A System for Analysis and Simulating Hydraulic and Hydrogeological Risks Through WebGIS 3D Digital Platforms

Abstract

The present research activity carried out demonstrated how simulation tools developed through WebGIS 3D digital platforms are capable of producing approximate forecasts of the effects of potentially catastrophic meteorological phenomena that may affect riverbeds in the territories observed. This work presents an analysis and simulation platform with graphic representation of the results in the form of three-dimensional animation. This methodology may represent a useful tool for all bodies and organizations that need to create hypothetical scenarios for the management of emergencies related to flooding events in watercourses, especially in areas of maximum hydrogeological vulnerability in the Italian territory. These scenarios are particularly useful in cases where watercourses are located near inhabited centers, industrial areas or strategic infrastructures, where the risk of material damage and danger to the population is greater. The simulation is based on the morphology of the land adjacent to the bed of an affected watercourse, taking elevation into account to determine the direction of the expansion of the water mass. An important aspect of the platform is the extreme speed of simulation resolution, which allows the tool to be used even in real time. This real-time forecasting approach is crucial for making quick and informed decisions, thus reducing reaction times and improving emergency management on the ground, with a potential positive impact on the safety of the population and the protection of infrastructure.

1. Introduction

The Italian territory has more than 1200 watercourses, which represent a fundamental element of the country’s natural landscape and hydrographic heritage. This extensive river network not only models the territory but also directly influences the lives of the population living along its riverbanks. From antiquity to the present, Italian rivers have played a crucial role in the development of civilizations, providing resources for agriculture, industry and transport. However, this water wealth also brings with it a significant risk: flood risk.

Floods are considered one of the most destructive natural disasters globally, causing significant human, economic and environmental losses. To address this issue, several global directives and international initiatives have been developed to improve risk management and adaptation to climate change. The objectives are as follows: significantly reduce disaster risk, including the risk of floods, by increasing the resilience of communities and infrastructure; improve the understanding of disaster risk through monitoring and data collection; strengthen risk management and coordination between governments and communities; invest in disaster risk reduction and climate adaptation; and promote sustainable recovery strategies after floods. Some of the main global flood policies and frameworks are as follows: the Sendai Framework for Disaster Risk Reduction; the United Nations Framework Convention on Climate Change (UNFCCC); the European Union Floods Directive (2007/60/EC); the Global Framework for Climate Services (GFCS); Integrated Flood Management (IFM); and the Global Facility for Disaster Reduction and Recovery (GFDRR).

The territory of the Italian State, characterized by a complex morphology ranging from the Alps to the Apennines and the coastal plains, is particularly vulnerable to flooding events. Intense rainfall, especially frequent in certain seasons, together with sometimes inadequate water management can cause floods with serious consequences for infrastructure, the economy and the safety of populations. Floods represent one of the most destructive natural disasters, and in recent decades, an increase in their frequency and intensity has been observed, a phenomenon aggravated by ongoing climate change.

As can be seen from the recent catastrophic events in the Emilia Romagna region, it is essential to provide structural flood risk mitigation measures, as well as to prepare an emergency management plan that limits as much as possible the damage caused by the increasingly frequent floods that occur as a result of climatic events that were once considered exceptional but are now becoming increasingly common and destructive.

The institution that deals with the study of possible measures to mitigate land-related risks, among which flood risk can certainly be counted, is the ISPRA (Istituto Superiore per la Protezione e la Ricerca Ambientale), which, among its many activities, also periodically produces reports on the hazard and health status of the Italian territory. In the hydrological sphere, the most recent is the Report on Flood Hazard Conditions in Italy and Associated Risk Indicators [1] in which hazard levels are defined, the mosaic of the territory according to hazard is outlined, and the most recent data for populations, cultural heritage and installations exposed to flood risk are presented.

Once the risk has been defined, it is necessary for organizations to equip themselves with tools to help create scenarios in the case of climatic events but also to forecast damage to people and structures in the case of events that are already underway. Given the developments of the last few years in computing power and connectivity, it is possible to support emergency management operations with IT tools that exploit internet connection to enable them to be used on portable devices or simple terminals equipped with a browser.

The research activity described in this study aims to create a platform for the real-time simulation of the overflowing of watercourses from their riverbeds, with an interactive three-dimensional visualization of the simulation results, by exploiting the powerful WebGL graphics libraries [2], the Three.js three-dimensional rendering libraries [3] and the two-dimensional maps provided by the Open Layers API [4].

The system uses the polygonal geometries of the waterways provided by OpenStreetMap [5], which generates vector maps for the entire planet and has various layers, including waterways; these geometries are cast in a portion of the surrounding territory, reproduced thanks to the DEM (Digital Elevation Model) raster maps of the TINITALY 1.1 project. 1 [6], which, with a resolution of 10 × 10 m, allow for the accurate modeling of the terrain. This is further enriched by textures from satellite images offered by the COPERNICUS and ESRI services [7], for the better locating of any roads, infrastructure or inhabited centers present in the area of interest.

These data, combined with the input data entered by the user, including the amount of rain falling in an hour (mm/h), the soil permeability index and the time data of the simulation, make it possible to calculate the expansion of the volume of water in the area concerned, making it possible to understand which areas will be most affected by the outflow of water from the riverbed and also having an indication of the heights reached and any built-up areas reached.

The system also proposes the consideration of, albeit with a simplified approach, the contribution given by the water precipitated on the slopes adjacent to the riverbed, which then inevitably fall into it, increasing the water flow involved.

The simulations are performed automatically at any point in the area of interest, which, for the purpose of this research study, was limited to the region of Liguria but can easily be expanded to the entire Italian territory for potential use in real cases; the user can in fact select a point of their choice, and all the data of a 5 × 5 km area around it are automatically downloaded, adapted and used within the simulation, which can also be repeated several times, perhaps with changing the input parameters, to provide a series of diversified scenarios that take into account different starting conditions.

