Sediment in Streams and Reservoirs

APPENDIX A SEDIMENT IN STREAMS AND RESERVOIRS

1). Stream suspended sediment data and reservoir sedimentation rates E. Tipping, B.M. Simon and A.J. Lawlor Centre for Ecology and Hydrology, Windermere, Cumbria

Rough Sike This is a stream draining blanket peat at Moorhouse in the northern Pennines. It was sampled by Crisp in 19631,2. In the present study the stream was resampled and the dependence of suspended sediment concentration on discharge was compared with the results for 1963. There was no significant difference in the relationship. Thus there is no evidence for a change in erosion rate at the site. Because the grazing density of sheep at the site is restricted, a change in rate due to overgrazing would not be expected. The C, N and P contents of the suspended matter are highest at low discharge (low suspended solids concentration). The bulk of the exported suspended solids have C, N and P contents of ca. 150, 10 and 0.5 mg g-1 respectively. Thus the solids are approximately 30% organic matter, and the current exports of N and P associated with the solids are ca. 1.1 and 0.06 g m-2 a-1 respectively, based on Crisp’s particulate load of 112 g m-2 a-1. These estimates are similar to those of Crisp1 for 1963. The particulate exports of N and P correspond to ca. 60% and 85% respectively of the total loads of the two nutrients, calculated using data for dissolved N3, and P (supplied by the Environmental Change Network) Reservoir data Turbidity data were obtained from Yorkshire Water for Keighley Moor Reservoir (Southern Pennines), a lone reservoir fed from a single catchment, which is blanket peat moorland. It is an impounding reservoir with no built structures to control sediment inputs. Data for turbidity, from 1979 until 1997, vary considerably with time, showing the sensitivity of the system. Climatic variations, in particular droughts, may explain the higher turbidities observed in the second half of the 1980s and in 1995/1996, but otherwise the data reveal no evidence for a consistent temporal increase or decrease in erosion rate. Literature data for streams Erosion rates estimated from stream sediment data yield values in the range 55-112 g m-2 a-1 for blanket peat catchments in the Pennines2,4 and Wales5. Mineral soil catchments in the Pennines6,7 gave 12-28 g m-2 a-1. Erosion of peat when severe, is due to gullying, which exposes the mineral substrate. Most of the eroded material is therefore not organic matter. Only approximately 19% of the material eroded from a blanket peat catchment in the southern Pennines was organic matter3 (cf. the value of 30% for Rough Sike). Literature data for reservoirs Measurements of sediment contents of reservoirs were used to estimate sediment yields for a number of catchments in the southern Pennines8. The values, which are averages over 100-150 years, range from 3 to 400 g m-2 a-1. The high values are associated with blanket peats in the Southern Pennines. Similarly high values have been reported for blanket peats in the Midland Valley of Scotland9. Historical trends Erosion of peats became severe in the Pennines in the 18th century8, 10, due to exacerbation by human influence. In SE Scotland, significant erosion of peat began 300-400 years ago, possibly due to natural processes (e.g. storms, burning episodes, Little Ice Age) and human influence (burning, grazing)11. There is evidence of a ca. 3-fold increase in erosion rate over the last 50 years at Strines reservoir (southern Pennines, peat catchment)12.

APPENDIX A - SEDIMENT IN STREAMS AND RESERVOIRS Cost of erosion It has been estimated13 that erosion in Pennine catchments operated by Yorkshire Water has cost £74m, for the construction of new reservoirs to compensate for loss in storage capacity.

/ 2 g m

i 1 l o s

d 0 n e p s u

s 1998-99 ( -1 g o l 1963

-2 -1.5 -1 -0.5 0 0.5 log (discharge in cumecs)

Figure 1 Distribution of suspended solids in discharge from Rough Sike, Moor House National Nature Reserve.

