road construction impacts on stream suspended sediment loads in a nested catchment system in nepal

land degradation & development

Land Degrad. Develop. 17: 343–351 (2006)

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/ldr.717

ROAD CONSTRUCTION IMPACTS ON STREAM SUSPENDEDSEDIMENT LOADS IN A NESTED CATCHMENT SYSTEM IN NEPAL

J. MERZ,1* P. M. DANGOL,2 M. P. DHAKAL,2 B. S. DONGOL,2 G. NAKARMI2

AND R. WEINGARTNER1

1Hydrology Group, University of Bern, Switzerland2People and Resource Dynamics in Mountain Watersheds of the Hindu Kush-Himalayas (PARDYP)/International Centre for Integrated

Mountain Development (ICIMOD), Kathmandu, Nepal

Received 18 February 2005; Revised 4 April 2005; Accepted 16 September 2005

ABSTRACT

In terms of erosion and elevated suspended sediment concentrations, road construction has a major impact on the environment,which is described in this paper. In the Andheri Khola catchment, Nepal, the sediment regime of a stream at different locationswithin the catchment suddenly changed between 1999 and 2000. The only explanation for this change was the construction ofthe Bardibas-Dhulikhel highway, which was initiated in this area in January 2000 and completed in March 2000. The changes insuspended sediment concentration could be shown both visually as well as statistically at three different locations in thecatchment with a catchment without any impact of the road as a control. Other possible reasons for the change could beexcluded by using the available data from the catchment. The impact of the road was estimated to range from 300 to500 per cent in terms of change in sediment yield per annum. Copyright # 2006 John Wiley & Sons, Ltd.

key words: road construction; sediment regime; middle mountains; Nepal

INTRODUCTION

The focus of watershed-management projects in Nepal, which aim to reduce sediment outputs from rural

catchments, has mostly been on agricultural land-use systems. While irrigated land is generally accepted to act as

a sediment sink (Carver, 1995), the steep, often marginal and rain-fed, agricultural land is believed to be one of the

main sediment sources with highest delivery mainly during the pre-monsoon season (Carver and Schreier, 1995).

In terms of soil-fertility loss in these areas, erosion poses a risk for farmers (Brown et al., 1999). However, in terms

of sediment output, degraded areas (i.e. barren areas, landslide scars, gullies and badlands, mainly located on red

soils in the catchment) can produce larger amounts of sediment (Carver and Nakarmi, 1995; Carver, 1995;

Nakarmi et al., 2000). In addition to this, rural roads and highways are proven sediment sources, often causing

slope instabilities and concentration of runoff, which can be hazardous if not properly managed (Deoja, 1994).

Careless maintenance of a mountain road can result in erosion rates of 100 t ha�1 y�1 according to the same author.

Tschanz et al. (1999) document a large landslide in a steep and forested area of the Yarsha Khola catchment in

the Dolakha district, Nepal, which was entirely caused by inappropriate road construction. A good overview of the

impact of roads, and rural roads in particular, in Northern Thailand is given in Ziegler and Giambelluca (1997).

In general they identify roads as having a major impact on the sediment regime and flood generation of a

Copyright # 2006 John Wiley & Sons, Ltd.

�Correspondence to: J. Merz, Hydrology and Water Resources, University of Bern, PARDYP, GPO Box 8975, EPC 2736, Kathmandu, NepalE-mail: [email protected]

Contract/grant sponsor: Swiss Agency for Development and Cooperation (SDC).Contract/grant sponsor: International Development Research Centre (IDRC).

catchment. Thus, in the context of the expanding road network in the region they need to be addressed with greater

emphasis. Merz and Mosley (1998) suggest that rural roads have a major impact on runoff as well as sediment

movement in a micro-scale catchment in New Zealand by intercepting subsurface flow and concentrating flow.

