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Visualising River Discharge With a Flood Hydrograph

Genevieve is a geography enthusiast who has a particular love for wetlands and deltas (don't ask why!)

What is a hydrograph?

A storm hydrograph or a flood hydrograph is a graph that

factors-that-affect-a-storm-hydrograph

Factors that affect a hydrograph

There are several different factors that can affect the storm hydrograph to a rainfall event, for example, the characteristics of the drainage basin. If the drainage basin is small then the water will reach the main channel faster decreasing the lag time. Also if the drainage basin is steeper then that will also decrease the lag time as water got to the channel is makes water flow more quickly. Extremes in temperature can also affect the shape of a hydrograph as extremely cold weather and extremely hot weather reduces rates of infiltration as soil becomes impermeable and increase rates of surface runoff so water gets to the channel more quickly decreasing lag time. The type of precipitation may also affect the shape of the hydrograph as prolonged rainfall means that the ground becomes saturated lowering the rate of infiltration to almost zero which increased overland flow. This in turn means that there is a decrease in lag time as water makes it way to the channel quicker and cause peak discharge to be higher. Also if there is a high intensity storm where the intensity of the storm is higher than the infiltration and discharge rate, there will be an increase in surface runoff causing the water to rise suddenly making the rising limb become steeper causing a flash flood. The land use adjacent to the river can also affect the storm hydrograph as a high density of vegetation on the adjacent land intercepts rainfall preventing soil compaction which reduces infiltration rates as well as storing water in their stems which is returned to the atmosphere through transpiration. This will increase the lag time as well as decrease peak discharge as more water is stored in the vegetation as well as making the rising limb less steep as water is slowed by vegetation on its way to the channel. Urban land use adjacent to the river channel can also affect the shape of a storm hydrograph as sewers and gutters decrease the lag time of the graph as water is a transported into the channel quicker. Urban areas are also mostly impermeable which does not allow infiltration to take place the rising limb is steep as water rapidly flows into the channel as it has no where else to go. The recessional limb is also steep as channels are often canalised meaning there is less friction in the channel so water flows more quickly to the sea. The soil and rock type of the channel and surrounding area can also affect the hydrograph as permeable rocks and soil (e.g. sandstone, sandy soils) allow for rapid infiltration so decreasing peak discharge and increasing lag time. However, impermeable rock (e.g. granite, clay soils) does not allow for infiltration so increases overland flow which increase peak discharge and decreases lag time making the rising limb steeper.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2022 Genevieve Brooks

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