Management measures analysis and comparison

The MRM have been implemented for different reasons. The most common measure target is habitat conservation, restoration or creation. Only a few cases combine this conservation target with a safety target (flood storage capacity), research target, and/or recreation opportunities. Half of the cases are driven by a compensation reason. The degree of target achievement is overall high: almost half of the measures are considered to have a high degree of target achievement, the other part a medium degree meaning that not all targets are completely reached. However in some cases it was proved that the degree of target achievement could be improved by making some adaptations to the MR site.

An MRM could be executed by different techniques. Half of the TIDE cases are implemented by dike breach and half by defence removal (large dike breach), with a dike breach between 3m and 2650m

Another type of estuarine habitat restoration is by Reduced Tidal Exchange (RTE). Within TIDE we have only one RTE example (S-Lip.). In half of the measures, the dike breach or defence removal is combined with land lowering. In many cases it was proven that different design aspects such as initial site elevation, slope of the area and hydrodynamics do influence habitat development and the success of the measure. In some cases the initial design was not optimal, but adaptations to the site were possible to improve the success of the measure.

The TIDE MRMs together transformed about 1000 hectares adjacent land into estuarine habitat, consisting mainly of marsh land and intertidal flat habitat (Figure 11). For the TIDE cases, about 90% of the created habitat surface (approx. 900 ha) was however implemented for compensation reasons meaning that it is not really new habitat because it was lost first somewhere else.

All TIDE MRMs have a monitoring program with duration between 3 to 15 years. The parameters mostly monitored (in at least half of the TIDE cases) are: vegetation, accretion and sedimentation on site, invertebrates, birds and fish.
MRMs generate many synergies between nature, flood protection, port development, recreation and natural resources, but also conflicts with agriculture and local inhabitants.
MRMs are expensive but could also generate large benefits. The relative implementation cost of the TIDE MRM cases amounts 280,000 €/ha with a large range between 16,000 and 1.4 Million €/ha. For some measures, only a rough estimation was available.

• Three TIDE MRMs are considered as outliers with a remarkable high relative implementation cost, because of a high amount of soil that had to be removed out of the area (E-Hahn.S.) and that had to be treated because of contamination (E-Sp.B.), or uncertainty about the total implementation cost (S-Pad.).
• Furthermore, different measure characteristics are studied to find reasons for the large variance in the relative implementation cost.
• Size and age: No significant relationship is observed between the relative implementation cost and the size of the measures, nor could we observe a temporal evolution in the relative implementation cost.
• Implementation techniques: A significant difference in the relative implementation cost is observed between the TIDE measures implemented by dike breach or by defence removal. The latter technique is, evidently, much more expensive. A positive relationship with the breach size was however not significant. Furthermore, measures with land lowering are expected to be more expensive but this difference was also not significant.
• Creek system implemented: Measures with the implementation of a creek system are expected to be more expensive but this difference was not observed for the TIDE cases.
• Overall it is not possible to give a clear indication about what causes a higher or lower relative implementation cost. It depends too much on local conditions.
• Critical note: By comparing measure characteristics with the relative implementation cost nothing could be concluded about the success of the measure. Indeed, the effectiveness of the measure to reach the objectives/requirements and to be sustainable is more important when considering the measure design than the implementation cost.

Besides the implementation cost of the measures, also the benefits are studied based on the Ecosystem Services (ES) concept. However, no scientific consensus exists yet on the monetary valuation of ES. Different approaches are explored with often also different outcomes.

• A simple approach was applied to get a rough idea of the order of magnitude of the monetary benefits of the MRMs. A recent overall literature review with global monetary data for different biomes was used and multiplied with the habitat creation in the MRMs. Based on this approach, the TIDE examples generate an average benefit of 133,000 € per hectare and year, ranging from 70,000 to 155,000  € per hectare and year. The monetary benefit calculated here is however an overestimation because it is limited to the benefits generated within the estuary itself without counting for the lost adjacent land. (See APA 2013 page27-28 for calculation method)
• A more detailed approach to calculate the local benefits of a measure is however recommended. Therefore, a guidance document is developed to support managers and decision makers in how to quantify and monetary value the changes in ecosystem services specifically for the study site (Liekens  Broekx 2013) .

