Soil erosion and conservation on land cultivated and drained for afforestation

Download Soil erosion and conservation on land cultivated and drained for afforestation

Post on 11-Jun-2016

212 views

Category:

Documents

0 download

Embed Size (px)

TRANSCRIPT

<ul><li><p>HYDROLOGICAL PROCESSES, VOL. 7, 317-333 (1993) </p><p>SOIL EROSION AND CONSERVATION ON LAND CULTIVATED AND DRAINED FOR AFFORESTATION </p><p>P. A. CARLING AND M. S . GLAISTER institute of Freshwater Ecology, Ambleside, Cumbria LA22 OLP, UK </p><p>AND T. P. FLINTHAM </p><p>Department of Civil Engineering, City University, London, EC1 V OHB, UK </p><p>ABSTRACT Erosion of soil from pre-afforestation plough furrows has been measured on four soil types in Scotland for 12 to 18 month periods between 1987 and 1990. Rainfall-run-off was also measured at one site. Run-off is directly proportional to furrow length and rainfall intensity, and for a wide range of intensities (typically &gt; 6mm hr-') a small amount of soil is flushed out of the furrows. However, for furrow spacings of 3.8 m, a critical downslope run-off increment associated with significant soil loss is of the order of 25 cm3 s-' m-I , which is in accord with a storm of five years return period and a maximum intensity of 25mmhr-'. The total run-off volume for any hydrograph is commensurate with the total rainfall in the rainstorm - typically 40-80% by the hydrograph peak and approaching 100% by the end of the hydro- graph; i.e. long term storage is negligible. A positive relationship was recorded between furrow length, slope angle and sediment yield, with deposition predominating in furrows less than 30m in length on slopes less than a few degrees. Soil loss is proportional to the excess streampower expended by the run-off with an exponent in the range 1 - 1.5. For the soils examined, significant differences in soil loss when comparing sites for low power expenditure become undifferen- tiated at high power expenditures. For the rainfall regimes monitored, maximum soil losses were in the region of 40 kg per meter run-length of furrow, when soil peds were ripped from the bed. Laboratory data concerning the critical erosion threshold power and shear stress to erode soil peds are in general accord with the threshold furrow run-lengths defined using the field data for a five year storm and the soil losses observed. </p><p>KEY WORDS Forestry Drainage Furrows Soil erosion Erosion control Rainfall-run-off Sediment transport Buffer zones </p><p>INTRODUCTION </p><p>The usual means of preparing an upland site for afforestation in the UK is to cultivate and drain the area using a combination of downslope furrows and open drains close to the contour. Guidelines exist for the design of forest drainage networks (Pyatt, 1990; Forestry Commission, 1991) and the management of stream courses (Mills, 1980) which, if followed, minimize the likelihood of excessive slope erosion and sedi- ment entering water courses (Carling and Orr, 1990). However, some turbid run-off is inevitable (Robinson and Blyth, 1982; Burt er al., 1983) but should be kept within acceptable limits. Usually, within one or two years, the furrows have revegetated so that the period of potentially high turbidity is not prolonged, although drain erosion can remain a problem. Despite the availability of guidelines, occasions occur when turbid run-off has caused problems for the water supply industry (Austin and Brown, 1982; Stretton, 1984) and claims have been made that drainage damages salmonid fish stocks (Stewart, 1963; Graesser, 1979; Mills, 1985). </p><p>Within the UK it is well known that erosive rainstorms can occur at any time of the year (Coppin and Richards, 1990) and so it is impossible to plan drainage work completely to avoid high risk periods. </p><p>0885-6087/93/0303 17- I7$13.50 0 1993 by John Wiley &amp; Sons, Ltd. </p><p>Received 26 May I992 Accepted 24 August 1992 </p></li><li><p>318 P. A. CARLING, M . S. GLAISTER AND T. P. FLINTHAM </p><p>However, there is an opportunity to address the design of drainage networks rigorously using hydraulic principles of open-channel flow. In this paper, soil loss and run-off from furrows have been measured with the specific intention of validating an hydraulic drainage model specifying the spacing, length and gradient of cross-drains to limit potential furrow and drain erosion. </p><p>Perspective Areas to be afforested are prepared by ploughing furrows [or mole drains; see Robinson (1990) for a </p><p>description] downslope. An excess of run-off down the furrow is prevented by ploughing or excavating low gradient drains across the slope to intercept run-off and sediment (Figure 1). In effect, furrow dis- charge is prevented from building up to levels at which scour is excessive. The gradient and length of drains need consideration and sediment exiting from drains is trapped either in buffer zones of rank vegetation or is collected in pits which are emptied regularly. The schematic sediment transfer system is shown in Figure 2. Sediment pits and buffer zones provide a secondary means of controlling sediment output, but the design of a cross-drain network provides the primary means of sediment control. It is correct