Urban Flood Management Dordrecht indonesia
UFMWP201
Water specific issues for Urban
Flood Management
The Stadswerven area as a case study to gain insight in relevant hydraulic issues for urban flood management.
Karin Stone Joost Beckers Reinaldo Penailillo
31 october 2008 www.ufmdordrecht.nl
© Deltares, 2008
Prepared for: BSIK, Leven met Water, City of Dordrecht
Water specific issues for Urban Flood Management
The Stadswerven area as a case study to gain insight in relevant hydraulic issues for urban flood management.
Karin Stone Joost Beckers Reinaldo Penailillo
Report
October 2008
COLOFON
Urban Flood Management (UFM) De steden Dordrecht, Hamburg en Londen werken samen in een project aan de ontwikkeling en toepassing van duurzaam Stedelijk Hoogwaterbeheer in 3 proefgebieden.
Voor UFM Dordrecht bestaat het consortium van publieke en private, lokale en nationale partijen uit: De gemeente Dordrecht, Waterschap Hollandse Delta, Rijkswaterstaat, Ministeris van Verkeer en Waterstaat, Provincie Zuid-Holland, Dura Vermeer, Deltares, Progrez en UNESCO-IHE.
UFM Dordrecht wordt ondersteund en gefinancierd door Leven met Water (LmW). LmW projectnummer: P-3075
Document code [vb. UFMWP201]
Water specific issues for Urban Flood Management Karin Stone, Joost Beckers, Reinaldo Penailillo Delft, 31 oktober 2008
Auteursrechten
Alle rechten voorbehouden. Niet uit deze uitgave mag worden verveelvoudigd, opgeslagen in een geautomatiseerd gegevensbestand of openbaar gemaakt, in enige vorm of op enige wijze, hetzij elektronisch, mechanisch, door fotokopieën, opnamen of enige andere manier, mits de bron op duidelijk wijze wordt vermeld, alsmede de aanduiding van de maker indien deze in de bron voorkomt.
Aansprakelijkheid
De partners van het UFM project en degenen die aan dit project hebben meegewerkt, hebben een zo groot mogelijke zorgvuldigheid betracht bij het samenstellen van deze uitgave. Nochtans moet de mogelijkheid niet worden uitgesloten dat er toch fouten en onvolledigheden in deze uitgave voorkomen. Het gebruik van deze uitgave en gegevens daaruit is geheel voor eigen risico van de gebruiker en UFM sluit, mede ten behoeve van al degenen die aan deze uitgave hebben meegewerkt, iedere aansprakelijkheid uit voor schade die mocht voortvloeien uit het gebruik van deze uitgave en de daarin opgenomen gegevens. Mocht u onvolkomenheden aantreffen, dan verzoeken wij u dit bij ons kenbaar te maken.
Contactadres: s.vanherk@ufmdordrecht.nl
Voorwoord eindpublicatie Urban Flood Management Dordrecht
Wonen in een laaggelegen delta, dat kun je niet alleen. Steeds meer zien overheden, kennisinstituten, burgers en het bedrijfsleven dat zij elkaar nodig hebben en dat zij bovendien samen tot betere oplossingen kunnen komen. Je hebt namelijk pas iets aan een veilig land als mensen het mooi genoeg vinden om er te willen wonen. En andersom is een mooie stad waar je te vaak natte voeten krijgt ook niet leefbaar. De kernvraag is: hoe zorgen we ervoor dat Nederland nu maar ook over 50 jaar en zelfs 100 jaar nog een veilig én aantrekkelijk land is om in te wonen?
Het Leven met Water project Urban Flood Management (UFM) Dordrecht laat zien hoe we samen kunnen werken aan deze vraag. Naast de waardevolle concrete uitkomsten van het project, zoals de stedelijk ontwerpen, de inzichten in hoogwater risico en potentiële schade, en in de juridische en communicatieve instrumenten, levert UFM vooral een manier van werken. Een innovatieve manier van werken die een nieuwe weg inslaat in het omgaan met hoogwater in stedelijk gebied.
De UFM aanpak is niet alleen nuttig bij waterveiligheidsvraagstukken. Deze vorm van ontwerpend onderzoek kan ook goed ingezet worden bij het zoeken naar integrale oplossingen voor wateroverlast, waterkwaliteit en andere opgaven. De interactie tussen verschillende disciplines is daarbij essentieel. Daaruit komen nieuwe inzichten en ideeën. Dat is niet alleen waardevol voor de uiteindelijke oplossingsrichtingen, maar ook een verrijking voor de deelnemende personen en organisaties. Door de interactieve samenwerking hebben inzichten uit UFM bij kunnen dragen aan beleidsontwikkelingen op verschillende schaalniveaus. Ik zie graag dat deze aanpak ook vervolg krijgt in daadwerkelijke uitvoering van de ideeën. We moeten als Nederland de durf en de wil hebben om voorop te blijven lopen. Laat u met deze publicatie dan ook inspireren om op soortgelijke wijze te werken aan een klimaatbestendige inrichting van Nederland, niet alleen buitendijks maar vooral ook binnendijks. Veel leesplezier!