By comparing the costs and benefits of the measures, the cost-efficiency of the TIDE cases is analysed:

• The first method is the earn-back time, i.e. the average time that the measure should be operational before the total implementation cost is earned back. For the TIDE MRMs this amounts on average 2.3 years, ranging from 0.1 year to 15 years.
• The second method is the benefit/cost ratio, i.e. the annual benefit (as calculated above) generated for every 1€ invested (as calculated above). For the TIDE cases the benefit/cost ratio is on average 2.82:1, meaning a benefit of 2.82 €/y for every 1€ invested. The benefit/cost ratio for the TIDE cases ranges from 0.07 to 13.35 €/y for every 1€ invested.
• The earn-back time and benefit/cost ratio both give an indication of the cost-efficiency of a measure, assuming that the measure targets are met completely. However, in reality the latter assumption is rarely the situation. It is therefore recommended to first check the success of measures to meet the development targets and additionally the cost-efficiency estimate could be used to make a selection between measures that are expected to be successful.

In the final section, the results of an ES assessment for the MRMs are analysed (based on the TIDE ES study (Jacobs 2013) ).

• In a first part, the target ES are indicated per measure based on the development targets (Table 33). Most TIDE MRMs target the habitat and habitat services. In a few cases, this target is combined with a regulating service (flood water storage, dissipation of tidal and river energy), and/or a cultural services (opportunities for recreation and tourism, and information for cognitive development).
• The TIDE MRMs have a positive expected impact (from slightly positive to very positive) on at least 12 of the 20 considered ES. (Measures of table 32 in table 26)
• The expected impact on the targeted ES is in most cases very positive. On average, only 10% of the ES with a positive expected impact (slightly positive to very positive) are also targeted. This means that the MRMs are expected to generate many co-benefits! (Measures of table 32 in table 26)
• Regarding the beneficiaries, the TIDE MRMs are mainly beneficial in an indirect way, at a longer term (for future use), and at a local and regional scale. (Measures of table 32 in table 27)

Table 33: Translation of measure targets in terms of ES

Target	Corresponding Ecosystem Service
Safety	R1 - Erosion and sedimentation regulation by water bodies R4 - Water quantity regulation: dissipation of tidal and river energy R12 - Reg. of extreme events: flood water storage
Habitat conservation/restoration	S - Habitat services (biodiversity)
Compensation	S - Habitat services (biodiversity)
Access opp. and education	C4 - Cult. Opportunities for recreation and tourism
Research	C3 - Cult. Information for cognitive development

Optimisation of MRMs with a focus on the sedimentation rate

The second part of the MRM report focusses on issues related to the sedimentation rate at MR sites. Sedimentation and erosion processes have an important role in the development of MR sites and hence in the success of the MRMs. It is however a complex issue and difficult to predict and anticipate on in practice. Although for many measures some modelling work on this topic was done in the planning stage, the reality after measure implementation turned out to be different and does not always suit the development goals. In some TIDE cases the sedimentation rate was therefore considered as a problem, e.g. because tidal water areas silted up quickly due to unexpectedly high sedimentation rates or because habitat development was curtailed due to unexpectedly strong erosion. However, if the situation arises where we require a system which is not in equilibrium this might be more a problem of setting the goal than of the sedimentation rate that is “too high”. Meaning: the project might be in the wrong place, the objectives might be unrealistic or the design of the project might be suboptimal.

Managers have to deal with the unpredictability of the dynamic estuarine system but this does however not mean that managers do not have the possibility to improve the success of the measure and for example reduce the need to dredge the sites. Different aspects of the MRsites are studied to analyse their relationship with the sedimentation rate on the site. It is the aim of this study to better understand the link between the MRsites (both the location within the estuary and the design of the site) and the sedimentation rate and to formulate recommendations to enable managers to improve the selection and design of the site and hence the success of the measure.

Sedimentation rate TIDE cases
In general, the sedimentation rate is highest immediately after implementation and then levels off after some years. The overall average sedimentation rate on the TIDE MR sites is 9 cm/yr, with the highest sedimentation rate measured at parts of the Kleinensieler Plate (75 cm/yr, W-Kl.P.) and the strongest erosion in some parts of Ketenisseschor (-30 cm/yr, S-Ket.). The average accretion at Kleinensieler Plate (W-Kl.P.) is very high compared to all other TIDE cases, and without W-Kl.P. the overall average sedimentation rate is only 5 cm/yr.