Froehlich and Walling (1997) conclude that unmetalled roads in a Polish catchment are a dominant sediment

source and contribute the majority of the suspended sediment to the local stream. They also show that on

unmetalled roads flow is initiated earlier than on other land surfaces. Gucinski et al. (2000) review the scientific

information on forest roads in the United States and conclude that the ‘geomorphic effects of roads range from

chronic and long-term contributions of fine sediment into streams to catastrophic mass failures of road cuts and

fills during large storms’. The hydrological effects are threefold according to the same authors: (1) roads intercept

rainfall directly; (2) they concentrate flow; and (3) they divert or reroute flow. The impact of roads on peak flows

are scale dependent. While in small catchments roads can effectively dewater the catchment and thereby increase

the peak flow considerably, in large catchments roads comprise only a small area of the entire catchment. However,

overall, the impacts of roads are road- (and site-) specific due to different geological, topographic, climatic,

vegetation and other conditions. Ives and Messerli (1989) provide an overview of the impact of roads on sediment

production in the Indian Himalayas with a particular focus on landslides.

A particularly vulnerable time is during construction. Road construction is a massive interference with the

environment (Schaffner, 1987). The main impact is felt by the removal of vegetation, mass wasting along the cut

slopes, and runoff over bare areas formed by excavation and by deposition of spoil material (Schuster and Huebl,

1995). This vulnerability only ends once the bare surfaces have overgrown and the, often loose, spoil material has

settled and is kept in place by vegetation. As the construction time is very short, and the impacts are often not

foreseen by the project’s hydrological services or research projects, quantitative impact studies of this effect are

limited. Sharma (1993) reports a total of 40 000 to 80 000m3 of debris generated during the construction of 1 km of

road in the mountains of Nepal. Deoja (1994) mentions 8000 t ha�1 y�1 of soil loss as a result of careless

construction. However, how much of this debris is washed out of the area is difficult to assess. Ives and Messerli

(1989) argue that there is no accurate assessment of how much sediment from road construction actually reaches

the stream.

Data from the People and Resource Dynamics in Mountain Watersheds of the Hindu Kush–Himalayas

(PARDYP) project’s site in the Jhikhu Khola catchment, Nepal, allows us to draw some quantitative and

qualitative conclusions on the direct impact of road construction in mountainous terrain of Nepal on the sediment

regime of the drainage network of this catchment. This paper aims to present the facts that show the impact as well

as the potential reasons behind such an impact.

BACKGROUND

His Majesty’s Government of Nepal supported by the Government of Japan is building a road from Dhulikel via

Bokundebesi-Nepalthok to Bardibas in Central Nepal. The total length of the road will be 158 km. About 9 km of

the road is within the Jhikhu Khola catchment, the PARDYP study catchment in Nepal, and is located at its

southern boundary. Excavation and construction work in this area was done in 1999 and 2000 mainly by means of

excavators and jack hammers (B. S. Rana, pers. comm., 2003) to rehabilitate and widen the existing alignment of a

village road. Only limited use of explosives was required. The excavated material was brought to deposition sites,

and not (as often happens in other cases) deposited downslope of the road. Significant efforts were put into proper

stabilization of the road slopes using both traditional engineering and bio-engineering methods.

One stretch of the new road lies within the catchment area of the Upper Andheri Khola (about 1�2 km) and the

Kukhuri Khola (0�8 km), where PARDYP Nepal maintains hydrological stations (sites 7 and 8 as well as site 2

further downstream).

A sudden change in the sediment regime at these sites indicated the major impact of the construction work.

According to H. Katagiri (pers. comm., 2003), construction work between kilometres 35 and 37 (in the catchment

area of the Upper Andheri Khola and Kukhuri Khola) were initiated in January 2000 and concluded in March

2000. The dates of the construction period were confirmed by the local readers monitoring the hydrological

344 J. MERZ ET AL.

Copyright # 2006 John Wiley & Sons, Ltd. LAND DEGRADATION & DEVELOPMENT, 17: 343–351 (2006)

stations downstream of the road. Not only did the sediment regime at the gauging stations change in this period,

but also the bed material in the streams changed dramatically. Before 2000 the river bed in the Kukhuri Khola was

mainly composed of boulders, but after 2000 it was sandy and of a finer texture.

