.
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