Flash floods – an increasingly common concern for local governments

Flash floods, rainfall floods, urban floods, or flash floods in English, are the most common terms for phenomena that regularly haunt modern Polish cities. The drought that has characterized recent years in our country could indicate that there is no threat of flooding and waterlogging in favor of a problem with access to water. However, it is not so obvious.

Climate change is bringing us a general upheaval of climate trends in favor of, among other things. evolution in the number of seasons and basically reducing them to two – winter and summer, higher average annual temperatures, lower average annual precipitation, but also intensification of extreme phenomena. According to CSO statistics, in 2021. registered nearly 30,000 fire department interventions due to excessive rainfall and flooding, in 2022. There were about 20,000 of these interventions.

In Poland, flash floods are most often the consequence of storm events, the accumulation of intense precipitation in a short period of time over a small area. The losses they generate are sometimes very high. Extreme phenomena, combined with the significant sealing of the urban catchment area and often inefficient sewer networks, designed according to standards that are no longer up-to-date, cause periodic retention of rainwater on surfaces that by design should be drained. The areas most vulnerable to such events are those below the surrounding terrain, as they become natural (through surface runoff or lack of drainage) recipients of rainwater.

However, we do not intend to analyze in detail the genesis and course of these phenomena in urban areas. The subject of the article is potential solutions at the planning and analytical level that can be implemented in cities, depending on the needs, the quality of the data available or the financial resources available.

DHI Polska has been successfully implementing projects for cities and local and government administrations, as well as private entities for over 16 years. They are related to the management of water in rivers, in the sewage and water supply system, and at sea. We deal extensively with water, analyzing and planning the most efficient management of resources, ways to deal with surplus or shortage.

For our analysis, we use modern tools and technology based on DHI Group’s proprietary software, MIKE. MIKE family tools support the work in most of the projects we carry out in three main areas – water resources, urban water, marine areas. Modeling sewer networks is one of our daily tasks. In the field of rainwater management, we have a wide range of local experience: developed strategies and concepts of rainwater management for Wroclaw, Raciborz, Toruń and Gdynia, analyses of unlined areas for Kalisz, Kolobrzeg, Katowice and Bielsko-Biala, or models with the concept of solutions for selected drainage basins/areas in Bydgoszcz, Gdansk, Katowice and Krakow.

Short animations summarizing the implementation of selected projects can be viewed online, QR codes are included at the end of the article. While providing our customers with tools from the MIKE family, we also provide technical support and training in their use. As part of the DHI group, we also have a great deal of international experience, and can draw knowledge from Scandinavian countries or Singapore, which are more advanced in this area.

The scope of support for cities and units managing the sewerage network in urbanized areas can be several levels or limited to a selected level of sophistication of analysis. The following are the next steps to prepare the city to make the right decisions, to identify the points that require the most urgent intervention, in order to continue in-depth analysis and action on the truncated area.

With such a huge increase in operating costs over the past year, it is crucial to spend funds as efficiently as possible, and effectively reduce the risk of losses. We take care not to exacerbate the financial problems caused by the current economic situation and the energy crisis.

The first step? Determination of probable precipitation

The first step in any analysis related to flood risk is the preparation of meteorological data, the adoption of specific values of probable precipitation to which the city is exposed.

The basis for determining authoritative values is the determination of the probability of occurrence and duration of precipitation, as well as the appropriate selection of the calculation method. So far, nationwide precipitation models – the Blaszczyk (1954) and Bogdanowicz-Stachy (1998) models – have been the most widely used in Poland.

In recent years, they have been systematically replaced by local precipitation models that take into account current measurement data and revise older methods, often indicating an underestimation of the calculation results in relation to currently recorded precipitation. Local models, based on high temporal resolution precipitation data for a network of stations across the country, are available in the form of off-the-shelf products, such as PANDa’s Polish Rainfall Intensity Atlas (paid data) or IMGW-PIB’s PMAXTP project (free data).

The choice of method for developing authoritative precipitation should take into account the conditions in the area under analysis, as not every method is universal. It is also necessary to determine the probability (frequency) and duration of precipitation accepted for calculation. The most common statements used in engineering practice are:

• Probability of precipitation p=10% (frequency C=10), duration t=15 min and t=120 min;
• probability of precipitation p=20% (frequency C=5), duration t=15 min and t=120 min, in accordance with the PN-EN-752:2017 stormwater drainage network design standard;
• 1% precipitation probability (frequency C=100), duration t=15 min and t=120 min, especially relevant for river network integrated modeling, where the 1% scenario is the most reference.

Adopting a longer rainfall duration (for example, 120 min) allows analyses to be carried out for the least favorable conditions, i.e. achievement of full saturation of the soil with water (depletion of infiltration capacity), sinking of culverts and their operation under pressure, partial or complete filling of storm drains, etc.

