Ungauged Streams Research Articles

Development of a Model to Identify the Flow at any Location of an Ungauged Stream for Selection of Sites for Small Hydel in the Kalimpong Subdivision of West Bengal

ABSTRACT Before the implementation of any small scale hydropower scheme, it is essential to determine the availability of sufficient and reliable amount of water to make the scheme economically viable. The sites for small hydels are generally located in the inaccessible tracts of hilly regions. This hinders the proper evaluation of the site. The investments in the small hydels are also comparatively very low as compared to a big hydel power plant; as such not much of fund can be allocated to the initial site suitability decisions. Once a best site in a stream having suitable head has been identified then proper and detailed survey of the site could be undertaken for the construction of the plant. The streams on which small hydel plants are planned are mostly ungauged, which again poses a problem in identifying the amount of flow at certain location. In this work an attempt has been made to create a model for identification of flow at any location of a watershed. The two components of flow that is the base flow and runoff has been taken into consideration for calculating the flow. The base flow has been identified by direct measurement and run off has been delineated using the distributed curve number method. Finally a regression has been done to come to a generalized equation for determination of total flow at any point of a watershed in the Kalimpong subdivision.

Flood Hazard Management in Public Mountain Recreation Areas vs. Ungauged Fluvial Basins. Case Study of the Caldera de Taburiente National Park, Canary Islands (Spain)

Las Angustias River is an ungauged stream in the Caldera de Taburiente National Park (Spain), where frequent intense flash-flood events occur. The aim of this research is to analyze the flood hazard at the Playa de Taburiente. Based on the limited information available (short time-series of daily precipitation), a statistical frequency analysis of 24 h rainfall was completed and the precipitation results were transformed into surface runoff. To determine if the model underestimates the flows that are generated in the basin, the dendro-geomorphological information available was used to calibrate results. The results of the HMS model were significantly lower. At this point, both the rainfall data and the rainfall-runoff model were re-analyzed to maximize the rainfall intensity values and the runoff generated (increasing the CN value for the basin). For the 1997 flood event, a 1250 m3·s−1 flood minimizes the RMSE for the disturbed tree sample; this flow value also clearly exceeds any peak flow derived from the rainfall-runoff analysis. It is only when rainfall intensity and surface runoff are maximized that the peak flows obtained approximate those associated with dendro-geomorphological data. The results highlight the difficulties of flood hazard management in ungauged torrential basins in mountain recreational areas (such as National Parks). Thus, in the absence of flow records, when considering the maximum rainfall intensity scenario may be a useful and effective tool for flood risk management.

Runoff prediction in ungauged catchments in Norway: comparison of regionalization approaches

Abstract Runoff prediction in ungauged catchments has been a challenging topic over recent decades. Much research have been conducted including the intensive studies of the PUB (Prediction in Ungauged Basins) Decade of the International Association for Hydrological Science. Great progress has been made in the field of regionalization study of hydrological models; however, there is no clear conclusion yet about the applicability of various methods in different regions and for different models. This study made a comprehensive assessment of the strengths and limitations of existing regionalization methods in predicting ungauged stream flows in the high latitudes, large climate and geographically diverse, seasonally snow-covered mountainous catchments of Norway. The regionalization methods were evaluated using the water balance model – WASMOD (Water And Snow balance MODeling system) on 118 independent catchments in Norway, and the results show that: (1) distance-based similarity approaches (spatial proximity, physical similarity) performed better than regression-based approaches; (2) one of the combination approaches (combining spatial proximity and physical similarity methods) could slightly improve the simulation; and (3) classifying the catchments into homogeneous groups did not improve the simulations in ungauged catchments in our study region. This study contributes to the theoretical understanding and development of regionalization methods.

