P1A 05 Flood Hydrographs

How is lag time on a hydrograph measured?

• It is the time between the peak rainfall event and the peak discharge.
• It is the time from the beginning of the rainfall event and the peak discharge.
• It is the time from the beginning and the end of the rainfall event.
• It is the time between the beginning of the rising limbe to the end of the falling limb.

The image shows a hydrograph. How long did the rainfall event last?

• 6 hours
• 1 day
• 4 hours
• 1 hour

Estimate the stream discharge at the time of the rainfall event.

• 50 cumecs
• 10 cumecs
• 4 cumecs
• 10 mm

How long was the lag time?

• 1 day
• 2 days
• 6 hours 
• 10 hours

What is indicated by the darker yellow area under the graph?

• Water added to the stream by surface runoff.
• Water added to the stream by groundwater.
• The amount of load within the stream.
• The amount of water that is infiltrating into the soil.

Choose the correct statement about the influence of urbanization on a previously forested area.

• Urbanization will increase both evapotranspiration and infiltration.
• Urbanization will increase evapotranspiration and reduce infiltration.
• Urbanization reduce infiltration and increase runoff.
• Urbanization increase infiltration and reduce runoff

A cause of a shorter lag time.

• deforestation
• ploughing
• soft persistent rain
• replacing concrete and asphalt with natural vegetation.

In the case of desert flash floods the steep rising limb can be caused by ........

• soft sands
• hard surfaces
• high temperatures
• low temperatures

True or False? Storm hydrographs can be used by emergency response teams to analyse flooding risks near streams.

• True
• False

What are the factors that influence the way how a stream reacts to rainfall events called.

• Risky factors
• Flash flood factors
• Risk asessements
• Drainage basin controls