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.