**2. Forensic analysis of floods**

It is true that we are used to hear about the application of forensic science in crimes and felonies. In fact, even the dictionary definition for forensic science leads us to the application of scientific practices within a legal process. This can be translated into the participation of highly speci‐ alized professional or criminologists, who do research, seek and locate evidence—many times laboratory based—that can provide conclusive proofs about a fact that needs to be clarified. Sometimes, the evidence cannot be seen at first glance and a much deeper exam is needed in order to determine the true causes of an event. Even engineering or hydrology in particular have not been mentioned so far, a lot of the concepts described seem to fit very well in these disciplines. At the end, any forensic investigation has the objective of establishing how an undesired event happened and eventually, what to do in order to prevent that it happened

Forensic engineering has evolved to an interdisciplinary approach, considering another specialty such as anthropology, sociology and economy among others. It can be state that the general purpose of forensic engineering is to determine or clarify the causes of failure of certain system with the objective of improve the designs or assist in the operation procedures.

Common losses or failures that forensic engineers usually address include among others, damages to structures caused by natural phenomena such as thunderstorms, hurricanes, storm surge, saline intrusion, fires, etc. In fact, the need of research in this field is associated to a combination of climate conditions, topographic and physiographic conditions and resilience of the population: for example, coastal zones in poverty and a high potential of hurricanes incidence are highly susceptible to suffer. Nevertheless, some of these fields have rarely been addressed by forensic analysis, missing out on the advantages it could provide investigations

All of these lead to derive from the environmental and earth sciences and forensics, the socalled forensic hydrology. Even though the term is often associated to the different quantitative components of the water cycle, forensic hydrology also includes topics as water pollution and contamination, floods, droughts, other water-related resources and water infrastructure operation among others. There will be occasions in which forensic hydrology would help to prevent or at least reduce severe damages and in some others, it would lead to better water

The adjective forensic applied to geosciences appeared around 1980s [2], but few investigations have been developed since then. Derived from cases associated to pollution of water or soil, researchers started to talk about forensic geochemistry or forensic geology, as a synonym of the process to describe the use of specific techniques to find the sources of the pollution.

Some 20 years later, the growing concern on nature protection we could see the beginning of environment sciences forensics. On the other hand, the so-called engineering forensics, do research on materials, products, structures or components that fail or do not work in the way they were designed, generating or contributing to the collapse or failure of a civil work.

It is in the intersection of environment science forensic and engineering forensics where we can locate forensic hydrology with the main objective of determine the probable cause of an event or the human sources that contribute to increase damages or even human losses. This

again.

44 Forensic Analysis - From Death to Justice

on this topic.

management, improve its use and allocation.

In the topic of extreme phenomena, forensic hydrology can be directly applied to the case of floods. Forensic analysis of floods consists on the application of a methodology immediately after the occurrence of the event. Its application will give us the reconstruction of the event in order to determine what really happened, what factors contributed, what failed, who were the main actors that have something to blame about the damages, etc. The analysis will then be directed towards performing an evaluation or study of the event after the points mentioned have become clear. The final objective of the analysis is to suggest what is needed to aid or improve the system and thereby prevent this type of disaster in the future, to whatever extent possible. Flood analyses need to integrate hydrometeorological, hydrological, hydraulic, social and political principles with the help of technological modelling and simulation tools. The likely causes of damages due to floods in a basin can thereby be determined and the key factors involved in causing the damage can be documented.

A methodological guide for flood forensics has been already developed by the authors of this chapter [3]. Particularly for the case of floods, the guide has been organized in five phases: (A) information gathering and integration, (B) hydrometeorological and hydrological analysis, (C) hydraulic analysis, (D) integrative analysis and (E) final diagnosis.

Each phase implies the development of several activities with the only objective of document‐ ing and looking forward to clarify the actions that occurred. Some actions involve a detailed scientific analysis, while other needs the construction of a Geographical Information Systems (GISs) or even the implementation of a numeric simulation model. Some stages include field works in order to determine how the systems and action plans functioned and in what way the emergency was managed or even how the population lived the event.

Flood forensics looks for the integration of geographic, hydrologic, hydraulic, social, economic and political aspects in order to identify which of these factors intervened as main drivers to favour o diminish the impacts of the flood.

