Peak Flood Stages Research Articles

Towards developing comparable optical and SAR remote sensing inundation mapping with hydrodynamic modelling

ABSTRACT Inundation mapping is an essential part of environmental monitoring, flood disaster management and risk mitigation. There are many earth observation sensors that can provide spatial information about inundation extent at different times. However, these extents need to be comparable to provide an accurate and consistent estimate of a flood’s progression. Monitoring inundation extent around the peak of a flood event is important because it captures the hazardous period during a large flood event, and it identifies connections between off-stream waterholes and flood waters for environmental monitoring. This paper presents results from a study comparing near-coincident flood inundation maps derived from optical (Landsat, MODIS, Sentinel-2 and VIIRS) and Synthetic Aperture Radar (SAR) (Sentinel-1 and NovaSAR-1) remote sensing imagery. We also compare all inundation maps derived from remote sensing data with near-coincident hydrodynamic (HD) modelling. The study was conducted for the Fitzroy floodplain in Western Australia, a large, complex and remotely located river basin. The results show that optical remote sensing data have average F1 scores ranging from 0.57 to 0.65 when compared to the HD model results, with Landsat and MODIS performing best. Sentinel-1 and NovaSAR-1 SAR have a poor agreement (average F1 score of 0.31 and 0.35 respectively) when compared to the HD model results, particularly within scattered vegetation adjacent to the river channel, with better results in open water on the floodplain and in the river. If comparisons are made only during the peak flood stage, the F1 scores improve for the optical data (0.61 up to 0.8). Comparisons of the remote sensing inundation maps show that the optical data are suitable for interchangeable mapping during large flood events.

Impact of Climate Change on Runoff Prediction in Ogbese River Watershed

Increased rainfall amounts are projected in the humid southern parts of Nigeria due to climate change. The consequence of higher rainfall in future years would result to higher peak runoffs and flood stages in streams in these parts. The focus of this study is to simulate peak runoff at the outlet of Ogbese river watershed for future years of 2030, 2040, 2050 and 2060. Local twenty years (2000-2019) historical rainfall depths were used to statically downscale General Circulation Model outputs in the future for RCP 4.5 climate scenario. Downscaled rainfall depths were inputted in HEC-HMS model version 4.2 for rainfall-runoff simulation. The watershed was delineated with DEM in ArcGIS while four land use and land cover classifications were extracted with QGIS. Maximum rainfall depths projected in years 2030, 2040, 2050 and 2060 were 38.5mm/hr, 39mm/hr, 42mm/hr and 46mm/hr respectively. Peak runoff discharge simulated for RCP 4.5 climate scenario in years 2030, 2040, 2050 and 2060 are 1771m3/s, 1826 m3/s, 1897 m3/s and 2200 m3/s respectively. This represents 24.2% increase peak discharge between 2030 and 2060. Land area delineated for the catchment is 1946.2 km2. The LULC classification areas for urban area, forest, rock outcrop and bare land are 81.59 km2, 1721.84 km2, 146.27 km2 and 4.11 km2 respectively. The soil types are sandy clay loam (92.51 %), sandy loam (6.84 %), and clay (0.65 %). Curve Number and Initial abstraction parameter values are 70.27 and 2.89 respectively. Keywords- Climate change, GCM, HEC-HMS , Ogbese river, Peak runoff

Using Small Unmanned Aircraft Systems for Measuring Post-Flood High-Water Marks and Streambed Elevations

