This paper presents a method for assessing the retention capacity of a floodplain in the course of flooding and for estimating the significance of its water storage for transforming a flood wave. The method is based on two-dimensional numerical modeling of the flood flow in a river channel and in the adjacent floodplains, and is suitable for cases when the morphology of the flooding area is variable and complex, e.g. broad inundation areas with meandering channels. The approach adopted here enables us to quantify the retention capacity for inundation areas of various characters and with various land uses, and provides a tool for estimating the efficiency of possible measures for increasing the water storage capacity of a floodplain. The retention capacity is estimated using an evaluation of a series of detailed flood flow modeling results; the flood wave transformation effect is predicted with the aim of creating a non-linear reservoir model. A parametric study of the floodplain retention capacity for the upper branch of the Lužnice River is presented here, and the results for the current state and for various hypothetical scenarios of changes in geometry and land use are evaluated and compared. V příspěvku je prezentována metodika pro stanovení retenční kapacity inundačního území při povodňových průtocích a jeho význam pro transformaci povodňové vlny. Metoda využívá dvourozměrný numerický model proudění vody korytem a přilehlým inundačním územím a je vhodná pro případy, kdy charakter nivy je proměnlivý a velmi komplikovaný, např. široká inundační území s meandrujícími toky. Navržený způsob řešení umožňuje kvantifikovat retenční schopnosti niv různého charakteru při různých způsobech využívání a umožňuje případně navrhnout úpravu inundačního území tak, aby transformační účinek při průchodu povodňové vlny byl co největší. Retenční kapacita inundačního území je stanovena na základě výsledků podrobného modelování proudění vody při různých průtokových stavech a transformace povodňové vlny je řešena pomocí iteračního postupu založeného na Bratránkově metodě. V příspěvku je uvedena parametrická studie kvantifikace retenční kapacity nivy na základě vyhodnocení a porovnání transformační schopnosti pro nivu Lužnice v jejím horním úseku pro současný přirozený stav a pro různé teoretické scénáře změněného charakteru a způsobu využívání nivy.KLÍČOVÁ SLOVA: Dvourozměrné numerické modelování, retenční kapacita nivy, transformace povodňové vlny.

No abstract

The river Lužnice floodplain in south Bohemia with a preserved floodplain ecosystem and hydrological regime has been investigated for selected ecosystem services (ES): flood mitigation, biodiversity refuge, carbon sequestration, and production of commodities. Two contrasting scenarios at 5 km long studying floodplain segment were compared: near-nature state as it is (Scenario A) and virtually transformed floodplain (Scenario B). For the flood mitigation service, we used an output from hydraulic analysis of modelled flood events (model FAST 2D) showing the effective retention volume 2.3 mil m 3 for A and 0.83 m 3 for B. For monetary evaluation, we used the method of replacement cost of this retention volume. For biodiversity refuge, the value of biotopes (real for A and arbitrary for B) and their contribution to the floodplain area were analysed according to the Hessen method. Carbon sequestration was measured by eddy-covariance method. All ES for both scenarios were converted to monetary value and scenarios A and B were compared. The contribution of particular ES to the total ecosystem value showed the importance of flood mitigation and biodiversity refugium compared to other services. Overall floodplain value was estimated as 17 213 EUR ha. .yr -1 for state as it is (A) and 6 254 EUR ha. .yr -1 for transformed floodplain (B), i.e. virtual floodplain transformation decreased its value to 36%.

Numerical modeling of flood flow and the evaluation of flood hazards can be based on various numerical models and modeling techniques. One-dimensional (1D), quasi two-dimensional (1,5 D), two-dimensional (2D) or three-dimensional (3D) variants of numerical models can be used. While 3D models are too demanding to be used for flood flow modeling on personal computers, quasi-2D and 2D models can be more widely used to solve even larger practical problems nowadays. Detailed two-dimensional numerical modeling of flood flow in flooded urbanized areas with complex geometry using the 2D depth averaged model is presented in this paper. The governing equations of the model are expressed with a set of depth averaged Reynolds equations consisting of the continuity equation and two momentum equations for the horizontal velocity components. The eddy viscosity, which influences the horizontal turbulent momentum exchange processes, is modeled with the aid of a depth average version of the two-equation k-e turbulence model. The partial differential equations are solved numerically with a control volume method using fine non-orthogonal curvilinear grids and a non-staggered variable arrangement. The applicability and advantages of this modeling approach for simulating the flood flow in floodplains with complex geometry, and in urban areas, are illustrated by the results of a pilot study in Choceň and Ústí nad Orlicí, towns which were severely affected during the 1997 and 1998 floods in the Czech Republic.

No abstract