2. Materials and Methods

2.1. Watercourses

The Italian territory boasts the presence of numerous watercourses (around 1200), which are relatively shorter than those in other European regions due to the country’s geographic conformation and the presence of the Apennine Mountain chain that divides the flow of water along the peninsula into two opposing sides. [8]

Among Italian rivers, the longest of those that flow entirely through our territory is the Po, with a total length of 652 km.

The flow and length of the watercourses are due to the characteristics of the soil over which they flow; of particular importance are the steepness of the terrain and its permeability, due to the geological conformation of the underlying soil.

The following considerations can be made about Italian watercourses:

• The rivers with the greatest flow and width are those in the Alpine region, due to the high elevations.

• Rivers flowing along the peninsula, on the other hand, due to the arrangement of the Apennine chain, cross short valleys and generally do not exceed 200 km in length, with a higher average length on the Tyrrhenian side than on the Adriatic and Ionian sides.

• The rivers flowing into the Tyrrhenian Sea are also longer because they initially follow longitudinal Apennine valleys and then flow in cross-river courses with respect to the chain in the sub-Apennine area.

Italian rivers can therefore be divided into 3 main categories: The Alpine rivers, of glacial origin, are subject to flooding in the spring and summer seasons due to the melting of glaciers, with lakes occupying the lower parts of the valleys whose function is to calm the impetus of the rivers and clarify the turbid waters. The Apennine rivers, on the other hand, are subject to sudden autumn flooding due to rainfall. They are supplied by springs that flow at the edges of areas characterized by permeable fissured rocks, due to the lack of snow and glaciers in the Apennines. Finally, Sardinian and Sicilian rivers are usually torrential in character and are therefore rich in water in winter and almost dry in summer. The location within which the flow of river water occurs is called the riverbed. It is created precisely by the erosive action exerted by the water, which acts on the rocky substrate and determines its progressive erosion. For each watercourse, three distinct riverbeds can be identified in the cross-section:

• Flood bed (or ordinary bed): This constitutes the flow channel during ordinary flood periods, usually in spring and autumn. It is delimited laterally by banks or sub-vertical slopes. This riverbed contains coarse materials, deposited as a result of current variations. The presence of tree vegetation is poor.

• Major riverbed (or flood bed): This consists of the maximum area unavailable to the river during flooding. It is elevated and larger than the ordinary flow bed. It is characterized by the presence of coarse material and sediments, deposited by the water during flooding.

• Leaning bed (or flow channel): This constitutes the channel in which the river current flows during the lean phases. It is not characterized by sufficiently defined banks to allow its immediate distinction from the ordinary channel in which it forms. It often tends to divide and merge seamlessly, depending on the excavation action of the sediments in the ordinary bed.

From a morphological point of view, the largest riverbed is the widest and contains within it the floodplain, which in turn includes the flow channel (see Figure 1).

2.2. Floods

The main risk associated with watercourses is flooding areas adjacent to the riverbed, which are usually not covered by water. This phenomenon is called flooding, and Italy has established a regulatory reference framework for the assessment and management of flood risks by implementing Directive 2007/60/EC on the assessment and management of flood risks (Floods Directive—FD) through the Legislative Decree 49/2010. [8]

The institute in charge of monitoring the hydraulic hazard of the areas delimited by the District Basin Authorities is the ISPRA (Istituto superiore per la protezione e la ricerca ambientale)—https://www.isprambiente.gov.it/it/istituto—accessed on 6 February 2025 which is a public research body, with legal personality under public law and technical, scientific and administrative autonomy. The ISPRA is under the supervision of the Minister for the Environment and Energy Security (MASE). The institute produces the mosaic of the aforementioned areas, according to the three scenarios of the Legislative Decree 49/2010 (see Figure 2):

• High hazard with a return time between 20 and 50 years (frequent floods);

• Medium hazard with a return time between 100 and 200 years (infrequent floods);

• Low hazard (low probability of floods or extreme event scenarios).

Figure 2

Floodable areas per high flood hazard scenario.

[Figure omitted. See PDF]

From this mosaic work, it can be seen that the areas of high hydraulic hazard in Italy amount to 16,224 km² (5.4% of the national territory), the areas of medium hazard amount to 30,194 km² (10%) and those of low hazard (maximum expected scenario) amount to 42,376 km² (14%) (mosaic v. 5.0—2020) [9].

One aspect to be considered when forecasting floods and assessing the effects these phenomena may have on the surrounding territory is the hydrogeological risk. This is the risk deriving from slope instability and is closely connected to the geological and morphological conformation of the slopes, which in turn is influenced by the characteristics of the deposits and rocks from which they are formed.

Flood risk thus derives from a combination of hydrogeological risk and meteorological, climatic and environmental conditions. A fundamental element is precipitation; rainwater, once it falls on the ground, follows a cycle that depends in turn on geological and geomorphological factors and, not least, the human use of the land [9].

Hydrogeological risk can also be expressed as the combination of hydrogeological hazard (the probability that a landslide or flood of a given magnitude will occur for a given time and area), the vulnerability of built-up areas and the potential damage (often very high as high-value cultural assets may be exposed to landslides and floods).

Another concept that influences potential flood damage is hydrogeological instability, which summarizes the set of processes and actions that cause land and soil degradation; it is well known that the activities that contribute most to this degradation are of a purely ‘anthropic’ nature and can have very serious consequences, to the point of making a territory particularly exposed to the risk of extreme phenomena. In industrialized and densely populated (and therefore built-up) countries like Italy (see Figure 3) [9], much of the hydrogeological risk depends on the fact that the territory has been extensively modified through the construction of roads, bridges and railways, which interfere with the environment and its natural evolution. Most of the Earth’s geological history is characterized by the formation of mountain ranges, which are the most extreme consequence of the movement of the Earth’s plates. Mountains, in turn, are subject to erosive phenomena that, over time, lead to the dismantling of slopes and the consequent accumulation of debris towards topographically more depressed areas. The erosive phenomena that lead to the progressive leveling of reliefs can develop through the gradual transfer of rocky debris towards the valley, or its sudden and violent transfer, as in the case of landslides and floods.