Annemi ek e Nijhof
Directeur-Generaal Water Ministerie van Verkeer en Waterstaat
www.ufmdordrecht.nl www.levenmetwater.nl
Eindpublicatie UFM: digitale reader van alle einddocumenten
Beste Lezer,
Het document dat u voor zich heeft is één van de einddocumenten van het Urban Flood Management (UFM) Dordrecht. Hieronder vindt u een overzicht van alle UFM einddocumenten die gezamenlijk het eindresultaat vormen: de digitale UFM reader.
Elk document is zowel zelfstandig leesbaar als aanvullend aan de andere documenten. Om niet in elk document een uitgebreide introductie op te hoeven nemen, is er gekozen voor een apart samenvattend document waarin de algemene projectopzet, aanpak en resultaten staan beschreven. Wij nodigen u graag uit om deze algemene UFM samenvatting te lezen voorafgaand aan de andere einddocumenten.
Bedankt voor uw interesse, namens het hele UFM consortium,
Ellen Kelder
Gemeente Dordrecht
Chris Zevenbergen
Dura Vermeer Business Development
Sebastiaan van Herk Projectcoördinator namens gemeente Dordrecht
Mocht u contact op willen nemen, vanwege specifieke vragen en/of opmerkingen, kunt u een email sturen naar: s.vanherk@ufmdordrecht.nl
Inhoudsopgave digitale UFM reader
Alle onderstaande documenten zijn te vinden op www.ufmdordrecht.nl .
Naa m/Code
Samenvatting UFM: introductie, conclusies en aanbevelingen
UFMWP201
Water specific issues for Urban Flood Management
UFMWP301
Resilient Building and Planning
UFMWP401
Ontwerpend onderzoek naar hoogwaterbestendige ontwikkeling buitendijkse stad
UFMWP501
Communicatie strategie buitendijkse gebieden
UFMWP601
Policy en Governance
Bijlagen
UFMWP202
Statistische Berekeningen
UFMWP203
Memo Watersysteem
UFMWP302
Summary of Results
UFMWP303
Summary Design Variants
UFMWP402
Werkdocumenten Werkpakket 4
Derden
Machteld Hillebrand: Kwalitatief onderzoek naar risicocommunicatie en risicoperceptie ten aanzien van overstromingen te Dordrecht
www.ufmdordrecht.nl www.levenmetwater.nl
ii
Summary
Introduction
The Urban Flood Management (UFM) project focuses at three cities that face similar challenges with regards to urban development in flood prone areas. These cities are Dordrecht, London and Hamburg. The UFM Dordrecht project focuses on the management of flood risk in the Stadswerven. The Stadswerven is an area unprotected by dikes located along the river Beneden Merwede where urban development is planned to take place. Within the Dordrecht project, urban design concepts for the Stadswerven have been developed which take actual and future flood risk into account.
Work package 2 evaluated the hydraulic and safety aspects of the water system around Dordrecht. The objective of the work package was to provide the proper information about the water system for the damage assessment (WP3) and urban planning process (WP4). Flood characteristics were implemented as design parameters for the damage assessment and urban planning process. By learning through practice, insight was gained on the usefulness of the different characteristics. Several design concepts were developed for the Stadswerven area. This was done through an inter-disciplinary approach where urban designers worked together with experts from other relevant fields. Through expert judgment from the water experts as well as through questions from the urban planners, useful and relevant flood parameters were selected which proved to be usefull in the urban design process. The information on the water system was derived through statistical analysis, hydraulic modelling, water quality modelling and through expert knowledge.
Evaluation of flood parameters
The following flood parameters were evaluated:
Water system parameter Relevance for urban flood proof design
Water levels, average and maximum Choice of building concepts and public water levels under flood conditions. Both area architecture the present situation as well as water Anticipation within design on changing levels due to climate change have been water levels due to climate change. taken into consideration
Anticipation within design on uncertainty. Duration of flooding
Choice of building concepts and public area architecture in relation to possible damage.
Predictability of floods Choice for building concepts and evacuation strategy in relation to safety.
Average velocity of the main channels Insight in safety aspects when building along a river.
Maximum reached water depth in the Choice of building concepts and public flooded areas
area architecture in relation to safety and possible damage.
Water velocities in the flooded areas Choice of building concepts and public
UFM Dordrecht ( www.ufmdordrecht.nl ), Deltares ( www.deltares.nl ) Summary UFM Dordrecht ( www.ufmdordrecht.nl ), Deltares ( www.deltares.nl ) Summary
The effect of the orientation of buildings on Choice of building concepts in relation to the flood pattern
possible damage.
Other water system parameters not evaluated, but possibly of interest for the design process are wind and wave dynamics. These can influence the impact of the water on buildings causing additional damage.
The flooding characteristics for Stadswerven were evaluated for a situation with a return period of 4000 years and for the expected design water level in 2100. The latter anticipates on rising water levels due to climate change. When looking at the flooding characteristics for Stadswerven, it is seen that the reached water depths and velocities are relatively low. For the greater part of the Stadswerven area the water depths reach up to 0.5 m. Only the zone along the river shows larger water depths. The water velocities are relatively low, mainly lower than 0.25 m/s, a velocity at which wading through the water is still safe. In addition the flood duration in the Stadswerven area is short, approximately 8 hours. This is due to the fact that the duration of a flood at Stadswerven is mainly influenced by the duration of a storm surge. Due to the short flooding time, only a thin film of sediment is deposited during a flood. Floods can be predicted at least 12 hours ahead, enough time to take measures or to evacuate the Stadswerven area. These factors provide a safe environment suitable for a flood proof urban development aiming at minimal disturbance of daily life. The limited amount of restrictions due to the flood characteristics of the area, make it possible to apply a large range of concepts and solutions to cope with floods.
