Ordinary Floods Research Articles

Assessing climate change impact on flood discharge in South America and the influence of its main drivers

A warmer atmosphere is able to hold more water which consequently intensifies the hydrological cycle. The projected increase in extreme precipitation has been associated with greater floods; however, most recent studies have argued that the reduced soil moisture could be causing the opposite effect. We aim to understand how the hydrometeorological variables affect flood discharge and what the projections are for South America, a vulnerable continent that has been barely studied regarding flood trends. We used climate data from Eta simulations nested in 4 global climate models (BESM, CanESM2, HadGEM2-ES, MIROC5) as input for the MGB-SA hydrological model to yield flood discharge estimates. Then we were able to project the climate impacts on extreme precipitation, antecedent soil moisture, and flood discharge for large rivers (>1,000 km2) and understand how these variables are related. Our results showed a strong sign that antecedent soil moisture is expected to be reduced in most of the continent except in Southeastern South America (SESA). On the other hand, there are mixed signs for rarer precipitation and a clear spatial pattern for 2-year precipitations (RP2), which is expected to increase on the SESA and west Amazon and decrease on Central South America (CSA). For basins>100,000 km2, results indicate a negative change sign for 2-year precipitations, meaning that rainfall events that generate ordinary floods in large South American rivers are expected to decrease in the XXI century due to climate change. The change signs for flood discharge and extreme precipitation are spatially similar but more basins show a decrease for flooding than for rainfall. While only half of the South American basins are expected to present reduced 2-year precipitations, nearly 70 % of the rivers present a negative sign for 2-year floods, which can be attributed to the reduced antecedent soil moisture.

ANÁLISE DA SÉRIE TEMPORAL SENTINEL 1 DE INUNDAÇÕES NA AMAZÔNIA CENTRAL

This study aimed to analyze the dynamics of the flooded areas of the Sentinel 1-SAR time series in a section of the Central Amazon between September 26, 2016, and February 8, 2020. The total of images was 59 for each polarization. In addition, the study calculated the average ordinary flood line (ALOF) from the heights of the fluviometric rulers between the years 1967 to 2020 and compared it with the values present in the radar time series. The pre-processing of the Sentinel-1 time series in the VV and VH polarizations used the following methodological sequence: Apply Orbit File, Radiometric Calibration (σ0), Range-Doppler Terrain Correction, Speckle Filter, and conversion to decibels (dB). The previous analysis of the adaptive filters showed different results for the two polarizations, obtaining the best result for the VV polarization using the Frost filter with 3x3 and the VH polarization with the Lee filter 3x3. The extraction of water bodies and wetlands used a threshold value, making masks for the entire period. The most considerable extent of the floodable area occurred on June 17, 2019, with 6,611.86 km2, representing 16.42% of the SAR scene in the VH polarization and 6,443.19 km2, representing 16.10% of the SAR scene in the VV polarization. The relationship between the VH and VV wetlands to the ruler's height was satisfactory, with coefficients of determination (R2) of 0.79 in the VH polarization and of 0.64 in the VV polarization and a p-value less than 0.05. Keywords: Remote sensing; Radar; Mapping of Water Bodies.

Temporary flood marks proposal: What we learned after losing the baroque artifact from Cracow, Poland

For centuries flood marks have been used as a way to preserve the memory of extreme events. In many cities around the world they are considered a cultural heritage and a reminder of historic flood disasters. Their timeless message has endured the ravages of nature over the centuries, but without adequate protection, they risk being irretrievably lost, and with them an important historical message. In Cracow, 21 such artifacts have survived to this day, with the oldest dating back to the 16th century. Yet, the fundamental problem relates to their lack of cataloging and surveying to date. As part of the presented project, we surveyed the preserved marks and recreated the exact location of a lost baroque flood mark using the photogrammetric method, based on preserved photos and images available on Google Street View. Subsequently, this data was uploaded into the public database of the Open Hydrology project. In order to facilitate the reconstruction of flood information in the future, we propose an innovative way of using smartphones to register temporary flood marks. The conclusion from tests on several popular smartphone models suggests the proposed guidelines allow for an inexpensive and quick creation of flood marks with an accuracy of 10 mm. The data collected in a digital format can be used to raise public awareness of floods and is easy to disseminate. Historical flood marks are usually represented in the form of plaques, but are difficult to produce on a mass scale, yet appeal to the imagination. Their digital substitutes can be temporary flood marks photographed with smartphones, so their associated cost is negligible. Furthermore, because they are taken immediately after the flood and not during it (unlike ordinary flood photography), they do not pose a threat to people's safety. The method proposed in this article takes the form of simple guidelines and can be successfully applied for both the quick creation of new temporary flood marks, and for documenting traditional historical marks.