The development of hydrogeological events such as floods is relatively simple to describe. They occur when the water of a river can no longer be contained within its banks and invade the surrounding areas. The triggering cause is always the amount of rainfall, which can exceed the absorption capacity of the land; this happens when rainfall is particularly heavy or in cases where land disruption is so severe that the regular flow of rainwater is impossible.

In the case of moderate rainfall, which falls on the land over an even, prolonged period of time, the volume of water is distributed over time and the river flow is sufficient to dispose of the surplus water through the riverbed. On the other hand, when a large amount of rain falls over a very short period of time, the volume of water channeled into the riverbed is greater than the river’s disposal capacity. In other words, the volume invading the riverbed is greater than the flow rate and the excess volume is forced out (overflowing) from the embankments.

Moreover, when an area falls prey to hydrogeological instability, the capacity of soils to absorb water can be reduced to zero, thus causing an exponential increase in the volumes of water that pour into rivers during heavy rainfall. Another activity that increases the propensity of an area to be affected by floods is so-called ‘sealing’: covering land with concrete has the effect of preventing the infiltration of rain into the ground, increasing the volume and speed of water flowing towards rivers. Cementing riverbeds is also responsible for reducing the runoff time of water, which can then flow downstream faster. Finally, floods are made even more destructive by the presence of bridges with too little a span, which is an obstacle to water flowing under them.

2.3. River Basins

A hydrographic basin is a portion of a territory that, due to its orographic conformation, collects rainwater or water from melting glaciers or snowfields, all of which flow towards an impluvial furrow giving rise to a watercourse or towards a basin or depression giving rise to a lake or marshy area (see Figure 4). It differs from a hydrogeological basin in that the latter does not only consider surface water runoff but also infiltration runoff that depends on the stratigraphy and geological conformation of the subsoil [10].

If the water collected is only due to precipitation, we speak of a catchment area. In a valley catchment area, it is possible to identify a place of convergence of water, known as the ‘closure section’, through which the entire volume of water collected on the surface passes. The altitude at which the closure section is located is the reference altitude in the basin’s altitude measurements [10].

The boundary perimeter of each basin, once the closure section has been identified, is recognized by mapping, from the closure section, the line beyond which the water flows over the land, following another path to collect in a different catchment area. This line is called the ‘watershed line’ or simply the ‘watershed’. The watershed can be defined on a topographical map by connecting the peaks of the maximum elevation with a line that is always perpendicular to the contour lines, thus having the direction of the inverse of the maximum gradient [10].

When we consider the water of a hydrographic area that flows underground, the watershed no longer coincides with the surface water identified with topographical methods, through which it is much easier to delineate it, and various aspects must be considered that modify its course and its shape. One of these very important elements to consider is the filtering motion of the water, a motion that develops mainly on horizontal paths, carried out by the water in a saturated soil zone above an impermeable bottom layer. A basin whose waters flow below the earth’s surface is also considered a hydrogeological basin; precisely because of the subterranean component of the water, it is much more difficult to identify the watershed, and thus delimit the hydrogeological basin, than the hydrographic area [10].

2.4. Basin Shape

Different basins can have very different shapes despite having identical drainage surfaces. The shape can vary from more compact to more elongated, and this difference between basins is therefore assessed by calculating additional parameters besides the simple drainage surface [10].

The first calculation that can be made is the ratio or circularity factor R_c, and it is calculated by comparing the area of basin A with that of a circle with the perimeter P, equal to the length of the watershed line of the basin considered:

$R_c=4\pi {\displaystyle \frac{A}{P^2}}$

The ‘uniformity ratio’ R_u is defined as the ratio of the perimeter of the basin to the perimeter of the reference circle with the condition that the surface area of the basin is equal to that of the circle:

$R_u=4\pi {\displaystyle \frac{P}{2\sqrt{\pi A}}}$

The ratio of the area of the basin to that of a square with the side L, equal to the length of the basin, is called the ‘shape factor’:

$F={\displaystyle \frac{A}{L^2}}$

The elongation ratio is as follows:

$E={\displaystyle \frac{2\sqrt{A}}{L\sqrt{\pi }}}$

One of the very important parameters through which the shape of a catchment area is characterized is the Gravelius coefficient (Φ); it represents the ratio between the perimeter P of the catchment area and the perimeter of a circle of an equal area (A). Depending on the value to which the coefficient tends, the shape of the basin appears differently:

• Φ = 1: round shape;

• 1 < Φ < 1.25: round–oval shape;

• 1.25 < Φ < 1.5: elongated round–oval shape;

• 1.5 < Φ < 1.75: elongated oval–oblong rectangular shape.

We measure the compactness of a basin in relation to a circle with the same area with the following formula:

$K_c={\displaystyle \frac{P}{2\sqrt{\pi A}}}$

2.5. Altitude Characteristics

The most interesting antimetrical characteristic is the slope of the basin, and it can be described by the hypsographic curve, which is a monotonically decreasing curve determined by the means of a Cartesian graph in which the ordinate heights and increasing percentages of the total basin area are plotted [10,11,12].

In order to determine it, it is necessary to have a small-scale cartography with a hydrographic grid and a detailed altimetry (identified with good approximation by contour lines). The most interesting quantities that can be obtained from the hypsographic curve are as follows.

