AN INTRODUCTION. Drainage Basins An area of land drained by a river and its tributaries. This is also called the catchment area. Any precipitation which

AN INTRODUCTION

Drainage Basins An area of land drained by a river and its tributaries. This is also called the catchment area. Any precipitation which falls into the basin is collected and drains into the main river or its tributaries by runoff, throughflow or groundwater flow. Its boundary is marked by a ridge of high land called watershed Drainage basins have one main stream and many tributaries. Each tributary of main stream forms sub-drainage basin.

Drainage Basins Geomorphologists and hydrologists often view streams as being part of drainage basins. A drainage basin is the topographic region from which a stream receives runoff, throughflow, and groundwater flow. Drainage basins are divided from each other by topographic barriers called a watersheds or water divides.

Important vocabulary Watershed: A ridge or other line of separation between two river basins or drainage systems. Drainage Basin: An area of land drained by a river system. River system: a system comprising of streams and a main river Tributaries: Streams or rivers that flow into larger rivers. They bring water into the main river increasing its discharge downstream. Distributaries: Small channels that branch out from the main river and are mainly found in the lower course and delta. River Source: Point where the river starts River Mouth: Place where the river meets the sea / ocean (River ends)

Diagram of a drainage basin

The following image shows the nature of drainage basins as determined from a topographic map sheet. The red lines delineate the watersheds for the drainage basins of first order streams. The yellow lines define the watersheds for two larger sub-drainage basins that form part of the larger basin

Drainage Basins: A Systems Approach They form part of a larger Global Hydrological Cycle and are commonly viewed by scientists as being open systems. Inputs to these systems include precipitation, snow melt, and sediment. Drainage basins lose water and sediment through evaporation, deposition, and streamflow. These are outputs.

Global Hydrological Cycle

A drainage basin may be described as an open system. It forms part of the hydrological cycle

Drainage Basins: A Systems Approach A number of factors influence input, output, and transport of sediment and water in a drainage basin. Such factors include topography, soil type, bedrock type, climate, and vegetation cover. These factors also influence the nature of the pattern of stream channels

The drainage basin as an open system The drainage basin forms part of the hydrological cycle and can be described as an open system involving a series of: INPUTS: ways in which water enters the system. OUTPUTS: ways in which water leaves the system. You must be able to define these terms! STORES: ways in which water is held in the system. TRANSFERS: ways in which water is moved through and within the system.

Characteristics of a Drainage Basin Inputs : - in the form of precipitation Outputs:- water is lost through evapo-transpiration and runoff Within the system, some of the water: Is stored in water storage such as lakes or soil Passes through a series of transfers or flow e.g. infiltration, percolation and throughflow

What happens to rainfall? Be lost through the system through evapotranspiration Be held in storage in lakes, the soil or underground Flow into a river to return, eventually to the sea as run-off. The amount of rainwater that reaches the river will be: Precipitation (evapotranspiration + storage) So, river runoff will be less than precipitation If we have excess precipitation then, we get? Flooding

As water cycles through a drainage basin via inputs, outputs, transfers and stores, it creates a basin hydrological cycle.

Important terms..... Interception : precipitation lands on vegetation. Infiltration : Water that reaches ground surface will soak into soil. Overland flow : water moving over the ground. Throughfall : precipitation passing through the plant canopy Percolation : Some water sinks deep into the earth (percolates) through openings in the soil. Stemflow : water that flow down tree trunks or plant stems. Groundwater : water transfer through permeable rocks from the soil above. Baseflow : water flows from groundwater stores to the stream Throughflow : water moving through the soil. Surface run-off : water that flows over the land surface

Inputs Main Input - precipitation. The types: rain or snow, hail, etc. Factors affecting the amount of water in the system: the intensity, the duration and frequency. Each subsystem of the drainage basin system will also have: inputs and outputs the output from one stage of the diagram forming the input for another.

Storage Water is stored - on the surface, - in lakes and channels - in the groundwater store. It reaches groundwater via infiltration and percolation. Some water will be stored in the soil and rock (groundwater store) The amount of water stored will vary depending on: the porosity of the soil and the permeability of the rock. Water can be temporarily stored via interception. in the storage of water on leaf and plant stems.

