Unitus DSpace

Flow Time estimation in ungauged basins

Wed, 31 Dec 2008 23:00:00 GMT

Title: Flow Time estimation in ungauged basins Authors: Grimaldi, Salvatore; Petroselli, Andrea; Alonso, Gustavo; Santini, Monia; Nardi, Fernando Abstract: The aim of this work is to focalize the attention on the design flood estimation on small ungauged river basin of limited extension (<150 Km2). In this contest we refer to the Width Function Based Instantaneous Unit Hydrograph (WFIUH) model, which optimizes, through the DEM, the distributed morphological basin information. The Width Function (WF) is defined as the distance-area function or the probability measure obtained by dividing the number of cells at given hydrologic distance from the outlet by the total number of basin cells (the distance is measured along the flow path and normalized by the maximum distance from the divide to the outlet). WF is easily obtained using common flow direction algorithms on elevation data, but in order to obtain the basin travel time distribution (IUH, or FT, Flow Time) two parameters have to be assigned: the channel and hillslope velocities (Vc, Vh). Indeed these two values, rescaling the WF expressed in terms of length, provide the travel time of each cell of the basin. The WFIUH model is not largely applied since these two velocity values have to be calibrated while this IUH approach should be adopted on small and almost ungauged basin. Further improvements can be obtained considering the spatial variability of hillslope velocities with the aim to reduce the number of parameter and to better determine the basin IUH. Overland flow velocities are recognized to vary with slope length, flow depth, land use and other geomorphic hillslope characteristics. Several approaches for the variability velocity field estimation applied for FT definition can be found in literature, for instance starting from classic Manning’s law and making assumptions on the hydraulic ratio, or linking hillslope velocity with power laws or local geomorphic properties such as slope or contributing area. Although several studies have already focused on the relationships between WF, channel flow velocity and hillslope flow velocity, in literature it was not deeply investigated the spatial variability of overland flow velocity and how this variability affects the basin hydrologic response. So, after a brief review of the main methods, aim of this work will be: A) to highlight if it is useful and what are differences in using a fully spatial distributed hillslope flow velocity field to rescale the FL, as respect to the standard approach which considers constant hillslope and river network velocity values. B) to evaluate approaches useful for the flow velocity estimation in term of the capability to reproduce appropriate values and in term of the number and type of parameter introduced.

Flow Time estimation with variable hillslope velocity in ungauged basins

Thu, 31 Dec 2009 23:00:00 GMT

Title: Flow Time estimation with variable hillslope velocity in ungauged basins Authors: Grimaldi, Salvatore; Petroselli, Andrea; Alonso, Gustavo; Nardi, Fernando Abstract: The automated spatial estimation of the hillslope runoff dynamics is used as a valuable tool for the estimation of the travel time distribution (flow time), a major factor for the hydrologic prediction in ungauged basins. In fact, while the flow time function is usually obtained by rescaling the flow paths with constant channel and hillslope velocities, in this work a spatially distributed kinematic component, as a function of terrain features and in particular slope and land use, is implemented and its influence on the hydrologic response is tested by means of the Width Function Instantaneous Unit Hydrograph (WFIUH)framework. Hillslope surface flow velocities are evaluated by applying different uniform flow formulas within an automated DEM-based (terrain analysis) algorithm. A comparison test of the performances of the Manning, Darcy, Maidment and Soil Conservation Service uniform flow equations is performed for several case studies in Italy pertaining to different climatic and geomorphic conditions. Results provide new insights for a better understanding of the flow time function also introducing a more parsimonious and physicallybased calibration scheme of the WFIUH. Description: L'articolo è disponibile sul sito dell'editore http://www.sciencedirect.com/

Modelli afflussi deflussi per piccoli bacini idrografici non strumentati

Thu, 31 Dec 2009 23:00:00 GMT

Title: Modelli afflussi deflussi per piccoli bacini idrografici non strumentati Authors: Petroselli, Andrea; Grimaldi, Salvatore; Nardi, Fernando; Alonso, Gustavo Abstract: In questo lavoro si presenta un’applicazione su piccoli bacini non strumentati di un modello afflusso deflussi di tipo geomorfologico per la determinazione dell’idrogramma unitario istantaneo (IUH) basato sulla funzione di ampiezza (WF) riscalata rispetto le velocità di versante e di canale (WFIUH). L’aspetto innovativo del modello proposto consiste nella caratterizzazione automatica - fisicamente e spazialmente distribuita - dei tempi di percorrenza dei deflussi superficiali su versante. A tal fine sono utilizzate alcune formule empiriche che permettono la stima della velocità in funzione della pendenza, dell’uso del suolo e dell’area contribuente, parametri di facile determinazione dai dati territoriali digitali. In tal modo, lo schema proposto permette di definire il modello WFIUH con un unico parametro di calibrazione, la velocità di deflusso canalizzato. Nel caso di studio descritto i risultati ottenuti con il modello proposto si confrontano con quelli dei tradizionali modelli geomorfologici GIUH e con i valori osservati di portata.