therefore to direct initial attention to drain spacing. Too long a run-length down a furrow between drains may result in furrow erosion, whereas drains spaced too closely are not cost effective. Guidance on the requisite spacing for cross-drains has been provided by Thompson (1979) and Pyatt (1990), but a rigorous examination of the problem is lacking. </p><p>In principle, if the rate at which run-off builds up downslope is known and the erodibility of the given soil type can be established, then a prescription can specify the critical run-length of furrows at which erosion is instigated or soil loss becomes unacceptable. This critical run-length can then be set equal to a recommended cross-drain spacing. Further consideration of drain hydraulics can provide recommended drain gradients and lengths. To have any validity, however, any theoretical model should be founded on measurements of run-off and more especially down-furrow erosion rates on a variety of soils. In this </p><p>A B </p><p>D C Figure 1. Definition diagram of a ploughed slope (a) on plane ABCD. Furrows of spacing Ware ploughed downslope until intercepted at distance X by drains running obliquely across the slope at angle 0. Sediment is trapped in traps or vegetated buffer strips. The </p><p>effective contributing area (shaded) for each furrow is equal to W </p></li><li><p>SOIL EROSION AND AFFORESTATION 319 </p><p>t Extt </p><p>Figure 2. Schematic sediment transfer network. Rectangular boxes indicate sediment sources and ellipsoids represent sediment control and storage. </p><p>paper, the field data used to scale a mathematical model of stable non-eroding drainage networks are reported. A companion paper details the mathematical model ( Flintham and Carling, this volume). </p><p>METHODS </p><p>Field data on furrow erosion were collected from three different soils (regarded by the Forestry Commis- sion as susceptible to erosion; Table I) between 1987 and 1989 and soil erosion and run-off were monitored </p><p>Table I. Site characteristics </p><p>Carron: Kintyre (NGR NR 936997); altitude about 250m Soil*: Iron stagnopodzols with 15cm of peat, very stony sandy silt loam texture with fragipan at 50cm depth, in </p><p>glacial till of Dalradian quartzite, phyllite and porphyry. Also stagno-orthic gley soils with no peat, moderately stony, gritty sandy loam texture, same parent material as above </p><p>Vegetation: MolinialRanunculuslJuncuslPotentiIla rough grassland Slopes: Monotonic 0.1900 and 0.2777 </p><p>Lambdoughty: Galloway (NGR NX 488970); altitude about 250 m Stagno-orthic gley soil with no peat, moderately stony, gritty, sandy loam texture with fragipan at 40cm depth, in glacial till of granite and Ordovician greywacke </p><p>Soil: </p><p>Vegetation: MolinialCallunalErica t./Potentilla Slopes: Monotonic 0.1039 and 0.1044 </p><p>Soil: Scotston: Perthshire (NGR NN 917437); altitude about 425 m </p><p>Mainly ferric stagnopodzols (some ironpan stagnopodzol and peaty stagno-orthic gley soil) with usually less than 5cm peat, moderately stony, sandy loam texture with fragipan at about 35cm depth, in glacial till of Dalradian mica schist </p><p>Vegetation: Slopes: </p><p>Soil: </p><p>Vegetation: MolinialJuncus a . &amp; s./Brachythecium Slopes: </p><p>CallunalVacciniwm m.lDeschampsia J &amp; c./Galium s. Monotonic 0.0750, 0.1250 and 0.2933 </p><p>Skible: Kintyre (NGR NR 896605); altitude about 150 m Stagno-orthic gley soil with up to 30cm peat, slightly stony, sandy loam texture, in glacial till of Dalradian mica schist </p><p>(see Figure 3) 0.2178, 0.2210, 0.2219, 0.2229, 0.2230 and 0.2259 </p><p>*Soil notation after Avery (1990). </p></li><li><p>320 P. A. CARLING, M. S. GLAISTER AND T. P. FLINTHAM </p><p>at a further site on a fourth susceptible soil type between 1989 and 1990, where mole drains were addition- ally considered. A total of 15 furrows were monitored in a similar fashion for erosion, and so only the method used at one site is detailed below. The latter site is situated on the north-west facing slopes of Cnoc nan Gabhar in Glen Skible, near Skipness in Kintyre. The soils here are stony surface water gleys of a loamy texture up to 70-t cm deep and underlain by Dalradian mica schist or till (Table I ) . Profiles are poorly drained and vegetation consists primarily of Molina and various species of Juncus. Three separ- ate blocks measuring approximately 200 m downslope by 70 m across the slope were isolated by cutoff drains above and below each site. Each site was selected for uniformity of slope and the slope profile of each furrow was surveyed. At Skible the slope of Block A was lo", whereas that of B and C was 12.5" (Figure 3; the average slopes for the other sites are given in Table I). As well as determining the overall slope profile, the local slope at each measurement station was also recorded and it is these latter data which are used for the hydraulic calculations. </p><p>Each block was divided into three strips and ploughed (on 8 and 9 August 1989) down the slope with a D45 and D45/T60 double mouldboard plough or mole plough, giving run-lengths of 200 m. Characteristic cross-sections are shown in Figure 4, where furrow depths ( D ) are 0-45 m and the plough-tine