Handout 2_River landforms & Hydrograph.pdf

St. Joseph’s Institution Year 4 IP Geography Topic: Changing Interactions between Land and Water Handout 2: River Landforms and Hydrograph Key questions: 1. How river processes shape the land in different ways? 2. What...

St. Joseph’s Institution Year 4 IP Geography Topic: Changing Interactions between Land and Water Handout 2: River Landforms and Hydrograph Key questions: 1. How river processes shape the land in different ways? 2. What are the causes and impacts of floods? Key concepts: Process Change Possibilities LANDFORMS ASSOCIATED WITH RIVER PROCESSES Erosional Features Deposition Features V-shaped valleys Alluvial fans Interlocking spurs Meanders and ox-bow lakes Potholes Floodplain and levees Rapids Deltas Waterfalls and plunge pools Deferred and braided streams Gorges and canyons River cliffs River terraces Fig. 1 Diagram showing various river features along the course of the river between its source and mouth. NOT FOR EXTERNAL CIRCULATION SJI Year 4 IP Geography – Changing interactions between Land and Water (Handout 2 – River Landforms and Hydrograph) Floodplains A floodplain is a wide and gently sloping plain of alluvium in the lower course of a river valley. Formation 1. In the lower course, the gradient is gentle and flat. The river carries large quantities of sediments, which are brought down from the upper and middle courses. 2. Flooding is common in the lower course due to large quantities of sediments and the largest volume of water from the tributaries. 3. In time of floods, the river level rises and overflows its banks. The flood water carries the sediments to the adjacent lands. When the flood water subsides, these sediments are deposited. 4. Repeated flooding will build up extensive layers of alluvium resulting in a floodplain. 5. On the floodplains, features such as levees, ox-bow lakes and meandering rivers are also commonly found. 6. Examples: Ganges Plain (India), North China Plain (China), Chao Phraya Plain (Thailand) and Mississippi Plain (USA). Levees Levees are raised embankments built up along the banks of the river, rising above the level of the surrounding floodplain. Formation 1. Levees occur in the lower course of a river, where the valley floor is gentle and flat. The widening of the valley floor in the lower course caused by lateral erosion, produces a wide floodplain. 2. When the river floods, the finer materials like silt are spread further away to build up a floodplain. Nearer to the river banks, coarser materials are deposited and ridges which are higher than the surrounding plain called levees are formed. Levees along the lower course of the River Mississippi have a height of 9 metres. Fig. 2 Stages in the formation of floodplain and levees NOT FOR EXTERNAL CIRCULATION SJI Year 4 IP Geography – Changing interactions between Land and Water (Handout 2 – River Landforms and Hydrograph) Deltas A delta is a flat depositional plain formed at the mouth of a river. It is continuation of the river’s floodplain. Formation 1. When a river is about to enter a body of quiet water such as a lake or sea, its speed is checked and its loads are laid down. 2. Heavy sediments (sand and gravel) are deposited near the river mouth whereas fine sediments (clay, silt and mud) are carried further out to the sea. 3. The accumulation of sediments in the river mouth causes the main channel to branch into separate streams called distributaries, carrying material outwards. Levees and lagoons may be produced along river banks. 4. When the deposition is at a greater rate than removal by currents, an extensive area of sediments in layers is built up at the river mouth called a delta. 5. For examples, Ganges Delta, Nile Delta and Yangtze Delta. Fig. 3 Formation of delta Favourable conditions for delta formation at the River: 1. The processes of weathering and erosion are active along the course of the river so that river carries down large quantities of load. 2. There are tributaries that help to increase the load in the river. 3. There are no or few large lakes along the river course to trap or take away some of the load in the river. Favourable conditions for delta formation at the Coast: 1. There is a shallow sea at the river mouth to allow accumulation of sediments. 2. The coastal waters are quiet and there are no strong tidal currents and waves to remove the deposited sediments. 