Moderate Flood Research Articles

Influence of basin connectivity on sediment source, transport, and storage within the Mkabela Basin, South Africa

Abstract. The management of sediment and other non-point source (NPS) pollution has proven difficult, and requires a sound understanding of particle movement through the drainage system. The primary objective of this investigation was to obtain an understanding of NPS sediment source(s), transport, and storage within the Mkabela Basin, a representative agricultural catchment within the KwaZulu–Natal Midlands of eastern South Africa, by combining geomorphic, hydrologic and geochemical fingerprinting analyses. The Mkabela Basin can be subdivided into three distinct subcatchments that differ in their ability to transport and store sediment along the axial valley. Headwater (upper catchment) areas are characterized by extensive wetlands that act as significant sediment sinks. Mid-catchment areas, characterized by higher relief and valley gradients, exhibit few wetlands, but rather are dominated by a combination of alluvial and bedrock channels that are conducive to sediment transport. The lower catchment exhibits a low-gradient alluvial channel that is boarded by extensive riparian wetlands that accumulate large quantities of sediment (and NPS pollutants). Fingerprinting studies suggest that silt- and clay-rich layers found within wetland and reservoir deposits of the upper and upper-mid subcatchments are derived from the erosion of fine-grained, valley bottom soils frequently utilized as vegetable fields. Coarser-grained deposits within these wetlands and reservoirs result from the erosion of sandier hillslope soils extensively utilized for sugar cane, during relatively high magnitude runoff events that are capable of transporting sand-sized sediment off the slopes. Thus, the source of sediment to the axial valley varies as a function of sediment size and runoff magnitude. Sediment export from upper to lower catchment areas was limited until the early 1990s, in part because the upper catchment wetlands were hydrologically disconnected from lower parts of the watershed during low to moderate flood events. The construction of a drainage ditch through a previously unchanneled wetland altered the hydrologic connectivity of the catchment, allowing sediment to be transported from the headwaters to the lower basin where much of it was deposited within riparian wetlands. The axial drainage system is now geomorphically and hydrologically connected during events capable of overflowing dams located throughout the study basin. The study indicates that increased valley connectivity partly negated the positive benefits of controlling sediment/nutrient exports from the catchment by means of upland based, best management practices.

Spatial Variability of Heavy Metal Contamination in Alluvial Soils in Relation to Flood Risk Zones in Southern Québec, Canada

The contamination of rivers and riparian soils is a growing problem for several catchments in southern Quebec due to agricultural pollutants and other sources of pollution. This study deals with the concentration of heavy metals in alluvial soils and their spatial variability following the various flood return periods (0-20 years and 20-100 years) and outside flood zones. The heavy metal concentration of some soils exceeds levels in government standards. The elements with the highest concentrations are mainly Ni, Pb and Zn. For instance, the levels of Zn and Pb can be as high as 310 and 490 mg kg-1. In general, heavy metal concentrations are highest in active sedimentation zones (0-20 yrs). In this instance, the ANOVA test was conducted to determine whether the differences in heavy-metal concentrations in the soils could be significant between the three zones (Frequent Flood (FF), Moderate Flood (MF), and No Flood (NF)). With the increase in the flood return rate and current hydroclimatic changes, a downstream remobilization of contaminants can be expected, leading to a larger riparian area of contamination.

Calibrated Hydrodynamic Model for Sazlıdere Watershed in Istanbul and Investigation of Urbanization Effects

Protecting and improving water resources play an important role for meeting the future demand for drinking water and preventing flood events. Increasing population tends to cause a shift from rural to urban areas, which results in the alteration of concentration time, water quantity, and flow rate. Because of these changes, specific solutions need to be investigated to prevent flood and overflow. For this purpose, calibrated hydrodynamic models should be developed to understand the hydrodynamic behavior of the catchment. Sazlıdere Watershed, which provides an average of 55 million m3 of water per year, is one of the most important watersheds of Istanbul, Turkey. There is a tremendous population increase in the area, and moderate flood events are frequently observed. Therefore, in this study, a calibrated hydrodynamic model for Sazlıdere Watershed was developed by using the Environmental Protection Agency Storm Water Management Model (EPA SWMM). The model is calibrated by using the rainfall and flow data obtained from the stations we set in the area. The most sensitive parameters of the hydrodynamic model are determined to be Manning’s roughness coefficient for both channels and subcatchments followed by the infiltration parameters, i.e., hydraulic conductivity and the initial soil-moisture condition. The flow developed on the watershed was simulated both under typical rainfall events and under extreme cases, namely, high urbanization and storm events with a high return period. It is observed that even under typical storms, floods developed on the watershed; and finally, several flood control options are suggested.

