A storm hydrograph/flood hydrograph is a graph showing the relationship between rainwater and discharge in a river. Discharge is the water flowing in a river in a given unit of time. The storm hydrograph is mainly used to observe discharges for given storm events. The storm hydrograph has these properties:
- Water supplied to the river by the slow seepage of water from groundwater. The response of baseslow to rain is slow and can be negligible.
- Rising Limb
- Water entering the river, steeper rising limbs indicate fast entering of water into the river
- Peak Discharge
- Flow of water is at its highest.
- Lag Time
- Lag time is the time between highest rainfall and peak discharge.
- Falling/Recession Limb
- Shows falling discharge in a river.
- Approach Segment
- This shows a river’s flow before a storm. When a storm begins rain does not readily enter into the river and fills it, instead, most contributions comes from overland flow(runoff) or throughflow. Short lag times indicate that rain is heavy and entering the river fast. Long lag times indicate that rain water(runoff) is slowly reaching the river.
Factors Affecting the Storm Hydrograph
Several factors affects the time water reaches a river and its discharge.
Heavy, sudden bursts of rainfalls leads to steep rising limbs and short lag times. Water is not given time to infiltrate thoroughly. This is why in deserts dried up rivers quickly replenish after a heavy storm (flash floods). Conversely, light rains favours gradual and thorough infiltration, hence less water reaches the river. This results in long lag times. But if rainfall continues for a long period, the ground may become completely saturated and water can flow as saturation overland flow, this is typical of tropical areas.
Nature of Surface
Ice covered surfaces hinders infiltration and encourage high run-off rates leading to higher discharges and short lag times. Baked surfaces in arid regions prevents infiltration and encourage more run-off. The results are high peak discharges and short lag times.
Vegetation can absorb and hinder surface runoff from reaching the river. In addition, densely vegetated areas in tropical regions can intercept rainwater and less water reaches the surface as runoff.
Porous soils such as sands or rocks such as limestone are permeable, therefore runoff is reduced and leading to long lag times and gentler hydrographs.
Overland flow (run-off) is highest on steep slopes because infiltration is reduced, therefore water reaches the river fast (short lag time) and discharge is high.
Basin Morphology and size
Small circular catchments will have short lag times as runoff water quickly reaches the river. High elevated basins results in high runoff rates which shortens the lag time.
Human Impacts Affecting Storm Hydrographs
Lack of vegetation encourages high runoff rates. In addition, interception is reduced and water readily reaches the surface. This result in short lag times and high peak discharge.
Conversely, dense vegetation cover hinders runoff and intercept more rainwater. In addition, roots absorb water which reduce runoff.
Roads and pavements impedes infiltration and favours high runoff, thereby increasing surface runoff and short lag times and high peak discharges. Storm drains channels water to rivers increasing discharge. Urbanisation produce intense rainfalls and which increase the overall river discharge.
Dams stores water and blocks off the natural flow of the river. This can create long lag times.
Poor farming methods, tractors and cattle cane compact the soil creating a hard layer which hinders infiltration and encourage high runoff which in turn shorten the lag time. The removal of vegetation for farming also mean that run-off increases which shortens the time water reaches a river.
Storm Hydrographs by region
|Tropics||Sub-tropics||Deserts (Hot & cold)||Temperate|
|shape||gentle||fairly steep||very steep||fairly steep|
NB: It is important to note that tropical storm hydrographs are gentler for a given storm event, but long term discharge is usually high as water is abundant and flows as saturation overland flow.
The regional storm hydrographs only shows a broader view of a typical storm hydrograph, but local and specific factors are the most determinants of a storm hydrograph.
For example, we can have pavements in a particular tropical region area or steep slopes as well which both increase discharge and shortens the lag time.
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