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Klimatilpasning af afløbssystemer og metodeafprøvning. Økonomisk analyse

Summary and conclusions

Introduction

One of the consequences of the man-made climate changes is an increased risk of extreme rain and increased seawater level. To some extent, adaptation to the climate changes will lessen the damage. However, in Denmark there is a limited practical experience of climate adaptation. The Danish Environmental Protection Agency has tendered a contract in an attempt to increase focus on climate adaptation by dewatering of urban areas. The contract was won by NIRAS and COWI in cooperation with Aalborg and Roskilde Municipalities.

The project focused on answers to the following three questions:

• What are the main problems resulting from extreme rain and increased sea water level?
• What damage will result from extreme rain and increased seawater level?
• Which climate adaptation initiatives can lessen the damage and which initiatives are feasible seen from a socio-economic point of view?

The project team participated in a two-day workshop to gather all existing experience from experts and practitioners. Prior to the workshop, maps were produced for the two cities to enable the participants to visualise the consequences of climate changes.

Following the workshop, COWI and NIRAS have analysed the results with a view to assess the socio-economic consequences of adaptation to extreme rain and increase in seawater level in central urban areas in Roskilde and Aalborg Municipalities. Results of this analysis are given below. The outline reflects that adaptation in the two cities has been analysed separately.

Method

The methodical basis is a partial socio-economic analysis. Hereby, only consequences directly or indirectly related to the adaptation initiatives are quantified and valued. Two kinds of socio-economic analyses are applied - cost effectiveness and cost-benefit analyses. In a cost effectiveness analysis, the costs for adaptation of different scenarios are compared given that a certain goal is achieved. In a cost-benefit analysis, costs are compared to the benefits of adaptation. Both methods primarily involve direct effects in the present study.

Adaptation to man-made climate changes can be divided into two types: spontaneous adaptation and planned adaptation. Spontaneous adaptation involves immediate adaptation by manufacturers and consumers, while planned adaptation is based on political interventions. The study only includes the socio-economic consequences of planned adaptation. A prerequisite of the socio-economic analyses is that consequences of the planned adaptation are compared to a reference scenario. Spontaneous activities are included in the reference scenario along with currently planned initiatives influencing the scope of damage arising from climate changes. It is recommended practise that municipalities in their wastewater master plans maintain a service level for frequency of damage caused by overload of sewerage systems. It is assumed that this practice is continued during the reference scenario equivalent to a gradual extension of the sewerage systems.

A specific problem in socio-economic analyses of adaptation to extreme rain and increased seawater level is that the results are very dependent of the timing of incidences and the scope of damage. This has been accounted for by calculating the average damage cost today and in the future related to specific climate adaptation initiatives. The average damage costs are calculated based on an assessment of incidences occurring approximately once every 100 years.

Roskilde

Extreme rainfall

Roskilde Municipality has not been affected significantly by extreme rain storms during the recent years. Thus, the assessment of expected damage is made based on hydraulic calculations. The calculations show that a 100-year incidence will result in the northern and southern part of Roskilde being affected with overload of up to 0.5 meter under terrain in most areas with combined sewers. Most basements in these areas as well as local depressions in the terrain are expected to be flooded. Based on an assessment of the topographical conditions in the areas, it is estimated that approximately 60 buildings will be flooded above ground floor as they are placed in local depressions. Approximately 5% will be flooded in areas with separate sewers. Apart from damage to buildings and inventory, the primary damage is expected to be road breach, power failure and increased health risk due to contact with water containing faecal matter.

Adaptation to extreme rain is analysed in two different scenarios. In the first scenario, initiatives eliminating damage caused by 100-year rain in Roskilde are analysed. The second scenario includes focuses on more focussed initiatives that has a high reduction in economical cost of damages relative to the investment needed to reduce these damages. Both scenarios are based on the assumption that Roskilde Municipality implements adaptation to the extent necessary to meet the desired service level.

