Mitigation of urban flooding - Taylor & Francis Online

Urban Water Journal, Vol. 2, No. 4, December 2005, 215 – 226

Mitigation of urban flooding: A simplified approach for distributed stormwater management practices selection and planning GABRIELE FRENI* and ELISA OLIVERI Dip. Ingegneria Idraulica ed Applicazioni Ambientali, Universita` di Palermo, Viale delle Scienze, 90128 Palermo, Italy

The urbanization process and the hydraulic insufficiency of drainage systems are two of the most common causes of urban flooding. In some technical regulations, distributed stormwater management practices (DSMPs) are regarded as a solution for urban flooding problems. They can prevent the formation of runoff, dispose of it locally, or dampen its peak before it reaches the drainage system. Due to their diffuse localization and the wide number of available solutions, the evaluation of their efficiency in terms of flood reduction is very difficult. The methodology proposed in the present paper relies on the concept that the mitigation effects of DSMPs can be expressed as a function of the changes of the hydrological parameters of a catchment. Once the relation between a DSMP and the equivalent hydrological parameter is established, the efficiency of DSMPs can be evaluated using mathematical models simulating the runoff formation and propagation in urban areas and applying methodologies similar to parameter sensitivity analysis and model uncertainty propagation. Studying the effect of parameter variation on model output, it is possible to analyse quickly several different stormwater management solutions and to identify the best distribution of measures in order to achieve a defined mitigation task. The simplified procedure has been compared with a more detailed approach obtained by fully integrate DSMPs in the drainage system hydrodynamic model. The procedure has been applied to the real case study of Mondello catchment in Palermo (Italy), and the analysis of the results allows the identification of some guidelines for the mitigation plan preparation. Keywords: Best management practices; Stormwater management; Urban flooding

1. Introduction Urban development has a profound influence on the modification of the water cycle. Trees, meadow grasses and agricultural crops that had previously intercepted and absorbed rainfall are removed and natural depressions that had temporarily ponded water are graded to a uniform slope. Cleared and graded sites erode, are often severely compacted and can no longer prevent rainfall from being rapidly converted into stormwater runoff. Rooftops, roads, parking lots, driveways and other impervious surfaces prevent rainfall soaking into the ground. Consequently,

most rainfall is converted directly to stormwater runoff. The increase in stormwater runoff can be too great for the existing urban drainage system to handle, causing flooding and damage to private properties (Maryland DoE 2000). The traditional objective in providing stormwater drainage has been to remove water from surfaces, especially roads, as quickly as possible and with minimum inconvenience to human activities. It is then disposed of, usually via a pipe system, to the nearest receiving waters or to a downstream existing drainage system. In recent years, the problems connected with the traditional design schemes were pointed out and new design approaches have been

*Corresponding author. Email: [email protected] Urban Water Journal ISSN 1573-062X print/ISSN 1744-9006 online ª 2005 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/15730620500386461

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developed which introduce more natural methods of disposing of stormwater (WEF/ASCE 1992). Different names have been used in different countries to describe similar techniques. In the US and Canada, we find BMPs, or best management practices, which has been translated in French as ‘Pratiques de Gestion Optimales’ (PGOs) (CERTU 2003); a global term which has also emerged since the late 1990s is LID (low impact development) (Prince George’s County 1999). In the UK, the term sustainable urban drainage (SUD) has been accepted in a number of publications (Wild et al. 2002). In Australia, there is WSUD (water sensitive urban design), which is used to describe a new approach to urban planning and design that aims to offer sustainable solutions for integrating land development and the natural water cycle. Those last terms including sustainability are more addressed to water quality issues; in the following paragraphs, we will refer more generally to distributed stormwater management practices (DSMPs). These include practices such as infiltration and storage, and the general intention is to attempt to reduce runoff volume and peak flow, increasing the time it takes to reach the receiving water system (Stahre 1993, Roesner 1999, Carre´ et al. 2004). This stormwater management philosophy led to a new trend of ‘minimum impact’ in the design of new drainage systems and in the revision of the existing ones. However, many problems, essentially connected with the selection of the ‘best’ (or most effective) management practices for a practical case are still to be solved (WEF/ASCE 1998). Decisional problems are also connected with the evaluation of the effectiveness of packages of measures distributed on the whole catchment. Different measures can influence each others performance and maintenance is required which makes the attainment of the optimal design solution more difficult. In the present paper, the DSMP techniques are analysed comparing them to ‘improvements’ of hydrological parameters of the catchment. In changing the value of a