Dam Design can Impede Adaptive Management of Environmental Flows: A Case Study from the Opuha Dam, New Zealand

The Opuha Dam was designed for water storage, hydropower, and to augment summer low flows. Following its commissioning in 1999, algal blooms (dominated first by Phormidium and later Didymosphenia geminata) downstream of the dam were attributed to the reduced frequency and magnitude of high-flow events. In this study, we used a 20-year monitoring dataset to quantify changes associated with the dam. We also studied the effectiveness of flushing flows to remove periphyton from the river bed. Following the completion of the dam, daily maximum flows downstream have exceeded 100m(3)s(-1) only three times; two of these floods exceeded the pre-dam mean annual flood of 203m(3)s(-1) (compared to 19 times >100m(3)s(-1) and 6 times >203m(3)s(-1) in the 8years of record before the dam). Other changes downstream included increases in water temperature, bed armoring, frequency of algal blooms, and changes to the aquatic invertebrate community. Seven experimental flushing flows resulted in limited periphyton reductions. Flood wave attenuation, bed armoring, and a shortage of surface sand and gravel, likely limited the effectiveness of these moderate floods. Floods similar to pre-dam levels may be effective for control of periphyton downstream; however, flushing flows of that magnitude are not possible with the existing dam infrastructure. These results highlight the need for dams to be planned and built with the capacity to provide the natural range of flows for adaptive management, particularly high flows.

Islands in a European mountain river: Linkages with large wood deposition, flood flows and plant diversity

Vegetated islands are characteristic landforms of braided mountain rivers. Long-term observations and recent morphological and botanical surveys conducted in the gravel-bed Czarny Dunajec, Polish Carpathians, were used to determine the processes and patterns governing initiation and development of islands and their floristic complexity. Moreover, dendrochronologically estimated years of island inception were compared with the timing and magnitude of flood flows in the period 1970–2011 to infer about controls on the formation and persistence of islands in the river. In the high-energy, braided river, islands originate as a result of deposition of large vegetative particles, mostly large wood, on gravel bars and the associated vegetative regeneration of living wood or the growth of seedlings and saplings in the shelter of wood accumulations. Tree-ring dating of the largest trees growing in particular zones of building and established islands indicated a predominant upstream island growth in the river. It results from repeated accumulation of living wood on the head of islands and its subsequent regeneration and contrasts with the progressive downstream growth of islands in the rivers supplied with large, stable logs of the tree species without the capability to re-sprout. The lack of islands from the years 1982–1996 most likely reflects the removal of relatively young islands by two major floods in the 1990s which were, however, unable to destroy older and larger islands. After 1997 the occurrence of low to moderate floods facilitated the formation and persistence of islands. The plant inventory demonstrated that species richness increased non-linearly with the increasing age, area and shoreline length of islands. Islands supported more plant species than the riparian forest and attained comparable species richness at an early stage of development. Fast developing, dynamic and supporting rich plant communities, islands contribute highly to the overall floristic complexity of the river corridor and their re-establishment should be viewed as an important factor in the restoration of hydromorphologically degraded mountain rivers.

Sediment rating curves in the Ningxia-Inner Mongolia reaches of the upper Yellow River and their implications

