Channel planform change on the river dee meanders, 1876–1992

Abstract

Channel planform change was investigated along an 18 km section of the River Dee on the Welsh-English border by overlaying information from historical maps and air photographs. The information on river planform change spans the last 115 years, during which time the river has been subject to increasing flow regulation, which may have affected its planform. The downstream location of the reach provides two additional factors which may have an impact on the nature and rate of planform change through their influence on energy conditions in the reach: a tidal backwater influence on the downstream section of the reach; and a low angle of slope.The reach has shown very small changes in planform over the last 115 years, which have been successfully identified by a geographical information system (GIs)-based data handling methodology, which not only allows the estimation of a variety of indices of change, but also supports the estimation of the potential errors associated with registering the historical sources to a common base and digitizing the channel boundary locations. The study is successful in identifying channel planform change because it has utilized a GIs-based as opposed to a manual approach, but it represents the lower limit to which bank movement can be confidently identified in a low-power river environment from 1: 10000 scale sources.The changes identified by the GIS-based methodology include a decrease in channel mobility in a downstream direction; a predominantly oscillatory movement pattern in locations where channel movement has occurred; and an apparent propagation of a decrease in channel width downstream through the reach during the period since 1949, which is the main period of increasing flow regulation.KEY WORDS Tidal backwater Historical map sources Overlay analysis Geographical information systems Channel planform change

Character of channel planform change and meander development: Luangwa River, Zambia

“…These errors need to be taken into account when using the data for analytical purposes. Gurnell et al (1994) found errors of similar magnitude in a comparable study with the conclusion that differences in channel boundary positions in excess of 5 m are likely to be the result of true planform change rather than errors introduced by data handling. Strictly, bank erosion distances less than the maximum error should be ignored (equivalent to 1Á8 m a À1 ).…”

High-magnitude flooding across Britain since AD 1750

“…The River Dee's source is in Snowdonia on the eastern slope of Dduallt (the Black Hill); the river then flows down to Llyn Tegid (Lake Bala), a natural lake with an area of 1.6 km 2 , the largest natural water body in the Dee catchment, before flowing eastwards through a broad valley and the Vale of Llangollen, meandering northwards (Gurnell et al, 1994) through the Cheshire plain to its tidal limit at Chester Weir (NRA, 1993). Llyn Tegid has a long management history, with the level raised in the 1790s to support the Ellesmere Canal (constructed Thomas Telford) and subsequently for water resources, in the 1960s the original Telford sluices were bypassed and the lake level lowered, with new sluices constructed downstream at the confluence of the Afon (river) Tryweryn, this enabled 18 Mm 3 storage within Llyn Tegid, permitting up to 0.235 Mm 3 for daily abstraction and additional flood storage (NRA, 1993).…”

Aplikasi Sistem Informasi Geografis (Sig) Untuk Identifikasi Perubahan Sempadan Sungai Musi Di Kota Palembang (1922 - 2012)

“…Pemanfaatan informasi historis dalam bentuk peta, foto udara, citra satelit, dan survey dapat memberikan informasi kondisi masa lalu untuk penelitian geografi khususnya di bidang geomorfologi (Trimble and Cooke, 1991). Penelitian perubahan garis sungai juga telah banyak dilakukan (seperti Gurnell, et al, 1994;Gurnell, 1997;Li, et al 2007;Ollero, 2010;Brookes, 1994dan Sarma, et al 2007). Tingkat kedetailan untuk analisis perubahan morfologi sungai ini tergantung dari skala peta dan jumlah peta historis yang tersedia (Gurnell, 1997).…”

“…Pemanfaatan Sistem Informasi Geografis (SIG) dalam analisis dinamika sempadan sungai untuk skala waktu yang berbeda dapat dilakukan secara cepat, akurat, memiliki sejumlah kelebihan dibandingkan prosedur manual, serta mampu merepresentasikan beberapa tipe fenomena secara keruangan (Gurnell, et al, 1994;Goodchild, 2009). Hasil olahan data SIG ini juga dapat digunakan untuk analisis area terdampak akibat perubahan sungai, serta prediksi dampak yang ditimbulkan pada masa mendatang.…”