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Selection aid of alternative techniques in urban storm drainage — Proposition of an expert system
~
Pergamon
W.". Sci. TICIt. Vol. 39, No.4, pp. 241-248. 1999.
e 1999IAWQ
PIT: 50273-1223(99)00078-5
Published by Eloevl., Science LId Printed In Great Britain. All nllhla raetved 0273-1223199 $19'00 + 0'00
SELECTION AID OF ALTERNATIVE TECHNIQUES IN URBAN STORM DRAINAGE - PROPOSITION OF AN EXPERT SYSTEM S. Barraud*, Y. Azzout**, F. N. Cres** and B. Chocat* • URGC. Urban Hydrology INSA ofLyon, 20. avo A. Einstein, Bdtimeflt 304 69621 Villeurbanne Cedex, France • .. GBE. UMR 5569, Hydrology research teamlCC056, University ofMontpellier 2, 34 095 Montpellier Cedex, France
ABSTRACT 1be choice of BMPs in urban stonnwater drainage is most often made with a poor understanding of lite constraints, and of the possibilities afforded by thesc techniques. This leads 10 extra costs and
malfunctioning. To reach feasible choices, we have formalised a decision·making process. lakin. inlO account multiple criteria and the large number of p8ltners involved in that kind or proceu. We have then developed a decision-aiding procedure splil up into Iwo phases: an elimination phase permilJ us 10 exclude solutions which are identified as unworkable according to site considerations and a decision phase which allows the decision makers to compare and at least 10 choose all:enario built up with feasible solutions. This paper more panicularly deals with the first phase. This elimination phase. which is suitable ror an e~pen system type of represcnlation. has been implemented usin. a software called DELTANOE constructed for operational design issues but also for pedagogic purposes including conception. realisation. maintenance and management recommendations as well as sketches and photo examples. After presentin, the conte~t and basic principles of the elimination phase. we present the way the associated tool hu been constructed and Validated. II:> 1999 IAWQ Published by Elscvier Science Ltd. All righb reserved
KEYWORDS Best management practices: decision aiding system; expert system; software; urban storm water. INTRODUCTION
Although alternative techniques in urban stormwater management, such as pond, infiltration basin, swale, roof storage, ditch, soakaway, porous pavement, offer important possibilities in terms of urban development, and are often less costly and allow good protection against the risks of flooding and pollution of the receiVing water, they are still seldom used. Our analysis of this situation is that they are much more complex than the traditional sewer system. This is essentially due to the multiplicity of the technical solutions formed from multiple elements in close relationship with the urban system, allowing supplementary and new uses, 241
242
S. BARRAUD et al.
evolving in different ways compared to traditional drainage systems and at least demanding a large scale of competencies and standpoints (hydrology. structure and soil mechanics. hydrogeology. environmental impact.... as well as sociology. law. economics) for design. construction and maintenance. Finally, it has seemed necessary to develop a tool. which helps in the choice of urban stormwater drainage strategies to provide the "decision-makers" with the means to make the right investments. The stakes are important and are not only economic (best management. best protection against the risks of flooding, contribution to the proper management of water resources (protection of receiving waters). and making technical investments pay). It also implies giving designers a guide that allows them to imagine solutions and to integrate the management of water not simply as a constraint but as a new way of conceiving urban spaces. Finally. it means capitalising on. structuring and co-ordinating our knowledge in this field. Model\jne of the decision process; basic principles The model of help in the decision-making process we are proposing has many objectives. They include means of (Azzout et al.• 1995); aiding one or more decision-makers to formulate objectives; learning and helping in finding solutions; help in the acquisition of information. design progress. and identification of possible solutions. Thus. the method is much inspired by the decision models developed by H. A. Simon (1981).
Elimination
phase
Decision
phase
0-.0-<)··· . .in•• r.... _
.
. _.... O
Figure 1. Overview of the help process for making choices.
We have referred to the large number of possible scenarios (use of different principles of stonnwater drainage. of different constructions on different scales, whether centralised or scattered in the landscape, ete.).To define the problem of decision properly. it has been necessary to use a two-phase process (cf. Figure I): an elimination phase which makes it possible to eliminate the solutions which, depending on the site, are not feasible; a decision phase which allows the decision-makers to compare and at least to choose a scenario (including possible solutions) in keeping with their demands and their aims. In this paper we develop the first phase. Details of the second phase can be found in Azzout (1996) and Barraud et al. (1998).
