Use of high-resolution geographical databases for rainfall-runoff relation in urbanised areas

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Waf Tech Vol. 39, NO. 9, pp. 87-94, Wal Sci. Sci. Tech Vol. 39, No.9, 87-94, 1999 1999 0© 1999 1999 Published Published by by Elsevier Elsevier Science Ltd Ltd on on behalf behalf of ofthe the IAWQ 1AWQ

Pergamon

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USE HIGH-RESOLUTION USE OF HIGH-RESOLUTION GEOGRAPHICAL GEOGRAPHICAL DATABASES DATABASES FOR FOR RAINFALL-RUNOFF RELATION IN RAINFALL-RUNOFF RELATION IN URBANISED URBANISED AREAS AREAS Y. and A. Y. Zech Zech and A. Escarmelle Escarmelle Department Civil Engineering, I, Department of a/Civil Engineering, UniversitP Universite catholique cathalique de de Louvain, Lauvain, Place Place du du Levant Levant 1, B 1348 Louvain-la-Neuve, B 1348 Lauvain-la-Neuve, Belgium Belgium

ABSTRACT ABSTRACT Distributed reasons is Distributed models models are are more more and and more more used used in in regional regional hydrology. hydrology. One One of of the the main main reasons is their their essential essential compatibility distributed compatibility with wilh raster raster data data in in Geographical Geographical Information Information Systems. Systems. Also Also in in urban urban hydraulics, hydraulics, distributed models are promising but their development depends on the availability of high-resolution data models are promising but their development depends on the availability of high-resolution data able able to to represent from satellite satellite imaging imaging are not yet The paper represent urban urban features. features. Public Public databases databases from are not yet adequate. adequate. The paper investigates investigates the possibility of using other kinds of databases designed more specifically for cartography. The advantages the possibility of using other kinds of databases designed more specifically for cartography. The advantages and inconveniences of such an approach are pointed out, based on two actual examples. 8 1999 Published by and inconveniences of such an approach are pointed out, based on two actual examples. © 1999 Published by Elsevier Science on behalf of the IAWQ. All rights reserved Elsevier Science on behalf of the IAWQ. All rights reserved

KEYWORDS KEYWORDS Geographical Information System; urban hydraulics; urban hydrology; urban planning Geographical Information System; urban hydraulics; urban hydrology; urban planning INTRODUCTION INTRODUCTION Most of the urban-hydraulics techniques have developed considerably during the last few years. However, Most of the urban-hydraulics techniques have developed considerably during the last few years. However, rainfall-runoff modelling is not limited to the accurate representation of the water profiles in the sewer rainfall-runoff modelling is not limited to the accurate representation of the water profiles in the sewer network. Also the realistic computation of the discharges entering the sewer manholes is a significant network. Also the realistic computation of the discharges entering the sewer manholes is a significant modelling step. Therefore, an accurate description of the watershed is needed giving the topographical modelling step. Therefore, an accurate description of the watershed is needed giving the topographical features as well as the land use. But introducing the data required for this description into the program features as well as the land use. But introducing the data required for this description into the program remains the most tedious part of the user’s work. Even in the cases where the data collection may benefit remains the most tedious part of the user's work. Even in the cases where the data collection may benefit digitising facilities, this step can be biased by the operator’s subjectivity. A paper map is often not easy to digitising facilities, this step can be biased by the operator's subjectivity. A paper map is often not easy to interpret univocally, above all in urban areas where information is so abundant that some of the needed data interpret univocally, above all in urban areas where information is so abundant that some of the needed data are not represented: for instance level contours are regularly not drawn in urban zones. are not represented: for instance level contours are regularly not drawn in urban zones. Making use of Geographical Information System (GIS) techniques could revolutionise this part of the work. Making use of Geographical Information System (GIS) techniques could revolutionise this part of the work. Indeed GIS deals with Digital Elevation Models (DEM) and land-use databases. DEMs include the Indeed GIS deals Elevation Models (DEM) maps, and land-use DEMs include and the topographical data with useful Digital for building slope and aspect determiningdatabases. catchment properties topographical useful for building slope knowledge and aspect allows maps, us determining and delineating the data water-path network. Land-use to describecatchment in detail properties the catchment delineating water-paththenetwork. Land-use allows us to describe detail the catchment characteristics.the Moreover grid structure oftenknowledge used in GIS databases is often in perfectly coherent with characteristics. Moreover the which grid structure used in GIS databases is often perfectly coherent with distributed hydrologic models continue often to develop. distributed hydrologic models which continue to develop. 87 87