Abstract In this study, the relation between water discharge and suspended-sediment concentration (SSC) was analyzed based on daily water discharge and SSC recorded at six stations in the Ningxia-Inner Mongolia reaches of the upper Yellow River in flood seasons from 1952 to 1986. The Pettitt statistical method was used to test the abrupt changes in the time series of discharge and sediment concentration. The results indicate that the time series of discharge in Ningxia-Inner Mongolia reaches of the upper Yellow River can be divided into two periods, i.e., before 1969 and between 1969 and 1986. In these two periods, the third-order polynomial function could be used to describe the relationship between water discharge and SSC during the flood periods, indicating that sediment transport in the Ningxia-Inner Mongolia reaches was influenced by the complex processes of water and sediment inputs from the upstream and tributaries and also by the channel boundary conditions. The relation between SSC and the water discharge of low and moderate floods ( Q 3 /s) was fitted by sediment rating curves of a power function form. The sediment rating parameter b is higher in the upper section of the Ningxia-Inner Mongolia reaches due to a higher stream power there, and both the cross-section shape and river bed material determine the value of log( a ). Although a 40% decrease in sediment transport during the flood periods due to the impoundment of the Liujiaxia Reservoir and sediment-checking dams and the implementation of soil and water conservation programmes in the Ningxia-Inner Mongolia reaches of the upper Yellow River, the sediment transport regimes during 1952–1986 did not change.

Morphological characteristics of flood refugia of cobble-bed vegetation

Periodic flooding alters riverine vegetation community composition; moderate flood disturbance maintains communities at an early seral stage, whereas severe floods may remove entire communities. After disturbance, plants may recolonize riverbeds from refugia.Here, we investigated the morphological characteristics of refugia where riverine plants survived large floods. At three sites, plant communities growing on cobble-bars were surveyed, and the hydraulic characteristics of each site were analyzed. The populations of an indicator plant, Anaphalis margaritacea sub-species yedoensis (Franch. et Savat.) were assessed after major floods in 2004 and 2009. At each survey, the particle size and bed composition were measured, and the critical shear-stress required to move particles at each site was calculated. A one-dimensional flow and the average and local shear-stress at ten different discharges, including that of the largest flood were calculated. The results demonstrate that the A. margaritacea community is maintained at locations where the local shear-stress at peak discharge in large-floods does not exceed a critical value. These conditions were achieved in locations with particular morphological characteristics, where the river is not constrained by levees and is restricted by a downstream narrow-pass; in this reach river width increases with discharge thus limiting the increase in water depth with increasing discharge and allowing the establishment of refugia where plant species may survive large floods.Flooding and reestablishment from refugia maintain a characteristic riverbed community in equilibrium.

The response of braided planform configuration to flow variations, bed reworking and vegetation: the case of the Tagliamento River, Italy

ABSTRACTMorphological features of braided rivers (bars, channels and pools) experience major changes in area, shape and spatial distribution as a response to (i) the pulsation of discharge during a flood and (ii) the bed evolution induced by floods. In this work, at‐a‐station relationships between water level and planform configuration were investigated on the Tagliamento River, a large gravel‐bed braided river in northeast Italy, over a 2‐year study period comprising three bankfull events and several small‐to‐medium floods. The analysis was performed on two 1‐km‐long reaches, characterized by different riparian vegetation cover. Ground‐based images with an hourly temporal resolution were acquired using software‐controlled, digital cameras. Bars, channels, pools and vegetated patches were manually digitized on more than 100 rectified images. Sequences of constant‐level images spanning the study period were used to quantify the impact of floods on the stability of at‐a‐station relationships and on the turnover rate of water bodies.The analysis shows that wetted area increased almost linearly with water level in both reaches. The average number of branches per cross‐section peaked at intermediate flow levels, increasing from 2 at low flow up to 6–7. The number of branches displayed the largest fluctuations over time, with significant changes produced also by moderate floods. Turnover rates were high in both reaches, with more than 30% of wetted areas at low flow converting into bare gravel in less than 2 months. Vegetation colonization was found to limit the mobility of the low flow channels over time by concentrating the flow in fewer, deeper anabranches. The number of channels per cross‐section was 30–40% less in the vegetated reach and the proportion of low flow water bodies in the same position after 12 months increased from 3% to 14%. Copyright © 2012 John Wiley & Sons, Ltd.