Alternative techniques in urban storm drainage
243
ELIMINAnON PHASE: METHODOLOGY and discriminating criteria are Thi.s phase .can only be carried out. if the notion of solution is clarified was composed of researchers one This group. expert an of help the WIth done been h~ defmed. .Thts work s, civil engineering, urban mechanic soil logy, hydrogeo and engmeers from dIffer~nt fields (hydrology. ent and environment managem urban of atives represent together groups It sc!ence). human d~v~lopment a?d drainage system and ent environm road, for le responsib s mlOIsters, of dIfferent servIces of local authoritie management. Set of technical principles was more to choose a principle At this ~tage of the deci.sion process (preliminary study stage), the problem SOlutions, we have checked potential the define of s~lutlon than a precise defined drainage facility. To 1995): al., et (Azzout thus s principle l structura and phySical y, basin, pond, roof storage, Type of morphology (porous pavement, trench, ditch, swale, soakawa storage pipe, etc.). Type of functioning, e.g. (cf. Figure 2): through the surface, the way of introducing the rainwater in the structure (infiltration ~ localised injection after runoff, both of them). with matenals or not, the way of storing the water (in the open air, in buried structure filled ~ etc.). the way of emptying the work (by infiltration, by drains, etc.). ~ treatment facilities, the use of complementary equipment (bollom waterproofness. ~ etc.). compartments, on the type of morphology, which These different considerations permit us to define II families based type of functioning. They are the the with tion combina in s principle e alternativ technical 73 comprise following: le surface and with (or without) infiltration (or detention) porous pavement with (or without) permeab partitions and water treatment facilities, and with (or without) partitions infiltration (or detention) trench with (or without) permeable surface and water treatment facilities, t facilities, infiltration (or detention) ditch with (or without) partitions and water treatmen t facilities, treatmen water and s infiltration (or detention) swale with (or withoUl) partition t facilities, trelltmen water and s partition without) (or with y soakawa ) infiltration (or injection with (or without) and surface le permeab infiltration (or detention) platform with (or without) facilities, t treatmen partitions and water , dry infiltration (or detention) basin with (or without) water treatment facilities facilities. t treatmen water without) (or with basin ) detention (or n infiltratio underground retention pond with (or without) water facilities, roof storage, storage pipes. as the principles are general. After, We consider this set of principles as finished and transposable inasmuch principles. This set of principles proposed the to reply to gies technolo adapted choose to it will be possible dUring the procedure). modified not and (defined stable is comprises a sel of potential actions which of other elements in the exclusive is sel the of element (each global and context, extemal independent of e (as exhaustive as it can be!) due to procedure of selection) (Roy, 1985). Moreover, it seems to us exhaustiv network. expert our of plurality the and ge knowled of Slate the actual
244
S. BARRAUD tl al.
DiscrimiDatin~
criteria
To define discriminating criteria properly, we have opted for not eliminating the technical principle wrongly. We constructed a coherent set, thus the criteria would be "exhaustive", representative, non-redundant but also practically usable (Roy, 1985; Roy et al., 1993). We took care of the fact that the demanded information should be standard and easy to obtain. The definition given for each of the criteria is crucial since it consequently conditions its evaluation by the different partners. In fact, we essentially found physical information but also a criterion depending on the object of the study such as drainage of roadways, buildings, pedestrian areas, small pieces of land and large areas (residential. commercial, industrial area, parks and gardens). In fact, it is obvious for example that draining a roadway does not require knowing the slope of the roofs and that the availability of space is unnecessary if the use of a porous pavement is wished. Finally, 19 criteria were determined. Table I collects the set of criteria and specifies the adopted scale for the evaluation. We have explained the means to evaluate them in Azzout (1996) more precisely. The validation of the axioms and the "exhaustivity", the non-redundancy was iterative. Initially, we formulated a first list of criteria that seemed to answer our problems. Then, we presented it to the members of our expert network. Finally, this list, modified and enriched, was stabilised. The expert system, presented later in this paper, made this verification easier. Table I. List of discriminating criteria (Azzout et al., 1995) INCIDENCE OF CONSTRAINT CLASS
CRITERIA
CONSTRAINT CLASS
Soil behaviour in the presence of water Vulnerability of underground water to rain waler Absorplion abihty at the soil surface Absorption abihty of the SOil below the surface High water level of the water table
Bad Yes
use of detention techniques use of detention techniques
bad «lo- m/s) ' bad «IO·1m/s)
use of detention techniques use of detention techniques
Risk of polluted water
