A flood hydrograph
Jonathan Burch 02/05/2007 Geography Essay
A flood hydrograph is a graph of two axis, ‘discharge’ and ‘time’. Plotted on the graph is the amount of discharge over a period of time.
By looking at a hydrograph, a lot of information and data can be gathered about the river, the precipitation, the surrounding area and vegetation etc. The gradient, height and length of a line can tell you a lot of this information.
There are many different factors that can affect the appearance and shape of a hydrograph. Certain conditions can cause the line on the hydrograph to be tall and thin and other conditions can cause it to be short and wide.
Peak discharge is the term used to describe the maximum amount of discharge from the river over the period of time recorded; this peak discharge can be high or low depending on a lot of conditions.
Climatic factors are the most obvious conditions that can affect the flood hydrograph. If the precipitation is very intense and there is a lot of it then the hydrograph is likely to have a high peak discharge where as if the precipitation is low-intensity and there is a small amount then the graph will have a gentle ascending limb.
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The ascending limb is the first part of the line on a hydrograph that rises to the peak discharge. If the gradient is steep (like the example on the right) then this can indicate that the amount of rainfall becoming overland flow is very high, the result of this is that all the water reaches the river very quickly and all in a short period of time, this gives the immediate steep ascending limb on the hydrograph.
Reasons for large amounts of rainfall becoming overland flow can be little vegetation and maybe impermeable rock below the surface. Being impermeable rock, the water cannot infiltrate through this and become ground water, instead it flows over the ground to the river. Urban development and agricultural practices contribute to overland flow.
The descending limb is the last part of the line on a hydrograph, showing the discharge dropping with time after the peak discharge. This line can also be either steep of gentle. If it is steep then it indicates that the river is very efficient because it takes the excess water from the flood away quickly and brings the river back to its base flow. If the descending limb is gentle then it could mean the river is less efficient, it could also mean that the storm endured over a long period of time and much water is still being contributed to the river by groundwater flow.
The rates of evapotranspiration affect the nature of a flood hydrograph. If there are high rates of evapotranspiration then the peak discharge maybe lower and the lag time greater. If the rates are low then the graph will not be affected by this factor.
Soil types and characteristics can have a large impact on a hydrograph. If the soil is dry and permeable then it can hold lots of water and the hydrograph is more likely to look like line B on the right. If the soil is impermeable or it is a high antecedent soil that is already waterlogged the line is more likely to look like line A on the right.
If the drainage basin has steep slopes then despite other factors the ascending limb can be a very steep gradient because the rainfall flows directly downhill to the river/streams without having time to infiltrate into soil. Lag time can be very small if the drainage basin has steep slopes.
The term ‘lag time’ is the name given to the amount of time between the peak rainfall and the peak discharge of the river. This can be affected by many factors. If the lag time is short it usually means that a lot of the water reached the river by means of overland flow, this being the fastest way to reach the river. If the lag time is long then the water has probably had to reach the river by means of infiltration and groundwater flow, this is a long process. In an area of urban development the lag time is often very short because of the impermeable surfaces and the direct routes for water to get to rivers by storm drains.
The main factors that affect a hydrograph are to do with the drainage basin, this includes the drainage density, slopes, rock type, soil type, vegetation, urban development, water stores and agricultural practices.
If the drainage density is high then the hydrograph will look like line A in hydrograph 2 above. This is because there are a large number of streams leading into the river allowing the water to take a quick route. If the drainage density is low then the hydrograph will look more like line B in hydrograph 2.
Vegetation can greatly influence a hydrograph. The more vegetation and ground cover that there is, the more rainfall is intercepted. Vegetation prevents the water infiltration the ground as quick and reaching the river by means of overland flow or groundwater flow. If the drainage basin is greatly vegetated then the lag time is likely to be greater, the limbs are likely to be gentler and the length of the flood is likely to be longer. Forests and woodland intercept much rainfall and the roots encourage infiltration. If the drainage basin is bare and has little vegetation then there is likely to be more surface water.
If there are lakes and backwater swamps upon the drainage basin then the movement of water to the channel is slowed.
Another factor affecting the flood hydrograph is if the snow or ice that feeds the stream and rivers is melting quickly or slowly, this is decided by the weather.
Flood hydrographs are useful and there are many different factors that can affect its from. Flood hydrographs could be slightly more useful if they were supplied with information about the precipitation, e.g. duration.