Pipeflow suspended sediment dynamics and their contribution to stream sediment budgets in small rainforest catchments, Sabah, Malaysia

Forest Ecology and Management 224 (2006) 119–130 www.elsevier.com/locate/foreco

Pipeflow suspended sediment dynamics and their contribution to stream sediment budgets in small rainforest catchments, Sabah, Malaysia Aime´e M. Sayer a, Rory P.D. Walsh a,*, Kawi Bidin b b

a Department of Geography, University of Wales Swansea, Swansea SA2 8PP, UK School of Science and Technology, Universiti Malaysia Sabah, Box No. 2073, 88999 Kota, Kinabalu, Sabah, Malaysia

Abstract Despite soil piping being increasingly reported from the humid tropics, quantitative assessments of the role of pipeflow in runoff generation and sediment supply in humid tropical catchments remain lacking. This paper assesses pipeflow and streamflow suspended sediment dynamics in small humid tropical rainforest catchments in Danum Valley, Sabah, Malaysian Borneo. Pipeflow and streamflow in two small catchments were continuously monitored between December 2002 and June 2004 using pressure-transducer probes behind V-notch weirs; suspended sediment was continuously monitored with turbidity probes. Monthly flow and sediment load data show that a single monitored pipe in one catchment accounted for 47.2% of a stream’s discharge and 21.6% of the suspended sediment load over the monitoring period. Anecdotal evidence from unmonitored pipes suggests the total contribution from pipes to streamflow discharge and sediment loads may be much higher. In general, absolute monthly pipe and stream sediment yields increase with monthly rainfall and pipe or stream discharge. Fourteen storm event responses are considered in detail. Pipe and stream discharge-suspended sediment responses to storm events are rapid and are characterised by clockwise hysteresis. An inverse relationship between the percent contribution of sediment to streamflow from a monitored pipe and rainfall intensity was identified, indicating that in large, intense rainstorms other sediment sources (including slopewash and unmonitored pipes) become more important. Mechanisms of pipe discharge and suspended sediment generation are discussed and implications for future runoff, sediment and channel development models are briefly outlined. # 2006 Elsevier B.V. All rights reserved. Keywords: Pipeflow; Streamflow; Storm responses; Suspended sediment; Rainforest; Sediment fingerprinting

1. Introduction Soil pipes can be defined (based on Uchida et al., 2001) as naturally developed chains of connected macropores parallel or near parallel to the ground surface capable of transmitting concentrated preferential subsurface flow known as pipeflow. Bryan and Jones (1997) identify two processes of pipe development: spring-sapping, where the seepage of water from a stream bank or gully wall washes away soil particles to form a conduit, and tunnel erosion, where existing conduits such as root channels or desiccation cracks are enlarged by the flow of water through them. A detailed discussion of the physical and chemical soil properties that aid pipe development is given by Jones (1981, 1990), but the wide range of environments in which piping occurs precludes the use of a single model of pipe development (Bryan and Jones, 1997).

* Corresponding author. Tel.: +44 1792 295228; fax: +44 1792 295955. E-mail address: [email protected] (R.P.D. Walsh). 0378-1127/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2005.12.012

Piping has been reported from environments ranging from sub-arctic Canada (Carey and Woo, 2000) to the seasonal tropics of southern India (Putty and Prasad, 2000) and the semi-arid Loess Plateau of northern China (Zhu et al., 2002). Most quantitative soil piping research has focussed on temperate regions (e.g. Gilman and Newson, 1980; Jones, 1981; Uchida et al., 2001; Terajima et al., 2000; Holden and Burt, 2002) whilst piping in the humid tropics has received less attention (Bonell and Balek, 1993). Baillie (1975) described the morphology of discontinuously collapsed sections of piping in Acrisols and Cambisols in Sarawak, Malaysia. The large pipe systems consisted of 20–30 cm diameter pipe outlets, trenches and gullies formed by pipe collapse and bridged sections of intact piping referred to as underground streams. In a short reconnaissance study in Dominica, Walsh and Howells (1988) monitored pipeflow from 10 cm diameter outlets into streams in a banana and coconut plantation catchment. Pipeflow discharge varied very little in response to rainstorms in the short monitoring period and the specific conductance of pipeflow most closely matched streamflow

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values at baseflow, suggesting that the pipes are deep-seated in the kaolin-rich soil profile providing baseflow rather than stormflow. Pipeflow was estimated to account for at least 20% of streamflow. At La Cuenca, western Amazonia, overland flow is often generated by return flow from pipes so that the different pathways are difficult to separate. Near-surface lateral flows dominate runoff responses due to a rapid decrease in soil permeability between the surface and 30 cm depth (Elsenbeer and Vertessy, 2000). In contrast to Walsh and Howells’ (1988) Dominican pipes, chemical analysis of 58 pipeflow samples from La Cuenca found that this quickflow pathway is dominated by event (i.e. new) water (Elsenbeer et al., 1995). Previous hydrological studies in Danum Valley, Malaysia identified piping (Sinun et al., 1992; Douglas et al., 1999) and found rapid water table responses to rainfall (Bidin et al., 1993) and high subsurface flow velocities (Sherlock, 1997) that suggest the presence of well-connected pipes and/or macropores. This brief review indicates that whilst soil piping has been linked by several authors to stormflow and baseflow generation, sediment and solute supply to streams and channel network development in the humid tropics, quantitative assessments of these roles remain lacking. The aims of this study are to quantify the suspended sediment responses of pipes and to assess the role they play in the suspended sediment dynamics and budgets of small rainforest catchments in Danum Valley, Sabah. It also seeks to assess factors influencing variations in sediment response between different storm events. These objectives were addressed by (a) analysing pipe and stream monthly discharge and sediment loads and (b) examining pipe and stream responses to individual storm events of contrasting magnitude, intensity and antecedent rainfall. 2. Study area Field research was conducted in undisturbed lowland dipterocarp rainforest in the Danum Valley Conservation Area (4.58N), located in eastern Sabah, Malaysian Borneo (Fig. 1). Mean annual rainfall (1985–2004) recorded at the Danum Valley Field Centre (130 m a.s.l.) is 2822.8 mm. Rainfall occurs throughout the year, but with two wetter periods from October to January and May to June. During the study period (December 2002–June 2004), maximum 15-min rainfall intensities exceeded 50 mm h 1 on average every 17 days and the highest 15-min rainfall intensity recorded was 148 mm h 1 on 4 October 2003. Daily rainfall exceeding 50 mm occurs on average 9.3 times per year and rainfall exceeding 100 mm occurs 0.9 times per year. The geology is a complex melange of slumped Miocene volcanics, sandstones and mudstones promoting a local heterogeneity of soil, but siltloam Ultisols greater than 1.5 m thick are the most common group (Marsh and Greer, 1992; Chappell et al., 1999). Soil piping was found in deeper Ultisols throughout the study area. Many ephemeral channels have developed from progressive headward pipe collapse; evidence for this includes the presence of remnant sections of intact pipe roof (c. 1–2 m length) and buttress roots spanning steep-sided U-shaped channels.

