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The sinuous bedrock channel of the Tapi River, Central India:

Its form and processes

Vishwas S. KaleT

Department of Geography, University of Pune, Pune 411 007, India

Received 15 September 2003; received in revised form 3 March 2004; accepted 14 February 2005Available online 19 April 2005

Abstract

The Tapi Gorge lies in the monsoon-dominated region of the Indian subcontinent. Because of the seasonality of rainfall and

flows all the fluvial activity in the bedrock gorge is confined to the monsoon season, in general, and during a few high-

magnitude monsoon floods in particular. Field investigations along a 30-km reach of the sinuous bedrock gorge indicate that the

river displays all the morphologic properties of a meandering alluvial channel albeit with a much higher level of energy

expenditure. Considering the perimeter lithology and channel morphology two types of reaches are evident in the field: a

predominantly rocky and relatively straight reach close to the gorge-head, and a longer, sinuous reach of gravel deposition

downstream. Hydraulic modeling of a rainfall-induced dam-failure flood indicates that large-magnitude events that exceed thethreshold of bedrock resistance for a sustained length of time are capable of erosion. It appears that the overall channel and

gorge morphology is adjusted to two types of thresholds. A threshold of boulder-transport, which is associated with large floods

that are competent to entrain boulders but are incapable of bedrock erosion; and another higher threshold that is exceeded by

truly high-energy processes that generate large total energy and exceed the threshold of bedrock resistance. The later threshold

is exceeded only episodically, with fairly long periods of little or no bedrock erosion in between.

Interestingly, meso-scale erosional features such as inner channels and well-developed potholes are nearly absent or

inconspicuous within the gorge section. Whilst this could be partly attributed to the bedrock resistance, it appears that under the

present hydro-geomorphic conditions the dominant fluvial activity is not directed towards the channel bed, but towards the

banks. This is evident from the concentration of erosion on the outer banks and deposition of coarse gravel on the inner banks,

and armoring of the channel bed. The main conclusion of the study is that the bedrock channel is increasing the flow resistance

and energy losses by developing and enhancing the meandering pattern.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Monsoon floods; Bedrock gorge; Meandering pattern; Gravel deposits; Tapi River; India

1. Introduction

In the monsoonal and seasonal tropics the geo-

morphic work is strictly confined to the wet season

0169-555X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.geomorph.2005.02.016

T Tel.: +91 20 2560 1248.

E-mail addresses: [email protected],

vs _ [email protected].

Geomorphology 70 (2005) 296–310

www.elsevier.com/locate/geomorph



and in the dry season the rivers are predominantly

inactive or dormant (Kale, 2002). Even during the wet

season, the response of the stream channels to high

monsoon flows is essentially determined by the litho-logy of the channel perimeter and channel geometry.

Whilst in alluvial channels the more commonly

occurring moderate flows are effective (Wolman and

Miller, 1960; Gupta, 1995), in bedrock channels

noteworthy fluvial activity is associated only with

high-energy processes t hat occur during infrequent,

large-magnitude floods (Baker, 1977, 1988; Kale and

Hire, 2004). Because of high boundary resistance and

coarse bedload, moderate- and low-magnitude flows

are incapable of transporting the coarse bedload and/or

changing the morphologies of the non-alluvial river channels (Baker and Costa, 1987). Consequently, the

geomorphically effective flows in bedrock-dominated

channels have larger recurrence intervals (Tinkler,

1971; Kale and Hire, 2004).

Given the relatively large recurrence interval of

geomorphically effective flows in seasonally wet

areas, channels incised into bedrock develop mor-

phologies that yield the optimum energy expenditures

of flow (Baker and Kale, 1998). Similar to the alluvial

channels, an adjustment occurs in the channel

morphology, sinuosity, and gradient to energy expen-

diture at a higher level associated with large-magni-

tude f loods. Whereas amazingly high energy

expenditure is achieved along relatively straight and

constricted reaches, a sinuous or meandering channel

pattern reduces energy expenditure and evens out the

loss of power along the flow path (Baker and Kale,

1998).

This paper will consider the morphological and

hydraulic characteristics of the sinuous bedrock

channel of the Tapi River in central India. The main

objectives are to record and explain the longitudinal

variations in the morphologic and sediment charactersof the sinuous bedrock gorge, and to understand the

role of lithology and the variations in the hydraulic

conditions along the bedrock channel.

2. Geologic and geomorphic setting

The Tapi River rises over the Betul Plateau (600–

730 m asl) in central India and flows west to meet the

Arabian Sea. The river covers a total area of about

65145 km2, but the focus of the present paper is on

the approximately 100-km-long bedrock reach in the

headwaters of the Tapi River, in general, and on a 30-

km reach from the gorge-head, in particular (Fig. 1).The headwater area receives up to 100 cm of annual

rainfall. For about 35 km the river flows over a board,

undulating rocky surface, before entering the Tapi

Gorge, near Dhanora (Fig. 1). In general, the upper

gorge is narrow, deep (40–80 m) and sinuous (Figs. 2

and 3A) and various types of basalts are exposed on

the bed and banks. The lower wide gorge is

characterized by, more or less, a straight channel

and granitic gneisses and sandstones are exposed on

the bed or banks (Fig. 1). Upon exiting the gorge, the

r iver has incised into t hick, late Quaternary alluvium(Kale and Hire, 2004; Fig. 1).