Bearing capacity Permanent outlet Availability of space (space beside a pavement, car park beside a building) Risk of lilt-bearing water
Approaching ground level average· high·· bad (P<2)
No No Yes
Type of traffic Presence of important shear forces Mountain climate
heavy (H2) Yes Yes
Site .lope (part to be drained, possible availability of .pace) Continual arrival of water Low water level of the water table
alight to steep
Roof .Iope .teep Buildmg suitable for atorage
No does not approach ground level average «5%)· steep (>5%)·· No
use of detention techniques, use of injection wells and water basins (.) use of water treatment faclhties (••) use of detention techniques and treatment facilities impossible to use infiltration permeable surface or linear techniques for drainage of a pavement problem to drain the area using detention techniques impossible to use techniques requiring a larger area than the area to be drained need for treatment facility and the use of waterproof lining use of waterproof lining for permeable pavements use of waterproof lining for permeable pavements use of waterproof lining for permeable pavements, no roof storage installation of partitions retenhon pond cannot be used retention pond cannot be used (.) installation of partitions on roof (..) no roof storage no roof storage
Alternative techniques in urban stann drainage
20
Table 2. Example of combinations to obtain a particular technical principle (empty sections do not require a response) (Azzout. 1996) CRITERIA
COMBINATION I
Soil behaviour in the presence of water
COMBINATION 2
COMBINATION 3
COMBINATJON4
Bad
Vulnerability ofundersround water
Yes
Bad
Absorption ability at the soil surface High water level of the water table
Approaching around level Weak
Weak
Weak
Weak
Permanent outlet
Yes
Ves
Ves
Ves
Risk of silt-bearing water
No
No
No
Risk ofpolluted water Bearing capacity
High
Type of traffic Presence of large shear forces
Ves Weak
Site slope TECHNICAL PRINCIPLE
No High
Ves Weak
Weak
Weak
~ I I ~ Detention porous pavement with waterproof lining
Construction of identification sheet For each potential solution, we have evaluated the different criteria, several combinations permitting us to obtain a similar principle (cf. examples in Table 2). These identification sheets have been translated into rules "if condilion then action". For example: IF
THEN
Soil behaviour with the presence of water is bad OR the groundwater is vulnerable OR absorption ability of the soil surface is bad OR absorption ability of the underground is bad OR high level of the water table is approaching ground level AND Risk of polluted water is low AND permanent outlet is possible AND the risk of silt-bearing water is weak AND the site slope is steep The use of" Detention trench with partitions" is possible. PRESENTAnON OF THE EXPERT SYSTEM: DELTANOE
Due to the organisation of the rules, this phase is suitable for an expert system type of representation. We therefore put it to the test with a prototype, which gave good results. We recall that the tests also allowed us to refine the set of discriminating criteria. The expert system is normally organised on the basis of rules constructed from discriminating criteria. The user enters the facts relative to each site. The rules and data are processed by an expert system developed incidentally by eres (1989) in Turbo Prolog. Now this expert system is developed in the Windows environment. It makes it more user-friendly and easier to use. This system allows different possibilities (search for a possible solution. search for the technical feasibility of a particular solution with request for explanation in case of failure). The system also edits the description
246
S. BARRAUD et al.
of each technique with its characteristics as well as gives general advice as regards design, execution and management. Recently, we have added a pre-dimensioning module. The different menus of the computer system are as follows.
Main menu Four choices are offered: - Files ("Etude"): to manage the different files. - Expertise: to search solutions adapted to a site or to verify the feasibility of a particular solution on a given site. Pre-dimensioning ("Predimensionnement"): to basically calculate the rainwater volume to store. - Help ("Aide"): to give some information about program, criteria, solutions, etc. o
"Expertise" Menu If the user wants to determine the solutions adapted to his site, he has to answer different questions about criteria. In the expertise phase, facts relative to a site are entered. A possibility of explanation is proposed if the question does not seem clear or is not in accordance with the problem.
Figure 2. Expertise phase - infonnation related to a solution deduced by the system.
At the end of the expertise, a set of solutions is proposed. Two levels of information can be reached for each of them (cf. Figure 2): - a short description; - a complete knowledge basis including advice for designing, construction, maintenance illustrated by figures and photos. "VALIDATION" AND LESSONS DRAWN BY THE USE OF DELTANOE We have validated our methodology according to two levels: an internal and an external validation. The internal validation turned to the model structuring (pertinence of the proposed processes, of criteria, of chosen rules). This validation took place during the construction of the method itself and leaned on the expert network and an important work of synthesis (Azzout et al., 1994).
