Overbank Flooding Research Articles

Water use in a riparian cottonwood ecosystem: Eddy covariance measurements and scaling along a river corridor

The survival of riparian forests depends on water input from their adjacent rivers. There are multiple, additional demands on river water that threatens to reduce the water supply, prompting the question, how much water must be left flowing within a river to sustain the native riparian ecosystems? To address this, we made eddy covariance measurements of riparian cottonwood forest evapotranspiration (ET) in a representative site and used remote sensing and the Penman-Monteith equation to up-scale ET along the 171km river corridor from the Oldman River Dam to Lethbridge, Alberta. Our study was conducted in two growing seasons with contrasting weather conditions, including one with over-bank flooding (2014). Measured cumulative forest ET from May-September (2014: 451±90mm; 2015: 411±18mm) was very similar in the two years despite contrasting May-September cumulative precipitation input (2014: 362mm; 2015: 181mm). Integrated over the 56km2 area of riparian forest along the Oldman River corridor, the cumulative forest ET during May-September was 19.1 million m3 (339mm/season), about 0.9% of the average cumulative Oldman River discharge during 2008–2013. Cottonwood forest ET during May-June was less than 1% of average river flow rates during 2008–2013, but the ratio of ET to average river flow rate increased markedly to peak values of 4–5% in late July and early August. Our analysis indicated that the water-use rates of riparian cottonwood forests were high, even for a broad-leaf deciduous forest functional type, particularly given the modest leaf area index (1.4m2m−2) we measured for riparian forests along the Oldman River corridor. The high ET rates were possible because of tree access to alluvial groundwater to support transpiration. Our ET measurements and calculations provided perspective on riparian forest water use in relation to precipitation inputs and total river discharge, knowledge that is important for successfully managing the multiple demands on river water use.

Vertical surface water–groundwater exchange processes within a headwater floodplain induced by experimental floods

AbstractRestoring hydrologic connectivity between channels and floodplains is common practice in stream and river restoration. Floodplain hydrology and hydrogeology impact stream hydraulics, ecology, biogeochemical processing, and pollutant removal, yet rigorous field evaluations of surface water–groundwater exchange within floodplains during overbank floods are rare. We conducted five sets of experimental floods to mimic floodplain reconnection by pumping stream water onto an existing floodplain swale. Floods were conducted throughout the year to capture seasonal variation and each involved two replicate floods on successive days to test the effect of varying antecedent moisture. Water levels and specific conductance were measured in surface water, soil, and groundwater within the floodplain, along with surface flow into and out of the floodplain. Vegetation density varied seasonally and controlled the volume of surface water storage on the floodplain. By contrast, antecedent moisture conditions controlled storage of water in floodplain soils, with drier antecedent moisture conditions leading to increased subsurface storage and slower flood wave propagation across the floodplain surface. The site experienced spatial heterogeneity in vertical connectivity between surface water and groundwater across the floodplain surface, where propagation of hydrostatic pressure, preferential flow, and bulk Darcy flow were all mechanisms that may have occurred during the five floods. Vertical connectivity also increased with time, suggesting higher frequency of floodplain inundation may increase surface water–groundwater exchange across the floodplain surface. Understanding the variability of floodplain impacts on water quality noted in the literature likely requires better accounting for seasonal variations in floodplain vegetation and antecedent moisture as well as heterogeneous exchange flow mechanisms. Copyright © 2016 John Wiley & Sons, Ltd.

