An approach to simulation of dual drainage

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War. Sci. Sci. Tech. Tech. Vol. Vol. 39, 39, No.9, No. 9, pp. pp. 95-103, 95-103, 1999 1999 War. 0 1999IAWQ 1999 lAWQ ©

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AN APPROACH APPROACH TO SIMULATION SIMULATION OF AN DUAL DRAINAGE DRAINAGE DUAL Y S. Djordjevic*, DjordjeviC*, D. Prodanovic* ProdanoviC* and and C. Maksimovic** MaksimoviC** S. *Faculty ofCivil of Civil Engineering. Engineering, University Universi(y ofBelgrade. of Belgrade, Bulevar Bukvar revolucije revolucije 73. 73, 'Faculty P. 0. Box Box 895. 895. 11000 II000 Belgrade. Belgrade, Yugoslavia Yugoslavia P.D. **Department ofCivil of Civil & & Environmental Environmental Engineering. Engineering, Imperial Imperial College College ofScience, of Science, "Department Technology && Medicine. Medicine, Imperial Imperial College College Road, Road, London London SW7 SW7 2BU, 2BlJ, UK UK Technology

ABSTRACT ABSTRACT The paper paper presents the development development in in the the field field of of urban drainage modelling modelling known known as as dual dual drainage drainage -- an an The presents the urban drainage approach to rainfall runoff runoff simulation simulation in in which which the the numerical numerical model takes into into account account not not only only the the flow flow approach to rainfall model takes through the sewer system, system, but but also also the the flow flow on ou the surface. The The steps model development development are are described, described, the surface. steps in in model through the sewer and necessary necessary data, data, assumptions assumptions used operations to to be be performed performed using using GIS GIS are are discussed. discussed. The The and operations and used and numerical simultaneously handles dynamic equations of flow flow through through the the sewer sewer system system and and numerical model model simultaneously handles the the full full dynamic equations of simplified of the the surface The surface water (due (due to the limited limited capacity capacity of of inlets inlets or or to to simplified equations equations of surface flow. flow. The surface excess excess water to the the hydraulic head in the sewer system reaching the ground level) is routed to the neighbour subcatchment the hydraulic head in the sewer system reaching the ground level) is routed to the neighbour subcatchment (not attached to node), using using surface surface retentions, retentions, if if any. any. 0© 1999 1999 (not necessarily necessarily the the one one attached to the the downstream downstream network network node), IAWQ Published by Elsevier Science Ltd. All rights reserved IAWQ Published by Elsevier Science Ltd. All rights reserved

KEYWORDS KEYWORDS Storm Storm sewer sewer systems; systems; dual dual drainage; drainage; GE. GIS. INTRODUCTION INTRODUCTION The development of high end numerical models that simulate the real world processes has been boosted up The development of high end numerical models that simulate the real world processes has been boosted up by the use of Geographic Information Systems (GE). Yesterday’s lumping and averaging skills have been by the use of Geographic Information Systems (GIS). Yesterday's lumping and averaging skills have been changed owing to the ability to understand and simulate the physical processes at a finer scale. The changed owing to the ability to understand and simulate the physical processes at a finer scale. The simulation of rainfall-runoff consequences in urbanized areas has reached the point where highly accurate simulation of rainfall-runoff consequences in urbanized areas has reached the point where highly accurate models of sewer pipe flow are available, while the surface flow component, being much more complicated, models of sewer pipe flow are available, while the surface flow component, being much more complicated, still needs improvement of conceptual abstraction and numerical solution techniques. still needs improvement of conceptual abstraction and numerical solution techniques.

A detailed description of further development in the field of urban drainage modelling known as dual A detailed description of further development in the field of urban drainage modelling known as dual drainage is presented in this paper. It is an approach to urban rainfall-runoff simulation in which the drainage is presented in this paper. It is an approach to urban rainfall-runoff simulation in which the numerical model takes into account not only the flow through the sewer system, but also the flow along the numerical model takes into account not only the flow through the sewer system, but also the flow along the streets under certain conditions. The GIS with its powerful analytical capabilities is to be used to cope with a streets under certain conditions. The GIS with its powerful analytical capabilities is to be used to cope with a vast amount of new information about the surface. vast amount of new information about the surface.

The dual drainage model has two interactive parts, or two networks. The underground part consists of a of a The dual drainage two interactive parts, or two networks. underground part consists sewer system, with model known has manholes, inlets and control structures. TheThe surface part is made of channels, sewer system, with known manholes, inlets and control structures. The surface part is made of channels, natural flow paths, retention basins in local depressions or artificial control structures (brinks, ponds). The natural flow paths, in local depressions artificial structures topology of the sewerretention network basins is defined a priori, whereasor the surfacecontrol flow paths depend(brinks, on theponds). terrain The and of the sewer network is defined a priori, whereas the surface flow paths depend on the topology on water levels themselves. The GIS is used to perform a set of terrain analyses, mainly in order toterrain create and the on water levels themselves. The GIS is used to perform a set of terrain analyses, mainly in order to create the 95

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surface surface flow flow pattern, pattern, extract extract information information about about natural natural ponds ponds and and link link them them with with retention retention basins, basins, create create the the subcatchment subcatchment for for each each sewer sewer manhole manhole and and link link those those subcatchments subcatchments to to aa surface surface flow flow network network (Maksimovic (MaksimoviC et al., 1994). 1994). et The The results results of of the the dual dual drainage drainage simulation simulation are the the standard standard sewer sewer output output hydrograph, hydrograph, the the hydrograph hydrograph and and the volume volume of of water water that left left the the system system or that stayed stayed in in ponds, ponds, and and levels levels in in flooded flooded areas. The The GIS GIS isis used used to to read read those those data data and to to create the time time series series of of flood flood maps. maps. For For aa selected selected time time frame, frame, the the GIS GIS can can assess assess of damage damage caused caused by flooding. flooding. the costs of