Challenges of defining the floodplain through the “mean ordinary flood line” approach using remote sensing in Brazil: a case study of the São Francisco River

Abstract The “Mean Ordinary Flood Line” (MOFL) is a conceptual line adopted by Brazil’s Federal Government to delineate land within the floodplain under its ownership and jurisdiction having major social implications. Past attempts at the cartography of this line have encountered strong difficulties brought either by a low precision or an excessive cost. In this article, we propose a method based on historical water gauge data to determine the water level corresponding to the MOFL. Satellite images coincident with past dates when the MOFL was reached are selected and processed to extract the water surface from which the MOFL can then be produced. The approach was implemented in a 600 km reach of the São Francisco River in Minas Gerais as a pilot project. A field survey served to validate the results. The positional accuracy of the MOFL was estimated at 24 m which was considered excellent since mostly Landsat images with a spatial resolution of 30 m were used.

Enhanced flood hazard assessment beyond decadal climate cycles based on centennial historical data (Duero basin, Spain)

Abstract. Current climate modelling frameworks present significant uncertainties when it comes to quantifying flood quantiles in the context of climate change, calling for new information and strategies in hazard assessments. Here, state-of-the-art methods on hydraulic and statistical modelling are applied to historical and contemporaneous flood records to evaluate flood hazards beyond natural climate cycles. A comprehensive flood record of the Duero River in Zamora (Spain) was compiled from documentary sources, early water-level readings and continuous gauge records spanning the last 500 years. Documentary evidence of flood events includes minute books (municipal and ecclesiastic), narrative descriptions, epigraphic marks, newspapers and technical reports. We identified 69 flood events over the period 1250 to 1871, of which 15 were classified as catastrophic floods, 16 as extraordinary floods and 38 as ordinary floods. Subsequently, a two-dimensional hydraulic model was implemented to relate flood stages (flood marks and inundated areas) to discharges. The historical flood records show the largest floods over the last 500 years occurred in 1860 (3450 m3 s−1), 1597 (3200 m3 s−1) and 1739 (2700 m3 s−1). Moreover, at least 24 floods exceeded the perception threshold of 1900 m3 s−1 during the period (1500–1871). Annual maximum flood records were completed with gauged water-level readings (pre-instrumental dataset, PRE: 1872–1919) and systematic gauge records (systematic dataset, SYS: 1920–2018). The flood frequency analyses were based on (1) the expected moments algorithm (EMA) and (2) the maximum likelihood estimator (MLE) method, using five datasets with different temporal frameworks (historic dataset, HISTO: 1511–2018; PRE–SYS: 1872–2018; full systematic record, ALLSYS: 1920–2018; SYS1: 1920–1969; and SYS2: 1970–2018). The most consistent results were obtained using the HISTO dataset, even for high quantiles (0.001 % annual exceedance probability, AEP). PRE–SYS was robust for the 1 % AEP flood with increasing uncertainty in the 0.2 % AEP or 500-year flood, and ALLSYS results were uncertain in the 1 % and 0.2 % AEP floods. Since the 1970s, the frequency of extraordinary floods (>1900 m3 s−1) declined, although floods on the range of the historical perception threshold occurred in 2001 (2075 m3 s−1) and 2013 (1654 m3 s−1). Even if the future remains uncertain, this bottom-up approach addresses flood hazards under climate variability, providing real and certain flood discharges. Our results can provide a guide on low-regret adaptation decisions and improve public perception of extreme flooding.

