10 Streams and Flooding

Renu Lata

 2.0 Objectives

Explain the hydrological cycle and its relevance to streams Explain different types of streams

Describe a drainage basin and explain the origins of different types of drainage patterns Explain the flood hazards (causes, effects & management) in India

Describe some of the important historical floods in India

Explain some of the steps that we can take to limit the damage from flooding

2.1 Introduction

Streams are the most important agents of erosion and transportation of sediments on Earth’s surface. They are responsible for the creation of much of the topography that we see around us. They are also places of great beauty and tranquility, and of course, they provide much of the water that is essential to our existence. But streams are not always peaceful and soothing. During large storms and rapid snowmelts, they can become raging torrents capable of moving cars and houses and destroying roads and bridges. When they spill over their banks, they can flood huge areas, devastating populations and infrastructure. Over the past century, many of the most damaging natural disasters in India have been floods, and we can expect them to become even more severe as the climate changes.

2.2 The Hydrological Cycle

Water is constantly on the move. It is evaporated from the oceans, lakes, streams, the surface of the land, and plants (transpiration) by solar energy (Fig. 2.1). It is moved through the atmosphere by winds and condenses to form clouds of water droplets or ice crystals. It comes back down as rain or snow and then flows through streams, into lakes, and eventually back to the oceans. Water on the surface and in streams and lakes infiltrates the ground to become groundwater. Groundwater slowly moves through the rock and surficial materials. Some groundwater returns to other streams and lakes, and some goes directly back to the oceans.

Fig. 2.1 Different components of the water cycle. Black or white text indicates the movement or transfer of water from one reservoir to another. Yellow text indicates the storage of water.

2.3 Stream: Meaning and concepts

A stream is a body of flowing surface water of any size, ranging from a tiny trickle to a mighty river.

A drainage basin is a portion of the Earth’s surface that contains a main stream and its tributaries or the area from which the water flows to form a stream. All of the precipitation (rain or snow) that falls within a drainage basin eventually flows into its stream, unless some of that water is able to cross into an adjacent drainage basin via groundwater flow.

Drainage divide- a ridge or strip of high ground dividing one drainage basin from another.It determines into which basin precipitation flows. The upland between tributaries is called interfluves (Fig. 2.2).

Streams within the drainage basin are either perennial or intermittent in flow.

Perennial streams– permanent streams that flow all year.

Intermittent stream — a stream that carries water only part of the year.

2.4 Stream order

One way of categorizing streams is by stream order. Most streams begin as springs or boggy seeps of groundwater. Some flow from lakes without an inlet. These fledgling streams are labeled first order. When a first order stream meets another first order stream, the resulting flow is a second order stream (Fig.2.3).

The second order stream picks up additional first order streams, but remains second order. Only when two second order streams meet is the result a third order stream.

The study of drainage network and its characteristics of a particular region can be classified in two ways namely descriptive approach and genetic approach. The descriptive approach includes the characteristics study of the stream forms and its pattern of a specific region whereas the genetic approach comprises the evolution process of stream based on tectonics, lithologies, structure and topography of a region. Therefore, drainage system means the origin and development of surface runoff (streams) through time and drainage pattern defines the different pattern of arrangement form by drainage system in terms of geometrical shapes in the areas. Drainage pattern are strongly influenced by the rocks types, geological structure, climatic conditions and denudational history.

Generally, the evolution of drainage system of an area is controlled by nature of initial surface and slope and geological structure. Streams and drainage system are classified into two classes based on in initial slope and geological set up.

1. Sequent stream: The stream flow along the natural slope and geological fault such as consequent stream, subsequent stream, obsequent stream and resequent stream.
2. Insequent stream: The stream that do not flow along the natural slope and drain over the geological structure such as antecedent stream and superimposed stream.

2.4.1     Sequent stream system

2. Consequent stream- The early stage of stream development and its courses in accordance with initial slope and geological fold is known as consequent stream (Fig. 2.4). The consequent stream which follow the natural landscape slope are called dip stream while stream developed along the folded structure formed the master consequent stream also called as synclinal consequent stream. The master consequent is the main stream of the drainage system of a particular consequent stream of a region. Domes and volcanic cones are the favorable topography for the formation of consequent drainage system. The drainage system of the Indian coastal plains is the best example of consequent stream.