Stream flow simulation and verification in ungauged zones by coupling hydrological and hydrodynamic models: a case study of the Poyang Lake ungauged zone

Abstract. To solve the problem of estimating and verifying stream flow without direct observation data, we estimated stream flow in ungauged zones by coupling a hydrological model with a hydrodynamic model, using the Poyang Lake basin as a test case. To simulate the stream flow of the ungauged zone, we built a soil and water assessment tool (SWAT) model for the entire catchment area covering the upstream gauged area and ungauged zone, and then calibrated the SWAT model using the data in the gauged area. To verify the results, we built two hydrodynamic scenarios (the original and adjusted scenarios) for Poyang Lake using the Delft3D model. In the original scenario, the upstream boundary condition is the observed stream flow from the upstream gauged area, while, in the adjusted scenario, it is the sum of the observed stream flow from the gauged area and the simulated stream flow from the ungauged zone. The experimental results showed that there is a stronger correlation and lower bias (R2 = 0.81, PBIAS = 10.00 %) between the observed and simulated stream flow in the adjusted scenario compared to that (R2 = 0.77, PBIAS = 20.10 %) in the original scenario, suggesting the simulated stream flow of the ungauged zone is reasonable. Using this method, we estimated the stream flow of the Poyang Lake ungauged zone as 16.4 ± 6.2 billion m3 a−1, representing ∼ 11.24 % of the annual total water yield of the entire watershed. Of the annual water yield, 70 % (11.48 billion m3 a−1) is concentrated in the wet season, while 30 % (4.92 billion m3 a−1) comes from the dry season. The ungauged stream flow significantly improves the water balance with the closing error decreased by 13.48 billion m3 a−1 (10.10 % of the total annual water resource) from 30.20 ± 9.1 billion m3 a−1 (20.10 % of the total annual water resource) to 16.72 ± 8.53 billion m3 a−1 (10.00 % of the total annual water resource). The method can be extended to other lake, river, or ocean basins where observation data is unavailable.

Modeled hydrologic metrics show links between hydrology and the functional composition of stream assemblages.

Flow alteration is widespread in streams, but current understanding of the effects of differences in flow characteristics on stream biological communities is incomplete. We tested hypotheses about the effect of variation in hydrology on stream communities by using generalized additive models to relate watershed information to the values of different flow metrics at gauged sites. Flow models accounted for 54-80% of the spatial variation in flow metric values among gauged sites. We then used these models to predict flow metrics in 842 ungauged stream sites in the mid-Atlantic United States that were sampled for fish, macroinvertebrates, and environmental covariates. Fish and macroinvertebrate assemblages were characterized in terms of a suite of metrics that quantified aspects of community composition, diversity, and functional traits that were expected to be associated with differences in flow characteristics. We related modeled flow metrics to biological metrics in a series of stressor-response models. Our analyses identified both drying and base flow instability as explaining 30-50% of the observed variability in fish and invertebrate community composition. Variations in community composition were related to variations in the prevalence of dispersal traits in invertebrates and trophic guilds in fish. The results demonstrate that we can use statistical models to predict hydrologic conditions at bioassessment sites, which, in turn, we can use to estimate relationships between flow conditions and biological characteristics. This analysis provides an approach to quantify the effects of spatial variation in flow metrics using readily available biomonitoring data.

Probabilistic flood inundation mapping at ungauged streams due to roughness coefficient uncertainty in hydraulic modelling

Abstract. Probabilistic flood inundation mapping is performed and analysed at the ungauged Xerias stream reach, Volos, Greece. The study evaluates the uncertainty introduced by the roughness coefficient values on hydraulic models in flood inundation modelling and mapping. The well-established one-dimensional (1-D) hydraulic model, HEC-RAS is selected and linked to Monte-Carlo simulations of hydraulic roughness. Terrestrial Laser Scanner data have been used to produce a high quality DEM for input data uncertainty minimisation and to improve determination accuracy on stream channel topography required by the hydraulic model. Initial Manning's n roughness coefficient values are based on pebble count field surveys and empirical formulas. Various theoretical probability distributions are fitted and evaluated on their accuracy to represent the estimated roughness values. Finally, Latin Hypercube Sampling has been used for generation of different sets of Manning roughness values and flood inundation probability maps have been created with the use of Monte Carlo simulations. Historical flood extent data, from an extreme historical flash flood event, are used for validation of the method. The calibration process is based on a binary wet-dry reasoning with the use of Median Absolute Percentage Error evaluation metric. The results show that the proposed procedure supports probabilistic flood hazard mapping at ungauged rivers and provides water resources managers with valuable information for planning and implementing flood risk mitigation strategies.

