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The Nature index tool quantifies the change in nature value as a result of interventions in a project area. The tool translates a nature value into nature points, with more points indicating a higher value. It requires four steps to determine the effect of a certain intervention on the number of nature points. The tool can be used when the feasibility of an intervention or project has to be demonstrated or when an (i.e. cost effective) alternative has to be chosen. Applying the tool requires ecological knowledge.

    General Tool Description

    Context, purpose & results

    A transparent project development process includes evaluating alternatives. An integral assessment of the impact of alternatives has to be undertaken in order to compare them. Quantification of nature effects is not easy; a widely accepted quantification methodology for nature values does not exist. Instead, nature values are mostly based on a qualitative description. In this way, the comparison of nature values to other values in a management summary, overview table or cost-benefit analysis is difficult to make. Nature values are mostly included as a reminder, a qualitative term, or they are not included at all. This tool outlines a methodology to include nature qualities in planning processes by defining a quantitative nature index.

    In short, the nature index assesses intervention alternatives by awarding points to areas (hectares) of nature. The higher the quality or the more highly valued, the more points an ecotype gets. The method then compares alternatives by the number of points received. The points are pereferably based on (inter)national criteria as set out in policy documents such as the Water Framework Directive or Natura 2000. When objective criteria are lacking, points will be based on expert judgement.

    The origin of the nature index tool lies with the Netherlands Environmental Assessment Agency (PBL), the national institute for strategic policy analysis in the field of environment, nature and spatial planning (Sijtsma et al., 2009). The agency has developed a method to assess the permanent effects of policy alternatives on terrestric nature. In the ‘Building with Nature’ innovation programme the method has been developed further and the results made accessible through the present wiki-based guideline. The enhanced method applies to all nature systems, including aquatic/ marine systems, and time effects to enable valuation of dynamic as well as static effects of interventions.

    Overview nature index tool

    The nature index tool quantifies the change in nature value due to the policy alternatives considered in a project area. The tool translates the nature value into nature points, where more points indicate a higher value. It requires four steps (visualized in the figure) to establish nature points per policy alternative.

    Usage skills

    The tool can be used in situations where the feasibility of a project has to be demonstrated or where the best (cost effective) alternative has to be chosen. Applying the tool requires ecological knowledge. The tool is based on an ecological system analysis with efforts to quantify as much as possible. One should be capable to program and operate a program like Excel.
    It turns out that the system analysis approach requires significant (research) efforts. At first sight, this may make the method less applicable to studies in the initiation phase of a project. Yet, during a project one always benefits from a sound ecosystem analysis, so for projects that survive the initiation phase the effort pays back. The system analysis produces all the information needed for a proper assessment. At first sight, this approach requires more input information than assessments based solely on target species, but note that the latter also require a system analysis.

    Quantification of ecological effects has advocates and opponents. On the one hand it can lead to transparent information to support decision making. However, it also points out what data is available, what information is lacking and what assumptions have to be made. A method like the nature index tool is vulnerable for criticism, as it explicates the information that is missing and that had to be filled in with expert judgement. The more qualitative approaches do have to deal with the same lack of information and uncertainty in evaluating the effects. But these methods have more margin in presenting the results.

    Relevance nature index tool for Building with Nature

    There are two main reasons why the development of the nature index tool is relevant for the Building with Nature programme:

    • The system analysis approach facilitates the assessment of changes in ecological quality rather than ecological status;
    • It enables a budget approach to nature values, which provides the flexibility needed for multifunctional (integral) design processes.

    Ad 1) Building with Nature strives for tuning and optimisation of infrastructural designs and natural processes. It also aims at increasing natural values, existing as well as new ones. Quantifying the effects of infrastructural works on natural processes requires a system analysis, which is provided by the nature index approach. The overall philosophy of the (eco)system analysis is that nature values can gain if the necessary abiotic conditions are there. Large infrastructure projects and/ or measures to improve the ecological status of an area do affect the abiotic conditions. Indirectly - via system relations – they may also affect the relevant indicator species. Therefore a system analysis is necessary to identify the ecological impact of such projects. Although this may seem obvious, many ecological studies and sometimes even legislation focus on the ecological status (by means of indicator species), rather than the change in ecological quality resulting from interventions in the system.

