A flood hydrograph shows river discharge over a period of time. It shows the response of a river to a specific rainfall event. There is usually a time-lag between the peak rainfall intensity and peak flood flow. There are a number of factors that influence a flood hydrograph. A flood hydrograph is split in two, with the baseflow (which is mostly groundwater flow through rock) and the stormflow (overland flow, throughflow and direct channel precipitation) stormflow accounts for most of the hydrograph.
The graphs different shapes depend on a number of things, the flatter hydrographs tend to occur if the storm is light whilst the steep ones when there has been a lot of overland flow. One factor that impacts the graph is the type of bedrock. Bedrock can be permeable or impermeable. Impermeable bedrock, like shale, does not allow water to pass though it whilst permeable rocks such as chalk does. There are two types of permeable rock; porous, which contains pours that fill with and store water as well as pervious.
Pervious bedrock is cracked and so allows water to run in its joints. An example of this is carboniferous limestone. A basin with impermeable bedrock is likely to produce a much more dramatic hydrograph than one with permeable bedrock. The basins drainage density is another factor. Drainage density is defines as the total length of the channel divided by the basin area. Rivers with a high density result in higher levels of discharge. This is because water finds its way quickly into a channel. This increases flood risk.
The characteristics of high density drainage are an impermeable bedrock, saturated soils, steep slopes with little vegetation and high precipitation levels. It is not just the drainage density that has an effect but also it’s pattern. There are six different drainage patterns and each is found in a different type of area. The dendritic pattern is most common in the UK as it occurs in well adjusted gently-sloping basins with a fairly uniform rock type. Its shape resembles the branches of a tree.
A radial basin appears where rivers flow down the side of a hull whilst parallel patterns are typical of steep relief where there is little vegetation. This is likely to be a mountainside. Land use also affects flood hydrographs. Urbanised land is often covered with tarmac and other hard, non porous surfaces and so water runs off very quickly. Storm drains are put into some urban areas, these mean lag times are very short and peak discharge is very high. Urban areas also have less vegetation than rural. Vegetation is important in reducing discharge as it intercepts precipitation.
Where there is little vegetation there is also less interception and transpiration. The roots of plants and trees take up water, this reduces throughflow. Flooding is therefore more likely in deforested (urban) areas. Another factor that affects the hydrograph is the size of the basin, and shape. The smaller the basin, the less time it takes for water to drain into the river resulting in a shorter lag time. In a long narrow basin, water takes longer to reach the river, also, the steeper the basin, the longer it takes to drain.