Flood Routing

Flood routing is the process of estimating the changes in flow characteristics as water moves through a river system or a reservoir. It is an essential tool in flood management and planning. Reservoirs are commonly used for flood control by storing excess water during heavy rainfall events and releasing it gradually to prevent downstream flooding. When a flood wave passes through a reservoir, the characteristics of the outflow hydrograph are influenced by the storage and release capabilities of the reservoir.

Inflow and Outflow Hydrographs

The inflow hydrograph represents the variation of flow entering the reservoir over a certain period of time. It is usually plotted as a graph with time on the x-axis and flow rate on the y-axis. The shape and magnitude of the inflow hydrograph depend on various factors such as rainfall intensity, duration, and watershed characteristics.

The outflow hydrograph, on the other hand, represents the variation of flow leaving the reservoir after passing through it. It is also plotted as a graph with time on the x-axis and flow rate on the y-axis. The outflow hydrograph is influenced by the storage and release characteristics of the reservoir and may have different characteristics compared to the inflow hydrograph.

Attenuated Peak with Increased Time-base

When a flood wave passes through a reservoir, several processes occur that influence the shape and magnitude of the outflow hydrograph. The storage capacity of the reservoir allows it to store excess water during the flood event, thereby reducing the peak flow rate in the outflow hydrograph. This is known as attenuation.

Additionally, the release of water from the reservoir is controlled, which can result in an increased time-base of the outflow hydrograph. The outflow hydrograph may be stretched over a longer period of time due to the controlled release of water. This is particularly evident when the outflow hydrograph is compared to the inflow hydrograph, where the duration of the flood event may be shorter.

Therefore, the correct answer is option 'B' - the outflow hydrograph will have an attenuated peak with an increased time-base. This means that the peak flow rate in the outflow hydrograph will be lower compared to the inflow hydrograph, and the duration of the flood event will be longer.