Sediment–water flows in mountain catchments: Insights into transport mechanisms as responses to high-magnitude hydrological events

Sediment transfer in mountain streams occurs via processes classified as debris flows, hyperconcentrated flows, debris floods, and water flows under the control of the water energy and the amount of involved sediment. Despite the extensive documentation of the channel changes caused by high-magnitude hydrological events, the analysis of the sediment–water flows occurring during such floods is currently a minimally explored issue. This study investigated how the transport mechanisms activated in a mountain stream during a high-magnitude flood differ from those triggered during ordinary floods. It also evaluated the effectiveness of three morphometric approaches in predicting high-magnitude flows expectable in a channel sub-reach. The study area is the Tegnas catchment (Dolomites, Italy), a mountain basin affected in 2018 by a severe hydrological event (Vaia Storm), whose recurrence interval is approximately 200 years. We determined the transport processes typifying the stream network in the catchment during ordinary floods through field surveys and direct monitoring and compared these flows with the high-magnitude flow types that occurred during the Vaia Storm. Additionally, we examined the flows predicted using the morphometric approaches for high-magnitude events. We observed water flow as a response to ordinary events occurring along the entire Tegnas main stem, whereas debris flow ordinarily determines sediment transfer at steep tributaries. During the Vaia Storm, water flow still dominated along the Tegnas Torrent, although debris flows and debris floods were also documented at several sub-reaches of the main stem. The morphometric approaches satisfactorily predicted debris flows but often failed to recognize the debris floods occurred during the high-magnitude 2018 flooding. The analysis of different flow types enabled us to infer relationships among transport mechanisms, hydraulic forcing, and channel dynamics and to gain new insights on the poorly explored debris flood processes. Water flows transitioned into debris floods under unit stream powers exceeding the threshold of 5000 to 5500 W m−2 or downstream of sediment-injection points. The occurrence of debris floods, which caused higher channel widening than that induced by water flows, appeared to be facilitated by the presence of tributaries prone to debris flow occurrence connected to a receiving stream, the injection of fine material into the flows, and channels characterized by high slope and narrow section. The morphometric approaches enabled adequate first-order discrimination of expectable high-magnitude flows, but a more detailed characterization that includes field observations is necessary to understand the transport mechanisms that can affect a specific channel site during high-magnitude hydrological events for a more accurate and reliable definition of flood hazard at the local scale.

Wood blockage and sediment transport at inclined bar screens

Semi-permeable sediment traps are designed to retain sediment during large floods and enable sediment transport continuity during ordinary floods by combining two barriers with a guiding channel. During floods, not only sediment but also large wood (LW) can be transported, that may block the inclined bar screen (mechanical barrier) and inhibit sediment transport. The design of the inclined bar screen has not been tested with LW. We conducted a first set of simplified flume experiments to study the effect of (1) initial conditions, (2) bar screen design, and (3) LW volume on sediment transport. First results demonstrate that a LW volume blocking 20% of the flow cross-section is sufficient to reduce sediment transport by 50%. Hence, LW volume is a relevant parameter for the design of inclined bar screens. We provide future research ideas to enable the local separation of LW and sediment.

Coarse sediment transfer and geomorphic changes in an alpine headwater stream

Headwater sediment dynamics is very important for fluvial processes in downstream reaches, but have been poorly studied until now, and existing sediment processes are not well understood. In fact, quantifying bedload transport and assessing river changes is challenging, in mountain environments, and traditional bedload formulas tend to overestimate sediment fluxes. This work aims to analyze the coarse sediment transfer processes acting in the high-altitude headwater reach of Rio Cordon (eastern Italian Alps). Pebbles provided by Passive Integrated Transponders (PITs) were deployed in the study site and their position was periodically monitored over 20 months, in order to quantify sediment mobility (i.e., travel distances and virtual velocities) and, the bedload volumes. The geomorphic changes due to flood events were analyzed through high resolution DEMs (0.05 m cell), derived with Structure from Motion (SfM) in three different periods (October 2017, September 2018 and June 2019), allowing to compute the DEMs of Difference (DoDs). The study period was characterized by two distinct flood events, a short and intense summer rainstorm in August 2018 (recurrence interval RI > 2 years) and a severe cyclonic rainfall event (i.e. Vaia storm, RI > 50 years) in October 2018. The combination of tracer and DoDs analysis shows that ordinary flood maintained the reach in an apparent equilibrium state, with balanced volumes of erosion and deposition. Differently, the high-magnitude flood was characterized by bedload fluxes an order of magnitude higher than ordinary event and an evident net erosion. However, in both events the volumes mobilized in the headwater did not contribute significantly to the sediment fluxes delivered at the Rio Cordon basin outlet. This highlighted the reduced efficiency in sediment propagation mainly due to a marked dis-connectivity associated with the presence of a hanging valley in the middle part of the basin. This study discusses the complex dynamism of headwater streams, stressing the importance of understanding sediment fluxes and connectivity conditions that can strongly affect human activities and infrastructures even in lowland reaches.