Y. Y. ZECH ZECH and and A. A. ESCARMELLE ESCARMELLE

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Such Such aa distributed distributed model model was was built built at at the the Universite Universite catholique catholique de de Louvain Louvain aa few few years years ago. ago. This This model model was was described et al., al., 1994) 1994) but but aa conclusion conclusion of of these these was was that that for for an an accurate accurate described in in former former papers papers (Zech (Zech et representation representation of of urban urban areas, areas, the the available available database database meshes meshes were were too too coarse coarse:: the the best best ground ground resolution resolution of of satellite m (SPOT satellite imaging imaging systems systems was was 10 10m (SPOT in in the the panchromatic panchromatic band) band) instead instead of of aa needed needed resolution resolution of of 55 m. m. During During the the last last few few years years resolution resolution of of public public satellite satellite data data has has not not significantly significantly increased increased but but some some developments developments of of databases databases for for automatic automatic cartography cartography are are really really promising. promising. Most Most of of the the information information needed needed for for instance instance in in the the frame frame of of urban urban planning planning may may be be useful useful also also for for hydrological hydrological modelling, modelling, as as the the used used scale scale in urban planning is very close to the needed accuracy for urban watershed description. in urban planning is very close to the needed accuracy for urban watershed description. The The paper paper describes describes aa new new database database developed developed by by the the Walloon Walloon Ministry Ministry for for Equipment Equipment and and Transport Transport to to improve urban planning approaches. It shows the exciting possibilities but also the limitations improve urban planning approaches. It shows the exciting possibilities but also the limitations of of such such aa database as support of for hydrological model in urban areas and finally gives some suggestions to amend database as support of for hydrological model in urban areas and finally gives some suggestions to amend the design of the database for hydrological purposes. Two Belgian urban watersheds were selected to the design of the database for hydrological purposes. Two Belgian urban watersheds were selected to calibrate the procedure: Wavre the centre of which is rather densely inhabited and a district of the new town calibrate the procedure: Wavre the centre of which is rather densely inhabited and a district of the new town of Louvain-la-Neuve characterlsed by a fast growth of the developed area. ofLouvain-la-Neuve characterised by a fast growth of the developed area. DISTRIBUTED HYDROLOGICAL

MODEL FOR URBAN AREAS

DISTRIBUTED HYDROLOGICAL MODEL FOR URBAN AREAS

Hydrological models based on spatial distribution of data are relatively common for large-scale watersheds Hydrological modelsdata based on spatial distribution of datafor arethe relatively common for large-scale watersheds where the available resolution is generally sufficient needed accuracy. However, in urban zones, where the available data resolution is generally sufficient for the needed accuracy. However, in urban zones, the distributed models are not yet much developed. Indeed, the required resolution is higher than the the distributed models are not yet much developed. Indeed, the required resolution is higher than available data. Also some specific difficulties are typical of the urban context: a part of the catchmentthe is availablewith data.relatively Also some of the urban systems context: leading a part of the catchment artificial highspecific runoff difficulties coefftcients are andtypical is drained by sewer to high-speed floodis artificialTherefore, with relatively runoff coefficients is drained by sewer leadingphysical to high-speed flood routing. somehigh adaptations of commonand models were carried outsystems for catchment description routing. Therefore, some adaptations of common models were carried out for catchment physical description as well as for hydrologic response modelling (Zech et al., 1994). as well as for hydrologic response modelling (Zech et al., 1994).