Evaluation of flood proofing
Three design concepts for Stadswerven were developed. Flood patterns for the different design concepts were calculated with a 2D hydraulic model. This was done to evaluate the designs on their capability to withstand a flood and for visualization purposes. From the experience gained with the case study Stadswerven, a simple method for assessing urban designs aiming at flood proofness was developed. This method was applied to the Stadswerven design concepts. All design concepts proofed to be able to cope with floods. For the areas designated to flood, one should be attentive to safety with regards to water depths and especially the water velocities.
Sedimentation and contamination
River waters carry heavy metals adsorbed to sediment particles. When an area has been flooded, a residue due to sedimentation of silt and garbage is left behind. The flooding of the sewer system provides additional residue as well as health risks. For the Stadswerven area a first impression was gained of the sedimentation after flooding as well as the effect of flooding on the sewer system. The results show that only a thin film of sediment is left behind after flooding of the Stadswerven area. This is due to the short period of flooding. The deposits could be slightly contaminated with phosphates, nitrates and heavy metals such as copper and zinc, although no exact information is available on the expected contaminants within the deposited matter. For areas which flood more frequently one should anticipate on the possibility of deposition of contaminated sediments. Each flooding event leaves a thin film of residue which could accumulate over time and become a thicker layer.
UFM Dordrecht ( www.ufmdordrecht.nl ), Deltares ( www.deltares.nl ) Summary
Inter-disciplinary approach
The inter-disciplinary approach in which experts from different fields worked together to achieve a robust flood proof design concept, has proven to be an effective process. The interaction between the water experts and urban developers made it possible for the urban planners to anticipate on flood characteristics in an early stage of the design process. And the cooperation resulted in awareness and understanding of each others field of expertise, which in turn improved the communication and effectiveness of information exchange.
UFM Dordrecht ( www.ufmdordrecht.nl ), Deltares ( www.deltares.nl ) Summary
1 Introduction
1.1 The Urban Flood Management project (UFM)
Traditionally, flood management policies in Europe were flood defence or flood prevention policies, dominated by construction of dikes. Flood events in the 1990s in the Netherlands and Germany have given rise to intense debate on the future direction of flood management policies. It was realised that one hundred percent safety cannot
be guaranteed. In the Netherlands this insight produced a new policy concept: flood accommodation now is part of the official flood management policy.
In urban areas with high social and economic values, more focus on the reduction of the consequences of floods may provide an important opportunity in flood risk reduction. New approaches on urban design need to be developed to adapt the urban environment to flood risk by enhancing the resilience of the urban environment to floods and thus reducing its vulnerability. In many cases accepting and preparing for some degree of flooding will be a more sensible solution, not only from a technical and financial perspective, but also from a social and environmental perspective.
The Urban Flood Management (UFM) project focuses at three cities that face similar challenges with regards to urban development in flood prone areas. These cities are Dordrecht, London and Hamburg. The UFM Dordrecht project focuses on the management of flood risk in the Stadswerven. The Stadswerven is an area unprotected by dikes located along the river Beneden Merwede where urban development is planned to take place. Within the Dordrecht project, urban design concepts for the Stadswerven have been developed which take actual and future flood risk into account.
1.1 Risk assessment
The UFM project consists of 8 work packages. This report describes the methods, results and conclusions of work package 2 (WP2) which evaluated the hydraulic and safety aspects of the water system around Dordrecht. The objective of the work package was to provide the proper information about the water system for the damage assessment (WP3) and urban planning process (WP4). The amount of flood damage is, amongst others, a function of water depths and water velocities. And the urban planners needed to have sufficient knowledge about the dynamics of the river and of flooding risks to be able to develop flood proof urban designs. This required the development of hazard maps, information on lead times of floods (how much time is available to take measures), prediction of water levels, currents and water quality (sewerage systems, sedimentation from the rivers). By learning through practice insight was gained on the use of hydraulic information for damage assessment and the urban planning process.
Flood maps (water depths and velocities)
WP3: ‘Resilient planning & building’
1. Hydraulic
WP2: ‘Risk
conditions
assessment’
WP4: ‘Case: creation of an
2. Designs
integrated urban flood risk management plan’
3. Flood maps for the design proposals
Figure 1.1
Relation between WP2 and the WP’s 3 and 4.
Several innovative aspects within WP2 have provided new insight in the fields of hydraulic modelling and urban design. These aspects were:
• The modelling scale. Flood modelling in the Netherlands is mainly done on a larger scale, e.g. on the scale of a dike ring. The resolution of the computation units for this scale is in the order of 100x100 meters. For the Stadswerven case study, it was necessary to model on a scale where individual housing blocks and streets are recognisable. This demanded a modelling resolution of 10x10 meters. Experience was gained on modelling on a detailed scale and provided information on the possibilities and limitations of the modelling software.