    Ad 2) Building with Nature designs aim at multi-functionality, including ecosystem services and nature values .This means that a balance has to be found between different functions. A budget approach can be of use here, provided that relevant values can be expressed in an adequate ‘currency’. When applied to nature values, with the nature index as a currency, such a budget approach leaves room for compensating negative effects with positive ones. So far, this budget approach remains to be widely accepted. Now it is still possible that a measure with a positive nature index gets a negative score in an environmental impact assessment because it has a negative effect on a legally protected indicator species. For this reason, we expect budget approaches to become more widely used in the future.

    How to Use

    The nature index tool quantifies the change in nature value due to the design alternatives considered. In four steps (visualized in the figure), the tool translates the nature value into a quantitative score: nature points, where more points indicate a higher value.

    A major challenge in the process of nature valuation is to determine the necessary level of detail, especially in the assessment of changes in ecotypes and ecotype qualities. This requires thoughtful consideration before carrying out the actual analyses. The challenge is to find a balance between the necessary detail for proper representation of the system and the data, time and money available for the analysis. This level of detail will be different for each case but in general, for the four steps above to be assessable, ecotypes, species groups and driving abiotic parameters need to be included.

    Phases to determine Nature Indexes

    Step 1: defining the project

    Step 1 provides the basis for the further analysis. It results in a map of the project area that shows the relevant ecotypes. Also, in table x (working spreadsheet) we quantify the ecotype area and the changes in this area over time. Step 1 consists of the following sub steps:
    1a. map the project area and its ecotypes (map or schematic drawing);
    1b. define the reference situation and the design alternatives;
    1c. define the components and system relations for each ecotype;
    1d. define the time period of the analysis;
    1e. quantify the ecotype areas per alternative over the time period.

    Ad 1a) The project area includes all areas that are positively or negatively influenced by any alternative. The project area is not necessarily continuous, but may consist of separate patches in the landscape. The project area should be the same for all alternatives, in order to be able to compare their nature indexes.

    To define ecotypes in the project area, legislative documents are the starting point. In Europe, these are the Water Framework Directive, Natura 2000, or national legislation derived from these European frameworks. These documents are the preferred basis for nature indexing since they provide a shared and broadly accepted frame of reference. When information from these documents is not sufficient, not available, or reasons exist for not using it, expert judgement may be used to complete the information. In that case it is strongly advised to organize expert meetings, in order to have the ecotype definition commonly accepted. If alternatives affect ecotypes that are not mentioned in legislative documents, they should also be included in the analysis.

    If only a part of an ecotype is affected, we recommend to split this ecotype into two: a) the ecotype affected, and b) the ecotype not affected. Finally, visualization clarifies matters for discussion and ensures that the entire project area is covered with ecotypes.

    Ad 1b) The nature index study considers a number of predefined design alternatives and the reference situation. In most cases the reference situation is the current situation plus the expected autonomous development. The definition of the latter may be non-trivial, especially if other projects are foreseen in the area.

    Depending on the type of project, design alternatives may lead to a wide variety of ecological effects within the project area. When defining these alternatives it is important to consider the availability of ecological information, such as species numbers, soil parameters and cause-effect relations. Doing so in the very beginning of the design process yields a first estimate of the scale of the nature index study to be carried out.

    Ad 1c) Each ecotype is a dynamic system of which the main components and their interrelations and interactions need to be identified and understood. Based on this understanding it is possible to indicate which effects influence which ecotype quality and to what extent. As this approach treats ecotypes as dynamic systems, it reaches beyond merely satisfying the legal obligation to protect certain species.

    In addition to the system identification, the driving abiotic factors need to be identified. Natural drivers such as tidal fluctuations and water currents, but also human activities such as nutrient loading or nature management, should be included. Also the time-variation of these abiotic factors, either autonomous or in response to the project, needs to be known.

    Ad 1d) t=0 is the start of the project, which mostly coincides with the implementation of measures or structures. The time period ends when all effects have materialised, or (for practical reasons) after a maximum of 100 years. Besides permanent effects, a project may causes significant temporary effects. The nature index takes both into account.

    Ad 1e) The area of each ecotypes has to be calculated (in hectares) and serves as the basis for the attribution of nature points. The ecotype areas have to be determined for the reference situation and for each design alternative considered. Make sure that the totals add up to 100% of the project area.
    The document - blank spreadsheet nature points shows an excel spreadsheet that can be used to calculate the nature points. As a result of Step 1 the first page of the spreadsheet has been filled in.