Table 1.

parameter, it is possible to simulate the operation mode of the corresponding DSMP technique: DSMPs performances can be analysed by observing the response of the catchment to the variation of the hydrological parameters. This methodology is based on the commonly known ‘sensitivity analysis’, which is widely used both in the professional practice and in the research for the calibration of mathematical models (Christiansen and Wallace 1998, Freni et al. 2002). The proposed methodology allows for exploitation of the experience developed in the model calibration to a planning and design problem; focusing the attention on the parameters which mostly influence the hydrological response of the catchment, it is possible to identify the most effective sets of distributed measures. The developed procedure has been applied to the case study of Mondello catchment in Palermo (Italy). 2. The proposed methodology In the present paper, the efficiency of the most common distributed stormwater management practices, aimed at urban flooding mitigation, has been analysed. This kind of flooding mitigation strategy can be usually widespread all over an urban catchment with several small ‘house-byhouse’ measures. The proposed methodology relies on the concept that, according to the diffuse distribution of DSMP measures in the catchment, the effect of DSMPs can be simplistically expressed as a modification of the hydrological parameters of the catchment (for example, the introduction of infiltration trenches can be simulated by an increase in natural soil infiltration). Table 1 shows how the hydrological parameters, which describe the urban catchment, can be linked to the DSMP measures having similar mitigation effects. ‘Effective percentage of impervious area’ is one of the most critical parameters in urban drainage models and it

Relations between DSMPs and equivalent hydrological model parameters.

Model parameters

DSMP techniques

Technological alternatives

Effective percentage of impervious area

Disconnection of impervious areas and disposal of water on pervious surfaces

Soakaways, infiltration structures, porous pavements

Retention parameters

Retention structures (runoff disposal during rainfall events is negligible)

Retention tanks Downpipe tanks Reuse storages

Roughness parameters

Detention structures (runoff volume is temporarily stored and gradually discharged to the drainage system)

Detention tanks Inlet controls

Infiltration parameters

Infiltration of runoff generated on pervious surfaces

Infiltration trenches and basins Wetlands

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can be used to simplistically simulate impervious area disconnection measures. Disconnection of impervious area is an attractive DSMP measure, both in terms of its easy implementation, especially in low density developments, and because of its direct hydraulic impact: runoff generated on impervious surfaces is disposed of to a neighbouring pervious area directly (if soil permeability allows for such practice) or through infiltration facilities that are usually not connected to the drainage system. ‘Retention parameters’ are usually expressed as surface wetting and depression losses; those parameters, usually different for pervious and impervious areas, can be increased in order to take into account constructed structures aimed to store and dispose of stormwater over a long period (24 – 72 h). These parameters allow simulation of all structures in which stormwater disposal during events can be neglected: retention ponds, infiltration basins in soils with low infiltration capacity (K 5 10 mm/h) or constructed impervious wetlands. ‘Roughness parameters’ describe the speed of runoff generation in hydrological models. They can be modified in order to simulate the presence of detention measures according to the fact that the basic effect of such practices is to attenuate runoff peak flow without changing the runoff volume. According to this concept, the possibility of simulating detention facilities is connected with the model’s capacity to attenuate the runoff hydrograph. ‘Infiltration parameters’ can be used to simulate mitigation measures on pervious areas that allow for the disposal of rainwater through infiltration during the event. In cases where the soil has high permeability and infiltration during the event cannot be neglected, increasing infiltration parameters for pervious areas allows analysis of the efficiency of infiltration facilities. Once the relations between DSMPs and equivalent changes of hydrological parameters are established, the efficiency of DSMPs can be evaluated using methodologies similar to parameter sensitivity analysis. Studying the effect of parameter variation on model output, it is possible to evaluate the efficiency of different stormwater management solutions. As the proposed comparison is only qualitative, it can be used at planning level and for evaluating the general mitigation trend associated to a specific mitigation measure. Moreover, as the proposed study can be carried out on a geographic basis, it can be used to identify the best distribution of measures in order to achieve a defined mitigation task, defining the most effective measure to be applied to a specific area. The methodological approach is general and can be applied to a wide range of problems both regarding quantity and quality management of stormwater simply changing the analysis objective function: in the present study, having the aim of flooding mitigation, the reduction