Hvdroloeical model Hydrological model

The simplest data structure is a raster or cellular organisation of spatial data, and among the possible The simplestmodels data structure a raster organisation spatial data, andresponse among to therainfall possible tessellation the most iswidely usedoriscellular the square grid. Alsoofthe hydrological is tessellationat models used 1isschematises the square the grid.hydrological Also the hydrological described the scalethe of most a grid widely cell. Figure behaviour ofresponse a cell. to rainfall is described at the scale of a grid cell. Figure I schematises the hydrological behaviour of a cell. Evapotranspiration

E

Evapotranspiration E

~Rainp Rain P

w = filrlction of W = function of

~~~~~

P P-

ETP

P-ETP

6%-s)

(S.-S) S/S” SIS•

.

Infiltration W

e = SIS

0

VW = a, +bi.s,

Vadose storage S

Q. = k S Groundwater flow Base flow Q. Groundwater SII

Figure 1. Hydrological

V=lps:Ip n v response scheme for a cell.

Figure 1. Hydrological response scheme for a cell.

Sawer

Rainfall-runoff relation in areas Rainfall-runoffrelation in urbanised wbanised areas

89

A part part of of the rain is intercepted by evaporation, another is infiltrated, infiltrated, depending mainly A mainly on the rainfall rainfall reservoir. intensity and on the soil moisture, intensity moisture, the latter being represented by the relative filling filling of of aa vadose reservoir. This storage supplies both interflow, interflow, according to Darcy’s Darcy's law law in unsaturated soils (Hillel, 1971), and This soils (Hillel, groundwater flow, flow, according according to standard Darcy's groundwater Darcy’s law. law. For For each cell of of the the grid, and and for each each time step, step, it is possible to determine the local runoff Interflow Q, Q; is added thus possible runoff flow flow Q,. QP.Interflow added to the surface flow flow but with with some delay. Groundwater only considered as as a global loss for for the watershed. Groundwater storage is only watershed.

ItIt is now flow routing routing from one node of velocity now possible possible to conduct the flow of the grid to the other one. one. The flow flow velocity depends on the land slopes s, T’he velocity velocity in the sewer sewer network 8\ and on the soil type. The network is estimated from the Manning hydraulic radius assumed equal to half hydraulic radius Manning formula formula with with the hydraulic half the pipe radius, since the hydraulic variation of discharges. variation is small in a large range of Urban Urban catchment description description Two Two main information information layers are useful useful for urban hydrology hydrology:: land-use models describing the type of of soil cover at each of the spatial structure, structure, and Models (DEMs) @EMS) giving giving the spatial cover at each cell cell of the spatial and Digital Digital Elevation Elevation Models the spatial distribution of elevations. distribution of In classification is is related runoff characteristics. characteristics. Digitised Digitised polygons polygons limiting In urban urban areas areas the the land-use land-use classification related to to runoff limiting homogeneous area and and obtained obtained from from any map are in raster raster structure structure where where aa homogeneous area any existing existing map are converted converted in conventional with surface corresponding to to distinctive distinctive hydrological behaviours conventional number number is is associated associated with surface types types corresponding hydrological behaviours (1 forests; 22 == crops; parking; 66 == open (1 == forests; crops; 33 == meadows meadows and and gardens; gardens; 44 = = buildings; buildings; 55 == roads roads and and parking; open water water surface). building representation, high-resolution models models have have to to be used. The The adopted grid cell cell of of surface). For For building representation, high-resolution be used. adopted grid 55 m x 5 m was found to be sufficient to detect building existence from land-use data file. m x 5 m was found to be sufficient to detect building existence from land-use data file. On other hand, DEM has has to to be take into into account account some some specific specific features features of of the the urban On the the other hand, common common DEM be adapted adapted to to take urban scene like the presence of buildings and the interception of runoff by sewerage network. The elevation of all all scene like the presence of buildings and the interception of runoff by sewerage network. The elevation of the to aa building increased by constant amount amount (for (for instance instance 10 10 m) in is automatically automatically increased by aa constant m) in the nodes nodes corresponding corresponding to building is such no more more able able to to cross buildings. On On the the other in order order to to simulate simulate the the such aa way way that that runoff runoff flow