Impact of site selection and site design aspects on the sedimentation rate

Outer-dike vs inner-dike measures
A first difference is made between outer-dike and inner-dike areas. In this study, outer-dike sites are defined as the areas that are under direct influence of the river and hence under influence of the full tidal range. Most TIDE cases are outer-dike sites. The inner-dike sites are defined as areas with a hydraulic constriction by a (narrow) construction (eg. sluice, sill or overflow barrier) in between the site and the estuary, resulting in a dampened tidal range on the site. A special case of inner-dike measures is a Controlled Reduced Tide (CRT), of which one example is analysed within TIDE (S-Lip.). It is expected that the sedimentation and erosion processes will differ between outer- and inner-dike sites due to the different site conditions, depending on water depth, residence time, concentration of suspended matter in the water column, erosion forces etc.. The Kleinensieler Plate is an example for a measure with more or less outer-dike character at the beginning which later on has been converted into an inner-dike site. By this water exchange, sediment entry and sedimentation rate has decreased significantly.

Indeed, based on the TIDE measures no significant relationship was found between the average sedimentation rate at the MR site and whether the site is located outer- or inner-dike (See APA 2013 page45).

Factors related to the location of the MR site in the estuary
Overall, the following location characteristics are considered as determining both global and local sedimentation and erosion processes: salinity gradient (TIDE-km and estuarine zone), Suspended Particulate Matter (SPM) and turbidity maximum, location at inner or outer side of a river bend, and hydrodynamics in the area.

Salinity gradient (TIDE-km and estuarine zone)
The first factor is the location of the MR site along the salinity gradient: at a certain TIDE-km or certain estuarine zone (freshwater, oligohaline, mesohaline and polyhaline). No relation with the average sedimentation rate at the MR site was found.

Suspended Particulate Matter (SPM) and turbidity maximum
The second factor is the SPM near the MR site and the location of the site at a turbidity maximum. For the TIDE cases, the average SPM amounts 200 mg/l, ranging from 38 mg/l to 700 mg/l. As expected, the sedimentation rate is higher at sites with a high SPM supply (Figure 13).

Factors related to the design of the MR site
Overall, the following site characteristics are considered as determining both global and local sedimentation and erosion processes: initial elevation (lower vs. higher zones), inundation (flood frequency and duration high vs low), slope (weak vs steep), opening to the river, vegetation at the site, drainage and creek system development.

Site topography: elevation and inundation
Spatial differences in elevation in the area will have an influence on spatial patterns of accretion and saltmarsh vegetation, with implications for the habitat development on the site such as benthic invertebrate diversity and bird usage of the site. It has previously been shown that an inverse relationship exists between elevation and accretion rates inside the realignment site. This is a consequence of the tidal regime in the area, i.e. lower parts will be flooded more frequent and for a longer time and hence more sediment could be deposited. It is proved that there is a positive relationship between inundation (frequency and duration) and the accretion rate and hence with elevation. This is also observed at the TIDE cases: sedimentation rates are higher at the lower areas (e.g. S-Lip., Figure 15 and W.Kl.P, Figure 16).
Inappropriate elevation could result in specific site objectives (e.g. marsh development) not being met. Areas that are located much lower than mean high water level (MHWL) for example are quasi constantly flooded and hence vegetation development is difficult. Old polders, frequently used as project sites, are however often located much lower than MHWL as a consequence of increasing water levels and alignment of the areas. In general, an elevation of the site at MHWL is considered as an optimal condition for realignments. The elevation of most TIDE cases is indeed situated around MHWL.

Slope
A causal relationship exists between the percentage of slope grade of the mudflat and the intensity of sedimentation and erosion: flat areas are characterised by more sedimentation and steep areas by less sedimentation or even erosion. In the TIDE example S-Ket., a sedimentation shift to erosion from a critical slope grade of 2.5% or more was determined. This also corresponds to the difference between the TIDE habitat types intertidal flat habitat (slope rate <2.5%) and intertidal steep habitat (slope rate >2.5%).

Opening to the river
The connection of the site with the river proved to influence the sedimentation and erosion processes in the site. The dimensions of the opening (width and elevation) will (partially) determine to which extent the site is under influence of the tidal prism. In addition, this will influence currents and water levels in the site and hence also the inundation and correspondingly the sediment inflow and the accretion rates. A larger opening (wider and/or low in elevation) can correspond with a larger water volume flowing in the area potentially bringing in also more suspended material. In addition, it is expected that a more or less proportion of suspended material that enters the area will also be deposited there and not return to the river. Hence, to control the sedimentation in the area it might be crucial to control the inflow of suspended material. From the TIDE cases no clear relationship was observed between the average sedimentation rate and the breach size (both absolute and relative to site surface), nor with the elevation of the opening. However, in the TIDE case W-Kl.P. the overflow barriers were heightened to reduce tidal range and by this the amount of suspended matter entering the project area, and indeed siltation tendencies were slowed down (see above). In another TIDE case (S-Heusd) it occurred however that the elevation of the opening was too high to properly drain the area, but this was solved by making an extra breach at MLWL.