MODEL ABSTRACTION ABSTRACTION MODEL The real real world world of of urban urban catchment catchment and processes processes involved involved in rainfall rainfall to runoff runoff transformation transformation are too The complex to be simulated simulated in detail detail by any any numerical numerical model. model. In In order order to create create aa usable usable simulation simulation tool, tool, some some complex kind of of simplification simplification has to be done, done, either either without without taking taking into into account account the the physical physical processes, processes, or or by kind simplifying some some of of them. them. simplifying The very very first first approach approach in in urban urban drainage drainage modeling modeling was was to to establish establish the the link link between between rainfall rainfail and and observed observed The out flow, flow, using using a number number of of different different nonphysical nonphysical parameters. parameters. The The improvement improvement of of the first first models models was when they were were applied applied to smaller smaller parts parts of of catchment catchment down down to the level level of of one one subcatchment. subcatchment. After After careful careful when they calibration of of such such aa model model on on aa single single catchment catchment on on the the basis basis of of measured measured rainfall rainfall and and out out flows, flows, itit was was calibration possible to estimate estimate the the sewer sewer system system response response to to aa certain certain range range of of storms. storms. However, However, the the model model accuracy accuracy possible to for heavy storms was was questionable. Another drawback drawback of of these these models models was was their their lack lack of of nonphysical nonphysical for heavy storms questionable. Another parameters, that the the model model created created without without calibration calibration was was rather rather aa fancy fancy hydrograph hydrograph generator generator than than aa parameters, so so that serious tool. serious simulation simulation tool. To the dubiousness dubiousness of nonphysical urban urban drainage drainage models, models, the the simulation simulation of of some some physical physical of nonphysical To overcome overcome the processes in the rainfall to runoff transformation was introduced. The entire process was divided into two processes in the rainfall to runoff transformation was introduced. The entire process was divided into two main phases: the deals with with the the rainfall rainfall and and its its conversion the effective effective runoff runoff from from each each main phases: the first first one one deals conversion into into the subcatchment account soil retention capacities capacities of the surface, surface, different different types types of of areas areas of the subcatchment (taking (taking into into account soil infiltration, infiltration, retention on the surface, flow along the subcatchment, etc.as input to the second one flow in the sewer system on the surface, flow along the subcatchment, etc.- as input to the second one - flow in the sewer system network structures. The The link link between those two is unidirectional: unidirectional: the the of pipes, pipes, manholes manholes and and control control structures. between those two parts parts is network of role of the first part was only to create the input for the second one. Many current commercial drainage role of the first part was only to create the input for the second one. Many current commercial drainage models abstraction. of abstraction. models belong belong to to this this type type of The are coarse coarse surface of interaction between of today’s today's models models are surface description description and and lack lack of interaction between The two two weakest weakest points points of surface and underground flow components. The first problem has been attacked by the distributed parameter surface and underground flow components. The first problem has been attacked by the distributed parameter grid based models (e.g. Abbott, 1993). They divide the surface into small homogeneous patches and apply grid based models (e.g. Abbott, 1993). They divide the surface into small homogeneous patches and apply full water balance and 2D dynamic equations for each of them. The physical processes on the surface have full water balance and 2D dynamic equations for each of them. The physical processes on the surface have been simulated fairly well, but it is too complex for urban drainage. Adding interaction with underground been simulated fairly well, but it is too complex for urban drainage. Adding interaction with underground system would make it even worse. The balance between the amount of entered data, computation time and system would make it even worse. The balance between the amount of entered data, computation time and accuracy of the output must be maintained. accuracy of the output must be maintained. The model presented in the paper tries to solve the probiem of full interaction between two components, It is The model presented in the paper tries to solve the problem of full interaction between two components. It is basically the extension of the existing physical models, with somewhat improved surface flow component basically the extension of the existing physical models, with somewhat improved surface flow component (still the non-homogeneous subcatchment area is used, but now it reflects the existing flow conditions much (still the non-homogeneous subcatchment area is used, but now it reflects the existing flow conditions much closer) and full interaction between surface and underground flow components (Fig. 1). As these two closer) and full interaction between surface and underground flow components (Fig. I). As these two components are linked and the water can be drained using either of the two, the term dual drainage is used. components are linked and the water can be drained using either of the two, the term dual drainage is used. Three assumptions are fundamental in the presented model. Three assumptions are fundamental in the presented model. Firstly, due to a limited inlet capacity, the sewer system will not necessarily be able to drain all runoff. Firstly, due to a limited inlet capacity, the sewer system will not necessarily be able to drain all runoff. Secondly, when a part of the system is pressurized, water can go out of the system. Thirdly, surface water Secondly, when a part of the system is pressurized, water can go out of the system. Thirdly, surface water that cannot be drained by a sewer system is to be routed further downstream. The direction of surface flow is to be The paths. direction of surface flow that cannotfrom be drained by aflow sewer can differ that of the in system sewer pipes - itrouted has to further follow downstream. the surface flow If the ponds exist, of the flow in sewer pipes it has to follow the surface flow paths. If the ponds exist, can differ from that they are to be taken into account as well. The surface flow can be captured by some downstream inlet as be well. The out surface can be capturedThe by proposed some downstream they are of to the be taken account manhole sewer into system, or can routed of theflow current catchment. model hasinlet its manhole of the sewer system, or can be routed out of the current catchment. The proposed its background in some earlier investigations on GIS usage in data preparation for physically model based has urban background in some earlier investigations on GIS usage in data preparation for physically based urban

An simulation of An approach approach to to simulation of dual dual drainage drainage

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drainage models (Elgy 1993). AA similar, similar, although although somewhat simpler approach found in drainage models (Elgy et al., al., 1993). somewhat simpler approach can can be be found in works works of (1995) and of Kinouchi Kinouchi et et al. al. (1995) and Mark Mark et et al. al. (1997). (1997).