When commencing phase A, information gathering and integration, it has to recognize that this is not an easy step, since in many cases, information may not exist or if it does, it may be disperse and difficult to compile (**Table 1**).


**Table 1.** Phase A of flood forensic analysis, based on Ref. [3].

The attention in this phase could be oriented towards the following:


**•** Socioeconomic and political: Information of interest in this regard includes action and development plans and programs, damages caused, emergency management, personal testimonies, historical records of past flooding events. Additional information may include data for soil use change in the basins, floodplain occupancy and poverty condi‐ tions of the population in high-risk areas. This can help in the verification of the preventive measures and the preparation actions that were taken in the zone of impact, the actions that have been implemented to improve the living conditions of the affected people and those actions oriented to the emergency management preparedness and the post-disaster reconstruction in order to avoid major damages. Personal testimonies become high value information because these are not usually included in technical reports. Interviews to direct affected people, observers and authorities can add significant value to the analysis.

With this information, a Geographical Information System (GIS) should be integrated. A base map can be used as basic platform for consultation and processing in the forensic analysis. One of the first processes would be the determination of the geomorphologic characteristics of the hydrologic system of the case study in the extents of the integral model. The use of this GIS results in advantages that are evident. If a GIS of the region exists at the moment of the flooding event, the possibility of uploading specific real time information would be of high value, transforming the GIS in a full dynamic tool. This can contribute in the execution of operative actions and in the strategic decision making facilitating the problems comprehension and eventual resolution.


**Table 2.** Phase B of flood forensic analysis, based on Ref. [3].

A methodological guide for flood forensics has been already developed by the authors of this chapter [3]. Particularly for the case of floods, the guide has been organized in five phases: (A) information gathering and integration, (B) hydrometeorological and hydrological analysis, (C)

Each phase implies the development of several activities with the only objective of document‐ ing and looking forward to clarify the actions that occurred. Some actions involve a detailed scientific analysis, while other needs the construction of a Geographical Information Systems (GISs) or even the implementation of a numeric simulation model. Some stages include field works in order to determine how the systems and action plans functioned and in what way

Flood forensics looks for the integration of geographic, hydrologic, hydraulic, social, economic and political aspects in order to identify which of these factors intervened as main drivers to

When commencing phase A, information gathering and integration, it has to recognize that this is not an easy step, since in many cases, information may not exist or if it does, it may be

**Phase Stage Before During After**

2. Status and quality of hydrometeorologic and

4. Geomorphological characterization of basins

**•** Geographic: Geographic and human aspects, theme maps, satellite imagery, aerial photographs, topographic surveys, photographic registers of the impacted zone of the

**•** Hydrometeorologic: Records of daily basis climatologic stations, automatic stations and meteorological observatories. Such information will provide information of the rainfall

**•** Hydraulic: Records of flow gaging stations, physical conditions of hydraulic infrastruc‐ ture as well as its design information; outflows from infrastructure and the official operation policy as well as the actual operation conditions during the flooding event. If it happens that the analyst can join the field inspections during the event, he can perhaps document or at least observe flood stages, depths, velocities and sediment transport rate

event regarding its genesis, extension, magnitude, duration and intensity.

1. Recopilation of geographic, hydrometerorological, hydrologic, hydraulic, socioeconomic and politic

hydraulic analysis, (D) integrative analysis and (E) final diagnosis.

the emergency was managed or even how the population lived the event.

information

The attention in this phase could be oriented towards the following:

streams and river, the basin and the urban environment.

hydrometric information

3. Geographical Information System

favour o diminish the impacts of the flood.

disperse and difficult to compile (**Table 1**).

**Table 1.** Phase A of flood forensic analysis, based on Ref. [3].

**A. Information gathering and**

46 Forensic Analysis - From Death to Justice

in critical reaches.

**integration**

In this stage of phase A, an analysis regarding the quality of the information should be carried out. In the case of missing climatic data, for example, hydrometric information could be an alternative. It should be noted that the estimation of missing values or the use of transposition techniques from neighbour basins significantly affect the reliability of the analysis.

During phase B, hydrometeorological and hydrological analysis, the most critical steps of the methodology are addressed. The comprehension of these processes in the basin constitutes the base of the study, giving support to the full understanding of the event that generated the flooding (**Table 2**).