The 30-km reach of the Tapi Gorge under review is

dominantly cut into horizontally bedded Cretaceous–

Eocene dDeccan TrapT basalts. Two types of struc-

tures, namely columnar–jointed and vesicular–amyg-

daloidal basalts are commonly observed. Columnar

basalt flows are observed at different levels, and

amygdaloidal basalts occur as intervening layers.

Evidence that the river encounters greater difficulty

in incising the columnar basalts is provided by the

presence of waterfalls and hanging tributaries at the

contact of the columnar and amygdaloidal basalts.

Wherever the river has managed to cut into the

resistant columnar basalts, however, it is confined to

narrow, deep, and box-shaped canyons. The river

basin occupies the Son-Narmada-Tapi (SONATA)

lineament zone, characterized by strong neotectonism

and moderate seismicity (Ravi Shankar, 1991). The

gorge morphology, therefore, appears to be strongly

related to the lithological and tectonic controls.

The log–log plot of the longitudinal profile of the

upper Tapi River shows noticeable convexity (Fig.

3B). The overall stream–gradient index of Hack (1973) is 95. From the source up to about 35 km

the gradient index value is less than 100, because the

river flows over the wide, rocky undulating Betul

Plateau, and the bedrock channel is shallow and

unincised. In the vicinity of the gorge-head, however,

the gradient index values increase significantly (Fig.

3B). The Tapi Bedrock Gorge, therefore, is charac-

terized by higher channel gradient and stream power.

Occurrence of early Holocene deposits within the

gorge (Kale et al., 2003) indicates that the gorge is

V.S. Kale / Geomorphology 70 (2005) 296–310 297



F i g .

1 .

G e o m o r p h i c m a p o f t h e u p p e r T a p i G o r g e .

V.S. Kale / Geomorphology 70 (2005) 296–310298



pre-Holocene in age. The preservation of the deposits

also suggests that probably the overall morphology of

the bedrock gorge has not changed significantly since

early Holocene times.

3. River hydrology

The Tapi River is entirely fed by monsoon rains.

Measurable flows occur only during the monsoon

season (June to October) and for a short time after the

end of the monsoon season. During the long dry

summer season, the channel is virtually dry. Only large

pools contain water. Consequently, all of the fluvial

activity takes place during the 4–5 months of the

monsoon season. During the remaining part of the year

the channel is totally inactive. Because the channel

boundaries are resistant and the channel bed material is

coarse, most of the common monsoon flows are incom-

petent to cause erosion or to move the coarse gravel.

Long continuous gauge data were not available for

any site within the study area. To get some idea about

the intra-seasonal variations in the monsoon dis-charges, daily discharge data for a sample year

(1988–1989) for the Dedtalai gauging station, located

ca. 190 km from the source, are presented in Fig. 4.

This figure shows that the monsoon flow (June to

October) is occasionally interrupted by large-magni-

tude events. These events occur in response to large

amounts of monsoon rainfall in the source region

(Kale et al., 1994). During such events, most of the

geomorphic activity takes place within the gorge. The

discharge pattern suggests that the bedrock gorge is

0 2 km

Goinda

Mendha

Fig. 2. Indian Remote Sensing satellite image (FCC) of the study area (acquired in November, 1996). Data Source: National Remote Sensing

Agency (NRSA), Hyderabad, India.

Fig. 3. (A) Cross-section across the Betual Plateau and the Tapi

Gorge. (B) Longitudinal profile of the upper Tapi River showing

values of Hack’s (1973) stream–gradient index.




subjected to extreme variations in discharge and

energy conditions even during the wet season (Kale

and Hire, 2004).

4. Methodology

The 30-km reach described in this paper forms the

upper part of the Tapi Gorge. The present study is

entirely based on field investigations in the upper

narrow section of the gorge. Observations regarding

channel morphology, erosional and depositional fea-

tures, bed and bank material, and lithology were made

in the field during the non-monsoon season and the

information was recorded on a map. The width and

depth of channel flow were measured at 24 cross-

sections along the 30-km reach of the gorge (Table 1).

The flow depths were estimated on the basis of high

water marks and information provided by the local people.

In July 1991, a rainfall-induced failure of a dam,

located upstream of the Tapi Gorge-head, generated a

flood far larger than possible from extreme amounts of

monsoon rainfall (Kale et al., 1994). This event was

unprecedented in the recent geomorphic history of the

Tapi River. Because the event provided an excellent

opportunity to understand the impact of a rare, high-

magnitude event on bedrock channels, post-flood

studies were undertaken downstream of the dam, and

high water marks and information regarding changes

in the channel morphology were recorded at 33 loca-

tions downstream of the dam (Kale and Gadgil, 1997).