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... pipeflow

'" pond outflow "===::> surface flow ~ flooded area ~surface runoff

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Figure 1. Model with dual drainage possibility Figure I. Model with dual drainage possibility.

GIS ASSISTED MODEL CREATION GIS ASSISTED MODEL CREATION Description of the model is given by the steps that a modeller has to go through. In order to avoid confusion Description of the model isingiven by the steps packages, that a modeller has GIS to gosystem through. to avoid confusion in naming some functions different existing the meta will Inbeorder considered. in naming some functions in different existing packages, the meta GIS system will be considered. Initial chase Initial phase Each urban drainage project starts with data gathering and verification processes. Data about the surface, Each drainage starts with data gathering verification processes. Data the surface, existingurban sewer networkproject (if any), historical rainfalls, soil and types, etc. are obtained from anyabout available source existing sewer network any), historical rainfalls, soil types, etc. are obtained available and in a variety of forms (if(papers, strip charts, films, digital unstructured or in GISfrom form).anyAfter carefulsource data and in a variety formsnetwork (papers,can stripbecharts, films, digitalby unstructured in GIS form).and After careful data inspection, the of sewer created (mostly digitizing orpaper maps) accompanying inspection, tiles the (manually, sewer network created by digitizing and accompanying parameter filling can somebe tables or (mostly dialog boxes, manually paper draftingmaps) subcatchment boundary parameter files (manually, fillingofsome tables or dialog drafting subcatchment boundary lines and calculating percentages different types of areasboxes, inside manually the subcatchment). lines and calculating percentages of different types of areas inside the subcatchment). The proposed urban drainage model assumes that the GIS will be used from the beginning of the model creation. Instead of drainage typing inmodel some assumes coordinates in used a CAD-like the highly model The proposed urban that of the manholes GIS will be from theenvironment, beginning ofthe structured Instead GIS inputof oftyping the sewer systemcoordinates should be used. The GIS inshould take care environment, of network connectivity, creation. in some of manholes a CAD-like the highly naming of GIS pipesinput and of manholes, of should diameters, etc. The It should also automatically generate connectivity, most of the structured the sewercheck system be used. GIS should take care of network data needed by the based on of entered data. etc. Finally, the basic GIS functions generate such asmost coordinate naming of pipes and model, manholes, check diameters, It should also automatically of the correction andbywarping will ensure positional accuracy of entered data. the model, based the on improved entered data. Finally, the basic GIS functions such as coordinate data needed correction and warping will ensure the improved positional accuracy of entered data. The GIS terrain creation functions should be used to enter elevation data of measured spot heights and/or heights manholecreation tops and to createshould the Digital Elevation (DEM). technique to be applied of measured spotisheights and/or The GISofterrain functions be used to enterModel elevation data Which for DEMof creation the usertheand on the GIS used, but (DEM). the Triangular Irregular Network (TIN) heights manhole depends tops and on to create Digital Elevation Model Which technique is to be applied model is strongly suggested. Conversion from TIN to grid based model is to be done as most of the surface for DEM creation depends on the user and on the GIS used, but the Triangular Irregular Network (TIN) spatial analyses, are to Conversion be used in later morebased efficient in isa grid selection of grid model is stronglywhich suggested. fromsteps, TIN are to grid model to besystem. done asThe most of the surface size is analyses, crucial, since grid might to are unacceptably longincomputation coarserof grid spatial whichtooarefine to bea used in laterlead steps, more efficient a grid system.time, The and selection grid might out some characteristics. As a rule oflong thumb, grid size oftime, 1 to and 2 metres should computation coarser grid size is filter crucial, since important too fine a surface grid might lead to unacceptably be usedfilter for out urban areas, and thousand-million rule for As rural areas - total number or pixels, might some important surface characteristics. a rule of thumb, grid sizeofofused I to 2cells, metres should should million and (Maidment, 1996). rule for rural areas - total number of used cells, or pixels, be usedbeforaround urbana areas, thousand-million should be around a million (Maidment, 1996).

