G1 - Storm Hydrographs - Sample Answer




                                                                                                                                    Figure 1



Figure 1 shows the storm (flood) hydrograph of a river during a high-flow event. The bankfull discharge is 30 cumecs.




(a) Use figure 1 to highlight the major features of this flood event. (5 marks)


A feature of this flood is the peak precipitation (50mm) occurring 4 hours after the start of the event. This caused the discharge to rise from its baseflow condition of 10 cumecs to a bankfull discharge of 30 cumecs in 51/2 hours and then to peak discharge of approximately 48 cumecs in another 3-4 hours (where the river had breached its banks) - the lag time is therefore approximately 9 hours. The discharge then slowly returned to baseflow within 14 hours of peak discharge and the baseflow level dropped back to normal within 23 hours of peak discharge. The whole flood event lasted for 36 hours.


(b) Highlight the demographic and social impacts of flooding. (10 marks)


A flood can have many damaging effects on people, the surrounding area and the future. The first obvious impact of a flood is a demographic one: deaths. Many people drown in the initial flood itself, as occurred during the River Limpopo flood in Mozambique in 2000. As well as initial drowning people can die as a result of secondary hazards such as diseases and starvation. The flood can destroy great areas of farmland submerging crops and livestock. All the dirty water that breaches the channel, runs along streets and sewage channels. The consequence of this is bacteria infections spread within the water systems including typhoid and cholera. People contract these water-borne diseases and increase the pressures on healthcare systems.

Houses, farms, gardens, hospitals, schools, churches and many more can all become flooded and unusable. This happened as a result of the 2007 floods in Tewkesbury (UK) when the River Severn breached its banks. People were forced to migrate to temporary accommodation or evacuate the area. This can lead to disrupted education to the younger society, disrupted medical care for the elderly and ill and general disruption to everyday life. All the roads go under water so the emergency services cannot get to the people that need the help. Gas, telephone, electricity and water supply to houses can be totally lost. Socially, everyday life is in turmoil. A positive social impact could be seen in how the people of Tewkesbury rallied together showing considerable community cohesion.

A flood can be a terrible disaster to have to live through and the Lynmouth flood of 1952 is an excellent example of this. Torrrential rain in the catchment caused the River Lyn to flow through the streets and deposited 100,000 tonnes of boulders on the streets. In the flood 34 died (demographic impact) 90 houses and hotels were destroyed (social) and 130 cars wrecked (social).


(c) Explain the importance of sampling strategy and the need to avoid bias when collecting data in the changing physical environment you have undertaken. (10 marks)



(Hint: With question (a), remember to mention key things such as the timescale of the whole event in hours and the discharge of the river and its lag time. Question (b) demographic impacts are birth rates/death rates and migration. Social impacts include - HESH  housing, education, sanitation, healthcare, and can be either good or bad.)