RIVER CHANNEL MORPHOLOGY

The study of the channel pattern and the channel geometry at several points along a river channel, including the network of tributaries within the drainage basin. Also known as River Channel morphology

4.1 Types of River Channel

When the river flows in its way, it follows a definite path, the path followed by the river during its flow is called river channel. The river follows different path according to the energy level, the velocity of river and the gradient of riverbed. There are three different types of river channel.

Straight

The river follows a straight path in it's high energy level. In this case erosion is predominant and the velocity of river is maximum. In hilly region, the river follows straight path.

                                                                                                                                                                                            

Meandering                                                                                            
 In this type of flow, the river follows the path like as snake's movement. In valley region, the river follows the meandering path. Erosion and deposition take place side by side in the same time.

                                      

Braided                                                                                                                 

In this type of flow, the river follows multiple channels. Deposition is predominant in this flow. The braided river tends to be very wide and relatively shallow. The river follows this type of path in Terai region.

Factors Responsible for River Channel Morphology

The river does not follow the constant path for all the time. The river channel may change time by time according to the factors affecting to it. There are different factors affecting for the river channel morphology.

Discharge

The volume of water flowing per unit time is known as discharge. The discharge of water may affect for the river channel morphology. The activities of river may depend on the discharge of water. The high discharge water may cause erosion, transportation and deposition in high rate, which activities ultimately may affect for the river channel morphology.

Velocity

The velocity has the significant role for river channel morphology. The fast moving rivers are capable of erosion and carrying a greater amount of materials. The velocity itself depends on the gradient and discharge of river.

Lithology

The nature of rocks along the channel and along the side may effect for the river channel morphology. Some types of rocks are more easily eroded by river water. The river channel, which passes through the limestone, will be straight.

Load

The river channel may also be affected by the load condition of the river. If the river is fully loaded it will not have further capacity to erode and transport the materials of the channel. But if the river is under loaded it will erode its channel effectively.

Activities of River

When a river flows continuously in its way it carries out different activities along with it's continue flow. The activities of river are different in different places according to the velocity of river, gradient of riverbed and energy level of flow. There are three different activities generally done by the river.

Erosion 

The process in which the existing materials are taking away by the river during its flow is known as erosion. Erosion takes place in high velocity and maximum gradient. River cuts side bank and erodes bed in this stage.

Transportation

The process in which the river carries erosion materials from one place to another place is called transportation. Transportation is completed in solid form, solution form and biological form. In solid form, there are two different loads of flowing materials, i.e. coarser materials as bed load and finer materials as suspended load.

Deposition

The process in which the flowing materials are left by the river and collected it in the river's side and bed is called deposition. Deposition takes place in small velocity and low gradient. This process causes the riverbed to be up and sometimes changing the course of river. 

Land Forms Developed by the River

Higher Terrace

It is the land formed by the river as it was flowing in these areas in the past history. The higher terrace is over the high flood level of the river in present. There may be finer and coarser material layers alternately in the higher terrace.

Middle Terrace

As its name it is the middle part of the higher and lower terrace. This terrace will also over the high flood level of the river.

Lower Terrace (Flood Plain)

During floods a river overflows its bank and submerges the adjacent low-lying areas where deposition of alluvial material takes place. A wide belt of alluvial plain formed in this way on either side of a stream, is called 'flood plain'.

Channel Bar

Many rivers are forced to rope a part of their load along their beds, especially in the flatter regions along their course. Most commonly the deposits so formed tale the shape of long narrow ridges called channel bars. Since they are made up of sand, they are also called sand bar. The river will flow from both side of the channel bar. They are temporary in nature because an increase in velocity, the river may cut down and take the sand along with it.  

Oxbow Lake

The meanders grow by eroding its outer bank and depositing sediments at the inner bank. During this process the sharpness of the river bends increases progressively and the neck of meander becomes narrow and narrow. Finally a stage comes when the r iver cuts through the neck and starts flowing straight leaving behind its roundabout course. Such left out old meanders which remain filled with stagnant water, are called “oxbow lakes”.    

Alluvial Fans

The alluvial material, which flows down from mountains, accumulates at foothills where the stream enters a plain. The deposition occurs due to abrupt cha nge in the gradient of river valley. Such deposits spread out in the shape of flat fans and are called “alluvial fans”. Usually the coarse material is dropped near the base of the slope while finer material is carried further out on the plain. Alluvial fans from many adjacent streams along a mountain may merge to form a long wedge of sediment called “alluvial aprons”.      