Alternative techniques in urban storm drainage
247
The ~xtema~ valida~ion was meant to legitimate the result of our methodology in assessing its contribution and Its pertmence m terms of results. This level of validation is difficult and is generally absent in the methodologies proposed in the literature. In order to "validate" the pertinence of such a tool we have compared decisions wit~ and without the proposed model. It has been done considering six real c~ studies of dIfferent natures at dtfferent scales (from the drainage of a simple parking lot to a zone of about SO hal (Azzout, 1996). To make the comparisons, we have used the same amount and the same quality of data available at the moment of the "real" design. In a great majority of cases, the solutions obtained by using the expert system were not contradictory with the ones really chosen. However, the list proposed by the system was much richer. Further prescriptions and constraints not verified during the real project were so with the expert system (need for partitioning or for pre-treatment facilities, or need for further study, for example). The cases where the solutions were not in accordance with those given by our methodology are interesting. The first one deals with the drainage of a parking lot where a porous structUre with permeable surface has been chosen and constructed. The expert system has not proposed this solution "arguing" that a probable amount of silt due to the state of the surroundings of the zone would risk clogging the permeable surface. This solution was ruled out by a second criterion: shearing due to traffic and frequent track manoeuvring might again clog the permeable surface. This zone has existed for seven years now. and is actually getting clogged. An analysis of this case shows that the desire to be innovative had blinded the designer and led to an under estimate of the constraints. We can notice other innovative solutions could have been chosen (porous pavement without permeable surface for example). The second case study deals with a public zone where soakaways have been implanted. The expert system did not propose such a structure because the water table was too fluctuant and thus the risk of direct pollution of the groundwater could have been serious. In this case. the analysis of the decision shows that no other solution had been considered because of a bad knowledge of other techniques. The discrepancy in findings here results from no alternative to a soakway having been envisaged. Moreover, questions regarding a certain number of criteria had not been examined, even though information would have been readily available (the problem for the local council being mainly to deal as quickly as possible with the overflow which was becoming chronic there). These examples well illustrate the usefulness of such an instrument. Not only does it make available all of the application conditions for the various techniques, but it also enables them to be combined so as to limit still all too common design faults. CONCLUSION The procedure we have proposed has already been partly validated by stormwater drainage professionals and town planners. The information necessary for the establishment of the different criteria and the definition of alternative techniques are based on a knowledge synthesis, and the expertise of a group coming from different fields (AZlOUt et aJ., 1994). The advantage of this phase of elimination is that it requires a relatively small number of criteria which are often already familiar to the designers. However, this tool, which may not seem to out-perform a competent engineer, In fact enabl~s exhaustive examination of conditions to be fulfilled for the techniques to be adopted, broadens the chOIces made by designers. who always tend to go for the same solutions and brings into play all the kinds of equipment backing up the techniques. Moreover, the proposed expert system, besides its help in the selection of viable techniques. turns out ~o be an excellent pedagogical tool (conception, realisation, maintenance and management recommendatlons, sketch and photo examples...).
248
S. BARRAUD et al.
REFERENCES Azzout, Y.• Barraud. S.• Ctes. F. N. and Alfakih. E. (1994). Alternative Techniques in Stormwater Drainage. Paris: Ed. Tec and Doc de Lavoisier. 372 p. (in French). Azzout. Y.• Barraud. S.• Ctes. F. N. and Alfakih. E. (1995). Decision aids fot alternative techniques in urban stonn management. Wat. Sci. Tech.• 32(1). 41-48. Azzout. Y. (1996). Decision aids for alternative techniques in urban stonn management. PhD thesis. URGC. Urban Hydrology. lnstitut National des Sciences Appliquees de Lyon. 245 p (in French). Barraud. Soo Azzout. Y. and Cres. P. N. (1998). Decision aiding methodology for designing and selecting alternative techniques in the field of urban stonn drainage. Journal ofDecision System (in press). Cres. F. N. (1989). The Contributions of Expert Systems to the Science of Water. PROMISE: a Project Simulator. MOISE: a Faultfinding System for Autonomous Sewerage Disposal. PhD thesis. Ecole des Mines de Paris et St Etienne (in French). Roy. B. (1985). Multicriteria Methodology in the Decision Helps. Paris: Ed. Economica. 423 p. (in French). Roy. B. and Bouyssou. D. (1993). Multicriteria Decision Help: Methodology and Cases. Paris: Ed. Economica. 695 p. (in French). Simon. H. A. (1981). The Science. of the Artificial. Cambridge MIT Press. 230 p.