Average elevation: this is the elevation that represents the integral average of the diagram and thus represents the height of a rectangle whose base is the percentage of the total area; its surface area is equivalent to that subtended by the hypsographic curve.

Median altitude: This is the altitude value that corresponds to half of the basin area in the graph of the hypsographic curve. Another widely used parameter is the mean slope, usually calculated according to the Alvar–Horton formula:

$I_m=\underset{D_h\to 0}{\lim }{\displaystyle \frac{D_h{L}_v}{A}}.$

where L_v is the total length of the contour lines of the assigned elevation difference D_h.

In practice, this parameter is often estimated approximately by superimposing a square grid on the topographic map of a basin, calculating at each node the difference in elevation between the two nearest contour lines and their distance. Local slopes are thus obtained, the arithmetic means of which can be calculated to obtain the average slope.

2.6. Topological Characteristics

The topological representation of a hydrographic network is carried out on the basis of its development on a two-dimensional plane and is based on certain easily classifiable topological properties. Regardless of the scheme adopted, a number of general definitions can be identified

• ‘External nodes’ (or sources) are defined as those nodes from which a single rod originates.

• ‘Internal nodes’ (or junctions) are defined as nodes consisting of the points at which more than one rod flows.

• ‘Internal branches’ are defined as elements connecting internal nodes and junctions or internal nodes themselves.

• ‘External branches’ are defined as elements connecting an internal node immediately downstream with a source.

• ‘Network magnitude’ is defined as the total number of sources.

• ‘Topological distance’ is defined as the number of branches from the source to the network outlet.

• ‘Topological level’ is defined as the topological distance between the upstream node and the network outlet.

• ‘Network diameter’ is defined as the maximum topological distance.

2.7. Horton–Strahler Sorting System

The direction of water flow and the shape of a water network establish a hierarchical relationship between the various branches that make up the network. The first to systematically propose a quantitative geomorphological analysis of hydrographic networks and their drainage basins was E.R. Horton in 1933 [13]; he based this analysis on the hierarchization of the constituent elements of a network according to certain rules. Horton’s classification was taken up in 1952 by Strahler, who proposed a classification procedure essentially based on five simple rules (see Figure 5):

• First-order channels are defined as those originating from sources.

• From the union of two branches of the order ‘n’ derives one branch of the order ‘n + 1’.

• From the union of two branches of different orders, the confluent with the greater order will be the stretch of the channel immediately downstream.

• The succession of two or more branches characterized by the same order ‘n’ constitutes channels of the same order.

• The channel characterized by the highest order ‘N’ determines the order of the channel itself.

Figure 5

Strahler sorting.

[Figure omitted. See PDF]

The Alvar–Horton formula, also known as Horton’s Law, describes the hierarchy and organization of the drainage network in river basins. Horton identified empirical laws to characterize river networks, which include the following.

Law of Order of Watercourses: Watercourses can be classified in an order system; when two watercourses of the order n join together, they form a watercourse of the order n+1. This hierarchy represents the structure of the drainage network.

Bifurcation Ratio: This is defined as the ratio between the number of watercourses of one order and the number of watercourses of the next highest order.

$R_b={\displaystyle \frac{N_n}{N_{n+1}}}$

where

• R_b is the bifurcation ratio;

• N_n is the number of watercourses of the order n;

• N_n+1 is the number of watercourses of the next order.

$R_l={\displaystyle \frac{L_n}{L_{n+1}}}$

where

• R_l is the average length ratio;

• L_n is the average length of watercourses of the order n;

• L_n+1 is the average length of watercourses of the next order.

• Basin Area Ratio: This defines how the basin area of a certain order compares with that of the next order.

$R_{\alpha }={\displaystyle \frac{A_n}{A_{n+1}}}$

where

• R_α is the ratio of the basin area;

• A_n is the basin area associated with watercourses of the order n;

• A_n+1 is the basin area for the next order.

2.8. Corrugation Time

The runoff time is a further morphological and hydrological characteristic and represents the time it takes for a hydraulically disadvantaged water particle, i.e., one falling at the furthest point from the closure section, to reach the section itself. The runoff time (t_c), measured in hours, is essentially determined using various analytical formulas.

Giandotti’s equation is an empirical formula used in hydrology to estimate the time of concentration (Tc) of a catchment area, i.e., the time required for all rainwater collected at the furthest point of the catchment to reach the outlet or control section. It is useful for calculating the peak runoff during heavy rainfall.

$t_c={\displaystyle \frac{4·\sqrt{A}+1.5·L}{0.8·\sqrt{H}}}$

where S is the surface area of the basin, expressed in km²; L is the length of the main stem, expressed in km; Hm is the mean elevation of the basin, expressed in m, referring to the mean sea level; and h0 is the elevation of the closure section, also expressed in m, also referring to the mean sea level. This formula is valid for catchment areas larger than 100 km².

Viparelli’s formula is used in hydraulics to estimate the runoff time in catchment areas, providing a similar measure to Giandotti’s formula, but it was specifically developed by Giuseppe Viparelli for hydrological evaluations in small basins under heavy rainfall conditions.

$t_c={\displaystyle \frac{L}{V}}$

where L indicates the length of the river shaft and V is the velocity of the water particle; the velocity, set at 5.4 km/h, is determined with the aid of an abacus that proposes a relationship between the velocity of the water, the average slope gradients and the type of terrain characterizing the basin.

Pezzoli’s formula is an empirical expression used to calculate the runoff time (t_c) in catchment areas, particularly in small mountain basins. This formula is mainly used in hydrology to estimate the response time of a basin to intense, short-duration rainfall and is particularly suitable for steeply sloping terrain.