Fig. 1. Location of Danum Valley in SE Asia.

Two small catchments known as W3 and W7 (Fig. 2 and Table 1) were investigated in this study. The upper and middle reaches of W3 are drained by an ephemeral stream with a channel gradient of 3–78 that is bordered by the steepest slopes in the catchment of 17–248. Several small pipes (2–3 cm diameter) are located in the banks of the stream channel head area. At the lower end of this channel, streamflow finds a subsurface route and on several occasions was observed to cascade into small vertical holes (<10 cm diameter) in the channel floor. Several trenches formed by collapsed piping are located between the upper and lower streams where the slopes are least steep (5–108). The trenches measure 1.0–1.5 m in length and are up to 1.5 m deep. Two pipe outlets discharge into the channel head area of the ephemeral stream in the lower part of W3. The larger of the two outlets was chosen for continuous monitoring and is located 75 cm below the top of the headwall and has a near-round cross-section measuring 18 cm in diameter. It extends for at least 1 m sloping gently downwards into the channel bank. The smaller outlet located 80 cm below the top of the headwall has a near-round cross-section that measures 12 cm in diameter. It extends for at least 60 cm also sloping gently downwards into the bank. The lower stream has a channel gradient of 7–108. The pipe outlet approximately 10 m northeast of the monitored pipe (marked ‘H’ on Fig. 2), has a round-elliptical cross-section measuring 30 cm in diameter situated approximately 35 cm below the soil surface. This outlet slopes more steeply (338) downwards into the hillslope. In W7, slopes are typically 2–188 with the steepest slopes in the centre of the catchment. A micro-gully in the centre of the catchment is evidence that localised concentrated surface flow occurs, but there are no surface streams. The gentlest slopes (2– 88) are found in the lower part of the catchment where sections of collapsed piping are located. There are two small pipe

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Fig. 2. The study catchments W3 and W7. Table 1 Topographic and soil physical properties in two catchments W3 and W7 ID

Type

Area (m2)

Maximum altitude difference (m)

Maximum slope (8)

Mean slope (8)

Mean silt– clay at surface (%)

Mean silt– clay at 110 cm (%)

Mean bulk density at surface (g cm 3)

Mean bulk density at 110 cm (g cm 3)

W3 W7

Pipes and surface streams Entirely piped

13204 5267

34 18

24 18

8 10

60.8 48.2

88.1 58.2

0.6 0.7

1.1 1.1

collapses measuring 40–50 cm in diameter and a maximum of 450 cm in depth. These round features are vertical shafts providing windows into the subsurface pipe channel. Farther down-catchment, there is a large collapse trench >5 m long, 1.8 m at its widest point and 1.3 m deep. Four pipe outlets are located in the walls of this trench; they have round or elliptical cross-sections measuring 9–22 cm in diameter and situated 80– 115 cm below the ground surface. The combined flow from these outlets was continuously monitored. Pipeflow from the trench enters an intact section of piping and at least some of the flow re-emerges down-catchment at the lower pipeflow gauging station indicated on Fig. 2. Numerous small vertical holes (<3 cm diameter and up to 40 cm deep) can be found in the ground surface of both catchments. Whether their origin is linked to soil faunal activity or to hydrogeomorphological processes, they certainly do have a hydrological function as they were witnessed to fill rapidly with water during storm events. The large soil pipes that were monitored in this study resemble large subsurface channels making them somewhat distinct from preferential flow networks of smaller pipes and macropores often described in runoff studies in other forested catchments (e.g. Sidle et al., 2000). Soil profiles in each catchment show increasing bulk density and silt–clay content with depth (Table 1). The reduction in permeability implied by these data is supported by studies of

saturated hydraulic conductivity (Ksat) at nearby sites (Table 2) (Sinun et al., 1992; Bidin et al., 1993; Sherlock, 1997). They reported that Ksat values are highly variable at the soil surface with a maximum value of 539 mm h 1 but decline rapidly with depth to maximum values of 105 mm h 1 at 0.3–0.4 m and 10 mm h 1 at 1.1–1.2 m depth. These data, when considered in conjunction with information on typical storm rainfall intensities, suggest that the ponding necessary to promote lateral subsurface flow frequently occurs. They also suggest that pipe development in the study area may be in part linked to the process of spring-sapping at the point of flow emergence in a stream bank (Baillie, 1975; Jones, 1981; Bryan and Jones, 1997). Table 2 Variation in saturated hydraulic conductivity (Ksat) values with depth measured at Danum close to study catchments (compiled from Sinun et al., 1992, Bidin et al., 1993 and Sherlock, 1997) Depth from ground surface (m)

Range of measured Ksat values (mm h 1)