Floods affected approximately two billion people around the world from 1998–2017, causing over 142,000 fatalities and over 656 billion U.S. dollars in economic losses. Flood data, such as the extent of inundation and peak flood stage, are needed to define the environmental, economic, and social impacts of significant flood events. Ground-based global positioning system (GPS) surveys of post-flood high-water marks (HWMs) and topography are commonly used to define flood inundation and stage, but can be time-consuming, difficult, and expensive to conduct. Here, we demonstrate and test the use of small unmanned aircraft systems (sUAS) and close-range remote sensing techniques to collect high-accuracy flood data to define peak flood stage elevations and river cross-sections. We evaluate the elevation accuracy of the HWMs from sUAS surveys by comparison with traditional GPS surveys, which have acceptable accuracy for many post-flood assessments, at two flood sites on two small streams in the U.S. Mean elevation errors for the sUAS surveys were 0.07 m and 0.14 m for the semiarid and temperate sites, respectively; those values are similar to typical errors when measuring HWM elevations with GPS surveys. Results demonstrate that sUAS surveys of HWMs and cross-sections can be an accurate and efficient alternative to GPS surveys; we provide insights that can be used to decide whether sUAS or GPS techniques will be most efficient for post-flood surveying.

Use of high-water marks and effective discharge calculation to optimize the height of bank revetments in an incised river channel

In deeply incised rivers, bankfull discharge (i.e. the flow filling the channel to the top of the banks) does not represent channel forming flow and increasingly large flows are associated with increasingly large boundary shear stress. In such rivers, solid bank revetments (rip-rap, gabions, retaining wall) are usually constructed to the top of the banks—similarly as in vertically stable rivers—despite the fact that the upper parts of the banks may never be flooded. To optimize the height of solid bank revetments in deeply incised channels, it is thus important to determine whether a flood magnitude can be identified, for which the combination of flow duration and bedload transport rate results in the highest river efficiency to transport bedload and perform geomorphic work. This question was explored in the Morávka River, Czech Carpathians, which deeply incised into non-resistant flysch bedrock over the past few decades. Observations of high-water marks (e.g. trash lines, wash lines) after a flood in 2014 enabled reconstruction of the peak flood stage in the deeply incised reach and the adjacent, vertically stable reach. These observations, together with post-flood measurements of cross-sectional channel geometry, distances between consecutive cross sections and estimates of channel roughness, were used in one-dimensional hydraulic modelling aimed to determine a peak discharge of the flood in a number of cross sections in both reaches. Despite the close proximity of both reaches, markedly higher discharge values were obtained for the incised reach and the discrepancy was used to calibrate roughness coefficients for the incised reach. A flow-duration curve determined on the basis of a 25-year series of daily discharges in the upstream gauging station together with data about channel geometry and roughness in the incised cross sections were used to simulate bedload transport at successive discharges with the BAGS sediment transport model. The calculations indicated the effective discharge for bedload transport in the incised reach to have the recurrence interval of 7–12 years. The upper limit of the reach-average effective discharge is associated with the stage slightly higher than a half of the bankfull depth. This highlights the disparity between effective and bankfull discharges and indicates that in incised channels, channel forming flow should be linked with the former. The vertical extent of the reach-average effective discharge is proposed as an indicator of the optimal, cost-effective height of solid bank-protection structures in the deeply incised channel and the suitability of this approach is discussed.

(Mis) management of floodplain vegetation: The effect of invasive species on vegetation roughness and flood levels

Floodplains are prone to plant invasions, which increase their roughness and decrease their flood conveyance capacity. In recent decades, extremely high floods have occurred in the Tisza River (Hungary) without an increase in discharge. This could be partly explained by land cover changes, as plough fields and pastures have been replaced by forest plantations and invasive plants have become widespread in the Tisza River floodplain. The aims of the present research were (1) to evaluate long-term land cover changes from the point of view of floodplain roughness, (2) to calculate vegetation density with and without the invasive shrub Amorpha fruticosa, and (3) to model (HEC-RAS) the flood conveyance in the case of unmanaged and managed vegetation (eliminating invasive plants). The study was carried out at three floodplain sections of the Tisza and Maros rivers, Hungary.In the eighteenth century, wetlands (61–93%) covered the studied floodplain areas, but as a result of mid-nineteenth-century channel regulation works, pastures and plough fields (42–72%) became widespread, and riparian forests (8–19%) appeared. In the late twentieth century, poplar plantations (43–86%) replaced pastures and plough fields and provided a perfect habitat for invasive plants. As a result of these land cover changes, the mean vegetation roughness of the floodplains increased from 0.021–0.032 (1783) to 0.066–0.092 (2017). However, at-site measurements indicate considerably higher vegetation roughness values (0.093–0.134) when the invasive Amorpha is also considered. Invasive species clearance could decrease the vegetation roughness by 86%.Based on our modelled data, peak flood stages could be decreased by 13–34 cm after the clearance of invasive plants. However, these values are influenced by the floodplain slope and characteristics of the modelled flood wave. The management of longer floodplain sections would have a considerable effect on flood stages, while the clearance of smaller patches would not have this effect.