    Step 2: assessing change in ecotype quality

    In this step the time-evolution of the ecotype quality is assessed. This step is probably the most challenging and complex of the tool. The quality of an ecotype may change autonomously or in response to the project. Step 2 consists of the following sub steps:

    2a. determine quality indicators per ecotype and describe their state in case of a
    100% intact ecotype;
    2b. determine the required input for the indicators that will vary when
    implementing design alternatives;
    2c. estimate the current quality of each ecotype (in % of the quality of an intact 
    ecotype) and the change in quality over time.

    Ad 2a) One measure for the quality of an ecosystem is biodiversity (the presence of species). In an international context, considerable effort has been put in standardization of indicators of biodiversity. UNEP has developed the Mean Species Abundance as an indicator of the quality of ecosystems. The method is based on a widely accepted set of species characteristic for an ecosystem. The European N2000 and the Water Framework Directive take a similar approach. The nature index tool uses a localized version of the Mean Species Abundance: for every local ecosystem one can define the species that are able to or supposed to live in that system (see also Sijtsma et al. 2009).

    The nature index tool expresses nature quality per ecotype in percentages. A nature quality of 0% reflects absence of species, while a nature quality of 100% reflects abundance of species matching a healthy or intact ecotype. The healthy ecotype is the desired quality of the ecotype and serves as a reference.

    To define indicators and their potential value (= intact ecotype), we take legislative documents such as the Water Framework Directive, Natura 2000, or national legislation derived from these European frameworks as the starting point (see step 1.1 for argumentation).
    The nature index tool offers much freedom in the choice of the level of detail of indicators, e.g.:

    • specify one lumped indicator that reflects the quality of the ecotype (in relation to the project goal), or;
    • specify indicators directly related to the project goals only, or
    • assess the status of the complete ecosystem. In this case it is advisable (for workability) to categorize the indicator species in species groups and take the average of the quality per group. For an example we refer to Wortelboer (2010) who performs an analysis of nature quality of the Dutch saltwater systems. He categorizes the indicator species in the groups phytoplankton, benthos, fish, birds and mammals.

    Ad 2b) Estimating the current or future quality of an ecotype is often difficult. Note that the absolute value of ecotype quality is not the most important information. More relevant are the relative quality differences between alternatives.

    One of the most challenging issues with indicator valuation is the fact that it should also be valid in the future, with and without project. Predicting the effect of the autonomous development and the project on the quality of species, such that alternatives become comparable, requires expert judgement and, where possible, modeling.

    There is a great variety in modeling tools with regard to ecological processes. For every case or plan area, the selection of suitable tools can be different. Moreover, these modeling tools are very actively being invented, renewed and improved. A summary of the currently existing, useful models is described in the STOWA report called ‘Ecologische Instrumenten’ STOWA, 2009. Here, Excel is used as a modeling environment. In addition, information from other tools, such as the dose-effect relations from de DELTARES HABITAT-WIKI, can be used to describe the interrelations between species groups and the changes of the abiotic parameters over time.

    Ad 2c) The quality of the ecotype at the end of the time period reflects the permanent effects of the project. In addition, temporary effects may seriously affect an ecotype for some time, after which it may recover. To express these effects – and thus enable comparison between alternatives – the nature index tool assesses the quality of ecotypes over the complete time period.

    Step 3: weighing different ecotypes

    Step 1 and 2 define the relevant ecotypes in the project area and their quality state, irrespective of their relative importance. Some ecotypes represent a higher nature value because they have a higher biodiversity or include rare species. In the nature index tool these elements have to be accounted for. If we don’t take the relative value of ecotypes into account perverse effects may occur. For example, changing a precious intertidal area with low-quality brackish water into a widespread deep area with high quality sea water would be beneficial in such an index. Therefore, the nature index tool has to correct for rarity and/ or larger-scale biodiversity.

    For terrestrial ecotypes different weighing factors are determined, based on the presence of rare species and biodiversity (Sijtsma et al, 2009). Ecotypes get more weight if they include species that are not hosted in (most) other ecotypes and they get more weight if they host a lot of different species (high biodiversity). These weighing factors balance the ecotypes at a larger scale and make nature point values comparable across projects.

    It is not possible to use this system for aquatic ecotypes, as the data to determine rarity and biodiversity are lacking. For aquatic ecotypes one therefore has to work with a local weighing system. One such system works as an indicator that takes into account the relative abundance of ecotypes in the project area. When an ecotype diverges extremely from the target situation (e.g the historic reference situation or the policy objective), than it should be weighed with a relative high factor.

