Ditch erosion processes and sediment transport in a drained peatland forest

Ecological Engineering 75 (2015) 421–433

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Ditch erosion processes and sediment transport in a drained peatland forest Leena Stenberg a, * , Tapio Tuukkanen b , Leena Finér c, Hannu Marttila b , Sirpa Piirainen c , Bjørn Kløve b , Harri Koivusalo a a b c

Aalto University School of Engineering, Department of Civil and Environmental Engineering, P.O. Box 15500, 00076 Aalto, Finland University of Oulu, Water Resources and Environmental Engineering Research Group, P.O. Box 4300, 90014 Oulu, Finland Finnish Forest Research Institute, P.O. Box 68, 80101 Joensuu, Finland

A R T I C L E I N F O

A B S T R A C T

Article history: Received 30 June 2014 Received in revised form 29 October 2014 Accepted 28 November 2014 Available online xxx

Ditch network maintenance in drained peatland forests increases sediment load and causes harmful effects on downstream watercourses. The objective of this study was to identify the immediate erosion processes in the ditch network and quantify the impact of the ditch network maintenance on suspended solids (SS) load at the catchment outlet. Ditch level processes were measured by conducting field campaigns and catchment-level responses were obtained by using the paired catchment approach. Measurements were conducted at two nested catchments in Koivupuro, Eastern Finland, where runoff and turbidity were monitored continuously at the catchment outlets and erosion and deposition were measured in selected ditches with a pin meter, erosion pins and sediment collectors. According to the results, the sediment load was at its highest during the first year after the ditch network maintenance and especially high during the first spring snow-melt period. Pin-meter measurements, sediment-collector data and results of ditch-bed erosion pins showed that the erosion was more severe in the ditch cut into mineral subsoil than in the ditch cut into thick peat. The SS load from the whole Koivupuro catchment during the first year after the ditch network maintenance was, depending on the estimation method, 185 or 250 kg treated-ha
1 a
1 (0.56 or 0.75 kg ditch-m
1), while the net erosion within the ditch network was much higher, 6300 kg treated-ha
1 a
1 (19 kg ditch-m
1), indicating that suspended solids were deposited before reaching the outlet of the catchment. Compared to the earlier studies which have focused on the catchment-level effects of ditch network maintenance on the SS loads, the current study succeeded in showing erosion processes and their variability within the ditch network and provided information for improving water protection during ditch network maintenance. ã 2014 Elsevier B.V. All rights reserved.

Keywords: Ditch network maintenance Suspended solids Pin meter Erosion pins Catchment

1. Introduction Peatland drainage for improving forest growth was a common practice in Finland in 1950s–1980s when half (4.7 Mha) of the pristine peatlands were drained. Today, the maintenance of the existing ditches is carried out on about 50,000 ha annually (Finnish Forest Research Institute, 2013) and new ditch networks are no longer dug to pristine peatlands. The term ditch network maintenance in the context of drained peatland forests is understood to include, in addition to the cleaning of existing ditches, the digging of new complementary ditches. Boreal

* Corresponding author. Tel.: +358 504073016. E-mail address: leena.stenberg@aalto.fi (L. Stenberg). http://dx.doi.org/10.1016/j.ecoleng.2014.11.046 0925-8574/ ã 2014 Elsevier B.V. All rights reserved.

peatlands have been drained for forestry also in Sweden, Norway, Canada, USA and Russia (Paavilainen and Päivänen, 1995). Drainage changes the water balance and flow paths, and their aggregated impact on runoff from the drained catchment varies depending on site conditions (e.g., Holden et al., 2004). When water table is lowered, the better aeration in root zone improves tree growth. Simultaneously, evaporation from the forest floor decreases, which is compensated by increased evapotranspiration from tree foliage, leading to smaller runoff. Due to peat subsidence, the effective porosity of peat in drained peatlands is smaller compared to pristine peatlands, but the capacity to temporarily store water can be higher (Päivänen and Hånell, 2012). The magnitude and timing of runoff from drained peatlands depends on the combination of water table position, antecedent soil moisture storage and ditch network conveyance. In many cases, the ditch network enables faster runoff to the catchment outlet and

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can open new pathways for groundwater discharge (Rossi et al., 2012). The drainage efficiency is gradually reduced, due to the shrinkage of the peat layer, decrease of the ditch depth and growth of vegetation to the ditches (Silver and Joensuu, 2005). Besides the hydrological effects, drainage and ditch network maintenance reduce water quality by increasing erosion and sediment transport (Joensuu et al., 2002; Kenttämies, 1981; Marttila and Kløve, 2010a; Nieminen et al., 2010) as well as the transport of nutrients, especially that of phosphorus, with eroded soil particles (e.g., Sallantaus, 1986). Different water protection measures (sedimentation ponds and pits, submerged weirs, surface runoff areas) are widely used, but the need and effectiveness of the measures in controlling the sediment load depends on site characteristics. The development of these measures requires identification of erosion sources and quantitative knowledge of the source-area processes. This study is motivated by the need to improve the knowledge for designing better and more appropriate water protection measures as well as preventing erosion in the ditch network maintenance areas. The erosion and sediment load after ditch network maintenance have been studied mainly at catchment scale (Ahti et al., 1995; Joensuu et al., 2001, 2002; Manninen, 1998; Marttila and Kløve, 2010a; Nieminen et al., 2010). These studies show that most of the erosion and sediment transport occurs during the first years following the ditch network maintenance. Finér et al. (2010) compiled data in Finland and reported that the excess load caused by ditch network maintenance without water protection methods is on an average 600 kg treated-ha
1 a
1 during the first year following ditch network maintenance. The load estimates vary widely between sites, since erosion processes differ depending on soil type in ditch banks and beds (Tuukkanen et al., 2014). When ditches are scoured in thin-peated sites, mineral soil layers can eventually be exposed. These areas have high risk for erosion (Joensuu et al., 2002), which can, if the soil is fine-textured, continue for decades (Joensuu et al., 2006). The mechanisms giving rise to the sediment load from the ditches in drained peatlands include flow erosion, undercutting and collapse of ditch banks (Marttila and Kløve, 2010a). Seepage (Fox and Wilson, 2010) and weathering due to e.g., frost (Lawler et al., 1999; Stott, 1997) are other important mechanisms affecting erosion in streams. While the key erosion mechanisms can be identified, knowledge about their functioning after ditch network maintenance is limited. Stenberg et al. (2015) studied small-scale bank erosion processes in a ditch with mineral soil banks and quantified the changes in bank dimensions during the first weeks after the ditch maintenance. It is still unclear how and how long the bank erosion continues, what is the role of the ditch bed compared to the ditch banks and what are the erosion processes at ditches dug in thick peat. Moreover, it is unclear how the erosion observed in ditches is connected to the sediment load observed at the outlet of the catchment. The first objective of this study was to quantify the immediate impacts of ditch network maintenance on sediment load from a ditch maintenance area. The second objective was to connect the sediment source-area processes with catchment-scale sedimentload processes. For this purpose, erosion in thin-peated and thickpeated source areas was measured in the catchments established for the quantification of the sediment load. More specific questions were (1) how and when does erosion occur in the source areas inside the ditch network after the ditch network maintenance, and (2) how and when is the sediment transported out from the catchment or stored within the ditch network? We had two main hypotheses: (1) the changes in the source areas are larger in the thin-peated ditches than in the thick-peated ditches and (2) the sediment load is highest during the first spring flood following the ditch network maintenance. We were focusing only on the first

