Bar deposition in glacial outburst floods: scaling, post-flood reworking, and implications for the geomorphological and sedimentary record

The appearance of a flood deposit in the geomorphological and sedimentary record is a product of the both the processes operating during the flood, and those that occur afterwards and overprinting the deposit with a record of ‘normal’ processes. Nearly half of the total discharge of the November 1996 jökulhlaup on Skeiðarársandur was discharged through the Skeiðará river. The flood deposits have been extensively reworked since, up until 2009 when the channel was abandoned, effectively leaving the Skeiðará as a terrace, when retreat of Skeiðarárjökull directed meltwater to the adjacent Gígjukvísl river system. This paper describes the creation and modification of jökulhlaup barforms in the Skeiðará river, relating the changes to post-flood fluvial processes and glacier retreat. Large compound bars formed from the amalgamation of unit bars up to 1.5 km long. The location of the compound bars was governed by the macro-scale topography of the flood channel, and their size by upstream channel width in accordance with bar-scaling theory. Jökulhlaup bars are therefore scale invariant and formed in a similar fashion to braid bars in non-jökulhlaup braided rivers. Post-flood fragmentation and reworking of the bars consistently increased the length-width ratio of preserved bar fragments from approximately two and one half to over five. These observations increase our understanding of the preservation potential and final form of jökulhlaup deposits and provide the basis for an improved model for the recognition of jökulhlaup deposits in the geomorphological and sedimentary record

The Evolution of Corryong/Nariel Creek since European Settlement: Implications for On-going Management Prioritisation

Geomorphological stability is a useful starting point to inform river management priorities, as it is critical to other river health parameters such as ecology and water quality. A key debate in channel stability is the relationship between climate and human activity. Corryong Creek is an ideal setting to study the interaction between climate and anthropogenic changes on channel evolution as it has experienced significant levels of both. Catastrophic floods have been induced by high rainfall, the floodplain has been completely cleared, the riparian zone is almost entirely invaded by willows, and every reach of the channel has experienced some form of channel modification. The impacts of both climatic and anthropogenic factors are visible in our channel change data, although at different spatiotemporal scales. Higher flows during La Niña resulted in channel widening while lower flows during El Niño resulted in channel narrowing. In addition, land clearing had caused the river to evolve into a higher-energy, straighter channel, while spatially variable and temporally irregular factors such as river engineering, willow density and stock trampling tended to intensify erosion on a reach scale. As our analysis shows that periodic increases in erosion during La Niña are expected, the local community needs to first accept and adapt to some level of channel erosion in order to avoid catastrophic damage during floods. As the second priority, since the reversibility of these factors are limited, erosion risk can be mitigated through strengthening willow management, limiting river engineering, practicing bushfire management, and fencing the riparian zone

Impact of large instream logs on river bank erosion

There has been abundant research into the effect of tree roots on stabilizing river banks, and also on the effect of trees on bed-scour after they have fallen into the stream, but there is little research into the effect of instream logs on bank erosion. Here we develop the hydraulic theory that predicts local and reach scale bank erosion associated with instream logs with various configurations and distributions and conclude that individual log can increase local bank erosion, but multiple logs can reduce overall reach erosion. Where there is consistent bank strength, the local erosion varies in a non-linear way with the angle, size and position of the log. The reach scale effect of multiple logs depends on the distribution of logs and the proportion of the reach occupied by logs. Erosion effects of instream logs are difficult to measure. We are testing the above theory of erosion associated with instream logs in a series of anabranches of different sizes that experience consistent irrigation flows each year (on the Murray River in SE Australia). These channels have high erosion rates, abundant logs, and are like a giant flume that allows us to measure erosion processes, as well as hydraulics, in a controlled setting

Iceberg jam floods in Icelandic proglacial rivers: testing the self-organized criticality hypothesis

In this paper, we describe a fluvial marginal process associated with the formation of iceberg jams in Icelandic proglacial lakes. The floods triggered by the release of these iceberg jams have implications for the geomorphic evolution of the proglacial fluvial system. The process of iceberg jam floods share some conceptual characteristics with Self-Organized Criticality (SOC) approach of complex systems. Using a simple numerical model and field observations, we test the hypothesis that iceberg jam floods exhibit SOC. Field observations and aerial photo-interpretations in southeastern Iceland demonstrate the occurrence of icebergs jam in ice-contact lakes. The mapping of the south Vatnajökull margins between 2003 and 2012 reveals an increase of the calving potentiality and a rise in the likelihood of iceberg jam flood occurrence. Based on the results of the numerical model and field observations, we suggest that iceberg jam floods should be recognized as a SOC phenomenon. Analysis of the simulated time-series show that the iceberg jam floods become less frequent and more similar in magnitude over time. This global trend is related to the gradual enlargement of the lake outlet channel

The impact of dams on floodplain geomorphology: are there any, should we care, and what should we do about it?