• The implementation of hydraulic aspects as design conditions for spatial planning. Because of the character of the Stadswerven study area, being an area unprotected by dikes, it is necessary to consider the hydraulic characteristics of a potential flood, as well as the hydraulic behaviour of the rivers around Stadswerven for the urban design. The use of hydraulic information as a design condition is a relatively new approach to the design phase. By doing so, insight was gained on the functionality of the technical hydraulic information for urban design purposes.
• Interdisciplinary approach in spatial planning. The exchange of information and knowledge between the water experts and the urban planners required an interdisciplinary work process. The water experts and urban planners carried out their activities simultaneously providing the possibility to give advice as well as react to requests for information from the other disciplines during the entire process. The intensive cooperation provided insight in each others field of work resulting in a better understanding of each others expertise and needs.
1.2 Guidance to the reader
Chapter 2 gives an overall explanation of the approach followed in work package 2. The results are treated in chapters 3 to 5 after which chapter 6 discusses a method to assess a flood proof urban development. Chapter 7 concludes with conclusions and recommendations.
2 Approach
2.1 Purpose and approach
The objective of the project was to implement flood characteristics as input parameters for the damage assessment and urban planning process. By learning through practice, insight was gained in the usefulness of the different flood characteristics for these work fields. The Stadswerven case study was carried out as a learning process. Through the case study, several design concepts were developed for the Stadswerven area. This was done through an inter-disciplinary approach where urban designers worked together with experts from other relevant fields. WP2 provided the following activities within the working process:
Generation of flood maps for input to damage assessment The parameters on maximum reached water depth and flow velocity for a flooded area, determine the amount of flood damage. WP2 provided these parameters for the non- protected areas on the Island of Dordrecht as input for the damage assessment carried out within WP3.
Design conditions as input for the urban design concepts The urban designers implemented hydraulic information as design conditions for spatial planning (WP4). This was done for the Stadswerven study area. In consultation with the urban designers and from expert judgment, it was decided to evaluate the possible use of the following parameters as design conditions:
• Water levels, average and maximum water levels under flood conditions. Both the present situation as well as water levels due to climate change have been taken into consideration. The effects of climate change were evaluated using the WB21 climate change scenarios.
• Duration of high water levels •
Predictability of floods. •
Average velocity of the main channels. •
Maximum reached water depth in the flooded areas. •
Water velocities in the flooded areas. •
The effect of the orientation of buildings on the flood pattern.
Evaluation of the Stadswerven design concepts Three design concepts for Stadswerven were developed by the designers. Each concept was tested on its flood proof capability.
Visualization of final designs Flood patterns for the different design concepts were calculated. The flood patterns as
a course of time were used for visualization purposes. Residue due to flooding of urban area
River waters carry heavy metals adsorbed to sediment particles. When an area has been flooded, a residue due to sedimentation of silt and garbage is left behind. The flooding of the sewer system generates additional residue as well as health risks. For the Stadswerven area a first impression is gained of the sedimentation after flooding as well as the effect of flooding on the sewer system. Information on sedimentation gives insight into the after effect of a flood. Sedimentation was not applied as a design condition for the Stadswerven case study, but alternative urban designs which incorporate or reduce the effects of sedimentation can be developed in addition to the proposed design concepts.
2.2 Applied methods
Information on the hydraulic system was gained through several methods. First of all use was made of expert knowledge of the project team on the water systems around Dordrecht. Information on the water levels was derived through statistical analysis. Hydraulic modelling was used to gain insight on flood duration, average flow velocities, orientation of buildings and overland flooding parameters such as water depths and flow velocities. A water quality model was applied to gain information on sedimentation patterns.
Statistical analysis The probability of a water level to occur was derived from statistical analysis. First of all use was made of historical water level measurements over a period of time. Also the statistical tool Hydra B was used. This tool combines the probability of the different parameters which influencing the water levels at Dordrecht to calculates the probability of the actual water levels. The uncertainty was also calculated and expressed as a confidence interval. Finally the effect of climate change on the water levels was evaluated through climate change scenarios. Appendix A gives an extensive description of the applied statistical analysis methods.
Hydraulic modelling The flooding patterns were simulated through hydraulic modelling. A 1D model was used to simulate the river flow. Overland flooding was calculated with a 2D model. Appendix B gives an extensive explanation of the development of the 1D and 2D models.
Water quality modelling The sedimentation of silt and clay during a flood, was simulated using a water quality model. In this case a combined 2D and water quality model were implemented. More information on the sedimentation modelling can be found in Appendix C.
2.3 The study area
History The city of Dordrecht is situated on the Island of Dordrecht. This Island resulted from the St. Elizabethsflood in 1421. Before the flood the city of Dordrecht was situated in the polder the "Groote or Zuid-Hollandse Waard" (Figure 2.1).
Figure 2.1
The city of Dordrecht before the St, Elizabethsflood of 1421.
Due to the flood, a large part of this polder was inundated and only the eastern part of the polder and the city of Dordrecht where the ground levels are higher, were spared. In time the inner sea caused by the flood, was reclaimed and protected by dikes (Figure 2.2).
Figure 2.2
The island of Dordrecht was recaptured over time.