penetrates to 0-60 m ( T ) . Open furrows were spaced 3.8 m apart and mole drains were 2 m apart at a nominal depth of 0.35-0.40 m. Site inspection following ploughing showed that furrow 'turn-out' on block A was poor, with many furrows blocked by turf, and the site consisted to a large extent of shallow peat rather than mineral soil. Consequently this block was abandoned. On blocks B and C, three D45, three D45/T60 furrows and three mole drains (see Figure 4) were selected for measurement of erosion rates, giving nine monitored drainage lines in total. In each furrow three monumented cross-sections 0.5m apart were established at 40, 80, 120, 160 and 195m downslope. Reference pegs were also sited at intervals down the three mole drains. At each furrow station the cross-sectional area (Figure 5) was resurveyed every three months for an 18 month period using a profile frame with 50 vertical measuring rods at 20mm spacings. In each mole drain the depth of the slot was recorded using a single rod. Data for replicate cross-sections were averaged at each station for each survey, and cross-sectional areas calculated. Deposition or erosion along the length of a furrow was recorded as a loss or gain in the total cross-sectional area between surveys (Figure 6). </p><p>An additional D45 furrow was instrumented to record run-off. Ninety-degree V-notch weirs were installed at stations 40, 80, 120, 160 and 195m from the upper end of the furrow. Stations 40, 120 and 195 were instrumented with non-invasive ultrasonic water level recorders to give continuous water level (H) records. Technical problems meant that only four hydrographs were recorded in detail and hydro- graph peaks tended to be artificially flattened (see Figure 7), but water level records were calibrated for discharge (Q) using a rating curve derived from direct weighing of run-off over the weirs. </p><p>E l m t k n (m) bop. </p><p>Figure 3. Slope profiles at Skible. Zero evaluation datum is the base of slope c. 130 rn OD </p></li><li><p>SOIL EROSION AND AFFORESTATION 32 1 </p><p>Interfurrow Interfurraw Turf ridge </p><p>Interfurrow lnlsrf urraw </p><p>Turf ridge Turf ridge </p><p>l - 4 0.30m </p><p>Figure 4. Typical furrow and drain cross-sections. (a) D45 furrow; (b) D45/T60 furrow; (c) mole drain; and (d) cross-drain </p><p>An autographic rain gauge was positioned [ca. 180 m Ordnance datum (OD)] at the upper end of the run- off furrow for the period October 1989 to December 1990, so that a crude water balance could be calculated for individual storms at Skible. In addition, reference was made to daily rainfall totals for Meteorological Office stations at Skipness House (NGR NR 19086576; 24m OD) and Whitegates (NGR NR 18656876; 5m OD). For comparison, a hourly rainfall record was obtained at the Scotston site for the period July 1988 to June 1989. Finally, a double ring infiltrometer was used to try to measure infiltration rates in the bottom of furrows, but the till subsoil proved to be impervious: an identical result to that reported by Ledger and Harper (1987) for till near Penicuik. </p><p>Undisturbed soil samples at each site were obtained using lengths of box-section steel channel (0.5 m x 0.3 m x 0.1 m) inverted and hammered into the planar surface of exposed uneroded furrow sedi- ment. These samples were used to measure soil bulk density under natural soil moisture conditions and were then used to line the bed of an hydraulic flume for erodibility tests. In the latter instance no unique threshold for erosion can be defined. Low stresses may entrain loose sand from the surface but available sediment is soon exhausted and the bed does not degrade. Erosion of significance to the investigation was deemed to occur when soil peds were ripped from the bed and locally the bed degraded by at least 10mm. A combination of slopes and discharges was used to define the critical shear stress and stream- power erosion thresholds for each soil, although in some instances these were little differentiated. Further information is provided by Flintham and Carling (this volume). </p></li><li><p>322 P. A. CARLING. M. S. GLAISTER AND T. P. FLINTHAM </p><p>Figure 5 . (a) View down newly cut D45 furrow at Skible. Note bed cut in mineral horizon and peat sidewalls. (b) Bed of D45/T60 furrow showing slight erosion of central tine-slot. </p><p>RESULTS </p><p>Rainfall- run-ofl relationships Daily rainfall totals at Skible exceeded 40mm on some occasions throughout the year and 80mm at </p><p>Scotston, but storm intensities were generally low. The peak intensity recorded was about 15 mm hr-', but 6-8mmhr-' was common. For the four hydrographs recorded (e.g. Figure 7), the intensity did not exceed 4.5 mm hr-' . Nevertheless, the hydrograph response is rapid, peak flow occurring about 15-20 minutes after peak rainfall intensities of 3-4.5mmhr-'. The growth of the contributing area for the gauged furrow was not always linear over the full length of the furrow owing to lateral seepage away from the lower end of the slope during high rainfall intensities (Figure 8). With negligible rainfall (&lt; 1 mm hr-'), seepage induced a steady linear increase in the downslope increment of drainage (about 0...</p></li></ul>