years after the operation, which are the most critical ones considering erosion and sediment load. In earlier studies, sediment load has mainly been estimated with yearly resolution but, in this study, monthly to seasonal time-steps were used. 2. Materials and methods 2.1. Catchment description and hydrometeorological data Erosion processes and transport of sediments were studied at Koivupuro catchment (Fig. 1), which has an area of 113 ha and lies in Sotkamo, Eastern Finland (63 530 N, 28 400 E). A sub-catchment of 5.2 ha was delimited within the catchment. Välipuro catchment (86 ha) that is located ca. 2 km from Koivupuro was chosen as the control catchment (e.g., Ahtiainen and Huttunen, 1999). In Koivupuro, forested peatlands cover 21%, forested upland soils 47% and open pristine mires 32% of the whole catchment area. Of the catchment, 24% is drained and forested. The forests are dominated by Scots pine (Pinus sylvestris L.) with some Norway spruce (Picea abies L.) and birch (Betula pendula Roth). Climate in the area is described by the mean annual precipitation of 591 mm and the mean annual air temperature of +2.3  C (Pirinen et al., 2012). Initial ditching was conducted in 1983 for an area of 32 ha. Ditch maintenance operation was started in August 15, 2011, and lasted for seven days on an area of 27 ha. A total of 9.1 km of old ditches was cleaned and new supplementary ditches dug, of which 25% had a mineral soil contact. In the sub-catchment (Fig. 1), the length of the cleaned ditches was 1.5 km in total, of which 20% extended to mineral soil. The ditches were dug to the depth of about 1 m and had a width of about 2 m. Koivupuro and Välipuro were instrumented with a measurement weir (double v-notch) in 1978 (Ahtiainen et al., 1988). For this study, another measurement weir was installed in August 2011 at the outlet of the sub-catchment (Fig. 1). The Koivupuro weirs were instrumented with pressure sensors (PR-26 W and PR-36 W, Keller Ltd.) to measure water level for discharge estimation and turbidity sensors (Analite NEP9500, McVan Instruments Ltd.) to estimate the concentration of suspended solids (SS) in runoff water at

Fig. 1. Location of the Koivupuro catchment (a) and the layout of the whole catchment and its sub-catchment and the measurement sites (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

L. Stenberg et al. / Ecological Engineering 75 (2015) 421–433

15 min intervals. For sensor calibration, water samples were taken at both outlets. Meteorological variables were obtained from Valtimo and Sotkamo weather stations operated by the Finnish Meteorological Institute and located 26 km and 30 km from Koivupuro, respectively. 2.2. Paired catchment analysis for estimating the ditch network maintenance effects

Rm;Koivupuro ¼ 0:8983  Rm;Va€lipuro þ 4:9079

(1)

2

The R value for Eq. (1) was 0.95 (p < 0.001, n = 55). Water samples at the discharge outlets of Välipuro and Koivupuro were gathered by the local environmental authorities 7–11 times per year and analyzed for SS concentration (mg l
1). During 2007–2008, the water samples were filtered through a 1.2 mm glass fiber filter, which was also used during 2009– 2010 together with a 0.4 mm polycarbonate filter. In 2011, only the 0.4 mm filter was used. The data from the water samples was linearly interpolated for each day between the sampling times. During 2009–2010, when both of the filters were applied, an average of the two concentration values was used in the interpolation. The interpolated values and the daily runoff were then used to calculate the daily SS loads, which were further aggregated as monthly totals. Monthly SS loads (kg ha
1 month
1) from January 2007 to July 2011 were used to adjust the linear regression equation between the Koivupuro and Välipuro catchments for the calibration period: S Sloadm;Koivupuro ¼ 1:0001  S Sloadm;Va€lipuro þ 0:0786 2

December 2012 to mid-May 2013. To fill this gap and few other minor gaps in the Koivupuro data, Välipuro was used here as a reference catchment. The daily runoff values (Rd mm d
1) from Koivupuro and Välipuro after the Koivupuro ditch network maintenance (from August 2011 to December 2012) were used to derive the linear regression equation between the two catchments: Rd;Koivupuro ¼ 1:0958  Rd;Va€lipuro þ 0:2547

To calculate the effect of the ditch network maintenance on runoff and SS load from Koivupuro, paired catchment analysis was applied. The paired catchment approach in peatland forest conditions has been used by e.g., Prévost et al. (1999),Joensuu et al. (2002), Nieminen (2004), Laurén et al. (2009), Marttila and Kløve (2010b), and Nieminen et al. (2010). Runoff Rm (mm month
1) from January 2007 to July 2011 was used for adjusting the linear regression equation between Koivupuro and Välipuro (control catchment) during the calibration period:

(2)

The R value for Eq. (2) was 0.72 (p < 0.001, n = 55). After the ditch network maintenance, water samples from Koivupuro outlet were taken more frequently. In addition to the samples collected and analyzed by the local environmental authorities (3–7 per year), samples were collected with an automatic ISCO sampler in 2011 and 2012 and analyzed for SS concentration, using the 1.2 mm Whatman glass fiber filter (GF/C). During the first autumn following the ditch network maintenance (August–November 2011), a total of 59 water samples were collected. In 2012 (from April to November), the total amounted to 47 water samples. Water samples with the ISCO sampler were also taken from the sub-catchment outlet inside Koivupuro. In 2011 (August–October) 59 and in 2012 (April–October) 74 samples were collected and analyzed. Both of these data were used in the interpolation of the SS concentrations and in the estimation of SS load from Koivupuro. Eqs. (1) and (2) were used to calculate the monthly estimates of runoff and SS load for no-treatment conditions in Koivupuro after the ditch network maintenance (from August 2011 to August 2013). These estimates characterizing no-treatment conditions are compared in the Section 3 to the measured values which are affected by the ditch network maintenance. The unit SS loads were calculated by dividing the load by (1) the whole catchment area and (2) the area affected by ditch network maintenance. As the freezing of water in Koivupuro weir led to a breakdown of the pressure sensor in December 2012, no data were available from

423

(3)

2

Eq. (3) (R = 0.89, p < 0.001, n = 443) and the data from Välipuro were then used to estimate the missing runoff values for the Koivupuro catchment. 2.3. Measurements and estimation of suspended solids loads Turbidity is reported to correlate well with organic SS concentration, but the data need to be calibrated for individual sites (Marttila et al., 2010; Marttila and Kløve, 2012). The calibration curves for Koivupuro and its sub-catchment were formed using the SS concentrations (SSC, mg l
1) of the water samples from the catchment outlets and comparing them with the associated turbidity (T, NTU) values: SSC Koivupuro ¼ 0:412  T Koivupuro þ 0:8851