In the first scenario, the initiatives necessary for eliminating damage are estimated as follows:

• Increased storage capacity (115,000 m³ and 50,000 m³, respectively)
• Establishment of sewers for stormwater drainage to Roskilde bay (5.9 km new sewers)
• Increased drainage from the town centre to the transport corridors (1.2 km new sewers)

The results of the first scenario are shown in Table 0C. It appears that implementation of initiatives eliminating damage caused by 100-year rain in Roskilde involves a socio-economic loss of 225 million DKK. Implementation of adaptation initiatives has been analysed for the city as a whole as well as for smaller parts of the city. This is also shown in Table 0C. The result is that only the city centre of Roskilde (zone 4) will have a socio-economic benefit from this level of adaptation.

It can be concluded that it will be most beneficial to focus initiatives in areas and on types of damage resulting in the highest damage reduction.

Table 0C: Result of the first scenario.

In the second scenario, locations are analysed where a socio-economic benefit can be expected by planning adaptation in addition to the gradual optimization of the sewerage system which is part of the spontaneous adaptation in the basic scenario.

In Roskilde, there are a number of buildings which due to the topographical conditions have high risk of flooding above ground floor during heavy rain. A rough estimate shows that this concerns approx. 60 buildings in the centre of Roskilde. The result of the second scenario shows that seen from a socio-economic point of view a further flooding protection of these buildings will pay off. In addition, installation of high water level closure is recommended for exposed basements in buildings connected to combined sewers. The socio-economic calculations indicate that expected damage in the basements does not justify large investments in upgrading of public sewer systems. Nor can it be recommended to implement further initiatives to protect against road breaches, sewer collapse, power failure etc. caused by extreme rain.

Increased seawater level

The analysis is based on the assumption that with the present climate a maximum seawater level of 1.8 meter above mean sea level is expected to appear once every 100 year. In year 2096, a similar incidence is expected to result in the seawater reaching a level of 2.37-3.13 meter above the present mean water level.

The area contains valuable cultural heritage and national treasure, i.e. the area is an essential part of the identity of the city of Roskilde. The total damage resulting from a 100-year water level increase is estimated to amount to 200-300 million DKK.

Roskilde Municipality has not initiated measures to avoid this catastrophe, nor have decisions on future initiatives been made.

At the workshop, two possible initiatives to secure the city against increased seawater level were identified:

• Establishment of a 3.5 meter embankment along the entire waterfront and seashore
• Establishment of dam from Hundested to Rørvig

An embankment along the waterfront was assessed to be unacceptable as this will cause large visual nuisance and result in a decline in house prices along the seashore. Furthermore, part of the experience when visiting the Viking Ship Museum is the sensation of feeling part of the bay. This impression would be spoiled by a 3.5 meter high embankment. Thus, establishment of a dam from Hundested to Rørvig was proposed at the workshop.

The present value of initial and operating costs for establishment of a dam amounts to 1.26 billion DKK. Thus, the costs involved with establishment of a dam does not compare to the damage reduction in the city of Roskilde.

The conclusion was thus that from a socio-economic point of view, it is not reasonable to remove the effects of increased water level in the city of Roskilde alone. However, the estimated damage costs only include the city of Roskilde. Establishment of a dam would provide for many other benefits and losses, e.g. other areas would be secured, and there would be derived effects. Therefore, a complete evaluation of this proposal is outside the scope of this study.

Aalborg

Extreme rainfall

Aalborg Municipality has experienced a number of incidences with extreme rain within recent years. Based on these incidences, the damage has been evaluated partly through hydraulic calculations and partly through accounts of the actual damage observed during heavy rain. Based on these evaluations, it was estimated that most likely future damage will be moderate if the sewerage system is gradually optimised corresponding to the present service level.

Aalborg Municipality has prepared a strategy for the sewerage system involving a complete separation of urban wastewater and stormwater. The time frame for total separation is 90 years. During this period, the risk of flooding will gradually be eliminated for flooding of basement in areas with combined sewers.

