Heavy Rainfall Risk Analysis

The town of Rodgau has commissioned the engineering firm BGS Wasser, based in Darmstadt, to carry out the heavy rainfall risk assessment.

Goal

The aim of the heavy rainfall risk analysis is to provide preparatory protection

  • the population
  • municipal facilities and properties

to protect the urban area from potentially more frequent and more intense heavy rainfall events in the future, and thus also to adapt to climate change.

 

Procedure

As part of the risk assessment, a model was developed in which relevant precipitation scenarios and heavy rainfall calculations were carried out using software specifically designed for this purpose.

To this end, standardised guidelines were issued by the State of Hesse, on the basis of which a simulation and analysis of runoff pathways during heavy rainfall were carried out for the Rodgau urban area.

Heavy rainfall events (rainfall scenarios) moving across the urban area from various directions were simulated. The urban drainage system, comprising canals and watercourses (the Rodau), was also taken into account.

Using a digital terrain model, assumptions have been made regarding flow paths in areas with varied topography.

Result

The study has produced neighbourhood-specific heavy rainfall hazard maps. These are used to spatially visualise and predict the risk posed by extreme rainfall. They show where water

  • flows above ground
  • is gathering
  • which areas and buildings might be at risk

Water depths and flow velocities are shown on the maps to help assess potential hazards and risks. The maximum values determined (water depth and flow velocity) have been plotted on 17 map sheets at a scale of 1:2,500.

The heavy rainfall hazard maps show that, in the districts of Rodgau, the calculated historical rainfall events result in significant impacts in some areas. This revealed a high proportion of low-lying areas within Rodgau’s flat topography.

The key findings and potential impact are recorded separately for each of the five districts. The heavy rainfall hazard maps are made available via the municipal and publicly accessible geographical information system ‘Bürger-GIS’ in order to alert the public to existing hazards and thus enable them to take their own precautions.

An information event will also be held to provide the public with direct information.

Information session on heavy rain hazards
The results of the heavy rain hazard analysis will be presented at an public information session for the residents of Rodgau on 10 August at 7.00 pm at the Dudenhofen Community Centre.

The maps mentioned above will also be made available for the public to view at the planned information event.

Measures

Municipal measures

The town of Rodgau uses the maps to identify potential risk hotspots. They serve as a basis for planning measures such as retention areas or the optimisation of the sewerage network (municipal heavy rainfall risk management).

  • Inclusion in urban land-use planning and in local authority building projects

    Integrating heavy rainfall risk prevention into local planning processes: urban and land-use planning:
    The maps are incorporated into the planning of new development areas to ensure that buildings are not erected in vulnerable low-lying areas or along watercourses in the first place.

  • Measures to reduce runoff

    Measures to reduce runoff include:

    • Unsealing
    • Nature-based measures along watercourses
    • Nature-based measures on the site (adapted design and management)
    • Strengthening support in outlying areas
    • Strengthening local emergency response capacity within the town
    • Improving drainage and flow control
    • Prevention of blockages (e.g. blockages in culverts)
    • Watercourse maintenance and upkeep

  • Preparation for emergency response (alert and operational plans)

    Civil Protection & Emergency Services:
    The maps help fire brigades and emergency services to prepare for various scenarios and to protect critical infrastructure in a targeted manner in the event of an emergency.

  • Decentralised on-site stormwater management

    Decentralised on-site stormwater management (also known as decentralised rainwater management) is used for the sustainable management of rainwater in built-up areas. This involves managing rainwater where it falls, rather than discharging it centrally via the sewerage system. The aim is to preserve or restore the natural water cycle as far as possible.