STUDY SITE AND DATA

The Andheri Khola catchment is located within the Jhikhu Khola catchment about 45 km east of Kathmandu

(Figure 1). Rain-fed agricultural land (in Nepali bari) dominated the land-use system in the catchments with

36�7 per cent in the catchment area of site 2, 55�4 per cent in the area of site 7 and 62�9 per cent in the area of site 8.Forest accounted for 39�9 per cent, 21�3 per cent and 14�9 per cent in the catchment areas of sites 2, 7 and 8,

respectively. The road alignment passes through the upper part of this catchment at an altitude of about

1450m a.s.l. Sediment data are available from hydrological stations below the intersection of the new road

with the monitored tributaries. The first sites are at a distance of 1�3 to 1�4 km from the road (Table I and Figure 1).

These stations, sites 7 and 8, monitor two separate catchments draining into the Andheri Khola catchment, which

is being monitored by site 2 at a distance of 5�2 km. The Andheri Khola is a tributary of the Jhikhu Khola, which

has a station at the outlet of the catchment at a distance of 10�8 km. Site 13 has been included in these analyses for

the purpose of control. The road impact potential is a function of the location of the measurement site in relation to

the road and the distance from the road.

The observations include manual staff gauges and automatic water level recording using floaters and pressure

transducers connected to loggers at natural, but stabilized cross-sections. Regular discharge measurements for

establishing water level–discharge relationships are done by the area–velocity method using a current meter and

the dilution method using salt as the tracer. Sediment sampling is done regularly as well as during flood events by

means of USDH-79 depth integrating sediment samplers at locations with turbulent flow and assume that the water

Figure 1. Location of the study area.

ROAD CONSTRUCTION IMPACTS ON SEDIMENT LOADS 345


and sediment is well mixed at this location. The samples are taken at the stream line approximately in the middle of

the stream. Regular samples were initially taken weekly as well as during flood events. An attempt was made to

take samples during both the rising and the falling limb of the hydrograph. After two complete seasons only flood

events were sampled. The samples obtained in the field were taken to the field laboratory, where they were filtered

using Whatman 40 filter papers and then oven-dried before weighing. Errors are mainly expected and associated

with the sampling location in the river. As the samples are always taken the same way, however, it is assumed to be

acceptable to compare the results relatively between the time before road construction and after road construction.

RESULTS

The comparison of the sediment-rating curves suggests a major change in the sediment regime between 1999 and

2000. This change can be clearly observed in the data from sites 7 and 8 (Figures 2a and b). In both cases the

suspended sediment concentrations in grams per litre at a certain discharge are on average higher after the road

construction in winter 2000 than before this intervention. This is true for the discharge amounts of the entire

range, although values at higher discharge levels are convergent partly due to the limited sample numbers at that

level. At site 2 (Figure 2c) the suspended sediment concentration of the samples after the construction

intervention are consistently above the concentrations sampled before. The impact of the construction is

Table I. Spatial parameters of the hydrological stations in the Jhikhu Khola catchment

Site Part of the AKC Down- Road impact Catchment Distance from the roadstream of AKC potential area [km2] (along Andheri Khola) [km]

Site 1 No Yes Limited 111�4 10�8Site 2 Yes Medium to high 5�4 5�2Site 7 Yes High 0�7 1�3Site 8 Yes High 1�8 1�4Site 13 No No None 1�5 —

AKC: Andheri Khola catchment.Source: PARDYP.

Figure 2. Sediment rating curves for four sites in the Jhikhu Khola catchment: (a) site 2; (b) site 7; (c) site 8; and (d) site 1 (logarithmic scale).

346 J. MERZ ET AL.


therefore still visible at this scale and distance from the road. At site 1, the outlet of the entire 111�4 km2

catchment, no distinct difference can be observed on the basis of the sample points at a given discharge (Figure

2d). However, the samples from before the construction time reflect generally higher discharges than the samples

after construction.

It is also important to note that the range of suspended sediment concentrations for any given discharge is much

greater after road construction than before. In addition all the peak concentrations for each discharge are elevated

Figure 2. Continued



after the construction. This suggests that there is not only more sediment available, but that some of the underlying

processes have been changed after the road was in place.

For comparison, the sediment rating curves of site 13, which is on the other side of the Jhikhu Khola catchment

and has no road impact potential, are shown in Figure 3. No obvious difference can be observed between the data

from before the construction of the road on the other side of the catchment and afterwards. This result would be

Figure 2. Continued

Figure 3. Sediment rating curve of site 13 without impact of the road (logarithmic scale).