Numerical modeling of three-dimensional stratified tidal flow over Camarinal Sill, Strait of Gibraltar

[1] The baroclinic response to barotropic tidal forcing in the Camarinal Sill area, within the Strait of Gibraltar, is investigated with a three-dimensional, fully nonlinear, nonhydrostatic numerical model. The aim of numerical efforts was the assessment of three-dimensional effects, which are potentially significant in the area because of rather irregular bottom topography, variable background stratification, and complex structure of barotropic tides. Model results reveal a complex baroclinic response under relatively moderate flood tidal currents, which includes the formation of internal hydraulic jumps upstream of the sill, internal cross waves close to the channel walls, and a plunging pycnocline at the lee side of the sill crest. These structures exhibit significant cross-channel spatial dependence and may appear to be aligned together across the channel. This fact makes their identification difficult from the surface pattern captured by remote sensing images. Under strong barotropic forcing (spring tides) the upstream hydraulic jumps are shifted to the lee side of Camarinal Sill, where a single internal hydraulic jump is formed. Significant first- and second-mode hydraulic jumps are also generated near smaller secondary sills in Tangier basin, thus extending the occurrence of intense water mixing and energy dissipation to other zones of the strait.

Motueka River plume facilitates transport of ruminant faecal contaminants into shellfish growing waters, Tasman Bay, New Zealand

Hydrographic and water quality surveys of the Motueka River and its river plume were conducted during a moderate flood event (peak flow of 420 m3/s) to assess the source and fate of faecal contaminants transported into Tasman Bay. Escherichia coli (E. coli) and enterococci concentrations in the river were up to 10000 and 7300 Most Probable Number (MPN)/100 ml during peak flow, respectively. A coastal survey revealed a shallow low-salinity plume that extended at least 6 km into Tasman Bay and the region's largest aquaculture management areas (AMAs). Mussels within the influence of the river plume, including those collected within an AMA, had E. coli and enterococci concentrations as high as 1300 and 2200 MPN/100 g tissue, respectively. Application of microbial source tracking markers using end-point PCR assays identified the presence of faecal contamination from ruminant animals (cows, sheep) in water and mussel samples. The detection of ruminant faecal contamination within shellfish located 6 km offshore highlights the close connection between land use and the quality of New Zealand's highly valued coastal resources.

Fusegates selection and operation: simulation–optimization approach

Fusegates present a reliable and cost-effective alternative, which increase flood protection and water supply benefits. This article develops a comprehensive simulation–optimization framework for practical selection, installation, and operation of fusegates. The computational model simulates the complicated hydraulic behavior of fusegates systems with varying design characteristics and consequential anomalous routing process in case of flood events. An efficient mixed genetic algorithm (GA) is subsequently developed and coupled with the highly nonlinear hydraulic simulation model to minimize the overall expected annual cost under structural, hydraulic, and operational constraints. Types, heights, and tipping heads of gates are explicitly treated as optimization decision variables. Furthermore, the frequent practice of installing all gates in the same level is practically improved to favorably help minimize water loss in case of moderate discharge floods. The proposed model is demonstrated and discussed for a case study of the Taleghan Dam fusegates installation project in Iran. A series of sensitivity analyses are also conducted to assess routing effect and uncertainty in water unit price and replacement costs and provide more insight and understanding of the design problem.

Stratigraphic, sedimentologic, and dendrogeomorphic analyses of rapid floodplain formation along the Rio Grande in Big Bend National Park, Texas

The channel of the lower Rio Grande in the Big Bend region rapidly narrows during years of low mean and peak flow. We conducted stratigraphic, sedimentologic, and dendrogeomorphic analyses within two long floodplain trenches to precisely reconstruct the timing and processes of recent floodplain formation. We show that the channel of the Rio Grande narrowed through the oblique and vertical accretion of inset floodplains following channel-widening floods in 1978 and 1990–1991. Vertical accretion occurred at high rates, ranging from 16 to 35 cm/yr. Dendrogeomorphic analyses show that the onset of channel narrowing occurred during low-flow years when channel bars obliquely and vertically accreted fine sediment. This initial stage of accretion occurred by both bed-load and suspended-load deposition within the active channel. Vegetation became established on top of these fine-grained deposits during years of low peak flow and stabilized these developing surfaces. Subsequent deposition by moderate floods (between 1.5 and 7 yr recurrence intervals) caused additional accretion at rapid rates. Suspended-sediment deposition was dominant in the upper deposits, resulting in the formation of natural levees at the channel margins and the deposition of horizontally bedded, fining-upward deposits in the floodplain trough. Overall, channel narrowing and floodplain formation occurred through an evolution from active-channel to floodplain depositional processes. High-resolution dendrogeomorphic analyses provide the ability to specifically correlate the flow record to the onset of narrowing, the establishment of riparian vegetation, the formation of natural levees, and ultimately, the conversion of portions of the active channel to floodplains.