Changes in Floodplain Vegetation Density and the Impact of Invasive Amorpha fruticosa on Flood Conveyance

Abstract Flood conveyance of floodplains is significantly influenced by the riparian vegetation cover, since vegetation affects flow velocity, therefore has a considerable impact on flood height and rate and pattern of sedimentation. However, climate change promotes the spread of invasive species, and their rapid growth results in dense vegetation stands, thus they have a significant impact on floodwater hydraulics. The aims of the present study are (1) to analyse the long-term changes in land-use and vegetation density on the Lower Tisza River, (2) to evaluate the role of the invasive Amorpha fruticosa in increasing vegetation density, and (3) to model the effect of dense floodplain vegetation on flood level and flood conveyance. Long-term (1784-2017) changes of land-use suggest that in natural conditions the study area was occupied by wetlands (92%), thus water covered the area for almost the whole year. In the 19th century, after levee constructions the wetlands were replaced by meadows and pastures (94%), then by the end of the 20th century planted and riparian forests replaced these land-covers. As a result, the mean roughness (0.14) of the floodplain has increased threefold until the early 21st century. Today forests are invaded by Amorpha fruticosa, which increases the vegetation density by 3% in riparian forests, by 23% in forest plantations, and by up to 100% in abandoned pastures and a rable lands. According to the results of HEC-RAS (Hydrologic Engineering Center’s River Analysis System) and CES (Conveyance Estimation System) models, if floodplain vegetation was managed and Amorpha fruticosa was cleared from the floodplain, peak flood level would decrease by 15 cm. Due to dense vegetation, the flood conveyance decreased by 4-6%, and the presence of Amorpha fruticosa reduced the flood flow velocities by 0.014-0.016 m/s. Accordingly, clearance of the floodplain from Amorpha fruticosa would have positive effects on flood protection, since peak flood stages would decrease and flood waves would shorten.

Quantifying the contribution of flood intensity indicators with the projection pursuit model

Abstract Identifying the various factors that affect the intensity of a flood event, such as its duration and volume, is essential for strategic planning and flood management. Further, quantifying the impacts of these major factors on flood intensity using the contribution rate is essential, but technically challenging. In this study, the authors have adopted the projection pursuit model to quantify the contribution rates of peak flood stage and peak flood discharge, flood duration, and total flood volume (the maximum 12-, 24-, and 72-hour flood volumes) in the Wujiang River in Southern China. This study showed that peak flood discharge and total flood volume were the two dominant factors impacting flood intensity. Although flood duration can be a major factor for some flood events, it contributed the least to flood intensity for most of the historic flood events studied. Likewise, the maximum 24-hour and 72-hour flood volumes contributed little to flood intensity. Findings from this study not only demonstrated the successful adoption of the projection pursuit model for contribution rates, but also provided critical information for planning and managing the regional hydraulic resources in the Wujiang River.

Effects of sea level rise on hydrology: case study in a typical mid-Atlantic coastal watershed

Abstract Sea level rise (SLR) can negatively affect the hydrology of coastal watersheds. However, the relevant information is incomplete and insufficient in existing literature. The objective of this study is to present a modeling approach to predict long-term effects of SLR on changes of flood peak, flood stage, and groundwater table with an assumption that the historical climate would reoccur in the future. The study was conducted for a typical coastal watershed in southeast USA. The results indicate that sea level had been rising at a rate of 4.21 mm yr−1 from 1948 to 1982 but at a faster rate of 5.16 mm yr−1 from 1983 to 2013. At such SLR rates and by 2113, the groundwater table beneath the eastern part of the watershed would be raised by 0.10 to 0.29 m, while the annual mean peak discharge and flood stage at the watershed outlet would be increased by 13.84 m3 s−1 (from 3.63 to 17.47 m3 s−1) and 0.92 m (from zero to 0.92 m), respectively. The other parts of the watershed would be relatively less affected by SLR. For coastal watersheds, SLR will probably raise the groundwater table, and increase the magnitude and occurrence of peak discharge and flood stage.