We undertook a review of the potential for dams to impact floodplain geomorphology, using both a conventional literature review and a systematic review using ‘causal criteria’ analysis. The literature review identified potential impacts on overbank flooding, scour and sedimentation, within-channel bank erosion, meander migration and cutoff frequency, and avulsion characteristics and frequency. The temporal scale of impacts ranged from years and decades, through to millennia. The causal criteria analysis indicated that with the exception of reduced meander migration rates, most impacts had been too poorly documented to be confident of their impact at present. We identify a distinction between ‘passive impacts’ (floodplain disconnection) and ‘active impacts’ (changes in geomorphological processes and functioning). Dams do impact floodplain geomorphology: many of the impacts will be subtle, and over very long timescales (1000s of years), but altered overbank sediment loads have the potential to change patterns of scour and deposition across floodplains. Further research is needed that specifically seeks to identify the impacts of dams on floodplain geomorphology, hydrology-geomorphology-vegetation interactions, and floodplain ecological response. Given the practical constraints on overbank environmental flow releases, there is relatively little that can be done to mitigate dam impacts on floodplain geomorphology. The main options include using within-channel flows to maintain meander migration and partial floodplain connectivity. We suggest that the major action should be that once dams come online, efforts should be made to prevent channel enlargement through scour, channel widening and wood removal, so that geomorphological processes can fully reestablish immediately once the dam ceases to operate

The Potential for Dams to Impact Lowland Meandering River Floodplain Geomorphology

The majority of the world’s floodplains are dammed. Although some implications of dams for riverine ecology and for river channel morphology are well understood, there is less research on the impacts of dams on floodplain geomorphology. We review studies from dammed and undammed rivers and include influences on vertical and lateral accretion, meander migration and cutoff formation, avulsion, and interactions with floodplain vegetation.The results are synthesized into a conceptual model of the effects of dams on the major geomorphic influences on floodplain development.This model is used to assess the likely consequences of eight damand flow regulation scenarios for floodplain geomorphology. Sediment starvation downstream of dams has perhaps the greatest potential to impact on floodplain development. Such effects will persist further downstream where tributary sediment inputs are relatively low and there is minimal buffering by alluvial sediment stores.We can identify several ways in which floodplains might potentially be affected by dams, with varying degrees of confidence, including a distinction between passive impacts (floodplain disconnection) and active impacts (changes in geomorphological processes and functioning). These active processes are likely to have more serious implications for floodplain function and emphasize both the need for future research and the need for an “environmental sediment regime” to operate alongside environmental flows

Interaction of ENSO-driven Flood Variability and Anthropogenic Changes in Driving Channel Evolution: Corryong/ Nariel Creek, Australia

This is an Accepted Manuscript of an article published by Taylor & Francis in Australian Geographer on 03/09/2015, available online: 10.1080/00049182.2015.1048595Understanding the relative contributions of climatic and anthropogenic drivers of channel change are important to inform river management, especially in the context of environmental change. This global debate is especially pertinent in Australia as catchments have been severely altered since recent European settlement, and there is also strong evidence of cyclical climate variability controlling environmental systems. Corryong/Nariel Creek is an ideal setting to further study the interaction between climate and anthropogenic changes on channel evolution as it has experienced both significant periods of flood and drought, controlled by the El Niño Southern Oscillation (ENSO), and extensive anthropogenic changes. Since European settlement the floodplain has been completely cleared, the riparian zone almost entirely invaded by willows, and every reach of the channel has experienced some form of direct channel modification. Through the combined analysis of channel evolution, climate changes and anthropogenic history of the river it was found that both the ENSO-driven climate and anthropogenic drivers are significant, although at different scales of channel change. Significant straightening in response to land clearing in the early twentieth century occurred before any records of direct channel modifications. Following this, most river management works were in response to instabilities created in the clearing period, or to instabilities created by flooding triggering a new phase of instability in reaches which had already undergone stabilisation works. Overall, human activities triggered channel instability via land clearing, and management works since then generally exacerbated erosion during high flows that are driven by climate fluctuations. This research raises the interesting question of whether rivers in Australia have become more responsive to the ENSO cycle since the clearing of catchment and riparian vegetation, or whether the past response to climate variability was different

Icelandic jökulhlaup impacts

Iceland contains the worlds largest and best-documented active glacial outwash plains or sandar that have been studied since the 19th century. Vigorous subglacial volcanic activity and the presence of numerous ice-dammed lakes, make Iceland the prime location for the study of glacier outburst floods or jökulhlaups and their geomorphological and sedimentological impact. Increasing attention is being focused on large jökulhlaup channels related to both modem and ancient processes. Jökulhlaup impact within Icelandic bedrock channels has so far received little attention despite the fact that such channels are abundant as sandar in Iceland. Despite clear descriptions within Icelandic literature of jökulhlaup impact on glacier margins, there have been attempts to link jökulhlaup feeder system dynamics with processes and products in the proglacial outwash system. This chapter presents the latest research on the impacts of jökulhlaups generated by three main mechanisms: volcanic, ice-dammed lake drainage, and glacier surge. This chapter identifies and discusses the main advances in the understanding of jökulhlaup impact in Iceland