There are two types of consequent stream:

1. i) Longitudinal consequent- Stream which follow along the syncline or depression axis in a geological folded structure.
2. ii) Lateral consequent- stream which follow from the lateral sides to join the depression axis. Thus the lateral consequent stream normally joins at almost right angle to longitudinal consequent stream.

2. Subsequent stream- The stream drained from the anticlines or ridges axis which developed after master consequent stream and joined the mainstream is called subsequent stream (Fig. 2.4). Generally, it defines as the transverse stream to the master consequent. For example, the Asan river and Song river are the subsequent stream which also a tributary of the

• Obsequent stream- The movement of stream current in reverse direction to the master consequent is known as obsequent stream (Fig. 2.4). The stream drained along the slope of landscape, thus, obsequent stream is also type of consequent. The stream developed from the northern slopes of the Himalayas ranges of east west direction flow northward to join the tributary drain east west direction (subsequent stream) of the southward flowing master consequent stream. For example, numerous stream originating from the Siwalik ranges flow northward due northern slopes and meet east west subsequent stream of the southward flowing master consequent of the Ganga and Yamuna river. Thus, the tributaries flowing northward from the Siwalik ranges are the obsequent stream.

2. Resequent stream- The stream (tributary) which lately developed and drained in the parallel direction to the master consequent is referred as requesent stream. Since, the stream is of recent evolution as compared with master stream, thus called requesent (Fig. 2.4).

The resequent stream is originated from folded geological structure during the starting of second cycle of erosion. The anticlinal ridges and synclinal valleys are converted into anticlinal valleys and synclinal ridges in due time during the first cycle of erosion in folded mountains. Thus, the longitudinal stream initiated by the end of first cycle of erosion in the anticlinal valleys and the development of new valleys in the syncline with the beginning of second cycle of erosion. Later, the stream originated in the synclinal area that flow parallel to the original longitudinal consequent form at the first cycle of erosion become resequent stream.

2.4.2    Insequent drainage system

1. Antecedent drainage system- The development of stream long before the upwarping of land surface or which antedate the upliftment of land or mountain by maintaining the constant water course through continuous downcutting of their valley is known as antecedent stream(Fig. 2.5). It is important to note that river or stream do not divert or deflect preexisting course with the subsequent uplift of the land area and further maintain the natural course by downcutting its valley at the equivalent rate with the rate of upwarping of the landmass. It is difficult to know the exact uplift rate of land as well as downcutting rate by the river in the particular area. Hence the concept of antecedent stream is totally related to both assumption and prediction.
   1. The degree of upwarping is an important factor for the initiation of antecedent drainage system because stream course following the same pathways depend on local environment conditions. In such case, stream or river undergo continuous erosion of valley through active incision at the equal rate of upliftment thus able to maintain the preexisting course. The dimension of uplift of land should be only local prospective and not the regional one for the evolution of antecedent stream. The stream originated in Himalayas regions are the examples of antecedent drainage system. Jhelum, Ganga and Yamuna river are the antecedent to the lesser and outer Siwalik ranges and Kamala, Rapti and Gola river are represent antecedent stream to only outer Siwalik range.

   

• Superimposed drainage system- A river flowing on a particular geological formation and structure, has inherited the characteristics of its previous form developed on upper geological formation of entirely different structural characteristics. In other words, a superimposed drainage is formed when nature and characteristics of the valleys and flow direction of a consequent stream developed on the upper geological strata underlying the lower strata of entirely different characteristics (Fig. 2.6). The current of consequent stream eroded the flat ground surface horizontally of bedded sedimentary rocks. With due time passage the stream excavated the underlying surface through repeated vertical erosion of the valley. Thus valley developed on the upper structure is superimposed on the lower strata and river maintains the normal flow direction and its drainage pattern in the area.