Patchy bed disturbance and fish predation independently influence the distribution of stream invertebrates and algae

1. The identification of factors determining the patchy distribution of organisms in space and time is a central concern of ecology. Predation and abiotic disturbance are both well-known drivers of this patchiness, but their interplay is still poorly understood, especially for communities dominated by mobile organisms in frequently disturbed ecosystems. 2. We investigated the separate and interactive influences of bed disturbance by floods and predation by fish on the benthic community in a flood-prone stream. Electric fields excluded fish predators from half of 48 stream bed patches (area 0·49 m(2) ) with contrasting disturbance treatments. Three types of bed disturbance were created by either scouring or filling patches to a depth of 15-20 cm or by leaving the patches undisturbed, thus mimicking the mosaic of scour and fill caused by a moderate flood. Benthic invertebrates and algae were sampled repeatedly until 57 days after the disturbance. 3. Disturbance influenced all ten investigated biological response variables, whereas predation affected four variables. Averaged across time, invertebrate taxon richness and total abundance were highest in stable patches. Algal biomass and densities of five of the seven most common invertebrate taxa (most of which were highly mobile) were higher in fill than in scour patches, whereas two taxa were more abundant in scour and stable than in fill patches. Furthermore, two common invertebrate grazers were more abundant and algal biomass tended to be reduced in fish exclusion patches, suggesting a patch-scale trophic cascade from fish to algae. 4. Our results highlight the importance of patchy physical disturbance for the microdistribution of mobile stream organisms and indicate a notable, but less prevalent, influence of fish predation at the patch scale in this frequently disturbed environment. Disturbance and predation treatments interacted only once, suggesting that the observed predation effects were largely independent of local bed disturbance patterns.

Flood of the Red River Basin in 2009 and Effectiveness of Rapid Mitigation Efforts

2009, the city government ordered a mandatory evacuation of the residents living in certain areas of the city, and public schools, including three universities in the region, were closed for up to two weeks. The Red River originates at the Bois de Sioux and Otter Tail rivers in the northern United States and flows through Fargo and Grand Forks, North Dakota, toward Winnipeg, Canada. The river is 885 km long, including 635 km in the United States and 255 km in Canada Minnesota Department of Natural Resources 2009. Given that the U.S. Midwest is geographically flat, the Red River Basin may be the critical floodplain. Thus, moderate floods of the Red River Basin have regularly occurred during the past 110 years. The peak was the 1897 flood that recorded a crest of 12.2 m Flood Information Center FIC 2009. Fargo is the largest city in North Dakota with a population of approximately 100,000 U.S. Census Bureau 2010. The Red River flows along the east side of the city, as shown in Fig. 1. This forum discusses the 2009 flood of the Red River Basin in the city of Fargo and the effectiveness of the rapid efforts to minimize the flood damage. Of particular interest are the forensic investigations into the extreme event, which attributed the flood to four major sources: the sudden change of temperature, snow melting, topography, and soil properties. Postflood activities are briefly discussed.

Drainage morphometry and its influence on landforms in volcanic terrain, Central Anatolia, Turkey

An evaluation of morphometric parameters was made to study drainage morphometry and its influence on landform processes, flood and erosion characteristic in subbasins which are located in volcanic terrain between Niğde and Aksaray Province SW Cappadocia, Central Anatolia. The study area was divided into 28 subbasins. The high flood potentiality exists on the high volcanic cones, dom, dissected foot of mountains and the high parts of plateaus covering the 13 subbasins, and are associated with high slope, shape of subbasins, impermeable geology and sparce vegetation. The moderate flood potentiality exists on the plateaus covering the 9 subbasins and is associated with high drainage density, high stream frequency, high bifurcation ratio and high slope conditions; although these subbasins have long narrow shapes. The low flood potentiality exists on plains covering 5 subbasins and is associated with low slopes and its long narrow shape. Analysis of drainage morphometry helps to understand inter-relationships between landforms and flood potentiality for planning and management in basins.