References 1Crisp, D.T. (1966) Input and output of minerals for an area of Pennine moorland: the importance of precipitation, drainage, peat erosion and animals. J. Appl. Ecol. 3, 327- 348. 2Crisp, D.T. & Robson, S. (1979) Some effects of discharge upon the transport of animals and peat in a north Pennine headstream. J. Appl. Ecol. 16, 721-736. 3Adamson, J.K., Scott, W.A. & Rowland, A.P. (1998) The dynamics of dissolved nitrogen in a blanket peat dominated catchment. Environ. Pollut. 103, 69-77. 4Labadz, J.C., Burt, T.P. & Potter, A.W.R. (1991) Sediment yield and delivery in the blanket peat moorlands of the southern Pennines. Earth Surface Processes & Landforms 16, 265-271. 5Francis, I.S. (1987) Blanket Peat Erosion in Mid-Wales: Two Catchment Studies. Ph.D. Thesis, University of Wales (Aberystwyth). 6Carling, P.A. (1983) Particulate dynamics, dissolved and total load, in two small basins, northern Pennines, UK Hydrol. Sciences. J., 28, 355-375. 7Butcher, D.P., Claydon, J., Labadz, J.C., Pattinson, V.A., Potter, A.W. & White, P. (1992) Reservoir sedimentation and colour problems in southern Pennine reservoirs. J. Instn. Wat. & Environ. Mngmnt. 6, 418-431. 8Butcher, D.P., Labadz, J.C., Potter, A.W.R. & White, P. (1992) Assessment of catchment erosion in the southern Pennines, United Kingdom, using reservoir sedimentation monitoring. In Erosion and Sediment Transport Monitoring Programmes in River Basins, IAHS publ. No. 210, 443-454.

APPENDIX A - SEDIMENT IN STREAMS AND RESERVOIRS 9Duck, R.W. & McManus, J. (1990) Relationships between catchment characteristics, land use and sediment yield in the Midland Valley of Scotland. In Soil Erosion on Agricultural Land (Boardman, J., Foster, I.D.L., Dearing, J.A.) Wiley, Chichester, 285. 10Tallis, J.H. (1995) Climate and erosion signals in British blanket peats: the significance of Racomitrium lanuginosum remains. J. Ecol. 83, 1021-1030. 11Stevenson, A.C., Jones, V.J. & Battarbee, R.W. (1990) The cause of peat erosion: a palaeolimnological approach. New Phytol. 114, 727-735. 12White, P., Labadz, J.C. & Butcher, D.P. (1996) The management of sediment in reservoired catchments. J. Chartered. Instn. Water & Environ. Mngmnt 10, 183-189. 13White, P. Labadz, J.C. & Butcher, D.P. (1997) Reservoir sedimentation and catchment erosion in the Strines catchment, UK. Phys. Chem. Earth, 22, 321-328.

Acknowledgement: The UK Environmental Change Network (ECN) is a multi-agency organisation. Thanks are due to all the ECN site managers and their sponsors for permission to use their data.

2) Comparison of turbidity and flow from moorland and forest catchments at Plynlimon G. Leakes, CEH, Wallingford Monitoring of turbidity levels and flow data measurements within the Afon Cyff catchment (3.13 km2), Plynlimon, Wales, an unforested moorland/grassland tributary to the River Wye, provided a comparison with a forested tributary of the River Severn (the Tanllwyth catchment (0.89km2). Bedload fluxes recorded for both catchments in the 1990’s were significantly lower than long term averages (1973-1999), which were heavily influenced by major geomorphological events caused by floods in 1973 and 1977, rather than stocking rates and thus confirms the stability of the current Plynlimon channel forms. Suspended sediment comprises the major proportion of sediment flux in both the Tanllwyth and Cyff catchments: land use changes in both catchments may underlie this shift. In the Tanllwyth catchment sediment fluxes may now be affected by replanting following cropping of the first forest rotation in the early 1990’s. Sediment yields, and particularly suspended sediment yields, in the Cyff catchment have been influenced by the construction of a new road on the hillslopes within the catchment.

Table 1 Suspended sediment concentration data collected from the Cyff and Tanllwyth catchments, 1995 to 1999 Suspended sediment Nant Tanllwyth Afon Cyff fluxes (forested) (moorland/grassland)

Year 1995 1996 1997 1996 1997 1998 1999 Mean SSC (g l-1) 5.3 6.5 11.6 1.2 1.3 2.7 2.6 Max SSC (g l-1) 319.2 417.7 843.1 146.2 73.1 125.0 150.5 Total SL (tonnes) 21.5 39.0 25.4 16.7 7.6 32.07 38.67 SS yield (t km-2yr-1) 24.3 43.8 57.1 5.3 4.9 20.5 18.5

APPENDIX A - SEDIMENT IN STREAMS AND RESERVOIRS