STREAMS AND FLOODING- Notes

STREAMS AND FLOODING- Notes:

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 e 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.

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 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

teway

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.

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.

Sequent stream system

1. 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:

Longitudinal consequent- Stream which follow along the syncline or depression axis in a geological folded structure.

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.

II. 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 Yamuna and Ganga river are again the master consequent stream.

Obsequent stream- The movement of stream current in reverse direction to the master consequent is known as obsequent stream. 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.

IV. 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.

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

I. 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 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.