Seven decades of hydrogeomorphological changes in a near‐natural (Sense River) and a hydropower‐regulated (Sarine River) pre‐Alpine river floodplain in Western Switzerland

ABSTRACTHydropower alteration of the natural flow and sediment regime can severely degrade hydromorphology, thereby threatening biodiversity and overall ecosystem processes of rivers and their floodplains. Using sequences of aerial images, we quantified seven decades (1938/1942–2013) of spatiotemporal changes in channel and floodplain morphology, as well as changes in the physical habitats, of three floodplain river reaches of the Swiss pre‐Alps, two hydropower‐regulated and one near‐natural. In the Sarine River floodplain, within the first decades of hydropower impairment, the magnitude and frequency of flood events (Q2, Q10, Q30) decreased substantially. As a result, the area of pioneer floodplain habitats that depend on flood activity and sediment dynamic, such as bare sediments, decreased dramatically by approximately 95%. However, by 2013 vegetated areas had generally increased in comparison to the pre‐regulation period in 1943, indicating general vegetative colonization. Between 1943 and 2013, the active channel underwent essential narrowing (up to 62% width reduction in the residual flow reach) and habitat turnover rates were very low (5% of the total floodplain area changed habitat type five to six times). In contrast, from the 1950s onwards, the near‐natural floodplain of the Sense River experienced recurrent narrowing and widening, and frequent changes between bare and vegetated areas, reflecting the shifting habitat mosaic concept typical for natural floodplains. In the three reaches investigated, we found that the active floodplain width and erosion of vegetated areas were primarily controlled by medium to large floods (Q10, Q30), which combined with reduced time intervals between ordinary floods ≥ Q2 most likely mobilized streambed sediments and limited the ability of vegetation to establish itself on bare gravel bars within the parafluvial zone. These findings can contribute to restoration action plans such as controlled flooding and sediment replenishments in the Sarine and other floodplain rivers of the Alps. © 2020 John Wiley & Sons, Ltd.

The elusive topic of rainfall-induced surge waves in rivers: lessons from canyon accidents

ABSTRACT Rainfall-induced surge waves in rivers are still today considered as an elusive and non-realistic phenomenon that has continued to mystify researchers and practitioners alike since the kinematic shock theory introduced in 1955. This is mainly due to scarce sightings of real cases, as well as physical and numerical uncertainties and sporadic calculations. In order to overcome such indefiniteness, a first set of documented real cases of surge waves in canyon rivers has been collected and their dreadful consequences highlighted. Then, the physical mechanism of surge wave development and propagation and the role of wave celerity differences are examined. A novel parameter (relating wave celerity and cross-section geometry) is introduced and used to highlight few relevant characteristics of rainfall-induced surge waves. The role of floods faster than ordinary flash floods (here defined as instant floods) is demonstrated. The conclusions are confirmed with a series of numerical tests for ideal and natural channels. Finally, differences from ordinary flash floods are identified in the framework of risk assessment, as well as the perspective to revisit real cases of surge waves in canyons and other streams.

Drivers of variability in large wood loads along the fluvial continuum of a Mediterranean intermittent river

AbstractAlthough in‐channel and floodplain large wood (LW) has been recognized as an important component of lotic ecosystems, there is still limited knowledge on the recruitment, mobility and retention of LW in rivers with an intermittent hydrological regime. In this study, we analysed the LW characteristics and related reach‐scale variables of 22 reaches in a Mediterranean intermittent river (Evrotas, Greece) in order to identify predictors of in‐channel and floodplain LW distribution. Our results indicated high downstream variation in LW volumes in the fluvial corridor (0.05–25.51 m3/ha for in‐channel LW and 0–30.88 m3/ha for floodplain LW). In‐channel and floodplain LW retention was primarily driven by the hydrological regime of the studied reaches (i.e. perennial or non‐perennial) with higher volumes of LW observed in perennial sections. The width of the riparian corridor was an important predictor of LW storage at the reach scale. Non‐perennial reaches had a disproportionally larger number of relatively small‐diameter living trees at the expense of mature trees with larger diameters typical for riparian stands functioning as LW recruitment areas in perennial reaches. The smaller dimensions of in‐channel LW in non‐perennial reaches, coupled with the dominance of loose LW pieces, implies frequent LW transport during ordinary flood events. Nevertheless, overall low LW retention in the fluvial corridor under non‐perennial flow regime predicts low volumes of mobilized LW. In contrast, the recruitment of relatively long and large‐diameter LW from mature riparian stands in perennial reaches, together with additional LW stabilization by banks, bed sediments, living trees or other LW pieces decreases the potential for further LW transport. © 2020 John Wiley & Sons, Ltd.