Channel response to extreme floods: Insights on controlling factors from six mountain rivers in northern Apennines, Italy

This work addresses the geomorphic response of mountain rivers to extreme floods, exploring the relationships between morphological changes and controlling factors. The research was conducted on six tributaries of the Magra River (northern Apennines, Italy) whose catchments were affected by an extreme flood (estimated recurrence interval>100years in most of the basins) on 25 October 2011. An integrated approach was deployed to study this flood, including (i) analysis of channel width changes by comparing aerial photographs taken before and after the flood, (ii) estimate of peak discharges in ungauged streams, (iii) detailed mapping of landslides and analysis of their connectivity with the channel network.Channel widening occurred in 35 reaches out of 39. In reaches with channel slope<4% (here defined as nonsteep reaches), average and maximum ratios of post-flood and pre-flood channel width were 5.2 and 19.7 (i.e., channel widened from 4 to 82m), respectively. In steep reaches (slope≥4%), widening was slightly less intense (i.e., average width ratio=3.4, maximum width ratio=9.6). The relationships between the degree of channel widening and seven controlling factors were explored at subreach scale by using multiple regression models. In the steep subreaches characterized by higher confinement, the degree of channel widening (i.e., width ratio) showed relatively strong relationships with cross-sectional stream power, unit stream power (calculated based on pre-flood channel width), and lateral confinement, with coefficients of multiple determination (R2) ranging between 0.43 and 0.67. The models for the nonsteep subreaches provided a lower explanation of widening variability, with R2 ranging from 0.30 to 0.38; in these reaches a significant although weak relation was found between the degree of channel widening and the hillslope area supplying sediment to the channels.Results indicate that hydraulic variables alone are not sufficient to satisfactorily explain the channel response to extreme floods, and inclusion of other factors such as lateral confinement is needed to increase explanatory capability of regression models. Concerning hydraulic variables, this study showed that the degree of channel widening is more strongly related to unit stream power calculated based on pre-flood channel width than to cross-sectional stream power and to unit stream power calculated with post-flood channel width. This could suggest that most width changes occurred after the flood peak. Finally, in terms of hazard, it is crucial to document the type and magnitude of channel changes, to identify controlling factors, and most importantly, to develop tools enabling us to predict where major geomorphic changes occur during an extreme flood.

Integrative neural networks model for prediction of sediment rating curve parameters for ungauged basins

Summary One of the most uncertain modeling tasks in hydrology is the prediction of ungauged stream sediment load and concentration statistics. This study presents integrated artificial neural networks (ANN) models for prediction of sediment rating curve parameters (rating curve coefficient α and rating curve exponent β ) for ungauged basins. The ANN models integrate a comprehensive list of input parameters to improve the accuracy achieved; the input parameters used include: soil, land use, topographic, climatic, and hydrometric data sets. The ANN models were trained on the randomly selected 2/3 of the dataset of 94 gauged streams in Ontario, Canada and validated on the remaining 1/3. The developed models have high correlation coefficients of 0.92 and 0.86 for α and β , respectively. The ANN model for the rating coefficient α is directly proportional to rainfall erosivity factor, soil erodibility factor, and apportionment entropy disorder index, whereas it is inversely proportional to vegetation cover and mean annual snowfall. The ANN model for the rating exponent β is directly proportional to mean annual precipitation, the apportionment entropy disorder index, main channel slope, standard deviation of daily discharge, and inversely proportional to the fraction of basin area covered by wetlands and swamps. Sediment rating curves are essential tools for the calculation of sediment load, concentration-duration curve (CDC), and concentration–duration–frequency (CDF) analysis for more accurate assessment of water quality for ungauged basins.