$t_c=0.055{\displaystyle \frac{L}{\sqrt{p}}}$

where L denotes the length of the main stem and p denotes the average slope of the riverbed. The ridge flow rate can be calculated using empirical formulas such as the rational formula:

$Q_p=C \cdot i \cdot A$

where

• Q_p is the ridge flow rate (in m³/s);

• C is the runoff coefficient, which varies according to permeability and land cover;

• i is the precipitation intensity (in mm/h);

• A is the basin area (in km²).

Also, the term S denotes the basin area and the term i denotes the rainfall intensity of a duration equal to the runoff time t_c, while the term φ denotes the runoff coefficient, i.e., the rainfall yield, the value of which depends on the permeability and the type of vegetation cover of the basin. If one wishes to express the flow rate in m³/s, and the surface area of the basin is expressed in km² and the rainfall intensity i in mm/hour, one must divide the obtained value of Qc by 3.6.

2.9. Risk Mitigation Measures

Floods are natural phenomena over which humans have no control (except in the acceleration of climate change that favors increasingly extreme weather events), but what can and must be performed instead is risk mitigation in the event of these events occurring.

Possible mitigation interventions vary depending on whether an area is built-up or non-built-up; in the former case, structural and non-structural interventions are necessary, ranging from engineering works to consolidate unstable slopes to instrumental monitoring and warning networks. An example is the Rercomf landslide monitoring network ARPA Piedmont, Sondrio Geological Monitoring Centre–ARPA Lombardia.

For areas that have not yet been built on, on the other hand, it is fundamental to place new urbanization areas in safe places, with particular attention to strategic buildings such as schools, hospitals and public offices; it is also necessary to implement proper territorial planning, through the application of constraints and land use regulations (PAI) [14].

• -. The Plan for Hydrogeological Structure is the most effective action for risk reduction in the medium–long term (see Table 1).

With a view to mitigate hydrogeological instability, in addition to the implementation of structural interventions, cognitive activity on a national scale is strategic, also in accordance with the provisions of Articles 55 and 60 of the Legislative Decree 152/2006.

In addition to risk mitigation policies, physical barriers can be built to protect areas most at risk from flood damage. These include the following:

• Weirs: Erosion by surface water depends on the speed of the watercourse, which can be slowed down by building steps, called weirs, along the watercourse. Between one weir and the next, the water flows with a small gradient, only to jump abruptly to the next step. This reduces the speed of the current and, consequently, also the removal of materials.

• Artificial embankments: In order to prevent flooding, watercourses can be prevented from overflowing by constructing artificial embankments. The design of these defense works must take into account the increase in water masses during floods. For this reason, embankments are erected at a certain distance from the riverbed so as to allow the water to escape if necessary.

• Flood reservoirs and spillways: Flood reservoirs and spillways are built in order to reduce the flow of flooding watercourses. Flood reservoirs are basins into which water is conveyed in order to retain part of the flood wave. Spillways are canals or tunnels that, when opened before the flood wave arrives, drain part of the water into a lake or sea.

• Containment systems: In many rivers, artificial embankments interfere with the normal functioning of river systems. For adequate protection from flood waves, containment systems are essential. Recently, reservoirs have also begun to be used to control and regulate river flows.

• Artificial channels: To contain flood waves along the lower reaches of rivers, artificial channels and riverbed widenings have been dug since ancient times. In some vast stretches of floodplains, they are left clear so that floodwater can drain away completely.

• Reforestation and vegetation control: since deforestation contributes to increasing the risk of floods and landslides, the adequate control of vegetation destruction and reforestation where necessary can be very important factors in maintaining a territory in equilibrium and preventing land disruption.

3. Case Study: Hydraulic and Hydrogeological Risk Simulator

This experimental work involves the creation of a geographical web platform for analyzing and simulating hydrogeological risk on Italian soil in the event of flooding events, using soil elevation and existing watercourse beds as input data. Using this application, it is possible to click on a point of interest, obtain elevation data for a 5 km area around the point itself and simulate the overflow of watercourses in that area in the event of rain, also considering the external contribution due to the slope of the surrounding land.

3.1. Area of Interest

We chose to limit the scope of this work to the area of the Liguria region, given the presence of numerous cartographic data that can be freely downloaded from a dedicated geoportal (https://geoportal.regione.liguria.it/, access on 6 February 2025).

These data include several thematic maps available in WFS and shapefile formats including the hydrographic network, river basins and many other maps useful for monitoring the region’s territory.

As far as the geometries of watercourses are concerned, however, it is preferable to start with a polygonal representation of a watercourse, as this allows a starting situation, in which the watercourse is still within its bed, to be as realistic as possible. Since the geoportal does not provide this type of model, we chose to use the free maps of OpenStreetMap.

3.2. Type of Application

The application WebGIS 3d is available and accessible via the project page of LabGeoInf, a research laboratory of the National Research Council in Italy, identified as SmartData—Hydro Simulator. (The web application service is available here: http://150.146.207.228:8080/idro_simul/, access on 6 February 2025) This choice allows it to be easily usable by any user with an internet connection without the need to install local software and makes it possible to exploit the capabilities of modern browsers which, by the means of dedicated libraries such as Three.js, allow the visualization of three-dimensional environments with a large quantity of polygons on the screen, as is often the case in the realization of simulations of this type.

The user interface consists of two different screens:

• -. The initial screen is a 2D visualization of the region of interest, within which it is possible to move, zoom and obtain information on the features of interest, as well as select an area in which to run the simulation;

• -. The second screen is a three-dimensional representation of the selected area, where the user can set parameters and start the simulation.

The simulation is displayed on the screen as an animation, in which both the time steps and the total duration are user-selectable parameters. At the end of the simulation, it is possible to compare the initial and final state of the water expanse to assess which surrounding areas might be most affected by flooding.

3.3. Input Data—DEM

The DEM used for this project is TINITALY/1.1, a new version, released in January 2023, of the original TINITALY/01 presented in 2007 (see Figure 6 and Figure 7).

