Assessing the impact of land use change on water flow in Hemlingby Creek : Comprehensive analysis of the cloudburst in Gävle August 2021

Sammanfattning

In the last 20 years, flood-related disasters have increased and with climate change and the increase in global temperature, heavy rainfall will become more common. Between 17 and 18 August 2021, large parts of Gävleborg County were affected by heavy rainfall. During a 24-hour period, the county was hit by 161.6 mm of rain. Hemlingby, located in Gävle Municipality, was severely affected due to flooding in Hemlingby creek. The aim of the study was to develop a hydrological and hydraulic model of Hemlingby creek to investigate the hydrological characteristics and the extent of the cloudburst on 17–18 August 2021. These models were also used to investigate if the land use change, of the newly developed area Hemlingborg, has an impact on the water flow in Hemlingby creek. The aim was also to gather the experiences of residents living near Hemlingby creek regarding the cloudburst in 2021, as well as to collect photographs and validation material. The method consisted of combined hydrological and hydraulic modeling to simulate the water flow of the cloudburst on 17–18 August 2021 in Hydrologic Engineering Center (HEC) – Hydrologic Modeling System (HMS) and to model the inundation in HEC - River Analysis System (RAS). Both an unsteady two-dimensional (2D) modeling and a steady one-dimensional (1D) modeling were performed to analyze which method was best suited to model the study area. An uncertainty analysis on the Manning’s n value was also performed. Additionally, interviews were conducted with two residents near Hemlingby creek to gather important local knowledge and perspectives regarding the creek's behavior, possible problems, and validation material. The simulated result of the cloudburst in August 2021 showed a peak flow of 12.6 m³/s at 05:00 on 18 August in Hemlingby creek. The study analyzed how the changed land use of the newly developed area Hemlingborg affects the water flow in Hemlingby creek, which showed a peak flow of 11.6 m³/s. The analysis on how the change in resolution affected the hydrological model showed that the lowest resolutions of 25 m and 50 m resolution created significant changes in the result compared with the 2 m resolution. For the hydraulic model result, the conclusion drawn is that the 2D model proved to be the most suitable model for this area. The impact of using a terrain model with or without "burned-in stream" was also investigated, resulting in an inundation difference of 138 730 m2 between the models. The uncertainty analysis showed that the Manning's n value could be confirmed as a sensitive parameter in a hydraulic model. The future for Hemlingby remains uncertain, but in line with climate change, more intrusive cloudbursts are expected to affect urban communities and measures need to be taken to cope with future severe rain events.