Development and Application of Spatially Parameterized Depth Duration Frequency Model for Estimation of Design Rainfall for Oromia State, Ethiopia

The magnitude and frequency of extreme rainfall events are required for planning, design and operation of many hydrological and water resources projects. Design rainfall depth is often used to estimate the severity and rarity of floods in areas where flow records are not sufficient enough to warrant direct flood estimation. The design of hydraulic structures on un-gauged streams and creeks, such as bridges, culverts, spillways, water harvesting and flood defense mechanisms depends upon proper estimation of extreme rainfall events. Quantification of design rainfall is generally done by using information contained in Depth-Duration-Frequency (DDF) relationships. Depth Duration Frequency relationships are currently constructed based on at site frequency analysis of rainfall data separately for different durations. These relationships are not accurate and reliable since they depend on assumptions such as distribution selection for each duration; they require a large number of parameters, experience intensive equations and regionalization is also very poor and coarse. In this study a DDF model with gridded set of parameters is developed for estimation of point rainfall frequencies for a range of duration for any location in Oromia regional state. A DDF model was fitted to series of annual maxima and its parameters were determined by a least squares method and these parameters were interpolated and mapped on a 1km grid. The model allows for a parsimonious and efficient parameterization of DDF relationships, and its performance is shown to improve the reliability and robustness of design storm predictions as compared with those achievable by interpolating the quantile predictions of extreme rainfall data for specific durations. Moreover, design rainfall estimates found from the scaling DDF model are comparable to estimates obtained from traditional techniques; however, the scaled approach was more efficient and gives more reliable estimate compared with the observed rainfall depth at all stations.

Morphometric Characteristics and the Relation of Stream Orders to Hydraulic Parameters of River Goro: An Ephemeral River in Dire-dawa, Ethiopia

Major morphometric parameters of Goro watershed were derived from Aster Global Digital Elevation Model (GDEM) using ArcGIS10.1 extension, Arc Hydro Tool 2.0 in order to characterize it quantitatively. The study also assessed the relation of stream orders to selected channel hydraulic parameters (width, depth, channel bed slope, velocity and discharge) at bankfull flow condition. Channel geometry data and grain size samples collected from 14 representative cross-sections used as input to compute hydraulic parameters using equations appropriate for ungauged streams. The analysis of morphometric parameters indicate that pattern of stream networks is less controlled by structural condition though the area is situated in the rim of the great east African rift valley; and its geomorphic development is at late youth stage. Though hydraulic components of the channel do have a positive relation with stream orders, only channel cross-section area and discharge well related to stream orders (each increased at the rate of 0.86 and 1 on average with R 2 of 0.92 and 0.8 respectively with increasing in stream orders). However, depth, velocity and bed slope of the channel are less explained by stream order, which may indicate us these hydraulic parameters are rather affected by other local channel factors.

Seasonal soil moisture patterns: Controlling transit time distributions in a forested headwater catchment

AbstractThe Transit Time Distribution (TTD) of a catchment is frequently used for understanding flow paths, storage characteristics, and runoff sources. Despite previous studies, the connections between catchment characteristics and TTDs are still not fully understood. We present results from a 2 year stable isotope tracer investigation in the forested Wüstebach headwater catchment (38.5 ha), including precipitation, stream, and tributary locations. We used the gauged outlet to determine effective precipitation (peff), subdivided for wet and dry catchment state, and assumed it to be spatially uniform. We then calculated TTDs of 14 ungauged stream and tributary locations where stable isotope tracer information was available and compared them to respective subcatchment areas and the proportion of riparian zone within the subcatchments. Our approach gave insight into the spatial heterogeneity of TTDs along the Wüstebach River. We found that hydrological hillslope‐riparian zone disconnection was an important factor, as the catchment shifted between two distinct, time‐variant hydrological responses that were governed by seasonal changes of overall catchment wetness. The difference in hydrological behavior of the riparian zone and hillslopes could explain the often encountered “old water phenomenon,” where considerable amounts of old water quickly appear as runoff. TTD results showed a negative correlation between riparian zone proportion and Mean Transit Time (MTT), corroborated by the dense network of soil water content measurements. No correlation between subcatchment size and MTT was found.