Fluvial system response to late Pleistocene-Holocene sea-level change on Santa Rosa Island, Channel Islands National Park, California

Santa Rosa Island (SRI) is one of four east-west aligned islands forming the northern Channel Islands chain, and one of the five islands in Channel Islands National Park, California, USA. The island setting provides an unparalleled environment in which to record the response of fluvial systems to major changes of sea level. Many of the larger streams on the island occupy broad valleys that have been filled with alluvium and later incised to form steep- to vertical-walled arroyos, leaving a relict floodplain as much as 12–14m above the present channel. The period of falling sea level between the end of the last interglacial highstand at ~80ka and the last glacial lowstand at ~21ka was marked by erosion and incision in the uplands and by deposition of alluvial sediment on the exposed marine shelf. Sea level rose relatively rapidly following the last glacial lowstand of −106m, triggering a shift from an erosional to a depositional sedimentary regime. Accumulation of sediment occurred first through vertical and lateral accretion in broad, shallow channels on the shelf. Channel avulsion and delta sedimentation produced widespread deposition, creating lobes or wedges of sediment distributed across relatively large areas of the shelf during the latest Pleistocene. Backfilling of valleys onshore (landward of present sea level) appears to have progressed in a more orderly and predictable fashion throughout the Holocene primarily because the streams were confined to their valleys. Vertical aggradation locally reduced stream gradients, causing frequent overbank flooding and lateral channel shift by meandering and/or avulsion. Local channel gradient and morphology, short-term climate variations, and intrinsic controls also affected the timing and magnitudes of these cut, fill, and flood events, and are reflected in the thickness and spacing of the episodic alluvial sequences. Floodplain aggradation within the valleys continued until at least 500years ago, followed by intensive arroyo cutting that abandoned the relict floodplains, forming alluvial terraces. Sedimentary evidence points to overgrazing and drought, followed by catastrophic flooding, in the mid-nineteenth century as factors that may have accelerated and dramatically enhanced arroyo formation on the island.

Early Mississippian sandy siltstones preserve rare vertebrate fossils in seasonal flooding episodes

AbstractFlood‐generated sandy siltstones are under‐recognised deposits that preserve key vertebrate (actinopterygians, rhizodonts, and rarer lungfish, chondrichthyans and tetrapods), invertebrate and plant fossils. Recorded for the first time from the lower Mississippian Ballagan Formation of Scotland, more than 140 beds occur throughout a 490 m thick core succession characterised by fluvial sandstones, palaeosols, siltstones, dolostone ‘cementstones’ and gypsum from a coastal–alluvial plain setting. Sandy siltstones are described as a unique taphofacies of the Ballagan Formation (Scotland, UK); they are matrix‐supported siltstones with millimetre‐sized siltstone and very fine sandstone lithic clasts. Common bioclasts include plants and megaspores, fish, ostracods, eurypterids and bivalves. Fossils have a high degree of articulation compared with those found in other fossil‐bearing deposits, such as conglomerate lags at the base of fluvial channel sandstones. Bed thickness and distribution varies throughout the formation, with no stratigraphic trend. The matrix sediment and clasts are sourced from the reworking of floodplain sediments including desiccated surfaces and palaeosols. Secondary pedogenic modification affects 30% of the sandy siltstone beds and most (71%) overlie palaeosols or desiccation cracks. Sandy siltstones are interpreted as cohesive debris flow deposits that originated by the overbank flooding of rivers and due to localised floodplain sediment transport at times of high rainfall; their association with palaeosols and desiccation cracks indicates seasonally wet to dry cycles throughout the Tournaisian. Tetrapod and fish fossils derived from floodplain lakes and land surfaces are concentrated by local erosion and reworking, and are preserved by deposition into temporary lakes on the floodplain; their distribution indicates a local origin, with sediment transported across the floodplain in seasonal rainfall episodes. These deposits are significant new sites that can be explored for the preservation of rare non‐marine fossil material and provide unique insights into the evolution of early terrestrial ecosystems.

