2 Hydrographs and flood terminology explained

A river hydrograph is simply a depiction of the flow in a river on a graph. When it shows the response in flow to a heavy rain event (storm), then it is referred to as a storm hydrograph. Figure 4 shows the key features of such a graph. The vertical axis on the left shows the flow (or discharge) of the river measured in cubic metres per second (sometimes abbreviated to cumecs), while the horizontal axis at the bottom denotes time (usually in days). The area under the curve (shaded in Figure 4) represents the total volume of water discharged during the flow event.

Figure 4 A storm hydrograph describing a river’s flow following a heavy rain event.

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A graph showing time on the x axis and water flow on the y axis . A rainfall event causes river levels to rise above base flow levels. The lag time is the time between the rain event and the peak discharge. Bank full discharge is not reached in this example.

Figure 4 A storm hydrograph describing a river’s flow following a heavy rain event.

The key items that the hydrograph depicts are:

• Baseflow: the steady level of the river with normal rainfall and no storm event.
• Lag time: the time that elapses between the storm event and the flood peak arriving at a particular location.
• Bank-full flow: the maximum flow rate that the channel can contain. If the flow rate exceeds this value, water will spill out of the channel onto the floodplain.
• Peak discharge: the maximum flow rate following a storm event.
• Time above threshold: the duration of water coming out of the river channel to inundate the floodplain.

In a densely populated country like the UK, most of its rivers are quite heavily managed and that management can be altered to change the shape of a storm hydrograph. For example:

• Clearing drains and ditches high in the catchment will help water move quickly to the river, reducing the lag time.
• Raising flood embankments will increase the bank-full flow.
• Water abstraction for irrigation will lower the base flow.

In terms of managing flood risk, the most important aspect of a storm hydrograph is the peak flow, as this determines the maximum height of the floodwater and therefore the area that would be affected. While the initial response to floods is to build higher flood defences (e.g. embankments to keep the water in the channel), this can only work to a limited degree. By keeping all the water in the channel through one vulnerable area, the risk to the next vulnerable area downstream increases. There is now a move away from such engineered solutions to more natural flood management by giving water the room to spread across floodplains. The effects of such Natural Flood Management (NFM) are shown in Figure 5.

Figure 5 Storm hydrographs from an urban monitoring station toward the bottom of a flashy rural catchment. They depict a) a storm event prior to floodplain restoration work; b) an equivalent storm event after the floodplain in the upper catchment has been reconnected to the river through the removal of embankments

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Two graphs in the same format as Fig 4, but graph a shows a sharp peak breaching the bankfull limit whereas graph b shows the peak being both delayed and flatten. Its peak flow is below the bankfull limit. A graph showing time on the x axis and water flow on the y axis . A rain fall event in a catchment with re-connected river and floodplain and other natural flood management measures, causes river levels to rise slowly above base flow levels. The lag time is long, with river levels reaching peak discharge slowly and dropping well below bank full discharge - no flooding occurs.

Figure 5 Storm hydrographs from an urban monitoring station toward the bottom of a flashy rural catchment. They depict a) a storm event ...