Mean runoff coefficient estimation for ungauged streams in the Kingdom of Saudi Arabia

Runoff coefficient estimation in ungauged watersheds has a priority for rain water harvesting and management of runoff water, for domestic and agriculture activities, in semi-arid and arid regions. To estimate mean runoff coefficient (CRe) for ungauged streams, Pearson's coefficient of linear correlation (r) was measured. The method of linear regression (y = mx + c) was applied for 16 gauged catchments representing several regions in the Kingdom of Saudi Arabia, such as Al Qassim, Al Madinah, Riyadh, Asir, Makkah, and Jazan. The studied catchments were equally divided into two groups based on their main streams slopes; group A includes eight gauged streams having main stream slope less than 0.01, where group B includes eight gauged streams having main stream slope equal to or greater than 0.01. The result yields the constant of the linear regression for each group and the mean runoff coefficient of basin by an independent value (basin slope) for group A and an independent value (stream slope) for group B. The results indicate that the measured runoff coefficient (CRm) and the estimated runoff coefficient (CRe) are almost equal to each other.

Application of Fuzzy Logic Techniques in Estimating the Regional Index Flow for Michigan

Knowledge of flow dynamics within streams is not only helpful in determining applications, such as water withdrawals, but is also a driving force in a watershed for other components, such as nutrients, dissolved oxygen, and ecological health. With increased pressure on water resources, along with growing attention to climate change and anthropogenic activity, the ability to accurately predict extreme conditions continues to be invaluable to decision makers and watershed managers. Our objective was to explore new concepts and to identify robust techniques for estimating the index water yield and subsequently the index flow for ungauged streams in the state of Michigan. Four different modeling methods (multiple linear regression, fuzzy regression, fuzzy expert, and adaptive neuro-fuzzy inference system, or ANFIS) were evaluated using 10-fold cross-validation. A combination of statistical and graphical analyses confirmed the advantages of 10-fold cross-validation in selecting the best model while avoiding overfitting or generating unexplained values such as outliers. Results showed that the fuzzy expert method was the best and most robust prediction model. Further evaluation revealed that the ANFIS model preserved many attributes of the data set, at the expense of overfitting and generating unexplained values such as outliers.

Using correlation of daily flows to identify index gauges for ungauged streams

Key Points Correlation of flows can be used to select index gauges for predicting flow

Reply to Marchi's comment on &quot;Geomorphic hazards and intense rainfall: the case study of the Recco Stream catchment (Eastern Liguria, Italy)&quot; by Faccini et al. (2012)

“Geomorphic hazard and intense rainfall: the case study of the Recco Stream Catchment (Eastern Liguria, Italy)” (Nat. Hazards Earth Syst. Sci., 12, 893–903, 2012) presented the impact of a heavy and short rainfall event in the Ligurian area (NW Mediterranean). It triggered several shallow landslides and flooded a part of the Recco alluvial plain. In the last decades, such events are increasing in Liguria and in the Mediterranean. In the Recco Stream catchment, we analyzed the conditions of geomorphic hazard related to the recent intensification of heavy rainfalls. We demonstrated the inadequacy of the Basin Master Plan for the slope environment. In fact, most of the instability phenomena were triggered in areas classified with medium and low geomorphic hazard. In the section “Information about the target event rainfall” the triggering weather condition was considered. The pre-frontal thunderstorm super cell caused heavy and short rainfalls concentrated on a very narrow area. These phenomena occur particularly in the central area of Liguria, due to AlpineApenninic orography and the Ligurian Sea. The radar image in Fig. 3c of the paper shows this situation from a geographical point of view. The isoyeth map of the 1 June 2007 event (Fig. 3b) was drawn on the basis of rain gauges data recorded in the six surrounding weather stations (Chiavari, Genova, Bargagli, Colonia Arnaldi, Polanesi, Bavari). This is supported by a hyetogram recorded at Polanesi station, located in the lower part of the Recco Stream catchment. The flood peak discharge has been preliminarly assessed on a rainfall/runoff evaluation. Further analysis in an un-gauged stream can be carried out by means of post-flood topographic surveys; the peak discharge was assessed by using six significant hydraulic cross sections of the Basin Master Plan (Provincia di Fig. 1. Locations of cross sections in the Recco Stream channel network.

