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In many parts of the Netherlands a dense network of watercourses is necessary to drain the land. A related aspect is thechanging transport capacity of the water courses due to thegrowth of aquatic plants in the growing season. In turn thisphenomenon requires aquatic weeds to be controlled by waterboards in order to maintain the water course's required capacity.The procedure outlined in this paper can be used to determinethe frequency of weed control based on hydrological andhydraulic conditions. The drainage from groundwater gives anexpected variation in discharge over the growing season. Thepermissible flow rate is governed by both the growth of weedsand the permissible water levels in the water course. Weedcontrol is necessary during the growing season when theexpected discharge is more than the permissible flow rate. Thisprocedure has been incorporated in the model MWW(Maintenance of Water courses by Water boards). An examplehow to use MWW is given.

Maaionderhoud in waterlopen is nodig om de watertransportfunctie ervan te waarborgen. Het benodigde maaionderhoud in waterlopen is vanuit hydrologisch oogpunt berekend. De belangrijkste aspecten hierbij zijn: verwachte afvoer, dimensies van de waterloop, drooglegging, stromingsweerstand, en groei van water- en oevervegetatie. Voor het berekenen van tijdstip en frequentie van onderhoud zijn de verwachte afvoeren berekend met een eendimensionaal grondwatermodel. Als de verwachte afvoer groter is dan het toelaatbare debiet, is onderhoud nodig. Het onderhoud is berekend voor twee waterlopen in Twente. De methode biedt de waterschappen de mogelijkheid om het maaibeheer te verantwoorden of te verminderen.

De voormalige zes waterschappen in Friesland hebben in 2001 besloten een proef op te starten waarbij vanuit de praktijk wordt gekeken naar kansen voor een meer natuurvriendelijk beheer en onderhoudsregime. Daarvoor is het vijf jaar durend onderzoek Onderhoud op Maat opgezet dat in 2001 is gestart en 2006 wordt afgerond. De proef richtte zich op watergangen in agrarisch gebied en zonder een natuurvriendelijke inrichting. Op basis van bestaande theoretische kennis, praktische ervaring en de kenmerken van een gebied zijn in vijf proefgebieden onderhoudsmethoden voorgesteld en vijf jaar uitgevoerd. Gedurende de proef zijn de ontwikkelingen gevolgd ten aanzien van ecologie, waterkwaliteit, waterkwantiteit, de beheeraspecten, kosten en draagvlak. Ter bevordering van het draagvlak zijn vanaf het begin van het project zowel de bestuurders als de ambtenaren van het waterschap en vertegenwoordigers uit de streek bij het onderzoek betrokken. Na vijf jaar is gebleken dat er praktische beheersvormen zijn toegepast waarbij de kwaliteit van de flora en fauna ook daadwerkelijk is toegenomen. Daarbij is een verschuiving te zien van soorten van voedselrijke naar voedselarmere omstandigheden. Bovendien is op veel plaatsen het aantal plantensoorten toegenomen. Vanwege de korte periode en de beperkte omvang van de proefvakken is er nog geen relatie te leggen tussen de onderhoudsvormen en de gemeten waterkwaliteit. Het meeste effect van een op maat gesneden onderhoud wordt verwacht op de vegetatie en de biologische kwaliteit van het water.

De voormalige zes waterschappen in Friesland hebben in 2001 besloten een proef op te starten waarbij vanuit de praktijk wordt gekeken naar kansen voor een meer natuurvriendelijk beheer en onderhoudsregime. Daarvoor is het vijf jaar durend onderzoek Onderhoud op Maat opgezet dat in 2001 is gestart en 2006 wordt afgerond. De proef richtte zich op watergangen in agrarisch gebied en zonder een natuurvriendelijke inrichting. Op basis van bestaande theoretische kennis, praktische ervaring en de kenmerken van een gebied zijn in vijf proefgebieden onderhoudsmethoden voorgesteld en vijf jaar uitgevoerd. Gedurende de proef zijn de ontwikkelingen gevolgd ten aanzien van ecologie, waterkwaliteit, waterkwantiteit, de beheeraspecten, kosten en draagvlak. Ter bevordering van het draagvlak zijn vanaf het begin van het project zowel de bestuurders als de ambtenaren van het waterschap en vertegenwoordigers uit de streek bij het onderzoek betrokken. Na vijf jaar is gebleken dat er praktische beheersvormen zijn toegepast waarbij de kwaliteit van de flora en fauna ook daadwerkelijk is toegenomen. Daarbij is een verschuiving te zien van soorten van voedselrijke naar voedselarmere omstandigheden. Bovendien is op veel plaatsen het aantal plantensoorten toegenomen. Vanwege de korte periode en de beperkte omvang van de proefvakken is er nog geen relatie te leggen tussen de onderhoudsvormen en de gemeten waterkwaliteit. Het meeste effect van een op maat gesneden onderhoud wordt verwacht op de vegetatie en de biologische kwaliteit van het water.