Urban areas unprotected by dikes Large areas on the Island of Dordrecht are not protected by dikes. Parts of these areas are urbanized. An example of such an area is the old city of Dordrecht. The study area Stadswerven is also located in this zone. The Stadswerven area used to be an industrial site dominated by shipyards. The river Wantij runs through the Stadswerven area dividing it into a northern and southern part (Figure 2.3).
Figure 2.3 City of Dordrecht. In black the urban areas which are not protected by dikes. The area of Stadswerven lies within the box. The old city of Dordrecht is encircled.
The ground levels within these unprotected areas are relatively high. In the Stadswerven the average elevation is approximately NAP +3m or higher. Only the grounds around the harbour in the old city lie at approximately NAP +2 à 2.5m, making this area more prone to flooding. But the buildings around the harbour are adjusted to
be able to cope with flooding. For many of these houses the floor level lies 0.2 – 0.4 m above the ground level, they are floored with tiles and are built such that they are able to withstand water flowing along the outer wall. Figure 2.4 shows a detailed aerial photo of the study area Stadswerven.
Noord
Beneden Merwede
Dordtse Kil
Wantij
Figure 2.4
Aerial photo of the Stadswerven case area (Google earth)
The rivers around the Island of Dordrecht The island of Dordrecht is enclosed by rivers. Clockwise starting at the north side, the island is bounded by the ‘Beneden Merwede’, ‘Nieuwe Merwede’, ‘Amer’ and ‘Dordtse Kil’ (Figure 2.5).
Beneden Merwede
Wantij
Nieuwe Merwede
Dordtse Kil
Amer
Figure 2.5 Island of Dordrecht. The city of Dordrecht is indicated in yellow. The green line shows the location of the primary dike. The study area Stadswerven is situated within the encircled area.
Dordrecht lies in the transition zone where the water levels are determined by both the discharge of the rivers Rhine and Meuse and the sea levels. The sea influence is also noticed by the fact that the river levels follow the tide cycle.
Figure 2.6 Rivers Rhine and Meuse. Island of Dordrecht is indicated in red.
The flow direction depends on the discharge of the Rhine and (to a lesser extent) the Meuse. The water flows towards the sea during low tides. Water flows into the sea via the Nieuwe Waterweg (Maasmonding) and through the locks in the Haringvliet (Figure 2.7). The Maasmonding is an open outlet. The discharge at the Haringvliet locks depends on the Rhine discharge at Lobith. The locks are shut when the river discharge
is low (< 1200 m 3/ s). The locks are fully open at a Rhine discharge of 10,000 m s.
Maasmonding
Haringvliet locks
Figure 2.7 Flow direction during low tides.
The water flow in an opposite direction during high tides up to a Rhine discharge at Lobith of 4000 m 3 /s as illustrated in Figure 2.8.
Figure 2.8 Flow direction during high tides for a Rhine discharge at Lobith up to 4000 m 3 /s.
The flow direction changes when the Rhine discharge at Lobith is larger than 4000 m 3 /s. From this point onwards the river discharge starts to dominate the incoming tide flow. This is illustrated in Figure 2.9.
Figure 2.9 3 Flow direction during high tides for a Rhine discharge at Lobith larges than 4000 m /s .
3 Design conditions for flood proof urban development
An important project goal was to develop design concepts for flood proof urban development through an inter-disciplinary process which involved both water and urban planning experts. By learning through practice, it was possible to gain an insight into which water system parameters are essential or useful within this inter-disciplinary design process. In the initial phase of the design process, flood characteristics were implemented as design conditions. During the process additional information was submitted. The selection of relevant parameters came about through expert judgment from the water experts as well as through questions from the urban planners. This chapter treats the water system parameters which proved to be essential or useful, both as initial design condition as well as during the design process. The evaluated water system parameters are listed in Table 3.1.
Table 3.1 Water system parameters and their relevance for the urban flood proof design
Water system parameter Relevance for urban flood proof design
Water levels, average and maximum Choice of building concepts and public water levels under flood conditions. Both area architecture the present situation as well as water Anticipation within design on changing levels due to climate change have been water levels due to climate change. taken into consideration
Anticipation within design on uncertainty. Duration of flooding
Choice of building concepts and public area architecture in relation to possible damage.
Predictability of floods Choice for building concepts and evacuation strategy in relation to safety.
Average velocity of the main channels Insight in safety aspects when building along a river.
Maximum reached water depth in the Choice of building concepts and public flooded areas
area architecture in relation to safety and possible damage.
Water velocities in the flooded areas Choice of building concepts and public area architecture in relation to safety and possible damage.
The effect of the orientation of buildings on Choice of building concepts in relation to the flood pattern
possible damage.
Other water system parameters not evaluated, but possibly of interest for the design process are wind and wave dynamics. These can influence the impact of the water on buildings causing additional damage. Sedimentation due to flooding was also Other water system parameters not evaluated, but possibly of interest for the design process are wind and wave dynamics. These can influence the impact of the water on buildings causing additional damage. Sedimentation due to flooding was also
3.1 Water levels
For the urban development of an area not protected by dikes or other defence systems, it is essential to understand which water levels can occur in advance of the design phase. A robust design anticipates on the possible water levels including expected water levels due to climate change. Ground levels and the expected water levels determine the choice for building concepts and public area architecture. A low area subject to frequent flooding calls for different building concepts than an elevated area which will flood only occasionally.