0.0–0.1 0.1–0.2 0.2–0.3 0.3–0.4 0.7–0.8 1.1–1.2

29–539 130–163 64–108 4–105 0.3–49 0.4–10

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3. Methodology The research design comprised: (1) monitoring pipeflow discharge and turbidity at three locations in two zero-order catchments; (2) a comparison of the pipeflow record with the streamflow record in one catchment to assess the contribution of pipes to streamflow discharge and sediment loads; (3) the collection of ancillary information on overland flow and its turbidity and (4) the exploratory application of sediment fingerprinting methodologies to compare pipeflow suspended sediment with potential sediment sources. This study also formed part of a wider investigation in which pipes were mapped; pipeflow solutes monitored and erosional changes of pipe features were assessed using repeat measurement techniques. The two small catchments were chosen for continuous hydrological monitoring on the basis of their contrasting drainage: one catchment (W7) is entirely piped up-catchment of the stream channel section whilst the other catchment (W3) contains pipes and surface streams (Fig. 2). Both catchments have Ultisol soils and exhibit similar topography (Table 1 and Fig. 2). In the partially piped catchment (W3), gauging stations were established in November 2002 in an ephemeral surface stream and at a large (18 cm diameter) pipe outlet that feeds into the stream 30 m up-channel from the stream gauging station. In the smaller, entirely piped catchment (W7), an upper pipeflow gauging station was installed in December 2002 and a second (lower) pipeflow site was instrumented in July 2003 (Fig. 2). These installations enabled assessments of the responses of different pipes to rainfall; and (in the case of W3) the quantification of the contribution of pipeflow to stream discharge and sediment budgets and comparison between pipe and stream responses during storms. The monitoring period (December 2002–June 2004) encompassed a wide range of storm events and antecedent conditions. Streamflow and pipeflow were continuously monitored using PDCR1830 Campbell Scientific pressure transducers and 195Analite turbidity probes, sited behind 1208 V-notch weirs and coupled to CR10X Campbell Scientific data loggers. The data loggers were programmed to record the mean flow depth and turbidity every 15 min (from readings taken at 1 min intervals). The turbidity probes were adapted by the manufacturer to have a measurement range up to 10,000 NTU to cater for the high turbidity expected. The box-shaped cross-sections of the collapsed pipe in front of the monitored outlets in W7 and the streamflow site in W3 permitted the installation of weirs engineered from 3 mm sheet-steel. The location of the monitored pipe outlet in W3 in the bank of a channel head did not lend itself to the installation of a weir. Flow from the pipe was therefore captured by inserting plastic guttering into the bank below the outlet to direct flow into a plastic tank containing the sensors with a 1208 V-notch weir at the exit-end of the tank. Rainfall amounts and intensity were measured using Casella standard and natural siphon rain gauges situated in an open site approximately 800 m from the study catchments. Stage data are converted to discharges (Q) in litres per second (L s 1) using the standard formula (Q = 1000(2.47h2.5) for a sharp-crested V-notch weir; where h is the height of water flow

above the bottom of the V-notch in metres) (Gregory and Walling, 1973). Total monthly and storm pipeflow and streamflow values are expressed as catchment runoff depths (in mm) (calculated by dividing total flow in litres by the catchment area in square metres). This method gives an estimate of the contribution from the monitored pipe to streamflow in W3 but does not give a true value of pipe runoff depth as the pipe’s catchment area is smaller (by an unknown amount) than that of the stream. Measurements of turbidity (T) (in NTU) were converted to suspended sediment concentration (SSC) using the calibration equation: SSC (mg L 1) = 0.674T. This relationship was derived by measuring the turbidity of 24 pipeflow and streamflow samples, which were then vacuum filtered through pre-weighed papers, then dried and re-weighed. A regression analysis (r2 = 0.90; standard error = 75 mg L 1) between the set of corresponding values provided the basis for the calibration. Total pipe and stream discharges and sediment loads for each month of the monitoring period and for 14 individual storm events were calculated from the continuous record of pipe and stream discharge and suspended sediment concentrations. To facilitate a discussion of the possible origins of pipeflow suspended sediment, an exploratory sediment fingerprinting study was undertaken. The approach is underpinned by the principle that eroded sediment particles within a catchment can be differentiated in terms of their source on the basis of their physical and/or chemical properties. A comparison of suspended sediment properties, which collectively form a fingerprint, with the same properties from potential upstream source areas then enables source–output linkages to be inferred. The sediment properties that can be used to create the fingerprint are wide-ranging and may include: mineralogy, mineral magnetics, geochemistry, organic carbon content, soil texture, isotopes and radionuclides. The applicability of each property varies between different environmental situations (Collins et al., 2001; Royall, 2001; Collins and Walling, 2002). This exploratory investigation, which forms part of a wider sediment fingerprinting study using a range of sediment properties and a composite-component approach (Walsh et al., 2004; Blake et al., submitted for publication), uses organic carbon content (which is preferentially contained in surface soil relative to the subsoil) to differentiate between surface and subsurface sediment sources. Pipeflow suspended sediment samples were collected from bottle-traps fixed in front of pipe outlets in W3 and from sediment deposited behind the weirs in W7. Two soil pits were dug in each catchment to permit sediment sampling at different depths in the soil profile. 4. Results and analysis 4.1. Monthly discharge and sediment loads Monthly discharge and sediment load data for the W3 catchment from December 2002 to June 2004 are shown in Table 3. Data for April–June 2003 are missing owing to data logger failure. A similar table cannot be presented for the W7 catchment because the probes were frequently buried by pipeflow sediment thereby disrupting the continuous record. Such repeated

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Table 3 Monthly pipe and stream discharge and sediment loads in W3 catchment December 2002–June 2004 Rainfall (mm)

Raindays

Total pipe Qa (mm)

Total stream Q (mm)

December

186.0

20

8.7

46.3

2003

January February March April May June July August September October November December

254.3 165.3 310.8 192.8 228.5 243.0 297.0 93.2 436.4 381.9 234.2 701.2

22 20 23 11 14 15 21 15 26 22 18 29

25.9 18.8 35.2 – – – 47.6 0.6 39.8 60.3 37.0 277.8

2004

January February March April May June

338.3 153.6 169.6 149.3 371.7 256.1

20 15 20 14 20 14

Total

5163.2

Total (excl. April–June 2003)