In deze bijdrage wordt over de ontsluiting te water een overzicht gegeven: waar lagen de waterwegen, welke werden gegraven en wanneer werden ze in gebruik genomen. Het betreft de Romeinse periode en de Middeleeuwen

In deze bijdrage wordt over de ontsluiting te water een overzicht gegeven: waar lagen de waterwegen, welke werden gegraven en wanneer werden ze in gebruik genomen. Het betreft de Romeinse periode en de Middeleeuwen

De natuur in Nederland raakt steeds meer versnipperd. Infrastructuur is op veel plaatsen mede verantwoordelijk voor deze versnippering. Om een overzicht te krijgen van de locaties waar zich knelpunten voordoen tussen de natuur en infrastructuur is eenlandsdekkende knelpuntenanalyse uitgevoerd. De knelpuntenanalyse richt zich wat natuur betreft op de ecologische hoofdstructuur (EHS), gestelde natuurdoelen buiten de EHS en de robuuste verbindingen. Wat betreft infrastructuur beperkt deze studie zich tot rijkswegen, provinciale wegen, spoorwegen en vaarwegen. Een significante vermindering van de levensvatbaarheid van dierpopulaties als gevolg van de aanwezigheid van infrastructuur is bij het achterhalen van de knelpunten als graadmeter gebruikt. Deze analyse is uitgevoerd met het expertsysteem LARCH. Hierbij zijn tien soortgroepen, zogenaamde ecoprofielen, doorgerekend. Deze soortgroepen zijn representatief voor circa driekwart van het Nederlandse (droge) natuurareaal. In totaal zijn 126 knelpunten vastgesteld, exclusief de knelpunten binnen robuuste verbindingen. Per knelpunt is de ecologische winst berekend wanneer het desbetreffende knelpunt zou zijn opgelost. Op basis van deze ecologische winst zijn de knelpunten geprioriteerd. Tevens is aangeduid wanneer knelpunten samenhang met elkaar hebben; dit betekent dat het oplossen van het ene alleen zinvol is wanneer ook het andere knelpunt wordt aangepakt.

De natuur in Nederland raakt steeds meer versnipperd. Infrastructuur is op veel plaatsen mede verantwoordelijk voor deze versnippering. Om een overzicht te krijgen van de locaties waar zich knelpunten voordoen tussen de natuur en infrastructuur is eenlandsdekkende knelpuntenanalyse uitgevoerd. De knelpuntenanalyse richt zich wat natuur betreft op de ecologische hoofdstructuur (EHS), gestelde natuurdoelen buiten de EHS en de robuuste verbindingen. Wat betreft infrastructuur beperkt deze studie zich tot rijkswegen, provinciale wegen, spoorwegen en vaarwegen. Een significante vermindering van de levensvatbaarheid van dierpopulaties als gevolg van de aanwezigheid van infrastructuur is bij het achterhalen van de knelpunten als graadmeter gebruikt. Deze analyse is uitgevoerd met het expertsysteem LARCH. Hierbij zijn tien soortgroepen, zogenaamde ecoprofielen, doorgerekend. Deze soortgroepen zijn representatief voor circa driekwart van het Nederlandse (droge) natuurareaal. In totaal zijn 126 knelpunten vastgesteld, exclusief de knelpunten binnen robuuste verbindingen. Per knelpunt is de ecologische winst berekend wanneer het desbetreffende knelpunt zou zijn opgelost. Op basis van deze ecologische winst zijn de knelpunten geprioriteerd. Tevens is aangeduid wanneer knelpunten samenhang met elkaar hebben; dit betekent dat het oplossen van het ene alleen zinvol is wanneer ook het andere knelpunt wordt aangepakt.

