Varyl R. Thorndycraft

The catastrophic floods of AD 1617 in Catalonia (northeast Spain) and their climatic context

Abstract This paper presents a reconstruction of the catastrophic floods of AD 1617 in Catalonia (northeast Spain). Compilation of archival data sources shows that heavy rainfall occurred from 2 to 6 November 1617 and that the resultant flooding caused severe damage throughout the region, including the destruction of at least 389 houses, 22 bridges and 17 water mills. Discharges of 2700–4500 m3 s−1 and ≤2000 m3 s−1 were estimated from dated palaeostage indicators for the Ter and Segre rivers, respectively, whilst 4680 m3 s−1 was calculated for the Llobregat River flood in a previous study (Thorndycraft et al., 2005). The magnitude of the 1617 floods of the Llobregat and Ter rivers exceeded the largest events of the instrumental data series (2300 and 2350 m3 s−1, respectively). The 1617 floods are also compared to the longer-term palaeoflood record, which shows that the largest floods in the region were associated with colder phases of climatic variability.

Prospects and challenges of simulating river channel response to future climate change

Due to the predicted impacts of future climate on hydrology, morphological changes to river channels are expected. Quantifying the magnitudes and rates of future channel change is important for sustainable river channel management. To date, reviews of simulation approaches for investigating river channels and the modelling of environmental change impacts on channel form and process have focused on contemporary process or palaeo perspectives. Hence, herein we review numerical modelling approaches available for reach-scale simulation of future river channels and the predicted in-channel hydro- and morphodynamic changes modelled. We found that despite their widespread availability, hydrodynamic, morphodynamic and cellular models have yet to be used routinely in future in-channel simulations, with cellular models in particular under-represented. Our review shows that predictions of within-channel changes vary greatly between hydro-climatic regions and under contrasting climate change scenarios, mainly due to varying input discharge scenarios; however, increased sediment transport and flood risk are usually predicted. Key challenges in simulating future channel change include representations of external forcing conditions, adequate temporal and spatial scales, transport equations, changing channel materials and lateral erosion; calibration and validation; simulation chains with multiple models; and identification of feedback systems and non-linearity. Nevertheless, despite these challenges, models with increasing complexity have recently been developed and so there is increasing potential in their application. One-dimensional hydro- and morphodynamic simulations, and cellular models, can be modified to reflect the requirements of future representations, such as grain size properties, whilst there is also now an increasing capability to include a greater quantity of external forcing conditions. Some studies, however, have demonstrated the need to develop two-dimensional models for application in centennial-scale studies. We recommend that a wider range of scenarios and the combined effects of multiple external forcing factors should be included, whilst studies are also needed from more hydrologically diverse reaches.

Bayesian age modelling applied to palaeoflood geochronologies and the investigation of Holocene flood magnitude and frequency

A new methodology, based on Bayesian age modelling, is presented for the analysis of palaeoflood geochronologies and palaeodischarge data. Bayesian age models were developed, using the Oxcal radiocarbon calibration software, for the geochronologies of three palaeoflood sites in Spain, namely the Gaudalentín, Tagus and Llobregat rivers in SE, Central and NE Spain, respectively. The age–depth plots resulting from the applied Sequence models enabled the construction of flood magnitude-frequency plots through substitution of the original stratigraphic depth data with the associated minimum discharge quantified by hydraulic modelling. The age models presented demonstrate that a Bayesian approach for analyzing Holocene flood magnitude and frequency prevents the loss of geomorphic and hydrologic information inherent in radiocarbon frequency methods previously used in the analysis of palaeoflood data sets. Frequency approaches do not allow proper consideration of flood magnitude information and only incorporate those geomorphic units specifically dated. The Bayesian age models calculate modeled ages for undated units as well, so that all the individual flood events identified in the field can be incorporated and visualized in the data output. The palaeoflood age models therefore illustrate: (1) the age range for clusters of palaeoflood events; (2) the number of events within each flood cluster, with an age estimate and palaeodischarge value for each event; and (3) the potential impact of discharge censoring on the record, for example the role of accommodation space infilling on the quantification of palaeodischarge. The methodology and results are briefly discussed within the wider context of fluvial palaeohydrology, in particular: (1) the role of Bayesian modelling in future fluvial palaeohydrology research; and (2) the value of bedrock gorge sites for investigating past flood–climate relationships, given the problems of deciphering allogenic and autogenic drivers in alluvial sedimentary records.

The glacial geomorphology of the Lago Buenos Aires and Lago Pueyrredón ice lobes of central Patagonia

ABSTRACT This paper presents a glacial geomorphological map of landforms produced by the Lago General Carrera–Buenos Aires and Lago Cochrane–Pueyrredón ice lobes of the former Patagonian Ice Sheet. Over 35,000 landforms were digitized into a Geographical Information System from high-resolution (<15 m) satellite imagery, supported by field mapping. The map illustrates a rich suite of ice-marginal glacigenic, subglacial, glaciofluvial and glaciolacustrine landforms, many of which have not been mapped previously (e.g. hummocky terrain, till eskers, eskers). The map reveals two principal landform assemblages in the central Patagonian landscape: (i) an assemblage of nested latero-frontal moraine arcs, outwash plains or corridors, and inset hummocky terrain, till eskers and eskers, which formed when major ice lobes occupied positions on the Argentine steppe; and (ii) a lake-terminating system, dominated by the formation of glaciolacustrine landforms (deltas, shorelines) and localized ice-contact glaciofluvial features (e.g. outwash fans), which prevailed during deglaciation.