flow is is no cross the the buildings. other hand, hand, in sewerage fictitious trench replaces the the sewer sewer nodes fixed elevation elevation decrease decrease (l (1 m), m), sewerage network network aa fictitious trench replaces nodes by by an an arbitrarily arbitrarily fixed in are automatically diverted to this artificial thalweg. The The pipe slopes and and in such such aa way way that that crossing crossing waters waters are automatically diverted to this artificial thalweg. pipe slopes lengths consequently stretched stretched but be corrected by adjusting adjusting aa fictitious fictitious roughness. roughness. Figure Figure 22 lengths are are consequently but this this may may be corrected by depicts such an depicts such an adapted adapted DEM. DEM.

Figure2. AdaptedDEM for urbanareas. Figure 2. Adapted DEM for urban areas. Standard methods are used to determine the catchment characteristics (Meijerink et al., 1994). From the Standard matrix, methodsit are used to todetermine (Meijerink et al., in 1994). elevation is possible derive thethe flowcatchment direction characteristics matrix, assuming that the runoff a cellFrom flows the to elevation it is among possiblethe to derive the flow direction that the runoff in drainage a cell flows to the lowestmatrix, elevation eight surrounding nodes.matrix, From assuming both matrices, automatic basin the lowest elevation amongout. theAlso eightthesurrounding nodes. From both element matrices,may automatic drainage basina delineation may be carried time of concentration of each be computed in such delineation be carried time of concentration of each element maytobethe computed way that themay contribution of out. any Also cell totheany outlet point may be calculated according time. in such a way that the contribution of any cell to any outlet point may be calculated according to the time.

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Y. Y. ZECH ZECH and and A. A. ESCARMELLE ESCARMELLE

USE OF COMMON COMMON DIGITAL-ELEVATION DIGITAL-ELEVAnON AND AND LAND-USE LAND-USE MODELS MODELS USE of a 30 m x 30 m DEM DEM the For the Belgian territory territory two two GIS databases are available available:: one consists consists of For of which which are given with with an accuracy accuracy of of 1I m, the other other is a land-use database from LANDSAT elevations of database from LANDSAT imaging systems systems with with the same resolution resolution of of 30 m m x 30 m. imaging not really really compatible with with hydrologic-models hydrologic-models requirements. Figure 3 shows The elevation accuracy is not shows the elevation model of of the neighbouring neighbouring area of of the town town Wavre Wavre (Belgium) (Belgium) where where the valley valley flows elevation flows from from southsouthwest to north-east. north-east. An An automatic delineation of of the catchrnent catchment contributing contributing to the downstream downstream point point of of the west valley valley clearly clearly failed, due to the appearance appearance of of an untrue untrue adverse slope in the thalweg. thalweg. This This inaccuracy inaccuracy may be be still still worse worse in urban areas where where the natural slopes have often been been smoothed.

LJ

•• ••

35m 55m 75m 95m 115 m 135 m

Figure 3. Elevation of the the neighbouring of town Wavre Wavre (Belgium) and automatic automatic delineation delineation of of the the Figure 3. Elevation model model of neighbouring area area oftown (Belgium) and catchment. catchment.

Also the land-use land-use database database is is not easy to to use for urban-area modelhng. Available source data data are are spectral spectral Also the not easy use for urban-area modelling. Available source LANDSAT images (pixels of 30 m x 30 m) translated into land-use classification. As this was designed LANDSAT images (pixels of 30 m x 30 m) translated into land-use classification. As this was designed for for agricultural purpose, only only 33 of the 54 urban areas, areas, depending depending on on the the habitation of the 54 proposed proposed classes classes concern concern urban habitation agricultural purpose, density: the habitat habitat represents represents less of the total area, suburban density density if if this rate is is density: rural rural density density ifif the less than than 33 33 % % of the total area, suburban this rate between 33 and 66 %, urban if more than 66 % of the surrounding pixels are identified as habitation, Figure between 33 and 66 %, urban if more than 66 % of the surrounding pixels are identified as habitation. Figure 44 depicts land-use map same region depicts such such a a land-use map for for the the same region as as above. above.

m • m • m • a

Downtown density Downtown density Suburban density Suburban density Rural density Rural density Crops - Forest Crops - Forest

IIT l._.J

Water Water

Figure 4. Land use and habitat in the neighbouring area of Wavre (Belgium). Figure 4. Land use and habitat in the neighbouring area ofWavre (Belgium).