Spatially distributed modelling of surface water-groundwater exchanges during overbank flood events – a case study at the Garonne River

Exchanges between surface water (SW) and groundwater (GW) are of considerable importance to floodplain ecosystems and biogeochemical cycles. Flood events in particular are important for riparian water budget and element exchanges and processing. However SW-GW exchanges present complex spatial and temporal patterns and modelling can provide useful knowledge about the processes involved at the scale of the reach and its adjacent floodplain. This study used a physically-based, spatially-distributed modelling approach for studying SW-GW exchanges. The modelling in this study is based on the MOHID Land model, combining the modelling of surface water flow in 2D with the Saint-Venant equation and the modelling of unsaturated groundwater flow in 3D with the Richards’ equation. Overbank flow during floods was also integrated, as well as water exchanges between the two domains across the entire floodplain. Conservative transport simulations were also performed to study and validate the simulation of the mixing between surface water and groundwater. The model was applied to the well-monitored study site of Monbéqui (6.6km²) in the Garonne floodplain (south-west France) for a five-month period and was able to represent the hydrology of the study area. Infiltration (SW to GW) and exfiltration (SW to GW) were characterised over the five-month period. Results showed that infiltration and exfiltration exhibited strong spatiotemporal variations, and infiltration from overbank flow accounted for 88% of the total simulated infiltration, corresponding to large flood periods. The results confirmed that overbank flood events played a determinant role in floodplain water budget and SW-GW exchanges compared to smaller (below bankfull) flood events. The impact of floods on water budget appeared to be similar for flood events exceeding a threshold corresponding to the five-year return period event due to the study area's topography. Simulation of overbank flow during flood events was an important feature in the accurate assessment of exchanges between surface water and groundwater in floodplain areas, especially when considering large flood events.

Hydrogeologic Processes Impacting Storage, Fate, and Transport of Chloride from Road Salt in Urban Riparian Aquifers.

Detrimental effects of road salt runoff on urban streams are compounded by its facilitated routing via storm drains, ditches, and flood channels. Elevated in-stream salinity may also result from seasonal storage and discharge of chloride in groundwater, and previous work has hypothesized that groundwater discharge to streams may have the effect of diluting stream chloride concentrations in winter and enriching them in summer. However, the hydrogeological processes controlling these patterns have not been thoroughly investigated. Our research focuses on an urban stream and floodplain system in Syracuse, NY, to understand how groundwater and surface water exchange impacts chloride storage, fate, and transport. We created a 3D groundwater flow and solute transport model of the floodplain, calibrated to the distributions of floodplain hydraulic heads and groundwater fluxes to the stream throughout the reach. We used a sensitivity analysis to calibrate and evaluate the influence of model parameters, and compared model outputs to field observations. The main source mechanism of chloride to the floodplain aquifer was high-concentration, overbank flood events in winter that directly recharged groundwater. The modeled residence time and storage capacity of the aquifer indicate that restoration projects designed to promote floodplain reconnection and the frequency of overbank flooding in winter have the potential to temporarily store chloride in groundwater, buffer surface water concentrations, and reduce stream concentrations following periods of road salting.

Sediment deposition patterns in a tropical floodplain, Tana River, Kenya

Floodplains exert strong controls on downstream sediment transport and are as such important in material budgets of river systems. To understand sediment budgets in the Tana River, we investigated sediment storage along a 380 km floodplain reach (Garissa-Garsen) in the lower Tana River (Kenya), using a combination of approaches: (i) measurements of sediment deposition after an important flood event and, (ii) quantification of sediment storage using fallout radionuclide activities (137Cs and 210Pbex). Event-based sediment deposition ranged between 2-15 mm vertical accretion, corresponding to an average of 0.58±0.42 g cm-2 (dry weight). Average annual sediment storage based on fallout radionuclide activities were at a 50 yr mean of 1.15-1.21 g cm-2 yr-1 and a 100 yr mean of 1.01 g cm-2 yr-1 (using 137Cs and 210Pbex, respectively). Sediment deposition rates were mainly dependent on distance relative to the main river, flood height and microtopography. The deposited sediments originated from various sources including deeply mobilized, radionuclide-poor sediments. Shallow overbank deposits (< 2 m depth), dominated by silty-clay sediment fractions were observed in most depositional areas. Floodplain sediment stocks are controlled by annual overbank flooding, with a 73 yr flooding frequency of 1.05 flood yr-1. On average ca. 1-2 Mt (15-30%) of the river sediment load is deposited in the Garissa-Garsen floodplain reach. However, the overall channel dynamics including channel sediment storage and reworking are important and may have a very high impact on short-term sediment storage and release.