Because the gorge section is ungauged, multiple

cross-section surveys were undertaken at two sites

with the help of an Electronic Distance Measurer

(EDM), and the flood hydraulics were reconstructed

by employing the step-backwater flow model—HEC2

(Hydrologic Engineering Centre, 1982) and the high

water marks (Kale and Gadgil, 1997). The hydraulic

step-backwater routine was used to estimate the

discharge of the common monsoon high flows, one

of the largest monsoon floods that occurred in 1994

and the July 1991 dam-failure flood. The common

high monsoon flow was assumed to be close to ca.

2000 m3 sÀ1 on the basis of information provided by

the local people.

5. Bedrock channel morphology

Fig. 5 shows the geomorphic map of the 30-km

reach under review. The data regarding channel width

and depth, channel characteristics and the average

intermediate diameter of boulders collected at 24

cross-sections are presented in Table 1.

Approximately 13 km downstream of the gorge-

head (knick point) the river flows through a narrow

bedrock gorge with steep to near-vertical walls. The

Fig. 4. Variation in the daily discharge for a sample year (1988–1989) for the Dedtalai gauging station, located ca. 190 km from the source.

Catchment area=3860 km2. Data source: Water Year Book, Central Water Commission, New Delhi.




channel width generally varies between 70 and 120 m

(Table 1). Downstream the channel widens and the

width varies between 130 and 275 m (Table 1). Fig. 5

also reveals that steep or precipitous gorge walls are

usually present along the outer banks of the sinuous

channel of the Tapi River.According to the lithology of channel perimeter and

channel morphology two main types of channel cate-

gories—(i) predominantly rocky reach, and (ii) reach of

gravel deposition, could be identified in the field. The

representative cross-sections are shown in Fig. 6.

5.1. Predominantly rocky reach (Reach-1)

This reach occurs close to the gorge-head (cross

sections 1 to 4; Fig. 5) and is characterized by a

relatively steep gradient, narrow channel, and low

sinuosity (Fig. 7). Erosional forms and features

dominate this reach. The gradient is high, about

0.0072. Two major breaks in longitudinal profile,

represented by waterfalls with plunge pools (at cross-

sections 1 and 3), are associated with this reach (Fig.7). The reach contains two large boulder berms

(Table 2). These berms are present downstream of

the waterfalls. The channel is box-shaped in appear-

ance (Fig. 7) with a flat rocky floor and steep rocky

banks (Fig. 6A). Concentrations of coarse gravel also

occur at the inside of the bends, but the extent of

these deposits is limited. Slackwater deposits (SWD)

of varying thickness are sometimes present at

tributary junctions. The channel reach between

cross-sections 2 and 3 (Fig. 7B) is completely

Table 1

Cross-sectional parameters along the upper Tapi River

Cross-section

no.a

Distance from the

gorge-head (km)

Channel floor

width (m)

Flow

depth (m) b

Max. grain

size (cm)

Dominant channel

bed characteristics

Geomorphic features

1 0 144 5.3 58 Bedrock Waterfall, plunge pool

2 0.3 153 7.5 46 Bedrock Boulder berm

3 2.1 79 7.5 88 Bedrock Waterfall, boulder berm

4 4.5 71 4.8 103 Coarse gravel Narrow gorge

5 5.1 120 6 50 Coarse gravel Narrow gorge


7 8.3 110 4.5 138 Coarse gravel Narrow gorge,

narrow floodplain


9 9.5 100 4.5 110 Bedrock Narrow gorge


11 10.6 112 6.3 69 Bedrock Narrow gorge

12 11.1 112 7.0 74 Bedrock Wide gorge

13 12.6 120 7.4 20 Coarse gravel Wide gorge, narrow

flood plain on the right

bank and gravel deposits

on the left bank


15 16.4 181 5.3 72 Coarse gravel Right bank cut in

gravel bar

16 16.9 182 6.2 92 Coarse gravel Wide gorge






22 29.7 131 9.0 79 Coarse gravel Wide gorge23 30.3 172 7.5 57 Coarse gravel Wide gorge


a See Fig. 5 for location of the cross-sections. b On the basis of high level marks.




devoid of any kind of sediments. The almost straight

reach and box-shaped appearance of the channel

(Fig. 6B) suggest that the channel geometry does not

provide any location favouring a decrease in the

stream power per unit area.Although the channel has been carved out by

fluvial erosion, very little evidence exists of intense

bedrock erosion along this study reach. Well-devel-

oped inner channels are conspicuously absent and

the frequency of potholes is remarkably low (Fig.

7B). Apart from the hydraulic and energy conditions,

this could be attributed to the bedrock structure.

The two waterfalls (knick points) are developed

over the resistant columnar basalts. Elsewhere,

whenever columnar basalts form the channel bed,

potholes are poorly developed and inner channels

are absent or inconspicuous (Fig. 7B). Columnar

basalt flows are encountered close the gorge-head.

Therefore, the main gorge and the higher order

tributaries cutting through the columnar basalts arenarrow and deep (Fig. 5). Elsewhere hanging tribu-

taries are common.