Two interesting conclusions may be drawn from this figure. On one hand, a land-use model obtained by Two interestingofconclusions mayaccording be drawntofrom this figure. On oneproperties hand, a land-use obtained by reclassification the grid nodes the neighbouring nodes leads to amodel clear definition of reclassification of the grid nodes according to the neighbouring nodes properties leads to a clear definition of homogeneous areas: for example this technique highlights the contours of urban centre, suburban and rural homogeneous example technique contoursOn of the urban centre, suburban and rural districts which areas: may for be useful forthis general designhighlights of urbanthedrainage. other hand, this interpolation

districts which may be useful for general design of urban drainage. On the other hand, this interpolation

91

Rainfall-runoff relation in urbanised urbanised areas areas Rainfall-runoff relation in

technique obliterates obliterates the detailed detailed description description needed for defining velocity: with technique needed for defining the flow flow paths and the runoff runoff velocity: with whole town centre would such an approach approach the whole town centre would be considered considered as as built built with with a runoff runoff coefficient coefficient close close to one.

USE HYDROLOGICAL USE OF OF URBAN-CARTOGRAPHY URBAN-CARTOGRAPHY TOOLS TOOLS FOR FOR HYDROLOGICAL MODELLING MODELLING The nromam The PICC PICC program The Ministry of Equipment and and Transport (MET) of of the the Walloon Region (Belgium) carried out from The Ministry of Equipment Transport (MET) Walloon Region (Belgium) has carried out from 1991 a very ambitious continuous continuous digital cartography program (PICC: Projet Informatique de Cartographie Cartographie very ambitious digital cartography program (PICC: Projet Informatique Continue). aim of of the PICC project project is the the progressive of a database database where where data of of different different the PICC progressive creation creation of Continue). The The aim sources distribution networks, networks, population population data, data, social social or or sources (land (land use, use, industrial industrial characteristics, characteristics, infrastructures, infrastructures, distribution health etc.) are to aa common co-ordinates system system (georeferencing). product is is health statistics, statistics, etc.) are referenced referenced to common co-ordinates (georeferencing). The The product expected tool, above above all all for for regional regional and urban planning, planning, but but also also for for natural natural expected to to become become aa decision-making decision-making tool, and urban resources environment protection, traffic optimisation, original principle principle of of the work resources management, management, environment protection, traffic optimisation, etc. An An original work is the as the Walloon territory 17000 km’) will be be included in the program (MET, (MET, the continuity continuity as the whole whole Walloon territory (about (about 17000 km2) will included in the program 1993). 1993). Several database: some some of are point point data data (topographical (topographical references, references, of them them are Several elements elements are are represented represented in in the the database: trees, street inlets, sewer manholes, etc.), most of them are line data (for (for example: example : road limits or gutters) them are line data road limits or gutters) trees, street inlets, sewer manholes, etc.), most of only little number of which form polygons (houses for for instance). Every element element is is associated to three three cocowhich form polygons (houses instance). Every associated to only aa little number of ordinates of which the guaranteed accuracy (mean (mean quadratic error) is is summarised summarised in in Table 1, These These values values ordinates of which the guaranteed accuracy quadratic error) Table 1. are to absolute co-ordinates, relative relative errors errors between points being being much smaller. are related related to absolute co-ordinates, between adjacent adjacent points much smaller. Table 1. Admissible quadratic errors errors in in PICC continuous-cartography program Admissible quadratic PICC continuous-cartography program Table I.

Geodesic points Geodesic reference reference points Clearly points Clearly identifiable identifiable points Other Other points points

Xx cm cm * 20 ±20 5 12 ± 12 *25 ±25

Y y cm cm *20 ±20 f 12 12 ± *25 ±25

z2 cm cm *3 ± 3 f 15 ± 15 *25 ±25