Natural Levees

“Natural levees” are the low ridges, which are formed on both sides of a river channel by the accumulation of sediment. They tend to confine the flow of river water into its channel between flood stages. The natural levees occur in rivers, which have broad flood plains. During floods the river overflows its bank and its velocity decreases rapidly. As a result most of the coarse sediment is deposited along the area bordering the river channel and finer sediments are deposited more widely over the flood plain. In this way, successive floods build up ridges on both sides of a river channel, which are called “natural levees”.   

Point Bars

In meandering rivers, sediment deposits occur as point bars. The point bars are the crescent shaped deposits, which occur at inside bends of a river channel.

Deltas

Deltas are deposits built at the mouths of streams. The deltas are usually triangular in shape with their apex pointed upstream. When a stream enters an ocean or lake, the currents of the flowing water dissipate quickly. This results in the deposition of the series of sedimentary layers, which make up the delta. The material of most deltas is well sorted and many deltas are uniformly graded

4.2 Description of river channel observed in field visit

Day 1^st                                                   Date:-2067/11/15                               

                                                               Time:-11:02am

                                                               Location:-Trishuli-Belkhu Khola

This was the second site before reaching Malekhu.

 This is the area where Belkhu River meets Trishuli River. To reach Malekhu we have to cross bridge over Belkhu River. Here we observed a foundation of a small heighted old bridge. The bridge had collapsed in 1993 A.D.

Following was the reason of bridge collapse

·         The foundation of the bridge looks strong but the height of the bridge was less than high flood level of the Belkhu River.

·         Lack of complete study of watershed.

The presence of huge boulders by the side of the broken bridge supports the fact. There was huge amount of debris flow in the Belkhu River at that time. The catchments area of the river is also very large and this is also one of the reasons for the flooding away of the old bridge of small height. So the main reason for collapse of the bridge lack of proper site investigation, low height of the bridge and big rock balls that hit the bridge along with debris. The site selection and foundation of the bridge are good but maximum discharge and proper site selection was not seen while constructing the bridge which caused the collapse of the bridge.

Morphology of Trishuli-Belkhu Khola

Ø  Meandering river features was observed.

Ø  Fine aggregate situated in lower level or below the coarse  aggregate because of which visitation grows (Graded bedding )

Ø  Presence of Boulders of big size in the bank of the Belkhu River indicates the deposition of it by the same river.

Ø  Small delta formation observed at the junction point of Belkhu and Trishuli River.

                                           

Day 1^st                           Date:-2067/1115                                

                                       Time:-1:15pm

                                       Location:-Trishuli –Thopal Khola

            Chainage :-200+000m away from suspension bridge along old track

We studied the river channel morphology of the Trishuli-Thopal confluence from the higher terrace of the river located about 200m towards Dhading Besi along old track from the suspension bridge. There was channel bar in the left side of the river. The discharge of the Trishuli river was high with compared the Thopal river. There was side bar in the right bank of the river. At the confluence point there was erosion band in the right side of the river. There was an island at the confluence of these two rivers. The site study of the field fortified us with the knowledge about the morphology of the river, the activities of the river, various landforms formed by the river cutting, deposition and erosion. Moreover the site provided us with the knowledge about the appropriate site for the construction of the bridge, i.e. the bridge must be constructed where the effect of river cutting is the minimum and it should be somewhere near the deposition bank so that we can easily get the constructions required. The site must also have a strong foundation bed to resist the impact due to river.

Following features of river was observed at the site:-

Ø  Meandering river type

Ø  Still water present in some part of river.

Ø  Rock island forms at the junction of Trishuli- Thopal Khola.

Ø  Erosional and depositional bank in Trishuli river

Ø  Point bar deposit in Trishuli river.

Ø  Large and wide striking bank.

Erosional bank

The bank of the river, which is weaker part, has the tendency to get eroded. This is the unstable bank of the river.

Deposition bank:

The bank on which deposition process takes place is called deposition bank.

Channel bar:

The accumulation of the materials at the mouth of the river is called channel bar.

Flood plane:

The plane or the area covered by the river when the river is in the flooding condition. At these places, materials from very fine to coarser conditions are found.

River terrace:

The area that the river used in the past to cover during flooding or normal condition is called river terrace. This is the area has left the channel level to lower level and the remnants of earlier deposits to form higher level. This area is very suitable for human settlement. Gravel and sand is found in such places.

Side bar or Point bar:

These are the remaining of deposits carried out by the river at the stage of higher discharge. This is also known as side bar.

Rock Island:

It is the area formed by the river deposition. It is not necessary to be at its mouth.

                                                     

  Right bank of the Trishuli River at the height of sixty meter above the old road from Malekhu to Dhading Besi three hundred meter upstream of suspension bridge. Conference of Thopla Khola and Trishuli River.