4498.9

Year

Month

2002

Total pipe SS load (kg)

Total stream SS load (kg)

18.9

8.9

108.0 66.0 132.7 – – – 88.0 0.6 101.4 159.1 58.6 411.3

24.0 28.5 26.5 – – – 54.1 98.7 39.3 37.9 63.2 67.5

93.8 8.2 8.6 0.2 19.3 10.2

189.7 23.6 20.6 1.0 89.1 56.6

353

–

–

318

681.9

1495.8

Stream Q from monitored pipe (%)

Stream SS from pipe (%)

Weighted mean pipe SSC (mg L 1)

30.8

29.0

77.4

50.5

19.4 11.0 26.7 – – – 34.2 0.5 22.3 30.2 23.8 165.6

86.5 41.8 181.4 – – – 619.5 0.7 80.4 129.2 66.1 388.4

22.4 26.3 14.7 – – – 5.5 65.9 27.7 23.4 36.0 42.6

56.6 44.3 57.5 – – – 54.4 66.3 42.3 38.0 48.7 45.2

60.7 48.0 103.6 – – – 533.2 99.2 60.1 61.5 85.5 71.5

49.4 34.9 41.8 25.4 21.7 18.0

51.2 5.3 11.3 0.2 12.6 6.3

119.5 17.3 24.9 1.1 95.9 101.5

42.8 30.7 45.5 19.3 13.1 6.2

41.3 48.8 99.7 67.1 49.4 47.0

47.7 55.5 91.6 88.7 81.6 135.9

–

–

–

–

–

–

47.2

429.5

21.6

46.2

96.3

1985.2

Weighted mean stream SSC (mg L 1)

Q, discharge; SS, suspended sediment. a N.B. Stream and pipe discharges are expressed in mm runoff units by dividing flow by the catchment area (13,204 m2) where 1 mm represents 13,204 L of discharge.

probe burial is itself, however, a qualitative indicator of the scale and frequency of pipeflow sediment transport. Overall, in W3 the single monitored pipe accounted for 47.2% of stream discharge and 21.6% of suspended sediment load in the stream over the monitoring period. Contributions from the monitored pipe to streamflow discharge and sediment loads during individual months ranged from 18.0 to 98.7% and from 5.5 to 65.9%, respectively. Absolute monthly pipe sediment loads are, in general, closely related to monthly pipe discharge (r2 = 0.99) and monthly rainfall (Fig. 3). The smallest pipe sediment loads of 0.2 and 0.5 kg occurred in the driest months with the least pipeflow, April 2004 (149.3 mm rainfall; 0.2 mm pipeflow) and August 2003 (93.2 mm rainfall; 0.6 mm pipeflow), respectively; the greatest pipe sediment load of 165.6 kg occurred in the wettest month with the maximum pipeflow, December 2003 (701.2 mm rainfall; 277.8 mm pipeflow). The stream similarly carried its smallest sediment loads of 0.7 and 1.1 kg in the driest months of April 2004 and August 2003, but the stream’s greatest sediment load of 619.5 kg occurred in July 2003, which was only the sixth wettest month (297.0 mm rainfall; 88.0 mm streamflow) in the monitoring period. The stream’s second greatest sediment load of 388.4 kg, however, occurred in December 2003, which was the wettest month with the greatest pipeflow discharge (411.3 mm). The very high load in July 2003 is anomalous in relation to the general stream sediment load–discharge pattern (Fig. 3).

The high streamflow sediment load relative to its discharge in July 2003 is reflected in the very high weighted-mean streamflow suspended sediment concentration (SSC) (533.2 mg L 1) for that month relative to the mean for the whole monitoring period (96.3 mg L 1) (Table 3). Pipeflow weighted mean SSC for July 2003 was lower (54.4 mg L 1) than that of the stream and closer to the overall weighted mean for the monitoring period (46.2 mg L 1), thus indicating the

Fig. 3. Scatter plot of relationship between monthly discharge and sediment load for the monitored pipe and stream in W3.

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Table 4 Comparison of runoff and suspended sediment loads for 14 runoff events in W3 Storm rainfall (mm)

Max 15-min RI (mm h 1)

API7 (mm)

10 January 11 January 11 February 2 March 27 March 8 July 11 July 26 July 2 October 3 October 11 November 14 December 18 December 28 December

23.8 29.5 17.8 35.0 45.7 24.5 96.8 52.8 62.5 56.6 7.5 109.5 6.3 99.1

32.0 34.0 36.0 82.0 9.1 60.0 96.0 108.0 104.8 108.0 12.0 n.d. 11.2 24.0

23.6 52.3 74.1 7.5 59.0 50.8 51.4 4.7 187.2 220.4 44.3 102.6 164.8 87.0

0.5 2.2 1.3 0.5 1.7 1.4 3.4 1.5 2.2 2.2 0.3 7.2 1.0 7.7

Total Mean

667.4 47.7

– 55.2

– 80.7

33.1 2.4

Date (2003)

W3 catchment Pipe Q (mm)

Stream Q (mm)

Stream Q from pipe (%)

Peak pipe Q (L s 1)

Peak stream Q (L s 1)

Pipe SS load (kg)

Stream SS load (kg)

Stream SS from pipe (%)

Peak pipe SSC (mg L 1)

Peak stream SSC (mg L 1)