What will be flooded? Detection of no-drainage sites

Drainless sites in cities are clearly vulnerable to flooding. In light of publicly available land surface data (Numerical Terrain Model, NMT), we can, with the use of GIS tools, quite easily make a simplified analysis and indicate where water accumulates as a result of heavy rainfall (we superimpose a predetermined rainfall on the land surface).

These areas can be delineated even in the absence of ground infiltration and the influence of evapotranspiration. Precipitation is usually distributed uniformly and evenly over the entire area. The next step is to verify the results obtained. In the course of their analysis, selected technical objects can be introduced to improve the relief of the NMT, which will naturally stop spilling water (e.g., dykes, retaining walls), which are water-covered surfaces (e.g., lakes, ponds), or which can allow water to flow (e.g., culverts, ditches), and which are not well mapped in the available NMT. The ability to introduce/revise the above structures significantly affects the correctness of calculations of rainwater runoff paths, which contributes to greater reliability of the correctness of the identified zones of potential risk.

Subsequently, the minimum area of flooded areas to be identified as problem areas (e.g. ^100m2 and above), as the results will indicate thousands of such points. However, this is typical post-processing and such verification is done only after the results are obtained and the scale of the problem in a given city is determined.

Although these analyses are considered to be the most simplistic, they quite accurately identify problem areas within the city and provide a very good starting point for further action.

What do we already have at our disposal? Designation of sites for green retention

The above-described simplified method, in addition to identifying sites without drainage and prone to flooding, makes it possible to identify points that can naturally perform a retention function. In order to do this effectively, the results of the analysis must be matched with information on existing land cover, where only areas defined as: forest and wooded land, shrub vegetation, grassland vegetation and agricultural cultivation, and uncultivated land are analyzed.

It is extremely beneficial when the problem area, which is a natural depression of the land, is also a site of potential retention. This could be an example of well-planned activities in the city, i.e. securing biologically active area in an area with flooding problems during heavy rainfall, which is reduced to some extent thanks to green areas.

This can also be a coincidence, so it is all the more reason to strive to increase or maintain already existing use in such places, which will help reduce the volume of flooding volume through transpiration or infiltration. In addition, it seems appropriate to be able to consider these areas for technical investments, for example: the construction of retention basins, squares or upgrading already existing biologically active areas due to the favorable terrain.

Of course, these are favorable situations, cases in which intensive development is not planned, as the change of development may cause the formation of local stagnation during heavy rainfall. Unfortunately, we encounter such situations in cities across the country. Each development should have a drainage plan, which, given the prospect of retention fees (a fee for reducing natural retention, for draining rainwater and snowmelt), should try to maximize water retention in its own area.

Where to react first? Prioritization of catchment areas for the introduction of a rainwater management program

In our experience, we usually receive hundreds of problem sites in the area of a medium-sized city. Due to budget limits, it is advisable to prioritize results in order to classify areas and identify those requiring the most urgent intervention.

There can be many such evaluation criteria. A sample set of conditions with weights is indicated below, but they are determined on a case-by-case basis, given local conditions and needs in a given city. Additional data and studies are required to conduct a reliable multi-criteria analysis, if only in terms of planned land use or data inventory from the State Fire Service.

Below is a summary of the catchment prioritization criteria with assigned weights for one of the cities.

CRITERION NAME:	WEIGHT:
1. number of facilities at risk of flooding – current status	10%
2. number of facilities at risk of flooding – past state	12,5%
3. volume of retained water in relation to impervious surface – current status	7,5%
4. volume of retained water in relation to impervious surface – past condition	10%
5. division of soils in terms of infiltration capacity in permeable areas	15%
6. number of localized areas particularly vulnerable to flooding	15%
7. number of interventions by the State Fire Service	17,5%
8. change in sealing of given catchment areas over time	12,5%
SUM:	100%

The values obtained from the relatively simple multi-criteria analysis clearly suggest which of the study catchments need priority treatment first. This provides a very good starting point for further analysis.

What’s the next step? Modeling and action planning

We then proceed with in-depth analyses of the selected problem areas or catchment areas. This is optimal, as it reduces the cost of performing analyses throughout the city. In-depth studies require the preparation of advanced hydraulic models that best reflect the hydrodynamics of the network and other facilities in the area under study, so that these further analyses and the planning of specific measures are as reliable as possible.

In order to prepare such models, a range of data is needed, which (excluding those from nationwide resources) are largely held by cities, municipal water utilities or other entities that manage the sewage network. The scope of necessary data mainly includes:

• Land surface data – numerical terrain model, numerical land surface model, orthophotos, data from GUGIK resources;
• Data on existing infrastructure – Topographic Object Database, data from GESUT resources, base map, network data from the city’s own resources;
• measurement campaign – data on flows, fills and precipitation conducted as part of a periodic measurement campaign of several months, the campaigns are conducted for a given project, necessary for reliable calibration of models;
• probable precipitation data – to be obtained or calculated, as described earlier;
• additional surveying data/measurements – often the data on selected network elements and objects are not reliable and require additional field inspections or surveying measurements.

