River channel planform changes in upland Scotland : with specific reference to climate fluctuation and landuse changes over the last 250 years

[摘要] Rates of river channel change in three contrasting Scottish uplandenvironments have been studied within the context of Climaticfluctuation and landuse changes over the last 250 years. The object ofthe research was to assess the spatial and temporal variation in channeltypes, the main controls on channel pattern and the dominant modes ofchannel adjustment. This was undertaken in a hierarchic framework withsites being investigated at three spatial scales.At a macro-scale, the spatial and temporal variation in channelpattern was evaluated through a random sample of river channel segmentsfor each study area, derived from the first and second editions of the1:10,560 0.S. maps plus the 1:10,000 third edition. Each channelsegment was classified within a map-based channel system typology,specifically constructed for upland Scotland. Measures of activitycollected for each sample incorporated sinuosity, braiding and lateralshift indices. Flood histories were reconstructed for each study areaon the basis of discharge records, long rainfall records andcontemporary accounts, to assess if there was any evidence for climaticchange, fluctuation or periodicities. Estimates of the recurrenceinterval of rainfall and runoff events of differing magnitude, frequencyand duration were assessed. Data, mainly of a qualitative nature, werederived from contemporary sources and estate plans to evaluate whetherany landuse changes could have changed the runoff regime and sedimentmobility within each catchment.At a meso-scale, 7 to 9 channel segments (already identified as"active" within the macro-scale study) were subject to a more detailedprocess-response analysis, using sequential aerial photographs. Finallyat a micro-scale, the unit stream powers at these sites were studied inrelation to specific runoff rates thereby relating channel process tochannel form.The strength of the controls on channel planform type varied indegree with the area studied. The glacial legacy, the positioning oflocal baselevels and sediment size were found to be dominant controls.In terms of channel dynamics, the position of the Channel planform inrelation to process thresholds and the existence of a quasi-equilibriumcondition were both very important. In terms of process-response, thefollowing general observations hold true. An extreme event of high RI(>100 years) will have a major disruptive impact if there is room forexpansion of the channel system and providing thresholds for sedimenttransport are exceeded. If these thresholds are high, the fact that thechannel has not recently been disrupted may also be important. Themodes of expansion across the active area depend on the type of channelinvolved. Different study areas have different types of Channelpattern present and thus a greater likelihood of certain types ofplanform adjustment. The role of more moderate events (10-50 years)varies principally with sediment size and Channel slope. Small-scalemodification may take place where stream powers associated with moremoderate events exceed competence thresholds.It was found that process rates were highly variable in both timeand space and that present rates were not necessarily representative ofthe past 250 years. Even within this timespan, there have been periodsof increased activity in response to increased discharges of moderatemagnitude (eg. 1870s-1880s within the Dee study area) and randomextreme magnitude floods (eg. between 1948-1956 in the Tweed studyarea). The impact of landuse change, especially in relation to sedimentmobilisation (Dee and Spey study areas), and speed of runoff (Tweed studyarea) also appeared to be important.

[发布机构] University:University of St Andrews;Department:Earth & Environmental Sciences and Geography & Sustainable Development (Schools of)