2.2 8.7 3.6 3.4 10.9 8.1 23.6 6.2 17.6 20.1 0.3 40.0 0.8 32.0

22.7 25.3 37.2 14.5 15.6 16.8 14.4 24.2 12.5 10.9 100.0 18.0 125.0 24.1

1.8 2.1 1.6 1.8 2.3 2.0 4.3 2.5 2.8 2.4 0.5 4.3 1.2 2.8

4.5 12.0 4.0 7.4 40.2 20.8 65.9 26.4 42.3 43.6 0.5 76.6 1.1 49.5

1.5 4.0 2.3 1.7 3.9 1.8 11.5 2.8 3.2 2.1 0.2 16.6 0.5 12.5

5.1 12.5 6.3 13.7 34.3 18.2 440.8 116.5 26.8 20.0 0.2 133.3 0.4 56.9

29.0 31.8 36.5 12.4 11.3 10.0 2.6 2.4 11.8 10.5 90.3 12.5 122.5 22.0

443 478 904 2656 1322 557 858 725 500 186 82 1014 71 443

599 246 797 2840 1174 1226 3320 4441 397 182 82 754 43 322

177.5 12.7

– 32.9

– 2.3

– 28.2

64.6 4.6

885.0 63.2

405.6 29.0

– 732

– 1173

Q, discharge; Max 15-min RI, maximum 15-minute rainfall intensity; API7, 7-day antecedent precipitation index; SS, suspended sediment; SSC, suspended sediment concentration; n.d., no data. N.B. Stream and pipe discharges are expressed in mm runoff units by dividing flow by the catchment area (13,204 m2) where 1 mm represents 13,204 L of discharge.

stream’s high sediment load was caused by factors which did little to affect the pipe’s sediment concentration and load. 4.2. Storm event responses Baseflow SSCs of the monitored pipe and stream in W3 were typically in the range of 25–42 mg L 1 and peaks in pipe and stream SSC were associated with rainstorm events. During the monitoring period, the SSC of the monitored pipe exceeded 500 mg L 1 during 31 rainstorm responses, in nine of which

SSCs exceeded 1000 mg L 1 and in three SSCs exceeded 2000 mg L 1. The highest recorded pipeflow SSC was 2656 mg L 1 on 2 March 2003. Peak stream SSC exceeded 500 mg L 1 during 24 rainstorm responses; they exceeded 1000, 2000 and 3000 mg L 1 in nine, three and two events, respectively. The highest recorded streamflow SSC was 4441 mg L 1 in the 26 July 2003 event. Data for a representative range of 14 storm event responses in W3 and W7 are shown in Tables 4 and 5, respectively. In W3, whilst peak streamflow varied between 0.5 and 65.9 L s 1, peak

Table 5 Comparison of runoff and suspended sediment loads for 12 runoff events in W7 Date (2003)

W7 Catchment Upper pipe Q (mm)

Lower pipe Q (mm)

Peak upper pipe Q (L s 1)

Peak lower pipe Q (L s 1)

Upper pipe SS load (kg)

Lower pipe SS load (kg)

Peak upper pipe SSC (mg L 1)

Peak lower pipe SSC (mg L 1)

10 January 11 January 11 February 2 March 27 March 8 July 11 July 26 July 2 October 3 October 11 November 14 December 18 December 28 December

0.5 2.9 1.2 0.3 8.5 7.3 32.2 – 11.7 8.5 – – – –

– – – – – – 19.9 – 6.5 4.0 – – – –

0.3 2.0 0.7 0.9 12.5 11.6 68.9 – 9.6 7.9 – – – –

– – – – – – 25.2 – 6.4 5.0 – – – –

0.1 1.3 3.5 1.7 43.9 5.7 154.1 – 78.0 22.2 – – – –

– – – – – – 128.0 – 73.0 – – – – –

932 648 655 854 1288 263 1630 – 2006 2323 2668 691 – 4717

– – – – – – 2363 – 3187 – 1173 4717 – 2168

Total Mean

73.1 8.1

30.4 10.1

– 12.7

– 12.2

310.5 34.5

201.0 100.5

– 1556

– 2722

Q, discharge; SS, suspended sediment; SSC, suspended sediment concentration. N.B. Pipe discharges are expressed in mm runoff units by dividing flow by the catchment area (5267 m2) where 1 mm represents 5267 L of discharge.

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pipeflow varied less (0.5–4.3 L s 1) for the 14 selected storms. Sayer et al. (2004b) noted that storm hydrographs for the W3 pipe often exhibited a plateau in peak discharge and that further rainfall inputs failed to increase pipe discharge despite rises in streamflow being recorded (see Fig. 4b for examples). During the monitoring period, pipeflow peaked at 4.3 L s 1 only twice (during the extreme rainfalls on 11 July and 14 December 2003) and otherwise rarely peaked in excess of 2.8 L s 1. Peak stormflow SSC for the W3 pipe during the 14 events averaged 732 mg L 1, with individual values rivalling and sometimes exceeding those of W3 streamflow (average 1173 mg L 1). In contrast, peak SSCs of the pipes in W7 were much higher, averaging 1556 mg L 1 (upper) and 2722 mg L 1 (lower). Relationships between pipe and stream SSCs and storm variables are, however, not simple. The highest SSC of the W3 pipe (2656 mg L 1) occurred when peak pipeflow reached 1.8 L s 1 in response to a moderate-size rainstorm (35.0 mm) on 2 March 2003 with a high maximum 15-min rainfall intensity (82 mm h 1) and very low 7-day antecedent rainfall (7.5 mm). A similar set of storm conditions (52.8 mm rainfall; maximum 15-min intensity of 108 mm h 1 and 7-day antecedent rainfall of 4.7 mm) on 26 July 2003, however, resulted in a much lower pipe peak SSC (725 mg L 1) despite a somewhat higher peak pipeflow (2.5 L s 1). In July 2003, two storms of very high maximum 15-min intensity (108 and 96 mm h 1) resulted in the two highest W3 streamflow SSCs of 4441 and 3320 mg L 1 despite peak streamflows (26.4 and 65.9 L s 1) being less than their highest levels. Peak pipeflow SSCs for the same storms were much lower (858 and 725 mg L 1). This finding suggests the importance of sediment sources other than the monitored pipe during intense storms. In W7, a small storm of 7.5 mm on 11 November 2003 resulted in anomalously high upper and lower pipe peak SSCs (2668 and 1173 mg L 1) relative to peak pipe and stream SSCs in W3 for the same storm (82 and 82 mg L 1). A possible explanation is that an episodic internal pipe collapse provided enhanced sediment availability to the W7 pipes in that event. Pipe and stream suspended sediment loads for individual storm events tend to increase with storm rainfall. The three greatest W3 pipe sediment loads (of 11.5, 12.5 and 16.6 kg) occurred following large rainstorms of 96.8–109.5 mm. The percentage contribution of pipe suspended sediment to stream sediment load, however, appears to fall with increasing maximum 15-min rainfall intensity. The smallest contributions (2.6 and 2.4%) occurred when maximum 15-min intensities were high (96 and 108 mm h 1, respectively) (Fig. 4b) whereas the largest contributions (90.3 and 122.5%) occurred when maximum 15-min rainfall intensities were low (12 and 11.2 mm h 1, respectively). This inverse relationship suggests that in high intensity events other sediment sources (e.g. other pipes, slopewash) become more important sediment contributors to the stream than the monitored pipe. Fig. 4 shows the suspended sediment responses of the pipe and stream in W3 to four storm events and responses by the upper pipe in W7 to two storm events. During the two smaller storms on 10 January 2003 (23.8 mm) and 11 February 2003 (17.8 mm) the W7 pipe reached peak SSC more quickly than