The comprehension of these hydrometeorological processes should include the analysis of the storms genesis. It is well known that a storm is the set of rains of well-defined characteristics that belong to a giving meteorological perturbation, but in this phase, the natural phenomena must be clearly identified and characterized in its relationship to the flood event. The fact that it was a single storm or a combination of events must be stated. The genesis of a storm that produces a flood event is a key to forensic flood analyses. Likewise, the temporal and spatial scales of the precipitations should be characterized, trying to represent their variability in the context of the basin or basins involved. For example, isohyet maps can be constructed in order to better visualize the distributions of the variables and be able to obtain information about the spatial evolution of the event. Graphs on the temporal evolution can also be obtained and further analyzed looking for tendencies, patterns of seasonal variations. The reconstruction of the rainfall field that occurred before and during the event is recommended. If the information is available, isohyet maps can be built with durations from 5 min (or even less) to up to 96 h, if this time window is relevant.

A basic but fundamental process is the determination of the descriptive statistical parameters of the event, since this could help to state in a few indicators, the whole set of observations of a single variable, making comparisons more precise and easier than those that can be done from graphs and plots. Monthly rainfall distributions as well as cumulative information of precipitation would help to identify seasonality and mass curves can give information on the temporal variation of the precipitation in the basin. All of this provides a background analysis and a reference framework.

In the hydrologic processes analysis, and due to the fact that the peak discharge and its corresponding hydrograph are associated to a lot of climatic and physiographic factors, their most reliable estimation is based on the probabilistic treatment of the information from historic observations, discharges or water depths. This process has been referred as flood frequency analysis. In addition, the probabilistic analysis of maximum data of precipitation makes possible the construction of intensity-duration return period curves, which characterize storms in the study region. These curves are also a valuable tool to study ungauged basins. Both for precipitation and discharge information, probability distribution functions must be analyti‐ cally fitted to data. The distribution that fits the best is then selected. Later on, this can be used to evaluate the magnitude of events with different exceedance probabilities or return periods.

Determining the difference between total precipitation and the hydrologic abstractions, that is the effective (excess) precipitation (also called direct runoff), is a must in the analysis. In the case of gaged basins, a simultaneous registry of precipitation and runoff from a storm will be available, and therefore excess rainfall will be calculated based on direct flow determined from the flood hydrograph. Dividing the direct runoff volume by the basin area can do this. On the other hand, if these abstractions are not known, such as in ungauged basins, specific methods can be used, such as the curve number model developed by the Natural Resources Conserva‐ tion Service [4]. The procedure to be applied in the forensic flood analysis is chosen according to the available information.

During phase B, hydrometeorological and hydrological analysis, the most critical steps of the methodology are addressed. The comprehension of these processes in the basin constitutes the base of the study, giving support to the full understanding of the event that generated the

The comprehension of these hydrometeorological processes should include the analysis of the storms genesis. It is well known that a storm is the set of rains of well-defined characteristics that belong to a giving meteorological perturbation, but in this phase, the natural phenomena must be clearly identified and characterized in its relationship to the flood event. The fact that it was a single storm or a combination of events must be stated. The genesis of a storm that produces a flood event is a key to forensic flood analyses. Likewise, the temporal and spatial scales of the precipitations should be characterized, trying to represent their variability in the context of the basin or basins involved. For example, isohyet maps can be constructed in order to better visualize the distributions of the variables and be able to obtain information about the spatial evolution of the event. Graphs on the temporal evolution can also be obtained and further analyzed looking for tendencies, patterns of seasonal variations. The reconstruction of the rainfall field that occurred before and during the event is recommended. If the information is available, isohyet maps can be built with durations from 5 min (or even less) to up to 96 h,

A basic but fundamental process is the determination of the descriptive statistical parameters of the event, since this could help to state in a few indicators, the whole set of observations of a single variable, making comparisons more precise and easier than those that can be done from graphs and plots. Monthly rainfall distributions as well as cumulative information of precipitation would help to identify seasonality and mass curves can give information on the temporal variation of the precipitation in the basin. All of this provides a background analysis