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Apart Apart from from DEM, DEM, data data about surface surface objects are to be entered into the GIS. Streets, big paved areas, houses, walls, green areas and other types of of urban elements that can can influence the large garages and hangars, large walls, surface flow characteristics, are to be be digitized and structured structured using an an flow and that have different different infiltration infiltration characteristics, cover data, appropriate layer scheme. scheme. All All those structured structured data are are called cover data, or cover image image (when (when exported cover image is time consuming, and can overload the project project system). The creation of into the grid system). of a detailed cover budget. Nevertheless, good DEM DEM and cover are essential essential for for the the dual dual drainage model. model. Time Time spent spent in in Nevertheless, good cover images are data preparation is longer than than in the classic would payoff payoff in later phases, phases, through automatic classic approach, but it would generation of of most most files needed needed by the model. Creation of sewer Creation of sewer network network Once sewer network network is the GIS, GIS, the user can can easily it, edit edit some some pipes, pipes, add add or or delete delete Once the the sewer is entered entered into into the the user easily manage manage it, some change parameters parameters of control structures to the the file file format format understood understood by by the the of control structures and and export export to some reaches, reaches, change simulation will keep objects in in the the sewer sewer system system and and the accompanying simulation model. model. The The GIS GIS will keep aa link link between between objects the accompanying database that holds holds all information) such such as as type type of of pipe, pipe, database that all necessary necessary attributes attributes (textual (textual and and numerical numerical information) diameter, of construction, date of of last etc. diameter, shape, shape, roughness, roughness, year year of construction, date last inspection, inspection, etc. A important role of GIS data inspection. inspection. The The GIS can check for disconnected disconnected A very very important role of GIS is is to to assist assist in in aa detailed detailed data GIS can check for parts of the pipe slopes slopes (compared (compared with along the the same same path), reduction of the the system, system, extreme extreme pipe with DEM DEM slopes slopes along path), reduction of the parts of pipe direction, the total length of the system or or length length between between certain certain points. points. the system pipe diameter diameter in in downstream downstream direction, the total length of System GIS, by automatic deletion and substitution substitution of pipes that that fall fall out out System simplification simplification can can also also be be done done by by GIS, by automatic deletion and of pipes of losing the the connectivity connectivity of of the the system. system. of the the selected selected criteria, criteria, without without losing Surface flow Surface flow analysis analysis The goal of of this familiarized with the general surface characteristics, such as as possible directions The goal this step step is is to to get get familiarized with the general surface characteristics, such possible directions of the surface flow, the existence of ponds or retention basins, and the automatic creation of the surface of the surface flow, the existence of ponds or retention basins, and the automatic creation of the surface drainage urbanized parts catchment, generally parts without without the the of the the catchment, generally upstream upstream parts drainage network network (applicable (applicable for for less less urbanized parts of underground drainage system). files are are DEM cover images, in raster raster fonn. form. ItIt is is necessary necessary underground sewer sewer drainage system). Input Input files DEM and and cover images, in to entered into to know know the the way way larger larger ponds ponds and and retention retention basins basins were were entered into the the DEM: DEM: either either with with true true ground ground elevations at surface with maximum elevation, while the cover image holds the description of it. it. Also, elevations or or as as at surface with maximum elevation, while the cover image holds the description of Also, the know if if DEM DEM was into account heights of of all all man objects was created created by by taking taking into account the the heights man made made objects the modeller modeller should should know (streets with gutter centre line, line, tops of houses houses and and walls, etc.), or if itit was was interpolated interpolated using using ground (streets with gutter and and centre tops of walls, etc.), or if ground points only. In the latter case (which is more often), DEM can be corrected using information from the cover points only. In the latter case (which is more often), DEM can be corrected using information from the cover image, raising and lowering pixels for some some pre-defined height. This operation is is called called height height image, by by artificially artificially raising and lowering pixels for pre-defined height. This operation correction of DEM. It produces DEM that is valid only for slope and aspect analyses. correction ofDEM. It produces DEM that is valid only for slope and aspect analyses. The GIS routine that will find all depressions within DEM should be invoked. It will create information

The GIS routine that will find all depressions within DEM should be invoked. It will create information about all found ponds, their lowest point, volume-stage relationship, maximal height and coordinates of exit about all found ponds, their lowest point, volume-stage relationship, maximal height and coordinates of exit point. The routine should discard all ponds whose volume is lower than some threshold value, by filling point. The routine should discard all ponds whose volume is lower than some threshold value, by filling them (like filtering of noise). Since computation is done in a raster system, it is possible to vectofize the them (like filtering of noise). Since computation is done in a raster system, it is possible to vectorize the delineated ponds and import them in to the GIS system. In order to link the ponds and create the till surface delineated ponds and import them in to the GIS system. In order to link the ponds and create the full surface network system, flow path analysis function of the GIS system is needed. It will first create the flow network system, flow path analysis function of the GIS system is needed. It will first create the flow accumulation image from DEM, the image where each pixel holds the volume of water that will pass over it accumulation image from DEM, the image where each pixel holds the volume of water that will pass over it during the surface flow process. By removing pixels with value smaller than some (user defined) threshold, during the surface flow process. By removing pixels with value smaller than some (user defined) threshold, the image of surface network is obtained. It needs some post-processing, mainly line thinning, to prepare it the image of surface network is obtained. It needs some post-processing, mainly line thinning, to prepare it for vectorization. The image of flow paths can be used only for visual network presentation. To make a for vectorization. The image of flow paths can be used only for visual network presentation. To make a surface flow network, it has to be vectorized, and ordered in an appropriate manner (from upstream to surface flow network, it has to be vectorized, and ordered in an appropriate manner (from upstream to downstream node), taking into account that loops might exist. So the smart vectorization routine able to downstream node), taking into account that loops might exist. So the smart vectorization routine able to create the full network topology has to exist in the GIS software. An example of automatically generated create the network has to surface flowfullnetwork is topology shown in Fig. 2. exist in the GIS software. An example of automatically generated surface flow network is shown in Fig. 2.

An An approach approach to to simulation simulation of of dual dual drainage drainage

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Figure 2. Surface flow network paths - after 2 x 2 kernel (left) and f%orn flow accumulation image(right). Figure 2. Surface flow network paths - after 2 x 2 kernel (left) and from flow accumulation image (right).

Results of the surface analysis are: removal of some errors found in DEM, detection of all surface ponds and Results of the surface analysis are: removal of some errors found in DEM, detection of all surface ponds and extraction of their parameters, assistance in making the decision about what ponds are to be used as retention extraction of their parameters, assistance in making the decision about what ponds are to be used as retention basins and with what strategy, creation of possible surface flow pattern as if there were no underground basins and with what strategy, creation of possible surface flow pattern as if there were no underground sewer system so that it can help during the design process of a new sewer system or in the reconstruction of so that can help network during the process a new sewer system or inwhere the reconstruction of sewer systemone. the existing The itgenerated willdesign not be used inofthe future model in areas a sewer system the existing The generated network will not the futurewith model in areassystem wherewhere a sewer system exists, but it one. is needed in semi rural and rural partsbeofused the in catchment no sewer rainfall is exists, but needednetwork. in semi rural and rural parts of the catchment with no sewer system where rainfall is drained by ittheis surface drained by the surface network.