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the pipe and stream in W3, and all three gauging stations recorded peak SSC between 15 and 45 min before peak discharge and between 0 and 45 minutes after peak 15-min rainfall intensity. Peak SSCs were short-lived and were already declining when peak discharges occurred; thus, all sites exhibited clockwise hysteresis. During the two larger storms on 11 and 26 July 2003 (96.8 and 52.8 mm, respectively) there were multiple peaks in stream SSC but only a single peak in pipe SSC, pipeflow and streamflow discharge as well as rainfall intensity. Again, this suggests that in larger, more intense storms sources of sediment other than the monitored pipe become important to the stream. During the 26 July 2003 storm the W3 pipe exhibited a clockwise hysteresis loop, whereas during the 11 July 2003 storm it exhibited a figure-of-eight shaped hysteresis loop in which the response began as an anticlockwise loop with peak discharge being reached prior to peak SSC, and then changed to a partially clockwise loop. 4.3. Ancillary information regarding catchment processes Observational evidence of runoff processes in W3 indicates the importance of other unmonitored pipes to catchment discharge and sediment loads. Pipeflow occurred frequently from a large (30 cm) unmonitored outlet (site H, Fig. 2) where it clearly commences to flow at a relatively low rainfall threshold. Thus, on 15 July 2004, a rainfall of 13.5 mm provoked a small discharge response (in which a sample bottle secured in front of the outlet only partially filled). On 8 July 2003, during a 24.5 mm rainstorm with a maximum 15-min rainfall intensity of 60 mm h 1, very turbid pipeflow was observed to emerge under hydrostatic pressure from the outlet and flow through the discontinuously collapsed pipe section to the east of the monitored pipe outlet and joined the stream above the gauging station. Gravel of up to 8 cm diameter deposited around the mouth of the hydrostatic pipe outlet demonstrates the erosive power of the pipe. Overland flow in the study area has previously been shown to be an important runoff mechanism and erosive agent although the percentage of storm rainfall that becomes overland flow is comparatively small. Sinun et al. (1992) reported that overland flow accounted for approximately 3% of throughfall and Clarke (2002) found that overland flow was frequent and widespread on higher gradient slopes (298) but somewhat less frequent and extensive on intermediate (238) and gentler (78) hillslopes. Sayer et al. (2004a) reported mean slopewash erosion rates, measured using the erosion bridge technique, of 3.4 mm a 1 on steep slopes (298) compared to 0.55 mm a 1 on gentler slopes (78). Overland flow samples collected during the current study in W3 and W7 during storms of 19.8 and 17.5 mm in July 2002 had high suspended sediment concentrations of 2523–8827 mg L 1 on 2–138 slopes in W3 and 1793– 6207 mg L 1 on 5–228 slopes in W7. The mechanism of overland flow development is not entirely clear; the high Ksat values reported by previous studies to at least 30 cm depth and often deeper (Table 2) would appear to rule out widespread generation of saturation overland flow. Equally, the mainly very high soil surface Ksat values (Table 2) would appear to render

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Fig. 4. (a) Discharge and suspended sediment responses by the pipe and stream in W3 and the upper pipe in W7 to the 10 January 2003 and 11 February 2003 rainstorms. (b) Discharge and suspended sediment responses by the pipe and stream in W3 to the 11 July 2003 and 26 July 2003 rainstorms.

A.M. Sayer et al. / Forest Ecology and Management 224 (2006) 119–130 Table 6 Preliminary measurements of total organic carbon (TOC) in pipeflow suspended sediment and with depth in the soil profiles of W3 and W7 Sample ID

Pipe SS Surface soil 10 cm depth 50 cm depth 110 cm depth

W3

W7

TOC range (%)

Mean TOC (%)

TOC range (%)

Mean TOC (%)

1.58–1.85 3.82–5.84 0.71–1.14 0.40–0.56 0.15–0.28

1.72 4.68 0.93 0.48 0.22

0.26–1.36 5.38–10.38 0.96–1.32 0.30–0.44 0.20–0.24

0.72 7.76 1.17 0.37 0.22

SS, suspended sediment; TOC, total organic carbon.