When considering water levels and urban planning, a distinction is made between the regular daily water levels and the less frequent high water levels. The Stadswerven development should be able to cope with the high water levels but should also provide
a pleasant and safe environment for daily life. These requirements apply to the present- day situation as well as to future conditions which are subject to climate change. The following sections therefore consider the regular water levels, as well as the extreme conditions. Both aspects are looked at in perspective of climate change.
3.1.1 Present water levels under regular conditions The water levels at Dordrecht in a regular situation are mainly influenced by the sea tides. Therefore two average water levels are distinguished, an average low and an average high tide water level. The average low tide level at Dordrecht is NAP +0.20m. The average high tide level is NAP +0.90m.
Due to rising sea levels as a result of climate change, it is expected that the average water levels at Dordrecht will increase. Figure 3.1 illustrates the effect of sea level rise on the water levels along the Rhine river under average discharge conditions. The dashed lines indicate the average low tide, the solid lines indicate the average high tide. Dordrecht is located approximately at river km. 980.
Water levels along the Bovenrijn, Waal, Beneden Merwede and Oude Maas – average discharge
Water level (cm +NAP)
Current sea levels sea level rise of 1 m sea level rise of 2 m sea level rise of 4 m sea level rise of 6 m
Location (km)
Figure 3.1 Effect of sea level rise on the average water levels along the Rhine river under average discharge conditions. Several values for sea level rise are shown in different colours. The dashed lines indicate the average low tide, the solid lines indicate the average high tide. Dordrecht is at km 980.
Figure 3.1 shows that the average water levels at Dordrecht will increase due to rising sea levels. To which extent the levels will increase, depends on the degree of sea level rise. The relation between the increase of average water levels at Dordrecht and the mean sea level rise, is illustrates in Figure 3.2.
High tide Low tide
Water levels at Dordrecht (m +NAP)
Sea level rise (m)
Figure 3.2 Relation between increase of average water levels at Dordrecht and the sea level rising due to climate change. The average high tide water level is denoted by the dashed line, the average low tide level by the solid line.
The present low and high tide levels at Dordrecht are plotted at a sea level rise of 0 metre. From Figure 3.1 it can the be seen that an average sea level rise of one meter, results in a water level rise at Dordrecht of approximately 95 centimetre. With this rule of thumb an approximate increase of the average water levels at Dordrecht due to sea level rise is calculated for the different WB21 climate change scenario’s (Table 3.2). see Appendix A for more information on the WB21 climate change scenarios.
Table 3.2 Approximate increase of water levels at Dordrecht due to sea level rise for the three WB21 climate scenarios.
WB21 scenario (KNMI)
3.1.2 High water levels Under extreme conditions, the water levels at Dordrecht can reach high values. Although these conditions don’t occur often, urban planners need to anticipate on extreme water levels. Through statistical analysis (frequency analysis), estimations are made of the probability of certain water levels to occur. This probability is expressed as the frequency per year that a certain water level is exceeded, the so-called ‘frequency of exceedance’. The frequency of exceedance is presented on a scale of 0 to 1. The inverse of this frequency is called the return period. A return period indicates on average how often a certain water level is exceeded. A return period of 10 years implies 3.1.2 High water levels Under extreme conditions, the water levels at Dordrecht can reach high values. Although these conditions don’t occur often, urban planners need to anticipate on extreme water levels. Through statistical analysis (frequency analysis), estimations are made of the probability of certain water levels to occur. This probability is expressed as the frequency per year that a certain water level is exceeded, the so-called ‘frequency of exceedance’. The frequency of exceedance is presented on a scale of 0 to 1. The inverse of this frequency is called the return period. A return period indicates on average how often a certain water level is exceeded. A return period of 10 years implies
The water level varies per location along the river. Because the variation of the water level is limited within a range of half a kilometre, one representative location was selected for which the water level analysis was performed. The calculated water levels at this point represent the predicted water levels for the whole Stadswerven area.
Noord
Papendrecht, Dike ring 16
Zwijndrcht, Dike ring 17
Beneden Merwede
Dordrecht, Dike ring 22
Oude Maas Figure 3.3
Extreme water level analysis location (arrow) and study area Stadswerven (box).
Extreme water levels at Dordrecht are caused by a combination of factors: - River Rhine discharge
- River Meuse discharge - Sea levels - Wind speed - Wind direction - The operation of the Maeslant- en Hartel flood defences (open, closed, failure) - The predictions of the sea levels at Hoek van Holland. These determine the flood
defence operation. - The operation of the Haringvliet locks
The performed frequency analysis of the water levels at Dordrecht takes all factors except the operation of the Haringvliet locks into account. The probability of an extreme water level at Dordrecht due to either an extreme sea level or an extreme river discharge is extremely small. The statistical analysis therefore considers combinations of either an extreme sea level and a moderately high river discharge, or an extreme river discharge in combination with a moderately high sea level. For the remaining The performed frequency analysis of the water levels at Dordrecht takes all factors except the operation of the Haringvliet locks into account. The probability of an extreme water level at Dordrecht due to either an extreme sea level or an extreme river discharge is extremely small. The statistical analysis therefore considers combinations of either an extreme sea level and a moderately high river discharge, or an extreme river discharge in combination with a moderately high sea level. For the remaining
The results of the frequency analysis are illustrated in Table 3.3. The table shows the return periods for the different water levels at Stadswerven. Figure 3.4 illustrates the values in a graph.