Controls Over Particle Motion and Resting Times of Coarse Bed Load Transport in a Glacier‐Fed Mountain Stream

AbstractCoarse bed load transport is a crucial process in river morphodynamics but is difficult to monitor in mountain streams. Here we present a new sediment transport data set obtained from 2 years of field‐based monitoring (2014–2015) at the Estero Morales, a high‐gradient stream in the central Chilean Andes. This stream features step‐pool bed geometry and a glacier‐fed hydrologic regime characterized by abrupt daily fluctuations in discharge. Bed load was monitored directly using Bunte samplers and by surveying the mobility of passive integrated transponder (PIT) tags. We used the competence method to quantify the effective slope, which is the fraction of the topographical slope responsible for bed load transport. This accounts for only 10% of the topographical slope, confirming that most of the energy is dissipated on macroroughness elements. We used the displacement lengths of PIT tags to analyze displacement lengths and virtual velocity of a wide range of tracer sizes (38–415 mm). Bed load transport in the Estero Morales shown to be size‐selective, and the distance between steps influences the displacement lengths of PIT tags. Displacement lengths were also used to derive the statistics of flight distances and resting times. Our results show that the average length of flight scales inversely to grain size. This contradicts Einstein's conjecture about the linear relationship between grain size and intervals between resting periods in a steep step‐pool stream in ordinary flood conditions.

Experiment on morphological and hydraulic adjustments of step‐pool unit to flow increase

AbstractExceptional flood events with a return period of about 50 years can be destructive to step‐pool channel segments. However, field investigations and flume experiments have not examined the hydraulic and morphological feedbacks of step‐pool morphology during unsteady hydrographs of exceptional flood events. We performed a series of flume experiments with a manually constructed step model, perturbed with three hydrographs that varied in the rate of water supply change. The bed texture, topography, flow regimes, surface flow field and water depth were characterized and measured as the flow rate was increased during the experiments. A distinct pool feature emerged downstream of the manually constructed step when the flow rate exceeded the threshold scaled to the peaks of ordinary flood events in well‐graded mountain streams. The pool feature was modified in several different ways with flow rate increase. The bed surface steadily coarsened, micro‐bedforms developed and became more pronounced, the bed topography became more spatially complex based on analysis using the Hurst exponent, and last, pool depth steadily increased. Pool modification was also linked to the flow regime: the impinging jet regime led to grain size segmentation in the pool while the jump regime contributed to decelerating flow velocity. The steeper rising limb of hydrograph led to a less developed pool feature, with smaller sized micro‐bedforms in the pool bottom to outlet, and higher discharge threshold for distinct coarsening and scouring in the pool. The estimated energy dissipation within the step‐pool unit decreased as a power function from low to high flow, quantified as the ratio hc/HS, where hc is the critical water depth and HS is scour depth. Our results highlight the interaction between morphology, hydraulics, and energy dissipation of step‐pool unit and the crucial role of hydrograph shape on the interaction during flow increase © 2019 John Wiley & Sons, Ltd.

Assessment of the Demarcation Method for Federal Riverine and Accreted Lands: Case Study of the Rio De Janeiro State Section of the South Paraíba River

This study analyzes the demarcation method of riverine and accreted land of the Brazilian Federal Heritage Department and proposes the incorporation of the flow rate corresponding to the recurrence interval of two years, as recommended by the State Environmental Institute of the state of Rio de Janeiro. The case study of the Rio de Janeiro section of the Paraiba do Sul River was investigated, and the results indicate that the Federal Heritage Department’s method does not consider the ongoing anthropization of the river, caused mainly by the construction and operation of hydroelectric plants. In addition, it was observed that the limnimetric scales of the studied gauging stations are influenced by constant changes in the riverbed and by riverbank occupation, making it difficult to estimate the ordinary flood level. The study concludes by suggesting the adoption of a flow rate with a recurrence interval of two years and the simulation of the runoff conditions for demarcation of the average ordinary flood line.