348 J. MERZ ET AL.


expected if the influence of the road were being put forward as the reason for the change in the sediment regime. It

can be presented as a control, and indicates no significant change in hydrology in the ‘before’ and ‘after’ periods.

These observations can also be supported by field observations in the river. It was observed that the upper

streambeds after road construction were filled with relatively coarse sediment of the sand fraction. Before the road

construction the bed was basically covered with pebbles and cobbles that were quite stable except during storm

events.

OTHER POSSIBLE REASONS

Up to now the only reason discussed for the sediment regime change has been the construction of the road. Other

possible reasons could be dramatic differences in rainfall pattern, major mass movements in the area or major land-

use changes.

A difference in the rainfall pattern between the period before and after 31 December 1999 cannot be observed on

the basis of the rainfall data from sites around the Andheri Khola catchment (Table II). The year 1999 was the year

with the highest rainfall, mainly due to the large event in October, which produced about 10 per cent of the annual

rainfall. The remaining years are all in the same range (1100–1300mm). The number of rainfall events does not

differ greatly either. In general, the number of events before 2000 was even higher. In terms of the largest events, at

all sites the October event of 1999 tops all the others. Otherwise there is no distinct difference between years with

the largest events usually around 55–65mm rainfall.

There appears to be no sudden increase in surface runoff and erosion, as can be seen at the 100m2 plot

(5m� 20m) scale within the catchment (Table III). The years 1997 to 2001 all show similar runoff values of about

345m3 ha�1. Soil loss varies in these years from 3 t ha�1 to 20 t ha�1. However, years with high values of annual

erosion are observed both before and after the end of 1999. It can therefore be concluded that the reason for the

changing sediment regime in the river was not based on a changing surface-erosion regime on agricultural land in

the catchment.

There were no major mass movements recorded in the area after 1999; these could have been responsible for the

sediment regime change. Land use was stable and no major changes occurred in the study period.

Table II. Rainfall parameters for the study period.

Site Annual rainfall [mm] Rainfall events [No] Largest rainfall event [mm]

1998 1999 2000 2001 1998 1999 2000 2001 1998 1999 2000 2001

4 1111�4 1442�5 1068�6 1203�0 114 94 71 92 52�2 149�1 69�9 58�46 1288�2 1545�6 1213�0 1208�1 114 106 95 82 65�3 170�7 54�8 65�312 1265�2 1418�8 1167�2 1109�8 95 62 92 109 65�6 129�4 51�4 46�414 1291�6 1481�3 1187�7 1259�7 79 91 88 102 60�5 133�0 64�0 47�816 1217�4 1464�3 1296�0 1214�7 85 na na 109 58�3 168�8 na 61�0na, not available.Source: PARDYP.

Table III. Annual runoff and soil loss from erosion plot within the affected catchment

1993 1994 1995 1996 1997 1998 1999 2000 2001

Runoff [m3 ha�1] 231�1 166�2 166�3 256�6 343�9 341�9 351�3 355�6 346�7Soil loss [t ha�1] 37�18 6�96 1�89 18�70 8�35 20�05 2�82 13�85 6�49Source: PARDYP.



IMPACT

The change in suspended sediment concentration has a major impact on the sediment regime in the catchments of

the Andheri Khola and the Kukhuri Khola (Table IV). The difference in suspended sediment yield between

undisturbed conditions (i.e. the sediment rating curve before the road construction was used to calculate the

sediment yield) and disturbed conditions (i.e. the sediment rating curve for 2000 and 2001 was used for

calculation) was estimated to be about 300 per cent at site 2, with increased sediment yield after the construction of

the road. The difference at site 7 is 400–600 per cent and at site 8 about 440 per cent with reference to the

undisturbed conditions.