Land cover changes in small catchments in Slovakia during 1990–2006 and their effects on frequency of flood events

Statistical approach to the analysis of the relationship between the frequency of flood events and land cover (LC) changes in small catchments of Slovakia is presented in this paper. The data for identification of LC changes were taken from the 1990 and 2006 CORINE LC (CLC) data layers. They were derived by computer-aided visual interpretation of satellite images under the CLC Projects. The data about frequency of flood events in small catchments are from the period 1996–2006. Two hypotheses were formulated: (1) the greater the area of LC changes, the more frequent flood events; (2) in catchments where LC changes accelerating formation of direct runoff (e.g. urbanization, deforestation, farming) dominates, flood events are more frequent than in catchments where the prevailing LC changes (e.g. afforestation) reduce formation of direct runoff. Validity of hypotheses was tested in the framework of flood potential of catchments by two-factor ANOVA method. The obtained results indicate that (1) flood event frequency increases with the increasing total area of LC changes in a catchment. This tendency clearly manifests itself in catchments with very high flood potential. It is somewhat less distinct in catchments with moderate and high flood potentials. (2) There were no differences in flood event frequency between the group of catchments, where LC changes accelerating the formation of the direct runoff prevailed and the group of catchments where LC changes decelerating the formation of direct runoff were dominated.

Spatial and Temporal Patterns in Fish Assemblages Following an Artificially Extended Floodplain Inundation Event, Northern Murray-Darling Basin, Australia

Water extraction from dryland rivers is often associated with declines in the health of river and floodplain ecosystems due to reduced flooding frequency and extent of floodplain inundation. Following moderate flooding in early 2008 in the Narran River, Murray-Darling Basin, Australia, 10,423 ML of water was purchased from agricultural water users and delivered to the river to prolong inundation of its terminal lake system to improve the recruitment success of colonial waterbirds that had started breeding in response to the initial flooding. This study examined the spatial and temporal patterns of fish assemblages in river and floodplain habitats over eight months following flooding to assess the possible ecological benefits of flood extension. Although the abundances of most fish species were greater in river channel habitats, the fish assemblage used floodplain habitats when inundated. Young-of-the-year (4-12 months age) golden perch (Macquaria ambigua) and bony bream (Nematalosa erebi) were consistently sampled in floodplain sites when inundated, suggesting that the floodplain provides rearing habitat for these species. Significant differences in the abundances of fish populations between reaches upstream and downstream of a weir in the main river channel indicates that the effectiveness of the environmental water release was limited by restricted connectivity within the broader catchment. Although the seasonal timing of flood extension may have coincided with sub-optimal primary production, the use of the environmental water purchase is likely to have promoted recruitment of fish populations by providing greater access to floodplain nursery habitats, thereby improving the ability to persist during years of little or no flow.

New York City Storm Surges: Climatology and an Analysis of the Wind and Cyclone Evolution

Abstract A climatological description (“climatology”) of storm surges and actual flooding (storm tide) events from 1959 to 2007 is presented for the New York City (NYC) harbor. The prevailing meteorological conditions associated with these surges are also highlighted. Two surge thresholds of 0.6–1.0 m and >1.0 m were used at the Battery, New York (south side of Manhattan in NYC), to identify minor and moderate events, respectively. The minor-surge threshold combined with a tide at or above mean high water (MHW) favors a coastal flood advisory for NYC, and the moderate surge above MHW leads to a coastal flood warning. The number of minor surges has decreased gradually during the last several decades at NYC while the number of minor (storm tide) flooding events has increased slightly given the gradual rise in sea level. There were no moderate flooding events at the Battery from 1997 to 2007, which is the quietest period during the last 50 yr. However, if sea level rises 12–50 cm during the next century, the number of moderate flooding events is likely to increase exponentially. Using cyclone tracking and compositing of the NCEP global reanalysis (before 1979) and regional reanalysis (after 1978) data, the mean synoptic evolution was obtained for the NYC surge events. There are a variety of storm tracks associated with minor surges, whereas moderate surges favor a cyclone tracking northward along the East Coast. The average surface winds at NYC veer from northwesterly at 48 h before the time of maximum surge to a persistent period of east-northeasterlies beginning about 24 h before the surge. There is a relatively large variance in wind directions and speeds around the time of maximum surge, thus suggesting the importance of other factors (fetch, storm duration and track, etc.).