(4)

SSC sub-catchment ¼ 0:1845  T sub-catchment þ 2:0

(5)

The R2 values were 0.90 (p < 0.001, n = 82) and 0.71 (p < 0.001, n = 74) for Eqs. (4) and (5), respectively. The outliers in turbidity values, such as sporadic peaks, were left out when forming the calibration curves. The calibration curve (Eq. (5)) for the subcatchment was adjusted to make the concentration of 2 mg l
1 to correspond to the turbidity value of 0 NTU. Without this modification, SS concentrations and further SS loads would have been overestimated. The SS concentrations of the discharge at the catchment outlets were estimated by two approaches. Firstly, the SS concentrations were calculated with Eqs. (4) and (5). The obvious errors in the turbidity data, such as the high values caused by algae growth on the sensor during the summer season, were treated as outliers. The gaps in the turbidity data were filled with a value calculated as an average of four values recorded before and after the erroneous data. In the second approach, the SS concentrations of the water samples were linearly interpolated between the sampling times. The SS concentrations were then multiplied with the discharge to produce two estimates of SS loads. The discharge measured at the outlet of Koivupuro was used in the load estimation for both catchments. The subdaily SS load estimates were totaled to daily (kg ha
1 d
1), monthly (kg ha
1 month
1) and annual (kg ha
1 a
1) SS loads. 2.4. Measurements in erosion source areas and processing of the data 2.4.1. Pin-meter method Pin meter (Fig. 2) was used to measure changes in ditch microtopography and provided proxy data for identifying the source of erosion. The measurements were conducted at two sides of a 4-meter-long ditch sections. The pin meter used in this study was described earlier by Stenberg et al. (2015) who applied it to measure ditch-bank topography. The supporting structures in the current study were different from those used by Stenberg et al. (2015). In our case, the pin meter was applied to measure both ditch banks at a measurement location. The pin meter had three different pin sizes (50, 75 and 100 cm in length), and the spatial resolution of the pins along the pin-meter steel support (see Fig. 2) was 2 cm. Pin-meter measurements were made during intensive field campaigns at two different locations in the Koivupuro

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changes in channel bed topography (slope) during the two years following ditch network maintenance. Spatial variation in ditch bed and bank topography (as measured by the erosion pins) was also compared with the measured changes in ditch cross-sections at the pin-meter locations.

Fig. 2. The pin-meter measurement setup.

catchment, in ditches A2 and B3 (Fig. 1). The first of these (A2) was a thick-peated site (four campaigns) where both ditch bank and bed were peat, and the second one (B3) a thin-peated site (five campaigns) where the ditch reached mineral soil layer. The pinmeter measurements were made at 20 cm intervals along the ditch. Thus the resolution of the pin-meter data was 20 cm in the direction of the ditch and 2 cm in the direction of the cross-section. Each measurement was photographed and the positions of the pins were then determined from the photographs. The raw pin-meter data included the distance of soil surface from the pin meter, and these data were then converted into x-, y-, and z-coordinates of the soil surface. The soil-surface data were interpolated over the whole measurement area for a 2 cm  2 cm grid with ordinary kriging. The interpolated data were then used to obtain erosion rates as a difference between the soil-surface elevations at different measurement times by assuming that a decrease in elevation indicated erosion. The accuracy of the pin-meter measurements was studied by repeating the measurements five times at the same location. The difference between the minimum and maximum values of the five measurements for each pin was calculated. The difference varied between 0.09 cm and 3.58 cm, the average being 0.96 cm with standard deviation of 0.86 cm. 2.4.2. Erosion-pin method To better encompass spatial variation in erosion and deposition in the ditch network, erosion pins (e.g., Lawler, 1993) were installed in the ditch bed and banks with approximately 20 m intervals at ditches A2, A3 and B3 (Fig. 1). The erosion pins were 60 cm long wooden sticks (diameter 5 mm) in peaty areas and 60 cm long threaded steel rods (diameter 6 mm) in inorganic soils. An erosion pin was installed at a depth of two-third of a pin (40 cm) by pressing the pin into the ground. One-third (20 cm) of it was left visible for follow-up measurements. Bed erosion pins were installed in the middle of the ditch bed, which was 35 cm wide. Bank erosion pins were placed in the middle of the bank slope, in alignment with the bed erosion pins. Using a tape measure, the length of each erosion pin above the soil surface was measured with the accuracy of 0.5 cm during the five field-measurement campaigns. Erosion and deposition over the given time period were defined by the differences between initial and final erosionpin readings. Bed erosion pins allowed us to define erosion and deposition areas within the drainage network. In order to explain the location of erosion and deposition areas, a leveling survey was carried out with 10 m resolution in ditches A2, A3 and B3. Additionally, the erosion pins were used to assess

2.4.3. Sediment collection Sediment collectors/traps were used to quantify the amount and distribution of bank erosion at the drainage network. For this purpose, 0.5 m wide plastic sediment traps (half-cylinder, radius 5 cm) were built and carefully installed in the ditch banks at the lower courses of ditches A1–A4 and B2–B3 (Fig. 1). Sediment traps were placed approximately in the middle of the slopes, where the water level in the ditch did not reach the collector. These sediment collectors were intended to measure sediment production due to weathering, groundwater seepage and rain splash rather than hydraulic erosion or bank collapse due to ditch runoff. Eroded sediments captured by each trap were repeatedly collected and weighed to estimate erosion rates between the ditches. The sediment traps were emptied once in October 2011 and 7 times during the summer and autumn 2012. Bank erosion rates (in kg) per unit area were calculated for each sediment trap, using the sediment dry weights and the bank area above the sediment trap. Assuming the average bank slope of 1 m, bank erosion rates (kg ditch-m
1) were calculated and compared with the erosion estimates from the pin-meter measurements. The total amount of bank erosion in the study catchment was defined by assuming the same erosion rate (average of all measurements) for all ditches. 2.5. Soil properties in the pin-meter measurement sites Soil samples were taken near the pin-meter measurement sites in ditches A2 and B3. Peat and mineral soil samples were taken at 10 cm vertical intervals from both ditch banks. Additional peat samples were dug from the field between the ditches down to a depth of 60 cm below the soil surface. In ditch B3, mineral soil was also sampled from the ditch bed. Peat samples were analyzed for peat type, degree of decomposition (von Post scale) and bulk density, whereas mineral soil samples were analyzed for soil type and bulk density. Soil type was determined from the particle size distribution that was obtained by sieving the dry samples. The results of the soil properties are presented in Table 1. Stoniness of the mineral soil under the peat layer next to the pin-meter measurement site at ditch B3 was approximated by systematically selecting 16 points and measuring the penetration of a steel rod into the mineral soil layer. The volumetric stone content y (%) was then calculated with the equation given by Viro (1951) as documented by Tamminen and Starr (1994) y ¼ 83
2:75x;

(6)

Table 1 Soil properties of the right and left banks of ditches A2 and B3 (see Fig. 1) near the pin-meter measurement sites.