The best solutions currently proposed are integrated models, including one- and two-dimensional models for both the sewer network and its receptors (watercourses, ditches, reservoirs). It’s very important that the analysis be carried out on a broader scale, taking into account the threat and situation on watercourses.

Advanced analyses of various scenarios are conducted on properly prepared hydraulic models. Various variants of technical solutions are being tested to eliminate the problem. The results of the analysis and the developed modeling products can also be used for other tasks of the unit, such as the process of obtaining permits for discharges from storm sewer overflows, etc.

How to protect yourself? Predictive systems

Despite efforts, there are times when the threat cannot be eliminated. There are various additional, non-structural tools that support cities in the crisis management process. We are mainly talking about forecasting systems, early prediction and warning systems. DHI Poland has developed an application, Network Flood Monitor (NFM), which forecasts flooding and warns of its occurrence in the city. Based on the sewer network model, the autonomous system predicts its behavior while analyzing the hydrodynamics of the entire system.

The application is integrated with the current meteorological forecast, which makes it possible to predict, thanks to the analysis carried out in advance, the impact of the forecasted precipitation on the behavior of the sewage network. Users are informed of the occurrence of potential flooding. The NFM application is flexible, i.e. it can forecast states in the stormwater, combined sewer, sanitary sewer networks, as well as combine models of all of them. The functions available in the app can be customized according to the user’s needs. Expansion with new functionality is also possible. The tool assumes integration with SCADA telemetry data, with radar and predictive data, or with a machine learning module. These systems have been implemented by DHI in several cities in Poland, including. in Gdynia.

Where to get the funds to implement the activities? Funding

The most common explanation for the lack of implementation of activities, or even neglect of network maintenance, is the lack of sufficient funds. In view of the current economic situation, this is even more difficult than years ago. That is why it is so important to direct your activities to reduce costs (increase energy efficiency) and avoid potential losses (flooding, floods, failures). European funds have been supporting investment processes in many areas of the economy in our country for many years.

EU funding sustains climate change adaptation and water and wastewater management in key objectives and priority axes. It is primarily in these two areas that the problems of rainwater and the sewer network fall into place. There was also a measure related to the development of green infrastructure, which may include retention in urban areas. The start of a new perspective is always a very good opportunity to mobilize and enter competitions for fundraising. Previously, additional scoring was given to sewer development projects that already had hydraulic, rainfall models. It is important, therefore, to start the preparatory work immediately and not lose the chance to raise valuable funds.

Summary

In Poland, the issues of floodwater management and flood protection planning are quite precisely regulated by the Water Law Act, which transposes the requirements of the Floods Directive into domestic conditions. Unfortunately, the area of flash floods, urban, is not included in the scope of these analyses. There are currently no uniform guidelines for planning and managing stormwater, standards that would make it easier to get started in less technically advanced cities.

Currently, Wody Polskie is implementing a project to create a guide to help cities identify areas at risk from rainwater, which will certainly organize the knowledge available on the market and provide possible directions for future work in an accessible way. Regardless of the commitment at the central level, cities that have experienced the negative effects of flooding and waterlogging in recent years, primarily related to the inefficiency of the sewage system or areas without drainage, should consider specific measures and plan resources for their implementation.

In many municipal climate change adaptation plans (MPAs), we are already seeing the identification of these elements as important and in need of action. The cities can definitely be supported by the next EU financial perspective, which once again indicates climate change adaptation as a priority.

The drive for urbanization of naturally flooded areas (depressions, river valleys) poses obvious risks to users, and unfortunately, the development of stormwater drainage networks and the greatest possible retention of water in developed areas often does not go hand in hand with the construction of more housing estates. Unconditionally shifting the problem to the network administrator is only a short-term solution, as this water will sooner or later appear in the area of the settlement, developed but not equipped with proper drainage and protection.

This is definitely a topic for improvement, and in the age of available tools and modern technological solutions that enable network modeling, it is possible to know the scale of the problem and react much faster than in the past. DHI Polska has for years been supporting Polish cities in solving problems with excess rainwater and the threat of flooding in their areas.

Stormwater Management Program for the City of Toruń

Rational management of rainwater – analysis of areas at risk of flooding in Kolobrzeg

Analysis of sites at risk of flooding due to torrential rain within the city limits of Kalisz

Spatial analysis of rainwater drainage basins for MPWIK Wrocław

Our articles on similar topics:

Modeling a way to save water

How to effectively retain water. A guide for local governments

Flooding – what is it and does it threaten me?