of the peak discharge and of the water volumes collected by the sewer system have been adopted as objective functions, together with the water volume collected at each flooding location. The use of these objective functions allows for monitoring of the drainage system insufficiency events either regarding the negative effect on population (analysing the distribution of flooding over the catchment), or regarding the cause of the problem (studying the runoff generation in the different parts of the area). An alternative approach focused on water quality can monitor wet weather outflow to the receiving waters using both quantity and quality objective functions. In the present study, the absolute index of weighted sensitivity has been calculated on the basis of the following relationship (James and James 1994, Lei and Schilling 1994): fobj ðhÞ fobj ðlÞ isen ¼

fobj ðmÞ Ph Pl Pm

ð1Þ

where: isen

Ph, Pl, Pm fobj(h) fobj(m) fobj(l)

absolute weighted sensitivity index of the objective function with respect to the parameter variation; respectively, maximum, minimum and average of the parameter range; value of objective function corresponding to the maximum parameter value; value of objective function corresponding to the average parameter value; value of objective function corresponding to the minimum parameter value.

A positive value of the sensitivity index indicates that the objective function increases with the parameter, while a negative value indicates that an increase of the parameter corresponds to a reduction of the objective function. If the sensitivity index is approximately equal to unity it indicates that a variation of the parameter value propagates to the objective function without substantial modifications; conversely, values close to zero show that variations of the parameter do not sensibly affect the objective function. When model calibration is carried out, the sensitivity analysis allows concentration on those model parameters that have a large influence on the model output and thus need further analysis. In the present study, the developed procedure is aimed to evaluate the mitigation effects of the DSMP techniques: the sensitivity index is here intended to express the effect of DSMP with respect to flood mitigation. Focusing the attention on the most sensitive parameters, it is possible to identify the most effective sets

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of DSMP, e.g. increase of the permeability through infiltration, pervious pavement, source detention, extension of flow paths, increase of the gutter roughness, etc. It should be observed, however, that the described methodology is based on two assumptions that represent both the limit and the approximation of the developed procedure:

routing has been solved coupling the continuity equation and the Manning equation (US EPA 2004):

. The objective functions, generally non-linear, are replaced by approximated linear relationships, as explained below in detail. The acceptability of such hypothesis depends on the characteristics of the adopted model and, in some cases, on the characteristics of the adopted mathematical algorithm. . The use of a linear relationship implies that the effects of variation of several parameters on the objective functions can be linearly superposed, i.e. the variation of the objective function due to the variation of several parameters is the sum of the effects of single variations.

