Exploring an Approach to Estimate Runoff in an Ungauged Mixed Urban Micro Catchment - A Case Study, Pune, India

Sustainable and integrated water resource management needs an hour, and achieving accurate estimation of runoff is key. The decision-making on urban landscaping planning for low-impact development techniques depends largely on the accuracy of rainfall. The haphazardly developed cities in India are encountering flooding crises due to the unexpected expansion. These mixed urban catchments comprise a muddle of residential, commercial, urban-rural, and industrial zones in any combination. Due to this change in urban catchments, the hydrological cycle gets affected and results in elevated runoff volume. The solutions to these are therefore necessary to be planned at a micro catchment level. This paper aims to explore an approach to calculate the runoff of such a micro mixed urban catchment. The geographical scope of this study is the fringe boundary of Pune city. For this ungauged basin, the basic mass balance equation was used to estimate runoff values compared with the runoff values calculated from empirical equations previously developed. From this comparison, it is observed that runoff values obtained from empirical equations were underestimated, which may be due to rapid land-use caused by urbanization. Hence, a need was felt to re-evaluate the coefficients of these empirical models, which take into cognizance the current scenario and its allied changes over the years. An attempt is made to modify the coefficients of empirical equations considering precipitation as the primary parameter. These modified coefficients fetched better runoff results than the runoff results obtained from the coefficients of previously established empirical equations. However, even with these modified coefficients, the runoff results were underestimated, which may be because of not considering the physical characteristics of the catchment in these equations. Therefore, to increase the accuracy of these results, a numerical model that considers these catchment characteristics was chosen. In the present study, a dynamic rainfall-runoff model - stormwater management models (SWMM) is used and compared to assess runoff for an ungauged micro-catchment. The runoff results achieved from these SWMM models better reproduced the hydrologic and hydraulic behavior of the study area (with RMSE of 2.51) by considering detailed catchment characteristics compared to those obtained from all the other empirical models.

Hydrological Similarity-Based Parameter Regionalization under Different Climate and Underlying Surfaces in Ungauged Basins

Hydrological similarity-based parameter regionalization is the dominant method used for runoff prediction in ungauged basin. However, the application of this approach depends on assessing hydrological similarity between basins. This study used data for runoff, climate, and the underlying surface of the Hulan River Basin and Poyang Lake Basin to construct a novel physical hydrological similarity index (HSI). The index was used to compare the efficiency of transfer of the parameters of commonly used regionalization methods and to finally apply parameters to ungauged basins. The results showed that: (1) Precipitation is the main climatic factor regulating magnitude of runoff in the Poyang Lake Basin. Spring runoff in Hulan River Basin was regulated by precipitation and temperature. (2) The GR4J and CemaNeigeGR4J models achieved reasonable simulations of runoff of Poyang Lake Basin and Hulan River Basin. Although CemaNeigeGR4J considers snowmelt, the model simulations of spring runoff in the Hulan River Basin were not accurate. (3) There was a significant correlation between climate, the underlying surface, and hydrological model parameters. There were fewer significant correlations between environmental factors and between environmental factors and hydrological model parameters in the Hulan River Basin compared to those in the Poyang Lake Basin, possibly due to less sub-basins in the Hulan River Basin. (4) The HSI based on a combination of principal component analysis and the entropy method efficiently identified the most similar gauged basin for an ungauged basin. A significant positive correlation existed between the HSI and parameter transfer efficiency. The relationship between the HSI and transfer efficiency could be represented by logistic regression and linear regression in the Poyang Lake Basin and Hulan River Basin, respectively. The HSI was better able to quantify the hydrological similarity between basins in terms of climate and underlying surface and can provide a scientific reference for the transfer of hydrological model parameters in an ungauged basin.