As the dimensions of the breach are important for the development of the area, much attention is addressed during the planning phase to create optimal dimensions. For specific measures it might be necessary that at a long term perspective dimensions remain stable. For instance, sedimentation and erosion processes could, depending on the dynamics, enlarge or diminish the opening and change the hydromorphological characteristics in the area. To improve the stability, breaches are frequently enforced by a sill. Also a sluice system (such as in case of the FCA-CRT S-Lip.) could offer a solution, because the dimensions are constructed in detail and fine-tuning is possible.

Another aspect of the opening to the river is the number of breaches. If only one breach connects the site to the river, the site will function as a reservoir which will cause a different hydrodynamic situation compared to a site with at least two openings by which the site will function as a flow through (e.g. S-Lip. with high inlet and low outlet to improve the flow through characteristic). In a flow through case, hydrodynamics will be higher causing less sedimentation. However, flow current could also be too strong causing strong erosion obstructing habitat development. This was the case in the TIDE example E-Wr.B. where one site of the creek had to be closed to stop erosion and make habitat development possible.

Overall, managers have several possibilities to control, at least to a certain extent, the sedimentation in the MR site and hence improve the success of the MRM. In the site selection phase, it is advised to take into account the location of the turbidity maximum in the estuary, the SPM concentrations along the estuary and the location of river bends. In the designing phase, many factors could be controlled: outer- or inner dike area with full or dampened tidal influence; initial elevation of the area relative to the tidal prism; elevation differences within the MR site to improve habitat diversity; the slope of the area (a slope of 2.5% and more has to be avoided to make habitat development possible); sheltered sites have higher sedimentation rates compared to exposed sites; and with a larger opening more suspended matter could enter the area and could hence be deposited.

General recommendations for successful MRMs

The overall success of a MRM depends on the possibility to meet the different development targets. Hence the targets have to be specific, measurable and achievable within the context of the project (IECS 2008). MRMs executed in an estuary have to deal with the dynamic and complex context of the estuary. Biotic and abiotic factors of the estuary interact constantly, ultimately resulting in a dynamic equilibrium situation. When intervening in the estuary, e.g. by implementing a MRM, the system is disturbed will evolve towards a new dynamic equilibrium. For a successful MRM, the development targets have to be in accordance with what can be expected to become the new situation in the long term. The manager has however also the opportunity to guide the development of the MR site towards a targeted equilibrium situation by a well-considered design and location. When understanding the impacts of a MRM it will become easier to manipulate the ecological and hydromorphological processes in such a way that the MRM will evolve to the targeted equilibrium situation. In practice it is however difficult to predict the resulting equilibrium situation when implementing a certain measure and hence if this will be in accordance with the development targets.

To limit the unpredictability of the success of MRMs it is recommended to formulate dynamic goals with a time trajectory that corresponds to the perceived and predicted changes in the project area and in the estuary, rather than a fixed target without temporal consideration. That implies that the goals do not only contain a qualitative description of the desired situation (eg. which habitat types and which species communities), but also a time frame to reach the target (eg. at year t, t+10 and t+20) (IECS 2008). Since the development of the restoration project does not end after the completion of the engineering phase, it is recommended to incorporate realistic predictions of the time frame of evolution in tidal wetland restoration planning (Williams and Orr 2002). Existing and on-going projects in similar conditions could be used as reference to estimate the evolution of habitat development and to determine feasible performance criteria for different habitats.

Formulating dynamic goals (eg. marshland with mudflats and creek development) has to follow from the understanding of both the ecological and the hydromorphological changes (IECS 2008). As sedimentation, erosion and the development of the vegetation are natural processes of the restored estuarine habitats, changes will occur (eg. mudflat will evolve to marsh). The character of the estuarine habitat will therefore inevitably change. The goals of restoration projects should hence be formulated with the ecological and the hydromorphological (desired and undesired) changes in mind because they are intrinsic aspects of the estuarine habitats. This means that it is advised to target certain habitat types and species communities, but not in quantitative terms (exact number of hectares of each habitat type or exact number of species).