Comparison between the TOPMODEL and the Xin’anjiang model and their application to rainfall runoff simulation in semi-humid regions

To study the application of the TOPMODEL and the Xin’anjiang model to rainfall runoff simulation in semi-humid regions, the Holtan excess infiltration runoff module was added to the TOPMODEL structure. The basin of the Heihe Jinpen Reservoir in Shaanxi Province, China, was selected as the study area. Rainfall and runoff data and digital elevation models were collected. The watershed topographic parameters and 21 floods that occurred from 2005 to 2013 were obtained to simulate rainfall runoff. Results show that the improved TOPMODEL and the Xin’anjiang model can effectively stimulate rainfall runoff. The average values of their Nash coefficient are 0.84 and 0.83, respectively, upon calibration, and 0.78 and 0.80, respectively, upon validation. The Xin’anjiang model performs slightly better than the improved TOPMODEL. The results of large flood peaks are better than those of ordinary floods. Both results can be used to simulate the rainfall runoff of a watershed.

Test of methodology for developing a large wood budget: A 1-year example from a regulated gravel bed river following ordinary floods

Wood budgeting is a current approach for quantifying the spatial and temporal variations of in-channel large wood (LW). Despite its importance for a proper LW management, the assessment of LW budget is still a challenge because of the complexity of LW dynamics. Focusing on a 3.7 km-long gravel-bed reach of the Piave River (North-eastern Italy), this study proposes an attempt to assess a short-term (1 year) LW budget as a consequence of ordinary floods (RI < 1 yr). Lateral recruitment, mobilization and deposition of LW were investigated by two field surveys conducted at the beginning and end of the study period. The lateral input was evaluated by measuring, positioning and tagging all standing trees (diameter ≥ 0.1 m) within a 20 m-wide buffer strip along the floodplain and island perimeters. Bank erosion was checked by measuring the riverbanks with a differential Global Positioning System (dGPS), during the first and second survey. Contemporarily, the position and characteristics of each fluvial wood element (diameter ≥ 0.1 m and length ≥ 1 m) within the active channel was determined. These short-term analyses revealed that ordinary floods are not able to induce relevant variations in the LW abundance and redistribution of in-channel wood. Nevertheless, the results highlighted the role of riverbank erosion in supplying LW to the active channel. Episodic inputs of LW from bank erosion were also observed during ordinary flood events, introducing 33% (0.21 m3·km−1) of the total LW input amount. Results on LW transport showed that, in the Piave River, both deposition and mobilization of LW may also occur during low flow conditions (i.e. 14% of bankfull stage) without causing important variations in the wood quantity. However, the low magnitude of floods was reflected in the small amount of LW involved in the input (0.4 m3·km−1) and output (0.7 m3·km−1) processes, pointing out that sites for deposition and mobilization of LW are conditioned by flood magnitude and morphological settings. The combination of recruitment, mobilization and deposition processes resulted in a slight decrease of LW storage (−0.07%), while including also the anthropic removal of LW the amount of wood storage decreased to −9.7%. Despite the short-term investigation, this work sheds further light on LW processes, providing useful results for the management of LW in wide piedmont rivers.

Temporal variations of large wood abundance and mobility in the Blanco River affected by the Chaitén volcanic eruption, southern Chile

The aim of this study is to analyze the temporal variation in abundance and transport of in-channel wood along a river affected by a volcanic eruption, considering the mobility and retention rates in detail. The 2008 eruption of Chaitén Volcano (Southern Chile) severely affected the Blanco River basin, causing changes in channel morphology and large wood (LW) dynamics along the fluvial corridor of the river, a fourth-order stream on the southern slope of the volcano. Temporal dynamics of LW were investigated, from January 2015 until March 2016, in three 80m-long study reaches of the active channel in the lower course of the river. Every wood element (≥1m in length and ≥0.1m in diameter) within the active channel of each reach was georeferenced, measured, tagged, and characterized. Wood mobility rate, defined as the number of tagged wood pieces repositioned or not found within each study reach to total tagged LW in the same reach; and retention rate, defined as the number of tagged LW pieces found within each study reach to total tagged wood pieces in the same reach, were calculated considering two periods, one in summer and one in autumn-winter. A regional flood frequency analysis estimated flood magnitude, defining a recurrence interval of ~1year for summer floods, and 10–25years for autumn-winter floods. Results highlighted how, eight years after the eruption, the types of LW are still influenced by the impacts of pyroclastic density currents on riparian vegetation. The characteristics and abundance of monitored LW are typical of a river system with frequent wood transport. The mobility rate ranges between 42 and 94% during the summer and autumn-winter period, respectively; while the wood volume decreases during the summer and increases in autumn-winter. Large wood mobility was also found to vary according to the predominant morphological settings, with lower values in narrower channels compared to wider wetted areas. In this context the greatest abundance of LW was detected after the higher floods, corresponding to an input of LW up to 285.35m3·ha−1. Along the Blanco River ordinary floods deliver LW mainly through fluvial transport, and bank erosion during higher floods appears as an additional and important wood recruitment process. The bank erosion processes, with up to 5200m2 of eroded area, confirmed the widening tendency occurring in river systems following volcanic disturbances. As there are dead riparian trees along the easily erodible banks, it is reasonable to suppose that the Blanco River will continue to widen for a long period, recruiting and transporting downstream huge volumes of LW. In this context these analyses appear of fundamental importance to better manage the river system, also for decreasing hazards to the downstream village of Chaitén.