Peat type Degree of decomposition (von Post scale) Bulk density of peat (g cm
3) Bulk density of mineral soil (g cm
3) Mineral soil type D50 of mineral soil (mm)

A2, thick peat

B3, thin peat

Right Left

Right

LSCa 5 0.06 – – –

LCSa LSa 5 5 0.15 0.20 2.72 1.96 Sandy till Sandy till 0.60 0.45

LSCa 5 0.05 – – –

Left

a LSC = woody Spaghnum-Carex peat, LCS = woody Carex-Sphagnum peat, LS = woody Sphagnum peat (Päivänen and Hånell, 2012).

L. Stenberg et al. / Ecological Engineering 75 (2015) 421–433

where x is the average depth of rod penetration in cm. The volumetric stone content was 48% and 64% in the left and the right side of the pin-meter measurement site, respectively. 3. Results 3.1. Runoff and suspended solids loads from the catchments before and after ditch network maintenance There was a clear and rapid increase in the SS loads after ditch network maintenance in Koivupuro (Fig. 3b). Based on the paired catchment approach, the SS load increase for the first month was 15 kg treated-ha
1 month
1 (1200%) (Fig. 3b). The SS load increased by 166 kg treated-ha
1 during the first year after ditch network maintenance and only by 20 kg treated-ha
1 during the second year (Table 2). Also runoff increased soon after the ditch network maintenance during September and October 2011. Before the ditch network maintenance the difference between the measured and estimated runoff varied between positive and negative values, but thereafter the measured runoff was always greater than the estimated no-treatment runoff (Fig. 3a). The SS load was estimated with two different methods for the whole Koivupuro catchment and its sub-catchment (Table 2). SS loads estimated using the turbidity data and calibration curves (Eqs. (4) and (5)) tended to give higher estimates but were in the same order of magnitude as those based on water samples only. In both years, the SS load from the sub-catchment was clearly lower than that from the whole Koivupuro catchment. Both catchments showed distinct decrease in SS loads during the second year. The maximum daily SS loads (Fig. 4) from the whole Koivupuro catchment were 23.2 and 28.7 kg treated-ha
1 d
1 depending on

425

whether the SS concentration values were interpolated from the water sample data or interpreted from the turbidity data, respectively. From the sub-catchment, the highest daily SS loads were 4.0 and 6.3 kg treated-ha
1 d
1 for the water sample- and turbidity-based estimates, respectively. In both areas, the highest peaks were observed in the first year following the ditch network maintenance, during spring snow melt or summer rainstorms depending on whether the SS load estimates were based on the water sample data or the turbidity data, respectively. 3.2. Erosion and deposition in the source areas 3.2.1. Ditch cross-sections The pin-meter measurements indicated only small changes in the average cross-section of the ditch on the thick-peated A2 site (Fig. 5a). From the ditch network maintenance in August 2011 to the last measurement in September 2012, the median vertical difference in the soil-surface elevation of the average cross-section of the ditch was
1 cm. In contrast to the ditch on thick-peated site, the average crosssection of the ditch on the thin-peated B3 site changed clearly after the ditch network maintenance (Fig. 5b). The changes were minor until the first spring flood when erosion was clearly observed in the lower part of the ditch bank, especially in the left bank of the ditch (Fig. 5b). During summer 2012, only minor erosion was detected, but it continued after the second spring runoff peak. This time erosion concentrated on the middle parts of the right ditch bank. From the time of the ditch network maintenance to June 2013, the median difference in the soil-surface elevation of the average cross-section of the ditch was
3 cm. As seen in Fig. 5b, most of the erosion occurred in the mineral soil part of the ditch bank. Of the

Fig. 3. Monthly runoff at Koivupuro and Välipuro catchments and the difference between the measured and estimated runoff at Koivupuro (a), and monthly runoff and suspended solids (SS) loads from Koivupuro and its sub-catchment (b). Catchment area = whole Koivupuro catchment, treated area = area of ditch network maintenance, SS load* = estimated SS load for Koivupuro without the ditch network maintenance.

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Table 2 Suspended solids (SS) load (kg ha
1) estimated before and after ditch network maintenance (DNM). SS load based on water samples Välipuro catchment, SS (kg catchment-ha
1 a
1) (86 ha) Average for years 2007–2010
1

Koivupuro catchment, SS (kg catchment-ha Before DNM (average for years 2007–2010) First year after DNM Second year after DNM

a


1

a b

SS load estimated without DNMa

2

–

–

3 44 8

–

) (113 ha)

Koivupuro catchment, SS (kg treated-ha
1 a
1) (27 ha) First year after DNM Second year after DNM Sub-catchment, SS (kg ha
1 a
1) (5.2 ha) First year after DNM Second year after DNM

SS load based on turbidity and calibration curves (Eqs. (4) and (5))

60 8

185 32

250 31

39 3b

75 12

– 5 3

19 12

– –

Estimate based on the paired catchment analysis with Välipuro (see Section 2.2). Data available from August 2012 to December 2012.

ditch bed, 10–20 cm was not covered by the pin-meter measurements on the thin-peated site. 3.2.2. Ditch-bank topography Besides the interpretation of the average cross-sections, pinmeter data were used for calculating the spatial variability of the evolution of the soil-surface elevation. The data were processed with ordinary kriging and the differences between the measurements were treated as either erosion or deposition, depending on the direction of the change. The changes that occurred in the thickpeated A2 site during the first year after the ditch network maintenance (from August 2011 to September 2012) indicated only small changes in the ditch banks and bed (Fig. 6a). Local deposition was observed in the ditch bed, and soil surface was slightly lowered in the upper parts of the banks. On the contrary, the changes in thin-peated site B3 were more substantial after the first year (Fig. 6b). Again deposition was observed at the ditch bed, whereas erosion was evident in the lower half of the left ditch bank (bright blue color in Fig. 6b). There were also clear spots of erosion in the lower half of the right ditch bank. Soil surface also seemed to be lifted higher in the middle of the right bank. The thin-peated B3 site was also measured after the second spring (Fig. 6c). The deposition at the ditch bed and the clearest erosion spot remained at the same location as one year earlier, but the right bank had undergone erosion that was more widely spread. There were also changes in the left ditch bank, as the soil surface in the middle of the bank was higher than before and the elevation of the upper bank was lower. The spatially interpolated data in Fig. 6 were further used to convert the elevation changes in the two ditches to changes in volume and mass (Fig. 7). The calculations were made separately for the lower and upper halves of the banks. Only the elevation changes that were greater than 1 cm were taken into account, as this was the approximated error of the pin-meter measurements. The volumetric erosion during the first months after the ditch network maintenance was similar on the thin- and thick-peated sites, but thereafter (October 2011–May 2012) it was higher in the thinpeated site (Fig. 7a). However, during May–September 2012 the thick-peated site had undergone more erosion in volumetric terms than the thin-peated site. Volumetric deposition was systematically smaller than the erosion in both sites, except during the winter period from October 2011 to May 2012 in the thick-peated site. Soon after the ditch network maintenance, the upper bank section dominated the volumetric erosion in both sites, and the domination continued in the thick-peated site over the whole measurement