The conceptual difference between superimposed and antecedent drainage system are superimposed stream developed valley in the upper geological formation and extending to lower structure irrespective of its different characteristics and without any upwarping of the landmass whereas antecedent stream maintain the previous water course in the valley besides continuous upliftment of the land area across the river pathway. Most the rivers of the Deccan trap region of Indian Peninsular are superimposed stream due to evolution of recent drainage system on the freshly formed surface by solidification and cooling of lavas erupted during late Cretaceous and early Tertiary periods. Such stream excavation of upper lava covers and superimposed on the underlying geological structure. The Subarnarekha river is an example of superimposed stream on Dalma and phyllite hills to the west in the south-eastern Chotanagpur plateau region of Jharkhand.

Drainage pattern

The drainage pattern defines the geometrical shapes of the stream system and the spatial array of stream in a specific area or region. The drainage pattern of a particular area is not identical with to another region which relatively depends on nature of slope, lithological and structural factors, vegetative area and environmental conditions. Generally, there are different types of drainage pattern namely, 1. trellised pattern, 2. dendritic pattern, 3. rectangular pattern, 4. centrifugal or radial pattern, 5. centrepetal pattern, 6. annular pattern, 7. barded pattern, 8. pinnate pattern 9. herringbone pattern, and 10 (Fig.2.7). parallel pattern.

2. Trellised pattern- It is formed by the array of tributaries lateral stream and major consequent stream flowing the natural land slope and well adjusted to the geological structures [Fig. 2.7 (i)]. Such drainage pattern is originated in the region having alternated parallel valleys (synclinal) and ridges (anticlinal). Numerous master consequent longitudinal streams is developed in the elongated synclinal valley whereas lateral stream originated from both side of the ridges and join at almost right angle to master consequent stream. Hence, the stream arrangement form by the combination of consequent longitudinal stream and lateral consequent stream is called trellised pattern. The main difference between trellised and rectangular pattern based on the stream space between them. In the trellised pattern, the stream gaps are closed whereas rectangular pattern is formed by the wide gaps of stream.
   2. 2. Centripetal pattern- Another name of centripetal pattern is known as inland pattern which is opposite to the radial pattern. Inland drainage pattern is developed in the low lying or cone of depression area where numerous streams converge or direct towards at a particular region forming water accumulating zone or a basin [Fig. 2.7 (v)]. A series of stream that originated from surrounding elevated land surface come together at a specific low central area which may be a basin or carter lake. The central low lying area at the top of Riagarh Dome in theRadial pattern- Stream which divert away water tributary from central high point to various directions along the natural gentle slope of landmass is called radial pattern [Fig. 2.7 (iv)]. Such stream also known as centrifugal pattern. The favorable land structure for the evolution of radial pattern are dome structure, volcanic cones, batholiths and laccoliths, residual hills and isolated uplands respectively. The above mention land reliefs collect the water at the particular central cavity point and drained out along the slope resemble the spokes of wheel forming consequent stream. The best example of radial drainage pattern is the entire drainage array of Sri Lanka. In India, the upland area of Ranchi city located south- west developed numerous stream namely, South Koel, Subarnarekha, Kanchi and Karo rivers which represent the radiate drainage pattern. lower Chambal basin is the example of centripetal drainage pattern.
      2. Annular pattern- The tributaries of master consequent stream are initiated in the circular shape is called annular pattern. The other term of annular pattern is known as circular pattern. Such pattern is appeared in the area characterized by a mature and dissected dome mountain having parallel and alternate group of soft and hard rock beds. The different rate of erosion of soft and hard rocks produce ringed structure with resistant hard rock strike outward and less resistant soft rock form circular fissure [Fig. 2.7 (vi)]. The master consequent stream develop at the dome diverse down the slope like radial drainage pattern and the tributary streams develop by the soft bed erosion have arcuate shape finally meet again the master consequent stream, thus formed the annular drainage pattern. An example of annular drainage pattern is the dissected Sonapet dome of Uttaranchal.