In comparison, vesicular–amygdaloidal basalt

flows are relatively more susceptible to weathering

and erosion. Therefore, reaches developed in amyg-

daloidal basalts have hummocky bedrock surfaces and

the channels are relatively wide and shallow.

Although small-scale erosional features, such as flute

marks and grooves, are present meso-scale erosional

features, such as potholes and inner channels, are

Fig. 5. Geomorphic map of the study reach showing the major morphologic features discussed in the text. 1=bedrock reaches; 2=gravel

deposition reaches; 3=steep gorge walls; 4=waterfalls; 5=deeply incised tributaries; 6=unincised tributaries; 7=large point bars; 8=main

settlements; 9=cross-section numbers. The inset map shows the channel gradient estimated for different sections on the basis of 1:50000

topomap.




poorly developed. At the gorge-head (cross section 1;

Fig. 5) the resistant columnar basalt flow is underlain

by relatively less-resistant vesicular–amygdaloidal

basalt flow. As a result of differing ero sional

resistance, it appears that the knick point (waterfall)

is receding upstream primarily by erosion and under-

cutting of the less-resistant amygdaloidal basalts.

Columnar basalt flows that occur along the gorge

walls yield large blocks. Consequently, the two large

boulder berms located downstream of the waterfalls

consist of large boulders (Table 2) supplied from the

gorge walls.

5.2. Reach of gravel deposition (Reach-2)

This reach has low channel gradient and thesinuosity is noticeably high (Fig. 2). The gradient

varies between 0.0011 and 0.0026 (Fig. 5). The

gradient decreases with an increase in channel

sinuosity and meander size. Although the gorge is

basically developed in bedrock, coarse gravel covers

the channel bed and occurs in the form of bars, riffles,

and point bars. Bedrock is exposed in patches on the

channel floor, particularly in the vicinity of large

pools or on the concave banks. Sometimes uncon-

solidated cobbly–bouldery gravel deposits form the

bank material (Fig. 6B). These situations represent

incision in gravel deposits.

Fig. 5 illustrates that downstream of the cross-

section 6 the meander bends are generally wide andopen. Only between cross-sections 14 and 17 is the

meander bend relatively tight. The variation in the

channel curvature has influenced the flow and sedi-

ment dynamics. The satellite image, Fig. 2, shows the

path of post-monsoon flow along the sinuous channel

of the Tapi River. The low monsoon f low is

consistently closer to the outer bank but mid-way

(inflection point) between the axes of meander bend.

This represents changes in the bed topography across

the river. The channel is wider and asymmetrical at the

meander bend axis but relatively narrow and sym-metrical between the bends. Scour pools are located

along the outer banks and broad, shallow point bars

wrap around the inner bends. During floods, the flows

are deeper along the concave banks and shallower

along the convex banks. This in turn indicates that the

zone of high velocity and bed shear stress shifts

towards the outer bank along the bend and leads to

higher erosion. Throughout the reach the point bar

deposits are dominated by coarse gravel (cobbles and

boulders). Riffles occur along reaches between the

scour pools. All these features are characteristics of

meandering alluvial channels. This, therefore, sug-

gests that, despite the differences in the channel

boundary resistance and level of energy expenditure,

the meandering alluvial and bedrock channels behave

in a similar manner.

Wherever the gorge walls have receded signifi-

cantly away from the channel banks, low and narrow

floodplains have developed. Such a situation is best

developed at cross-section 13 (Fig. 5). The flood-

waters of the 1994 flood (one of the largest in recent

years) covered the floodplain and deposited pebbles

and cobbles besides fine sand and silt. Betweencross-sections 20 and 24, the meanders are larger, the

gradient is lower, the channel is wider and shallower

(Table 1) and the width of bars is significantly

higher. These are, therefore, reaches of gravel

deposition because of significant loss of energy and

flood power.

Slackwater deposits occur at several places along

the gorge. The deposits are about 1–3 m thick and are

mostly present on the inner banks and at tributary

junctions. Radiocarbon dates indicate that these

Fig. 6. Representative channel cross-sections across—(A) rocky

reach (Reach-1) near Dhanora and (B) gravel depositional reach

(Reach-2) near Ghuttigarh. Based on EDM survey.




deposits were emplaced about 150–400 years ago

(Kale et al., 2003).

6. Flood hydraulics and dynamics

Large monsoon flows through the rocky and

gravel bed reaches were modeled by generating

water–surface profiles for different discharges by

using the step-backwater model (HEC-2; O’Connor

and Webb, 1988). The generated water–surfaces

profiles were compared with various high stage

indicators to infer the discharges. Two sub-reaches

were modeled. Fig. 8 shows the water–surface

profiles generated for a 650-m-long rocky sub-reach

at the gorge-head (cross-sections 1–2; Fig. 5) and

Fig. 9 illustrates the simulated water–surface profiles

for a 800-m sub-reach dominated by coarse gravel at

Fig. 7. (A) A view of the Tapi Gorge cut into the Betual Plateau. The gorge-head is seen in the center, and a gravel bar is present in the lower

left-hand corner of the photograph. (B) The box-shaped, rocky channel of the Tapi River. A major break in the bed profile, represented by a

waterfall (dry) is visible in the lower left-hand corner. The flat channel bed is developed in a resistant columnar basalt flow. Another columnar

basalt flow occurs at the top, above the scour line (SL). Note the absence of meso-scale erosional features or sediments on the rocky channel

bed. The scour line (SL) is developed in weathered amygdaloidal basalt.