Beekherstel in Nederland vindt op vele locaties met diverse methoden plaats. De kennis over deze beeksystemen is echter beperkt. Daarom is in gezamelijk overleg tussen de Provincie Noord-Brabant, waterschap De Aa, waterschap De Dommel en DLO-Staring Centrum besloten tot het uitvoeren van een monitoringsproject van drie beekherstelprojecten (De Aa bij Helmond, Keersop-Gagelvelden en Tongelreep-Achelse Kluis) gedurende een periode van twee jaar. De monitoring werd gericht op ontwikkelingen in geomorfologie en macrofauna na hermeandering. Er blijken grote verschillen te zijn tussen de beken onderling; niet alleen verschilt de dynamiek, ook het verschil in bodemtype blijkt een grote rol te spelen.

IntroductionSince the publication of 'Our Common Future' (1987) by the World Commission on Environment and Development, sustainability or sustainable development has become one of the most important aims of environmental policy throughout the world. But it has proved difficult to work out in a definition, in policy targets and in policy instruments. There are many definitions of sustainability for rural and urban regions, as well as for activities such as agriculture, nature conservation, outdoor recreation and water resource management; there seems to be no need for yet another.In this thesis, sustainability is defined as a task of planning. The central topic is, therefore, how sustainability is defined in planning, practice and implemented in plans. But what are planning practices lid what are plans? To answer these questions an analytical tool was developed by looking for a definition of Planning that can accommodate new developments in the relation between Dutch government and society. Inspired by Friedmann planning is defined in this thesis as an attempt to link knowledge to action in the public domain; this was elaborated for land use planning in Dutch rural areas. A distinction is made between three task: rural development, steering and planning. The first concerns the knowledge about the physical environment and the action (design and evaluation) taken to achieve the desired situation in the physical environment. The second involves transforming the visions of private actors and the concepts of policy makers into claims on the physical environment, so that it can be decided which claims are legitimate and feasible in relation to development. The third involves the making a plan by organizing the interaction between the development and the steering tasks within the framework of political institutions and regulations. It can be defined in different ways because the content of the tasks can vary. In this thesis, the term 'planning style' is used to describe this. A distinction is made between four such styles: planning as reform, planning as policy analysis, planning as a learning process and planning as exchange process. These styles are elaborated for the tasks of steering and of development, the definition of the plan and the relation between design and implementationTable I Styles of planningSource: this theses table 2.4Three planning systems deal with the physical environment in the Netherlands: spatial planning, environmental planning and water management They have a common goal: to achieve a sustainable physical environment.But their definitions of the tasks of steering and development differ, as do their planning traditions. These differences are not a problem as long as the planning systems have distinct planning practices. However, intensive land use in the rural areas (agriculture, bulb growing and horticulture) and a high population density is causing the planning practices to converge, bringing about the need for coordination and integration of planning practices and their concepts.Central topicThis thesis is a contribution to the debate about the opportunities and likelihood of coordinating and integrating of the existing planning traditions, embodied in the planning systems, into a new planning practice directed to sustainable development in Dutch rural areas. The central topic is to clarify the differences between the concepts of steering and the concepts of development and to construct a model for assessing and designing of integrated planning practices. To this end, the following research questions are posed:1. what are the features of the planning systems?2. what are the dominant concepts about the physical environment?3. what are the dominant concepts of steering in relation to the physical environment?4. who are the adressees of these planning systems?5. what conditions apply to the integrated planning practices and what are the main aspects for assessing existing planning practices?6. what conclusions can be drawn about the task of planning in the future and the usefulness of the evalution model?MethodSeveral methods were used to answer these questions. To detect the features of the planning systems the literature was studied, planning practices were evaluated and interviews were conducted. In an attempt to find possible connections between the dominant concepts of development an analysis was done of the water system approach (Ministerie Verkeer en Waterstaat, 1985), the 'environmental utilisztion space' (Opschoor, 1990) and the model of the spatial organization (Kleefmann, 1990), drawing on scientific literature and documents produced by the Dutch government. The same procedure was followed for the dominant concepts of steering: the classic model, the network model and the market model (Koppenjan et al. 1993) in relation to the adressees: the 'active citizen' in spatial planning, the 'target group' in environmental planning and the 'user of the land' in (regional) water management. A model based on these analyses was built for assessing and designing of integrated planning practices.Model for assessment and designThe relation between Dutch government and society has changed in the last decade and forms of network steering are becoming more important. As a result integrated planning practices can only succeed when there is a need to cope with diffuse pollution and a change in behaviour. Three changes have come about a) private actors are participating in the planning process, b) the existing concepts of development and steering are being transformed into wholly of partically new concepts and c) the plan is now a joint product containing a vision of how the region should develop. These aspects are elaborated in Table 11. In short, the planning process ought to have the signature of two styles: planning as learning process and/or planning as exchange process. Planning as policy analysis and planning as reform are less suitable.Table II. Aspects for assessing and designing planning practicesSource: this thesis table 9.3CasesA wide range of cases is needed in order to analyse the state of the art of integrated planning practices in the Netherlands. The cases were selected according to the following criteria. The plans had to express the willingness of government (national, provincial or local) to construct integrated planning practices. The selection had to contain plans of all three tiers government. The plans had to embody a relation between at least two concepts of development and a change in the concept of steering, compared with the pre- plan situation. Both regular and non-regular plans had to be included. This resulted in the following cases being selected (Table Ill): The National Memorandum on Environmental Policy II (Ministry of Housing, Spatial Planning and Environment, 1993); The Management Plan for the Waters of national importance (Ministry of Transport, Public Works and Water Management, 1992); especially for the IJsselmeergebied (Ministry of Transport, Public Works and Water Management, 1991), North Sea (Ministry of Traffic and Water Management, 1991) and the Rijntakken (Ministry of Transport, Public Work and Water Management, 1994); The Ammonia Reduction Plans for Southeast Friesland (forthcoming) and the Gelder Valley (1994); The coordination and integration between the Water Management Plan, the Environmental Policy Plan and the Spatial Plan of Gelderland Province (Provincie Gelderland, 1996abc); The vision for the Regge river basin (Goossensen et al., 1997).In cases one, two and three, seven plans were analysed and assessed. Cases four and five differ because the researcher participated in the planning processes. As a result the aspect of the design of integrated planning practices received more emphasis; an insider's assessment.Table III Cases of integrated planning practicesResultsThe results of the assessment have been compiled in Table IV. This overview shows that some planning practices can be classified as integrated: the Ammonia Reduction Plans and the vision for the Regge river basin. In the Ammonia Reduction Plans a combination of spatial zoning and a bulb (marketable emission rights) for the reduction of ammonia creates a new balance between agricultural development and protection of the nature conservation areas. The vision on the Regge river basin combines spatial hydrological zoning and the water management area defined as a bulb in which the problems of eutrophication and the fall in hydraulic head can be solved. In both planning practices, the participation of private actors is important in the discussion about the vision and the relation between this joint vision and the opportunities for fitting this in with the evryday practices of the private actors.Conclusions and discussion1. The three planning systems, current in the Netherlands today each have their own signature. In the last decade the development task has changed as a result of new insights into the relation between the components of the physical environment. The task of steering has changed too, as a result of a changing relation between Dutch government and society. The planning systems have reacted differently to these changes. The system of spatial planning is evolving from planning as policy analysis into planning as a learning process; environmental planning is evolving from planning as reform to planning as exchange process; water management is evolving from planning as reform to planning as policy analysis. The modernisation of the systems is proceeding in different directions.2. Interesting experiments are going on in Dutch planning practice in the countryside, where private and public actors are looking for a practical definition of sustainable development. It can be concluded that integrated planning practices can only succeed when public and private actors are able to achieve a creative combination of elements of the concepts of development and the concepts of steering and when this combination is based on clear relation between the steering and development task.3. The distinction between the planning task, the development task and the steering task and the combination in different styles of planning turns out to be a promising analytical tool for analysing planning practices with regard to the physical environment. But despite this there is a need for elaboration in more detail.