This model has a resolution of 10 m and is downloadable in the GeoTIFF format via the WMS, WMTS and WCS. In particular, the WCS (Web Coverage Service) [15] was used in the present study because it allowed us to download a map portion of a specific size. In this case, a size of 500 × 500 pixels was chosen where each pixel covered an area of 10 × 10 m, thus obtaining an elevation map for an area of interest of 5 × 5 km. However, the parameterization of this resolution allows even larger portions of terrain to be downloaded if necessary.

3.4. Geometries of Watercourses

For the areas covered by watercourses, it was decided to use map data from OpenStreetMap, a series of open-source maps managed by the OpenStreetMap Foundation and maintained by a large community of contributors. These maps form the basis of thousands of websites and guarantee an accurate and up-to-date representation of the world’s territory.

We chose to use such maps because watercourses are represented vectorially in the form of polygons rather than lines, as is the case with several other cartographic sources representing water networks. Polygonal representation makes it possible to simulate as realistically as possible the initial situation of a watercourse and thus the simulation of its spread in the surrounding terrain in the event of a flood [16].

The data were downloaded using the QuickOSM plugin for the QGIS software, which allowed us to download data from the various layers of which the OpenStreetMap map was composed using filters on the type of layer and a bounding box to delimit the area of interest.

Specifically, all geometries were selected in which the key-value pair consisted of natural = water, which characterized the geometries of the water mirror layer in the OpenStreetMap map.

Once downloaded to QGIS in the shapefile format, the geometries were then saved on the PostgreSQL database using the integrated functions of QGIS to be subsequently retrieved in the GeoJSON format by the services of the application for visualization in the form of layers for the OpenLayers Javascript libraries used by the platform.

In addition to the polygonal geometries of the watercourses, it was decided to add a layer to the platform containing the hydrographic network in the geoportal of the region of Liguria. This layer shows the linear geometries of the hydrographic network; the information provided includes the name of the watercourse, the indicative specific surface area, the indicative surface area of the underlying basin (referring to the selected segment), whether it is hydraulically investigated and the relative Strahler order, which measures the branching complexity of the selected stretch.

The geometries were downloaded from the geoportal in the form of shapefiles. As before, they were imported into the QGIS software and then saved to the PostgreSQL database, whose built-in geographical functions were used to filter the results, retaining only sections that intersected the polygonal geometries downloaded from OpenStreetMap.

3.5. Simulation Algorithms—Calculation of External Contribution

In order to simulate as realistically as possible the increase in water volumes following floods caused by major meteorological events, it is necessary to take into account not only the amount of rain that falls in the river basin but also that which is deposited in the surrounding areas and then, due to gravity, pours from higher-elevation areas into the riverbed, thus increasing the mass of liquid that could cause the river to flood.

This type of rainwater movement is called surface runoff. In order to take this ‘external contribution’ into account, while keeping the simulation streamlined and executable in near-real time, it was decided to approach the problem with a simplification: for each pixel of the elevation map, i.e., for each 10 × 10 m area within it, the platform selects the preferred path of the water towards neighboring cells, based on the slope, and of all the neighbors, it chooses the one with the steepest downward slope.

The first step is then to calculate, for each cell of the elevation grid, the slope of the four directions with respect to the current cell. These four slopes are then compared with each other to decide which direction the water will take on its path to the lowest point.

In the example in Figure 8, if we had to choose a path for the water from cell A towards the adjacent cells B, C, D and E, the path towards E would be chosen because there is a negative slope towards B and C, while between D and E, E is the one with a greater downward slope than A (see Figure 8).

The path is then continued until a flat area is found, or until an area within the initial riverbed is reached (or the boundaries of the map under consideration). Each pixel affected by the trickle of water thus created is increased by a certain value; repeating the operation on all the pixels of the map will thus result in ‘accumulation zones’ in which the water tends to stop, forming a static map that is kept in memory. When the dynamic simulation is performed, for each pixel reached by the mass of water simulated, a certain amount of volume is added depending on the external contribution of that pixel, thus taking into account the accumulated rain from the surrounding areas.

Figure 9 shows the complete map calculated for one of the zones of interest in the study area.

The areas of greatest water accumulation are visible in orange. It can also be seen that at the base of the slopes, water accumulates more, making a strong contribution to the flooding of those areas; the effect diminishes as the elevation increases, until there is no external contribution at the top of the heights.

The starting data for the simulation are as follows:

• -. The uniform grid containing the elevation data of the area of interest, from which a three-dimensional plane is derived to represent it;

• -. The initial geometry of the watercourse, represented by several cubes (one for each 10 × 10 m cell of the DEM), with a very small initial height to simulate the initial body of water.

The execution of the simulation starts after the user has filled in the fields that determine the control parameters through the interface. It is possible to define the amount of water due to rainfall, expressed in mm/m² per hour of rain; the time step between one frame and the next of the simulation, which can range from 15 to 120 min; and the total time of the simulation duration, in the range of 6 to 48 h. The last parameter to be selected is the soil permeability index, which affects the percentage of water that is considered absorbed by the soil and therefore does not increase the total volume of the overflow. This value can range from 0.0 (no permeability, found in rocky soils) to 1.0 (maximum permeability, which allows the soil to absorb the entire amount of water that falls, a condition that is obviously impossible in real cases).

Once these data are collected, the platform begins by first calculating a static map for external contributions. After that, it raises the geometries of the entire watercourse of the decided step, calculated as follows:

((rainfall amount + external contribution) × (1 − permeability index))

× (time step/total time).

An object representing the edge of the watercourse geometry (called the fringe in the platform) is then generated by selecting all the cells that lie on the outer contour of the geometry. This is the area that is compared with the elevation, to be possibly expanded if the top of the cube of each of its cells is at a higher elevation than the direction in which it would like to expand.