Pergamon
W.". Sci. TICIt. Vol. 39, No.4, pp. 241-248. 1999.
e 1999IAWQ
PIT: 50273-1223(99)00078-5
Published by Eloevl., Science LId Printed In Great Britain. All nllhla raetved 0273-1223199 $19'00 + 0'00
SELECTION AID OF ALTERNATIVE TECHNIQUES IN URBAN STORM DRAINAGE - PROPOSITION OF AN EXPERT SYSTEM S. Barraud*, Y. Azzout**, F. N. Cres** and B. Chocat* • URGC. Urban Hydrology INSA ofLyon, 20. avo A. Einstein, Bdtimeflt 304 69621 Villeurbanne Cedex, France • .. GBE. UMR 5569, Hydrology research teamlCC056, University ofMontpellier 2, 34 095 Montpellier Cedex, France
ABSTRACT 1be choice of BMPs in urban stonnwater drainage is most often made with a poor understanding of lite constraints, and of the possibilities afforded by thesc techniques. This leads 10 extra costs and
malfunctioning. To reach feasible choices, we have formalised a decision·making process. lakin. inlO account multiple criteria and the large number of p8ltners involved in that kind or proceu. We have then developed a decision-aiding procedure splil up into Iwo phases: an elimination phase permilJ us 10 exclude solutions which are identified as unworkable according to site considerations and a decision phase which allows the decision makers to compare and at least 10 choose all:enario built up with feasible solutions. This paper more panicularly deals with the first phase. This elimination phase. which is suitable ror an e~pen system type of represcnlation. has been implemented usin. a software called DELTANOE constructed for operational design issues but also for pedagogic purposes including conception. realisation. maintenance and management recommendations as well as sketches and photo examples. After presentin, the conte~t and basic principles of the elimination phase. we present the way the associated tool hu been constructed and Validated. II:> 1999 IAWQ Published by Elscvier Science Ltd. All righb reserved
KEYWORDS Best management practices: decision aiding system; expert system; software; urban storm water. INTRODUCTION
Although alternative techniques in urban stormwater management, such as pond, infiltration basin, swale, roof storage, ditch, soakaway, porous pavement, offer important possibilities in terms of urban development, and are often less costly and allow good protection against the risks of flooding and pollution of the receiVing water, they are still seldom used. Our analysis of this situation is that they are much more complex than the traditional sewer system. This is essentially due to the multiplicity of the technical solutions formed from multiple elements in close relationship with the urban system, allowing supplementary and new uses, 241
242
S. BARRAUD et al.
evolving in different ways compared to traditional drainage systems and at least demanding a large scale of competencies and standpoints (hydrology. structure and soil mechanics. hydrogeology. environmental impact.... as well as sociology. law. economics) for design. construction and maintenance. Finally, it has seemed necessary to develop a tool. which helps in the choice of urban stormwater drainage strategies to provide the "decision-makers" with the means to make the right investments. The stakes are important and are not only economic (best management. best protection against the risks of flooding, contribution to the proper management of water resources (protection of receiving waters). and making technical investments pay). It also implies giving designers a guide that allows them to imagine solutions and to integrate the management of water not simply as a constraint but as a new way of conceiving urban spaces. Finally. it means capitalising on. structuring and co-ordinating our knowledge in this field. Model\jne of the decision process; basic principles The model of help in the decision-making process we are proposing has many objectives. They include means of (Azzout et al.• 1995); aiding one or more decision-makers to formulate objectives; learning and helping in finding solutions; help in the acquisition of information. design progress. and identification of possible solutions. Thus. the method is much inspired by the decision models developed by H. A. Simon (1981).
Elimination
phase
Decision
phase
0-.0-<)··· . .in•• r.... _
.
. _.... O
Figure 1. Overview of the help process for making choices.
We have referred to the large number of possible scenarios (use of different principles of stonnwater drainage. of different constructions on different scales, whether centralised or scattered in the landscape, ete.).To define the problem of decision properly. it has been necessary to use a two-phase process (cf. Figure I): an elimination phase which makes it possible to eliminate the solutions which, depending on the site, are not feasible; a decision phase which allows the decision-makers to compare and at least to choose a scenario (including possible solutions) in keeping with their demands and their aims. In this paper we develop the first phase. Details of the second phase can be found in Azzout (1996) and Barraud et al. (1998).