      VII.  Barbed pattern- It is rare kind of stream pattern generally formed due to river capture. Barbed pattern is developed when the tributaries flow in opposite direction and join the master stream in hook-shaped bend [Fig. 2.7 (vii)].

      VIII. Pinnate pattern- Pinnate stream similar the feature of leaf veins. Such pattern is formed when master stream is developed at the narrow valley and their tributaries from the steep sides of parallel ridges meets the longitudinal main stream at acute angle [Fig. 2.7 (viii)]. The upper Son and Narmada river network represent pinnate drainage pattern.

      2. Herringbone pattern- The term herringbone is derived from the bone pattern of herring fish. Such drainage pattern is known as rib pattern [Fig. 2.7 (ix)]. It is developed in the board valley mountainous area which is flanked by steep slopes of parallel ridges. The master
         • Rectangular pattern- It is normally originated in the land surface where numerous surface runoff (rills) along weathered and eroded rocks along the interfaces of joints, faults and fractures form rectangular pattern. These small rills increase the width and length with passage of time become channels which drained at the lines of faults. Thus, the tributaries stream strike the master stream almost at right angles, thus formed the rectangular drainage pattern [Fig. 2.7 (iii)].
           2. Dendritic pattern- This stream pattern is far most commonly developed in the area where the unconsolidated rock formation can be easily eroded in tree-shaped structure [Fig. 2.7 (ii)]. Such dendritic pattern is independent of structural and lithological controls but associated with homogenous lithologies, slope and permeability of rock is the most ideal condition for the development of stream. Moreover, the stream network is more extensive in such land surface having flat surface, gentle slopes and permeable rocks. This pattern develops in different structural and lithological environments such as in the mountains (Himalayas), plateau (Deccan plateau), peneplain surface (Indian Peninsular), alluvium plain (North Both rectangular and trellised pattern have certain similar properties showing each tributary stream connect their master stream at right angle. But the difference between them is defined by the confluence angle is guided by strikes and dip angles of the bed rocks and narrow space tributary in trellised pattern while rectangular pattern have the confluence angle is determined by the weak faults lines and wide streams gaps.stream occupies the longitudinal valley and the lateral consequent tributaries develop from the parallel ridges join the longitudinal consequent stream almost at right angle. The upper Jhelum river in the Kashmir Vale feeds by numerous tributaries from the both sides is an example of herringbone drainage pattern.

2. Parallel pattern- It is characterized by numerous stream flowing along the regional slope and parallel to each other [Fig. 2.7 (x)]. Such type of drainage pattern is commonly occurred in continuous uniform slope and sinking rock beds like newly emerged coastal plains. The stream originated from the ridges of Western Ghat drained along the slope and maintained their water course towards west to feeds the Arabian Sea. Similarly, the eastern coastal plains of India also formed parallel drainage network.

2.6   Flood: Meaning and concepts

Flood – an event during which the volume of water in a stream/river exceeds the streams normal limits.

Runoff – Water that flows on the land’s surface. About 10% of Earth’s water is runoff

Flooding occurs when a river’s discharge exceeds its channel’s volume causing the river to overflow onto the area surrounding the channel known as the floodplain. The increase in discharge can be triggered by several events. The most common cause of flooding is prolonged rainfall. If it rains for a long time, the ground will become saturated and the soil will no longer be able to store water leading to increased surface runoff. Rainwater will enter the river much faster than it would if the ground wasn’t saturated leading to higher discharge levels and floods.

As well as prolonged rainfall, brief periods of heavy rain can also lead to floods. If there’s a sudden “burst” of heavy rain, the rainwater won’t be able to infiltrate fast enough and the water will instead enter the river via surface runoff. This leads to a sudden and large increase in the river’s discharge which can result in a flash flood.

Although many floods are triggered directly by precipitation just a few hours after it falls some floods can be triggered by precipitation that fell many months ago. Precipitation that falls as snow can remain as snow on the ground until it melts. This mightn’t be until the end of winter, so potentially several months. When the snow does melt, large volumes of meltwater will enter the  river increasing its discharge and triggering floods. These floods are often annual, occurring every year when snow melts in the spring. In Bangladesh, for example, melting snow in the Himalayas triggers annual floods in the summer.

