For systematic discussion about streams, it is advantageous to classify them; classification of streams enables one to make generalization about a group of streams having similar attributes. As can be seen below this classification is done using objective, qualitative or quantitative criteria. According to Rosgen (1996) such classification often helps in (i) prediction of river behaviour from its appearance;
(ii) development of specific hydraulic and sediment transport relations for a given stream type, (iv) extrapolating site specific data to stream reaches having similar characteristics; and (v) providing a frame of reference for communicating about stream morphology among different disciplines.
As discussed earlier Davis (1899) divided the streams into youthful, mature and old, depending on their stage of development in the cycle of erosion. This classification gives only qualitative attributes of each type. Davis (1890) also distinguished between consequent streams following the natural slope of land surface; subsequent streams flowing into consequent streams from the sides at right angles to the dip and parallel to the strike; resequent streams as tributaries to subsequent ones more or less parallel to consequent main streams; obsequent streams flowing against the dip of the beds; and insequent streams, which show no apparent relation to the dip of the beds. These are shown in Fig. 4.6 and discussed in detail by Worcester (1948). However, the classifications of Davis do not take into account the main hydraulic variables on which stream size, shape and plan form depend.
Fig. 4.6 Relation of drainage to topography and geological structure
(Davis 1890)
Depending on the variation flow in the stream with time, streams can be classified into three categories. Perennial or permanent streams are those, which flow throughout the year.
These get their water from lakes, snow banks or glaciers, or land from direct precipitation, and which maintain regular flow. Those perennial streams, which have cut deep into sediment or other strata, may receive ground water flow also. Intermittent streams are those whose sources of water fail intermittently. They occur mainly in regions of seasonal rainfall or snowfall, and particularly common to semi-arid regions. Ephemeral streams flow only in response to precipitation; they are not fed by springs or by slowly melting snow.
As discussed in Chapter II, Horton, Strahler and others have developed a system of ordering channels in a drainage network; channels of the same order show similar characteristics, as shown by Rznystin (1960).
Plan-form or channel patterns can be defined as the traces of the channel in plan as obtained from air-photos or as presented on the map. Plan-forms of alluvial streams are of importance to hydraulic
C - consequent, S - subsequent, R - resequent, O - obsequent, I - insequent stream L - original land surface
S L C
O B
engineers as well as to geo-morphologists and sedimentologists. For the hydraulic engineer not only the plan-forms but also their spatial and temporal variation is important to decide the location of bridges, barrages, levees and other structures. For the geo-morphologist they are an indication of modern river behaviour; plan-forms also throw light on the past morphology of the stream. A sedimentologist studies plan-forms and the associated sedimentary deposits in order to develop knowledge about old streams.
Lane (1957) analysed data from sand-bed rivers and rivers flowing through coarser material from USA and other countries and broadly classified the streams according to plan-forms into straight, meandering and braiding patterns. He further indicated that plan-forms are essentially a function of slope and bankful discharge. Leopold and Wolman (1957) followed the same classification as that of Lane. Plan-forms can be classified depending on whether the stream flows in a single channel or in multi-channels.
Streams flowing in a single channel can be straight or meandering. However, in nature, streams do not flow straight for more than 10 to 20 channel widths and even in straight channels the talweg shows a meandering pattern. Plan-form classification is shown in Fig. 4.7.
Fig. 4.7 Classification of plan-forms Plan form of streams
Single channel streams
Straight Meandering Anabranching Reticulate Deltaic Braided
Incised meanders Meanders in flood plains
Irregular Regular
Simple Compound
Simple
(sine, parabolic, circular, etc.) Compound Regular
Irregular
Classification according to shape
Classification according to valley width
Underfit Overfit Classification according
to movement Free (Lateral migration) Inactive
Migrating downstream Multi channel streams
The meanders can be either incised or in plain and can take various shapes in plan. Meandering streams can be further classified depending on whether the meanders move downstream, laterally or are stationary. Chitale (1970) classified meanders into regular and flat, irregular and flat, regular and acute, irregular and acute, simple, and compound meanders (see Fig. 4.8 (a) and Fig. 4.8 (c)). He also stated that a particular stream might have a single channel in one reach and multiple channels in other reach, a fact noticed on many streams. The multi-channel streams are classified into braided, deltaic, reticulate and ana branching. These are schematically shown in Fig. 4.8(b).