Alternative techniques in urban storm drainage
243
ELIMINAnON PHASE: METHODOLOGY and discriminating criteria are Thi.s phase .can only be carried out. if the notion of solution is clarified was composed of researchers one This group. expert an of help the WIth done been h~ defmed. .Thts work s, civil engineering, urban mechanic soil logy, hydrogeo and engmeers from dIffer~nt fields (hydrology. ent and environment managem urban of atives represent together groups It sc!ence). human d~v~lopment a?d drainage system and ent environm road, for le responsib s mlOIsters, of dIfferent servIces of local authoritie management. Set of technical principles was more to choose a principle At this ~tage of the deci.sion process (preliminary study stage), the problem SOlutions, we have checked potential the define of s~lutlon than a precise defined drainage facility. To 1995): al., et (Azzout thus s principle l structura and phySical y, basin, pond, roof storage, Type of morphology (porous pavement, trench, ditch, swale, soakawa storage pipe, etc.). Type of functioning, e.g. (cf. Figure 2): through the surface, the way of introducing the rainwater in the structure (infiltration ~ localised injection after runoff, both of them). with matenals or not, the way of storing the water (in the open air, in buried structure filled ~ etc.). the way of emptying the work (by infiltration, by drains, etc.). ~ treatment facilities, the use of complementary equipment (bollom waterproofness. ~ etc.). compartments, on the type of morphology, which These different considerations permit us to define II families based type of functioning. They are the the with tion combina in s principle e alternativ technical 73 comprise following: le surface and with (or without) infiltration (or detention) porous pavement with (or without) permeab partitions and water treatment facilities, and with (or without) partitions infiltration (or detention) trench with (or without) permeable surface and water treatment facilities, t facilities, infiltration (or detention) ditch with (or without) partitions and water treatmen t facilities, treatmen water and s infiltration (or detention) swale with (or withoUl) partition t facilities, trelltmen water and s partition without) (or with y soakawa ) infiltration (or injection with (or without) and surface le permeab infiltration (or detention) platform with (or without) facilities, t treatmen partitions and water , dry infiltration (or detention) basin with (or without) water treatment facilities facilities. t treatmen water without) (or with basin ) detention (or n infiltratio underground retention pond with (or without) water facilities, roof storage, storage pipes. as the principles are general. After, We consider this set of principles as finished and transposable inasmuch principles. This set of principles proposed the to reply to gies technolo adapted choose to it will be possible dUring the procedure). modified not and (defined stable is comprises a sel of potential actions which of other elements in the exclusive is sel the of element (each global and context, extemal independent of e (as exhaustive as it can be!) due to procedure of selection) (Roy, 1985). Moreover, it seems to us exhaustiv network. expert our of plurality the and ge knowled of Slate the actual
244
S. BARRAUD tl al.
DiscrimiDatin~
criteria
To define discriminating criteria properly, we have opted for not eliminating the technical principle wrongly. We constructed a coherent set, thus the criteria would be "exhaustive", representative, non-redundant but also practically usable (Roy, 1985; Roy et al., 1993). We took care of the fact that the demanded information should be standard and easy to obtain. The definition given for each of the criteria is crucial since it consequently conditions its evaluation by the different partners. In fact, we essentially found physical information but also a criterion depending on the object of the study such as drainage of roadways, buildings, pedestrian areas, small pieces of land and large areas (residential. commercial, industrial area, parks and gardens). In fact, it is obvious for example that draining a roadway does not require knowing the slope of the roofs and that the availability of space is unnecessary if the use of a porous pavement is wished. Finally, 19 criteria were determined. Table I collects the set of criteria and specifies the adopted scale for the evaluation. We have explained the means to evaluate them in Azzout (1996) more precisely. The validation of the axioms and the "exhaustivity", the non-redundancy was iterative. Initially, we formulated a first list of criteria that seemed to answer our problems. Then, we presented it to the members of our expert network. Finally, this list, modified and enriched, was stabilised. The expert system, presented later in this paper, made this verification easier. Table I. List of discriminating criteria (Azzout et al., 1995) INCIDENCE OF CONSTRAINT CLASS
CRITERIA
CONSTRAINT CLASS
Soil behaviour in the presence of water Vulnerability of underground water to rain waler Absorplion abihty at the soil surface Absorption abihty of the SOil below the surface High water level of the water table
Bad Yes
use of detention techniques use of detention techniques
bad «lo- m/s) ' bad «IO·1m/s)
use of detention techniques use of detention techniques
Risk of polluted water