Flash floods can also be triggered by slightly more catastrophic events. Erupting volcanoes can trigger very large flash floods called jökulhlaups when glaciers are partially or even fully melted by an erupting volcano or some other form of geothermal activity. The meltwater can enter rivers and greatly increase the river’s discharge leading to a flood.

Factors that influence runoff

• Ø Topography
• Ø Soil and bedrock
• Ø Land use

Floods in major cities especially during rainy season are proving to disastrous not only to the environment but also have serious implications for human life and property.

2.7 Urbanization and Flash Floods

• Ø Floods may be normal natural events, but urbanization causes fast flooding events called flash floods, to be more likely (Fig. 2.8).
• Ø In a natural setting, rainwater can infiltrate into the ground and vegetation will slow sheetwash.
• Ø In highly urbanized settings, rainwater cannot infiltrate asphalt and cement and there may be little or no vegetation to slow sheetwash.
• Ø As a result, the increased discharge in rivers lasts a shorter time, but increases faster and by a larger amount (Fig. 2.8).

Streams are the main sources of water for human consumption, agriculture and industrial needs. Streams are the cheapest means for goods transportation, hydroenergy and disposal of wastes. Flooding is a natural geological phenomenon due to continuous hydrological cycle in the earth and changing environment. However, the increase magnitudes of flood in present scenario are related to change in landuse pattern, deforestation, urbanization and human encroachment resulting modification of stream hydrology. Flood is the most common and costly types of natural and human induced disasters. Floods occur when high amount of stream discharge to be accommodated in the normal stream channel. When the water flow is high, the stream widens its channel by overflowing its banks and submerged the adjoining low-lying area. Such low-lying flooded areas are known as foodplains. In developing country like India, human casualties from flood due to lack of proper flood control and technical emergency response to flood affected area, absence of abating zoning regulations, high population density and lack of pre-warning system. Whereas the loss of life in developed countries is normally lower because of well preparedness of flood disaster management and early warning system. The disruption of life, property damage and economic loss is still facing in such countries.

2.8 Causes of floods

There are many factors responsible for casing flood in the area. They are:

1. Basic causes- High amount of stream flow beyond the carrying capacity of the channel and overflow from the bank to the surrounding floodplains causes floods. The two key reasons responsible for the occurrence of floods are i) excess water flow from the stream catchment area or basin and ii) deposition of sediments resulting reduce water carrying capacity of the stream.
2. Excess flows- Heavy rainfall and cloudburst, melting of large ice blocks or snow resulting of dam breakage and bursting of man-made or landslide built dams are the responsible for releasing excess flow in particular stream.
3. Carrying capacity of river- The maximum amount of water the stream could hold in a confine channel stretch is called carrying capacity of river. With the passage of time, every channel capacity is reduced to accommodate water due to accumulation of sediments carried down from the erosion of catchment basin, landslide debris, deposition of earth material for bridges construction and human settlement, industrial development and other activities obstruction the floodway.
4. Runoff versus Infiltration- The flood stream discharge depends on the amount of surface runoff or the recharge flow of groundwater from rainwater. The surface runoff is determined by the percolating water into the soil strata, green vegetative cover, soil texture and nature of land slope and geological conditions. Extensive green covers and permeable soil or rocks act as catalyst in absorbing rainwater in considerable proportions, thus slow down the rate of water flow down the slope. When the precipitation rate is higher than infiltration potential of the soil cause excess surface runoff contributing stream flood. On the other hand, permeable soil have better percolating capacity of water increases water table during monsoon season and give rise to numerous springs and seepages at foothills or down the slope, thus help in recharging stream during non-monsoon season. The rapid unplanned urbanization led to construction of building, paving and asphalted road construction has reduced the water infiltration and increased surface runoff in large cities like Delhi, Kolkata, Chennai, Bangalore and Mumbai causing urban floods.
5. Stream sedimentation and changing of river course- The suspended sediment load carried by stream water is managed by the formula that directly depend on, if the stream velocity becomes doubled then the sediment transportation would increases 64 times. The equation for sediment transport of sandy-bed rivers is gsαqx.

where gs = sediment discharge per unit width of the channel and q = stream discharge per unit width of the channel and x = the exponent range from 2 to 3.