period. However, during the spring, most of the erosion in the thinpeated site occurred in the lower half of the bank. The changes in mass indicating erosion or deposition (Fig. 7b) were calculated using the soil bulk density determined for both sites (Table 1). The changes in mass were evidently greater in the thin-peated site (Fig. 7b). Seasonality of both erosion and deposition was observed as the values were clearly higher during the winter and spring than during the summer and early autumn months. With an average ditch spacing of 35 m the overall erosion from the thick-peated site during the first year was 1032 kg ha
1 a
1. For the thin-peated site with average ditch spacing of 50 m the erosion during the first year was 21,104 kg ha
1 a
1. The net erosion (erosion subtracted with deposition) for thick-peated site was 873 kg ha
1 a
1 and for the thin-peated site 12,183 kg ha
1 a
1, both in favor of erosion. By assuming the same erosion rates for all of the thin- and thick-peated ditches in the network the erosion was 10,469 kg treated-ha
1 a
1 and 7541 kg treated-ha
1 a
1 for the whole Koivupuro catchment and the sub-catchment, respectively. The net erosion was 6292 kg treated-ha
1 a
1 and 4588 kg treated-ha
1 a
1 for Koivupuro and the sub-catchment, respectively. Only 1.8 or 2.4% of the eroded soil material was carried down to the Koivupuro outlet and 2.5 or 3.3% to the sub-catchment outlet depending on the SS load estimate (Table 2). 3.2.3. Spatial variability of ditch erosion Erosion-pin data collected 22 months after the ditch network maintenance showed that either erosion or deposition had occurred simultaneously in the sub-catchment (Fig. 8a and b). On an average, the erosion dominated the banks, but the average changes in the beds of ditches A2 and A3 was close to zero. Deposition of up to +10 cm and +6 cm was measured in ditch A2 and ditch A3, respectively. The highest change in ditch-bed elevation (erosion) was
15 cm in ditch A2 (Fig. 8a), while in the A3 ditch only minor erosion was observed. Erosion in ditch A2 was mainly found in the lower reach of the ditch and in the upper parts of the ditch where inorganic subsoil was exposed. Bed erosion in ditches A2 and A3 was not clearly dependent on the bed slope; instead the highest deposition was often observed in flat areas (data not shown). The erosion-pin data from ditch B3 (Fig. 8b) revealed that, in addition to bank erosion, there was clear erosion of the ditch bed. Bed deposition was measured only in one location in the upper reach of the ditch. Erosion in ditch banks varied from
11 to 0 cm and from
9.5 to 0 cm in ditches A2 and A3 and in ditch B3, respectively (Fig. 8). At all measured locations, bank erosion was recorded in at least one

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427

Fig. 4. Daily precipitation and air temperature (a), suspended solids (SS) concentrations and timing of the pin-meter measurements (b), SS loads in the sub-catchment (c), and runoff and SS loads in the Koivupuro catchment (d) during the two years following the ditch network maintenance.

side of the ditch. A few positive changes in bank elevation (deposition) seen in Fig. 8 were explained by bank collapses and thus represented bank erosion as well. Overall, observed ditchbank erosion (Fig. 8) was in agreement with the changes estimated with the higher resolution pin-meter data both at the thin- and thick-peated sites (Fig. 5). Sediment-collector data from ditches A1–A4 showed that on an average the bank erosion rate in the sub-catchment was 0.35 kg ditch-m
1 a
1 (standard deviation 0.20 kg ditch-m
1 a
1, n = 8) during the first year after ditch network maintenance (August 2011–October 2012). Total sediment supply due to ditch-bank erosion in the sub-catchment (5.2 ha) was, thus, about 101 kg ha
1 a
1. The corresponding bank erosion rate in thin-peated area (ditches B2 and B3) was 1.33 kg ditch-m
1 a
1 (standard deviation 0.50 kg ditch-m
1 a
1, n = 4). In the thick-peated area, the results

are of the same order of magnitude as the net erosion rates (erosion–deposition) calculated with the pin-meter data (3.1 kg ditch-m
1 a
1, Fig. 7b). However, net erosion in the thin-peated area was substantially higher when calculated from the pin-meter data (60.9 kg ditch-m
1 a
1, Fig. 7b). In the pin-meter measurement site in ditch B3, the erosion of the lower ditch bank and that of ditch bed by flowing water were the dominant erosion mechanisms which explained the large difference in measured erosion rates. When the pin-meter data were used to estimate net erosion only from the upper half of the ditch banks, an average net erosion rate of 12.1 kg ditch-m
1 a
1 was obtained for the measurement site in ditch B3 and 3.1 kg ditch-m
1 a
1 for the measurement site in ditch A2. These values are still high, but they are more comparable with the range of erosion rates measured with the sediment collectors.

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L. Stenberg et al. / Ecological Engineering 75 (2015) 421–433

Fig. 5. Average cross-section of the pin-meter measurement site in ditch A2 (a) and B3 (b) at different measurement times. The cross-sections are viewed from upstream direction. In the B3 ditch, the measurements did not cover the whole ditch bed. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4. Discussion The paired catchment analysis showed the striking impact of ditch network maintenance on sediment load in Koivupuro: the SS loads were evidently greater than they would have been without the ditch network maintenance. A remaining question is what the appropriate value for the SS load was when different methods gave different estimates. The initial ditching in Koivupuro (32 ha in 1983) resulted in an excess load of 57 kg treated-ha
1 a
1 during the first three years following the ditching (Ahtiainen, 1988). That load is clearly lower than the SS loads obtained in this study after the ditch network maintenance (Table 2). The national scale SS load estimate for ditch network maintenance without water protection measures is 600 kg treated-ha
1 a
1 for the first year and 200 kg treatedha
1 a
1 for the second year after the ditch network maintenance (Finér et al., 2010). Compared to these values, our estimates 185 and 250 kg ha
1 a
1 for the first year and 31 and 32 kg ha
1 a
1 for the second year indicated that although notable erosion

occurred in the ditches at Koivupuro there were considerable deposition areas inside the ditch network. As a result, the SS load at the catchment outlet was relatively low. It should be noted that no water protection methods were used in Koivupuro in order to reveal erosion source-area processes that are not mixed with impacts of water protection on sediment loads. Thus, our load estimates are likely to be greater than they would be after treatment with water protection measures. In the sub-catchment where ditch erosion in terms of mass changes was low, also the catchment-scale loads were smaller (e.g., 39 and 75 kg treatedha
1 a
1 for the first year) than in the entire Koivupuro catchment. This difference reflects the sizes and soil types of the two catchments. The sub-catchment was 95% smaller than the Koivupuro catchment, which resulted in smaller discharge volumes and presumably lower flow velocities at the subcatchment outlet. In the sub-catchment, a larger fraction of the ditches was in moderately decomposed peat compared to the whole Koivupuro drained area, where there were more ditches with their bed reaching mineral soil. Peat with moderate degree

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429

Fig. 6. The difference in the soil-surface elevation (cm) measured with the pin meter between August 2011 and September 2012 for the sites located in ditches A2 (a) and B3 (b). The difference between August 2011 and June 2013 for the site in B3 is also presented (c). Blue colour indicates erosion and red deposition.