where Q is the discharge, S is the sub-catchment surface area, i* is the net rainfall intensity, L is the sub-catchment width, s is the sub-catchment average slope, h and h0 are the water depth and the depression storage depth, respectively, and n is the Manning roughness coefficient. Since hydraulic insufficiency shall be analysed, hydraulic phenomena such as backwater and surcharge of sewers need to be taken into account. The complete 1D De Saint Venant equations and Preissman slot scheme are appropriate to model these phenomena and were used in the present study. In order to simulate the spatial distribution of surface flooding, a simplified ‘basin–weir’ schematisation, shown in figure 1, has been used (Danish Hydraulic Institute 2003). A fictive basin has been inserted above every manhole in order to analyse the temporal and spatial distribution of flooding (Akan 1988, Djordjevic et al. 1991, Freni et al. 2002) and a set of weirs were adopted in order to take into account flooding volumes propagation on the catchment surface. According to this scheme, it is possible to take into account two basic phenomena connected to urban flooding surface propagation:

In the case of traditional sensitivity analysis applied to model calibration, the linearity hypothesis is supported by the limited parameters variation range. In the proposed procedure, however, the variations of the hydrological catchment parameters are much larger (e.g. up to 100% with respect to the medium parameter value). Therefore, the linearity hypothesis needs to be verified (Schilling and Fuchs 1986). An automatic procedure has been implemented, supporting the calibration of the most common commercial urban drainage models. It allows the completion of a comprehensive sensitivity analysis in a quick and effective way. 3. The models used for the analyses The developed procedure has been applied to some mathematical models simulating the runoff formation and propagation in urban areas. The adopted models are widely used, both in professional practice and in research applications. They are all implemented in commonly applied commercial simulation software. Thus, the proposed procedure is generally applicable and does not depend on the hydrologic/hydraulic simulation software. A distributed ‘non-linear reservoir’ model has been adopted to simulate the surface runoff, taking into account both the detention storage and the infiltration phenomena. Concerning surface retention storage, a constant hydrological loss has been applied, concentrated at the beginning of the rainfall: different unit losses have been considered for pervious surfaces and for impervious ones. Infiltration has been simulated using the Horton equation. Rainfall–runoff

S

@h ¼ Si @t

1 Q ¼ Lðh h0 Þ5=3 s1=2 n

ð2Þ ð3Þ

. flooded water flows along the roads according to the longitudinal slope and enter the drainage system in the manholes which have sufficient collection capacity; . flooding propagation along the roads is simulated trough the use of fictive basins connected to each manhole in order to collect flooding water. For the case study the EPA SWMM (US EPA 2004) model has been used, motivated by two considerations: . the model showed a good stability also while analysing very complex drainage systems; this stability derives from the possibility for the user to select different

Figure 1. Weir – reservoir modelling scheme.


mathematical solvers according to the peculiarities of the case study instead of leaving this choice to the software itself; . model source code is public, so it is available for any further development and improvement. 4. The case study The Mondello catchment, shown in figure 2, is approximately 25 km2. Until the beginning of the last century, the analysed area was a semi-rural zone, exclusively covered by green surfaces and small fishing villages. During the past century, it has been progressively transformed into a tourist area, coupled with strong urban expansion. Due to this

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urbanization trend, the analysed area shows a remarkable variety of building types: detached houses surrounded by small gardens as well as block buildings with scarce pervious areas. During the last thirty years, fast urbanisation processes have not been coupled with the construction of the needed drainage systems aimed to collect stormwater. During rainfall events, therefore, the runoff volumes mainly propagate along the roads, and remarkable flooding is generated in depressed areas, where water volumes can reside for up to several days. The tourist relevance of the catchment and the strong urbanisation of its downstream portion make the adoption of centralised mitigation measures unsuitable; Mondello

Figure 2. Mondello contributing catchment overview.