Bearing capacity Permanent outlet Availability of space (space beside a pavement, car park beside a building) Risk of lilt-bearing water
Approaching ground level average· high·· bad (P<2)
No No Yes
Type of traffic Presence of important shear forces Mountain climate
heavy (H2) Yes Yes
Site .lope (part to be drained, possible availability of .pace) Continual arrival of water Low water level of the water table
alight to steep
Roof .Iope .teep Buildmg suitable for atorage
No does not approach ground level average «5%)· steep (>5%)·· No
use of detention techniques, use of injection wells and water basins (.) use of water treatment faclhties (••) use of detention techniques and treatment facilities impossible to use infiltration permeable surface or linear techniques for drainage of a pavement problem to drain the area using detention techniques impossible to use techniques requiring a larger area than the area to be drained need for treatment facility and the use of waterproof lining use of waterproof lining for permeable pavements use of waterproof lining for permeable pavements use of waterproof lining for permeable pavements, no roof storage installation of partitions retenhon pond cannot be used retention pond cannot be used (.) installation of partitions on roof (..) no roof storage no roof storage
Alternative techniques in urban stann drainage
20
Table 2. Example of combinations to obtain a particular technical principle (empty sections do not require a response) (Azzout. 1996) CRITERIA
COMBINATION I
Soil behaviour in the presence of water
COMBINATION 2
COMBINATION 3
COMBINATJON4
Bad
Vulnerability ofundersround water
Yes
Bad
Absorption ability at the soil surface High water level of the water table
Approaching around level Weak
Weak
Weak
Weak
Permanent outlet
Yes
Ves
Ves
Ves
Risk of silt-bearing water
No
No
No
Risk ofpolluted water Bearing capacity
High
Type of traffic Presence of large shear forces
Ves Weak
Site slope TECHNICAL PRINCIPLE
No High
Ves Weak
Weak
Weak
~ I I ~ Detention porous pavement with waterproof lining
Construction of identification sheet For each potential solution, we have evaluated the different criteria, several combinations permitting us to obtain a similar principle (cf. examples in Table 2). These identification sheets have been translated into rules "if condilion then action". For example: IF
THEN
Soil behaviour with the presence of water is bad OR the groundwater is vulnerable OR absorption ability of the soil surface is bad OR absorption ability of the underground is bad OR high level of the water table is approaching ground level AND Risk of polluted water is low AND permanent outlet is possible AND the risk of silt-bearing water is weak AND the site slope is steep The use of" Detention trench with partitions" is possible. PRESENTAnON OF THE EXPERT SYSTEM: DELTANOE
Due to the organisation of the rules, this phase is suitable for an expert system type of representation. We therefore put it to the test with a prototype, which gave good results. We recall that the tests also allowed us to refine the set of discriminating criteria. The expert system is normally organised on the basis of rules constructed from discriminating criteria. The user enters the facts relative to each site. The rules and data are processed by an expert system developed incidentally by eres (1989) in Turbo Prolog. Now this expert system is developed in the Windows environment. It makes it more user-friendly and easier to use. This system allows different possibilities (search for a possible solution. search for the technical feasibility of a particular solution with request for explanation in case of failure). The system also edits the description
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of each technique with its characteristics as well as gives general advice as regards design, execution and management. Recently, we have added a pre-dimensioning module. The different menus of the computer system are as follows.
Main menu Four choices are offered: - Files ("Etude"): to manage the different files. - Expertise: to search solutions adapted to a site or to verify the feasibility of a particular solution on a given site. Pre-dimensioning ("Predimensionnement"): to basically calculate the rainwater volume to store. - Help ("Aide"): to give some information about program, criteria, solutions, etc. o
"Expertise" Menu If the user wants to determine the solutions adapted to his site, he has to answer different questions about criteria. In the expertise phase, facts relative to a site are entered. A possibility of explanation is proposed if the question does not seem clear or is not in accordance with the problem.
Figure 2. Expertise phase - infonnation related to a solution deduced by the system.
At the end of the expertise, a set of solutions is proposed. Two levels of information can be reached for each of them (cf. Figure 2): - a short description; - a complete knowledge basis including advice for designing, construction, maintenance illustrated by figures and photos. "VALIDATION" AND LESSONS DRAWN BY THE USE OF DELTANOE We have validated our methodology according to two levels: an internal and an external validation. The internal validation turned to the model structuring (pertinence of the proposed processes, of criteria, of chosen rules). This validation took place during the construction of the method itself and leaned on the expert network and an important work of synthesis (Azzout et al., 1994).