In this way, it is possible to obtain an approximate simulation of the expansion in the volume of water as the amount of rain received increases, with an expansion stop in the case of slopes and areas with a higher ground elevation.

The water volumes displayed are also colored in a graduation from white to deep blue as the height of the water volume in a given cell increases to make it even clearer which areas are most affected by flooding.

In Figure 10 is the end result of a simulation that considers a time frame of 48 h from the start of precipitation, assuming a constant amount of precipitation over time.

3.6. Simulation Platform

The platform uses the classic client–server scheme (for more details on the platform, see the Supplementary Material), thus allowing for its use by several simultaneous users and publication on the web, possibly integrated within more extensive web portals that allow for different analyses of a territory for the mitigation of various types of risk (see Figure 11).

3.7. Three-Dimensional Rendering

The key part of the platform is the screen that runs the simulation and displays it in 3D. This choice is based on two main reasons: the first is that the three-dimensional X-, Y- and Z-coordinates of the points of the terrain model are necessary for the execution of the simulation, as the expansion of the water volume is decided on the basis of the height of the surrounding terrain in relation to the height of the water; the second is the possibility for the user to rotate and zoom in on the model while the simulation is running, for a better evaluation of the effects of flooding.

The capabilities of modern browsers allow the simulation to run in real time directly on the user’s terminal, not overloading the back-end server when running multiple instances of the platform.

The Three.js library provides, in the form of an API, the tools needed to create a 3D Scene complete with polygonal objects, lighting effects, cameras and post-processing within a normal web page, exploiting the potential of the WebGL graphics libraries. Figure 12 shows the conceptual scheme of a Three.js Scene.

Three.js is based on a number of software objects that encapsulate the main functionality of the software: The Renderer is the main object and is responsible for representing everything in a Scene on the screen, framing it as if the user were positioned at the point where the Camera is. This is modeled as a real camera with its own position, orientation and field of view (FOV), framing the Scene (see Figure 13).

Within the Scene, we find all the objects that make up the structure of the Scene that we frame. The Meshes are the actual 3D objects. These are, in turn, composed of Geometry, the polygonal geometries that describe their appearance, and Material, the materials that define their color, light reflection properties, etc.

Materials may in turn refer to Textures, raster images that are applied to the Geometries, suitably extended or repeated to cover the entire model.

The Scene is then illuminated by Light objects, which may be of different types to simulate various types of light, such as spot, ambient and omnidirectional light and so on.

One of the aspects to which attention must be paid for the correct visualization of three-dimensional space is the co-ordinate system; although they always consist of the three axes X, Y and Z, some applications consider Z as the vertical axis, and others consider Y. Three.js uses the latter convention, as shown in Figure 14.

Since in geographic systems longitude and latitude values are often traced back to X- and Y-co-ordinates, it would be more convenient, in the case of a geographic application, to use a reference system in which the Z-axis corresponds to the vertical axis, representing the elevation of the terrain; the library allows the reference system to be changed via a simple function: THREE.Object3D.DefaultUp = new THREE.Vector3(0, 0, 1).

This creates a unit vector in the Z-direction and assigns it to the one that is considered ‘high’ (Up); in this way, the convention is restored to the desired condition.

3.8. User Interface

The application consists of two different screens. The initial screen is a classic two-dimensional map showing the watercourses, the hydrographic network and the boundaries of the area of interest. The second is a 3D screen, which can be viewed after selecting an area on which to run the simulation, where it is possible to set parameters and launch the actual simulation.

The two-dimensional map view of the territory is the screen that is displayed when accessing the application URL (see Figure 15) [17].

The interface is very simple: the main element is the map, freely navigable and clickable. In addition to the zoom buttons on the left, which are provided directly by the OpenLayers API, there are two buttons for displaying the DEM in a 5 km area around the clicked point and a button for the 3D view, which is activated when the DEM is displayed (see Figure 16).

On the map, there are, in addition to the OpenStreetMap base layer, two layers: the first displays the watercourses downloaded from OpenStreetMap in vector form, while the second represents the hydrographic network downloaded from the geoportal of the Liguria region.

Clicking on a given point on the map will search for information in the latter layer. If elements are found, these will be colored in yellow and a pop-up window will open containing various information on the selected section of the grid (see Figure 17).

The reported data are as follows:

• -. The name of the watercourse;

• -. The Strahler order;

• -. The indicative specific surface area, expressed in square meters;

• -. The indicative surface area of the underlying basin, expressed in square meters;

• -. Whether the watercourse is hydraulically surveyed or not.

When clicking on the ‘View DEM’ button, the user will be asked to click on a point on the map, and once a point has been selected, the TINITALY WCS service will be called up, which returns the GeoTIFF of the elevation for a 5×5 km area around the selected coordinates. The GeoTIFF, in addition to being scanned to send elevation data to the simulation, is also displayed on the map for a better understanding of the area over which it operates (see Figure 18).

At this point, the ‘3D View’ button will be made active and, if there are watercourses within the area, the user can move on to the actual simulation [18,19].

3.9. Three-Dimensional Visualization Map

The simulation platform is activated with the ‘3D View’ button, at which point the 3D terrain model, the riverbed of the watercourse in a blue color and all elements of the user interface for interacting with the simulation are displayed, as shown in Figure 19 [20].

In particular, thanks to the panel on the left of the screen, it is possible to select the following (see Figure 20):

• -. The amount of rain falling in the area in an hour (mm/h);

• -. The soil permeability index, from 0.0 (completely impermeable) to 1.0 (completely permeable);

• -. The time increment for each ‘frame’ of the simulation;

• -. The total simulation time.

Figure 20

Simulation control panel.