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thus seems to be an ideal case study in order to estimate the efficiency of a set of distributed mitigation measures. An extended portion of the semi-rural catchments surrounding the urbanised area does not contribute to the runoff generation due to the catchment topography; some impervious areas retain the runoff volumes generating local flooding; other impervious areas are connected to pervious areas where the runoff volumes are infiltrated; some roads are actually equipped with underground infiltration structures. For all these reasons, the effective contributing area is approximately 8 km2 and it can be divided into 11 independent catchments, whose drainage systems (mainly superficial) concur into a strongly urbanised area near to the sea (as shown in figure 3) where the greatest flooding and the greatest damage take place. This area is delimited by an ancient groundwater drainage channel, the so-called ‘Ferro di Cavallo’ (Horseshoe). The channel was transformed into an underground sewer at the beginning of the twentieth century collecting both stormwater and wastewater from the developing urban area. Now the wastewater connections to the channel are discontinued and it mainly drains stormwater and some groundwater. The area delimited by the Ferro di Cavallo is characterised by an underground looped drainage network;

Figure 3.

specifically, it is constituted by 12 loops, with 500 mm circular pipes. In order to protect Mondello bay environment, drainage system outflows are not free but they are connected to two pump stations (each with a maximum flow rate equal to 0.5 m3/s). During winter, the pumped volumes are discharged to the sea out of the bay, while, during summer, tourist activities do not allow for discharging stormwater near the cost and the water volumes are disposed of to a neighboring wastewater treatment plant. As stated above, the drainage system is often surcharged, also during low return period rainfalls, and local floodings occur in many zones. The nature of soils and the characteristics of urban areas allows for adopting either infiltration or retention/detention measures in all the Mondello catchment. The use of swales and vegetated measures is not appropriate because of climatic conditions: long dry periods with high temperatures make the implantation of evergreen vegetal species difficult. For the same reason, in order to prevent septicity and population health consequences, underground measures should be preferred if possible, even if generally more expensive. No discharge measurements in the Ferro di Cavallo channel are available in order to carry out a traditional

Mondello seaside area drainage system.


calibration procedure. Starting from literature, also taking into account a parameter’s physical significance, a weak calibration has been carried out by a classical mean square error minimizaion method, using flooding volumes as the objective function. The calibration has been called ‘weak’ stated that the adopted objective function only partially reflects the hydraulic behaviour of the whole system. With the aim to estimate suitable values for the parameters, 13 historical rainfall events (recorded between 1993 and 1998) have been studied, which generated flooding in the past at many different locations inside the analysed area. The rainfall data have been collected at the Parco d’Orleans meteorological station, located near the analysed catchment, and have a temporal resolution of 5 seconds. After each of the selected rainfall events, the municipal Fire Brigades measured the flooded volumes in 12 different locations, and these values have been compared with the flooded volumes computed by the model. According to this procedure, the Manning roughness and the surface retention parameters have been calibrated; literature values, identified on the basis of the local soil characteristics, have been adopted for the infiltration parameters, because the flooded volumes results to be almost insensitive to these parameters. After calibration, simulations have been carried out using synthetic rainfalls. A classical three parameters IDF relationship formulation has been used here (Chu and Keifer 1957); IDF relationships have been obtained applying Gumbel probability law and interpolation. With the aim of testing the performance of each DSMP measures, different Chicago hyetographs have been constructed, changing all the possible parameters: duration, peak position and return period. Namely, different durations between 30 minutes and 10 hours have been considered, with peak positions at the beginning, in the middle and at the end of the event (defining the peak position r as the ratio between time to peak (tp) and rainfall duration d, 0 4 r 4 1). Three return periods have been analysed, namely 2, 5 and 10 years.