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L. Stenberg et al. / Ecological Engineering 75 (2015) 421–433

Fig. 7. Volumetric erosion and deposition changes (a) and erosion and deposition mass changes (b) calculated from the pin-meter data from ditches A2 (thick peat) and B3 (thin peat). Thick-peated site in ditch A2 was not measured for the period from September 2012 to June 2013.

of decomposition also resists erosion better than well-decomposed peat (Tuukkanen et al., 2014). Even though the origin of most of the eroded mass was traced to the mineral soil banks and beds of the ditches, it should be noted that the high degree of stoniness of the mineral soil was likely to decrease erosion and can explain why the load estimates for Koivupuro were clearly lower than the national estimate of Finér et al. (2010). When the fine fraction of the mineral soil was washed away, an armor layer of stones was developed and prevented further erosion from the bed surface (e.g., Lisle and Church, 2002), which was also visually perceived in the area. Joensuu et al. (1999) studied monthly excess SS loads in 37 catchments after ditch network maintenance. The average monthly unit SS loads of the catchments during the first year after the treatment (April–October) were clearly higher than those of Koivupuro based on water samples. The largest difference (66 kg treated-ha
1 month
1) between our estimate and the estimate by Joensuu et al. (1999) was in April and the smallest difference (11 kg treated-ha
1 month
1) in July. In relative terms, the values by Joensuu et al. (1999) were 1.7 (April)–7.4 (June) times higher than our estimates.

In the study made in Southern Finland with sandy loam ditch banks, the bank erosion was 8300 kg ha
1 during the 19-day-long study period(Stenberg et al., 2015). During the first two months, erosion in the thin-peated area of Koivupuro was 2985 kg ha
1, which is clearly lower than the erosion measured by Stenberg et al. (2015). The thin-peated site with stony ditch bed in Koivupuro was less prone to erosion than the ditch dug in fine mineral soil in Stenberg et al. (2015), even though the erosion process on the southern site was speeded up with artificial irrigation. Zaimes et al. (2008) reported that bank erosion rates caused by different riparian land-uses were the lowest for a forest buffer zone clayloam banks (5–18 kg m
1 a
1) and the highest for row-cropped field banks (304 kg m
1 a
1). The bank erosion rate measured with sediment collectors in the thin-peated ditches of Koivupuro (1.33 kg m
1 a
1) and the rates measured in the thick-peated site (3 kg m
1 a
1 with the pin meter and 0.35 kg m
1 a
1 with the sediment collectors) compare well with the forest buffer zone values by Zaimes et al. (2008) even though the bank soil is different. However, the local net erosion rate for the thin-peated site in Koivupuro was clearly higher when measured with the pin meter (61 kg m
1 a
1) but falls well within the range of bank

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431

0.20 Right bank Bed Left bank

Deposition

0.15 0.10 0.00

Pin meter site

(m)

0.05 -0.05

Erosion

a) Ditch A2

-0.10 -0.15 -0.20 27

43

63

83 103 124 144 164 185 205 225 235 255 265 280 300 320 340 av. Distance (m)

Deposition

0.20 Right bank Bed Left bank

0.15 0.10

b) Ditch A3

(m)

0.05 0.00

Erosion

-0.05 -0.10 -0.15 -0.20 20

40

60

80

100

120

140

160

180

200

220

av.

Distance (m) 0.20 Right bank Bed Left bank

Deposition

0.15 0.10 0.00

Pin meter site

(m)

0.05 -0.05

Erosion

c) Ditch B3

-0.10 -0.15 -0.20 50

70

90

110

144

164

171

191

221

241

261

281

301

av.

Distance (m) Fig. 8. Erosion and deposition in ditches A2 (a), A3 (b), and B3 (c) obtained 22 months after ditch network maintenance. The horizontal axis is the distance of erosion pins upstream from the ditch outlets. The abbreviation “av.” refers to the average erosion/deposition value of the ditch.

erosion rates reported by Zaimes et al. (2008). Volumetric bank erosion in small agricultural streams was reported to be 0.046 m3 m
1 over a 11-month period (Laubel et al., 2000), which compares well with the net erosion rates measured with the pin meter in the thin- and thick-peated ditches in Koivupuro, 0.050 and 0.056 m3 m
1 a
1, respectively. The mean bank erosion measured with erosion pins by Laubel et al. (2000) was 2.7 mm, while Zaimes et al. (2008) report 15–46 mm a
1 for the riparian forest buffer zone and up to 239 mm a
1 with row-cropped fields. Our erosion-pin measurements fall within these ranges, as the mean bank erosion over the 13-month period was 14 mm. 4.1. Accuracy of the measurements Although the turbidity sensors provided an indication of shortterm fluctuations in sediment transport, the interpretation of the sensor data for quantifying SS loads was found to be problematic. The probe was affected by a severe bio-fouling, which the wiping system cleaning the instrument could not overcome. Manual

cleaning of the optical surface in this study (about once per month) was not frequent enough to produce accurate readings at all times. However, most of the erroneous readings caused by this problem were obvious enough to be removed from the data. The removed values were either sudden, sporadic peak values or values that were growing steadily without having any explanation by discharge. Linjama et al. (2009) explored the operation of turbidity sensors in agricultural ditches and noted that reliable readings were achieved only when the sensor was cleaned with compressed air before every measurement. In addition to uncertainties in the turbidity sensor data, the discharge measurements in the sub-catchment were uncertain and their reliability could not be assured. The factors affecting the discharge measurements were water freezing in spring time, dirt and debris flowing in the water and uncertainty in the delineation of the catchment. The catchment delineation was based on the ditch network, but in some parts of the catchment there was groundwater entering the ditch network, which implied that the catchment area might be underestimated. Thus, the discharge measurements from