Riparian vegetation as a marker for bankfull and management discharge evaluation: The case study of Rio Torbido river basin (central Italy)

Bankfull discharge estimation is a crucial step in river basin management. Such evaluation can be carried out using hydrological and hydraulic modelling to estimate flow-depths, flow velocities and flood prone areas related to a specific return period. However, different methodological approaches are described in the scientific literature. Such approaches are typically based either on the assumption that the bankfull discharge corresponds to a narrow range of return periods, or on the correlation to the river geomorphological or local descriptors, such as vegetation. In this study, we used high-resolution topographic data and a combined hydrological-hydraulic modelling approach in order to estimate bankfull discharge in the ungauged basin of Rio Torbido River (Central Italy). The field survey of plant species made it possible to investigate the link between the riparian areas and the bankfull discharge. Our results were in line with previous studies and showed a promising agreement between the results of the hydraulic modelling and the plant species present in the investigated river cross sections. The plant species position could be indeed used for a preliminary delineation of the riparian areas to be verified more deeply with the hydrological-hydraulic approach.

Mitigation of Scour Failure Risk of a River Bridge Located in an Ungauged Basin

In this study, scour failure risk of the Çatalzeytin Bridge located in the Western Black Sea Basin, Turkey, was assessed for possible future flood events and appropriate scour countermeasures were considered based on economic and constructability considerations. Waterway adequacy in the spans of the bridge and scour criticality around bridge foundations considered for risk calculations in HYRISK were estimated by hydrological and hydraulic analyses of the watershed and stream. Since the watershed of the bridge is ungauged, geomorphological instantaneous unit hydrograph concept was adopted to estimate the peak discharges with various return periods to be used in hydraulic modelling. Monte Carlo simulation results indicated that most of the simulated peak discharges were in the 95% confidence interval. Hydraulic model results from HECRAS indicated that waterway adequacy and scour criticality were critical for discharges with 200 and 500-year return periods. Scour failure risk of the Çatalzeytin Bridge was classified as high and it was proposed to reduce the risk by constructing partially grouted riprap as the most feasible alternative that would consequently increase the expected lifespan of the bridge. Following this methodology, river bridges may be prioritized based on the risk analysis.

Analysis of a short-duration severe precipitation event in a small ungauged basin through a semi-distributed hydrological model

<p>A cyclone passed over Western Crete in October 17, 2006 and caused a heavy precipitation event producing a flash flood in a small agricultural basin. The only rain gauge in the studied basin recorded daily rainfall of 196.2 mm with a time-step of 15 minutes while 117 mm was recorded in 4 hours. Simulation of the flow hydrograph was performed with the semi-distributed hydrological model HBV-light and the calibration with the post-flood field data from witnesses that indicated the time to peak flow and the maximum water depth of the passing flood wave. The warming-up period of the model was sixteen days and the previous observed rainfall was 21 mm which was recorded on October 12<sup>th</sup>. Potential evaporation was estimated through the Blaney-Criddle method. The basin was divided into various elevation zones representing three vegetation classes. The parameters regarding the soil moisture routine were applied per vegetation class. Sensitivity analysis, performed by changing one parameter at a time shows that the parameters concerning the response and routing routine affected mostly the peak hydrograph. Initial results for the peak hydrograph were compared with the one validated with HEC-HMS model and produced a very good Nash-Sutcliffe coefficient. There is on-going research of the effect of HBV-light parameters and further results will appear on the poster.</p>

Pataruman Watershed Curve Number Determination Study Based on Indonesia Land Map Unit

The rainfall-runoff model is commonly used in flood discharge computation. One of the most frequently employed methods to estimate the flood discharge in the ungauged basin is NRCS-CN. This study is aimed to determine the CN value with/without soil drainage capacity from the soil information in SPT Indonesia. The location used is Pataruman watershed by calibrating the simulation discharge results with the observed discharge. Soil conditions in the Pataruman watershed show that the soil texture has a fine texture (HSG D), a slightly fine texture (HSG C), and a slightly coarse texture (HSG A). The average composite CN in the Pataruman watershed without soil drainage capacity was 85.58 while soil drainage capacity was 81.01. The results of the analysis show that with/without taking into account the soil drainage capacity in the Pataruman watershed there is no significant difference in discharge with a relation coefficient of 0.734 (without soil drainage capacity) and 0.732 (with soil drainage capacity). CN calculations with/without soil drainage capacity are recommended for small watersheds.