Contents preface (VII) introduction 1—37

Table 12.2 Average rates of migration of the Kosi river (9)

Period	Period of	Approximate distance	Rate of movement
	Movement (years)	moved (km)	(km/year)
1736-1770	34	10.8	0.32
1770-1823	53	9.3	0.18
1823-1856	33	6.1	0.18
1856-1883	27	12.9	0.48
1883-1907	24	18.5	0.77
1907-1922	15	10.9	0.73
1922-1933	11	29.0	2.63
1933-1950	17	17.7	1.04

RIVERS AND RIVER TRAINING METHODS

415

12.3.5. Delta Formation
As a river approaches the sea, it dumps its sediment load into it and the river mouth extends towards the sea on account of the sediment deposition. This lengthening of the river further reduces the already small slope of the river at its mouth. Thus, the sediment transporting capacity of the river decreases and the river deposits the sediment on the bed and banks of the river channel raising the river stage. In general, the rise in the river stage results in spilling over the banks and cutting through the banks if they are not sufficiently resistant. The spilled water may form branch channels or spill channels which, after their full development, start behaving like the parent river. As a result, the sediment transport capacity decreases considerably and the river bed starts rising resulting in the formation of delta (Fig. 12.6).

. R
Mahanadi

	R	.
Daya

Chilka

lake

Baitarni R.

Brahmani R
.

Bay	of	bengal

	R	.
Bhargavi
	Puri

Fig. 12.6 Delta formation of the Mahanadi river
12.3.6. Aggradation
When the sediment load in a river is in excess of its sediment-transporting capacity, the excess sediment gets deposited on the river bed. Therefore, the sediment load entering a given reach is greater than the sediment load leaving the reach during the same time. This causes a rise in the bed level (and, hence, the flood level) and increase in the bed slope with time. The phenomenon itself is known as aggradation. Aggradation usually occurs because of an increase in the sediment load at a section without change in discharge and sediment size. Aggradation occurs most commonly upstream of a reservoir. Because of the construction of a dam, the sediment-transporting capacity of the river upstream of the reservoir is reduced. The coarser sediment deposits farther away from the dam while the finer sediment is deposited closer to the dam. Aggradation can also occur when a tributary brings into the main river sediment load in excess of the river’s sediment-carrying capacity. Also, when rivers are divided into two or more branches, as in the case of deltaic rivers, it may not be possible for every branch to

416 IRRIGATION AND WATER RESOURCES ENGINEERING
maintain the state of equilibrium in respect of sediment flow, and, hence, aggradation may result. If an offtaking canal takes relatively sediment-free water, it would result in aggradation in the main river downstream of the offtake point.
12.3.7. Degradation
When the amount of sediment load being transported by a river is less than the sediment-transporting capacity of the river, the excess sediment needed to satisfy the capacity of the river will be eroded from the erodible bed, thereby lowering the bed. If the banks are also erodible, widening of the river would also result. This phenomenon of erosion of bed and banks is known as degradation. Degradation occurs in streams downstream of a reservoir. The reservoir stores a large percentage of the sediment load carried by the river. The flow downstream of the reservoir is relatively free of sediment, and, therefore, cause degradation resulting in lowering of the bed levels.
An interesting example of large bed level changes is that of degradation on the Ratmau torrent downstream of a level crossing on the Upper Ganga canal in UP (10). The flood discharge in the Ratmau torrent has been of the order of 1000 m³/s with a maximum of 2250 m³/s in 1947. The level crossing was constructed in 1850 to pass the torrent across the canal carrying a discharge of 300 m³/s. During the period from 1854 to 1977, the bed level of the torrent immediately downstream of the level crossing was lowered by 8.0 m. The bed slope of the torrent downstream of the crossing has decreased from 1.56 × 10^–3 to 5.5 × 10^–4. The lowering of the bed required frequent modifications in the stilling basin (10).
If degradation has occurred in a river near its confluence with a tributary, the tributary slope increases. Thus the sediment-transporting capacity of the tributary increases near the confluence, resulting in degradation near the confluence and this effect extends further upstream and also to other sub-tributaries. Degradation can also occur if the water discharge in a river is increased without increase in sediment load or sediment size. If the sediment-free waste water of an irrigation project is added to a river, it may result in degradation in the river.
Downstream of a spillway, a hydraulic jump is usually formed to dissipate the excess energy. Due to lowering of the bed and water levels on account of degradation, jump may shift downstream of the apron and endanger its safety. Dams on pervious foundations are subjected to uplift pressure which depends on the effective head which is equal to the difference between the reservoir and tail-water levels. The lowering of the tail-water level due to degradation increases the effective head and, hence, the uplift pressure on the dam. Lowering of water level at the intakes on account of degradation may make the diversion of water for irrigation more difficult. Lowering of bed in navigable rivers due to degradation may, at times, make the navigation locks inoperative.
Degradation can sometimes be beneficial too. Because of lowering of tail-water levels, the effective head increases. This would cause an increase in hydro-electric power generation. Degradation increases the river capacity to carry the flood flow. This lowers the high flood levels of a river. Lowering of the water level in a river on account of degradation lowers the ground water table in the adjoining areas.