Ghuttigarh (cross-sections 14–16; Fig. 5; Kale andGadgil, 1997).

The scour line carved by the July 1991 dam-failure

flood (Fig. 7B) corresponds with discharges ranging

between 9000 and 9500 m3 sÀ1, and the top of the

large boulder berm is approximately associated with

discharge close to 2000 m3 sÀ1 (Fig. 8).

Similar reconstruction for the Ghuttigarh reach

shows that the 1991 catastrophic discharge was close

to 10000 m3 sÀ1. The 1994-flood indicators correlate

with about 6000 m3 sÀ1. The slackwater flood

deposits, occurring at lower level, correspond with

discharges between 1000 and 2000 m3 sÀ1 (Fig. 9).

Apart from the water–surface profiles, the HEC-2

step-backwater program was used to derive estimatesof flow width, depth, mean velocity, energy slope,

boundary shear stress, and stream power per unit

boundary area. The longitudinal variations in these

values were studied to understand the distribution of

erosion and deposition dominated areas of the channel.

One important hydraulic feature of the channel that

is apparent is the increase in the hydraulic efficiency

with discharge. Because of narrow and incised nature

of the channel, flows get deeper and faster as the

discharge increases. The increase in discharge and

stage is associated with a decline in the width/depthratio. Consequently, the velocity and the energy per

unit area increases (Kale and Hire, 2004). Large

flows, therefore, are more effective in terms of

sediment transport and channel erosion. This seems

particularly applicable to the rocky reach close to

gor ge-head (Reach-1).

Fig. 8 gives the longitudinal variation in the unit

stream power for the flood from the dam-failure. The

unit stream power values for 2000 m3 sÀ1 were also

computed because this is assumed to be the relatively

Fig. 8. Generated water-surface profiles for different discharges for the bedrock reach near the gorge-head (Reach-1). Inverted triangles

represent the scour line carved by the July 1991 dam-failure flood. Unit stream power at 9500 m2 sÀ1 is shown in the upper part of the figure.

After Kale and Gadgil (1997).

Table 2

Dimension and grain size of boulder berms

Parameters (m) Boulder berm at

cross-section 2

Boulder berm at

cross-section 3Distance from the waterfall 260 70

Length of the berm 258 96

Width of the berm 67 8

Height of the berm 8 3

Mean long axisa 1.19 1.78

Mean intermediate axisa 0.88 1.27

Mean short axisa 0.48 0.83

Location of the berm Confined to

right bank

Linear and close

to mid-channel

a Based on 10 largest boulders.




frequent high monsoon flow. At the Dhanora sub-

reach (Fig. 8) the high monsoon flows have unit

stream power values in the range of 98 and 1700 W

mÀ2 and along the Ghuttigarh sub-reach (Fig. 9) the

values vary between 400 and 1000 W mÀ2. The

longitudinal distribution of the unit stream power

values indicate that the boulder berm (Fig. 8) and

gravel-bedded sections (Fig. 9) are associated with a

decrease in stream power per unit boundary area. In

comparison the rock-floored reaches are characterized

by relatively higher values of unit stream power. This

implies that gravel deposition occurs at all the

locations where a decrease occurs in the stream power

per unit area of the large flows (Wohl, 1992, 1993).If this inference is applied to the entire study reach,

it would suggest that close to the gorge-head (Reach-

1) the channel geometry and hydraulic conditions do

not favour large-scale deposition of gravel and large-

magnitude events are capable of boulder transport.

Downstream (Reach-2) as the sinuosity and channel

size increases and the gradient decreases, however, a

decline occurs in the boundary shear stress and unit

stream power, and widespread gravel deposition is

induced.

6.1. Hydraulic characteristics of the July 1991 flood

from dam-failure

The Chandora Dam is located about 16 km

downstream from the source of the Tapi River. The

catchment area up to the dam site is about 71 km2.

Intense precipitation from low-pressure systems dur-

ing a few days was the major cause of high-magnitude

floods in the area (Kale et al., 1994). Such was the

case in the last week of July 1991. As much as 40% of

the annual total rainfall was received in 1 day. The

very intense rainfall was followed by breaching of the

Chandora Dam (gross storage capacity 18.2Â106 m3)

and the reservoir was drained in less than an hour (according to eyewitness accounts).