[Figure omitted. See PDF]

In addition to these input parameters, there are two checkboxes: The first allows the user to display, instead of the textured terrain, the map of external contributions, with a graduation from green to red depending on the contribution each cell makes to the volume of water. The second, on the other hand, allows the display of the simulation result to be activated/deactivated after the simulation is completed, as can be seen in Figure 21.

This option allows a better comparison between the initial and final situation and a better understanding of which areas have been flooded compared to the initial riverbed. Below the two checkboxes are the two main buttons: the first allows the simulation to be started, while the second allows it to be interrupted if intermediate effects with respect to the selected time horizon are to be assessed. During the execution of the simulation, the time relative to the current frame is indicated by the timer at the bottom of the screen.

4. Conclusions and Future Work

This research activity describes the design and implementation of a tool for the simulation of hydrological risk in the event of rainfall-related flooding in a watercourse, showing the results through a three-dimensional representation of the affected water mass and the surrounding terrain. The simulation is based on the morphology of the terrain adjacent to the affected watercourse bed, taking elevation into consideration to determine the direction of expansion of the water mass.

A distinctive aspect of the platform is its extreme speed in the resolution of simulations, which also allows the tool to be used in real time. This means that during the occurrence of a meteorological event, the platform can be used to monitor the evolution of the situation and predict the effects of any persistence of the phenomenon over time. This real-time forecasting capability is crucial for making quick and informed decisions, thus reducing reaction times and improving emergency management on the ground, with a potential positive impact on the safety of the population and the protection of infrastructure.

A very important feature of the research work performed is the possibility of running simulations on any device with an internet connection, without the need for specific hardware or particularly high resources. This makes the platform accessible from laptops, tablets or even smartphones, offering considerable operational flexibility for users. In this way, the platform can be used not only by emergency operators at the various sites of the agencies involved but also by those who are directly in the field, perhaps during monitoring and control operations in areas at risk.

This feature makes it possible to obtain up-to-date information in real time, even in emergency situations, where quick access to data and the ability to develop up-to-date scenarios can make a difference in managing operations. Mobile use allows, for example, technicians and operators in the field to monitor the evolution of weather and hydrological conditions directly on site, to quickly share simulation results with coordination centers and to make decisions based on concrete and up-to-date data without having to worry about the availability of powerful workstations or computing servers. Moreover, this accessibility reduces operational costs related to the purchase and maintenance of specific equipment, making the platform a particularly cost-effective solution for public authorities, civil protection and land management organizations, even in resource-limited settings.

A further aspect of the simulation that could be worked on is the refinement of the analysis performed; for example, with a point or area description of the permeability of the terrain, one could use these data to determine the amount of water absorbed by the soil instead of relying on a static index entered by the user that remains static for the entire area under consideration. One could also use elevation maps at a higher resolution to model the area, and thus also the riverbed, more accurately; the simulation would also be more accurate as one would be modeling a greater number of cells, each with a smaller size and thus with greater fidelity than in the real case.

It would also be possible to exploit less approximate algorithms for the simulation of surface runoff, i.e., the movement of rainwater on the slopes that flow into the riverbed. In any case, a compromise will have to be found between the fidelity of the simulation and the speed of execution, since excessive complexity in the analyses carried out could compromise the platform’s usability in real time, defeating its purpose and leading to the use of more complex software that exploits dynamic fluid simulations to predict the movement of the mass of water involved in the outflow from the riverbed.

Author Contributions

Conceptualization, Mauro Mazzei; methodology, Mauro Mazzei; software, Davide Quaroni; validation, Mauro Mazzei and Davide Quaroni; formal analysis, Mauro Mazzei; data curation, Mauro Mazzei and Davide Quaroni; writing—review and editing, Mauro Mazzei and Davide Quaroni; supervision, Mauro Mazzei. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The sources of the data of the maps are the ISPRA (Institute for Environmental and Environmental Research) the INGV (National Institute of Geophysics and Volcanology) which are used for the project TINITALY/1.1.

Acknowledgments

The authors thank Istituto Superiore per la protezione e la Ricerca Ambientale and the l’Istituto Nazionale di Geofisica e Vulcanologia Research Center as sources of documentation necessary for the conduct of this paper.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Table

Figure 1. River in full and river in low period.

Figure 3. Resident populations in flood-prone areas for the three flood probability scenarios at the national level—ISPRA Mosaic, 2020.

Figure 4. A map of the Lambro–Seveso–Olona basin.

Figure 6. DEM Tinitaly/1.1.

Figure 7. Watercourse layer and DEM portion within platform.

Figure 8. Example of runoff based on relative slope.

Figure 9. Static map of external contributions based on surface runoff.

Figure 10. The final result of a simulation.

Figure 11. System architecture.

Figure 12. A conceptual diagram of a Scene.

Figure 13. Camera-related concepts.

Figure 14. Axis directions in Three.js.

Figure 15. User interface—two-dimensional display.

Figure 16. Button panel.

Figure 17. Pop-up with details of selected feature.

Figure 18. Visualization of DEM portion.

Figure 19. User interface—three-dimensional visualization.

Figure 21. A comparison of the situation at the beginning and end of the simulation.

Table 1

Outline of structural and non-structural measures for hydrogeological risk mitigation.

Hydrogeological risk mitigation strategy	- Knowledge activities: landslide census and mapping (IFFI inventory), mapping of flood areas (Floods Directive)
	- Risk assessment
	- Correct spatial planning (hydrogeological planning)
	- Structural risk reduction interventions for population centers and linear communication infrastructures
	- Communication and dissemination of information to public
	- Land maintenance and good agro-sylvo-pastoral practices
	- Relocations
	- Monitoring networks and warning systems
	- Civil protection emergency plans

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/ijgi14020073/s1. Additional material describes the functionality of the backend, frontend and simulation algorithm.

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