    The use of local weighing factors requires the definition of a so-called target situation, i.e. the ecological situation that we strive for in the project area. It can be the same as the ‘natural’ situation, but not necessarily. In some cases human intervention cannot be completely overcome. For example the storm surge barrier affects the ecological conditions in the Eastern Scheldt estuary significantly. Yet, the barrier will not be removed because of its coastal protection function (if it would be technically possible to remove it, at all). This should be taken into account when defining a realistic ecological target situation for the estuary. In Europe, the target situation can be abstracted from existing legislative documents like the Water Framework Directive.

    Scheme for determining the weighing factors for the current situation


    target situation (%)

    current situation (%)(part of step 1)

    weighing balance

    ecotype X




    ecotype Y




    ecotype Z








    When the (local) weighing factor is established by means of deviation from the target situation, the weighing factor can change in time - for example when the area of the ecotype changes over the years. In that case a new weighing factor has to be calculated (i.e. a new comparison with the target situation). In the document - blank spreadsheet nature points for calculating nature points the weighing factors are calculated automatically after the ecotypes (step1) and the target situation are defined.

    Step 4: assessing the nature index

    With all the data generated - area, quality index and weighing factors - it is easy to calculate the nature points as the formula only multiplies the elements:

    Σ relative area per ecotype * index ecotype quality * weighing factor.

    where Σ denotes summation over all ecotypes in the plan area. The nature points are calculated for the reference situation and each design alternative (see for example the table).

    Scheme for calculating the nature points:














    area ecotype X













    area ecotype Y













    area ecotype Z













    quality ecotype X













    quality ecotype Y













    quality ecotype Z













    weighting factor X













    weighting factor Y













    weighting factor Z


























    In principle the nature points are calculated per year (as the scheme for calculating nature points illustrates). The question is how to determine nature values for the whole time period from these yearly ones. There are two options:

    • by adding up the yearly results;
    • by discounting and determining a ‘net present value’.

    The second option is more familiar to economists than to ecologists. By discounting one takes into account that short-term effects are preferred over long-term effects. Discounting also reflects the uncertainty whether the rated effects will materialise. A high discount rate therefore corresponds with a strong preference for short-term effects and a high uncertainty. 
    Although in principle there is nothing wrong with discounting ecological effects, it seems a bridge too far for present nature valuation practice. Moreover, there is no accepted practice to determine realistic discount rates.

    Practical Applications

    The nature conservation plans in the Dutch Eastern Scheldt estuary serve as an example to illustrate the application of the nature index tool. The illustration follows the consecutive steps of the nature index tool.

    Step 1 Defining the project: sand deficit in the Eastern Scheldt estuary

    After the 1953 flood in the southwest of the Netherlands, a movable storm surge barrier was built in the mouth of the Eastern Scheldt, by that time a branch of the Rhine-Meuse delta (see figure). Additional works have separated the Eastern Scheldt from the other branches of the delta and have turned it into an elongated tidal basin. The works have increased flood safety in the area and stimulated recreation (sailing, diving, etc.).

    Several decades later, however, a downside of this icon of civil engineering becomes manifest: the reduced tidal flow in the estuary requires less wide channels than before and the narrowing of the channels goes at the expense of the intertidal flats. This was foreseen before construction, but it was expected to be a temporary effect that would be compensated once sediment from the outer delta would reach the shoals. Now that the barrier turns out to act as an almost perfect block to in- and outgoing sediment transport, the expected supply of sediment from the outer delta does not take place and the system faces a permanent sand deficit. 
    This has consequences for the unique and highly valued ecosystem in the area and for the flood defence system. First, the tidal flats and intertidal area are of vital importance to wading birds, which feed on organisms exposed during low tide. The decrease of the intertidal area induced by the sand deficit causes a decline of the foraging area for more than 100.000 waders. Moreover, the loss of relief of the intertidal shoals which takes place at the same time reduces the time available for foraging. Second, the loss of intertidal area results in less wave damping and a heavier wave attack on the downwind dikes. This may have consequences for the maintenance of those dikes.

    The Netherlands government, represented by its agency Rijkswaterstaat, aims to develop a policy for conservation of the intertidal areas. Scientists expect that - without intervention - almost all intertidal area will be lost by the end of this century. Sand nourishment on top of the intertidal shoals is one measure to compensate for erosion, be it only with a temporary effect. A pilot application has been realised on the Galgeplaat, one of the major shoals in the basin. Shoal nourishment will have a positive effect on nature in the long run, but a negative short-term effect (burial) in the nourished area.