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The ~xtema~ valida~ion was meant to legitimate the result of our methodology in assessing its contribution and Its pertmence m terms of results. This level of validation is difficult and is generally absent in the methodologies proposed in the literature. In order to "validate" the pertinence of such a tool we have compared decisions wit~ and without the proposed model. It has been done considering six real c~ studies of dIfferent natures at dtfferent scales (from the drainage of a simple parking lot to a zone of about SO hal (Azzout, 1996). To make the comparisons, we have used the same amount and the same quality of data available at the moment of the "real" design. In a great majority of cases, the solutions obtained by using the expert system were not contradictory with the ones really chosen. However, the list proposed by the system was much richer. Further prescriptions and constraints not verified during the real project were so with the expert system (need for partitioning or for pre-treatment facilities, or need for further study, for example). The cases where the solutions were not in accordance with those given by our methodology are interesting. The first one deals with the drainage of a parking lot where a porous structUre with permeable surface has been chosen and constructed. The expert system has not proposed this solution "arguing" that a probable amount of silt due to the state of the surroundings of the zone would risk clogging the permeable surface. This solution was ruled out by a second criterion: shearing due to traffic and frequent track manoeuvring might again clog the permeable surface. This zone has existed for seven years now. and is actually getting clogged. An analysis of this case shows that the desire to be innovative had blinded the designer and led to an under estimate of the constraints. We can notice other innovative solutions could have been chosen (porous pavement without permeable surface for example). The second case study deals with a public zone where soakaways have been implanted. The expert system did not propose such a structure because the water table was too fluctuant and thus the risk of direct pollution of the groundwater could have been serious. In this case. the analysis of the decision shows that no other solution had been considered because of a bad knowledge of other techniques. The discrepancy in findings here results from no alternative to a soakway having been envisaged. Moreover, questions regarding a certain number of criteria had not been examined, even though information would have been readily available (the problem for the local council being mainly to deal as quickly as possible with the overflow which was becoming chronic there). These examples well illustrate the usefulness of such an instrument. Not only does it make available all of the application conditions for the various techniques, but it also enables them to be combined so as to limit still all too common design faults. CONCLUSION The procedure we have proposed has already been partly validated by stormwater drainage professionals and town planners. The information necessary for the establishment of the different criteria and the definition of alternative techniques are based on a knowledge synthesis, and the expertise of a group coming from different fields (AZlOUt et aJ., 1994). The advantage of this phase of elimination is that it requires a relatively small number of criteria which are often already familiar to the designers. However, this tool, which may not seem to out-perform a competent engineer, In fact enabl~s exhaustive examination of conditions to be fulfilled for the techniques to be adopted, broadens the chOIces made by designers. who always tend to go for the same solutions and brings into play all the kinds of equipment backing up the techniques. Moreover, the proposed expert system, besides its help in the selection of viable techniques. turns out ~o be an excellent pedagogical tool (conception, realisation, maintenance and management recommendatlons, sketch and photo examples...).
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REFERENCES Azzout, Y.• Barraud. S.• Ctes. F. N. and Alfakih. E. (1994). Alternative Techniques in Stormwater Drainage. Paris: Ed. Tec and Doc de Lavoisier. 372 p. (in French). Azzout. Y.• Barraud. S.• Ctes. F. N. and Alfakih. E. (1995). Decision aids fot alternative techniques in urban stonn management. Wat. Sci. Tech.• 32(1). 41-48. Azzout. Y. (1996). Decision aids for alternative techniques in urban stonn management. PhD thesis. URGC. Urban Hydrology. lnstitut National des Sciences Appliquees de Lyon. 245 p (in French). Barraud. Soo Azzout. Y. and Cres. P. N. (1998). Decision aiding methodology for designing and selecting alternative techniques in the field of urban stonn drainage. Journal ofDecision System (in press). Cres. F. N. (1989). The Contributions of Expert Systems to the Science of Water. PROMISE: a Project Simulator. MOISE: a Faultfinding System for Autonomous Sewerage Disposal. PhD thesis. Ecole des Mines de Paris et St Etienne (in French). Roy. B. (1985). Multicriteria Methodology in the Decision Helps. Paris: Ed. Economica. 423 p. (in French). Roy. B. and Bouyssou. D. (1993). Multicriteria Decision Help: Methodology and Cases. Paris: Ed. Economica. 695 p. (in French). Simon. H. A. (1981). The Science. of the Artificial. Cambridge MIT Press. 230 p.