Transfer Runoff: All the water flowing over the drainage basins surface made up of: streamflow - flow through permanent river channels surface flow / overland flow Overland flow - transfers water through the basin either as: sheetwash - across the surface, or rills: tiny surface channels Beneath the surface, water is transferred via throughflow, which is the movement of water through the lower soil towards rivers, and groundwater flow (base flow). Water that has been intercepted by foliage may also be transferred, either directly as throughfall, or by running down branches and stems via stemflow.

Output final release of the water in a drainage basin flow into the sea (river runoff) will be the main output of a drainage basin. Some water will also be lost via evapotranspiration. Evaporation from water bodies and soils Transpiration from plants

Definitions

Hydrographs A hydrograph may be used to show how the water flow in a drainage basin (particularly river runoff) responds to a period of rain. A hydrograph shows variations in a rivers discharge over a short period of time, usually during a rainstorm.

Parts of a Hydrograph

This type of hydrograph is known as a storm or flood hydrograph and it is generally drawn with two vertical axes. One is used to plot a line graph showing the discharge of a river in cumecs (cubic metres per second) at a given point over a period of time. The second is used to plot a bar graph of the rainfall event which precedes the changes in discharge. The scale on the horizontal axis is usually in hours/days and this allows both the rain event to be recorded and the subsequent changes in river discharge to be plotted

The shape of the hydrograph varies according to a number of controlling factors in the drainage basin but it will generally include the following features: The baseflow of the river represents the normal day to day discharge of the river and is the consequence of groundwater seeping into the river channel. The rising limb of the hydrograph represents the rapid increase in resulting from rainfall causing surface runoff and then later throughflow. The falling limb (or recession limb as it is sometimes known) is when discharge decreases and the rivers level falls. It has a gentler gradient than the rising limb as most overland flow has now been discharged and it is mainly throughflow which is making up the river water. Peak discharge occurs when the river reaches its highest level. The time difference between the peak of the rain event and the peak discharge is known as the lag time or basin lag time.

The line graph shows the discharge. The bar graph shows the rainfall. When a storm begins, discharge does not increase immediately as only a small amount of rain will fall directly into the channel. The first water to reach the river will come from surface run-off. Water arriving in the river later comes from through-flow. The increase in discharge is shown by the rising limb. The decrease in discharge is shown by the falling limb. The gap between the time of peak (maximum) rainfall and peak discharge (highest river level) is called lag time. A river with a short lag time and a high discharge is more likely to flood than a river with a lengthy lag time and a low discharge.

Analysing a Hydrograph

At Point Z there are no longer surface run-off inputs and throughflow and ground flow are both in decline. The discharge of the river is almost back to its minimum base flow. In the hydrograph we can see that discharge increases rapidly due to a period of extended rainfall. Point W on the graph marks the peak rainfall. At this moment, precipitation has not entered the river. After this point in time surface run-off will input water into the river. At Point X surface runoff and discharge are increasing rapidly. At Point Y discharge is in decline. Surface run-off is still feeding water into the river but it has also passed its peak.

Controls on the shape of a Hydrograph

Drainage Basin Controls A number of physical and human factors (known as drainage basin controls) influence the way in which a river responds to precipitation and have an effect on the shape of the hydrograph. These include the size and shape of the basin, the steepness of slopes, the type of geology within the basin, antecedent rainfall and land use within the basin. Prolonged heavy rain causes more overland flow than light drizzly rain.

Hydrographs are graphs which show discharge (the amount of water passing a particular point in a river at a particular time). Therefore, factors that affect discharge within the drainage basin will affect the shape of the hydrograph. What affects the shape of a hydrograph? Land useType and amount of precipitation Geology and soilGradient of the valley sides Drainage Basin Shape

Physical Factors affecting flood hydrographs

Influence of Basin Shape The size, shape and relief of the basin are important controls. Water takes longer to reach the trunk stream in a large, round basin than in does in a small, narrow one.