Research Spotlight: New method to estimate daily flow rates at streams that lack gauges

Records of daily streamflow rates can help scientists estimate future runoff volumes or determine whether fish and other aquatic species have enough water to survive an upcoming season. Many streams are monitored by gauges; however, the sheer number of catchments throughout the United States makes it impractical to have instruments at every stream. As a result, scientists and water resources managers often face the problem of needing to know flow rates within ungauged streams. Archfield and Vogel have developed a method to estimate such flow rates. Using southern New England as a case study, the authors first selected a gauged stream and then, using statistical methods, compared the record of daily streamflow rates from that stream spanning the past 50 years to 27 other streams in the area. On the basis of this comparison, they made a map that marked out the region surrounding their stream where rates at the gauged stream were correlated to other streams about 98% of the time. The researchers then drew borders around the area where their stream matched other streams 95% of the time. Through a series of decreasing percentages of correlation, the authors built a spatial view of how closely their reference‐gauged stream matched other streams in the region. (Water Resources Research, doi:10.1029/2009WR008481, 2010)

Hydrographs and Estimates of Scour Depth Excess for Pier Scour Prediction

A practical procedure for using the Scour Rate in Cohesive Soils (SRICOS) method to predict bridge pier scour depth in ungauged streams is presented. The procedure is illustrated by the Interstate 90 westbound bridge over Split Rock Creek in South Dakota. Instead of generating a continuous hydrograph for the design service life of a bridge, the simplified approach considered a series of maximum annual floods. The parameters of peak flow distribution were calculated with estimates of peak flow magnitude obtained from regional regression equations. Flood duration was estimated with a rainfall atlas and a Soil Conservation Service runoff curve number based on a design storm. A set of equally probable future hydrographs was generated and used with the SRICOS method to compute the final scour depth. The predicted final scour depth followed the normal distribution. The risk values associated with different design scour depths were calculated with the mean and standard deviation of the normal distribution. The scour predictions were consistent with a bridge founded on highly erosion-resistant cohesive soils. The cumulative effect of scour from the small floods in a long sequence of floods could be represented by an equivalent time; the scour depth produced by the whole sequence could be modeled by using only the large floods if the equivalent time were small compared with the combined duration of the large floods.

Characterisation of selected extreme flash floods in Europe and implications for flood risk management

summary High-resolution data enabling identification and analysis of the hydrometeorological causative processes of flash floods have been collected and analysed for 25 extreme flash floods (60 drainage basins, ranging in area from 9.5 to 1856 km 2 ) across Europe. Most of the selected floods are located in a geographical belt crossing Europe from western Mediterranean (Catalunia and southwestern France) to Black Sea, covering northern Italy, Slovenia, Austria, Slovakia and Romania. Criteria for flood selection were high intensity of triggering rainfall and flood response and availability of high-resolution reliable data. Hydrometeorological data collected and collated for each event were checked by using a hydrological model. The derivation and analysis of summarising variables based on the data archive has made it possible to outline some characteristics of flash floods in various morphoclimatic regions of Europe. Peak discharge data for more than 50% of the studied watersheds derive from post-flood surveys in ungauged streams. This stresses both the significance of post-flood surveys in building and extending flash flood data bases, and the need to develop new methods for flash flood hazard assessment able to take into account data from post-event analysis. Examination of data shows a peculiar seasonality effect on flash flood occurrence, with events in the Mediterranean and Alpine–Mediterranean regions mostly occurring in autumn, whereas events in the inland Continental region commonly occur in summer, revealing different climatic forcing. Consistently with this seasonality effect, spatial extent and duration of the events is generally smaller for the Continental events with respect to those occurring in the Mediterranean region. Furthermore, the flash flood regime is generally more intense in the Mediterranean Region than in the Continental areas. The runoff coefficients of the studied flash floods are usually rather low (mean value: 0.35). Moderate differences in runoff coefficient are observed between the studied climatic regions, with higher values in the Mediterranean region. Antecedent saturation conditions have a significant impact on event runoff coefficients, showing the influence of initial soil moisture status even on extreme flash flood events and stressing the importance of accounting soil moisture for operational flash flood forecasting. The runoff response displays short lag times (mostly <6 h). The identified relations between watershed area, stream length and response time enable determination of a characteristic mean velocity of the flash flood process (at basin scales less than 350 km 2 ), defined as the ratio of characteristic length (mean river length) and time (response time or lag time), equal to 3 m s � 1 . This is related to the celerity with which the flood wave moves through the catchment. The analysis of the response time provides information on the time resolution and the spatial density of the networks required for monitoring the storms that generate flash floods.

