Decentralized flood protection
Decentralized flood protection includes measures, which can be implemented extensively to a variety of locations in the entire examined catchment area. This mainly includes:
- the measures for stormwater management in urban areas (e.g. rainwater seepage, drainage ditch systems, retention soil filters and basins, unsealing)
- promoting infiltration on agricultural land (e.g. conservative tillage, subsoil cultivation, afforestation, conversion of field to grassland, greening of depression contours)
- increasing the flood protective effect of forests (e.g. adjusted forest management and conversion, road construction and skidding technology)
- the retention of runoff in small storage spaces (depression storages, small reservoirs, very small basins according to DIN 19700)
- restoring or promoting natural retention in floodplains (water development bands, water retention in the floodplain, changing the runoff cross section, course extension, riverside forests)
Decentralised measures are usually integrative measures that have, in addition to the flood protection, other positive effects on the hydrological cycle, especially on groundwater recharge and the environment. They often exhibit synergies between the objectives of the Flood Risk Management Policy and the Water Framework Directive.
Decentralised measures will not only apply at the river itself, but in the entire catchment area and can be implemented almost within every form of land use. They may be well positioned throughout the catchment area and therefore are often closer to the places of origin of the flood damage. Effects and costs of the individual measures are often small, but rise in areal implementation and with an increasing number of measures.
The effect of the decentralized measures and their interaction depends i.a. on their location, the catchment area characteristics and the type of precipitation. They are especially effective when they exist in appropriate large numbers and are optimized or combined according to the site-specific conditions.
The decentralized measures to reduce flood runoff intervene at various points of the runoff process. For the flood reduction the runoff, the runoff concentration and the wave flow are relevant.
Quantitative data on the effect of the decentralized measures concerning the reduction of peak discharge and runoff volume can be detected with simulation models (precipitaion-runoff models, water balance models, etc.). They describe the relationship between the causative rainfall and the resulting hydrograph. Firstly, models must be justified in detail and physically adequate according to the various issues. Secondly, the model input data must be available in sufficient spatial and temporal resolution.
Hydrological effect of decentralized measures:
- increased retention in the area
- runoff delay (usually beneficial effect on wave superposition, longer flow paths)
- increasing the infiltration (reduction of the wave volume, strengthen groundwater recharge)
- extensive effect throughout the catchment area
- reduction of peak discharge at areal implementation even for rarer events
- compensation of human interventions into the water balance
- mitigating the peak flows and the effects of droughts
Advantages and limitations
Advantages of decentralized measures :
- flexibility for choice of location
- extensive implementability throughout the catchment area
- flexible expandability and implementability through a variety of individual measures
- a close spatial link between the measure and the protection goal
- usually a good sharing of the areas claimed by the measures with other uses
Limitations of decentralized measures:
- need for a clear and comprehensive flood protection concept as well as a consistent realization strategy
- mostly larger area claims compared to centralized systems
- need for a greater number of measures into the respective catchment area for a decisive reduction of the runoff peak (usually only a limited effect of each measure)
- dikes, dams and large retention basins are needed despite decentralised measures
- the combination with other usages requires compromises