Cover image analvsis

Cover image analysis

Cover image, if created during the initial phase of the project, has a dual function: one is in height correction if created during the initial of the project, hasand a dual function: one is in height correction Cover of DEMimage, (as mentioned), and another is aphase link between graphical attribute databases, where a number of ofDEM (as and another a linkobject between graphical where a number of attributes arementioned), stored for each kind of iscover (such are typeand of attribute area, its databases, permeability, surface storage attributes are stored for each kind of cover object (such are type of area, its permeability, surface storage retention, connectivity with sewer system, number of storeys and/or population density, property value for retention, connectivity with sewer system, number of storeys andlor population density, property value for flood damage assessment, etc.). flood damage assessment, etc.).

If a complete set of structured cover data is not available in GIS form, most drainage projects could not If a complete structured data Inis such not available in aGIS form, most projects could that not afford enteringset of of such data fromcover scratch. a situation, simplified coverdrainage image can be created affordcomprises entering of scratch.objects, In suchstreets a situation, a simplified cover image cansame be created that only thesuch few data most from significant and clearly delineated areas of the cover type only comprises few most significant streets andFor clearly areas thehas same cover type (areas of similarthe housing, large paved or objects, green areas, etc.). everydelineated such zone, the of user to specify the (areas of similar housing, large paved orareas, greenroofs areas,directly etc.). For every such zone,system the user to specify the percentages of pervious and impervious connected to sewer andhas those with some percentages perviousetc., andi.e. impervious areas, directly to sewer system those with some sort of sourceofcontrol, parameters thatroofs are used by aconnected surface component of the and model in rainfall-torunoff source control, etc., i.e. parameters that are used by a surface component of the model in rainfall-tosort ofconversion. runoff conversion.

The GIS classification tools could be applied over aerial photographs to speed up the calculation of percentages of different areas coverover types, but photographs this techniquetois speed still useless the detection The GIS classification tools within could selected be applied aerial up theforcalculation of of main cover streets, etc. Thecover result types, of classification can have plenty of small unclassified percentages of areas, different areashouses, within selected but this technique is still useless for the detection pixels within example, andetc. thatThe will result influence the surface flow programs. of main covera street, areas, for streets, houses, of classification can path have analysis plenty of small unclassified pixels within a street, for example, and that will influence the surface flow path analysis programs.

Subcatchment creation

Subcatchment creation

Automatic subcatchment creation (delineation) is the process of partitioning the surface into smaller areas that are drained by particular network nodes. is The take into account into DEMsmaller and cover partitioning the surface areas Automatic subcatchment creation (delineation) theprocedure process ofshould images, should the subcatchments the surface A well designed GIS that are and drained by create particular network nodes.that Thefollow procedure should flow take patterns. into account DEM and cover routine timecreate in thethe model creation by avoiding thethe slowsurface processflow of manual delineation. images, will andsave should subcatchments that follow patterns. A well designed GIS routine will save time in the model creation by avoiding the slow process of manual delineation.

Input data for the subcatchment creation procedure are the coordinates of manholes that will receive the surface water pipes and nodes in sewer systemareshould not be considered), DEMthatand cover images, of manholes will receive the Input data for (transit the subcatchment creation procedure the coordinates and certain command files that are used to interact with the delineation procedure. In partly urbanized areas surface water (transit pipes and nodes in sewer system should not be considered), DEM and cover images, without an underground sewer system, the generated surface network is used for delineation of and certain command files that are used to interact with the delineation procedure. In partly urbanized areas subcatchments.

without an underground sewer system, the generated surface network is used for delineation of subcatchments.

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Subcatchment Subcatchment delineation delineation is is carried carried out out mostly mostly in in aa raster raster system system and and the the result result isis aa total total catchment catchment area area classified into subcatchments subcatchments and and into into non-drained non-drained area. area. Using Using vectorization vectorization GIS GIS routine, routine, the the subcatchment subcatchment polygons are are imported into into GIS GIS system system and and linked linked with with appropriate appropriate manholes manholes or or surface surface channels. channels.

Figure 3. 3. Dual Dualdrainage drainagemodel model parameters. parameters. Figure

The needed by each delineated delineated subcatchment: subcatchment: its its area, area, The GIS GIS can can compute compute the the parameters parameters needed by the the model model for for each average slope, percentages percentages of different cover subcatchment, shape, shape, centre average or or weighted weighted slope, of different cover areas areas within within the the subcatchment, centre of of gravity, for each each subcatchment subcatchment are are required required for for the the dual dual drainage model (see (see gravity, etc. etc. Some Some additional additional parameters parameters for drainage model Fig. 3): “I Fig. 3): The VS = j(Z), assumingthat the boundaryis a vertical wall. This The subcatchment subcatchment stage-volume stage"volume function, function, Vs =: f{Z), assuming that the boundary is a vertical wall. This hmction will be used to calculate the flooded level function will be used to calculate the flooded level in in subcatchment. subcatchment. The the area connectedroofs, of stage, stage,As = The area area of of subcatchment subcatchment reduced reduced by by the area of of directly directly connected roofs, as as aa function function of As ==fo f{Z) = 1 RoofedPart (Z)) x &,uBc&). In fact, As is the area left after flooding the downstream part of the I - RoofedPart (2» x AsuBcAT(Z). In fact, As is the area left after flooding the downstream part of the subcatchment level Z. It is whenpart of the subcatchment is Z. It is used used in in rainfall rainfall to to runoff runoff conversion conversion when part of the subcatchment is subcatchment up up to to the the level flooded. flooded. Coordinates of gravity centre,asa ftmction of stage,G =fTZ), computedfor floodedareaof subcatchment Coordinates of gravity centre, as a function of stage, G = f{Z), computed for flooded area of subcatchment exclusiveof directly connectedroofs.Centreof gravity of floodedareais usedfor computationof lengthof exclusive of directly connected roofs. Centre of gravity of flooded area is used for computation of length of surface flow if downstreamrecipientis anothersubcatchment; or elseit is computedfrom the surfaceflow surface flow if downstream recipient is another subcatchment; or else it is computed from the surface flow pathto the next subcatchment or to the downstream pond. path to the next subcatchment or to the downstream pond.

