Last Wednesday 25th May 2022, The Royal Geographical Society (with IBG) organised a memorial event to celebrate Professor Ken Gregory legacy on Fluvial Geomorphology and Palaeohydrology. The organisation was led by Angela Gurnell and Catherine Souch (RGS). On the following link you may access the video of the event, with the participation of Ken Gregory’s family and relevant academic professors and scientists including Des Walling (Unv Exeter),Tony Brown (Unv. Southampton), Chris Jenks (GC), Heather Viles (BSG), Rita Gardner (RGS), among others.

http://www.rgs.org/geography/online-lectures/ken-gregory-memorial-event/

- A stratigraphic study of flood-related sediments enabled the reconstruction of significant flood records for two steep mountainous bedrock canyons in the southern Judean Desert.  

 - The record for the Ze'elim stream provided evidence of 22 floods with peak discharges of 100-900 m³s^-1 for a period of about 500 years. The largest flood of 900 m³s^-1, radiocarbon dated to 1980–1982 CE, was attributed a recurrence interval of <50 years, based on the systematic record only, and 150-200 years based on the combined systematic and palaeoflood records.

 - The palaeoflood record for Nahal Rahaf, include 13 floods, three of which were very large, exceeding 1100 m³s^-1. The radiocarbon ages of these floods are around 1667–1941 CE. These floods were deposited in a rock shelter (RAF-G), 7.3 m above the channel bed, which was found to overlie an Upper Palaeolithic archaeological site dated by radiocarbon to about 30,000 years BP.

 - In both streams, the FFA for the systematic data only, overestimated the frequencies of the large floods, most probably due to the short and discontinuous records. This was also true for other conventional frequency analyses applied by other agencies based on the systematic record alone.

 - The additional palaeoflood data extends the record to several hundred years, reducing the peak discharge values for different return periods and significantly enhancing FFA reliability.

- Sediments deposited in a small lake of the Basque Country holds a high-resolution record of the major floods that have affected the Basque Country over the last millennium.

- A total of 126 flood layers were identified over the last 1500 years. A subsequent textural and geochemical characterization of clastic lake sediments classified 50 layers as coarse detrital laminae (high- magnitude floods) and 76 fine detrital laminae (moderate-magnitude floods).

- The paleoflood record was completed with daily gauged flow records (2001-2020 CE) and documentary flood records from the Basque Rivers (1400-2000 CE) and Ebro headwater tributaries (1300-2000 CE). 

- The highest flood frequencies occurred between the 8-9^th centuries and during the last 500 years, showing a clear increase between 1830 and 1870 coinciding with the end of the Little Ice Age

- The documented flood-rich episodes correspond to periods of rapid climate change.

- Long-term paleoflood records demonstrated a high sensitivity of flooding to anomalous atmospheric circulation patterns occurring during climate transitions.

- Glacier outburst floods are a major hazard in glacierized catchments. In the Baker River (central Patagonia) at least twenty-seven outburst floods from Lago Cachet II occurred during 2008-2017 causing peak flows of the Río Baker to reach ~2000-4000 m3/s, some largest than any hydrometeorological flood.
- A study of paleofloods in the Baker River in central Patagonia reveals the occurrence of at least 89 extreme floods over the last ten millennia were even larger that recent glacier floods.
- The largest catastrophic outburst flood from Lago General Carrera occurred 9600 years ago, that reached 110,000 m3/s and ~70 in water depth, marking the final stages of the Late Glacial Interglacial Transition.
- After this megaflood, followed by five flood-phases coeval or post-dating Holocene neoglacials. Highest flood frequencies occurred at 4,200-4,400 years, with 26 floods of minimum discharges of 10,000-11,000 m3/s, and 600 years ago with 10 floods exceeding 4600-5700 m3/s. The largest modern outburst flood recorded surpassed ~3810 m3/s.
-Thus glacier flood magnitude declines two orders of magnitude over the Early to Mid Holocene and three orders when compared to the instrumental record

- Glaciers can block springs of karstic systems modifying the karst hydrology and cave sedimentation. Few clear examples wordwide have been studied over millenial scales with well-constrained chronology.