Essential strategy for urban seasonal-river restoration

Recently, many urban rivers are being restored due to positive environmental effects while debris flow may occur in seasonal-rivers of cities in arid regions. Therefore, as a part of river restoration studies, the impacts of debris flow on an urban seasonal-river (Vardavard River, Tehran) were analysed based on field investigations and hydrologic, hydraulic and sediment analyses. The results show that high-concentration of sediments in debris flow increases peak flow and hydraulic roughness, resulting in river flooding. The steep slope of the river basin increases the debris flow peak up to 1.2 times more than an ordinary flood peak with the similar return period. Hence, designing a slit dam before the river enters the city is suggested as an essential strategy to control the debris flow and improve safety of the downstream urban areas. The proposed approach, with enforced watershed management schemes, can be implemented to restore other urban seasonal-rivers and to prevent probable debris flow in arid regions.

Essential strategy for urban seasonal-river restoration

Recently, many urban rivers are being restored due to positive environmental effects while debris flow may occur in seasonal-rivers of cities in arid regions. Therefore, as a part of river restoration studies, the impacts of debris flow on an urban seasonal-river (Vardavard River, Tehran) were analysed based on field investigations and hydrologic, hydraulic and sediment analyses. The results show that high-concentration of sediments in debris flow increases peak flow and hydraulic roughness, resulting in river flooding. The steep slope of the river basin increases the debris flow peak up to 1.2 times more than an ordinary flood peak with the similar return period. Hence, designing a slit dam before the river enters the city is suggested as an essential strategy to control the debris flow and improve safety of the downstream urban areas. The proposed approach, with enforced watershed management schemes, can be implemented to restore other urban seasonal-rivers and to prevent probable debris flow in arid regions.

Two million years of river and cave aggradation in NE Brazil: Implications for speleogenesis and landscape evolution

This study characterizes and provides ages for an extensive sedimentary record occurring in Lapa Doce and Torrinha caves, NE Brazil. With >40km of surveyed passages, these caves integrate a distributary cave system fed by allogenic recharge from the surrounding sandstone plateaus. Sediment petrography together with descriptions of depositional facies and architectural elements shows four depositional units related to fluvial and standing water environments. These include, from bottom to top: (1) a channel unit including lateral bars deposited during an ordinary flood regime; (2) a sandy flood unit including minor channel and scour fills derived from bank-full equivalent flood events; (3) mud caps deposited in standing water that often reach the ceiling; and (4) intraclast breccias associated with collapse of the mud caps under saturated conditions. The deposits were dated using a combination of cosmogenic nuclide burial dating and U-series dating of flowstone. Cosmogenic nuclide data point to fluvial aggradation being active since 1.91+0.12My and extending until 0.36±0.08My, with intensive cave and valley aggradation events between 0.78±0.10My and 0.44±0.12My. Long term alluviation of the cave system seems to be important in forming passages, determining their configuration, and setting up a general distributary pattern evident in passage morphology and sedimentary sequences. Mud caps overlapping the fluvial deposits are interpreted as the products of successive rising and lowering of the water table (static level). Radiometric ages of interstratified flowstones and speleothems show that these oscillations were active at least since 115ky ago and finally ceased around 12ky ago, indicated by the recurrent age obtained from uneroded capping flowstones. These long-term water table oscillations may drive paragenetic expansion of the whole cave system. Valleys, caves and other landforms in our study area are part of an ancient landscape with multiple episodes of burial and exhumation. Despite the possibility of much older sediment being preserved somewhere in the cave system, our data indicate a younger Quaternary age for the bulk of the sediment filling the study caves.