Influence of Basin Size Large drainage basins catch more precipitation so have a higher peak discharge compared to smaller basins. Smaller basins generally have shorter lag times because precipitation does not have as far to travel.

Influence of Steepness Where gradients are steep, water runs off faster, reaches the river more quickly and causes a steep rising limb.

Influence of Type and Amount of Rainfall If the drainage basin is already saturated (antecedent rainfall) then surface runoff increases due to the reduction in infiltration. Rainwater enters the river quicker, reducing lag times, as surface runoff is faster than baseflow or through flow. Thus, if its been raining heavily previously, the ground may be waterlogged so the lag time will be reduced because water will be unable to infiltrate and will instead travel via overland flow. The amount of precipitation can have an affect on the storm hydrograph. Heavy storms result in more water entering the drainage basin which results in a higher discharge. The type of precipitation can also have an impact. The lag time is likely to be greater if the precipitation is snow rather than rain. This is because snow takes time to melt before the water enters the river channel. When there is rapid melting of snow the peak discharge could be high.

Influence of Geology If a river is surrounded by non-porous and impermeable rocks (e.g., mudstone) its going to have a high peak discharge and a short lag time. Impermeable rocks wont let water percolate through them, forcing the water to travel via overland flow. This is much faster than groundflow, interflow and throughflow so the lag time is reduced. Furthermore, non-porous rocks cant store water so the peak discharge of a river is increased as more water enters the river rather than being stored in the drainage basin.

Influence of Soil Type The soils ability to let water infiltrate has a similar effect to the dominant rock type in a drainage basin. Unconsolidated soils allow water to infiltrate and so act as a store in a drainage basin. In addition, water travels slowly through soil via throughflow. This reduces the peak discharge while increasing the lag time of a river. On the other hand, extremely fine clay soils dont allow water to infiltrate. As a result, water travels quickly as overland flow, reducing the lag time of a river.

Influence of Geology and Soils Drainage Basins with impermeable underlying rock are also associated with higher drainage density on the surface. Drainage density refers to the density of river channels and tributaries. A higher drainage density is a consequence of impermeable underlying rock. Areas of permeable rocks and soil allow more infiltration and so less surface run off.

Influence of Vegetation If a drainage basin has a significant amount of vegetation this will have a significant affect on a storm hydrograph. Vegetation intercepts precipitation and slows the movement of water into river channels. This increases lag time. Water is also lost due to evaporation and transpiration from the vegetation. This reduces the peak discharge of a river.

Human Factors affecting flood hydrographs

Human Factors There are a range of human factors that affect the shape of a storm hydrograph. These include: Drainage systems that have been created by humans lead to a short lag time and high peak discharge as water cannot evaporate or infiltrate into the soil. Areas that have been urbanised result in an increase in the use of impermeable building materials. This means infiltration levels decrease and surface runoff increases. This leads to a short lag time and an increase in peak discharge.

Influence of Land Use The way in which the land is used will also have an influence on the hydrograph Vegetation intercepts precipitation and allows evaporation to take place directly into the atmosphere so reducing the amount of water available for overland flow The large number of impermeable surfaces in urban areas encourages run off into gutters and drains carrying water quickly to the nearest river.

time discharge (cumecs) and rainfall (mm) Shorter lag time as water quickly reaches the channel via surface runoff, through drains, sewers etc Steeper rising limb due to impermeable surfaces Urbanisation and the storm hydrograph Higher peak flow as less water is stored; more water reaches the river Rural Urban

We need to be careful not to exaggerate the importance of urbanisation in creating flash responses in rivers; after all the city or town only represents a small area of the drainage basin as a whole. The construction of roads and parking lots does of course increase run-off as well as the building of houses and businesses on the floodplains. In addition, the design of roads and drainage infrastructure to transport water quickly into the river further reduces lag time. However, with the exemption of major metropolitan areas, these impacts need to be observed more generally at a local scale. On their own they are not solely responsible for flooding.

Flooding The removal of trees increases the risk of flooding. As urban areas grow, the risk of flooding increases.

Documents

AN INTRODUCTION. Drainage Basins An area of land drained by a river and its tributaries. This is also called the catchment area. Any precipitation which