List of neighboursubcatchments (thosethat sharepart of the sameboundary).The list is usedin the process List of neighbour subcatchments (those that share part of the same boundary). The list is used in the process of surfacenetworkcreation,to makea systemof equations. of surface network creation, to make a system of equations.

The lengthof boundaryline betweentwo neighbours,againasa function of stage,B! =flZ). The boundary The length of boundary line between two neighbours, again as a function of stage, B1 = f{Z). The boundary line lengthis usedfor flow computation,andcareshouldbe takenthat the boundaryline doesnot include line length is used for flow computation, and care should be taken that the boundary line does not include the crossedhouseor wall. the crossed house or wall.

For partsof subcatchment boundarythat arewithout neighbouringsubcatchment, the checkfor surfaceflow For parts of subcatchment boundary without neighbouring checktogether for surface path network shouldbe done. If therethat is aare flow path, the link to thatsubcatchment, pathshouldbethe created, withflow the path networkofshould be done. there is a flow path, link to is that pathto should created, together with has the coordinates exit point fromIf the subcatchment. Thisthe situation likely occurbe when the subcatchment of exit point from the subcatchment. This situation is likely to occur when the subcatchment has coordinates no downstream neighbour. no downstream neighbour.

An approach approach to simulation of An to simulation of dual dual drainage drainage

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Creation of surface network

The network consists The surface surface network consists of of two two parts. parts. First, the area the underground sewer system the network network is is created using links First, in in the area where where the underground sewer system exists, exists, the created using links to to all all neighbours of each each subcatchment, subcatchment, plus plus the surface flow path, if if existing. existing. There is no no preferred preferred direction direction of of the surface flow path, There is neighbours of flow from subcatchment, subcatchment, so so the might be highly looped. parameters, such such as as length and flow from the network network might be highly looped. Network Network parameters, length and width of flow are to be computed computed at at each each time time step, step, since since they they depend depend on flow path, path, are to be on flooded flooded area. area. The The storage storage width of ponds are included in the surface network, the same as retention basins with natural over flow or ponds are included in the surface network, the same as retention basins with natural over flow or controlled controlled outlet. This network, system, forms forms the dual drainage drainage model. outlet. This network, together together with with underground underground system, the dual model. Second, in in the areas without without underground system, the surface network network is as the one created created in Second, the areas underground system, the surface is the the same same as the one in the the surface flow analysis step. However, the result of automatic procedure needs some checking by the user and, surface flow analysis step. However, the result of automatic procedure needs some checking by the user and, possibly, some editing. The cross-sections of natural channels could be found from DEM, but it can also be possibly, some editing. The cross-sections of natural channels could be found from DEM, but it can also be assumed that width is some empirical function of the channel rank. assumed that width is some empirical function of the channel rank. There is no need to treat both parts within the same model. The second network can be computed separately There is no need to treat both parts within the same model. The second network can be computed separately from the first one. If it is situated upstream, the result of the simulation can just be applied as the input to the from the first one. If it is situated upstream, the result of the simulation can just be applied as the input to the first system, the dual drainage one. For a downstream located surface channel network, the result of the dual first system, the isdual one. For a downstream located surface channel network, the result of the dual drainage model useddrainage as its inppt. drainage model is used as its inppt. DUAL DRAINAGE SIMULATION PROCEDURE DUAL DRAINAGE SIMULAnON PROCEDURE The simulation of the surface runoff is done by the appropriate module of BEMUS program (MaksimoviC et The 1995). simulation of the runoff isbased donemodel by theinappropriate of BEMUS programrainfall (Maksimovic al., In short, it surface is a physically which the module transformation of effective to runoffet al., 1995). In isshort, is asolving physically model inequations. which the These, transformation runoff hydrographs doneit by the based kinetic-wave together of witheffective other rainfall possible toinflows hydrographs is done by solving the kinetic-wave equations. These, together with other possible inflows (waste water, pipe infiltration etc.), create the input to the underground sewer system. (waste water, pipe infiltration etc.), create the input to the underground sewer system. Prior to the calculation of the pipe flow, the node matrix is compressed using the row-indexed sparse storage Prior to the calculation the pipe flow,between the nodethematrix compressed using pumps the row-indexed storage scheme. In doing so, notofonly the links nodesisby pipes, culverts, and weirs sparse are taken into scheme. In so, notthat only the links nodesflooding. by pipes, culverts, pumps and weirs are taken into account, butdoing also those might be thebetween result ofthe surface account, but also those that might be the result of surface flooding. As long as the heads in all the manholes are below the appropriate ground levels, the simulation runs in a As long as the heads in all the ofmanholes are below the appropriate ground levels, simulationfour-point runs in a standard manner. Elimination the internal variables along each pipe (using thethePreissmann standardmethod), manner. together Elimination the internal variables alongends eachandpipe (using the Preissmann implicit with of Bernoulli equations for pipes’ manholes, enables dischargesfour-point at each implicit togetherinwith Bernoulli ends and manholes, enables pipe end method), to be expressed terms of waterequations levels atfor thepipes' corresponding nodes. These createdischarges the inflowsat each and pipe end to expressed at the corresponding These create the inflows and outflows at be a node (see in Fig.terms 4), of at water which levels then the continuity equationnodes. is solved by a modified Eulertrapezoidal atmethod. system is solved conjugate equation gradient method. water levels at outflows a node The (see matrix Fig. 4), at which thenbythethecontinuity is solvedOnce by athemodified Eulerthe nodes are calculated, Saint-Venant each pipegradient are solved. Clearly, is an levels iterativeat trapezoidal method. The matrix system isequations solved byalong the conjugate method. Once this the water procedure. Surcharged is handled equations by the improved open and supercritical flow is the nodes are calculated,flowSaint-Venant along each pipeslot are technique solved. Clearly, this is an iterative simulated bySurcharged reducing theflow convective termsbyand centering. procedure. is handled theupstream improved open slot technique and supercritical flow is simulated by reducing the convective terms and upstream centering.