    This case description is real, but some of the numbers in this document, especially those for the ecotype quality, are necessarily fictive or based on an educated guess. It is still early days for the ecological part of the sand deficit study (2012).

    Sub step 1a) map the project area and its ecotypes

    The figure on the right outlines the boundaries of the project area of the shoal nourishment case study (the Eastern Scheldt estuary) in purple. The total area amounts to 35.000 hectare and covers the entire the Eastern Scheldt estuary area outside the primary dikes. Both the nourishment and the sand extraction take place in this area. The area within the blue contour in the figure is the back-up location for sand mining if insufficient sand is available in the Eastern Scheldt estuary. This scenario is not included in the example case study.

    Both Natura 2000 and the Water Framework Directive have formulated goals for the Eastern Scheldt estuary. These provide input for determining the ecotypes.

    Natura 2000

    The Eastern Scheldt estuary is a Special Protected Area (SPA) within the Natura 2000 network. Natura 2000 is an ecological network of protected areas in the territory of the European Union. Member states are required to maintain SPAs in favourable state of conservation. The shoal nourishment aims at contributing to the Natura 2000 goals of the Eastern Scheldt estuary for the ecotype (habitat) ‘large shallow inlets’ profile document H1160. The quality objective for this ecotype is the conservation of the present tidal flats and channels in both variance and area. Essential for the ecotype quality are gradients in morphology (tidal flats vs. deeper channels), processes (dynamic vs. sheltered) and soil conditions (sand vs. mud). The conservation of the ecotype ´large shallow inlets´ is a precondition to (and serves to) maintain bird populations, another Natura 2000 goal in the area. The quality of the ecotype is mainly judged in the light of conservation of these bird populations. The Natura 2000 profile document (2008) provides a description of this habitat in a good state regarding structure and function:

    • presence of tide;
    • presence of natural channels;
    • variation of sandy and sludgy parts, including transition areas;
    • variation in height with both tidal flats and permanently flooded area;
    • variation in high- and low-dynamic area;
    • good water quality (transparency, salt);
    • presence of sea grass and ruppia fields
    • presence of rich mussel beds;
    • presence of algae or a layer with diatoms and cyanobacteria;
    • presence of species: benthos, fish, birds, mammals.

    Water Framework Directive (WFD)

    All water bodies in the EU have received a classification as WFD water type. The Eastern Scheldt estuary is a WFD water type ‘coastal water, shielded and polyhaline (K2)’. The WFD sets nature conservation goals for each of its water types. The following goals of the Eastern Scheldt estuary are relevant for the shoal nourishment case study: a balance between intertidal area/ shallow water/ mussel beds, % of natural coastline and hectares sea grass area. The WFD sets target values for the formulated goals based on a realistic reference situation, which is deduced from the natural (historic) reference situation. The degree of goal realization is expressed in % of the target value.

    The level of detail of the WFD conservation goals fits this analysis best and therefore it is taken as the basis for further analysis. The description of the habitat in Natura 2000, however, is more comprehensive and includes the ecotype deep water that is subject to sand extraction activities. We consider the two documents as complementary and supplement the WFD ecotypes with the deep water ecotype defined in Natura 2000.

    The figure on the right visualizes the ecotypes schematically. Not all deep water is subject to sand extraction activities; therefore only a few patches are included in the project area. Note that the figure is schematic, and only shows two tidal flats and sand extraction areas, instead of the multiple areas that actually occur in the Eastern Scheldt estuary.

    Substep 1b) Define the reference situation and the design alternatives

    The case study considers the following situations:

    • Reference situation: autonomous development without tidal flat nourishment;
    • Shore alternative: nourishment of tidal flats along the shoreline;
    • Tidal flat alternative: nourishment of all tidal flats.

    Reference situation/ autonomous development

    In this situation no tidal flat nourishment takes place and therefore the tidal flat area (ecotype 1) in the Eastern Scheldt basin decreases strongly. In addition, the sand deficit reduces the duration of the dry stage and increases the speed of flooding. So there will be less food and less time to forage on it. Both effects are detrimental to the wading bird population. In 1986, the tidal flat area was more than 11.000 hectare. By 2011, about 1.100 hectare had been lost. Without interventions, half the intertidal area will have disappeared in 2050. Before 2100, scientists expect the intertidal area to have come down to 1500 hectare, at which point a new equilibrium state will have established (Van Zanten & Adriaanse, 2008). The submergence of tidal flats increases the shallow water area (ecotype 2). The deep water area (ecotype 5) will not increase or decrease significantly.