- In the central Pyrenees (Spain) a clear relation between the evolution of the Ara glacier and the deposition a 60-m thick fine sediment record deposited in the Granito cave during the Last Glacial Cycle was established.

- These clastic sediments are related to the blockage of the cave entrance by the glacier.

- Three stabilization glacial phases were recognized (OSL ages in moraines)  also observed in other Pyrenean valleys.

- IRSL ages from the detrital in-cave sequence reveal that it was formed between 71-26 ky (MIS 4-MIS 2), in agreement with the U/Th ages indicating speleothems growing pre (75 ky) and post (11 ky) the detrital sequence.

- Different techniques including mineralogy, texture analysis, stratigraphic descriptions and pollen analyses, have provided a multiproxy approach to understand past environmental changes.

This chapter reviews concepts and methodological approaches commonly used in fluvial geomorphology to understand and analyse flood hazards, spanning from catchment to reach spatial scales. Modern fluvial geomorphology applied to flood hazard studies has developed close links with hydrology and engineering to provide a holistic approach for flood hazard assessment.

Flood geomorphology is being applied to (i) extend the flood record into the past from sediments, (ii) hydro-morphological mapping of channel and floodplain landforms, (iii) analysis and quantification of morphodynamic processes such as channel migration and sediment transport in response to individual or sequential flooding, and (iv) understand natural processes at the landscape scale as a means of reducing flood risk by providing nature-based alternatives to conventional structural solutions.

General and specific approaches on the study of flood hazards are considered here for four different fluvial environments: mountain streams, bedrock rivers, alluvial fans, and alluvial rivers. Geomorphologic and stratigraphic signatures of floods are critical to understanding the linkages among climate change, environmental change, flood hydrology, and the geomorphic development of fluvial landscapes.

- Floods from failures of constructed dams may trigger exceptional peak flows.

- On the night of January 9th, 1959, the Vega de Tera dam failed releasing a peak flow of 13,000 m3 s-1. The flood killed 144 people into its way to Lake Sanabria (9 km downvalley).

- Unsteady flow modelling shows a peak flow attenuation to 7900 m3/s at the village of Ribadelago and to 5150 m3/s in Lake Sanabria.

- We observed a repeated spatial distribution of landforms along the stepped longitudinal profile (steep-gentle slope segments), in close relationship with changes in the hydraulic regime and energy flow conditions.

- The flood carried well-mixed waters into Lake Sanabria as a dense, high energy sediment-laden flow resulting in an underwater hyperpycnal current. This caused the deposition of a distinctive up to 10 cm silty layer within the lake.

- Most of the sediments mobilized from the upper river valley were re-deposited along the flood pathway and only a minor volume (10-20%) reached the lake.

- A major post-flood impact is the reduction in sediment connectivity between the river and the lake, as some new sedimentation areas (pools) were generated.

- The watershed and lake impact of the flood demonstrates the role of extreme floods as geomorphic agents and landscape evolution.

- Stochastic weather generators outputs relies upon the length and spatial distribution of the precipitation data series.

- In this work, we present a new approach for the estimation of extreme floods based on the continuous synthetic simulation method (GWEX) supported with inputs of:

(a) A regional study of extreme precipitation to improve the calibration of GWEX

(b) Non-systematic flood information (i.e., historical information and/or palaeoflood records) for the validation of the generated discharges with a fully distributed hydrological model (TETIS).

- The results showed that this complementary information of extremes allowed for a more accurate implementation of both the weather generator and the hydrological model.

- This, in turn, improved the flood quantile estimates with return periods higher than 50 years but also for higher quantiles (up to approximately 500 years).

- For first time, it was found that the past three decades (1990–2016 ) were among the most flood-rich periods in Europe during the past 500 years, and that this period differs from others in terms of its extent, air temperatures and flood seasonality.
- Regarding the temperature of the air, from 1500 to 1900, floods used to take place with higher frequency during cold climate phases, while after 1990, floods increased within the context of global warming.
 - The data analysis identified nine periods of floods that were more abundant and the associated regions. The most notable are 1560-1580 (western and central Europe), 1760-1800 (most part of Europe), 1840-1870 (western and southern Europe), and 1990-2016 (western and central Europe).
 - Current flood-rich phase is the third most severe regarding floods. However, this data is at the expense of the duration of the current phase of abundant floods, to be concluded.
- A present floods cause annual damages accounting for more than 100,000 million euros, and the general tendency of abundant floods is increasing.
- Due to the change in flood generating mechanisms, this study recommends the use of tools to assess the risk of floods that capture the physical processes involved, and management strategies that can incorporate recent changes in the risk analysis. 