Use of PICC data data for characterisation Use ofPICC for urban-catchment urban-catchment characterisation The PICC project deals with cartography : that means that for any application the final document will be a The PICC project deals with cartography: that means that for any application the final document will be a map. The structure of the database is deeply influenced by this starting point. For example if a line is a map. The structure of the database is deeply influenced by this starting point. For example if a line is a common limit of two distinct areas, this limit will appear in only one of the concerned feature types. Figure common limit of two distinct areas, this limit will appear in only one of the concerned feature types. Figure 5a shows the layer describing the road-limits for the Lauzelle district in Louvain-la-Neuve : the missing Sa shows the layer describing the road-limits for the Lauzelle district in Louvain-la-Neuve: the missing contours are classified in other layers (footpath limits for instance). Such a specificity does not yield any contours are classified in other layers (footpath limits for instance). Such a specificity does not yield any problem for cartographic use : reconstruction of the whole map is conducted by addition of the various problem for cartographic use: reconstruction of the whole map is conducted by addition of the various layers and the complete contour results from this superposition (see Figure 5b where line data corresponding layers and the complete contour results from this superposition (see Figure 5b where line data corresponding to motorway margins and trail and footpath limits have been added). to motorway margins and trail and footpath limits have been added). However, distributed hydrologic models generally require area data (rasterisation), which is easy to get if the However, distributed hydrologic require data (rasterisation), which easy sparse to get iflines the original data is a polygon. If not, models it resultsgenerally in tedious work area to reconstruct the polygon dataisfrom original data is a polygon. If not, it results in tedious work to reconstruct the polygon data from sparse lines belonging to various layers. Moreover it appears often difficult to make a single curve from separate paths belonging and to closetoit.various layers. Moreover it appears often difficult to make a single curve from separate paths and to close it. Habitation delineation is easier, since this type of information is supplied in the PICC program directly as Habitation delineation easier, this or type of information in the PICC program directly polygon data. Polygons is may be since adjacent nested depending isonsupplied the urbanisation characteristics. Classicas on the urbanisation characteristics. Classic polygon data. Polygons may be adjacent or nested depending functions provided by GIS software packages convert automatically vector polygons to raster structures. functions provided by GIS software packages convert automatically vector polygons to raster structures.

Y. and A A. ESCARMELLE y, ZECH ZECH and ESCARMELLE

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6:MET-D432 ©:MET-D432 Licence no :: 0.432/9801/00 D.432/9801/OOl/UCL-Zech Licence nO IiUCL-Zech aa

0:MET-D432 ©:MET-D432 Licence D.432/9801/001/UCL-Zech Licence no n° :: 0.432/9801/001IUCL-Zech

bb

Figure 5. 5. Roads Roads of of LauzelIe Lauzelle district Belgium. Figure district at at Louvain-la-Neuve, Louvain-Ia-Neuve, Belgium. a. only; b. b. From From motorway-, motorway-, pathpath- and and footpath-limit footpath-limit layers a. From From road-limit road-limit layer layer only; layers

This time consumption land-use raster raster database. database. This results results in in aa significant significant advantage advantage in in time consumption for for establishing establishing aa land-use Figure 6a gives an example of land-use classification for a part of the Lauzelle district at Louvain-la-Neuve. Figure 6a gives an example of land-use classification for a part of the Lauzelle district at Louvain-la-Neuve. This map was easy to the class class habitation. habitation. The The complete complete land-use land-use map map was was much much more more to build build for for the This map was rather rather easy difficult achieve due to the conversion from vectors to to closed closed polygons which is is to to be be difficult to to achieve due to the needed needed conversion from separate separate vectors polygons which done generally by hand. done generally by hand.

a

b b

a • •

Habitations Roads, paths and parkinQ

D

Gardens and meadolM> Forest

Figure 6. Land-use map of the south-west part of Lauzelle district: a) by automatic conversion of PICC data and b) Figure 6. Land-use map of the south-west part of Lauzelle district: a) by automatic conversion ofPrCC data and b) from interpolation of aerial photographs. from interpolation of aerial photographs.