Table 3.3 Water levels for different return period at Dordrecht, Stadswerven Return period
Water level
(years)
(m +NAP)
statistical analysis trend line data from water board
water level (m+NAP)
Figure 3.4 Return period for extreme water levels at Dordrecht, Stadswerven.
The Flood Management Legislation prescribes that the dikes around Dordrecht should
be able to withstand the maximum water level that is expected to occur every 2000 years. This means that the dikes around the island of Dordrecht are designed to be able to avert floods up to a water level of NAP +3.01m. This water level is called the design water level.
3.1.3 Changing water levels due to climate change It is expected that due to climate change, extreme water levels at Dordrecht will occur more often. This is a result of sea level rise and an expected increase of extreme river discharges (though the average river discharges are not expected to rise due to expected increase of dry spells in summer). Both these factors influence the water levels at Dordrecht. As a consequence the water levels for a given return period will increase. This effect is outlined in Table 3.4. The results are based on the WB21 average climate scenario. This scenario assumes a moderate climate effect.
Table 3.4 Future water levels (m +NAP) based on the WB21 average climate scenario. return period
Table 3.4 shows that the design water level will rise from NAP +3.01m up to NAP +3.41m in 2100. This water level corresponds to a return period larger than 20,000 years in the present situation. The return period for the current design water level (NAP +3.01m) will increase to 100 years in 2100. On average the increase in water level at Dordrecht is approximately 20 cm in 50 years as is illustrated in Figure 3.5.
average scenario WB21
water l
return period (year)
Figure 3.5
Return period for water levels in 2001, 2050 and 2100.
For the determination of the height of dikes, wave dynamics, seiches e.g. are taken into account by adding approximately 50 cm. to the design water level. The ground levels of Stadswerven are NAP +3m or higher. If 50 centimetres is taken as a measure for the influence of wave dynamics and seiches, it can be expected that on average slight flooding can be expected once every 50 years in the present situation. In future the frequency of flooding will increase considerable. The old city of Dordrecht, which lies at
a level of NAP +2 to +2.5 m already experiences occasional flooding events. In future these events will occur more often.
3.1.4 General uncertainty The given return periods are estimated values. It is possible to determine a confidence interval for these values, considering a number of uncertainties in the calculation method. For the development of the urban designs an indication of the confidence interval is needed. Certain design concepts assume frequent flooding of specific areas.
E.g. if houses designed to float during a flood are implemented, then it will be expected that these houses will float on a regularly basis. It is necessary to design the areas assigned for this type of housing at a proper elevation taking the confidence interval into account to assure the flooding of the area. This requires that the return period confidence interval is known. The confidence interval can also be used to determine the necessary flexibility of the design. The uncertainty for short term predictions is smaller than for long term predictions. A flexible design concept anticipates on the long term uncertainty by reserving room for adaptation within the design.
Uncertainties in the calculations are mainly caused by the following factors:
1 Uncertainty in the course of the frequency of exceedance function (Figure 3.4). This function is a best fit to measurements from an observation period that is much shorter (37 years) than the return periods for the design water levels (2000 1 Uncertainty in the course of the frequency of exceedance function (Figure 3.4). This function is a best fit to measurements from an observation period that is much shorter (37 years) than the return periods for the design water levels (2000
2 Even if it were possible to determine the frequency of exceedance of a certain water level perfectly, it is not certain that this water level will actually be reached within the given return period. The frequency of exceedance is the average or expected frequency. To draw a parallel with the throwing of a dice: although the probability of dicing a three is 1/6, it is not said that when throwing six times, a three will be thrown exactly once. However we can calculate the probability of at least one three in six throws, or no three at all.
3 The uncertainty in development of climate change processes. It is not possible to incorporate the uncertainty due to climate change in the confidence interval. Instead, the uncertainty due to climate change is evaluated through several climate change scenario’s (see paragraph 3.1.5).
The first two types of uncertainty are included in the confidence interval shown in Figure
3.6. The graph is based on observations made in the period 1970 - 2007. It is expected that when applying additional measurements and physical knowledge, the uncertainty range will be smaller. Therefore no extrapolation beyond a return period of 250 years was done.
return period (years)
Figure 3.6 Estimated 95% uncertainty interval (outer lines) based on a limited number of observations (red dots).
3.1.5 Uncertainty due to climate change The results illustrated in Table 3.4 and Figure 3.5 are based on the WB21 average scenario. This scenario assumes a moderate climate effect. Two other WB21 scenarios 3.1.5 Uncertainty due to climate change The results illustrated in Table 3.4 and Figure 3.5 are based on the WB21 average scenario. This scenario assumes a moderate climate effect. Two other WB21 scenarios
return period (years)
Figure 3.7 Average scenario (top solid line) and maximum scenario (dashed line) for 2050. The lower solid line with observation points in red, represents the present situation.
return period (years)
Figure 3.8 Average scenario (top solid line) and maximum scenario (dashed line) for 2100. The lower solid line with observations points in red, represents the present situation.