At the workshop it was assessed that damage caused by extreme rain can be reduced by a combination of the following 3 initiatives:

• Dissemination
• Retention of water in Østerådal
• Various local initiatives

The local initiatives primarily involve establishment of new pumping stations, new outlets and extension of existing retention bassins. With a cost of close to 96 million DKK, these local initiatives are the most expensive. The initiatives will primarily reduce the risk of flooding of basements and above ground floor. As mentioned above, the elimination of damage is dependent on the separation of sewers. Consequently, damage costs will be reduced in line with progress in separation of sewers. Table 0D gives an overview of adaptation initiatives and reduction of damage costs resulting from implementation of these initiatives.

Table 0D: Socio-economic costs and present value resulting from adaptation and reduction of damage costs.

By adjusting the socio-economic costs to 2007-prices with a market discount of 3 %, it is possible to evaluate whether the adaptation will result in socio-economic profit or losses. The present scenario would result in a profit of 5.1 million DKK. The analysis only includes quantifiable damage costs, thus e.g. reduced public welfare as a result of uninhabitable houses is not taken into account.

Increased seawater level

The increase in seawater level was evaluated based on the same principles as for the city of Roskilde. Increased extreme water level will be most problematic in the densely built-up urban areas in Nr. Sundby and Aalborg. The high water level at a future 100-year rain involves the risk of the tunnel under Limfjorden and the runways in Aalborg airport partly being flooded. At the workshop it was agreed that the extent of this potential flooding risk was unacceptable.

At present there are no plans for adaptation initiatives against high water level. It is, however, expected that gradually spontaneous adaptation will be included in district plans to provide for the risk of flooding.

Three adaptation initiatives were suggested at the workshop:

• Sluice in Hals
• Higher walls surrounding quay and sluice in Østerå
• Hydraulic gates across Limfjorden.

The three initiatives were evaluated based on a cost-effectiveness analysis. The second initiative, i.e. establishment of higher walls surrounding quay and sluice in Østerå, was by far the most cost-effective initiative with a socio-economic cost of close to 100 million DKK, whereas the other initiatives resulted in a socio-economic cost which as a minimum was 7 times higher.

The two last initiatives are both local initiatives directed towards urban areas. Consequently, implementation of these initiatives might result in the areas east of Aalborg being flooded. Thus, it might be questioned whether these initiatives can be implemented in practice. Evaluation of the social benefits and losses involved in establishment of a sluice in Hals is outside the scope of this study.

General conclusions

The results are based on relatively rough technical and financial analyses primarily focusing on the direct costs and benefits involved with the individual initiatives. A thorough analysis with extensive pricing of especially indirect costs should be carried out prior to implementation of specific initiatives. Below, the recommendations and conclusions of the project team are summarized:

• The method used for this study, i.e. gathering information at a workshop followed by technical and financial analyses at a strategic level, makes sense. The results have been sturdy and have provided for indicative action.
• The socio-economic calculation shows that the adaptation method for sewerage systems suggested in Spildevandskomiteens Skrift 27 (the climate addendum) is close to optimum for the cities of Roskilde and Aalborg. On this basis, it is estimated that the adaptation suggested in the Spildevandskomiteens Skrift 27 is sufficient in cities with good topographical conditions. The consequences of overload of wastewater in terrain should always be considered, including a separate assessment of the hygienic conditions.
• It is difficult for the individual municipality to adapt to increased seawater level. On the one hand, the consequences of a flooding catastrophe are extreme, and on the other hand the socio-economic calculations show that apparently it is not worthwhile to implement large adaptation initiatives. There is a need for further analyses. Based on such analyses, it would be relevant to prepare national guidelines.
• The municipalities must be open for broad and transverse solutions, allowing for new initiatives to have a double function in relation to climate adaptation, e.g. in connection with planning of new recreational areas or leisure-time facilities.
• The financial analyses are based on a very simple model. Extreme incidences of rain or high water level will have long-term effects which are not included in the financial analyses. Especially, it might be stressed that extreme seawater level will affect several towns at the same time. Consequently, the economic losses must be expected to be larger than the sum of the individual losses from a number of limited incidences in different urban areas.

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Version 1.0 September 2007, © Miljøstyrelsen.