Optimisation of measure success
To optimise the success of the MRMs it is recommended to start in the planning phase with incorporating lessons learned from previous and on-going projects. Indeed, the general knowledge on how to develop realignment sites has already been greatly advanced through practical experiences in many case studies. Knowledge sharing could be improved by an iterative approach, i.e. follow and further develop best practices established in the past. The evaluation of previous and on-going projects will provide valuable information on the short- and long-term development of restoration projects. This could help to understand the impact of management interventions on overall developments and this can also indicate which other tools are required to guide restoration projects towards their goals (IECS 2008). A deeper going analysis of comparable successful measures realized under similar conditions could also minimize the risk of associated problems (eg. additional maintenance effort after measure implementation; reconstruction of overflow barriers; etc.). Exchange of experiences, also across estuaries, is hence necessary to improve the overall success of MRMs and this TIDE report aims to be a first step in that direction.

The success of MRMs also depends on the pre- and post-project monitoring.

• This is indeed necessary in order to check whether the targeted results finally have been achieved. And more important to identify unwanted changes or a lack of change in certain aspects for which interventions may be required to steer the development in the aimed direction (IECS 2008).
• Adaptive management, both during and after implementation, forms an important part of the management strategy to improve the overall success of the restoration project.
• Previous and on-going projects could also help to identify which factors are important to monitor, as well as identifying which monitoring techniques should be used.
• Regarding the success of MRMs, it is recommended to consider (at least) tidal prism, breach design (and breach flow speeds), the role of site morphology in delivering particular habitats, and how future accretion may influence site development (Scott et al.).
• The time-scale of the monitoring program has to follow the time-frame of project and hence of the development goals. Because long-term monitoring is in practice often difficult to establish within the project, it is recommended to incorporate the monitoring and possibly the evaluation in a regular long-term monitoring program (IECS 2008).

MRMs generate many ecosystem services and many synergies, but also conflicts between different stakeholders could occur. An effective, clear, honest and early communication strategy with the public, stakeholders and regulators is hence also a key aspect in the overall success of MRMs. It is indeed important to optimise the social support for the measure: by securing landowner involvement and allow sufficient time for landowner negotiations (Scott et al.), by emphasizing the multiple socio-economic benefits of the measure, and if necessary by explaining that the design has changed as far as possible to minimise negative effects on public.

Success related to sedimentation issues
The success of MRMs depends, among many others, on the induced sedimentation and erosion processes (Vandenbruwaene et al. 2011) because these processes are key factors in realising most development goals, i.e. to ensure a site is at the right elevation and receives sufficient tidal inundation for habitat development and for flood storage capacity. However, the real sedimentation and erosion processes on the site are not always in favour of the development goals. When sedimentation rates are higher or lower than expected this could be a disadvantage for certain goals. Reduction of the sedimentation rate in the realignment site could be beneficial to meet for instance the goal flood water storage and additionally this could also reduce the need for maintenance efforts in the future which is then beneficial for vegetation, fauna and water structures.

The presented study (part 2) illustrates that by considering certain aspects of the site selection and design, the expected sedimentation and erosion processes could be manipulated to a certain extent in favour of specific development goals. A first recommendation is to evaluate existing and on-going projects to use one or several reference states from a comparable setting (in terms of geomorphology, tidal range and elevation) as basis to establish the design on a target state for the restoration site (IECS 2008). Furthermore, the conclusions from the presented study (part 2) could be used as guideline for optimal site selection and design. Depending on the development goals (habitat development and/or safety), the sedimentation and erosion processes could be guided in a favourable way by designing certain site aspects in a specific way. For many realignment sites the development goals are however a combination of the development of different habitat types. It is therefore recommended to adapt the design of different zones of the site in favour of the different goals. This means a large spatial variation in elevation, slope, etc.

An overall rule for designing realignment sites should be to minimise land manipulation and work with the existing topography as far as possible. It is hence recommended to maximise the advantage from natural physical and vegetative processes and natural sources from the site (e.g. materials for dike enforcement). Furthermore, the extent of any landform manipulation must be justified due to the consideration of project objectives, the potential gains and the likely cost (Scott et al.)

Overall, it is important to keep always in mind that the estuary is a highly dynamic ecosystem and the most important rule for successful management is to work with the system, not against it!