Figure 4. Node inflows and outflows. Figure 4. Node inflows and outflows.

As from the start of flooding, track is kept of surface water levels. The excess of water is stored in that subcatchment, the of stage-volume function is used to compute water The level. Then,offor computed level, the As from the start flooding, track is kept of surface waterthe levels. excess water is stored in that reduced subcatchment area is computed as well as the centre of gravity for the flooded part (see Fig. 3). subcatchment, the stage-volume function is used to compute the water level. Then, for computed level, the Using the list of neighbours, the check is done for possible outflows, and for each one the boundary width is reduced subcatchment area is computed as well as the centre of gravity for the flooded part (see Fig. 3). found. Using the list of neighbours, the check is done for possible outflows, and for each one the boundary width is found.

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S. DJORDJEVI6 S, DJORDJEVIC et et al ai,

Also, all neighbours that will the flow flow length length as as the Also, for for all neighbours that will receive receive the the water, water, the the distance distance between between appropriate appropriate centres of gravity gravity is is computed. The check flow paths computed flooding computed. The check for for surface surface flow paths at at the the computed flooding level level is is then then centres of performed. length of flow together conditions is is taken performed. If If there there is is any, any, the the length of flow together with with downstream downstream conditions taken Tom from aa prepared prepared surface There is two subcatchments surface flow flow path path network. network. There is aa possibility possibility to to have have a a pond pond between between two subcatchments or or inside inside the the current one level reaches height of of their their current one (see (see Fig. Fig. 3). 3). Those Those ponds ponds will will receive receive the the water water when when flooding flooding level reaches the the height input point. continuity equation takes the account, and also the possibility to input point. The The continuity equation takes the stored stored volume volume of of water water into into account, and also the possibility to have controlled out out flow have the the controlled flow from from certain certain retention retention basins. basins.

The model flow. This solving the of momentum and The model now now routes routes the the surface surface component component of of flow. This is is done done by by solving the set set of momentum and continuity equations (applied to individual subcatchment) in a way similar to the one applied by Kinouchi et continuity equations (applied to individual subcatchment) in a way similar to the one applied by Kinouchi et al. (1995). However, possible flow directions are not predetermined. Instead, they depend on water levels, al. (1995), However, possible flow directions are not predetermined. Instead, they depend on water levels, and may alternate during one event. Also, distances and boundary widths are not constant - they depend on and may alternate during one event. Also, distances and boundary widths are not constant - they depend on water stages. Finally, the simulation of two flow components is fully interactive in terms that water stages. Finally, the simulation of two flow components is fully interactive in tenns that inflows/outflows from one to another are adjusted at each time step, and because the surface runoff inflows/outflows from one to another are adjusted at each time step, and because the surface runoff hydrographs are not calculated prior to the sewer system simulation, but rather by considering possible hydrographs are not calculated prior to the sewer system simulation, but rather by considering possible flooding of part of the subcatchment. flooding of part of the subcatchment. The latter is illustrated in Fig. 5, where runoff is simulated from twenty-minutes of rainfall of constant of slope rainfall constant The latteronis the illustrated in Fig. 5, where runoff simulated from intensity impervious subcatchment havingis area of 1 ha andtwenty-minutes squared root of overofManning of 1 ha and squared root of slope over Manning intensity on the impervious subcatchment having area coefficient equal to 5. Line A is a ‘regular’ hydrograph calculated wit&out taking flooding effect into coefficient equal to starts 5. Line is min, a 'regular' calculatedis wit.\lout taking effect account. If flooding at t A = 10 and thehydrograph entire subcatchment flooded at t = 15flooding min, then line Binto is account. If at tgraphs: = 10 min, and the entire subcatchment is flooded at t = 15 min, then line B is obtained byflooding summingstarts up two the first one is a hydrogmph calculated as runoff from a portion of the obtained by applied summing up on twothe graphs: the firstpart one of is athe hydrograph calculated runoffand from portion one of the hyetograph only non-flooded subcatchment (dottedas line), thea second is hyetograph applied only on the non-flooded of the subcatchment line), the subcatchment. second one is instantaneous transformation of the portion ofpart hyetograph applied to the(dotted flooded partand of the instantaneous of the portioncomputation, of hyetograph to attempt the flooded part ofprecipitation the subcatcbrnent. Clearly, this istransformation still not an exact hydraulic butapplied rather an to include in the Clearly, this is still not an exact hydraulic computation, but rather an attempt to include precipitation in the flooded area into the continuity equation in a timely more logical manner. flooded area into the continuity equation in a timely more logical manner. t min

of flooding)

of flooding

~1-----j~~~~4L,L,.l~~--+-----l 10

20

30

t(min)

Figure 5. Influence of flooding on effective runoff.