In the absence of systematic gauge data or even

estimates of flood discharges, water–surface profiles

were calculated for a series of discharges ranging from

1000 to 15000 m3 sÀ1 by using the HEC-2 step-

backwater program (Hydrologic Engineering Centre,

1982; Kale and Gadgil, 1997). The most reliable

evidence of maximum flood-stage along the canyon is

a scour line carved in weathered amygdaloidal basalt

at Dhanora (Figs. 6A and 8). The hydraulic calcu-

Fig. 9. Water-surface profiles along the gravel dominated reach near Ghuttigarh (Reach-2). Shaded area at the bottom indicates unit steam power

at 10000 m2 sÀ1. After Kale and Gadgil (1997).




lations demonstrate that flood discharges between

9000 and 9500 m3 sÀ1 are required to match the high

stage evidence in the form of a scour line near the

gorge-head (Figs. 7A and 8). Sensitivity analysisusing different Manning’s n-values reveals that the

scour line is invariably associated with discharges

between 9000 and 10 000 m3 sÀ1 (Kale and Gadgil,

1997). Hydraulic modeling further indicates that

during peak flooding, energy slopes ranged between

0.0014 and 0.0057, mean channel velocities varied

between 8 and 11 m sÀ1, and calculated flow depths

ranged from 14 to 16 m. During the flood, Froude

numbers were in the subcritical domain, ranging

between 0.50 and 0.86 (Kale and Gadgil, 1997).

Similarly, results of the hydraulic modeling indi-cate that a peak discharge of ca. 10000 m3 sÀ1 is

consist ent with the high water mark at Ghuttigarh

(Fig. 8). The mean velocities in the channel during the

flood peak ranged from 4.4 to 6.6 m sÀ1 and the

Froude numbers varied between 0.36 and 0.55.

Because of the relatively higher form ratio and lower

channel gradient along this sub-reach, the values of

mean velocity and Froude number are lower (Kale and

Gadgil, 1997).

The results indicate that the July 1991 flood was an

exceptional geomorphic and hydrologic event. The

potential of such a large flood flow to modify the

landscape can be evaluated in terms of unit stream

power and boundary shear stress, rather than dis-

charge alone (Baker and Costa, 1987). Calculations

yielded a maximum stream power per unit area of

1300–6300 W mÀ2 for the peak discharge and in

some deep bedrock pools the values would have been

much higher. Such high power per unit area is

sufficient to accomplish a variety of unusual geo-

morphic and hydraulic phenomena (Baker and Costa,

1987; Wohl et al., 1994). Comparison of the threshold

values of coarse sediment transport estimated by usingthe equations developed by Williams (1983) and the

values generated by the step-backwater routine

indicates that the peak flow was capable of boulder

transport. Interestingly, the record-breaking event did

not produce spectacular, dramatic, and widespread

geomorphic impacts (Kale and Gadgil, 1997). About

60 km downstream of the dam, the channel experi-

enced little effect from the flood.

Morphological characteristics of the pre-flood

channel are not known. The present account, there-

fore, is based on the information provided by the local

inhabitants of our study area. A checklist of the effects

of the flood was prepared and investigations were

carried out at 33 cross-sections along t he Tapi River between the dam site and Teska (Fig. 1). The

summary of the results is presented in Table 3.

Limited movement and redistribution of channel

boulders and localized channel scour and scabland

development (Table 3) implies that the peak flood

velocity and depth did not exceed the threshold

conditions necessary to cause significant channel

modifications. Although evidence exists of damage

to human structures (temples/bridges) along the

channel, eyewitness accounts reveal that the demoli-

tion mainly resulted from hammering by large treesuprooted and washed by the flood. The survey,

therefore, suggests that the flood from dam-failure

caused less net channel change.

Significant channel modifications occur only when

the critical competence values are exceeded for a

sustained length of time, such that sediment is

entrained and the bedrock is eroded (Costa and

O’Connor, 1995; Wohl, 1998). Systematic records or

observations of the duration of the flood in the gorge

section are not available to estimate the total energy

expended over a flood hydrograph, as suggested by

Costa and O’Connor (1995). Local inhabitants, when

questioned, invariably mentioned that the flood peak

passed in less than half-an-hour. It, therefore, appears

Table 3

Summary of geomorphic impacts of the July 1991 flood from a

dam-failure based on observations at 33 locations (after Kale and

Gadgil, 1997)

Geomorphic effects Extent/magnitude/remarks

Bank/gorge wall erosion Local, not significant

Erosion of SWD/floodplain Relatively common, but limitedChannel widening Minor, insignificant

Channel bed erosion Insignificant to absent

Channel deposition Moderate to high in some areas

Scabland formation Local, reported from 1 to 2 areas;

scabland formed by removal of

top weathered layer

Erratic boulders Local

Overbank gravel deposition Local, confined to wide gorge

Sand deposition Widespread

Bar reorganization Relatively widespread

Link channels across bends Reported from 1 site

Filling of deep pools Observed at many places




that the peak flood was characterized by short

duration and low total energy expenditure. This in

turn implies that the pre-flood bedrock channel

morphology of the Tapi River is largely adjusted inthree dimensions to geomorphically effective flows

that have a much longer return period. Evidently, the

July 1991 flood was not a threshold-passing flood for

bedrock erosion.

