After the floods of 1897, 1932 and 1954, the flood event of August 2002 highlighted the increasing risk due to intense precipitation or rapid snowmelt in the Ore Mountains. Even regional precipitation events (e.g., 1931 Schwarzwassertal, 1999 Marienberg, 2000 Ruebenau) caused extreme floods with high local damage. Meteorologists predict a general increase in the quantity and dimensions of extreme events in the coming decades in Central Europe (MPI for Meteorology 2004, Bárdossy & Pakosch 2005), signalling the need for action and implementation.
The extreme flood events are primarily the result of runoff in the flood-generating areas, which are mostly located in the German-Czech border area. The enormous damage caused by the 2002 flood event in Germany alone illustrates the problem of flood generation in the headwater areas. For example, the damage within the catchment of the Ruebenauer Bach (area: 8 km², length: 4 km, in the upper basin of the Natzschung) amounted to about 2 million € (Datenbank M-Bau – Zentrale Schadenserfassung und Sanierungsmanagementssoftware, LTV Sachsen Hochwasser 2002).
Owing to runoff concentration, the amount of damage increased considerably downstream along the Natzschung as well as at the other tributaries of the Upper Floeha above and in the town of Olbernhau (Caemmerswalder Dorfbach, Frauenbach, Moertelbach, Schweinitz, Natzschung, Doerfelbach, Rungstockbach, Bielabach) with an overall damage of 18 million € upstream of Olbernhau.
High discharges had and still have a disastrous impact on the town of Olbernhau since an HQ20 was the maximum that could pass through without damage (current protection goal HQ50). During the flood in 2002 the total damage within the town reached 62 million € (loss database and private losses). A similarly high risk potential exists at the southern escarpment in the Czech Republic (e.g., Prunerovsky potok).
Flood damage is closely linked with the flood-generating areas, which therefore need to be taken into greater account for a sustainable flood protection. Whereas central flood protection measures (e.g., reservoirs) only respond to runoff concentration, decentralised measures respond more to surface runoff generation.
The revised version of the Water Act of Saxony emphasises the role of the runoff generation areas: “Runoff generation areas are areas, especially in the low mountain ranges and hilly landscapes, in which intense precipitation events or rapid snowmelts may cause heavy surface runoff and therefore increase the flood risk in the rivers and endanger public safety and order…” (According to Water Act of Saxony §100b,2). For this reason, “water infiltration and retention is to be preserved and improved…” (According to Water Act of Saxony §100,2).
Over the past years – especially after the flood in 2002 – flood protection concepts have provided essential knowledge about the precipitation-runoff characteristics of the river basins in the Ore Mountains and the necessary central flood protection measures. In addition, the upper catchments, particularly the runoff generation areas and the floodplains, offer considerable additional retention potential. The use of this potential may decrease the risk potential and reduce the damage to downstream riparians (e.g., the town of Olbernhau) as well as to the cross-border catchments and settlements in the ridges.
The main objective of the research project “DINGHO” is to improve cross-border flood protection in the catchments in the ridges of the Central Ore Mountains with special regard to second-order rivers in the subcatchments of the Upper Floeha (upstream of the town of Olbernhau). Decentralised, integrated flood protections measures support a sustainable and ecological development of the border area and reduce environmental risks (reduction of risk potentials by flood protection), thus contributing to the improvement of cross-border cooperation and networking in terms of flood protection.
The main questions of the project:
1. Are the given potentials in the ridge locations of the Ore Mountains efficient for decentralised flood protections measures to reduce risk potential and attenuate the flood wave?
2. What potentials come with forestry areas?
3. Is it possible to provide flood protection management for the entire cross-border area in a concentrated and coordinated way?
4. Is it possible to improve the implementation of flood protection measures through an interdisciplinary and bilateral cooperation?
The concept of decentralised and integrated flood protection comprises the following moduls, which may be combined depending on the hydrological and geomorphological conditions. Current and future land use has to be considered.