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having 2, 5 and 10 years return periods, respectively, and a duration of d ¼ 5 hours and r ¼ 0.5 (figure 4). Subsequently, distributed measures, showing a better performance with regards to the selected events, have been compared changing input rainfall duration and peak position in order to evaluate DSMPs robustness. Disconnection of impervious areas and detention of runoff from impervious areas proved to be the most effective measures. Runoff retention on impervious areas can be effective on short and frequent events but its efficiency decreases rapidly when increasing the rainfall return period. Measures applied to the pervious part of the catchment are only effective for high return period events. This is due to the fact that pervious areas have large natural retention and infiltration capacities that are able to ‘mitigate’ low intensity rainfalls. The performed analysis allowed for obtaining a first DSMPs classification on the basis of their mitigation efficiency. The next step has been the robustness analysis of the most promising DSMPs in order to evaluate their efficiency for different rainfall temporal pattern. Figure 5 shows the sensitivity of flooding volume changing rainfall event peak position and duration with respect to three mitigation measures selected in the previous stage, namely pervious area detention, impervious area disconnection and impervious area detention. Observing the graphs shown in figure 5, the following conclusions can be drawn: . detention on pervious surfaces is ineffective for short and frequent rainfalls, but becomes progressively more effective increasing duration, peak position and return period of the rainfall. For example, for the case study it

5. The simulation results As discussed in the previous paragraphs, a linearized sensitivity model has the great advantage of being simple and time-efficient, even if the linearity hypothesis must initially be verified. In the case study, the linearity hypothesis has been verified for all analysed parameters, for all input rainfalls. The maximum error resulting from the assumption of linearity was less than +10%. According to the procedure described above, the sensitivity of flooding volume has been analysed with respect to all the available mitigation measures. Initially, in all simulations the same input hyetographs have been used

Figure 4. Catchment-wide flooding volume reduction for synthetic events with duration d ¼ 5 hours, r ¼ 0.5 and variable frequency (Tr ¼ 2, 5 and 10 years).

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Figure 5. Catchment-wide flooding volume reduction for a 5-year event: (a) detention on pervious area; (b) disconnection of impervious area; (c) detention on impervious area.

was found that 100 m3/104 m2perv detention volume (equivalent to a 150% increase of Manning roughness) allows for a reduction of about 45% of the flooding volume, generated by a 5-year rainfall with 10 hours duration; . disconnection of impervious area is the most effective measure for flooding reduction in the Mondello catchment, confirming the results obtained from the runoff analysis: also for a 5-year event with 10 h duration, the flooding reduction efficiency is 0.43, i.e. 0.43% of flooding reduction for each percent of disconnected impervious area; . detention on impervious areas can be effective especially when it is not possible to disconnect impervious areas. For example, a 50 m3/104 m2imp detention volume (equivalent to 200% increase of the natural surface storage) allows for a reduction of about 24% of the flooding volume generated by a 5-year rainfall with 10 hours duration. Detention is also robust with respect to the temporal rainfall pattern. The results can also be spatially displayed in order to identify the locations where a specific DSMP measure is

most effective. Thus, sub-catchments where a measure is effective can be easily identified, allowing for a more effective planning decision. Figure 6 shows the results of a simulation where runoff peak flow sensitivity is related to impervious area disconnection for a short synthetic rainfall event. The figure shows efficiency variations for different sub-catchments: most urbanised areas in the central part of the catchment are characterised by better performance; lighter areas represent highly pervious sub-catchments where runoff generation from impervious areas is negligible. 5.1

Validation of the simplified methodology

The presented methodology compares distributed stormwater management practices to ‘improvements’ of catchment hydrological parameters. Changing the value of a parameter, it was possible to simulate the operational mode of the corresponding DSMP technique. Even if some of these correlations seem instinctively reliable, some doubts can arise from the application of a subcatchment-scale simplified approach for analysing house-by-house flooding mitigation measures. A validation procedure has been then carried out, comparing the proposed methodology (which simulates


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Figure 6. Impervious areas disconnection: peak flow reduction (‘Chicago’-type synthetic rainfall event with 2 hrs duration, r ¼ 0.5 and 5 years return period).