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Koivupuro catchment were used instead of the sub-catchment data. Rossi et al. (2012) showed how groundwater–surface water connections can lead to notable subsurface flow from an upslope esker toward drained peatland areas. The accuracy of pin-meter measurements was on average 1 cm, which was found to be tolerable since the changes in the soilsurface elevation were clearly larger than 1 cm. Even though the accuracy test indicated that the maximum error was higher (3.58 cm), the kriging procedure smoothed the soil surface, making the error in individual pin position less significant. To decrease the impact of the errors on the erosion and deposition estimates, elevation changes less than 1 cm were disregarded in the analysis of pin-meter data. It should be noted that erosion and deposition are not the only processes affecting the measured changes in the topography of ditch banks and beds. Peat has a high porosity and is affected by shrinkage, swelling and frozen-ground processes (Kennedy and Price, 2005; Oleszczuk and Brandyk, 2008), which may lead to volumetric changes that can cause uncertainty to our net erosion estimates. 4.2. Erosion processes in the source areas The volumetric erosion rates obtained in this study for thick- and thin-peated areas were of the same order of magnitude (18.8 m3 ha
1 a
1 and 14.8 m3 ha
1 a
1, respectively). However, when converted to mass units the results for the thick- and thin-peated areas were quite different (1032 kg ha
1 a
1 and 21,104 kg ha
1 a
1, respectively). The local absolute erosion rates (kg ha
1 a
1) were also much greater than the SS load values at the catchment outlet in this or earlier studies (e.g., Joensuu et al.,1999; Nieminen et al., 2010). It is obvious that only a minor fraction of the material that is eroded is carried down to the catchment outlet, which highlights the sediment processes occurring within the ditch network. Local eroded material is deposited to be later re-transported (acting as a source) during next higher flow events or to be stabilized (Marttila and Kløve, 2008, 2010a). Compared to the whole Koivupuro catchment, in the thickpeated sub-catchment a larger proportion of the eroded material was carried downstream to the outlet. Thus, we can assume that even though erosion rates were greater in the thin-peated areas the organic particles or colloids (Marttila and Kløve, 2014a) with low density and settling velocity were more likely to be transported than the mineral soil particles. In contrast to mineral soil particles deposited close to the drained area, particulate organic matter causes problems further downstream by changing physical conditions at stream beds by e.g., siltation (Laine, 2001; Laine and Heikkinen, 2000; Marttila and Kløve, 2014b). Clear temporal and spatial variation in ditch erosion and deposition were observed in both thin- and thick-peated areas (Figs. 7 and 8). In ditches A2 and A3, both erosion and deposition were observed along the ditch bed. In ditch B3, deposition did not occur with the same extent as in the thick-peated sub-catchment. This was most probably due to sustained base flow in ditch B3, which was likely to be triggered by the open pristine mire draining to ditch B3 (Fig. 1). In contrast to the erosion-pin data, the pinmeter measurements indicated that soil was deposited at the bed of the B3 ditch (Fig. 6b and c). Visual observations suggest that this deposited material was likely to represent stones that fall from the ditch bank as the bank erodes. The stones then remain at the edges of the ditch bed, as they are too heavy to be transported further by flowing water and thus mask the erosion of finer material in the pin-meter data. Bank erosion was observed almost in all parts of the study catchment although its rate varied widely between locations and observation periods (Figs. 7 and 8). It is thus suggested that most of the deposited peat in the drainage network originate from ditch banks. This conclusion is supported by the measured sediment supply (sediment-collector data and pin-

meter measurements) from the ditch banks. The pin-meter measurements further suggest that, in the thin-peated B3 ditch, erosion from the lower bank section dominated during spring season (Fig. 7a and b), which indicates that high discharges and snow melt affect the ditch bed and lower section of the ditch bank. Field visits revealed that vegetation started to develop and stabilize the banks during the first summer following the ditch maintenance. However, it seemed that erosion was still continuing after the second spring (Figs. 5b, 6c, and 7). This goes well with the fact that SS load in the catchment outlet was still higher than the no-treatment estimates (Fig. 3). The discharge of ditch B3 is assumed to include waters from the open pristine mire located on the west side of the ditch network. Thus, it is important to note that the discharge in the ditch is relatively high and the importance of bed and bank material on sediment load was increased, especially since ditch B3 was located close to the outlet weir. If the bed and bank material in ditch B3 had been peat, the situation might be different not only in the ditch but the influence could probably be seen also at the measurement weir. The high discharge of ditch B3 is also likely to cause overestimation in the erosion calculated for the whole ditch network since in these calculations the pin-meter site in the B3 ditch was assumed to represent all thin-peated ditch areas. Considering the results from the point of view of water protection, it has to be kept in mind that water protection is planned and implemented for a time period of years while majority of the SS loads occur during a much shorter period. The need for ditch network maintenance should also be considered carefully to avoid unnecessary ditching and, thus, SS loads at some of the sites where evapotranspiration of the tree stand is capable of keeping water levels at reasonable levels (Sarkkola et al., 2013). This study further stresses the importance of mineral soil contact for erosion processes. Therefore, it might be reconsidered whether the ditch depth could be diminished from the commonly used depths in ditch network maintenance. With lower ditches, less mineral soil is exposed in thinpeated areas and, thus, the risk for erosion is smaller. This is especially important in areas with highly erosive soils. In those areas, ditch network maintenance should, if possible, be avoided. 5. Conclusions This study focused on the quantification of erosion processes and sediment transport during two years following ditch network maintenance. Monitoring SS load both at catchment outlet and local erosion inside the catchment provided new insight into forest ditch maintenance impacts on water quality. SS loads at catchment outlets were clearlyaffected by the ditch network maintenance, and the loads were the highest during the first spring following the maintenance. The first-year SS load was, depending on the estimation method, 185 or 250 kg treated-ha
1 a
1 from the entire Koivupuro catchment and 39 or 75 kg treated-ha
1 a
1 from its subcatchment. During the second year, the SS load levels at the catchment outlets decreased to 31 or 32 kg treated-ha
1 a
1 in Koivupuro and 3 or 12 kg treated-ha
1 a
1 in the sub-catchment. The results indicated that local net erosion in the source areas (6290 kg treated-ha
1 a
1 in Koivupuro and 4590 kg treatedha
1 a
1 in the sub-catchment) was much greater than the SS load at the catchment outlets, which leads to a conclusion that not only erosion but also deposition is an important mechanism functioning inside the ditch network. The results also show that there is a large spatial variability in the erosion and deposition in both thin- and thick-peated areas. Results of pin-meter, sediment-collector and ditch-bed erosion-pin data indicate that erosion rates were higher in the thin-peated areas where ditches reach mineral soil. However, all the different erosion measurements suggest that most of the SS load originates from ditch banks. Local erosion in terms of