     Examples include:

    • Infiltration via troughs, infiltration ditches or infiltration shafts on the property
    • Rainwater harvesting systems (cisterns) for watering or flushing toilets
    • The evaporation or retention of precipitation by green roofs, green façades, green courtyards and much more.
    • Improving water retention in outdoor areas
    • Preparation of local emergency evacuation routes

  • Enhancing water retention in outdoor areas & preparing emergency water flow routes within built-up areas

    Both are key elements in preventing the effects of heavy rainfall. The aim is to retain water across the area (‘sponge landscape’) in order to delay peak runoff into the town and, should flooding nevertheless occur, to channel the water through the town in a way that minimises damage.

    Measures to improve water retention in outdoor areas include:

    • Soil management (agriculture) – such as increasing the humus content, tillage
    • Forestry measures – such as forest conversion and the widespread infiltration of water into the forest soil
    • Structural landscape features – such as the restoration of the Rodau, retention basins and troughs

Private measures – recommendations

Notwithstanding the measures taken by the municipal drainage authority and other local authorities, private homeowners and landowners also have a responsibility to effectively minimise damage caused by natural disasters.

In addition to the information below, you can also find further details in an excellent leaflet published by the City of Offenbach on the topic of ‘Heavy rain! What should you do?’.


  • Raising risk awareness amongst the general public, businesses, and the agricultural and forestry sectors

    Awareness-raising:
    The information shown on the maps makes the abstract risk of heavy rainfall tangible, as – unlike rivers – it can occur anywhere across the area.

  • Information, behavioural and structural precautions, as well as property protection,

    Simple tips on how to protect yourself and your home:

    • Do not store valuable or important items in rooms at risk of flooding
    • Ensure that water can flow freely through the roof and drainage channels
    • Carry out regular maintenance on the gully covers on the premises (not on the road)
    • Install backflow preventers/valves or check that they are working properly on a regular basis
    • If necessary, install lifting systems or check that they are working properly on a regular basis
    • Unprotected areas can be sealed off using sandbags at doors and windows
    • Arrange for any necessary structural measures, such as flood barriers for windows and doors, to protect against water ingress
    • Heating oil tanks must be suitable for the ‘external water pressure’ load condition and must be securely fixed in place
    • Install cables and sockets as high up as possible in basement rooms
    • Store substances that are hazardous to health or the environment in a safe and dry place

    Checklist:
    This checklist can help you assess the risk of damage to your building caused by heavy rain. The more questions you can answer ‘No’ to, the better protected your house is. For questions you answer ‘Yes’ to, protective measures are advisable and worth considering:

    1. Does the location of your building mean that water flows towards you from the road or other properties?
    2. Is your building situated in a low-lying area, on a slope or near a body of water?
    3. Has there already been any flooding in your area?
    4. Are the entrances and basement windows at ground level?
    5. Are there any basement access points or light wells through which water could enter?
    6. Is there a slope on your property leading down towards the house?
    7. Isn’t there a high kerb in front of your property?

  • Financial risk provisioning

    To ensure your home is fully protected against heavy rain, it is essential to take out home insurance that includes cover for natural hazards (natural hazards insurance). Conventional home insurance policies do not usually pay out in the event of heavy rain , as this is classified as a natural hazard rather than water damage from pipes. You should therefore check whether heavy rain is explicitly mentioned or included as a natural hazard . If so, the policy will cover damage to the building caused by flooding, backflow and heavy rain.

    In addition, household contents should be covered separately by contents insurance that includes cover against natural hazards.

    Key steps to ensure protection:

    • Include natural hazards cover: Existing home insurance policies can often be extended to include the ‘natural hazards’ cover option, with the addition of cover for heavy rain.
    • Check for cover against damage caused by backflow: Make sure that damage caused by backflow from the sewer system is included in your cover.
    • Prevention (personal precautions): Fitting a backflow valve and sealing basement windows and doors to ensure that insurance cover is not jeopardised by negligence.
    • Check the risk: Premiums depend on the ZÜRS zone (risk class); cover is often cheaper than you might think.
    • Keep important documents safe: - Do not store documents or valuables in the cellar or in rooms at risk.