The Role of Upstream Midtropospheric Circulations in the Sierra Nevada Enabling Leeside (Spillover) Precipitation. Part I: A Synoptic-Scale Analysis of Spillover Precipitation and Flooding in a Leeside Basin

Abstract Pacific-originating storms that produce heavy leeside liquid precipitation in the Sierra Nevada are rare compared to those that generate windward slope rainfall. However, these leeside precipitation events have a profound effect on the flood hydrology of leeside basins in the Sierra Nevada. This study identified 12 storms that affected the Truckee River basin in northeastern Nevada. The storms produced both moderate and extreme flooding in this leeside basin. A synoptic-scale analysis of conditions leading to leeside storms was produced using a compositing procedure. Composites for multiple pressure levels and multiple parameters were produced for class 1 storms—those storms producing moderate flood flow in the Truckee River basin—and class 2 storms—those producing extreme flooding [>10 000 cubic feet per second (cfs), or 283 m3 s−1] in this basin. The analysis confirms that the two flood populations are in fact generated by Pacific-originating storms with observably different synoptic-scale circulations. The class 2 storms are moister through a great depth in the troposphere (saturated to 750 hPa), and they occur coincident with warmer conditions in the lower and midtroposphere. Class 2 events exhibited more favorable upper-level jet streak structures in the eastern Pacific and over western North America. Both classes of leeside storms were shown to differ substantially from Pacific-originating storms that exclusively affect the windward slope of the Sierra and the coastal mountain ranges of California (California storms). The leeside storms were much warmer than California storms through much of the lower and midtroposphere, and the onshore flow was predominantly from the west-southwest in leeside storms compared to southerly flow in California storms. The findings suggest the existence of a midlevel atmospheric river delivering moisture to leeside basins of the Sierra Nevada.

Palynological records of climate and oceanic conditions in the late Pleistocene and Holocene of the Nile Cone, Southeastern Mediterranean, Egypt

Variations in palynomorph concentrations and relative abundances, including pollen and spores, organic‐walled dinoflagellate cysts, and amorphous organic matter (AOM), were studied in sediment core NC Core 2 from the upper slope (1,030 m water depth) of the Nile Cone, southeastern Mediterranean, Egypt. Each sample represents palynomorph deposition for intervals of around 500–1,000 years, and this study provides the first detailed palynological record for the Nile Cone. The dinoflagellate cyst assemblages and AOM production reflect changes in surface water temperature and river discharge in response to climate forcing as marked by variations in pollen and spore indicators of semi‐desert vegetation and tropical Monsoon intensity. The basal sediments (approximately 14,600–9400 yr before present [BP]) contain high concentrations and relative abundances of Polysphaeridium zoharyi, Polysphaeridium spp., Spiniferites elongatus, and Spiniferites spp. during the transition from postglacial to interglacial conditions. Variations in these taxa and total Impagidinium spp. suggest a gradual temperature increase, interrupted by a pre‐Holocene cool interval around 11,000 yr BP. The high African monsoon index for this pre‐Holocene time corresponds with pollen evidence of relatively high Nile discharge. The organic‐rich S1 sapropel sediments, dated as around 9,500–6,270 yrBP, were deposited several thousand years later, during a time of lower Monsoonindex, wanner surface water, and increased dinoflagellate cyst production. This was accompanied by moderate Nile flooding and oxygen‐deficient or anoxic bottom water on the upper Nile Cone, mainly supporting the increased production and carbon import hypothesis for eastern Mediterranean S1 formation. Dinoflagellate cyst assemblages in the Nile Cone S1 differ from those of the deeper, more northern Levantine and Cretan basins in the near absence of heterotrophic protoperidinioid cysts, despite the uniformly high organic S1 carbon content. The prevailing climate during the formation of sapropelic sediments in the southeastern Mediterranean was tropical to subtropical, but was interrupted by an early cooler interval with reduced humidity, as marked by re‐appearance of Spiniferites elongatus and increased Ephedra pollen. The carbonate‐rich surface layers were probably deposited under warm dry climatic conditions with minimum Nile input, but evidence of oxidation prevents detailed interpretation.