Tracing the record of early alluvial tin mining on Dartmoor, UK

Abstract Although there has been considerable speculation, there is as yet no conclusive evidence to confirm Dartmoor as a tin producing region in prehistory. This is in part due to extensive tin mining on the moor during the Medieval period destroying evidence of earlier mining activity. This paper presents the preliminary results of an integrated mineralogical and geochemical geoarchaeological approach to identifying the record of early mining activity. Floodplain sedimentary successions, that have not themselves been mined, and are downstream of known areas of tin streaming, retain a geochemical record of the mining activities because the early tin streaming released large quantities of mine waste tailings. Where these successions can be dated, they provide an indirect means of testing the record of mining activity on Dartmoor. Geochemical and mineralogical analyses of datable fluvial sediments aimed to (a) distinguish between natural and mine contaminated sediments, and (b) to date the onset of sediment aggradation caused by tin mining activity in the headwaters. Results from the Avon Valley show negligible Sn concentrations in basal silts that are dated to pre-2845 ± 45 yrs bp. Overlying silts are enriched in Sn and were deposited after 1560 ± 40 ad, suggesting that the Sn enrichment corresponds with tin streaming in the 16th century. In the Erme Valley, significant Sn enrichment post-dates an organic-rich silt dated at 1280 ± 45 ad. At both sites there is sediment aggradation coupled with an enhancement in Sn concentration consistent with mine waste contamination as a result of Medieval tin streaming.

High resolution lake sediment record reveals self-organized criticality in erosion processes regulated by internal feedbacks

Reconstruction of high‐frequency erosion variability beyond the instrumental record requires well‐dated, high‐resolution proxies from sediment archives. We used computed tomography (CT) scans of finely laminated silt layers from a lake‐sediment record in southwest Oregon to quantify the magnitude of natural landscape erosion events over the last 2000 years in order to compare with palaeorecords of climate, forest fire, and seismic triggers. Sedimentation rates were modeled from an age–depth relationship fit through five 14C dates and the 1964 AD 137Cs peak in which deposition time (yr mm‐1) varied inversely with the proportion of silt sediment measured by the CT profile. This model resulted in pseudo‐annual estimates of silt deposition for the last 2000 years. Silt accumulation during the past 80 years was strongly correlated with river‐discharge at annual and decadal scales, revealing that erosion was highly responsive to precipitation during the logging era (1930–present). Before logging the frequency–magnitude relationship displayed a power‐law distribution that is characteristic of complex feedbacks and self‐regulating mechanisms. The 100‐year and 10‐year erosion magnitude estimated in a 99‐year moving window varied by 1.7 and 1.0 orders of magnitude, respectively. Decadal erosion magnitude was only moderately positively correlated with a summer temperature reconstruction over the period 900–1900 AD. Magnitude of the seven largest events was similar to the cumulative silt accumulation anomaly, suggesting these events ‘returned the system’ to the long‐term mean rate. Instead, the occurrence of most erosion events was related to fire (silt layers preceded by high charcoal concentration) and earthquakes (the seven thickest layers often match paleo‐earthquake dates). Our data show how internal (i.e. sediment production) and external processes (natural fires or more stochastic events such as earthquakes) co‐determine erosion regimes at millennial time scales, and the extent to which such processes can be offset by recent large‐scale deforestation by logging.

High resolution lake sediment record reveals self-organized criticality in erosion processes regulated by internal feedbacks: Drivers of erosion variability estimated by a high-resolution record

Reconstruction of high‐frequency erosion variability beyond the instrumental record requires well‐dated, high‐resolution proxies from sediment archives. We used computed tomography (CT) scans of finely laminated silt layers from a lake‐sediment record in southwest Oregon to quantify the magnitude of natural landscape erosion events over the last 2000 years in order to compare with palaeorecords of climate, forest fire, and seismic triggers. Sedimentation rates were modeled from an age–depth relationship fit through five 14C dates and the 1964 AD 137Cs peak in which deposition time (yr mm‐1) varied inversely with the proportion of silt sediment measured by the CT profile. This model resulted in pseudo‐annual estimates of silt deposition for the last 2000 years. Silt accumulation during the past 80 years was strongly correlated with river‐discharge at annual and decadal scales, revealing that erosion was highly responsive to precipitation during the logging era (1930–present). Before logging the frequency–magnitude relationship displayed a power‐law distribution that is characteristic of complex feedbacks and self‐regulating mechanisms. The 100‐year and 10‐year erosion magnitude estimated in a 99‐year moving window varied by 1.7 and 1.0 orders of magnitude, respectively. Decadal erosion magnitude was only moderately positively correlated with a summer temperature reconstruction over the period 900–1900 AD. Magnitude of the seven largest events was similar to the cumulative silt accumulation anomaly, suggesting these events ‘returned the system’ to the long‐term mean rate. Instead, the occurrence of most erosion events was related to fire (silt layers preceded by high charcoal concentration) and earthquakes (the seven thickest layers often match paleo‐earthquake dates). Our data show how internal (i.e. sediment production) and external processes (natural fires or more stochastic events such as earthquakes) co‐determine erosion regimes at millennial time scales, and the extent to which such processes can be offset by recent large‐scale deforestation by logging.