Rivers, Flood and Management
The draining basin hydrological cycle:
The drainage basin is the area of land that is drained by a river and its tributaries. Also; the catchment area from which a river system obtains its water. Drainage basin largely ‘closed’ system- watershed – boundary of a water basin- generally follows a ridge of high land. Movement of water in a drainage basin-Drainage Basin Hydrological Cycle- open system with inputs and outputs LOOK AT PAGE 9 IN NEW TEXTBOOK FOR DIAGRAM
Evaporation-Process by which liquid water is transformed into water vapour. Evapotranspiration-The total amount of moisture removed by evaporation and transpiration from a vegetated land surface. Base Flow-Slow transfer of water through rocks.
Infiltration-The passage of water into the soil. Takes place relatively quick at the beginning of a storm but as the soil becomes saturated infiltration rates fall rapidly. Interception-When vegetation intercepts some precipitation on its way to the ground. Water is then lost back to atmosphere by evapotranspiration. Plants also use some water for growth; vegetation reduces and slows down waster transfer to river. Percolation-The deeper transfer of water into permeable rock under the soil surface. Precipitation-Water in any form that falls from the atmosphere onto the surface of the Earth. Overland Flow-Rapid transfer of waster over surface of the ground- most likely to occur during periods of very heavy rainfall or when soil has become completely saturated. Through Flow-Downhill transfer of water through soil layer to the river. Particularly effective when impermeable rocks prevent percolation. Water Balance:
Water balance helps give understanding of unique hydrological characteristics of an individual drainage basin. Also known as the balance between inputs and outputs. Precipitation (P) = Streamflow (Q) + Evapotranspiration (E) +/- Changes in storage (S) P=Q + E +/- S
Factors affecting River Discharge:
River Discharge- The volume of water passing a measuring point or gauging station in a river in a given time. Discharge (m3 per second) = cross sectional area (m2) X velocity (metres per second)
Factors affecting Discharge of river:
Distance Downstream- River Discharge tends to increase with distance downstream. The main reason for this is the addition of water to the river channels due to smaller tributaries joining the main river. Climatic Characteristics- River Discharge usually represents levels of precipitation. Snowfall delays discharge as the water is stored until the snow melts but then there is a surge in discharge. Factors such as temperature can govern evaporation rates and vegetation growth. Land Use-Afforestation (tree planting) tends to reduce discharge and make it more constant. Urbanisation increases discharge as there is rapid water transfer of water through the means of surface run off over impermeable surfaces such as tarmac and concrete. Pipes and sewers also speed up the transfer of water. Channel Modifications- Constructing a reservoir regulates discharge and makes it more constant. Modifications to the river channel such as channel straightening and enlargement can increase discharge whereas river restoration and creation of flood storage can reduce discharge because small-scale flooding is encouraged.
Storm Hydrographs are graphs that show the discharge of the river following a storm event.
Rising Limb- Increase in Discharge
Receding Limb- fall in Discharge back to base level
Peak Discharge- Highest Flow in channel
Lag Time- Time delay between peak rainfall and peak discharge
Said to be two main types of hydrographs:
Drainage Basin and precipitation characteristics
| ‘Flashy’ hydrograph with a short lag time and high peak
| Low, flat hydrograph with a low peak
| Basin size
| Small basins often lead to a rapid transfer of water
| Large basins result in a relatively slow water transfer
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