However some defects remain in the so-obtained land-use map that can affect lightly the right hydrologic However some remain in the so-obtained thatPICC can affect lightly 6a) the to right modelling of thedefects catchment. If we compare the mapland-use capturedmap from data (Figure the hydrologic same map modellingbyofinterpretation the catchment. we compare the map fromdifferences PICC data clearly (Figureappear. 6a) to Some the same map obtained of If aerial photographs (Figurecaptured 6b), some of them of aerial photographs (Figure 6b), some differences clearly appear. Some of them obtained by interpretation can be explained by the fact that the aerial photograph was taken some months later, at a time where can be explained by the factOther that discrepancies the aerial photograph taken some at a time where urbanisation had developed. seem morewassignificant. For months examplelater, the continuity of the urbanisation had developed. discrepancies more significant. For example the continuity of the roads is not guaranteed in theOther automatic conversionseem of the PICC data, because some paths are too small for roads guaranteed the automatic the photograph PICC data, because are too small for the 5 mis not mesh, while thisin continuity can conversion be forced byof the interpreter,some Onpaths the other hand, some the 5 m mesh, while this continuity can be forced by the photograph interpreter. On the other hand, some

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Rainfall-runoff relation relation in urbanised areas areas Rainfall-runoff

misinterpretations are are also also observed observed in in the the manual manual procedure: procedure: some some paths paths are are interpreted interpreted as forest forest due due to to misinterpretations presence of oftrees trees hiding hiding the the soil soil level. level. presence of information information needed needed for for hydrological hydrological modelling modelling is the the elevation elevation model model:: for for the the Another important important piece piece of Another construction instance of the the latter, latter, PICC PICC data data may may also also play play an an interesting interesting role. role. Most Most of of the the point point data data (for (for instance construction of the manhole manhole covers) covers) represented represented in in the the cartographic cartographic system system are supplied supplied with with their their level level in in such such a way way that that a the relatively dense dense unstructured unstructured grid grid of of ground ground elevations elevations is is available. available. Classical Classical GIS GIS functions functions make possible relatively make itit possible to such a to interpolate interpolate those those points points to to the the fixed fixed grid grid needed needed for for rasterisation. rasterisation. Figure Figure 7 gives gives an example example of of such work work for for Lauzelle. Lauzelle.

Figure 7. DEM of Lauzelle district. Figure 7. DEM of Lauzelle district.

II

•• •• ••

55 10 10

m

15 15

I

20 20

m

25 25

m

30 30

m

35 35

•• •• •• m

40 40

m

45 45

m

50 50

m

55 55

m

60 60

m

65

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Figure 8. Centre of Wavre: catchment delineation, sewer network, times of concentration (in minutes). Figure 8. Centre of Wavre: catchment delineation, sewer network, times of concentration (in minutes).

Runnine the hydrologic model with PICC data Running the hydrologic model with PICC data One piece of primordial information is not available in the PICC database: the sewerage-network One piece of The primordial not available in the location, PICC database: characteristics. network information plan may beisdeduced from manhole at least ifthe the sewerage-network general scheme is characteristics. The network plan may be deduced from manhole location, at least if the general scheme is known, but no information may be found in PICC data about sewer elevations and diameters. Unfortunately known, but noinformation information escaping may be found PICC data about requirements sewer elevations and included diameters.in Unfortunately underground totallyin to cartographic is not the database not included in thenetwork database underground to cartographic leading to the information usual tedious escaping work of totally introducing them. Figurerequirements 8 shows an is example of the sewer in leading theWavre. usual tedious work of introducing them. Figure 8 shows an example of the sewer network in the centreto of the centre ofWavre.

Y. ESCARMELLE Y. ZECH ZECH and and A. A. ESCARMELLE