Ephemeral stream dynamics, land cover changes and climate variability in the marginal grabens of the northern Ethiopian Rift Valley

This study focused on ephemeral streams morphodynamics and their relation with hydro-climatic and bio-physical characteristics in their headwater and graben floors in the marginal grabens along the northern Ethiopian Rift Valley. The Raya graben was taken as a case study area, which is the largest marginal graben in northern Ethiopia. The link between rainfall variability and land cover changes shows that there was a negative correlation between precipitation and deforestation which implies that an increase in precipitation decreases deforestation deterring the peasants from tree cutting. Regarding the relationship between upper catchment characteristics and river morphology, results show that upper catchment area is the most important controlling factor of the length of and the area occupied by the rivers in the graben bottom. A simple hydraulic analysis based on the change of specific discharge as the river flow approaches the bridges demonstrates that the abrupt change in channel width as the river approaches the bridge is the main cause of the thick deposition and, consequently, of the increased frequency of overbank floods upstream of the road bridges in the study area. The results related to channel bank erosion capacity of flash floods show that all the peak discharges were equally important in triggering channel bank erosion. Concomitantly, vegetated channels are much more resistant to channel bank erosion than non-vegetated banks. Land changes in graben bottoms dictate that the changes are not simply related to a distributary river system but that human intervention (e.g., the conversion of bushland and forests into cropland, settlement on alluvium, and avoidance of losing farmland to other land units) and natural vegetation regeneration (e.g., alluvial deposits into shrubland) are also important. Overall, the study suggests that to control the morphodynamics of the ephemeral rivers and their impact on farming activities in the graben bottom, catchment rehabilitation activities need to focus both on the upper catchment and the river channels in the graben bottom.

Hydrological and hydraulic modelling applied to the mapping of flood-prone areas

Overbank flooding caused by historically high flows, such as that in the Rio Mundau watershed (lying between the states of Alagoas and Pernambuco) in 2010, has been the cause of widespread damage. The purpose of work described in this paper was to propose a mapping of areas liable to flooding in the township of Rio Largo (Alagoas) in the Rio Mundau watershed by means of an “off-line” coupling of the hydrological/hydraulic models (MGB-IPH/HEC-RAS), through consideration of extreme floods with different return periods for discharge. The hydrological model had a parameterization appropriate for extreme floods, using as input rainfall data with different return periods. Calibration and validation of the hydrological model were adequate in drainage areas larger than 1500 km2, but were less acceptable in headwater drainage areas where different geology and soil cover gave rise to surface runoff. The hydraulic model showed good agreement with point observations of flood levels in 2010 in both rural and urban areas along the water-course (R2 = 0.99; RMSE = 1.41 m and CV (RMSE) = 0.04). In urban areas distant from the river, however, flood levels were over-estimated, indicating a need to use more detailed Digital Elevation Models. Flood events with return period greater than 50 years have the potential to cause great damage (floods exceeding 0.46 km2 in the urban area). The study showed that the use of coupled models was a viable approach for mapping areas liable to flooding, when it is not possible to analyse local flow frequencies in support of a hydraulic model.