3. The climate favours the fast growth of vegetation, which binds and stabilises the sediment. NOT FOR EXTERNAL CIRCULATION SJI Year 4 IP Geography – Changing interactions between Land and Water (Handout 2 – River Landforms and Hydrograph) Types of deltas Arcuate delta Bird’s foot delta Estuarine delta Triangular or fan-shaped Looks like a bird’s foot Materials are deposited in a Composed of coarse Composed of fine sediments long narrow submerged sediments such as gravel called silt. estuary. and sand. Have a few distributaries that Delta does not grow beyond Great number of extend far out to sea. the general coastline as the distributaries. Example: Mississippi Delta, deposits are washed away by Example: Deltas found at the USA waves and currents. mouths of the Nile, Ganges, Example: Seine Delta, France Indus, Mekong and Irrawaddy Rivers. RIVER DISCHARGE River discharge refers to the volume of water flowing through a particular point in the river in a given period of time. It is usually calculated by multiplying the velocity by the cross-sectional area and is usually expressed in cubic metres per second. It is obtained by using the following formula: Discharge = Area of cross section (m²) x velocity of flow (metres per second) NOT FOR EXTERNAL CIRCULATION SJI Year 4 IP Geography – Changing interactions between Land and Water (Handout 2 – River Landforms and Hydrograph) HYDROGRAPHS Fig. 4 A storm hydrograph A hydrograph is a graph that shows the variation in river discharge at a particular point in a river over a certain period of time. It is also used to show how long a river takes to flood. A hydrograph usually shows both the level of baseflow, which originates from the groundwater supply and the stormflow. The stormflow consists of the rising limb and the falling limb. The steeply rising limb represents the increasing river discharge when the initial surface runoff and later the throughflow (water moving through the soil) of the storm eventually reach the river. The falling limb represents the decreasing river discharge as the storm dies. It is not as steep as the rising limb. The peak represents the highest discharge. It occurs after the most intense rainfall has ceased. A storm hydrograph may have more than one peak if there are several consecutive heavy downpours. The time difference between the heaviest rainfall in the storm and the peak of the river discharge is called the lag time. The length of the lag time varies from one river basin to another and from one storm to another. For example, a short lag time means water has got into the river quickly and it is more likely to flood. The opposite will happen when it takes a long time for the water to enter the river, with a long lag time and a gentle rising limb. Factors influencing the lag time in the storm hydrograph 1. Physical characteristics of drainage basin Basin size – If a drainage basin is small, it is likely that rainfall will reach the main channel more rapidly than in a large drainage basin where the water has much further to travel. Lag time will therefore be shorter in the smaller drainage basin. Basin shape – A more circular basin will have a shorter lag time and a higher peak flow than an elongated basin. All the points on the watershed of the former are approximately equidistant from the gauging station whereas in the latter it takes longer for water from the extremities of its basin to reach the gauging station. Relief – The slope of the basin and its valley sides also affect the hydrograph. In steep-sided upland valleys, less infiltration takes place and water is likely to reach the river more quickly than in gently sloping lowland areas. NOT FOR EXTERNAL CIRCULATION SJI Year 4 IP Geography – Changing interactions between Land and Water (Handout 2 – River Landforms and Hydrograph) 2. Types of precipitation Prolonged rainfall – This is the most frequent cause of flooding. After a long period of heavy rainfall, the ground becomes saturated and infiltration is replaced by surface runoff. Intense storms – When heavy rain occurs, the intensity may be greater than the carrying capacity of the soils. The resultant surface runoff is likely to produce rapid rises in river levels. 