Storm Hydrographs

• Discharge is the volume of water passing through a cross-sectional point of the river the river at any one point in time, measured in Cumecs.
• Cumecs are cubic metres per second (m3/sec).
• Rising limb is the line on the graph that represents the discharge increasing.
• Falling limb is the line on the graph that represents the discharge decreasing.
• Lag Time is the time between peak rainfall and peak discharge.
• Baseflow is the level of groundwater flow.
• Peak rainfall is the hour of greatest rainfall during the storm.
• Peak discharge is the point/time of maximum river discharge carried by the storm.
• Storm runoff is the discharge of the river above normal flow, caused by the storm event.
• Storm Event is the storm (period of rainfall).
• Normal flow is the average level of discharge of the river caused by water flowing into the river from tributaries and groundwater.
• Discharge = Cross sectional area x Rivers mean velocity
• A flood hydrograph is used to represent rainfall for the drainage basin of a river and the discharge of the same river on a graph.
• A flash hydrograph has a short lag time and high peak discharge, most likely to occur during a storm event, with favourable drainage basin characteristics.
• A low flat hydrograph has a long lag time and a low peak discharge.
• Features of a Flashy Hydrograph:

  • Shorter lag time
  • Steep rising and falling limb
  • Higher flood risk
  • High peak discharge
• Features of a low-flat hydrograph:

  • Long lag time
  • Gradually rising and falling limb
  • Lower flood risk
  • Low peak discharge
• High rainfall intensity increases a higher discharge potential from the river and more likely for soil to reach it's field capacity. Thus, increasing the surface runoff and decreasing the lag time.
• Antecedent rainfall increases the surface runoff potential as the ground is saturated and the soil has reached its field capacity.
• Antecedent rainfall is rainfall that has occurred the day before the event.
• Impermeable underlying geology decreases percolation and therefore greater levels of throughflow.
• High drainage density is where there are many tributaries leading to the main river, increasing the speed of drainage and decreasing the lag time.
• A small basin allows the rainfall reaches the central river more rapidly, decreasing the lag time.
• A circular basin allows rainfall to reach the central river more rapidly, decreasing the lag time.
• Low temperatures mean that less evapotranspiration occurs so there is a greater peak discharge.
• The type of precipitation, such as snow or hail takes time to melt before moving towards the river, so rainfall increases the flooding risk.
• Vegetation cover, such as forests, intercept more rainfall, decreasing the flood risk, however exposed areas will transfer water to the river more rapidly, decreasing the lag time.
• Urbanisation creates more impermeable surfaces, so runoff and surface storage increases, and infiltration rates are reduced.
• Pastoral farming results in the ground being trampled so less interception and more surface runoff.
• Deforestation means there is less interception by trees, so water reaches the ground and river more quickly. Thus, there is more surface runoff and greater flood risk.