Depositional environments of overbank sedimentation in the lignite-bearing Grey Clays Member: New evidence from Middle Miocene deposits of central Poland

A detailed sedimentological study of Polish lignite-bearing successions has not previously been undertaken. This contribution focuses on the lignite lithotypes and lake and crevasse splay lithofacies that together constitute the Grey Clays Member. This lithostratigraphic unit strictly refers to the Middle Miocene overbank fluvial environments in central Poland. It mostly consists of the First Mid-Polish Lignite Seam (MPLS-1) in the Tomisławice lignite opencast mine. This lignite seam, containing four lithotypes, is interbedded by sandy and silty-clayey lithofacies. The lithotype associations are characteristic of the low-lying mires representing: fen or open water (<2m deep), bush moor and wet-forest swamp. Thus, during their formation the water table was close to the depositional surface. On the other hand, the occurrence of a muddy association composed of clayey-silty (fines) lithofacies both within and on top of the lignite seam corresponds to the presence of small and deep (>2m) lakes in the mire area. This is additional evidence of the relatively long-lasting and slow sedimentation of these very fine-grained clastics from suspension in standing lake water. Conversely, the sandy lithofacies associations, representing crevasse splays, were deposited suddenly during overbank flooding. Crevasse splay deposits are typical of initial stages of avulsion and are moderately diverse both texturally and structurally. Exceptions here are slurry flow deposits that contain rip-up clasts of paleosol aggregates. These crevasse splay deposits provide the first evidence of the fluvial environments of the Mid-Miocene mires (backswamp) in central Poland, and they split the currently exploited lignite seam, MPLS-1, in the study area. Thus, identification and description of lithofacies and lithotypes, and determination of their spatial distribution can contribute to a better understanding of the mire development, of which the examined lignite seam arose.

Quantifying the impacts of artificial flooding as a salt interception measure on a river-floodplain interaction in a semi-arid saline floodplain

The natural flow regime of the Lower Murray River in South Australia has been altered and resulted in severe decline in the condition of long-lived vegetation on the floodplain. This paper aims to quantify the relative impacts of artificial flooding on the flow and solute dynamics of the floodplain aquifer and its ecological implications. It is shown that artificial flooding can temporarily form less saline groundwater and soil profiles which in turn improves water availability for vegetation. From ecological point of view, artificial flooding delivers some of the same benefits as natural overbank floods. However, the washed-off solute mass from the soil profile was only sustained for 4–6 months. It appears that artificial flooding is an intervention technique that is limited spatially to the flooded area and temporally to the flood duration. Hence, it may be considered as a short term management technique in arid and semi-arid areas.

Damming ephemeral streams in the Sonoran Desert, Arizona, USA: biogeomorphic analysis of riparian area growth

In this study, two ephemeral streams are compared in assessing bio-geomorphic change to channels intersected by the Central Arizona Project (CAP) canal. Water and sediment are fully or partially restricted to move downstream due to the CAP canal, which acts as a dam to these streams. Upstream of traversed channels, vegetation cover has increased over time causing the development of a green-up area. Along the canal, several channels remain longitudinally connected via culvert or overchute to the downstream sections of the stream channels. The two streams examined both had longitudinal connectivity, but only one of the streams had a green-up area upstream. Field work and channel surveys reveal that this is due to the culvert’s peak discharge outflow. The stream with a developed green-up area had a culvert size that is approximately three times less than the discharge of this ephemeral stream causing the partial damming upstream during flow events. Green-up zones slowly enlarge over time as bed levels and adjacent desert surface heights increase from continual sediment deposition upstream causing the floodplain to laterally increase due to overbank flooding. It is estimated that the green-up area for the partially dammed stream will increase by approximately 1570 m2 over the next 36 years. Suggestions for urban and agricultural development are presented in this paper in relation to these dynamic green-up areas. Understanding biogeomorphic processes along dammed ephemeral streams lends valuable insight to riparian conservation efforts and future urban development plans in desert regions.

