Performance of infiltration swales with regard to operation in winter times in an Alpine region.

In cold climate regions winter conditions significantly influence the performance of stormwater infiltration devices. Frozen soil and water storage by snow changes their operation. In this paper winter operation of a grassed infiltration swale was investigated using on-site and laboratory measurements. The field investigation of a grassed swale at a parking place in an Alpine region showed that the swale fulfilled its function properly. Although the top layer was frozen for some time, the storage capacity of the swale was sufficient to store the precipitation until the conditions improved. The soil attenuated the air temperature, at 20 cm below ground surface the soil was only frozen for one week. winter maintenance proved to be a problem, together with the snow from the parking place a lot of gravel and fine particles were deposited at one end of the swale. This decreased the hydraulic conductivity at that point significantly. The laboratory tests with soil columns showed an increase of flow time through the soil column with decreasing soil moisture content. For soil temperatures below 0 degrees C the hydraulic conductivity was reduced for increasing initial soil moisture contents. All in all the hydraulic conductivity was best around 0 degrees C for all soil water contents. However, also at minus 5 degrees C the coefficient of hydraulic conductivity was always at least above 10(-6) m/s, thus within the range of tolerated hydraulic conductivity specified in the national guidelines. Nevertheless, the handling of the soil was found to have high influence on the results. The results indicate that in the Alpine region infiltration swales operate sufficiently under winter conditions although with decreased performance.

Nowcasting of rainfall and of combined sewage flow in urban drainage systems.

Nowcasting of rainfall may be used additionally to online rain measurements to optimize the operation of urban drainage systems. Uncertainties quoted for the rain volume are in the range of 5% to 10% mean square error (MSE), where for rain intensities 45% to 75% MSE are noted. For larger forecast periods up to 3 hours, the uncertainties will increase up to some hundred percents. Combined with the growing number of real time control concepts in sewer systems, rainfall forecast is used more and more in urban drainage systems. Therefore it is of interest how the uncertainties influence the final evaluation of a defined objective function. Uncertainty levels associated with the forecast itself are not necessarily transferable to resulting uncertainties in the catchment's flow dynamics. The aim of this paper is to analyse forecasts of rainfall and specific sewer output variables. For this study the combined sewer system of the city of Linz in the northern part of Austria located on the Danube has been selected. The city itself represents a total area of 96 km2 with 39 municipalities connected. It was found that the available weather radar data leads to large deviations in the forecast for precipitation at forecast horizons larger than 90 minutes. The same is true for sewer variables such a CSO overflow for small sub-catchments. Although the results improve for larger spatial scales, acceptable levels at forecast horizons larger than 90 minutes are not reached.