The difference between sediment yields in the undisturbed conditions, and the conditions after road construction

was about 10 000 t y�1 (135 t ha�1 y�1) at site 7 and 17 000 t y�1 (95 t ha�1 y�1) at site 8. This was related to the

road length within the two catchments: about 0�8 km within catchment 7 and about 1�2 km within catchment 8,

resulting in a unit area sediment yield of about 170 t ha�1 y�1 per 1 km road at site 7 and 80 t ha�1 y�1 per 1 km

road at site 8. At site 2 the unit area sediment loss due to the road impact decreased to about 25 t ha�1 y�1 per 1 km

of road. The impact at the scale of the Jhikhu Khola catchment is limited to negligible, as shown with the rating

curve in Figure 2d and is therefore assumed to be less than 1 t ha�1 y�1. Plotting the sediment yields per kilometer

against catchment area, an inverse power function is observed in the form sediment yield per 1 km road

[t ha�1 y�1]¼ 15965� catchment area [ha]�1�0351.Compared with the figures indicated by Deoja (1994) of 8000 t ha�1 y�1 for careless road construction these

values show that the impact of road construction was limited in this case. However, for a few people, the Andheri

Khola and its tributaries are a lifeline. Seventy-two irrigation diversions receive their water from these streams

(Nakarmi, 1995). An increase in sediment load might block the intakes and change the river ecology. An increase

in the level of the riverbed will further endanger the irrigated fields along the river. Again it must be stressed that

the constructors did a commendable job in terms of environmentally friendly road construction by proper

deposition of the excavated material and engineering and bio-engineering stabilizations of the road cuts. However,

the impact of the construction cannot be fully mitigated.

CONCLUSIONS

The visual comparison of sediment rating curves before and after the road construction shows a clear impact of the

road on the sediment regime. The same is shown in the field by means of the changing stream beds and increased

deposits in the streams. However, statistical analysis of the means and the rating curve parameters cannot be

treated rigorously as the suspended sediment concentrations are too variable in nature and small changes on

the basis of other minor factors are also likely to have an impact on the regime. Other potential reasons, such as

Table IV. Sediment load of the Andheri Khola and its tributaries

Site Parameter Undisturbed Disturbed Difference 2000 Difference 2001

2000 2001 2000 2001 [t] [%] [t] [%]

2 Average annual flow [m3 s�1] 0�068 0�066 0�068 0�066 No diff. No diff.Average yield [t ha�1 y�1] 24�9 21�0 77�1 65�7 52�2 310�1 44�7 312�3Total yield [t y�1] 13 394 11 337 41 542 35 402 28 148 24 065

7 Average annual flow [m3 s�1] 0�012 0�011* 0�012 0�011 No diff. No diff.Average yield [t ha�1 y�1] 27�0 43�0 160�8 180�8 133�8 595�6 137�8 420�9Total yield [t y�1] 1997 3179 11 897 13 382 9900 10 203

8 Average annual flow [m3 s�1] 0�032 0�030 0�032 0�030 No diff. No diff.Average yield [t ha�1y�1] 28�9 27�4 129�7 120�1 100�8 448�1 92�7 438�8Total yield [t y�1] 5151 4870 23 080 21 372 17 929 16 502

*Due to a major shift in the discharge rating curve without observed discharge measurements at site 7, the post-monsoon flow of 2001 had tobe estimated on the basis of previous years.

350 J. MERZ ET AL.


change in precipitation pattern, increased surface runoff and erosion, land-use changes, or mass wasting, can be

excluded as possible reasons for the change in sediment regime.

According to the consulting engineers all measures were taken to minimize the sediment mobilization. So the

current impact of 200–400 per cent increase in sediment yield at the three sites would have been even higher if

these measures had not been taken. This was also shown by comparing the calculated sediment yields for the sites

in the catchment with values for sediment yields due to careless road construction reported in the literature.

In order to monitor the effectiveness of the erosion and landslide control measures along the road it will

be interesting to review the data over the next 5–10 years until the vegetation stabilizes both the road slopes and the

excavation deposits. This analysis also needs to consider hydrological parameters with questions about the

potential of changing hydrological processes over this period.

acknowledgements

The authors would like to acknowledge the financial support of the donors Swiss Agency for Development and

Cooperation (SDC), the International Development Research Centre (IDRC) and ICIMOD. Many thanks to

Pravakar B. Shah, former country coordinator PARDYP Nepal, Bhuban Shrestha, country coordinator PARDYP

Nepal, and the entire PARDYP team for their support. The data collection would not have been possible without

the efforts of the local readers. A very special thanks therefore goes to them.

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road construction impacts on stream suspended sediment loads in a nested catchment system in nepal

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