Fig. 4.8(a) Plan-forms of rivers
It is appropriate to describe two other forms of streams based on the relative width of meander and the valley. A mis-fit stream (Dury 1969) is defined as one, which occupies a valley formed by a stream of considerably larger or smaller discharge. An under-fit stream occupies the valley the valley formed by a stream of greater discharge. Most of the streams, which are under-fit, now have had their channel forming discharge reduced due to climatic changes. An over-fit stream occupies a valley formed by
Incised meanders (irregular) Meander
beltMB
Compound meanders Buyuk meanders river (Turkey) Definition sketch for meandering stream Straight reach of valley creek, Pa (U.S.A.)
0 100 m 150 m
150 790 m
ML Meander length
MW
Width Point bar b
Talweg
much smaller discharge; however an over-fit stream will usually remove all signs of small stream channel and widen its valley to conform to its present flow. Therefore, over-fit stream is a transient stage and is rarely found. Dury (1969) has discussed about another type of under-fit stream, called Osage type, which is named after the Osage river in Missouri (USA). This type of stream lacks meanders; however, it has pool and riffle sequence spaced at an interval of five channel widths. It behaves as if it were straight; however it does not reflect the curves of the valley. The apparent width to depth ratio of streams of Osage type is about forty, larger than ten as observed on meandering rivers; but actually in an under-fit stream of Osage type it is the wave length of the former stream and width of the shrunken present day stream. These types of plan-forms are shown in Fig. 4.8 (c).
Fig. 4.8(b) Plan-forms of rivers S
Under-fit meandering stream Osage-type
LL R LL
S
Valley meander lobe R Stream
Valley meander scar L
S S
Deltaic pattern Reticulate pattern
Diamantina river (Australia)
Anabranching pattern of Darling river (Australia) High land
Braided stream
Darling river High land
Free meanders oxbow lakes and meander scars Pembina river near Monola (Canada)
Schumm (1968, 1977) has classified stream channels flowing through sandy materials, based on the mode of sediment transport (i.e., predominantly suspended load, mixed suspended load, and predominantly bed-load), percent of silt-clay in the perimeter of the channel, and channel stability (graded, depositing i.e. excess sediment load, and eroding i.e., with sediment load deficiency). This is given in Table 4.2.
It may be mentioned that Blench (1955), and Simons and Albertson (1963) have also recognized the importance of bed and bank material in shaping the geometry of stable channels. Allen (1965) has diagrammatically represented Schumm’s ideas in terms of size and sinuosity, which is shown in Fig. 4.9.
Kellerhals et al. (1972, 1976) have given a further refinement in the classification of river channels, which is primarily based on the interpretation of air photos and detailed survey of Canadian rivers. The detailed data needed for classification include: (i) whether the stream is aggrading, degrading, partly entrenched, or entrenched with no flood plains, (ii) channel plan-form description, namely straight, sinuous (MB < 2 W), irregular or regular meanders, or tortuous meanders (q between channel axis and valley trend greater than 90o), (iii) presence of islands and basis; and (iv) lateral activity namely meanders moving downstream, downstream progression and cut-offs, entrenched loop development, avulsion etc. Figure 4.10 gives Kellerhal’s classification of lateral activity. This classification is very exhaustive but rarely used in engineering design. Further, some of the attributes cannot be quantified.
Fig. 4.8(c) Meander classification according to Chitale (1970) Compound meanders in Rind river (U.P.) India
Irregular and sharp meanders in Sai River (U.P.) India Regular and sharp meanders in
Mississippi river
Irregular and flat meanders in Ken river (U.P.) India Regular and flat meanders in
Mahi river (Gujrat) India
Compound meanders Simple meanders
Irregular and acute meanders Regular and acute meanders
Irregular and flat meanders Regular and flat meanders
Fig. 4.9 Diagram relating stream channel stability to sinuosity and character of stream load (Allen 1965) Table 4.2 Classification of channels according to Schumm (1968, 1977)
Mode of sediment transport
Percent of silt-clay
W/D: width to depth ratio, Si: sinuosity
Stable (graded) Depositing (excess
- W/D greater than 40 Si less than 1-3
Finally, streams can also be classified depending on the type of material on their bed, character of the sediment transported, and the slope. Boulder rivers have large size cobbles and boulders on their bed; they are found in mountainous regions with very steep slopes and they carry much finer material eroded from the catchments. Only in catastrophic floods do the boulders on the bed move. These rivers are usually entrenched. Gravel- bed rivers have gravel and sand on their bed, have steep slopes and are paved during normal flows. During the floods the pavement is destroyed. These are found in the foothills and have large width/depth ratio. Rivers in flood-plains flow through the material deposited by them, carry material forming the bed and banks of the river, and have relatively much flatter slope as compared to that of gravel-bed and boulder streams. Their bank material may be slightly cohesive and they carry varying amount of wash load.