Influence of transient flooding on steel strip reinforced soil walls

The paper presents the results of three full-scale tests that were carried out to investigate the influence of transient (cyclic) flooding on the performance of steel strip reinforced soil walls (SSWs). The walls were constructed to a height of 6m and then flooded and drained to about mid-height in four cycles. The walls were constructed with three different granular soils varying with respect to permeability, fines content and shear strength. Earth pressures and reinforcement loads were recorded at end of construction and at the end of each flooding cycle prior to draining. Hence, for the purposes of analysis, the walls were treated as either in a drained or flooded steady state condition. In-situ steel strip pullout tests were also performed. The wall facings were very permeable and thus prevented unbalanced hydrostatic and seepage forces from developing during drawdown that could increase reinforcement strip loads beyond drained condition values. The effects of soil type on measured loads at the connections and peak tensile loads located within the reinforced soil zone are identified. Measured reinforcement tensile loads at end of construction and at the end of peak flood stages are compared to predictions using different analytical models for the (dry) EOC condition. Similar comparisons are made using measured pullout test results and predictions using different pullout models. Implications for current design practice and wall performance in transient flooding environments are reported.

강변저류지 홍수위 저감효과 분석을 위한 수치모형 적용

강변저류지는 홍수량을 일시적으로 분담하여 홍수위험도를 저감시키는 시설로 국내에서도 대하천 정비사업을 통하여 여주, 나주, 영월 지역의 홍수 저감을 위하여 강변저류지가 새로이 설치되었다. 본 연구에서는 신규 도입된 강변저류지 중 여주 강변저류지를 대상으로 1차원 및 2차원 수치모의를 수행하여 대하천에서의 강변저류지에 의한 홍수위 저감효과를 분석하였다. 1차원 모의는 HEC-RAS 및 FLDWAV 모형의 저류지 기능을 적용하여 부정류 모의를 통한 홍수위 저감효과를 분석하였으며, 2차원 모의는 CCHE2D 모형을 이용하여 1차원 모형과 동일한 홍수 사상을 적용한 후 홍수위 저감효과를 분석하였다. 1차원 모의에서는 월류제 부근에서 첨두홍수위가 가장 크게 감소하였으며, 2차원 모의에서는 모의 구간의 가장 상류에서 홍수위 저감 효과가 가장 크게 나타났다. 또한 첨두홍수위는 1차원 모형에서 최대 0.13 m, 2차원 모형에서는 최대 0.20 m가 감소하는 것으로 나타났다. 검토 결과 2차원 모형이 하도 흐름에 대한 지형의 물리적 특성이 반영되어 보다 합리적인 모의 결과를 얻는 것이 가능한 것으로 보이며, 횡월류위어의 부근에서 하도 내 흐름 거동뿐만 아니라 월류 유량이 유입된 후 강변저류지 내 거동을 검토하기에는 1차원 모형보다 유리한 것으로 나타났다. The retention basin is a river-facility for the flood mitigation by storing the river flow temporarily. The new 3 retention basins are installed in these regions YeoJu, NaJu, YoungWol by the Large River Management Project. In this study, 1D and 2D numerical flow simulation are conducted to evaluate the reduction effect of the peak flood stage for the YeoJu retention basin. HEC-RAS and FLDWAV models are used for 1D simulation with the option of retention basin. CCHE2D model is used for 2D simulation with the same hydrograph used in 1D simulation. It is verified that the peak flood stage is reduced very largely about 0.13 m near the overtopping section of the levee in 1D simulation. It is verified that the peak flood stage is reduced very largely about 0.20 m at the upstream-end of the simulated reach in 2D simulation. 2D simulation for the retention basin is more reasonable because physical characteristics of topography in the model, and also more advantageous for the evaluation of the flow characteristics of the in- and outside of the retention basin on the results of simulation of this study.