L. Stenberg et al. / Ecological Engineering 75 (2015) 421–433

volumetric change of ditch-bank and -bed elevations was similar between the thick- and thin-peated areas, whereas the mass changes were clearly larger in ditches reaching mineral soil at thinpeated sites. According to this study the most important thing to be considered in the practical ditch management work is to avoid unnecessary mineral soil contact wherever possible. Acknowledgements Technical staff from participating institutes was involved in field measurements, sampling and laboratory work, and we are grateful for their contribution. The study was funded by the VALUE and RYM-TO doctoral programs, the Academy of Finland (ModStream project), Maa- ja vesitekniikan tuki ry (Finnish society of soil and water technology), and Ministry of Agriculture and Forestry (MAHA project). Metsähallitus (state-owned forestry administration enterprise) is acknowledged for providing the study site. References Ahti, E., Joensuu, S., Vuollekoski, M., 1995. Laskeutusaltaiden vaikutus kunnostusojitusalueiden kiintoainehuuhtoutumaan. In: Saukkonen, S., Kenttämies, K. (Eds.), Metsätalouden vesistövaikutukset ja niiden torjunta. METVE-projektin loppuraportti., 2. Suomen ympäristö, pp. 139–155 (in Finnish). Ahtiainen, M., 1988. Effects of clear-cutting and forestry drainage on water quality in the Nurmes-study. Symposium on the hydrology of wetlands in temperate and cold regions, Vol. 1, Joensuu, Finland 6–8 June 1988. Publications of the Academy of Finland 4/1988, 206–219. Ahtiainen, M., Holopainen, A-L., Huttunen, P., 1988. General description of the Nurmes-study. Symposium on the hydrology of wetlands in temperate and cold regions, Vol. 1, Joensuu, Finland 6–8 June 1988. Publications of the Academy of Finland 4/1988, 107–121. Ahtiainen, M., Huttunen, P., 1999. Long-term effects of forestry managements on water quality and loading in brooks. Boreal Environ. Res. 4, 101–114. Finér, L., Mattsson, T., Joensuu, S., Koivusalo, H., Laurén, A., Makkonen, T., Nieminen, M., Tattari, S., Ahti, E., Kortelainen, P., Koskiaho, J., Leinonen, A., Nevalainen, R., Piirainen, S., Saarelainen, J., Sarkkola, S., Vuollekoski, M., 2010. Metsäisten valumaalueiden vesistökuormituksen laskenta, 10. Suomen ympäristö (in Finnish). Finnish Forest Research Institute, 2013. Finnish Statistical Yearbook of Forestry. Finnish Forest Research Institute. Fox, G.A., Wilson, G.V., 2010. The role of subsurface flow in hillslope and stream bank erosion: a review. Soil Sci. Soc. Am. J. 74, 717–733. doi:http://dx.doi.org/ 10.2136/sssaj2009.0319. Holden, J., Chapman, P.J., Labadz, J.C., 2004. Artificial drainage of peatlands: hydrological and hydrochemical process and wetland restoration. Prog. Phys. Geogr. 28, 95–123. Joensuu, S., Ahti, E., Vuollekoski, M., 1999. The effects of peatland forest ditch maintenance on suspended solids in runoff. Boreal Environ. Res. 4, 343–355. Joensuu, S., Ahti, E., Vuollekoski, M., 2001. Long-term effects of maintaining ditch networks on runoff water quality. SUO 52 (1), 17–28. Joensuu, S., Ahti, E., Vuollekoski, M., 2002. Effects of ditch network maintenance on the chemistry of run-off water from peatland forests. Scand. J. Forest Res. 17, 238–247. Joensuu, S., Vuollekoski, M., Karosto, K., 2006. Kunnostusojituksen pitkäaikaisvaikutuksia. In: Kenttämies, K., Mattsson, T. (Eds.), Metsätalouden Vesistökuormitus. MESUVE- projektin loppuraportti, 816. Suomen ympäristö, pp. 83–90 (in Finnish). Kennedy, G.W., Price, J.S., 2005. A conceptual model of volume-change controls on the hydrology of cutover peats. J. Hydrol. 302, 13–27. doi:http://dx.doi.org/ 10.1016/j.jhydrol.2004.06.024. Kenttämies, K., 1981. The effects on water quality of forest drainage and fertilization in peatlands, 43. Publications of the Water Research Institute, pp. 24–31. Laine, A., 2001. Effects of peatland drainage on the size and diet of yearling salmon in a humic northern river. Arch. Hydrobiol. 151, 83–99. Laine, A., Heikkinen, K., 2000. Peat mining increasing fine-grained organic matter on the riffle beds of boreal streams. Arch. Hydrobiol. 148, 9–24. Laubel, A., Kronvang, B., Larsen, S.E., Pedersen, M.L., Svendsen, L.M., 2000. Bank erosion as a source of sediment and phosphorus input to small Danish streams. In: Stone, M. (Ed.), The Role of Erosion and Sediment Transport in Nutrient and Contaminant Transfer. IAHS, pp. 75–82 publ. no. 263. Laurén, A., Heinonen, J., Koivusalo, H., Sarkkola, S., Tattari, S., Mattsson, T., Ahtiainen, M., Joensuu, S., Kokkonen, T., Finér, L., 2009. Implications of uncertainty in a pretreatment dataset when estimating treatment effects in paired catchment studies: phosphorus loads from forest clear-cuts. Water Air Soil Pollut. 196, 251–261. doi:http://dx.doi.org/10.1007/s11270-008-9773-1. Lawler, D., 1993. The measurement of river bank erosion and lateral channel change: a review. Earth Surf. Proc. Land. 18 (9), 777–821.

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Lawler, D.M., Grove, J.R., Couperthwaite, J.S., Leeks, G.J.L., 1999. Downstream change in river bank erosion rates in the Swale–Ouse system northern England. Hydrol. Process. 13, 977–992. Linjama, J., Puustinen, M., Koskiaho, J., Tattari, S., Kotilainen, H., Granlund, K., 2009. Implementation of automatic sensors for continuous monitoring of runoff quantity and quality in small catchments. Agric. Food Sci. 18 (3–4), 417–427. Lisle, T.E., Church, M., 2002. Sediment transport-storage relations for degrading gravel bed channels. Water Resour. Res. 38 (11), 1219. doi:http://dx.doi.org/ 10.1029/2001WR001086. Manninen, P., 1998. Effects of forestry ditch cleaning and supplementary ditching on water quality. Boreal Environ. Res. 3, 23–32. Marttila, H., Kløve, B., 2008. Erosion and delivery of deposited peat sediment. Water Resour. Res. 44, W06406. doi:http://dx.doi.org/10.1029/2007WR006486. Marttila, H., Kløve, B., 2010a. Dynamics of erosion and suspended sediment transport from drained peatland forestry. J. Hydrol. 388 (3–4), 414–425. doi: http://dx.doi.org/10.1016/j.jhydrol.2010.05.026. Marttila, H., Kløve, B., 2010b. Managing runoff, water quality and erosion in peatland forestry by peak runoff control. Ecol. Eng. 36 (7), 900–911. doi:http://dx.doi.org/ 10.1016/j.ecoleng.2010.04.002. Marttila, H., Kløve, B., 2012. Use of turbidity measurements to estimate suspended solids and nutrient loads from peatland forestry drainage. J. Irrig. Drain. Eng. 138, 1088–1096. doi:http://dx.doi.org/10.1061/(ASCE)IR.1943-4774.0000509. Marttila, H., Kløve, B., 2014. Spatial and temporal variation in particle size and particulate organic matter content in suspended particulate matter from peatland-dominated catchments in Finland. Hydrol. Process. (In press) http:// dx.doi.org/10.1002/hyp.10221. Marttila, H., Kløve, B., 2014b. 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