In the hydrologic processes analysis, and due to the fact that the peak discharge and its corresponding hydrograph are associated to a lot of climatic and physiographic factors, their most reliable estimation is based on the probabilistic treatment of the information from historic observations, discharges or water depths. This process has been referred as flood frequency analysis. In addition, the probabilistic analysis of maximum data of precipitation makes possible the construction of intensity-duration return period curves, which characterize storms in the study region. These curves are also a valuable tool to study ungauged basins. Both for precipitation and discharge information, probability distribution functions must be analyti‐ cally fitted to data. The distribution that fits the best is then selected. Later on, this can be used to evaluate the magnitude of events with different exceedance probabilities or return periods. Determining the difference between total precipitation and the hydrologic abstractions, that is the effective (excess) precipitation (also called direct runoff), is a must in the analysis. In the case of gaged basins, a simultaneous registry of precipitation and runoff from a storm will be available, and therefore excess rainfall will be calculated based on direct flow determined from the flood hydrograph. Dividing the direct runoff volume by the basin area can do this. On the other hand, if these abstractions are not known, such as in ungauged basins, specific methods can be used, such as the curve number model developed by the Natural Resources Conserva‐

flooding (**Table 2**).

48 Forensic Analysis - From Death to Justice

if this time window is relevant.

and a reference framework.

As part of the hydrologic process analysis within flood forensics, return periods of precipita‐ tion and discharge events should be stated. That is, to determine the time interval in which an event of a given magnitude can be equalled or exceeded on average and over time [5]. It is important to remember that given that the rainfall-runoff relationship is non-linear, the return periods for rainfall and flows need to be differentiated. The return period for a particular rainfall is not the same as the one for the runoff generated by the same rainfall. This is naturally complicated process involving key variables, namely the soil moisture content at the moment of the precipitation as well as changes in vegetation, land use and anthropogenic activities in the basin. Therefore, a rainfall with a 100-year return period does not necessary generate a flow having a 100-year return period. In a basin subject to increasing urbanization or deforestation, the same rainfall generates runoffs having increasingly longer return periods.

One key aspect of flood forensics is to determine the flow associated with the flood event in the list of assigned probabilities using a plotting position formula such as that by Weibull [5]. It should be considered that the record does not include the value of the event under study but rather only historical values, that is, only records up to the year prior to the flood event. If the magnitude of the flow lies within the magnitudes registered by the historical data, a return period for the event can easily be determined empirically directly from the sample. Never‐ theless, if the flow is located outside the magnitudes found in the historical record then the return period *T* of the flood event can be calculated by fitting the sample to a PDF, considering that the fitting process does not include the value of the event in question. For this case, although the magnitude of the flow is larger than those in the historical record, a return period can be assigned to the flow through extrapolation. These two ways to assign *T* are intended to prevent that the length of the record or the magnitude of the event influence the value. The value of the flow associated to the flood event can be later incorporated into the historical records, which will result in a modification of the fitting of the records to the PDF as well as a change in the recurrence interval values through the plotting position. Thus, the assigned value of *T* will also change. The above is intended to highlight the fact that the assignment of the occurrence probability is evolutionary and not static.

Nevertheless, if hydrometric records are not available or are too limited to obtain a reliable interpretation or extrapolation, then rainfall-runoff relationships can be very useful because of their ability to infer flow information based on precipitation records.

For this, the application of models becomes very important since it turns the process more efficient and more reliable. Regardless of the model used to simulate the rainfall-runoff process, a calibration process must be included. The calibration makes possible to determine the value of the errors of the model when compare to a measurement or a reference pattern, all of this with sufficient precision and under specific conditions. It is crucial that these errors be sufficiently small and that they are determined with the highest precision possible. Regardless of the different rainfall-runoff models used, it is important to consider the limita‐ tions involved in applying each one to the zone, as well as the information restrictions that persists.

With this, a solid base for next phase, phase C. Hydraulic analysis is set. In phase C as in all the others, we look for the process to be efficient but always taking into consideration the limitations of the information and tools available. In this phase, the representation of the hydraulic behaviour of the systems should be obtained and studied; so, the conditions that intervened or favour the event. Simulation models of the river networks, floodplains, impacted urban zones; infrastructure operation and protection works efficiency should be addressed (**Table 3**).


**Table 3.** Phase C of flood forensic analysis, based on Ref. [3].

It is precisely for the hydraulic modelling and simulation of the river network, floodplains and urban areas where the information gathered and integrated in GIS would be of great help.