    Shore alternative

    In this alternative the nourishment is limited to the tidal flat areas directly adjacent to the shoreline, 500 hectares in total. The nourishment contributes to nature value as well as flood safety. The nature quality effects in this alternative are both permanent and temporary. The tidal flat area (ecotype 1) still reduces permanently, since the nourished volume is relatively small. In addition, the nourishment acitivity has two temporary negative effects on the tidal flat area. Firstly, burial by sand causes a temporary loss of species (Holzhauer & van der Werf, 2009). After completion, species recolonize the sand layer within a period of about 3-5 years. This recolanization rate depends on the thickness and characteristics of the sand layer, the physical distance to the surviving ecosystem and the migration characteristics of surviving fellow species. Secondly, sand transport away from the nourishment may disturbs the wading birds and mammals (seals) in the surrounding tidal flat area. The actual disturbance also depends on the timing of the activity.

    The shallow water area (ecotype 2) increases when tidal flats become submerged. As this will not be the case for nourished areas, this effect will be smaller than in the autonomous development. The nourishment therefore has a temporary negative effect on the shallow water area. Depending on how the nourishment is realised, the activity results in more or less suspended matter in the water column directly adjacent to the nourished area. The resulting turbidity may temporarily affects benthos and fish, and if much of this sand is deposited this may harm filter feeders (mussels, oysters).

    The deep water area (ecotype 5) will not increase or decrease significantly. The sand extraction does have a more or less permanent negative effect on the borrow area. The benthos in the borrow area is probably destroyed completely and, depending on the depth of the sand pit, will take time to recover or will not recover, at all. In addition, dredging may have a temporary negative effect via the turbid plume it creates. Depending on the intensity and the duration of the ensuing stress, this may affect benthos and fish (see also the tool on species response curves).

    Tidal flat alternative

    In the tidal flat alternative, the aim is to maintain the current tidal flat area (10.000 hectare) and its quality. To maintain the current area, 8.500 hectare tidal flat is nourished. The other 1.500 hectare does not erode since these flats are located in sheltered area. The tidal flats are nourished one by one. Nourishing the whole area at one time is both unfeasible (in terms of amount of sand required) and undesirable (in terms of environmental damage). For this case study example, the maximum area affected by a single nourishment is set at 10%. Furthermore, after a number of years the nourished areas have to be renourished for maintenance. The ratio tidal flat/ shallow water in this alternative remains the same as in the current situation. The negative side-effects of nourishment and sand mining are similar to those .described for the shore alternative.

    Sub step 1c) Define components and system relations for each ecotype

    First, the biotic players in the plan area will be identified. Note that their interrelations are not necessarily predator-prey related, but can also be non-food relations such as vegetation providing shelter for fish to grow up. In the biotic domain, the most important players are:

    • filter feeders that affect and are affected by turbidity and are an important food source to birds;
    • vegetation, which is both legally protected (Zostera spec. and Chara spec.) and important as food source, habitat for fish and water quality filter;
    • fish, which are an important food source for birds and seal, and feed on filter feeders and vegetation;
    • birds, highly diverse, most of which are legally protected and feed on filter feeders, vegetation and fish.
    • predators, mainly seal (legally protected), which feed on fish and are at the top of the local food chain.

    The second step is to define the most important abiotic influences. These can be either natural or human-induced. The focus of this step is on identifying the abiotic parameters that will change over time. The Eastern Scheldt estuary system can roughly be defined as dynamic, brackish and semi terrestrial. The driving processes are:

    • tide: not as strong as before the storm-surge barrier, but still present. Tidal fluctuation influences water depth and morphology;
    • water depth: influenced by tidal fluctuation and morphology, affects vegetation, birds and filter feeders;
    • salinity: has decreased since the construction of the storm-surge barrier, and is not influenced by any of the above;
    • turbidity: mainly due to suspended particles, influenced by water depth and morphology; affects filter feeders, vegetation and fish.

    The visual overview of the relations named above is shown in the figure on the right.