7. Discussion and conclusion

All the geomorphic activity is confined to the wet

season because of the seasonality of the flows in

monsoon areas. Along the Tapi Gorge, monsoonflows that exceed the threshold of boulder transport

and bedrock erosion are geomorphically effective.

Reconstruction of flood hydraulics implies that bed-

rock erosion in the resistant columnar basalts is

associated with rare, high-magnitude floods that

generate large values of total energy and exceed the

threshold of bedrock erosion. Even a flood from a

dam-failure that occurred in July 1991 could not

generate such conditions because the total energy was

low. It thus appears that the overall channel morphol-

ogy of the sinuous bedrock channel of the Tapi River

is adjusted to two types of thresholds: a threshold of

boulder-transport, which is associated with high-

magnitude floods that are competent to entrain

boulders but are incapable of bedrock erosion; and

another higher threshold of bedrock erosion, which is

exceeded by truly high-energy processes that generate

large total energy and exceed the threshold of bedrock

resistance. While the former threshold is exceeded

from time to time, the later threshold is exceeded only

episodically, with fairly long periods of no bedrock

erosion in between (Kale and Hire, 2004).

The incised bedrock channel of the Tapi River issinuous in planform. The channel reveals all the

characteristics of alluvial meandering channels. The

outer and the inner bends are characterized, respec-

tively, by scour pools and point bar deposits. The

location of bars, pools, and riffles is controlled by

the hydraulics of the flows. Most coarse gravel bars

are developed at the inside of bends because the

power per unit area declines in these locations

(Kelsey, 1988; Wohl, 1992; Baker and Kale, 1998).

Unlike the alluvial meandering channels, however,

the channel morphology appears to be strongly

related to infrequent large-magnitude floods because

only large floods are likely to be effective in terms

of erosion of the outer banks and transportation of coarse gravel.

An interesting aspect that has emerged during the

course of the present study is that within the bedrock

gorge the present channel is incised into coarse

gravel at some places. For instances, a large cobbly–

bouldery bar has been cut at Ghuttigarh (Fig. 6B),

indicating incision of rock to the present channel bed

level, followed by infilling of N10 m of coarse

gravel, and subsequent removal. Because stream

power is inversely related with channel width (Baker

and Costa, 1987), any increase in width associatedwith lateral erosion (particularly along the concave

banks) decreases the specific energy. The river is

compelled to incise into its own deposits and decrease

the width/depth ratio to maintain its hydraulic

efficiency and geomorphic effectiveness at a level

required to sustain water and sediment flux (Deodhar

and Kale, 1999).

The bedrock reach close to gorge-head (Reach-1),

with waterfalls (knick points), plunge pools, and large

boulder berms, falls in the zone of greatest concen-

tration of energy dissipation (Wohl, 1998). The

absence of inner channels and well-developed pot-

holes in this reach, however, is intriguing. The

channel is developed in horizontally layered bedrock

with differing erosional resistance. Sometimes it flows

over more resistant columnar basalts and sometimes

over less-resistant vesicular–amygdaloidal basalts.

According to some workers, under such situations

incising streams exhibit waterfalls or steep rapids at

the point of greatest incision, rather than inner

channels (Wohl, 1993). This explanation seems

unlikely in the present case, however, because a

well-developed inner channel in columnar basaltsexists several kilometers upstream of the gorge-head.

The meandering channel planform, the erosion

along the outer bends, deposition along the inner

bends and armoring of the channel bed by coarse

gravel imply that vertical incision is not the primary

activity under present hydraulic and gradient con-

ditions. Experimental studies by Shepherd (1972)

have demonstrated that sediment deposition at the

inside of the bends and erosion along the outer banks

is associated with a decrease in channel gradient




following intense vertical erosion. It appears that the

present gradient conditions do not favour channel

incision and downcutting. Preservation of early

Holocene flood deposits and thick gravel deposits at some places in Reach-2 supports this inference

because aggradation is unlikely if vertical erosion

and incision is the dominant process.

The tendency to meander and increase the sinu-

osity is a fundamental characteristic of some rivers

that are attempting to increase the flow resistance and

energy loss of high-magnitude events. The develop-

ment of a meandering pattern is one way to reduce the

energy expenditure of extreme discharges that are

competent to erode the channel bed and entrain coarse

gravel (Baker and Kale, 1998).The Tapi River provides an interesting example of

channel response in a high gradient, bedrock system

that is controlled by layered bedrock with differing

erosional resistance and is episodically subjected to

extreme discharges with large expenditure of total

energy.

Acknowledgement

The results presented in this paper are largely

based on studies carried out in connection with aresearch project undertaken by the author and

supported by the Indian Department of Science and

Technology, New Delhi. The author acknowledges

the support received from Pramod Hire and V.R.

Nagarale in the field. The author also thanks Paul

Carling, David Bridgland, and John Vitek for their

constructive and helpful comments.

References

Baker, V.R., 1977. Stream channel response to floods with examples

from central Texas. Geological Society of American Bulletin 88,

1057–1070.