Incorporation of GIS Based Program into Hydraulic Model for Water Level Modeling on River Basin

Water resources management usually requires that hydraulic, ecological, and hydrological models be linked. The Hy- drologic Engineering Center River Analysis System (HEC-RAS) hydraulic model and the Hydrologic Engineering Center Geospatial River Analysis System (HEC-GEORAS), imitates flow and water profiles in the Neka river basin’s downstream flood plain. Hydrograph phases studied during the flood seasons of 1986-1999 and from 2002-2004 were used to calibrate and verify the hydraulic model respectively. Simulations of peak flood stages and hydrographs’ evaluations are congruent with studies and observations, with the former showing mean square errors between 4.8 - 10 cm. HECRAS calculations and forecast flood water levels. Nash-Sutcliffe effectiveness (CR3) is more than 0.92 along with elevated levels of water which were created with some effectiveness (CR5) of 0.94 for the validation period. The coupled two models show good performance in the water level modeling.

TEHRI DAM-BREACH VERSUS MONSOON FLOOD ROUTING IN THE GANGA RIVER SYSTEM

ABSTRACT Dam breach or dam break causes release of large quantity of storage water from the reservoir creating major flood wave capable of causing disastrous damage to the downstream residents and property. The preparedness to withstand such an eventuality by predicting the possible extent of inundation of downstream zone and by formulating the emergency action plan can mitigate the disaster. The present study aims at predicting the characteristics of the flood wave such as peak flood stage, peak flood discharge and their times of occurrence and stage and discharge hydrographs at different locations downstream in the river due to Tehri dam-breach flood and also for monsoon flood. NWS-BREACH model is used for Tehri dam on the Ganga river, to predict the breach characteristics and the breach hydrograph. The predicted dam-breach flood hydrograph is routed through the Ganga river system using NWS-FLDWAV model, assuming that all the major tributaries joining the downstream Ganga river are already in flood state. The monsoon flood is also routed through the Ganga river system using NWS- FLDWAV. The dam-breach flood wave characteristics at different locations along the downstream of main river and its tributaries are predicted, and the results are discussed. The results of dam-breach flood routing are compared with those of estimated from the monsoon flood routing.

Identification and micro‐stratigraphy of hyperpycnites and turbidites in Cretaceous Lewis Shale, Wyoming

AbstractSandy hyperpycnal flows and their deposits, hyperpycnites, have been documented in modern environments and, more recently, in Cretaceous and Tertiary strata; they may be more common in the rock record, and within petroleum reservoirs, than has been previously thought. Muddy hyperpycnites also occur within the rock record, but these are more difficult to document because of their finer‐grained nature and lack of common sedimentary structures. This paper documents the presence of submarine slope mudstone and siltstone hyperpycnites (and muddy turbidites) in the delta‐fed, Upper Cretaceous Lewis Shale of Wyoming; based on field measurements, analyses of rock slabs and thin sections, and laser grain‐size distributions. Four lithofacies comprise laminated and thin‐bedded mudstones that are associated with levéed channel sandstones: (L1) grey, laminated, graded mudstone with thin siltstone and sandstone interbeds; (L2) dark grey to tan, laminated mudstone with very thin siltstone and sandstone stringers; (L3) light grey, laminated siltstones; and (L4) laminated mudstones and siltstones with thin sandstone interbeds. Two styles of mudstone grain‐size grading have been documented. The first type is an upward‐fining interval that typically ranges in thickness from 2·5 to 5 cm. The second type is a couplet of a lower, upward‐coarsening interval and an upper, upward‐fining interval (sometimes separated by a micro‐erosion surface) which, combined, are about 3·8 cm thick. Both individual laminae and groups of laminae spaced millimetres apart exhibit these two grain‐size trends. Although sedimentary structures indicative of traction‐plus‐fallout sedimentary processes associated with sandier hyperpycnites are generally absent in these muddy sediments, the size grading patterns are similar to those postulated in the literature for sandy hyperpycnites. Thus, the combined upward‐coarsening, then upward‐fining couplets are interpreted to be the result of a progressive increase in river discharge during waxing and peak flood stage (upward increase in grain‐size), followed by waning flow after the flood begins to abate (upward decrease in grain‐size). The micro‐erosion surface that sometimes divides the two parts of the size‐graded couplet resulted from waxing flows of sufficiently high velocity to erode the sediment previously deposited by the same flow. Individual laminae sets which only exhibit upward‐fining trends could be either the result of waning flow deposition from either dilute turbidity currents or from hyperpycnal flows. The occurrence of these sets with the size‐graded couplets suggests that they are associated with hyperpycnal processes.