For the analysis, information will be considered according to the scale needed. Initially, the geometry of the river, both longitudinally and cross-sections, and the hydrodynamic charac‐ teristics of the system have to be given. Among the physical characteristics of the river that are needed for the study are: soil type and vegetation in order to better estimate roughness coefficients (usually Manning's *n*), longitude, slope, elevations and depths, full cross sections and obstacles. Modelling and simulation can be reproduced both, the normal conditions and those present during the flooding event. The hydraulic modelling of a network of channels will provide enough information to determine the overall behaviour and that of the major event in the network. The information also enables determining the boundaries of floodplains during normal functioning and flood zones during major events. This information is important since once the floodplain is delimited and a determination is made as to whether human settlements are located there, a hydraulic analysis of the urban zone can be performed to establish the degree to which the dynamics of a city will be affected. To this end, the hydro‐ dynamic functioning of streets can also be simulated.

Erosive processes and sedimentation cause damages, including the reduction in the produc‐ tivity of the soil, loss and degradation of soil and sedimentation in reservoirs, drainage ditches and channels, as well as damages to hydraulic infrastructure. For the study case, the damages of most interest are those caused by scouring around infrastructure near or in the river, as well as changes to the flow capacity of the river due to sedimentation.

Changes in the hydraulic capacity of a river can be affected by the degree to which it and its respective floodplains are composed of unconsolidated sediments, which are quickly eroded by floods and high water levels. If the river transports fairly thick sediments during a flood, these will tend to be deposited along the bottom of the river and cause natural dykes to form. This could raise the bottom of the river, thereby increasing water levels. When this occurs there is a very high risk of flooding. Natural and induced landslides are also cases that can increase the risk of flooding, in which the amount of sediments transported by the river increases, causing the hydraulic capacity to decrease or, in the worst of circumstances blocking the river.

With this, a solid base for next phase, phase C. Hydraulic analysis is set. In phase C as in all the others, we look for the process to be efficient but always taking into consideration the limitations of the information and tools available. In this phase, the representation of the hydraulic behaviour of the systems should be obtained and studied; so, the conditions that intervened or favour the event. Simulation models of the river networks, floodplains, impacted urban zones; infrastructure operation and protection works efficiency should be addressed

**Phase Stage Before During After**

It is precisely for the hydraulic modelling and simulation of the river network, floodplains and urban areas where the information gathered and integrated in GIS would be of great help.

For the analysis, information will be considered according to the scale needed. Initially, the geometry of the river, both longitudinally and cross-sections, and the hydrodynamic charac‐ teristics of the system have to be given. Among the physical characteristics of the river that are needed for the study are: soil type and vegetation in order to better estimate roughness coefficients (usually Manning's *n*), longitude, slope, elevations and depths, full cross sections and obstacles. Modelling and simulation can be reproduced both, the normal conditions and those present during the flooding event. The hydraulic modelling of a network of channels will provide enough information to determine the overall behaviour and that of the major event in the network. The information also enables determining the boundaries of floodplains during normal functioning and flood zones during major events. This information is important since once the floodplain is delimited and a determination is made as to whether human settlements are located there, a hydraulic analysis of the urban zone can be performed to establish the degree to which the dynamics of a city will be affected. To this end, the hydro‐

Erosive processes and sedimentation cause damages, including the reduction in the produc‐ tivity of the soil, loss and degradation of soil and sedimentation in reservoirs, drainage ditches and channels, as well as damages to hydraulic infrastructure. For the study case, the damages of most interest are those caused by scouring around infrastructure near or in the river, as well

Changes in the hydraulic capacity of a river can be affected by the degree to which it and its respective floodplains are composed of unconsolidated sediments, which are quickly eroded by floods and high water levels. If the river transports fairly thick sediments during a flood, these will tend to be deposited along the bottom of the river and cause natural dykes to form.

2. Infrastructure operation and protection works efficiency

**C. Hydraulic analysis** 1. Simulation models of the river networks, floodplains, impacted

urban zones

verification

**Table 3.** Phase C of flood forensic analysis, based on Ref. [3].

dynamic functioning of streets can also be simulated.

as changes to the flow capacity of the river due to sedimentation.

(**Table 3**).