Wide‐area estimates of evapotranspiration by red gum (Eucalyptus camaldulensis) and associated vegetation in the Murray–Darling River Basin, Australia

AbstractFloodplain red gum forests (Eucalyptus camaldulensis plus associated grasses, reeds and sedges) are sites of high biodiversity in otherwise arid regions of southeastern Australia. They depend on periodic floods from rivers, but dams and diversions have reduced flood frequencies and volumes, leading to deterioration of trees and associated biota. There is a need to determine their water requirements so environmental flows can be administered to maintain or restore the forests. Their water requirements include the frequency and extent of overbank flooding, which recharges the floodplain soils with water, as well as the actual amount of water consumed in evapotranspiration (ET). We estimated the flooding requirements and ET for a 38 134 ha area of red gum forest fed by the Murrumbidgee River in Yanga National Park, New South Wales. ET was estimated by three methods: sap flux sensors placed in individual trees; a remote sensing method based on the Enhanced Vegetation Index from MODIS satellite imagery and a water balance method based on differences between river flows into and out of the forest. The methods gave comparable estimates yet covered different spatial and temporal scales. We estimated flood frequency and volume requirements by comparing Normalized Difference Vegetation Index values from Landsat images with flood history from 1995 to 2014, which included both wet periods and dry periods. ET during wet years is about 50% of potential ET but is much less in dry years because of the trees' ability to control stomatal conductance. Based on our analyses plus other studies, red gum trees at this location require environmental flows of 2000 GL yr−1 every other year, with peak flows of 20 000 ML d−1, to produce flooding sufficient to keep them in good condition. However, only about 120–200 GL yr−1 of river water is consumed in ET, with the remainder flowing out of the forest where it enters the Murray River system. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Dryland anabranching river morphodynamics: Río Capilla, Salar de Uyuni, Bolivia

The dryland anabranching river Río Capilla is characterized by nonvegetated and vegetated reaches with prominent channel morphology. To identify the morphodynamics of such dryland anabranching systems and their controls, we investigated the Río Capilla of the southern Altiplano Plateau using high-resolution satellite imagery and field measurements. Comparison of high-resolution satellite data reveals that erosion exceeds deposition for the main channel, accompanied by changes in channel planform, such as meander and channel morphology. On-site surveys combined with high-precision GPS and high-resolution satellite imagery show that channels are characterized by shallowness and poor development of levees. The study area of the Río Capilla is divided into two zones of different slopes: zone 1 with a high slope and zone 2 with a low slope. Zone 1 has a relatively straight main channel with few anabranches and grass-covered banks that are stable despite the high gradient; whereas zone 2 is typified by more anabranches with nonvegetated banks, and the main channel experiences prominent bank accretion and erosion. Excavations show that point-bar deposits are fine-sand-dominated in two reaches and that river banks primarily consist of silt and clay. The limited vegetation cover and abundance of desiccation cracks and macropores make the river bank more erodible leading to pronounced lateral migration in this low-gradient dryland river system. Shallow channels and poor development of levees in combination with in-channel accretionary benches result in frequent overbank flooding, which results in a high density of crevasse splays over unconsolidated river banks and accretionary benches. Connection of headcuts and crevasse channels together with lateral migration and chute channels and reactivation of partially abandoned meanders produces an anabranching pattern in such dryland river systems.

Episodic overbank deposition as a dominant mechanism of floodplain and delta-plain aggradation

The common view that frequent overbank flooding leads to gradual aggradation of alluvial strata on floodplains and delta plains has been challenged by a variety of studies that suggest that overbank aggradation occurs in a strongly episodic fashion. However, this remains a largely untested hypothesis due to the difficulty in establishing age models with sufficiently high resolution. Here we use 39 optically stimulated luminescence (OSL) ages from proximal overbank deposits in the Mississippi Delta to demonstrate for the first time that alluvial aggradation over centennial to millennial time scales is predominantly episodic, with aggradation rates of 1–4 cm/yr that can persist for centuries. OSL ages from three separate study areas produce age clusters that are distinctly different yet complement each other. These findings suggest that a substantial portion of the continental stratigraphic record consists of patchworks of relatively discrete, centennial- to millennial-scale sediment bodies assembled by autogenic processes.