widespread Hortonian overland flow unlikely. A feasible explanation for the observed occurrence of overland flow even during small storms, however, is the thatched roof mechanism of essentially Hortonian overland flow described by Ward and Robinson (2000) whereby the alignment of roots in the dense near-surface root mat parallel to the hillslope impart a preferential permeability along the roots so that water is evacuated through and over the shallow thatch. The reason why this may be poorly detected by permeameter measurements may be a practical one—i.e. permeameter insertion may often necessitate or involve prior clearance or avoidance of decaying litter and the surface root mat. Sediment contained in pipeflow may comprise (a) surface sediment provided by slopewash entering the pipe system through vertical holes and cracks, (b) subsurface material provided by the progressive enlargement of pipes or (c) a combination of the two. Additionally, in W3 some pipe sediment may derive from the upper stream section if a connectivity exists (Fig. 2). Provisional suspended sediment fingerprinting results from total organic carbon analysis of samples can be reported at this stage (Table 6). In W3, the mean TOC content of pipeflow suspended sediment was 1.72%, which is lower than that of surface soil (4.68%), somewhat higher than soil at 50 and 110 cm depths (0.48 and 0.22%, respectively) and corresponds most closely to soil at 10 cm depth (0.93%). In W7, the mean pipe TOC content was rather lower at 0.72%, which is considerably lower than the surface soil (7.76%) and intermediate between values for soil at the 10 cm depth (1.17%) and at 50 and 110 cm (0.37 and 0.22%, respectively). 5. Discussion and implications 5.1. The importance of pipe erosion in total and monthly stream sediment budgets Sediment from pipes clearly constitutes a major contributor to streamflow sediment budgets in the study area. In W3, the single monitored pipe accounted for 21.6% of the stream suspended sediment load over the monitoring period. Furthermore, in intense rainstorm events, when the percentage contribution from the monitored pipe was small, some of the remaining stream sediment load was delivered by unmonitored pipes within W3, notably the hydrostatic pipe (H, Fig. 2) and

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the alternative pipe outlet near the monitored pipe. Thus, the total contribution from pipes to the W3 stream sediment budget will be substantially higher than 21.6% though the precise proportion remains unknown. Although, in general, monthly pipe and stream sediment yields increased with monthly rainfall and pipe or stream discharge (Fig. 3), the exceptionally high stream (but not pipe) sediment yield of July 2003 constitutes a major anomaly. A hypothesis of sediment flushing by slopewash following a long dry antecedent period (Walling, 1974; Bird, 1987) in which surface soil particles are loosened by drying and soil faunal activity, although suggested by a comparatively low frequency of rain-days in April–June, is not supported by the daily rainfall record. There were six daily rainfalls >50 mm in April–June, including ones of 79.5 mm on 10 June and 82.5 mm on 30 June, which should have ensured that the ground in July was in a wet rather than dry condition. A more likely explanation may be an episodic input of material during one of the two storms with particularly high maximum 15-min rainfall intensity (96 and 108 mm h 1, Table 4) in July 2003 from roof collapse or blockage release within an unmonitored pipe; the ephemeral, actively collapsing system downslope of, and fed by flow from, the hydrostatic pipe is perhaps the most likely source. There was no evidence in July of other possibilities, such as a channel bank collapse or a debris dam burst along the stream. 5.2. Sources of pipeflow sediment The findings from the exploratory sediment fingerprinting study in which TOC values of transported pipe sediment are compared with vertical differences in TOC within soil profiles need to be treated with some caution. The higher TOC values (i.e. closer to surface soil than subsurface values) obtained for suspended sediment from the W3 monitored pipe compared with the trapped pipe sediment in W7 would seem to suggest that the proportion of material of subsurface (pipe wall) origin is lower from the W3 pipe than from the W7 pipe. This would seem to be in accordance with the discontinuous nature of piping in W3 (with links to stream sections upstream) compared with the entirely piped nature of the W7 catchment. The sediment sampled from the W7 pipe, however, was material deposited behind a V-notch weir; this may have been deficient in TOC in part because less dense organic matter particles might have been preferentially floated and transported over the weir and therefore been under-represented in the coarser and/or denser sediment deposited behind the weir. In contrast, the W3 pipe samples were suspended sediment samples, which arguably should not have suffered as much from such a bias. Firm conclusions, therefore, regarding the origin of pipe sediment should not be drawn from this exploratory study as further sediment property analyses are required to obtain a more rigorous composite-component fingerprint. Consideration of selectivity in erosion, transport and deposition of different sediment particle sizes will also be necessary. The study does however suggest promise for the novel application of sediment fingerprinting to pipeflow studies. Similar sediment fingerprinting studies have not

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previously been applied in pipeflow research and rarely has the technique been attempted in catchments as small as those in this study. 5.3. Storm event responses The very rapid responses, high SSC concentrations and mostly clockwise hysteresis responses of all the pipes and the W3 stream are consistent with the idea that pipes provide much of the stream sediment load and storm runoff. The more rapid response of the W7 pipe than the W3 pipe to rainfall may reflect a shorter travel path from the smaller catchment area of the entirely piped W7 catchment. With streams, rapid, clockwisehysteretic responses are usually considered to result from: (1) principal sediment sources being located close to the gauging station, (2) a change in the relative contribution of runoff processes during a storm response, with runoff processes with higher SSC dominating the early part of the hydrograph, (3) a progressive decline in sediment availability during a response, or (4) a combination of these factors. The much reduced SSC responses of both the W7 and W3 pipes in the second compared with the first event of 11 February and the clockwise hysteresis patterns in both storm responses (Fig. 4a) strongly suggest that the ‘progressive decline’ mechanism is dominant in pipe responses in the study area. Given the wetness of the Sabah environment, this suggests that even short dry periods between storms are sufficient for preparation of easily entrained sediment through drying and soil faunal activity. The complex, multiple-peaked SSC responses of the W3 stream to the July 2003 storms (Fig. 4b) imply the arrival of separate pulses of sediment and suggest that the sources of the later sediment are either distant from the gauging station and/or only connected to the stream channel under high moisture conditions (Seeger et al., 2004), or are associated with an unmonitored slowerresponding pipe or channel bank erosion or minor landslips as water levels recede. The figure eight-shaped Q–SSC response of the monitored W3 pipe on 11 July 2003 implies either that the sediment was derived from farther away than during the clockwise responses, thereby delaying its arrival at the gauging station relative to the discharge peak, or that the latter part of the sediment response was enhanced by a sudden minor pipe roof or wall collapse. 5.4. A maximum transmission capacity of the W3 pipe? Evidence from hydrograph form analysis and records of peak flow of the W3 pipe suggest that the pipe has a transmission capacity of approximately 2.8 L s 1, but that during very high magnitude and intensity rainstorms the threshold may be exceeded with peak discharges reaching 4.3 L s 1. A possible explanation is that during smaller intensity and magnitude storms the pipe’s discharge is constrained by a low connectivity between pipe and macropore segments in the catchment but during larger intensity and magnitude storms as soil moisture increases pipe and macropore segments become more connected thereby increasing the contributing area of the pipe allowing pipeflow