Upper left: Flood-produced damages near Murcia after the 1879 Santa Teresa flood. American card.  Juan Almagro Roca, 1879 (Archivo General de la Región de Murcia).

Upper right: Detailed description of the damage caused by the Almanzora River in September 1580 in the town of Albox and the city Cuevas del Almanzora (Almería). Source: Historical files of Vera. Volume of the years 1550-1600. Photo: Carlos Sánchez-García, PaleoRisk Research Group, UB.

Bottom: Flood seasonality within and outside flood-rich periods. a, Time series of smoothed frequency of floods in four seasons (green line, spring; red, summer; brown, autumn; blue, winter) and flood-rich periods indicated by coloured bars. b, Frequency of floods in four seasons. Left bars, interflood periods; middle bars, flood-rich periods of the past; right bars, flood-rich period IX (1990–2016). Error bars show 90% confidence bounds (after Blösch et al., 2020).

LINKS TO SELECTED PRESS NEWS:

- https://www.csic.es/es/actualidad-del-csic/la-mayor-frecuencia-actual-de-las-inundaciones-es-excepcional-en-comparacion-con

- https://www.ub.edu/web/ub/es/menu_eines/noticies/2020/07/038.htm

- https://www.sciencedaily.com/releases/2020/07/200722112655.htm

-The influence of climate variability on flooding was evaluated by an analysis of a 500-year palaeoflood and documentary records at Montlleó River, eastern Spain.

- Five paleofloods exceeded 740–950 m3 s−1; vs. 129 m3s-1highest gauged flood s.1971.

- Flood occurred in cluster episodes (30-year duration) either during cooler than usual climate (about 0.3 °C and 0.2 °C), or during high climate variability (floods and droughts).

- Flood quantiles from palaeoflood data showed 30%–40% higher peak discharges than those using only instrumental records.

- These extended flood records can assist on low-regret actions for flood-risk adaptation.

We provide the first detailed reconstruction of catastrophic glacial lake outburst flooding (>110,000 m³s^-1) from the Southern Hemisphere, focusing on the Río Baker catchment in central Patagonia.

- We evaluate morphological changes, sediment tranfers and functional sediment connectivity caused by flood events.

- Quantification of morphological change and sediment budgets was provided by Photogrammetric flights.

- A sediment connectivity index is proposed.

- The 2004 flood (470 m3 s−1) in the hyperarid, ungauged Nahal Hatzera ephemeral stream (45 km2), transported and deposited 0.85–2.1 m concrete boulders.

- A 600 years paleoflood record of 23 floods with peak discharges of 200–760 m3 s−1, indicates an average return period of 120 years for this event. 

-The shear stress and stream power indicate that the moderate-large floods are the most geomorphically effective floods rather than the largest floods in Nahal Hatzera.

- The Kuiseb River flash floods  travel ~280 km downstream, before recharging the aquifers that sustain a 130-km long green belt across the hyperarid Namib desert.

-  This study reveals the importance of flood properties on the oasis life cycle and emphasizes the impact of drought and wet years.

- The downstream aquifers are fed only by the largest flood discharge that allows the infrequent germination of vegetation.

-Variations in NDVI index was revelead as an effective tool in determining occurrences of large floods in these headwater-fed hyperarid regions. 

- Alluvial chronologies in two contiguous valleys show out-of-phase sedimentation periods under low connectivity conditions (9-3 ka), and in-phase cut-and-fill cycles after valley re-connection (post-3.0 ka).

- Vertically stacked alluvial sequences were dominant during periods of low connectivity.

whereas stair-case fill terraces due to cut-and-fill cycles occurred on connected reaches

- During the late Holocene alluvial activity periods coincide with morpho-stratigraphic data from the lower Guadalentín, indicating that fluvial connectivity throughout the catchment was only completed in the late Holocene.