River morphology

RIVER MORPHOLOGY AND SEDIMENT PROPERTIES

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Fluvial geomorphology

It studies the shape and evolution of the riverbed and its interaction with the drainage system of its collecting basin.[hr height=”20″ ]

Geomorphological cycle of the basin

Although the hydrological cycle is a continuous process that feeds water to the basin through precipitation, the feeding of solid matter to the basin occurs in a short geological time and is the result of the mechanism of the orogenesis of the formation of the mountains. With reduced tectonic activity, the mountain (fractured or folding or eruptive) represents a limited reserve of solid matter and potential energy, which by disintegration, erosion and deposition in the course of time is transformed into an almost flat relief of minimum potential energy (base level of erosion).

The geomorphological cycle comprises the total remodeling of the original relief to a quasi-plain over a period of many millions of years. Analogously to the time period of human life, the geomorphological cycle is divided into three stages:

Youth stage: characterized by strong erosion and the formation of deep valleys.
Maturity stage: valleys gradually widen and mountains flatten little by little.
Old age stage: landscape changes less and less.

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Figure N ° 1.1 River system idealized by Schumm (1977)

After having used up all the potential energy, the reshaping of the relief will find its limit at sea level, since this level is the most general basis for erosion.
In Figure N ° 1.1, a basin is shown as a sediment-producing system, where, ideally, 4 zones are defined:

Upper zone 1 or reception basin: corresponds to the main sediment-producing area. Area of interest for hydrologists and geomorphologists related to the evolution and growth of drainage systems.
Transport zone 2: where the stable channel predominates and is in equilibrium, that is, the amount of sediment that enters is equal to the amount that leaves. Area with predominance of hydraulic engineering and river control, as well as river morphology.
Deposition zone 3
Drainage zone 4: carries clear water after deposition has occurred

This zoning is artificial, since in any of the first three zones there may be erosion, transport and sedimentation processes. However, there is generally a predominant process in each of the zones. In some cases, the deposition zone may be truncated by the presence of a receiving river. In other cases, there is no transport zone because the production zone is directly interconnected with the deposition zone. The variables of the river system are presented.

Weather
Initial relief
Geology (lithology and structure)
Weather
Vegetation (type and density)
System relief or volume above base level
Hydrology of runoff and sediment production per unit area within Zone 1.
Morphology of the drainage network
Mountain morphology
Hydrology of water and sediment discharge to Zones 2 and 3.
Channel and valley morphology, and characteristics of the sediments in Zone 2.
Morphology of the deposition system and the characteristics of the sediments in Zone 3.

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Formation of the drainage pattern of a river

The most characteristic feature of a drainage basin is its drainage network, which develops in a variety of patterns. Howard (1967) presents 8 types of drainage patterns according to different soils and hydrological and topographic conditions.

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A.- Dendritic: it is formed on rocks with uniform resistance to erosion and on regions of gentle slopes.

B.- Parallel: it is formed in regions of steep slopes.

C.- Lattice: it occurs in areas of folded rocks with main divisions formed along outcrop of resistant rocks and easily erodable rock valleys.

D.- Radial: it occurs in volcanic areas where there is no effect of the difference in the resistance of the rocks.

E.- Rectangular: is formed in areas where joints and faults intersect at right angles.

F.- Annular: occurs in basins and eroded structural domes, where resistant outcrops form the main division and weak rocks form valleys (a concentric type of lattice pattern).

G.- Multicuenca: it is formed in areas where the original drainage pattern has been modified by glaciation and recent volcanism.

H.- Contorted: it occurs in areas of complex geology where the drainage pattern is controlled by metamorphic rocks, faults, seams and dikes.

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Figure N ° 1.2 Basic drainage patterns

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Sinuosity of the river (S_R)

SR is the quotient between the length of the thalweg (LT) and the length of the valley (LV). The thalweg is the flow path corresponding to the deepest part of the river.

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Rivers Classification

Straight
When SR £ 1.5 the river is considered straight.
Meandering
When SR> 1.5. A meandering river is presented in plan as sinuous and consists of a series of elbows or curves connected by straight or semi-straight sections. In curves, the deep part in the sections is located in the concave part, due to the relatively high speeds that occur there. Centrifugal forces in the elbows generate cross currents that flow towards the convex part of the bend.
Braided
A braided river divides into several channels, which successively meet and divide again. A good synonym for braiding is the medical term “Anastomosing” which means splitting and gathering of the blood vessels. Generally, these rivers are wide, poorly defined and with unstable banks, forming many channels and sub-channels that flow around sandy bars and islands, and that change position over time in an unpredictable way.

Lane (1957) states that the two main causes of river braiding are:
- Sediment overload: excess sediment that the flow can carry leads to sediment deposition.
- Strong slopes.
Another cause of braiding is the presence of easily erodable margins or banks. If the banks are easily erodible, the channel subjected to high flows widens and when low flows occur, bars are formed, which when stabilized, islands are formed.

Geometry of meanders

In addition to the sinuosity defined above, some empirical relationships have been developed between the meander’s length, width, and mean radius. The geometric relationships in Table N ° 2 are useful to characterize meanders.

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Table N ° 1.2 Empirical relationships for meanders

In the geometry of rivers, it is necessary to define some terms:

Bars: with bed shapes that have lengths of the same order or greater than the width of the channel and heights comparable to the average depth of the flow that generates them.
Point bars: they are sediment deposits that appear in the convex part of the curves of rivers. Their shape may vary as flow conditions change, but they do not move significantly relative to the bend or bend of the meander. However, the size and location of this bar vary with download.
Alternating bars: these are bars that are periodically distributed along the margins of the current. Its width is much less than the width of the channel. These types of bars move slowly downstream.

Cross bars: these are bars that occupy almost the entire width of the section of the channel, and appear in an isolated or periodic way along a given stretch of river and move downstream.

Tributary bars: these are bars that appear immediately below the entrance of a tributary to the main channel.

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Formation of Floodplains

The formation of flood plains is usually located in Zones 2 and 3 of the river system, where the occurrence of floods loaded with sediments exceeds the capacity of the channel and causes overflows towards the river banks, where the flow velocity is reduced. drastically reduces producing the deposition of sediments.

These continuous overflows on the margins as a hydrological response and the geomorphological process of Zone 1, form flood plains that in some cases have positive effects for the fertilization of the lands and in other cases they have destructive effects on agricultural areas and human settlements that they are located in the floodplains.

The alluvial plains, bordering the rivers and channels, have always offered advantages for human settlements and are conducive to agriculture, since their proximity to rivers offer facilities for water supply, effluent evacuation and other advantages. However, alluvial plains also serve to allow water flows that exceed the conduction capacity of the channel in times of floods to pass through them, forming natural fluvial floods.

Flow propagation in the floodplains is complex because the hydraulic characteristics of the floodplain are different from those of the riverbed, dead storage areas are formed, and the overflow flow of the riverbed into the floodplain and propagation the flow of water over it is still a problem to be solved.