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Figure of the distributed-model Figure 8 also depicts one of of the most most attractive attractive results results of distributed-model approach approach:: the automatic delineation of of time of of concentration concentration contours. contours. ItIt is interesting interesting to observe observe that the relatively relatively long time of of concentration concentration for for the whole whole catchment is due to small parts parts of of the watersheds watersheds corresponding corresponding to less developed developed areas. areas. It is now It now easy to use the model presented in the first first section. For For Louvain-la-Neuve Louvain-la-Neuve catchment, some measurements were were carried carried out at the outlet of of the district district of of Lauzelle. Comparisons Comparisons between between results results from the distributed distributed models, other results results from from classic classic software software packages packages and the measured discharges discharges reveal that the new new model is closer closer to the reality reality at least for for such a type of of catchment catchment (Zech (Zech et et al., ai., 1994). 1994). CONCLUSION CONCLUSION For of an an urban urban catchment, catchment, distributed distributed models models seem seem promising. promising. Such Such aa For modelling modelling the the hydrologic hydrologic behaviour behaviour of model has shown accuracy for for model developed developed some some years years ago ago at at the the University University of of Louvain Louvain has shown an an excellent excellent accuracy predicting predicting rainfall-runoff rainfall-runoff relation, relation, compared compared to to field field measurement. measurement. The The main main difficulty difficulty lies lies in in the the necessity necessity to (5 m) m) for for reaching reaching this this accuracy accuracy and and such such aa resolution resolution is is not not commonly commonly to use use aa rather rather refined refined mesh mesh (5 available. available.

The It may of using using recent recent databases databases developed developed for for automatic automatic cartography. cartography. It may The paper paper investigates investigates the the possibility possibility of be of the the line line be concluded concluded that that for for land-use land-use determination determination such such aa tool tool is is use&l useful but but not not perfect perfect because because of structure of most supplies really most data. data. For For elevation-model elevation-model construction construction the the cartographic cartographic system system supplies really aa bargain bargain structure of while so helpful helpful in in urban urban hydraulics, hydraulics, is is unfortunately unfortunately not not taken taken into into while underground underground information, information, that that could could be be so account database made account in in aa database made for for cartography. cartography. If some some adaptation addition to If adaptation and and addition to tools tools designed designed for for cartography cartography could could be be performed performed for for the the future, future, urban urban hydrology friendly as as accurate accurate and and user user friendly as urban urban hydraulics. hydraulics. hydrology could could become become as ACKNOWLEDGEMENTS ACKNOWLEDGEMENTS We appreciated the collaboration of “Direction de de la We appreciated the kind kind collaboration of the the "Direction de la la topographie topographie et et de la cartographic” cartographie" (director: (director: Mr A. Collignon) of the Walloon Ministry for Equipment and Transport and the significant Mr A. Collignon) of the Walloon Ministry for Equipment and Transport and the significant assistance assistance of of the the “Centre d’Etudes en Amenagement du Territoire” (director: Prof. P. Govaerts) who helped in the "Centre d'Etudes en Amenagement du Territoire" (director: Prof. P. Govaerts) who helped in the interpretation of the databases. interpretation of the databases. REFERENCES REFERENCES Hillel. D. (1971). Soil and Water. Physical Principles and Processes. Academic Press, London, UK Hillel. D. (1971). Soil and Water. Physical Principles and Processes. Academic Press. London, UK Meijerink, A. M. J., de Brouwer, H. A. M., Mannaerts, C. M. and Valenzuela, C. R. (1994). Introduction to the use of Geographic Meijerink. A. M. J.• deSystems Brouwer.forH. A. M.• Mannaerts, and Valenzuela. C. R. (1994). Introduction to the use of Geographic Information practical Hydrology. C. M. UNESCO International Hydrological Programme IHP-IV M 2.3. Information Systems Publication number 23. for practical Hydrology. UNESCO International Hydrological Programme IHP-IV M 2.3. Publication number 23. MET (1993). Projet informatique de cartographic continue (PICC). Les cahiers du MET. Collection Techniques. Minis&e wallon MET (1993). Projet informatique de cartographie de l’bquipement et des transports. Namur, continue Belgium (PICC). Les cahiers du MET. Collection Techniques. Ministere wallon l'equipement et des transports. Namur. BelgiumA. (1994). Rainfall-runoff Zech, Y.,deSillen, X., Debources, C. and Van Hauwaert, modelling of partly urbanized watersheds, Zech, Y.,Comparison Sillen. Debources, C. and Van (1994). partly between a distributed modelHauwaert, using GISA.and other Rainfall-runoff models. Sensitivitymodelling analysis.ofWat. Sci.urbanized Tech., 29(watersheds. l-2), 163Comparison between a distributed model using GIS and other models. Sensitivity analysis. Wat. Sci. Tech., 29(1-2), 163170. 170.

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