hydrographs - factors influencing storm

• Area

  • Large basins recieve more precipitation than small basins, therefor have larger run-off
  • Larger size means longer lag time as water has a longer distance to travel to reavh the trunk river
• Shape

  • Elongated basin will produce a lower peak flow and longer lag time than a circular on of the same size
  • A basin with fewer tributaries relies heavily on soil and grround based water movement. A basin with many tributaries has a steep rising limb as water uses the chanel to reavh the trunk stream quickly.
• Slope

  • Where valley side gradients are steep, water runs off faster, reaches the river more quickly and causes a steep rising limb. Prolonged heavy rain causes more overland flow than light drizzly rain.
• Rock type and soil

  • Permeable rocks mean rapid infiltration and little overland flow, therefor a narrow rising limb
  • Impermeable rocks will lead to an increase in overland flow and a steep rising limb
  • infiltration if generally greater on thick soil, although less porous soils act as impermeable layers. Saturated soil results in little infiltration.
• Vegetation

  • Forest areas will intercept the precipitation, creating a shallow rising limb and lengthen the lag time
  • Vegetation will intercept precipitation and store and absorb it, therefor preventing the water moving quickly into the river
• Land-use (urbanisation)

  • concrete and tarmac form impermable surfaces, creating a steep rising limb and shortening the lag time
  • Drains and sewers will transport water rapidly to river channels, reducing the lag time
  • Straightening of river channel and lining with concrete - this leads to faster deliverly of water downstream of urban areas and increases the flood risk
  • Less water reaches the channel by throughflow and base flow, mostly overland flow
• Land-use (Afforestation)

  • Intercepts the precipitation, creating a shallow rising limb and lengthening the lag time
  • Vegetation intercepts precipitation and allows evaporation to take place directly into the atmosphere. This reduces the amount of water available for overland flow.
• Precipitation and temperature

  • Short intense rainstorms can produce rapid overland flow and steep rising limb
  • If there has been extreem temperatures, the ground can be hard (either baked or frozen) causing rapid surface run-off
  • Snow on the ground can act as a store, producing a long lag time and a shallow rising limb. Once a thaw sets in the rising limb will become steep.
• Tidal Condiitons

  • High spring tides can block the normal exit for the water, therefor extending the length of time the river basin takes to return to base flow.