groups of houses as a single subcatchment) with a more sophisticated hydrodynamic approach which refines catchment discretization to the house level for simulating single DSMP measures hydraulic behaviour. Several examples are available in the literature for simulating DSMP structure hydraulic processes (Freni 2004, CIRIA 1996). These approaches have been used for integrating DSMP inside detailed urban drainage models in order to evaluate the impact of these practices on catchment response. As the sensitivity of the catchment to these practices will be investigated, a quantitative comparison between DSMPs detailed model representation and the proposed simplified approach is not needed because we will only compare sensitivity trends (figure 9). For this purpose, a small area in Mondello urban catchment has been selected. Applying the proposed simplified approach, the selected area (about 104 m2) has been analysed as a single subcatchment. In the detailed comparison approach, single surfaces (rooftops, gardens, streets, parking lots, etc.) have been analysed separately according to the scheme visualized in figure 7. In this detailed approach, the effect of impervious area disconnection can be analysed redirecting generated runoff over nearby pervious surfaces without admitting any

simplification. Storage devices can be analysed inside the drainage system model using a full hydrodynamic approach as described in the previous paragraphs. A generic infiltration structure can be analysed coupling storage hydrodynamic simulation with an infiltration model that, in the specific case study, has been taken from the widely used CIRIA 156 methodology (CIRIA 1996). The CIRIA approach makes the hypothesis that infiltration is possible only through mitigation structure sides, using a constant infiltration rate equal to saturated soil infiltration capacity; infiltrated discharge linearly depends on water level inside the structure. Comparing peak flow computed with the two approaches for different mitigation measures (figure 8), it can be noted that the simplified approach does not reduce model accuracy in estimating peak flow variation. The same behaviour has been found for runoff volume allowing for the acceptance of the simplified approach. In order to evaluate the influence of rainfall input on the accuracy of the simplified approach, a peak flow sensitivity index has been computed with the different approaches and compared (figure 9). This behaviour can be specifically explained considering that in the simplified approach the impervious areas disconnection fictitiously increases

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Figure 7. Validation sub-catchment: a coarse discretization (on the left) for simplified methodology application; a detailed discretization for hydrodynamic simulation (on the right).

Figure 8. Computed peak flow with the two different approaches for different mitigation measures (synthetic event: Tr ¼ 5 yrs; d ¼ 30 min; r ¼ 0.5): (a) impervious area retention; (b) impervious area disconnection; (c) impervious area detention.

the catchment pervious area, generating a greater effectiveness of disconnection measures. Even if differences increase according with the rainfall duration and peak position, they remain lower than 5% and can be accepted in a stormwater planning procedure. The differences can be connected to the coarser catchment discretization that generally increases concentration time.

6. Conclusions In the present paper, the DSMP techniques are analysed comparing them to ‘improvements’ of hydrological parameters of the catchment. Changing the value of a parameter, it was possible to simulate the operation mode of the corresponding DSMP technique: the DSMPs performances


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Figure 9. Peak flow reduction using disconnection measures for a 5-year event: (a) r ¼ 0; (b) r ¼ 0.5; (c) r ¼ 1.

can be analysed observing the response of the catchment to the variation of the hydrological parameters. To the aim of such analysis, an automatic procedure has been implemented which permits one to carry out—quickly and effectively—a widened sensitivity analysis involving all the parameters used in the proposed approach. The described procedure has been applied to some mathematical models simulating the runoff formation and propagation in urban areas. The adopted models are widely used ones, both in the professional practice and in the research activities, and are already implemented into the most common trading software aimed to simulate drainage systems. The analysis of the results allows for the identification of some guidelines for the mitigation plan preparation: in order to mitigate short and low return period rainfalls, the attention should be focused only on impervious areas, neglecting the runoff contributions coming from pervious surfaces; increasing rainfall duration, return period and peak position, the pervious areas runoff contributions becomes more important, so that a mitigation plan has to contain also interventions on these areas. While the impervious areas fast hydrological response does not allow for the use of retention facilities, which are only able to mitigate very short events, the presence of natural capacities on pervious areas allow for an efficient use of both detention and retention measures even if, for high return periods, retention facilities show the same limitations presented on impervious surfaces.

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