Late Holocene upper bounds of flood magnitudes and twentieth century large floods in the ungauged, hyperarid alluvial Nahal Arava, Israel

The impact of large twentieth century floods on the riparian vegetation and channel morphology of the relatively wide anabranching and braided Nahal Arava, southern Israel, was documented as part of developing tools to (a) identify recent large floods, (b) determine these flood's respective magnitudes in alluvial ungauged streams, and (c) determine long-term upper bounds to flood stages and magnitudes. Along most of its course Nahal Paran, a major tributary that impacts the morphology, floods and sediments of Nahal Arava at the study reach, is a coarse-gravel, braided ephemeral stream. Downstream of the Arava–Paran confluence, aeolian and fluvial sand delivered from eastern Arava valley alters the channel morphology. The sand has accreted up to 2.5 m above the distinct current channels, facilitating the recording of large floods. This sand enhances the establishment of denser riparian vegetation (mainly Tamarix nilotica and Haloxylon persicum) that interacts with floods and affects stream morphology. A temporal association was found between specific floods recorded upstream and tree-ring ages of re-growth of flood-damaged tamarix trees (‘Sigafoos trees’) in the past 30 years. This association can be utilized for developing a twentieth century flood chronology in hyperarid ungauged basins in the region. The minimum magnitude of the largest flood that covered the entire channel width, estimated from flood deposits, is approximately 1700–1800 m 3s − 1 . This is a larger magnitude than the largest gauged flood of 1150 m 3s − 1 that occurred in 1970 about 30 km upstream in Nahal Paran. Our estimation agrees with flood magnitude estimated from the regional envelope curve of the largest floods. Based on Holocene alluvial stratigraphy and OSL dating in the study reach we also conclude that flood stages did not reach the late Holocene (∼ 2.2 ka) surface and therefore we estimate a non-exceedance upper bound of ∼ 2000 m 3s − 1 flood magnitudes for Nahal Arava during that interval. This study indicates that in unfavorable areas the combination of hydrology, fluvial morphology and botanic evidence can increase our understanding of ungauged basins and give information crucial for hydrology planning.

Three-dimensional modeling of hydrodynamic processes in the St. Lucie Estuary

Comparing with the studies on large estuarine systems, such as the Chesapeake Bay and the San Francisco Bay, the processes of stratification and transport in small and shallow estuaries are relatively less studied. The St. Lucie Estuary (SLE) is a riverine estuary located on the east coast of south Florida. It is small and shallow, with mean depth of 2.4 m. To study the estuarine processes in the SLE, a hydrodynamic model was developed based on the Environmental Fluid Dynamics Code (EFDC) [Hamrick, J.M., 1992. A three-dimensional environmental fluid dynamics computer code: theoretical and computational aspects. The College of William and Mary, Virginia Institute of Marine Science, Special Report 317, 63 pp.]. The model was calibrated and verified using observational data obtained in 1999 and 2000, respectively. The model variables used for model data-comparisons are water elevation, velocity, temperature, and salinity. The model is then applied to study the hydrodynamic processes in the SLE. It is found that freshwater inflow plays a major role in the stratification and net flushing of the SLE. Stratification generally increases with freshwater inflow. But when the inflow is persistently large for a relatively long period, the estuary can suddenly change from very stratified to well mixed within a few tidal cycles and the stratification collapses. This finding suggests that large and persistent freshwater inflows do not always increase estuarine stratification. Instead, it may cause the stratification to collapse within a short period of time. In addition to gauged tributaries, ungauged lateral inflows can also be important to small and shallow estuaries like the SLE. Although small individually, the ungauged streams and surface runoffs can be a significant portion of the total inflow and affect salinity distribution significantly. Flushing time affects a wide range of hydrodynamic and water quality processes in the estuary. The model results indicate that commonly used formulas, such as the tidal prism formula and the Knudsen formula, may significantly underestimate the flushing time.