Soil erosion decreases soil fertility of the uplands and causes siltation of lakes and reservoirs; the lakes and reservoirs in tropical monsoonal African highlands are especially affected by sedimentation. Efforts in reducing loads by designing management practices are hampered by lack of quantitative data on the relationship of erosion in the watersheds and sediment accumulation on flood plains, lakes and reservoirs. The objective of this study is to develop a prototype quantitative method for estimating sediment budget for tropical monsoon lakes with limited observational data. Four watersheds in the Lake Tana basin were selected for this study. The Parameter Efficient Distributed (PED) model that has shown to perform well in the Ethiopian highlands is used to overcome the data limitations and recreate the missing sediment fluxes. PED model parameters are calibrated using daily discharge data and the occasionally collected sediment concentration when establishing the sediment rating curves for the major rivers. The calibrated model parameters are then used to predict the sediment budget for the 1994–2009 period. Sediment retained in the lake is determined from two bathymetric surveys taken 20 years apart whereas the sediment leaving the lake is calculated based on measured discharge and observed sediment concentrations. Results show that annually on average 34 t/ha/year of sediment is removed from the gauged part of the Lake Tana watersheds. Depending on the up-scaling method from the gauged to the ungauged part, 21 to 32 t/ha/year (equivalent to 24–38 Mt/year) is transported from the upland watersheds of which 46% to 65% is retained in the flood plains and 93% to 96% is trapped on the flood plains and in the lake. Thus, only 4–7% of all sediment produced in the watersheds leaves the Lake Tana Basin.
The Bavarian Danube River has experienced numerous large flood events in recent years which make flood management an urgent matter. The propagation of flood waves along the river is heavily influenced by controlled and natural flood retention. Over the past centuries, natural flood retention areas were lost due to river training, and the hydraulic characteristics of the channel-flood plain system were modified. The purpose of this paper is to understand the effect of river training on the flood retention characteristics along the Bavarian Danube. Systematic two-dimensional hydrodynamic modelling shows that extreme floods are attenuated more strongly in the present state of the channel-flood plain system than they were historically. This is because the retention areas are filled later during the event, so the attenuation effect is much larger for the same magnitude of the retention volume. Natural flood retention is therefore not an effective management option for reducing extreme floods on the Bavarian Danube. Controlled flood retention measures provide a higher efficiency regarding peak attenuation to retention volume ratio. On the other hand, the delay of flood peaks due to natural retention may be beneficial for the superposition of the flood waves with contributions from downstream tributaries.
Sídelní poloha situovaná při okraji terasy würmského stáří uprostřed labské nivy byla s většími přestávkami užívána od časného eneolitu do počátku starší doby bronzové. Poté ji z větší části překryl písečný přesyp, který uchránil nálezově bohatou kulturní vrstvu. Podle analýzy keramiky se zde vystřídalo pět sídelních fází. Z nejstarší, michelsberské fáze je půdorys domu č. 3, první tohoto typu v Čechách. Také další půdorys, č. 4, představuje první objev svého druhu v zemi, tentokrát ze staršího (baalberského) období kultury nálevkovitých pohárů. Stejně unikátní objekty jsou v kontextu své doby další tři půdorysy: loďovitý, č. 1, ze čtvrté fáze, patřící kultuře zvoncovitých pohárů, a dvojice úzce obdélníkovitých domů č. 5, 6 ze staršího období únětické kultury. Výzkum a sběry umožnily stanovit přibližnou velikost sídelního areálu, včetně hrubého odhadu plochy osazené v té které fázi. Pro funkci sídliště měla zřejmě podstatný význam poloha na aktivním toku Labe, jež bylo zároveň jednou z nejvýznamnějších dálkových komunikací Čech. Zánik sídelního areálu někdy po r. 2000 př. Kr. lze pak vysvětlit jako důsledek přesunu řeky jižněji, čímž se meandr, při jehož břehu areál ležel, změnil ve slepé rameno. and The settlement area situated at the edge of a Würm-dating terrace in the centre of the Elbe flood plain had been used – with longer breaks – since the Early Eneolithic until the beginning of the Early Bronze Age. Thereafter, a sand dune mostly covered it and protected the rich in finds cultural layer. Pottery analysis attests for five different settlement phases. The earliest, Michelsberg phase yielded the ground plan of house no. 3, the first of this type in Bohemia. The ground plan of another house, no. 4, also represents the first detected instance of this type in the country, dating to the early (Baalberg) phase of the Funnel Beaker culture. The ground plans of three more houses are equally unique in their chronological context: ship-shaped ground plan, no. 1, belongs to the 4th phase and dates to the Bell Beaker culture, and two narrow rectangular houses, no. 5 and 6, belong to the early phase of the Únětice culture. Excavation and surface survey enabled assessment of the approximate size of the settlement area, including a rough estimate of the area occupied in each particular phase. The position of the settlement next to the active Elbe watercourse seems to have significantly determined its function, since the Elbe river represented one of the most important means of long-distance communication in Bohemia. The decline of the settlement area approx. after year 2000 BC may then be explained as consequence of the river’s relocation southward, which turned the meander where the settlement existed in a dead-end channel (palaeochannel).
The impact of lateral momentum transfer (LMT) on channel conveyance is examined through applications of a new mathematical model for routing unsteady flows in compound channels. The model accounts for LMT through three parameters that relate: (i) 'actual' to 'isolated' sub-section discharge, (ii) main channel to flood plain lengths between stations, and (iii) flood plain to main channel depths. The model was applied to route a series of flood events in a hypothetical compound channel consisting of a deep smooth main channel between two wide and rough flood plain zones. The routing exercise was repeated with and without LMT in the analysis. Three empirical methods to account for LMT were investigated by comparing their corresponding simulated stage and discharge hydrographs. In general, LMT was found to have little impact on simulated stage hydrographs, however, for the case of small flood plain flows, LMT was responsible for some attenuation in the discharge hydrographs. and Príspevok skúma vplyv priečnej zložky hybnosti na prúdenie použitím nového matematického modelu neustáleného pohybu v zložených otvorených kanáloch. Použitý model zohľadňuje efekt priečnej hybnosti prostredníctvom troch parametrov, daných pomerom: (i) aktuálneho a ''oddeleného'' prietoku v danom úseku koryta, (ii) dĺžkou inundácie a dĺžkou úseku a (iii) plochy inundácie a plochy hlavného koryta v úseku. Model bol aplikovaný na výpočet niekoľkých povodňových udalostí v hypotetickom zloženom kanále s hladkým hlavným (hlbokým) korytom s obojstrannou drsnou inundáciou. Výpočty sa realizovali s akceptovaním a s vynechaním efektu priečnej hybnosti. Pri tom boli skúmané tri empirické metódy výpočtu priečnej hybnosti porovnávaním výsledkov na prietokových a hladinových hydrografoch. Výsledky preukázali, že účinok priečnej hybnosti sa ukázal len pri prietokoch, pri čom ich vplyv na priebeh hladín bol nevýznamný. Okrem toho výpočty pre korytá s malou inundáciou ukázali, že priečna hybnosť spôsobuje zoslabenie prietokov.