Hydrologic and geomorphic flow thresholds in the Lower Brazos River, Texas, USA

River science and management often require a design or reference discharge. The common (and sometimes unavoidable) use of such discharges may, however, obscure the fact that the magnitude and frequency of critical flows can differ due to various hydrological, geomorphological, and ecological criteria. Threshold stages and discharges were identified for six lower Brazos River, Texas gaging stations corresponding to thalweg connectivity, bed inundation, high sub-banktop flows, channel–floodplain connectivity (CFC), and overbank flooding. Critical flows were also identified for estimated thresholds for sandy bedform and medium gravel mobility, critical specific stream power for potential channel modifications, and cohesive-bank channel erosion. These thresholds have variable relationships to mean, median, and maximum flows. For four of the six stations, daily recurrence probabilities for all but flood flows are at least 1%, and as high as 11%. All stations achieve channel–floodplain connectivity at stages less than banktop. Estimated threshold flows for sediment mobility and channel erosion occur relatively frequently, with daily probabilities of 2–77%. Critical flows for bank erosion occur least often, and for sandy bedform and gravel mobility most often. Thalweg connectivity is always maintained at all sites, while bed inundation flows have a daily probability of about 80% or more. Overall, results suggest that no single flow level is dominant in hydrological or geomorphic dynamics, and that the frequency of a given threshold varies considerably even along a single river. The results support the idea that multiple flow levels and ranges are necessary to create and maintain the hydrological, geomorphological, and ecological characteristics of rivers, and that no single flow level is a reliable determinant of fluvial state.Editor Z.W. Kundzewicz; Associate editor Q. Zhang

Urban stream overflow probability in a changing climate: Case study of the Seoul Uicheon Basin, Korea

Flooding from the overflow of rivers and streams can cause major disruption in urban areas that is likely to have significant effects on human activities and the environment. Such consequences could be exacerbated by enhanced levels of precipitation resulting from future climate change. Various options are available for responding to flooding; however, further studies are needed to improve the design flood criteria in order to cope with the uncertainties of a changing climate. This study investigated an improved methodology for the evaluation of the overflow probability of urban streams. This was achieved through the application of Monte Carlo simulations (MCSs) and climate change scenarios that incorporated an increased probability of overbank flooding. An estimation of the probability of future rainfall in the Uicheon Basin of Korea, using chaos disintegration with regional climate model (RCM) scenario data, indicated a projected increase of 4.4%–9.6%. The results for 100-year flooding under projected conditions of climate change, based on a hydrologic overflow inundation model, showed that flooded areas could increase by 58.1% compared with current levels, depending on the climate change scenarios. However, forecasts based on MCSs indicated that extreme rainfall could increase by 94.9%. Thus, an overflow analysis that reflects both extreme hydrologic events and more frequent flooding due to climate change could provide a more reliable means of forecasting extreme events, as well as helping to prevent natural disasters associated with unexpected extreme flooding. The results obtained in this study would provide useful data for stakeholders and decision makers to both enhance policy standards and formulate measures to reduce the risk of urban flooding within the context of a changing climate.

Stable isotopic insights into paleoclimatic conditions and alluvial depositional processes in the Kaiparowits Formation (Campanian, south-central Utah, U.S.A.)