3. Weather Snow melt in spring – In areas with hills or mountains, the snow in the higher elevations melts and the water runs down into the streams. If the snowmelts are heavier than usual, floods may occur. Also, spring rains often occur at the same time as the snow is melting, contributing more water to the streams and rivers. Weather events – The torrential rains that accompany a hurricane or other tropical storm can also cause flooding in rivers. 4. Rock type Rocks which allow water to pass through them are said to be permeable. There are two types: Porous rocks, e.g. sandstone and chalk, contain numerous pores which can fill with and store water. Pervious rocks, e.g. carboniferous limestone, allow water to flow along bedding planes and down joints through the rocks. Both types of rocks permit rapid infiltration of water so there is little surface runoff and limit the number of streams. In contrast, impermeable rocks, e.g. granite, do not allow water to pass through them, so they produce more surface runoff and many streams. 5. Soil type Soil type controls the speed of infiltration, the amount of storage and rate of throughflow. Sandy soils, with large pore spaces, allow rapid infiltration and do not encourage flooding. In contrast, clays have much smaller pore spaces thus reducing infiltration and throughflow but encouraging surface runoff and increasing the risk of flooding. 6. Vegetation Vegetation, including trees, intercept rainfall, slowing its movement. Some of this moisture is stored on its leaves before it evaporates back into the atmosphere. Trees also absorb water from the soil allowing greater infiltration into the soil and reducing surface run-off. Flooding occurs more quickly in deforested areas. When vegetation is removed, infiltration and interception reduce and surface run-off increases. This leads to a greater risk of flooding as more water reaches the river channel. E.g. the increasingly frequent and serious flooding in Bangladesh is attributed to the removal of trees in Nepal and other Himalayan areas. Flooding is less likely to occur in areas of afforestation. 7. Urbanisation Urbanisation increases flood hazard. Constructing buildings, the surrounding roads and pavements create impermeable surfaces, which increase surface run-off. Water cannot infiltrate through tarmac and concrete. Gutters and drains also carry water more quickly to the nearest river, increasing the flood risk. In urban areas, small streams may be canalised so that, with friction reduced, the water flows away more quickly, shortening the lag time, increasing the flood risk. 8. Modification of river channel course Rivers that have been straightened or shortened face a higher chance of floods due to lesser river for the water to flow in. Natural or human constructed dams that are built may also fail with the sudden release of water into the downstream drainage, causing flooding downstream. NOT FOR EXTERNAL CIRCULATION SJI Year 4 IP Geography – Changing interactions between Land and Water (Handout 2 – River Landforms and Hydrograph)  Effect of vegetation on storm Effect of rock permeability on storm Effect of urbanisation on storm hydrograph hydrograph hydrograph HYDROGRAPHS OF TWO RIVERS, AFFECTED BY THE SAME RAINSTORM DESCRIBE shape of hydrograph Characteristics: Characteristics: Steep rising limb (i.e. short lag time) Gentle rising limb (i.e. long lag time) High peak discharge Low peak discharge Less steep falling limb (i.e. quick return to normal Gentler falling limb (i.e. slow return to normal flows) flows) EXPLAIN shape of hydrograph Features of the drainage basin: Features of the drainage basin: Impermeable rocks Permeable rocks Steep slopes Gentle slopes More urbanised More rural Sparse vegetation Dense vegetation / forests References 1. Chong, M., (1998) Aspects of Physical Geography For O Levels, Pekoe Publisher. 2. C. (2018). Collins Cambridge IGCSE – Cambridge IGCSE Geography Student Book (Third Edition,Third edition). HarperCollins UK. 3. Davies, D. (2018). Cambridge IGCSE® and O Level Geography Revision Guide (Cambridge International IGCSE) (2nd ed.). Cambridge University Press. 4. Guinness, P. (2018). Cambridge IGCSE and O Level Geography 3rd edition (3rd ed.). Hodder Education. 5. Harrison, L., & Broadbent, A. (2019). Geography for the IB MYP 4&5: by Concept. Hodder Education. 6. River FEATURES. (n.d.). Retrieved March 14, 2021, from https://www.geographypods.com/21-river-features.html NOT FOR EXTERNAL CIRCULATION SJI Year 4 IP Geography – Changing interactions between Land and Water (Handout 2 – River Landforms and Hydrograph)