There are likely to be many effects of building a town on a brook. We can see that the most probable responses would be that more water flows into the river and the flood risk increases. However, there are likely to be many other contributing factors and other responses to these rainfall events. In this essay, I am going to examine how the changes to the response of the river occur and what these changes actually are.
The river responded to rainfall very differently in three different time periods. In 1950, the peak discharge was about 1.8 cumecs. In 1950, the lag time would have been much longer due to the fact that there were no drainage systems. Water flows into the drains, and is taken straight to the nearest river which would greatly decrease the lag time. As the drainage system developed, the lag time would decrease, and this is shown as in later years the lag time is much lower. The flood risk for the area in 1950 is quite low because there is not a lot of water being discharged from the river. In 1950, the rising limb and falling limb are gentle.
In the second time period, 1958-62, the peak discharge is 5.2 cumecs. We can see that the discharge has increased from the discharge of 8 years ago. The lag time is also much shorter in this time period, which is a definite contrast to the previous one. The rising and falling limbs are also different to the previous time period. The main difference is that the rising and falling limbs are much steeper. The flood risk is quite high in this time period as the discharge is quite high. Also, the two limbs come to more of a point, rather than the curve that was present in 1950-54.
In the third time period, 1966-68, the peak discharge was 8 cumecs. This is a huge increase from the previous two periods. The lag time is has decreased from before, which is also a contrast from the previous two time periods. The flood risk has increased, from the last two periods, because there is more discharge from the river. The rising and falling limbs are much steeper compared to the other time periods because there is more water flowing into the river at a quicker rate.
The land use has changed dramatically in these three time periods. In the first of the three time periods, from 1950-54, we can see that the discharge was very low. This suggests that there were not many buildings or industrialised areas. Instead, there were more arable and rural areas with less tarmac that allows. However, as the new town was beginning to be built, more tarmac was being put onto the roads, which meant that there were less permeable rocks for water to infiltrate through, and there would have been less trees to intercept and absorb the water since most of the trees would have been cut down to make way for roads and buildings. At the third time period, the town had industrialised, and there were many new buildings and roads covered with tarmac. Overall, the land use has changed significantly from being mainly arable and rural, to an urbanised area. The drainage basin is mainly used for water extraction, which is used for industry, and irrigation.
The rivers response has changed dramatically over time. In the 50’s, there had not been mass urbanisation, and the land use was much different. This meant that there were not many buildings or a lot of tarmac covered roads. Therefore, there would have been less water going into the river as there would have been more trees, which would have intercepted and absorbed some of the water, and also there would have been far more permeable rocks that water could infiltrate through, instead of tarmac which is impermeable. This would all lead to a decrease in the peak discharge. However, we can see that the peak lag time for 1958 is dramatically higher, at 5.1 cumecs, and for 1966 is 8 cumecs which is an enormous difference from 1.8 cumecs in 1950. This is for a number of reasons, due to the land use changing and urbanisation. This involves the roads being covered in tarmac, buildings being erected, trees being cut down to make way for roads, which all means that there will be less infiltration and less interception and absorption which would mean that there is more water going into the river.
The flood risk has also changed because of the fact more water is now flowing into the water over a shorter time. This corresponds to the discharge of the river becoming higher and the falling and rising limbs becoming steeper. In later years, the water has been flowing to the river through surface run-off because the tarmac does not allow any percolation. This means that there will be more water in the river at a given time and the river can to discharge this quickly enough so the flood risk increases dramatically for the later years. For the first time period, 1950-54, there would have been less tarmac so the water could have percolated through the permeable rocks and as through flow and groundwater flow, it would have reached the river much slower meaning the basin had more time to discharge water which corresponds to the flood risk decreasing.
There are a number of different changes that have occurred to the storm hydrograph. As the town became more urban in the later time periods, we can see from the graph that the discharge greatly increased from an initial 1.8 cumecs (1950-54) to 8 cumecs in 1966-68. This is due to a number of reasons. In the earlier years, there were more rural areas which meant that there was more permeable rock. The water could then percolate through the rock and move as through flow and groundwater flow. This would also decrease the lag time which is a vital factor in reducing the flood risk. However, as the town urbanised, more tarmac and concrete would have been used to make roads meaning that there would be less permeable rocks, instead there would be impermeable tarmac. This would mean that the water could travel at its fastest speed as surface run-off to the river which would decrease lag time and increase the flood risk. Moreover, the water could now not percolate and infiltrate through the impermeable rocks which would lead to the flood risk increasing, and the lag time decreasing.
Furthermore, the trees would have also been cut down. This would mean that there would be less interception and absorption and transpiration which greatly contributes to the amount of water flowing into the river. Also, the fact that more drains would have been introduced once the town had become urbanised, would mean that the lag time would decrease and the flood risk would increase.
In conclusion, in believe that there were many impacts of building Harlow new town on Canon’s Brook. By building the town, and urbanising the area, it means that there would be more tarmac on the roads and buildings being erected. Therefore, trees would be needed to be cut down in order fro this to happen and many permeable rocks would be covered by the tarmac. Consequently, this corresponds to the flood risk increasing for the area. This is a major drawback as if the river did flood; it would lead to many possessions being damaged and could even cause death.