    Sub step 1d) Define the time period of the analysis

    The Netherlands government considers nourishment as the policy line for the coming decades. Other alternatives, such as adaptation of the Delta works, come in view in 2100 at the earliest. Nourishing requires regular maintenance and therefore is a continual activity with long-term effects. Therefore it was decided to start the analysis in 2010, at the start of the shoal nourishment case study, and take a time frame until 2100.

    Sub step 1e) Quantify the ecotype areas per alternative over the time period

    Table 1 presents the ecotype areas for the reference situation (autonomous development) and the design alternatives in 2010 and in 2100.


    area (ha in 2010)

    autonomous development (ha in 2100)

    shore alternative (ha in 2100)

    tidal flat alternative (ha in 2100)

    1. tidal flats
    a. tidal flat
    b. suppleted tidal flat



    2.000 ***


    2. shallow water
    c.  shallow water
    d. sea grass
    e. mussel bed

    8.000 *




    3. deep water
    a. deep water
    b. sand gaining










    1 year 2001-2006, source: STOWA 2007-32

    Starting points in completing the accompanying excel spreadsheet are in general:

    • submerged tidal flats become shallow water;
    • the recovery time of a nourished tidal flat is 5 years, thereafter the area is completely recovered and becomes tidal flat again in the spreadsheet.

    Specific starting points for the shore alternative are:

    • the nourishment of 500 hectare tidal flat takes place in 2015 and is repeated in 2060. This area does not submerge in the period 2015-2100;
    • a sand borrow pit of 25 hectare is dug in 2015 and deepened in 2060.

    Specific starting points for the tidal flat alternative are:

    • a maximum of 10% (1000 hectare) of the tidal flat area is affected by one . nourishment. The 8500 hectare of tidal flats at risk are nourished in two rounds, where already submerged area is treated first. After the recovery time of 5 year, a new area of 1000 hectare is nourished. In the period 2015-2100 every hectare is visited twice. After the second nourishment the area remains intertidal until 2100;
    • a sand borrow pit of 50 hectare is dug for every nourishment round.

    Step 2: Assessing change in ecotype quality

    Sub step 2a: determine quality indicators

    In this part of the analysis, the quality change of the ecosystem over time will be analyzed for each alternative. Following the defined ecosystem (see step 1), first the abiotic parameters will be discussed, and then their influence on each of the species groups. After that the method to determine a quality index will be explained.

    In step 1 the main species groups in the ecosystem of the plan area have been identified (see figure on the right).

    Abiotic parameters

    The abiotic parameters can be defined to an arbitrary degree of complexity, but for the purpose of this qualitative analysis it is convenient to only define parameters that will change over time or between alternatives. In the Eastern Scheldt example, the following parameters are defined:

    • turbidity;
    • substrate type;
    • light penetration;
    • water depth;
    • drought duration.

    For more explanation see The abiotic parameters.

    Biotic relations

    The most important biotic components are:

    • benthos;
    • vegetation;
    • fish;
    • birds.

    For more explanation see The biotic relations.

    Step 3 Determining the weighing factors

    For the shoal nourishment project the local weighing factors are derived from the Water Framework Directive. That is to say, the target situation and the current situation are taken from the WFD. In this table the relative areas of the ecotypes in the Eastern Scheldt are presented for the target and current situation.

    Weighing factors for the Eastern Scheldt estuary


    target situation

    current situation (%)

    weighing factor

    tidal flat




    shallow water




    sea grass




    (lithoral) mussel bed









    This example of the Eastern Scheldt shows that the weighing factor can vary widely. A hectare of sea grass can be worth 27 times more points than a tidal flat. This is not surprising as the sea grass is almost extinct, while it should cover about 7% of the area. Damaging the last remaining hectares of sea grass should be attributed a high negative value, measures to develop sea grass meadows a high positive value.

    The example of the Eastern Scheldt estuary also points out is that it is difficult to calculate the weighing factor for ecotypes that have completely disappeared - like for instance the littoral mussel beds. This ecotype should cover about 1,8% of the area, but in the current situation no area of mussel beds is left. If the percentage is zero, no weighing factor can be calculated (the formula is dividing by 0). A pragmatic solution is to work with a minimum of 1% of the target situation - resulting in the presented weighing factor. It also raises the discussion wether the target situation is realistic. Perhaps the littoral mussel bed should not be part of the list if reintroduction of the ecotype is necessary.


    • HABITAT-wiki of DELTARES
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