The water levels increase as a result of sea level rise and increase of high river discharges. It is assumed that the confidence interval does not change in the future.
The National Federal agreement on water (Nationaal bestuursakkoord Water, 2003) states that for area development at a minimum the WB21 average scenario for 2050 should be considered. For long term planning (time horizon of 200 year or more) it is advised to apply the maximum scenario to be certain of a robust development.
3.2 Flood duration
The impact of a flood is partly determined by its duration. If an area is flooded for a long period, a greater disturbance of daily life can be expected. If, in addition, the water depth is large, extensive damage to buildings and public space can occur. Building in an area which will be flooded often and for longer periods of time, requires a different planning concept than an area which only floods occasionally and for short periods of time.
The duration of a flood peak caused by extreme river discharges, is mainly determined by the duration of the discharge wave. The duration of a flood peak caused by extreme sea levels, is mainly determined by the duration of the storm surge. In Dordrecht, flooding will occur as a result of a combination of river discharge and storm surge. The flood duration is therefore determined by the duration of the storm surge.
An average storm surge duration lies in the order of 30 hours. The duration of a flood on the Rhine or Meuse is about 3 to 4 weeks. But the actual time that the maximum water level is reached, and thus that the Stadswerven area is flooded, is small. The design flood wave causing a water level of NAP +3.01m at Dordrecht will hold at the very maximum for only half an hour after which the water levels will start to lower. This is due to the fact that extreme water levels are only reached by a combination of river discharge and storm surge. From the calculations made with the overland flow (2D) model, it is seen that the flooding of Stadswerven had duration of approximately 8 hours.
The effects of high river discharges and storm surges on the duration of flood waves near Dordrecht are illustrated in Figure 3.10 and Figure 3.11 for several locations along the Beneden Merwede. The locations are shown in Figure 3.9. Location N38 lies upstream of Stadswerven where the Lek bifurcates into the Beneden Merwede and Nieuwe Merwede. Location N37 lies opposite Stadswerven and location N34 lies down stream of Stadswerven along the Dordtse Kil. Figure 3.10 shows the water levels for these locations for a river discharge dominated flood. Figure 3.11 illustrated the water levels for these locations for a storm surge dominated flood.
N37 N38
N34
Figure 3.9 Location of measurement points
Figure 3.10 Water levels for 3 locations along the Benden Merwede. Rhine river discharge at Lobith is 10000 m3/s, maximum sea level at the Maasmond is NAP +2.96m.
Figure 3.11 Water levels for 3 locations along the Benden Merwede. . Rhine river discharge at Lobith is 2100 m3/s, maximum sea level at the Maasmond is NAP +4.73m.
The difference in flood duration due to a river discharge or storm surge dominated flood is clearly seen for location N38. The locations N37 and N34 show a slight increase of the water levels due to dominating river discharge, but the duration of the flood is relatively short. For the locations N34 and N37 it is seen that the influence of the storm surge is greater than the influence of the river discharge. The figure clearly illustrates the location of Stadswerven (N37) in the transition zone between river and sea dominated water levels.
3.3 Prediction of floods
How far ahead can a flood at Dordrecht be predicted? If the prediction time and thus the warning time is small, there will be little time to evacuate people. In that case it will be necessary to construct for example safe locations within the urban design. On the other hand, if the prediction time is large, there will be enough time to evacuate and evacuation possibilities should be included in the plan.
The predictability of floods at Dordrecht depends on the predictability of a river flood or
a storm surge. River dominated floods can be foreseen within a time span of at least 5 days. This is due to the location of Dordrecht at the lower end of both the Rhine and Meuse catchments. With a considerable accuracy a storm surge can only be foreseen twelve hours ahead. The accuracy of the predictions for a larger time span decrease considerably.
3.4 Average flow velocity main channels
When building along a river one can assume that the river shall be used for recreational purposes. Before building along a river, one should therefore question the suitability of the river with regards to safety. An aspect which can be explored in a preliminary stage, is the average water velocity in the main channels.
Information on the water velocities in the main rivers around Dordrecht is gathered through the measuring program of Rijkswaterstaat. The measuring points of interest for Dordrecht are ‘Dordrecht’ (Beneden-Merwede) and ‘’s Gravendeel’ (Dordtse Kil). The measuring points are illustrated in Figure 3.12. No information on water velocities is readily available for The Wantij. To gain an indication of velocities on the Wantij, use was made of the 1D SOBEK simulation model (see Appendix B).
Beneden Merwede
Dordtse Kil
Figure 3.12 Location of relevant measuring points Rijkswaterstaat
Under influence of the tide, the water flows in two directions. The magnitude of the water velocity and duration of the low and high tide flows, depend on the intrusion of the tide, the discharge of the Rhine and Meuse and the regime of the Haringvliet locks. The duration of the outgoing flow increases with an increasing river discharge and for certain parts of the river at a certain level of river discharge, only an outgoing flow occurs.
Table 3.5 gives an impression of the maximum low and high tide velocities which occur under average tide conditions and for three different Rhine discharges. Negative velocities indicate a flow in inland direction. Positive velocities indicate a flow in the direction of the sea.