Baker, V.R., 1988. Flood erosion. In: Baker, V.R., Kochel, R.C.,

Patton, P.C. (Eds.), Flood Geomorphology. Wiley, New York,

pp. 81 – 95.

Baker, V.R., Costa, J.E., 1987. Flood power. In: Mayer, L.,

Nash, D. (Eds.), Catastrophic Flooding. Allen and Unwin,

London, pp. 1–21.

Baker, V.R., Kale, V.S., 1998. The role of extreme floods in

shaping bedrock channels. In: Tinkler, K.J., Wohl, E. (Eds.),

Rivers Over Rock: Fluvial Processes in Bedrock Channels,

Monograph vol. 107. American Geophysical Union, Washing-

ton, DC, pp. 153–165.

Costa, J.E., O’Connor, J.E., 1995. Geomorphically effective floods.

In: Costa, J.E., Miller, A.J., Potter, K.W., Wilcock, P. (Eds.),

Natural and Anthropogenic Influences in Fluvial Geomorpho-

logy, Monograph vol. 89. American Geophysical Union,

Washington, DC, pp. 45–56.

Gupta, A., 1995. Magnitude, frequency, and special factors affecting

channel form and processes in the seasonal tropics. In: Costa,

J.E., Miller, A.J., Potter, K.W., Wilcock, P. (Eds.), Natural and

Anthropogenic Influences in Fluvial Geomorphology, Mono-

graph vol. 89. American Geophysical Union, Washington, DC,

pp. 125 – 136.

Deodhar, L.A., Kale, V.S., 1999. Downstream adjustments in

allochthonous rivers: Western Deccan Trap upland region,

India. In: Miller, A.J., Gupta, A. (Eds.), Varieties of Fluvial

Form. Wiley, Chichester, pp. 292 – 315.

Hack, J.T., 1973. Stream-profile analysis and stream-gradient index.U.S. Geological Survey Journal of Research 1, 421–429.

Hydrologic Engineering Centre, 1982. HEC-2 Water surface

profiles user’s manual, U.S. Army Corps of Engineers, Davis.

Kale, V.S., 2002. Fluvial geomorphology of Indian rivers: an

overview. Progress in Physical Geography 26, 400–433.

Kale, V.S., Gadgil, A.S., 1997. Evaluation of the flood hydrology in

the upper Tapi basin. Using Palaeoflood and historical records.

Project report submitted to the Department of Science and

Technology, New Delhi.

Kale, V.S., Hire, P.S., 2004. Effectiveness of large monsoon floods

on the Tapi River, India: role of channel geometry and

hydrologic regime. Geomorphology 57, 275–291.

Kale, V.S., Ely, L.L., Enzel, Y., Baker, V.R., 1994. Geomorphic and

hydrologic aspects of monsoon floods on the Narmada and TapiRivers in central India. Geomorphology 10, 157–168.

Kale, V.S., Mishra, S., Baker, V.R., 2003. Sedimentary records of

palaeofloods in the bedrock gorges of the Tapi and Narmada

rivers, central India. Current Science 84, 1072– 1079.

Kelsey, H.M., 1988. Formation of inner gorges. Catena 15,

433–458.

O’Connor, J.E., Webb, R.H., 1988. Hydraulic modeling for palae-

oflood analysis. In: Baker, V.R., Kochel, R.C., Patton, P.C.

(Eds.), Flood Geomorphology. Wiley, New York, pp. 393–402.

Ravi Shankar, 1991. Thermal and crustal structures of SONATA: a

zone of mid continental rifting in Indian shield. Journal,

Geological Society of India 37, 211–220.

Shepherd, R.G., 1972. Incised river meanders; evolution in

simulated bedrock. Science 178, 409–411.

Tinkler, K.J., 1971. Active valley meanders in south-central Texas

and their wider significance. Geological Society of America

Bulletin 82, 1783–1800.

Williams, G.P., 1983. Paleohydrological methods and some

examples from Swedish fluvial environments: I. Cobble and

boulder deposits. Geografiska Annaler 65A, 227 – 243.

Wohl, E.E., 1992. Bedrock benches and boulder bars: floods in the

Burdekin gorge of Australia. Geological Society of American

Bulletin 104, 770–778.

Wohl, E.E., 1993. Bedrock channel incision along Piccaninny

Creek, Australia. Journal of Geology 101, 749–761.




Wohl, E.E., 1998. Bedrock channel morphology in relation to

erosional processes. In: Tinkler, K.J., Wohl, E. (Eds.), Rivers

Over Rock: Fluvial Processes in Bedrock Channels, Monograph

vol. 107. American Geophysical Union, Washington, DC, pp.

133–151.

Wohl, E.E., Greenbaum, N., Schick, A.P., Baker, V.R., 1994.

Controls on bedrock channel incision along Nahal Paran, Israel.

Earth Surface Processes and Landforms 19, 1–13.

Wolman, M.G., Miller, J.P., 1960. Magnitude and frequency of

forces in geomorphic process. Journal of Geology 68, 54–74.


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