Architecture of pre-vegetation sandy-braided perennial and ephemeral river deposits in the Paleoproterozoic Athabasca Group, northern Saskatchewan, Canada as indicators of Precambrian fluvial style

The Late Paleoproterozoic Athabasca Basin contains more than 1.5 km of predominantly sandy strata, most of which are of braided fluvial origin. In the eastern part of the basin, at McClean Lake, sandstones and minor conglomerates of the Read Formation at the base of the succession are preserved within a steep-walled valley system. They consist predominantly of meter-scale sheet elements, characterized by massive and flat-laminated fine- to very coarse-grained sandstone with minor discontinuous cobble and boulder conglomerate along lower set boundaries. These are interpreted as sheet-flood and stream-flood deposits of a terminal dry-land system, deposited in an arid climatic setting. Analysis of closely spaced drill-core indicates that late stage run-off was confined to small shallow channels, typically only a few tens of meters wide and a few meters deep. Overlying strata of the Bird Member of the Manitou Falls Formation are predominantly coarse- to very coarse-grained sandstones with abundant small-scale cross-stratification and minor granule and small pebble conglomerate. At McClean Lake, these appear to have been deposited as sheet-elements by semi-ephemeral to perennial braided rivers under more humid conditions. At Key Lake, 160 km to the southwest, architectural analysis of a 184 m wide section indicates that at least part of the Bird Member was deposited in deeper, sandy-braided rivers characterized by seasonally varied flow. The closest modern analogue is the South Saskatchewan River, in which large simple flow-transverse bedforms become exposed and dissected during falling stage, and act as nuclei for sand-flat development. The presence of numerous low-relief 4th order surfaces suggests continued reworking of bar-tops during rising stage. The incremental character of downstream accretionary elements suggests periodic migration of barforms during peak-flood stages, separated by periods of low flow. This indicates similarities with modern perennial braided systems, and is counter to the idea that all pre-Devonian fluvial systems should consist of stacked sheets formed by individual flood events. Thin gravel layers accumulated as lags on fourth order surfaces, with discontinuous mud layers suggesting deposition within temporary ponds in channel thalwegs after major floods. Strata in overlying units indicate a return to semi-perennial conditions.

Distribution of the extreme peak floods of the Tiber River from the XV century

The peak stages of the extreme floods inundating the town of Rome are known since the XV century. This record covering more than 500 years can be considered as a censored sample of the population of the peak flood stages at the site. To evaluate the peak flow of the observed events the following information is available: (a) a long record of daily stage measurements up to the XVIII century; (b) several records of daily rainfall depth measurement at two rain gauges in the Tiber catchment, extending at least up to the middle of the XIX century; (c) detailed surveys executed immediately after the great flood of 1870, including the elevation plan of the town, a survey of the riverbed and the flood hydrograph of the 1870 flood; (d) a less detailed survey of the riverbed executed in 1744; (e) a rich iconography, showing the conditions of the Tiber banks from the XVI century onward; (f) contemporary descriptions of several extreme floodings; and (g) a rich series of flow measurements and bed surveys after the great flood of 1870 to present days. A two-dimensional hydrodynamic model was used to simulate the flooding of the old town in order to determine the flow–discharge relationship, thus allowing reconstruction of a record of the past extreme floods. A statistical analysis was developed for evaluating the risk hazard, using the censored sample of all the extreme floods from the XV century and the sample of all the maximum annual floods from the end of the XVIII century.