The degree of erosion index is defined as the sediment amount drained by the river that is eroded from its catchment area and channel banks. The huge amount of sediment carried by floodwater caused siltation of floodplain. The sediment debris collected from mining and road construction, deforested and wasteland area and landslides are deficient of organic nutrients spread on the floodplain affecting agricultural productivity. Table 2.1 illustrates both natural and man- made factors leading to causes the floods

2.9 Flood hazard in India

1. Mountain barrier and rainfall- The Indo-Gangetic plains received heavy precipitation and cost huge economic lost and human life due to barrier of the great Himalaya Mountain (Fig. 2.9). The moisture laden clouds from the Bay of Bengal moving northwards are block by the high mountain of Himalaya causing torrential rainfall in the foothills and great Indian plain. Similarly, the Nilgiri-Anamudi act as barrier for rain-clouds and responsible for massive rainfall in Kerala and the Sahyadri Range in Western Ghat (Fig. 2.9 & 2.10).
2. Brahmaputra Basin- Brahmaputra basin being the largest basin in India’s river received frequent flood due substantial change in stream hydrology related to natural disaster and unplanned development in highly earthquake sensitive area (Fig. 2.9). Such factors accelerated the sediment deposition and force the Brahmaputra currents to flow southwards causing continuous erosion along the south bank.
3. Flood in Western India dryland- Indian States of Western region namely, Rajasthan and parts of Gujarat are characterized as desert areas with an annual average rainfall of 15 cm, have catastrophic floods even through erratic and localized scope (Fig. 2.9). Such unwanted events may be due to lack of proper drainage system and green covers, unable to channelize the excess water from sudden cloudburst in the region. In other words, floods in the desertarea are direct consequence of excessive runoff and underdeveloped proper drainage system.
4. Flood frequency- India receives an average annual rainfall equivalent to about 4,000 billion cubic meters (BCM). This only source of water is unevenly distributed both spatially as well as temporally. The estimated precipitation during the monsoon season (June to September) is of the order of 3000 BCM. Approx 75% rainfall during monsoon period caused flood due to excess water discharge more carrying capacity of river and stream channels. However, rainfall data from Meteorological Department shows no variation in precipitation level in the last hundred years, but flood frequency and their intensity have been increasing. Such increased in flood frequency may be attributed to deforestation, unplanned urbanisation, change in stream hydrology, siltation of stream channels and geological disaster. Flood frequency of any river is calculated from the relations curve of a particular stream discharge and flood recurrence intervals in years.

References and Recommended Reading

• Leopold, L.B., Wolman, M.G and Miller, J.P. (1964)    Fluvial processes in geomorphology.
• W.H. Freeman, San Francisco, California.
• Pandit, C. (2009) Some common fallacies about floods and flood management. Current Science, 97: 991-993.
• Sinha, R. and   Jain, V. (1998) Flood Hazards of North Bihar Rivers, Indo-Gangetic Plains.
• In: Flood studies in India; Memoir Geological Society of India, 41: 27-52.
• Soni, V., Gosain, A.K., Datta, P.S. and Singh, D.(2009) A new scheme for large-scale natural water storage in the floodplains: the Delhi Yamuna floodplains as a case study, Current Science, 96: 1338-1342.
• Waters, T.F. (2000) Wildstream: A Natural History of the Free Flowing River. St. Paul:Riparian Press.
• Wells, N. A. and Dorr, J. A. (1987) Shifting of the Kosi river, northern India, Geology, 15: 204-207.
• http://www.iitk.ac.in/waterwoes/brochure%20-%20Flood%20Disasters%20English.pdf
• https://www.nap.edu/read/1807/chapter/7
• http://www.kean.edu/~csmart/Observing/13.%20Streams%20and%20floods.pdf