50 Forensic Analysis - From Death to Justice

Another factor is the development of tides, where the over height of the sea level can worsen flooding inland in areas relatively near the coast or cause coastal flooding. Coastal floods are primarily involved in obstructing natural runoff into the sea and blocking the flow of drainage systems, of course with their respective effects on the coastal area.

In general, to evaluate the hydraulic analysis of an urban zone, parameters such as water depth, velocity, permanence of flow and supply of solids should be taken into account.

Particularly in the revision of hydraulic works, as ending part of the hydraulic analysis, observation regarding the failure has to be carried out. In order to establish if the failure was caused by an event that surpassed the design conditions. Among others, the following has to be reviewed.


Up to this moment, flood forensics has been focused on the analysis of engineering processes, however, in to order to converge to strong and objective conclusions, factor beyond engineer‐ ing have to be taken into account. Because of this, the next phase is dedicated to the integration of the analysis itself, with the socioeconomic and political part, being named as Phase D, integrative analysis, with its respective stages as can be seen in **Table 4**.


**Table 4.** Phase D of flood forensic analysis, based on Ref. [3].

This integrative analysis should consider the revision of action and development plans and programs in the region or zone where the flood took effect. The policies and strategies involved in these development plans should be reviewed for clarity and accuracy, and of course they should be evaluated as to whether they have been implemented or are in the process of being implemented. The follow-up of these actions should be reviewed since they are strategic visions for the future and the solutions they offer need to be maintained over time, making them crucial to the population, its safety and well-being. The designs of the plans need to be reviewed to verify that they are sustainable and contain improvements that will remain in the society after the plan has been completed. Thus, a development plan should be aimed at teaching the population to manage latent risks and not only be directed towards restoration actions. Even though this measure is secondary to the main action, it promotes self-sufficiency. Development plans should not neglect issues such as reforestation, protected zones that are directly related with levels of felling and deforestation in the disaster zone, as well as other changes in land use in supply basins which can be analyzed with historical vegetation and land use maps to see the evolution in the zone. Land planning should also be investigated to identify the location of high-risk lands sold at low values. The concentration of vulnerable human settlements should be determined as well as invasion into natural floodplains by urban settlements, commercial zones and other land uses. In particular, the legality of settlements in connection with the plan itself should be evaluated. It is also important to identify marginal‐ ization levels in high-risk settlements, as well as factors that influence the settlement of populations in these locations. Furthermore, the government's sensitivity to the risk situation should be identified, as well as any actions that it may have taken in this respect.

It will also significant the analysis of the emergency management itself, because it will consider actions taken prior to the event up to those implemented for the immediate attention of the population and infrastructure during the disaster. Actions in response to the population during an emergency could form the basis of all the procedures to be implemented in the event of an emergency, since everything that is done should be based on the protection and safety of the population and not only on economic losses. Since the actions taken are the result of planning and the projections generated by programs, adequate preparation should be ensured to reduce the impact during a disaster. A review of programs in the study zone should include their specific progress; efficiency and legitimate implementation, since the objectives proposed by each of the programs depend on their correct application. If programs are not executed as planned, then the reason for this should be identified since it can become a determining factor in increasing the magnitude of a disaster.

**Phase Stage Before During After**

3. Integration of hydrologic and hydraulic analysis with other 4. Generations of flooding maps and determination of impacts in

This integrative analysis should consider the revision of action and development plans and programs in the region or zone where the flood took effect. The policies and strategies involved in these development plans should be reviewed for clarity and accuracy, and of course they should be evaluated as to whether they have been implemented or are in the process of being implemented. The follow-up of these actions should be reviewed since they are strategic visions for the future and the solutions they offer need to be maintained over time, making them crucial to the population, its safety and well-being. The designs of the plans need to be reviewed to verify that they are sustainable and contain improvements that will remain in the society after the plan has been completed. Thus, a development plan should be aimed at teaching the population to manage latent risks and not only be directed towards restoration actions. Even though this measure is secondary to the main action, it promotes self-sufficiency. Development plans should not neglect issues such as reforestation, protected zones that are directly related with levels of felling and deforestation in the disaster zone, as well as other changes in land use in supply basins which can be analyzed with historical vegetation and land use maps to see the evolution in the zone. Land planning should also be investigated to identify the location of high-risk lands sold at low values. The concentration of vulnerable human settlements should be determined as well as invasion into natural floodplains by urban settlements, commercial zones and other land uses. In particular, the legality of settlements in connection with the plan itself should be evaluated. It is also important to identify marginal‐ ization levels in high-risk settlements, as well as factors that influence the settlement of populations in these locations. Furthermore, the government's sensitivity to the risk situation

should be identified, as well as any actions that it may have taken in this respect.