Figure 5. Influence of flooding on effective runoff.

Such a model generates the data about flooded areas and surface water velocities, dynamically, allowing further assessments. Most about of described model has velocities, been developed, and fewallowing details Such a damage model generates the data flooded dual areas drainage and surface water dynamically, still need to be worked out. It Most is hoped that the results the model application real problems be further damage assessments. of described dual of drainage model has beentodeveloped, andwill fewsoon details ready for publishing. still need to be worked out. It is hoped that the results of the model application to real problems will soon be ready for publishing.

An approach to to simulation simulation of drainage An approach of dual dual drainage

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CONCLUSION CONCLUSION One of of the the possible possible approaches simulation is presented presented in One approaches to dual dual drainage drainage simulation in the paper. paper. Apart from from automatic automatic data data creation classical physically physically based Apart creation for for classical based models, models, the GIS GIS forms forms the core core in in dual dual drainage modelling, where where georeferenced about the needed. Using Using standard standard and specially specially drainage modelling, georeferenced data data about the surface surface is needed. developed GIS functions, functions, the surface surface component component of of numerical model is created created and and linked linked to the the developed GIS numerical model underground sewer system. underground sewer REFERENCES REFERENCES Abbott, M. The electronic electronic encapsulation encapsulation of knowledge in in hydraulics, hydrology and and water water resources. resources. Advances Advances in in Abbott, M. B. B. (1993). (1993). The of knowledge hydraulics, hydrology Water 16,2 l-39. Water Resources, Resources, 16,21-39. Elgy, J., Prodanovic, D. (1993). Using systems for for urban hydrology. In:, In:, Application Elgy, J., Maksimovic, Maksimovic, C. C. and and Prodanovic, D. (1993). Using geographic geographic information information systems urban hydrology. Application of Geographic Information Water Resources Resources Management, K. Kovar Kovar and and H. Nachtnebel (eds). (eds). ofGeographic Information Systems Systems in in Hydrology Hydrology and and Water Management, K. H. Nachtnebel IAHS 211, IAHS Wallingford, 151-160. IAHS Publication Publication No. No. 211, IAHS Press, Press, Wallingford, 151-160. Kinouchi, T., and Tanaka, (1995). Prediction of local local inundation an urbanized urbanized watershed. In: 2nd 2nd Int. Conf on on Y. (1995). Prediction of inundation in in an watershed. In: Int. Con! Kinouchi, T., Kuriki, Kuriki, M. M. and Tanaka, Y. Innovative Storm Drainage, NOVATECH 41 l-41 8. Innovative Technologies Technologies in in Urban Urban Storm Drainage, NO VA TECH 95, 95, Lyon, Lyon, 411-418. Maidment, D. (1996). GIS GIS and and hydrologic hydrologic modeling -- an assessment of of progress. progress. In: In: Third Third Int. Int. Con! Co@ on on GIS GIS and and D. R. R. (1996). modeling an assessment Maidment, Environmental Santa Fe. Fe. Environmental Modeling, Modeling, Santa Maksimovic, L., Prodanovic, Prodanovic, D. and Djordjevic, Djordjevic, S. (1994). Gluing routines routines for for matching matching standard standard GIS GIS C., Elgy, Elgy, J., J., Fuchs, Fuchs, L., D. and S. (1994). Gluing Maksimovic, C., packages and design water projects. projects. In: In: Int. Conf on on Remote Remote Sensing Sensing and and GIS GIS in in Urban Urban packages with with simulation simulation and design models models for for water Int. Con! Waters, UDT ‘94, C. C. Maksimovic, Maksimovic, J. Elgy Elgy and (eds.), Moscow, Moscow, 91-104. 91-104. Waters, UDT '94, J. and V. V. Dragalov Dragalov (eds.), Maksimovic, A., Djordjevic, S., Prodanovit, and DraBkovif, M. (1995). of simulation simulation with with updated updated data data C., Rajcevic, Rajcevic, A., Djordjevic, S., Prodanovic, D. D. and Dra~kovic, M. (1995). Results Results of Maksimovic, C., and BEMUS model. In: Urban Urban Drainage in Italy, Calomino, C. C. Maksimovic Maksimovic and Drainage Experimental Experimental Catchments Catchments in Italy, F. F. Calomino, and and modified modified BEMUS model. In: B. Molino (eds.), (eds.), Editoriale Cosenza, 263-276. 263-276. B. Molino Editoriale Bios, Bios, Cosenza, Mark, O., van T., Rabbi, Rabbi, K. and Kjelds, Kjelds, J. (1997). A A MOUSE GIS study study of the drainage drainage in Dhaka city. city. Technical Technical report, report, Mark, 0., van Kalken, Kalken, T., K. and J. (1997). MOUSE GIS of the in Dhaka Danish Hffi'Sholm. Danish Hydraulic Hydraulic Institute, Institute, Horsholm. Prodanovic, S. and (1998). GIS assisted model model for dual drainage simulation. In: In: Hydroinformatics Hydroinformatics Prodanovic, D., D., Djordjevic, Djordjevic, S. and Maksimovic, Maksimovic, C. C. (1998). GIS assisted for dual drainage simulation. ‘98, Vol. Babovic and Larsen (eds), (eds), Balkema, Balkema, Rotterdam, '98, Vol. 1, I, V. V. Babovic and L. L. C. C. Larsen Rotterdam, 535-542. 535-542.