The Kaiparowits Formation contains an exceptionally rich history of tectonic, climatic, and biologic conditions within the Western Interior of North America during the Campanian. Here we reconstruct aspects of the southern Cordilleran foreland basin's paleohydrology using δ18O and δ13C values determined from unionoid bivalve shells and pedogenic carbonate nodules derived from a suite of lithofacies associations. Unionoid shells derived from fluvial deposits display average water δ18O estimates of −13.7‰ ± 2.1 (1σ) (VSMOW) and shell δ13C values of −4.0‰ ± 1.5 (VPDB), whereas pedogenic carbonate nodules display average values of −6.0‰ ± 0.5 and −8.7‰ ± 0.8, respectively. Unionoid shells derived from pond deposits fall in between the two other environments with average values of −9.5‰ ± 1.8 and −5.7‰ ± 2.1, in δ18O and δ13C values respectively. Water δ18O estimates are interpreted to represent high altitude runoff within river systems, low elevation precipitation within the basin onto floodplain soils, and varying degrees of mixing between these two components within floodplain ponds. δ13C values track the isotopic composition of dissolved inorganic carbon within river, soil, and pond waters with high values likely reflecting greater contribution from chemically weathered marine carbonates exposed in the hinterland and lower values reflecting greater contributions from the in situ degradation of plant matter. Up-section there is a shift to lower δ18O values and higher δ13C values in fluvially-derived unionoid shells that post-dates an incursion of the Western Interior Seaway, but coincides with a shift in sediment provenance, an increase in basin sedimentation rates, and a change to a more anastomosed-style channel morphology within the basin foredeep depocentre. By combining the isotopic patterns with previously published sedimentologic, climate model, and paleofloral records we find: 1) additional evidence for humid, wet, and potentially monsoonal conditions within the region, 2) support for a tectonic uplift event, potentially related to Laramide deformation, and 3) greater aggradation and overbank flooding within the alluvial system in response to the uplift event.

Detecting Flood Variations in Shanghai over 1949–2009 with Mann-Kendall Tests and a Newspaper-Based Database

A valuable aid to assessing and managing flood risk lies in a reliable database of historical floods. In this study, a newspaper-based flood database for Shanghai (NFDS) for the period 1949–2009 was developed through a systematic scanning of newspapers. After calibration and validation of the database, Mann-Kendall tests and correlation analysis were applied to detect possible changes in flood frequencies. The analysis was carried out for three different flood types: overbank flood, agricultural waterlogging, and urban waterlogging. The compiled NFDS registered 146 floods and 92% of them occurred in the flood-prone season from June to September. The statistical analyses showed that both the annual flood and the floods in June–August increased significantly. Urban waterlogging showed a very strong increasing trend, probably because of insufficient capacity of urban drainage system and impacts of rapid urbanization. By contrast, the decrease in overbank flooding and the slight increase in agricultural waterlogging were likely because of the construction of river levees and seawalls and the upgrade of agricultural drainage systems, respectively. This study demonstrated the usefulness of local newspapers in building a historical flood database and in assessing flood characterization.

Hydraulic and geomorphic processes in an overbank flood along a meandering, gravel‐bed river: implications for chute formation

AbstractHydraulic interactions between rivers and floodplains produce off‐channel chutes, the presence of which influences the routing of water and sediment and thus the planform evolution of meandering rivers. Detailed studies of the hydrologic exchanges between channels and floodplains are usually conducted in laboratory facilities, and studies documenting chute development are generally limited to qualitative observations. In this study, we use a reconstructed, gravel‐bedded, meandering river as a field laboratory for studying these mechanisms at a realistic scale. Using an integrated field and modeling approach, we quantified the flow exchanges between the river channel and its floodplain during an overbank flood, and identified locations where flow had the capacity to erode floodplain chutes. Hydraulic measurements and modeling indicated high rates of flow exchange between the channel and floodplain, with flow rapidly decelerating as water was decanted from the channel onto the floodplain due to the frictional drag provided by substrate and vegetation. Peak shear stresses were greatest downstream of the maxima in bend curvature, along the concave bank, where terrestrial LiDAR scans indicate initial floodplain chute formation. A second chute has developed across the convex bank of a meander bend, in a location where sediment accretion, point bar development and plant colonization have created divergent flow paths between the main channel and floodplain. In both cases, the off‐channel chutes are evolving slowly during infrequent floods due to the coarse nature of the floodplain, though rapid chute formation would be more likely in finer‐grained floodplains. The controls on chute formation at these locations include the flood magnitude, river curvature, floodplain gradient, erodibility of the floodplain sediment, and the flow resistance provided by riparian vegetation. Copyright © 2015 John Wiley &amp; Sons, Ltd.