discharge to exceed 2.8 L s 1. Sidle et al. (2000) proposed the existence of such preferential flow networks of macropores that expand in a stepped, non-linear manner as soil moisture (antecedent wetness) increased to explain stormflow generation in a temperate forest soil. An alternative explanation is that pipe transmission capacity may be stable only over comparatively short periods but subject to episodic change when pipe blockages (leading to a reduction in transmission capacity) or pipe enlargement or dam-bursts (leading to an increase in transmission capacity) occur, particularly in large storm events when pipeflow hydrostatic pressures and positive pore water pressures in soil material may be exceptionally high. 5.5. Wider implications The findings from this study also raise important questions as to the appropriateness of existing catchment runoff, sediment and drainage network initiation models for piped catchments not only in the humid tropics, but in all areas where pipes are found. The speed with which pipeflow is generated in both the W3 and W7 catchments cannot be explained by vertical infiltration and percolation through the soil matrix, as measured hydraulic conductivities (Ksat values) are not high enough to allow infiltrating rainfall to penetrate to pipe depths in a sufficiently short time period to generate the observed peaks in pipeflow. The speed of pipe response implies a preferential flow mechanism, in which vertical cracks, roots and root-holes (perhaps concentrated around tree-trunks and therefore systematically missed by hydraulic conductivity assessments), small pipe-roof collapses and animal holes provide conduits for infiltrating water (including localised overland flow) to bypass the soil matrix. Models such as TOPMODEL, which are based on an assumption that soil hydrological response is governed by permeabilities of a layered soil matrix, are not appropriate to the storm runoff processes found in such monitored catchments, which are dominated by preferential flow, pipeflow and also, to some extent, Hortonian overland flow. Similarly, because the results indicate that a significant proportion (at least 21.6% and perhaps more than 50%, if unmonitored pipes were included) of stream sediment load is provided by pipes, there is a need to incorporate piping parameters (such as pipe length per unit area) in models of stream sediment generation at sites where piping is significant. Also, the fact that some of the stream channels in the study area have clearly developed from pipes implies that: (1) drainage initiation models in such areas need to include a focus on the factors governing pipe initiation and development and (2) models based solely on Hortonian or saturation overland flow and throughflow are inappropriate. The frequency and widespread nature of overland flow in the area (Clarke, 2002; Sayer et al., 2004a) and the presence of some channel heads maintained by overland flow and not linked to piping in the adjacent parts of the study area, however, suggest that a pipe erosion model alone (Dunne, 1990) is also inappropriate.

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6. Conclusion In conclusion, pipeflow is a highly significant contributor to streamflow sediment budgets in the study area. In one catchment, a single monitored pipe alone accounted for 21.6% of monthly stream sediment loads and 29.0% of stream stormflow sediment loads for the 14 events studied in detail. As observational evidence shows that flow also occurs from unmonitored pipes, the total contribution of pipes to stream sediment loads is likely to be substantially higher, though slopewash is also a significant process. Suggested foci for future work on piping and its erosional role in the humid tropics include: the use of a sediment fingerprinting and tracer approach to assess the relative importance of erosion within pipes compared with inputs of sediment from overland flow; assessments of the density of piping system both in the wider study area and in areas of contrasting soil types elsewhere in the humid tropics; more systematic assessments of the routes by which water enters the piping system; and the response of piping systems to logging disturbance. Acknowledgements The research was funded by NERC studentship number NER/S/A/2001/06422. The authors would like to thank the Economic Planning Unit of the Prime Minister’s Department of Malaysia and the Danum Valley Management Committee for research permission. Thanks are also due to Muhammad Jamil Hanapi for field assistance at the Danum Valley Field Centre and to Dr W.H. Blake, University of Plymouth, UK, for help and advice with catchment instrumentation. This paper is number A/411 of the Royal Society’s SE Asia Rain Forest Research Programme. References Baillie, I.C., 1975. Piping as an erosion process in the uplands of Sarawak. J. Trop. Geogr. 41, 9–15. Bidin, K., Douglas, I., Greer, T., 1993. Dynamic response of subsurface water levels in a zero-order tropical rainforest basin, Sabah, Malaysia. In: Proceedings of the Hydrology of Warm Humid Regions, Yokohama, IAHS Publ. no. 216. pp. 491–496. Bird, S.C., 1987. The effects of hydrological factors on river suspended solids contamination from a colliery in South Wales. Hydrol. Process. 1, 321–338. Blake, W.H., Walsh, R.P.D., Sayer, A.M., Bidin, K., submitted for publication. Quantifying fine-sediment sources in primary and selectively-logged rainforest catchments using geochemical tracers. Proceedings of the Interactions between Sediment and Water, Lake Bled, Slovenia, IASWS. Water Air Soil Pollut. Bonell, M., Balek, J., 1993. Recent scientific developments and research needs in hydrological processes of the humid tropics. In: Bonell, M., Hufschmidt, M.M., Gladwell, J.S. (Eds.), Hydrology and Water Management in the Humid Tropics. Cambridge University Press, Cambridge, pp. 167–260. Bryan, R.B., Jones, J.A.A., 1997. The significance of soil piping process: inventory and prospect. Geomorphology 20, 209–218. Carey, S.K., Woo, M.K., 2000. The role of soil pipes as a slope runoff mechanism, Subarctic Yukon, Canada. J. Hydrol. 233, 206–222. Chappell, N.A., Ternan, J.L., Bidin, K., 1999. Correlation of physiochemical properties and sub-erosional landforms with aggregate stability variations in a tropical Ultisol disturbed by forestry operations. Soil Till. Res. 50, 55–71.

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