It will also significant the analysis of the emergency management itself, because it will consider actions taken prior to the event up to those implemented for the immediate attention of the population and infrastructure during the disaster. Actions in response to the population during an emergency could form the basis of all the procedures to be implemented in the event of an emergency, since everything that is done should be based on the protection and safety of the population and not only on economic losses. Since the actions taken are the result of planning and the projections generated by programs, adequate preparation should be ensured to reduce the impact during a disaster. A review of programs in the study zone should include their specific progress; efficiency and legitimate implementation, since the objectives proposed by each of the programs depend on their correct application. If programs are not executed as

**D. Integrative analysis** 1. Revision of action and development plans and programs

human settlements.

**Table 4.** Phase D of flood forensic analysis, based on Ref. [3].

52 Forensic Analysis - From Death to Justice

2. Analysis of the emergency management

In this phase, technical factors involved in the flood should be integrated with those of political, social and economic character. Main determining factors in the occurrence of the disaster should be identified. Also the factors that contributed or favoured the impacts of the disaster should be noted. This can be done through a hierarchic procedure in order to determine objectively the causes and effects of the flood event. It would be idyllic to think that all driving forces are taken into consideration; however, the analyst must integrate all elements that could have influence in the problem. If an adequate integration of each of the elements considered is reached, it would be possible to have a close picture of what really happened, minimizing uncertainty.

Finally, it is a good moment to generate the flooding maps and with these to determine related impacts. It should be remembered that flooding maps establish water depths in their relation with the topography for different discharges of interest. To generate flood maps with auto‐ mated procedures, software that jointly performs hydrological and hydraulic modelling at the street level can be used. A semi-automated procedure can generate flood maps using separate hydrological and hydraulic modelling and perform external integration using GIS software. Flooding maps can be generated as follows. First, a Digital Elevation Model (DEM) is generated and a map of the basins obtained. The DEM is used again and with the detailed topography, the alignment of rivers and the characteristics of the banks and cross-sections are alternately obtained. The hydrological model is used to obtain flows, which serve as input for the hydraulic model. After performing the hydraulic simulation, flood levels are obtained from the cross sections. Finally, these levels are processed and the geographic and hydraulic information are combined to generate the flood maps.

After generating the theoretical flood maps based on the modelling, they can be compared to the actual flood area by analyzing the differences between them, attempting to locate zones that are more problematic and finding their associations with all the factors analyzed previ‐ ously. The relationships between these factors and the flood zones can thereby be established. Theoretical flood maps delimit the risk zones in a general way and the map of the flood under study delimits the effects that have occurred. This enables objectively the determination of the main reasons why the event reached a particular magnitude.

After using the hydraulic modelling to determine the levels occurring in the urban zones, the percentage of damage can be identified according to type: direct (housing, educational buildings, health infrastructure, public facilities, etc.), indirect (supply of goods, interruption in services and communication systems, loss of work hours, among others) and intangible economic loss (those affected, the injured and loss of human life). The methodology used for this is divided into two steps—quantification of goods affected and the quantification of the costs of these effects.

With all of the above, phase E final diagnosis can be finally established. This corresponds to the culmination of the forensic analysis. The final objective is the identification of factors and giving relative weight to each one. This is be done by establishing a contrast study with historical events, the objective conclusion on the causes and effects, as well as the lessons learned and clearly the proposed actions (**Table 5**).


**Table 5.** Phase E of flood forensic analysis, based on Ref. [3].

Based on the history of flood events and the destruction caused in the study area, records from communications media and personal testimony from those affected, observers and authorities should be gathered into a document to perform a general analysis. Nevertheless, if complete analyses of previous events are available, those should undoubtedly be used. Thus, it will be possible to compare these events to the one that is currently occurring in order to identify recurring factors that influence floods in the study zone, with similar magnitudes or within a range of association. This will serve as a guide to discover whether the structural and nonstructural actions have been adequately applied over the history of the study zone or if other actions not previously considered need to be taken.