On the basis of a study of a number of streams in USA, Rosgen (1996) has proposed a hierarchical classification of streams. His classification provides the physical, hydrologic and geomorphic way of linking the driving forces and response variables at different levels of inquiry. Thus as one moves from Level I to Level IV, one progressively takes into account geomorphic characterization, morphological classification, stream condition and validation level. To facilitate the classification Rosgen used
Suspended load
Entrenchment ratio ER (= width of flood prone area at an elevation twice the bankfull depth/
bankfull width)
Width to depth ratio W/D = (Bankful width/mean bankfull depth) Sinuosity Si = (Stream length/valley length) and Slope S
Thus at Level I, based on ER, W/D ratio, sinuosity, slope and channel pattern, the streams are classified into nine types designated as Aat, A, B, C, D, DA, E, F and G as indicated in Table 4.3.
Table 4.3 Rosgen’s stream classification at level – I (Rosgen 1996)
Stream Type Aat A B C D DA E F G
ER < 1.4 < 1.4 1.4 – 2.2 > 2.2 N.A. > 2.2 > 2.2 < 1.4 < 1.4 W/D < 12 < 12 > 12 > 12 > 40 Highly < 12 > 12 < 12
variable
Si 1.0 – 1.1 1.0 – 1.2 > 1.2 > 1.4 N.A. Highly > 1.5 > 1.4 > 1.2 variable
Slope S > 0.10 0.04 – 0.02 – < 0.02 < 0.04 < 0.005 < 0.02 < 0.02 0.02 –
0.01 0.03 0.039
N.A.-Not applicable
The brief description of these nine types of streams is given below:
Aat: Very deep, entrenched torrent streams, mildly curved in plan, high relief, zone of deposition, step-pool morphology
A: Steep, entrenched step-pool streams, high transport of debris; erosional or depositional character, mildly curved in plan.
B: Moderately entrenched, moderate slope, very stable plan, longitudinal profile and stable banks, mildly curved in plan
C: Low gradient, meandering, point bar, riffle/pool topography, alluvial channel with moderate entrenchment and W/D ratio, broad valley.
D: Braided channel with longitudinal and transverse bars-eroding banks with very wide channel, abundance of sediment supply, aggradational tendency.
DA: Anatomising channels, well vegetated flood plain, stable stream banks, broad valley, low bed-load and high wash bed-load.
E: Low gradient, highly meandering, low W/D ratio, broad valley flood plain with alluvial material, high meander width ratio.
F: Entrenched meanders on low gradient, and high width/depth ratio, meanders very unstable laterally with high bank-erosion, pool-rifle morphology
G: Entrenched gullies, step-pool morphology, narrow valleys, unstable high erosion rates.
The Level II in the classification subdivides the streams in each class into a maximum of six categories, namely 1, 2, 3, 4, 5, 6 depending on the channel material i.e. (1) bed rock (2) boulders (3) cobbles (4) gravel (5) sand, and (6) silt and clay. These are written as A1, A2, A3, A4 …A6 etc. Thus A5
stream will be of A type with sandy material. It also takes into account bankfull discharge and corresponding hydraulic parameters in determining quantities such as entrenchment ratio, W/D and Manning’s n.
Aim of Level III classification is to provide description of stream condition as related to stability of stream, its potential, and function. This is based on additional inputs about hydrology, biology, ecology, and human activity. It evaluates and quantifies the channel stability, bed-stability (aggrading, degrading or stable), and bank erosion. Level IV classification is based on reach specific observations for verification of process based assessments of stream condition, potential and stability predicted from preceding analysis. The book by Rosgen contains valuable information for practicing river morphologists. Since a large number of sketches are included in the book, the text connects easily with the field conditions.
4.9 TOPOGRAPHY PRODUCED BY STREAMS
During the cycle of erosion as the streams develop they bring down a large quantity of sediment which eventually goes into the sea. While streams perform the erosional work in the upper reaches and
Fig. 4.11 Idealized fluvial system
deposition of sediment in the lower reaches various types of topography are produced. According to Schumm (1971) the fluvial system can be divided into three zones, named Zone 1, Zone 2 and Zone 3 in the downstream direction. The upper most part of the drainage basin is primarily the sediment source area (Zone 1); the water and sediment are derived here. Zone 2 is the transfer zone where for stable channel, the input is equal to output.
Zone 3 is the sediment sink or the area of deposition. Since the sediment is stored, transported and eroded in each zone, within each zone one process is predominant as mentioned above. The three zones are schematically shown in Fig. 4.11 are discussed below.