Engineered Levee Breaches for Flood Mitigation

Inland flooding from excessive precipitation and surface runoff remains the cause of extensive damage, loss of property, and human suffering worldwide. In this project, an engineered levee breach was considered as a measure to mitigate floods, and design attributes of the optimal levee breach were examined. The engineered levee breach creates depression waves that destructively interfere with the flood wave to crop peak flood stages. Furthermore, the engineered levee breach permits control over flood stage at a point along a channel where flood stage reduction is most beneficial in terms of hazard mitigation. The roles of flood plain storage, breach size, flood discharge, flood duration, and breach timing in the optimal design of engineered levee breaches were examined using a shallow-water model, and a strategy to design levee breaches is obtained by scaling the model results. It is shown that substantial flood stage reduction can be achieved with an engineered breach. This approach to flood fighting could prove useful in the mitigation of relatively short, extreme floods.

A leaky reservoir model for ephemeral flow recession

The flood wave in ephemeral streams can often be portrayed accurately by assuming ‘instantaneous ’ rise to peak flood stage (depth) and specifying a recession curve. Here recession is modeled as a conceptual analog of discharge from a single leaky reservoir. Physically, the reservoir may be considered to approximate that portion of the ephemeral stream channel that is flowing at the onset of recession. The discharging reservoir is described by a continuity equation and by discharge‐stage (stream rating) and storage‐stage (reservoir configuration) relations. No input is routed through the reservoir; at the onset of recession the reservoir has water in storage. A good fit to observed curves is obtained by calibrating two parameters, reservoir leakage rate and initial storage (reservoir shape). The agreement is most sensitive to changes in initial storage. The best fit parameter values are physically plausible.

Bar evolution and development of stratification in the gravelly, braided, Kicking Horse River, British Columbia

The Kicking Horse River is a gravelly, braided stream characterized by very low winter discharge, a peak spring flood (70 m3s−1 in 1973), and summer diurnal discharge fluctuations (from 18 to 44 m3/s−1 in 1973) related to daily melting on the glaciers that supply the river. The reach studied, near Field, B.C., is characterized by abundant braid bars, and was subdivided into three parts.The upstream reach is characterized by only one major channel, with few unit bars within it. Clast movement takes place at peak flood stages, and the clasts move in 'diffuse gravel sheets' on the channel floor. These sheets are only one to two clast diameters in thickness, and when the clasts stop rolling, the sheet becomes a coarse lag. By contrast, in the midstream and downstream reaches, the channel is more anastomosing, and there are many in-channel bars, mostly transverse and diagonal unit bars. The diagonal bars mostly lack foreset slopes at their downstream margin, and it is suggested that massive to crudely horizontally stratified gravels would be deposited. Transverse bars normally have a foreset slope, giving rise to cross-stratified gravels.Several bars were monitored continuously at their active stages during the diurnal rise in discharge in the late afternoon – evening. Measurement of water slope, current velocity, depth, clast sizes, and rate of bar migration were made every 15 minutes at each of the stations on each bar. The bars tended to advance episodically during periods of velocity increase, as recorded at the upstream monitoring station. However, clast size at the bar front does not seem to increase as velocity increases, possibly due to shielding effects of the largest, stationary, clasts on the bed. This effect made existing initiation-of-bedload-movement equations difficult to apply, and of severely limited use with respect to the poorly sorted gravels.Stratification could not be directly observed in the bars, but we suggest that upstream, the diffuse gravel sheets would form crudely horizontally stratified gravels, probably with a long-axis-transverse imbrication. In the midstream and downstream reaches, in finer gravels, more cross-stratification would be present, related to migration of transverse bars.