July 18th 2021: The Day with Extreme Rain and Extreme Flow in HOAL Catchment. Modelling with HEC-HMS

<p>The Seitengraben river is situated in the Erlauf basin, a tributary to the Danube. Since 2010 it is object to intensive studies by HOAL (Hydrologic Open-Air Laboratory). Its catchment area is 65.8 ha. In the time period 1990-2021, the mean annual precipitation is 819 mm, the evaporation 633 mm and the annual average temperature is 9.3°C.</p> <p>The extreme precipitation event from July 18<sup>th</sup> 2021 also produced an extreme direct flow, showing the limits of the existing measuring devices. The limited height of the H-flume cut the peaks of the flood waves. The flood waves at the catchment outlet (MW) were also influenced by the backwater from the retention reservoir situated downstream of the outlet, which was completely full for the first time since its construction in 2004.</p> <p>The 100 years design precipitation for this region is 130 mm in 24 hours (according to ÖKOSTRA).</p> <p>Our goal was to estimate the complete flow hydrograph at the catchment outlet, and for this, we chose the model HEC-HMS 4.10 (Hydrological Modeling System, U.S. Army Corps of Engineers).</p> <p>In order to complete the computation task, all measured data were used (weather station, measured flow from all stations, soil moisture sensors, survey cameras). The model HEC-HMS was fed with detailed information such as digital elevation model, soil type and texture, soil physics, soil moisture, land use and vegetation cover and rainfall.</p> <p>The model parameters were calibrated using the event from August 16<sup>th</sup> 2021, when 35 mm of rain produced 2.3 mm direct flow volume at the catchment’s outlet. Direct flow volume into the same section, computed by the HEC-HMS, is 2.6 mm. The Nash-Sutcliffe model efficiency coefficient NSE is 0.94 and R<sup>2</sup> =0.95, percent bias is 8% and RMSE Std. Dev. is 0.256. The calibrated model was used for the July 18<sup>th</sup>, 2021, event modelling.</p> <p>On July 18<sup>th</sup> convective precipitation occurred dispersed over 18 hours in 7 nuclei with the maximum intensity of 30 mm/hour and an average intensity of 15 mm/hour. From 152 mm of total rainfall, 64.3 mm were held back by vegetation, soil surface retention and soil infiltration.</p> <p>Due to several influences and disturbances of the measuring device, we did not get a complete hydrograph of the whole event with its multiple waves. So, part of the computed flood hydrograph cannot be compared with observations.</p>