Interception plays an important part in flood risk, as it dictates how much water will reach the soil to be carried as surface run-off or slower processes. To measure interception rates, insert large funnels into jars to collect rainwater, and place them in different vegetation areas, as well as in open air. By measuring these every 24 hours, we can see how the amount of rainfall reaching the floor varies. For example, 100% of the rain will hit the floor in uncovered areas; the deciduous areas will provide part cover, but only in spring and summer, and the coniferous areas provide little cover all year round. The amount of water intercepted is useful as it tells us how much water we are dealing with when it comes to the permeability of the soil.
Infiltration rates are the final measure before we get to the river. Measuring infiltration rates gives us a key insight into how much of the water getting through the vegetation is going to be absorbed or repelled. This is done by inserting an open ended impermeable cylinder in to the soil at various locations such as areas of deciduous vegetation, coniferous vegetation and pastoral land around Epping Forest. With the cylinder in about 10cm, it is filled up and measured when full, and then using a stopwatch, measure the level every 30 seconds. Other methods use a Constant Head Infiltrometer to maintain constant pressure, but this is the easiest and simplest method.
This will tell us how permeable the soil is, indicating whether rain water will be absorbed and be carried slowly by through flow or groundwater flow, or whether it won’t absorb and be carried much more quickly as surface run-off.
The more surface run-off, the shorter the lag time of the river discharge which means the river has a large volume of water to deal with in a short space of time, increasing flood risk.
However, we must also assess the river’s ability to deal with large volumes of water.
To do this, we have to find, amongst other things, the cross-sectional area of the river. Using a tape measure and a 1m rule, we can measure the width of the river and then measure the depth every 10cm across, creating a rough guide to the shape of the river, and therefore be able to see how much water the river can hold before it bursts its banks.
Useful measurements such as wetted perimeter, which measures the length of river bed submerged by water, is useful as it can back up the CSA by making us aware of any massive changes in between the measurements we took. 10cm readings are not very accurate but they are a sensible rough guide. This is also useful as it is necessary in the calculation of the hydraulic radius.
Hydraulic radius, calculated by CSA/Wetted Perimeter, is a useful tool in assessing flood risk, as it denotes the efficiency of a river. A perfect channel would be that of a perfect square, as it has ultimate efficiency. We can see by looking at the shape of the river if it is efficient enough to handle large quantities of water; if not, it will be more prone to flooding.
Velocity is also a useful factor to consider here; by measuring a distance and placing a cork at the start, timing how long it takes to reach the end and calculating its velocity, we can see how quickly the river flows, and therefore how long the water from precipitation will take to get back to its source. If this is quick, we can expect a higher risk of flooding.
However, most of this fieldwork is useless without some research about the area and previous events.
Antecedent conditions are perfect to combine with our results from infiltration rates; by looking at the weather from the past few weeks, we can judge how the ground will be. For example, if there has been hot dry weather for a prolonged period, the ground will be baked hard, and water will not be absorbed but run-off it. Similarly, if there has been rainfall over the past few days, the soil may be holding all the moisture it can, and so the water will just run off it. Using this information, we can already see why there may be a flood risk at certain times.
In conjunction with this, looking at previous flood events is also a helpful resource. The flood of 2000 happened in summer, an example of how sunny warm conditions can make increase flood risk if it was to rain.
Ordinance surveys are useful after having taking measurements for interception. We can see the difference in water intercepted in deciduous areas and coniferous areas, as well as the contrast between woodland and open air, so an OS map showing how much land is covered by which type of vegetation, if any, will give us an indication of how much water is lost through interception around the whole catchment.
A final method would be to look at how the brook is being managed; for example, in Loughton brook the grilles are cleared once every month, and repairs are carried out on a regular basis. If the local council or the body in charge do not carry out repairs and maintain the river due to lack of money or whatever reason, flood risk increases immediately, regardless of any other factors.