An integrated analysis of technical factors (hydrometeorological, hydrological and hydraulic) along with the social and economic dimensions explained previously enables the production of an objective evaluation of the causes and effects. The result will provide documented evidence of how major was the event from the probabilistic point of view and to what degree other external factors contributed to magnifying the impact of the flood. It is important to note that a combination of all the factors may be the best explanation possible in most cases. In that case, a weighting of the causes is recommended, assuring of course that this is done based on objective findings from the analysis.

Undoubtedly, the best way to capitalize the findings and results of a forensic analysis of a flood event consists in the opportunity of learning and the potential development of actions oriented to reduce the impacts of future similar events. Among the lessons learned from this analysis, answers to several questions can be obtained. Questions such as: How extraordinary was the flooding causing event? What is the probability of exceedance (or return period) of precipita‐ tion or discharge? To what level, timely and effective warning about the magnitude of the event could have diminished damages? Were the major damages located in floodplains and high risk areas? What was the role of the hydraulic infrastructure? Was the infrastructure well operated? Did the operation of infrastructure help to control the event? The operation policy was the one pre-established or a change was made? The hydraulic design criterion of structures is the adequate? Plans for urban development were respected? Are these plans adapted for flood cases? If a risk atlas exists, is it necessary to make adaptations to it in order to consider flood risks? In what measure, lack of conservation of higher basins contributed to damages? How adequate was the response to the emergency? Was the coordination among institutions favourable?

Duration for each phase and stage is given in **Table 6**.

historical events, the objective conclusion on the causes and effects, as well as the lessons

**Phase Stage Before During After**

Based on the history of flood events and the destruction caused in the study area, records from communications media and personal testimony from those affected, observers and authorities should be gathered into a document to perform a general analysis. Nevertheless, if complete analyses of previous events are available, those should undoubtedly be used. Thus, it will be possible to compare these events to the one that is currently occurring in order to identify recurring factors that influence floods in the study zone, with similar magnitudes or within a range of association. This will serve as a guide to discover whether the structural and nonstructural actions have been adequately applied over the history of the study zone or if other

An integrated analysis of technical factors (hydrometeorological, hydrological and hydraulic) along with the social and economic dimensions explained previously enables the production of an objective evaluation of the causes and effects. The result will provide documented evidence of how major was the event from the probabilistic point of view and to what degree other external factors contributed to magnifying the impact of the flood. It is important to note that a combination of all the factors may be the best explanation possible in most cases. In that case, a weighting of the causes is recommended, assuring of course that this is done based on

Undoubtedly, the best way to capitalize the findings and results of a forensic analysis of a flood event consists in the opportunity of learning and the potential development of actions oriented to reduce the impacts of future similar events. Among the lessons learned from this analysis, answers to several questions can be obtained. Questions such as: How extraordinary was the flooding causing event? What is the probability of exceedance (or return period) of precipita‐ tion or discharge? To what level, timely and effective warning about the magnitude of the event could have diminished damages? Were the major damages located in floodplains and high risk areas? What was the role of the hydraulic infrastructure? Was the infrastructure well operated? Did the operation of infrastructure help to control the event? The operation policy was the one pre-established or a change was made? The hydraulic design criterion of structures is the adequate? Plans for urban development were respected? Are these plans adapted for flood cases? If a risk atlas exists, is it necessary to make adaptations to it in order to consider flood risks? In what measure, lack of conservation of higher basins contributed to damages?

2. Objective conclusions on causes and effects 3. Lessons learned and proposed actions

learned and clearly the proposed actions (**Table 5**).

54 Forensic Analysis - From Death to Justice

**E. Final diagnosis** 1. Comparison with historical events

**Table 5.** Phase E of flood forensic analysis, based on Ref. [3].

actions not previously considered need to be taken.

objective findings from the analysis.


**Table 6.** Suggested duration to complete stages and phases of a flood forensic analysis, based on Ref. [3].

Another product flood forensic analysis can provide is the proposal of actions, all oriented to reduce damages produce by future flood events. Clearly, general ideas always arise, but the courses of actions will depend on the particular case. Just to name a few of them, these can include:

