A Model-Based Tool for Assessing the Impact of Land Use Change Scenarios on Flood Risk in Small-Scale River Systems—Part 1: Pre-Processing of Scenario Based Flood Characteristics for the Current State of Land Use

Abstract

Land use changes influence the water balance and often increase surface runoff. The resulting impacts on river flow, water level, and flood should be identified beforehand in the phase of spatial planning. In two consecutive papers, we develop a model-based decision support system for quantifying the hydrological and stream hydraulic impacts of land use changes. Part 1 presents the semi-automatic set-up of physically based hydrological and hydraulic models on the basis of geodata analysis for the current state. Appropriate hydrological model parameters for ungauged catchments are derived by a transfer from a calibrated model. In the regarded lowland river basins, parameters of surface and groundwater inflow turned out to be particularly important. While the calibration delivers very good to good model results for flow (Evol =2.4%, R = 0.84, NSE = 0.84), the model performance is good to satisfactory (Evol = −9.6%, R = 0.88, NSE = 0.59) in a different river system parametrized with the transfer procedure. After transferring the concept to a larger area with various small rivers, the current state is analyzed by running simulations based on statistical rainfall scenarios. Results include watercourse section-specific capacities and excess volumes in case of flooding. The developed approach can relatively quickly generate physically reliable and spatially high-resolution results. Part 2 builds on the data generated in part 1 and presents the subsequent approach to assess hydrologic/hydrodynamic impacts of potential land use changes.

A Model-Based Tool for Assessing the Impact of Land Use Change Scenarios on Flood Risk in Small-Scale River Systems—Part 2: Scenario-Based Flood Characteristics for the Planned State of Land Use

“…In the GIS-DSS, this land use map represents the reference state for all future changes of land use. In addition, different levels of protection (risk classes) were defined for each land use class according to their vulnerability (see [22]). The level of protection is expressed by the return period which describes the probability of recurrence of a natural event, in this case a statistical model rainfall of a certain duration.…”

“…The level of protection is expressed by the return period which describes the probability of recurrence of a natural event, in this case a statistical model rainfall of a certain duration. For hydrologic/hydraulic modeling [22], the 48 classes were generalized to 13 land use classes (Figure 2a). The history of the city of Rostock goes back to the 13th century.…”

“…For the watercourses and catchments of the study area, spatially high-resolution flood characteristics for the current state were preprocessed using combined rainfall-runoff and hydrodynamic stream models [22]. The flood characteristics are available for 18 precipitation events of different durations and return periods (see Table 1).…”

“…For the development of the storm water routine, functions were derived beforehand on the basis of the existing model results (see Table 2), which enable the calculation of the peak runoff coefficient for new land use polygons on the basis of a specified degree of sealing. In the first step, the peak runoff coefficients are determined for each subcatchment of a reference model site (Schmarler Bach site, see [22]) for the 18 simulated model rainfall events. The coefficient describes the ratio of the maximum runoff rate per area to the corresponding maximum rainfall rate per area (Equation ( 1)) and depends primarily on the proportion of paved areas (degree of sealing), the slope of the terrain and the rainfall intensity [29].…”

“…• a physical process model of the catchment hydrology and river hydraulics, set up for the current state of land use [22] and • a GIS-routine calculating the additional runoff for land use change scenarios and its routing through the stream system.…”