Regional and landscape analysis by GIS and remote sensing
Survey of the structural and environmental characteristics of the project area by the use of ArcGIS tools and remote sensing data (aerial photographs, satellite images, laser scanning). Accurate representation and the option of linking spatial data and data query enable a detailed analysis of the area in terms of potentials for flood protection in runoff generation areas and the choice of appropriate locations for decentralised flood protection measures.
Changes in agricultural techniques
The cultivation of agricultural areas has a strong impact on laminar and linear runoff generation. While ploughing destroys the macropore structure and inhibits infiltration, other techniques such as grubbing have less negative effects on the infiltration conditions of the soils. In addition, mulch and catch crops reduce the splash effect of raindrops and mitigate silting effects which promote surface runoff and soil erosion. Changes in agricultural techniques are incorporated quantitatively into rainfall-runoff models.
Forest conversion, forest extension and forestry techniques
Forestry measures such as forest conversion and extension provide the possibility to improve the regional water balance. In this context, increasing evaporation and enhancing soil water storage should be mentioned. A long-term change in the forest structure may improve the humus layer and the soil structure. Better infiltration conditions and a higher storage capacity of the soil reduce surface runoff. Higher soil roughness through changes in the understorey and the soil cover may also reduce surface runoff by cushioning falling raindrops before they hit the ground. Whereas forest conversion mainly affects the regional water balance, afforestation of previously unforested or only minimally forested areas attenuates runoff generation and reduces the flood peak. Furthermore, adapted forestry techniques are needed to decrease soil compaction caused by the use of heavy machinery and to preserve or improve the infiltration conditions and storage capacity of the soils.
Water retention in areas of settlement and infrastructure
Paths in forested and agricultural land and settlement areas linked with local morphometry form direct pathways for surface runoff. In this case appropriate measures such as infiltration trenches enhance the water retention. Water retention in drainage trenches Drainage channels in forested and agricultural areas as well as wetlands promote the rapid drainage of areas after heavy precipitation or during snowmelt. The installation of low-cost barriers enables these channels to be partly used for temporary surface runoff retention. Particularly drainage trenches in wetlands with a cross section area of 8 – 10 m² and more (e.g., the Moosbeerheide in the Natzschung river basin) provide a significant retention capacity, whereas the soils are still drained in dry periods.
Renaturation of straightened river sections
The renaturation of straightened river sections slows down the flow velocity within the channel and reduces runoff concentration and damage in floodplains and runoff concentration.
Increasing roughness within the channels and the floodplain
Channels and floodplains are often characterised by a low roughness, which leads to rapid runoff or discharge. This may makes sense within settlements or endangered facilities. Elsewhere it is possible to increase roughness by suitable measures to reduce flow velocity (e.g., by floodplain afforestation).
Construction of decentralised, integrated retention basins
Retention basins located below areas generating high runoff are an essential part of flood protection measures. Dams that are a few metres high, with shallow slopes and bottom outlets, fit into the landscape and still attain an effective retention capacity (up to 500,000 m³) at low specific costs (cf. Hartmann et al. 2005). The green retention basins may be used in different ways (e.g., grassland, pasture land or nature protection areas).
Construction of cascading retention basins in steep areas
In areas of steep relief, cascades of retention basins may provide effective flood protection. Especially on the steep southern escarpment of the Ore Mountains the retention effect of cascades should be investigated. The Czech Water Management Administration is already considering the partial reconstruction of steep river sections. A rainfall-runoff model should be used to analyse the effect.
Quantification, verification and evaluation of the effect of combined flood protection measures
Rainfall-runoff models will be used to analyse how combined measures and their location affect flood runoff. The hydrological impact of the measures can then be quantified and verified, enabling an interdisciplinary evaluation of their efficiency in terms of decentralised flood protection in order to propose a reliable catalogue of measures.
Project management in terms of identification, networking and integration of land management stakeholders and their requirements
Identification of the requirements and objectives of the land management stakeholders is very important in order to implement decentralised flood protection measures. Flood protection is compatible with nature protection, agriculture and forestry since the concept includes existing land management structures. An effective networking requires coordinated actions and meetings of all stakeholders.