Beekherstel in Nederland vindt op vele locaties met diverse methoden plaats. De kennis over deze beeksystemen is echter beperkt. Daarom is in gezamelijk overleg tussen de Provincie Noord-Brabant, waterschap De Aa, waterschap De Dommel en DLO-Staring Centrum besloten tot het uitvoeren van een monitoringsproject van drie beekherstelprojecten (De Aa bij Helmond, Keersop-Gagelvelden en Tongelreep-Achelse Kluis) gedurende een periode van twee jaar. De monitoring werd gericht op ontwikkelingen in geomorfologie en macrofauna na hermeandering. Er blijken grote verschillen te zijn tussen de beken onderling; niet alleen verschilt de dynamiek, ook het verschil in bodemtype blijkt een grote rol te spelen.

IntroductionSince the publication of 'Our Common Future' (1987) by the World Commission on Environment and Development, sustainability or sustainable development has become one of the most important aims of environmental policy throughout the world. But it has proved difficult to work out in a definition, in policy targets and in policy instruments. There are many definitions of sustainability for rural and urban regions, as well as for activities such as agriculture, nature conservation, outdoor recreation and water resource management; there seems to be no need for yet another.In this thesis, sustainability is defined as a task of planning. The central topic is, therefore, how sustainability is defined in planning, practice and implemented in plans. But what are planning practices lid what are plans? To answer these questions an analytical tool was developed by looking for a definition of Planning that can accommodate new developments in the relation between Dutch government and society. Inspired by Friedmann planning is defined in this thesis as an attempt to link knowledge to action in the public domain; this was elaborated for land use planning in Dutch rural areas. A distinction is made between three task: rural development, steering and planning. The first concerns the knowledge about the physical environment and the action (design and evaluation) taken to achieve the desired situation in the physical environment. The second involves transforming the visions of private actors and the concepts of policy makers into claims on the physical environment, so that it can be decided which claims are legitimate and feasible in relation to development. The third involves the making a plan by organizing the interaction between the development and the steering tasks within the framework of political institutions and regulations. It can be defined in different ways because the content of the tasks can vary. In this thesis, the term 'planning style' is used to describe this. A distinction is made between four such styles: planning as reform, planning as policy analysis, planning as a learning process and planning as exchange process. These styles are elaborated for the tasks of steering and of development, the definition of the plan and the relation between design and implementationTable I Styles of planningSource: this theses table 2.4Three planning systems deal with the physical environment in the Netherlands: spatial planning, environmental planning and water management They have a common goal: to achieve a sustainable physical environment.But their definitions of the tasks of steering and development differ, as do their planning traditions. These differences are not a problem as long as the planning systems have distinct planning practices. However, intensive land use in the rural areas (agriculture, bulb growing and horticulture) and a high population density is causing the planning practices to converge, bringing about the need for coordination and integration of planning practices and their concepts.Central topicThis thesis is a contribution to the debate about the opportunities and likelihood of coordinating and integrating of the existing planning traditions, embodied in the planning systems, into a new planning practice directed to sustainable development in Dutch rural areas. The central topic is to clarify the differences between the concepts of steering and the concepts of development and to construct a model for assessing and designing of integrated planning practices. To this end, the following research questions are posed:1. what are the features of the planning systems?2. what are the dominant concepts about the physical environment?3. what are the dominant concepts of steering in relation to the physical environment?4. who are the adressees of these planning systems?5. what conditions apply to the integrated planning practices and what are the main aspects for assessing existing planning practices?6. what conclusions can be drawn about the task of planning in the future and the usefulness of the evalution model?MethodSeveral methods were used to answer these questions. To detect the features of the planning systems the literature was studied, planning practices were evaluated and interviews were conducted. In an attempt to find possible connections between the dominant concepts of development an analysis was done of the water system approach (Ministerie Verkeer en Waterstaat, 1985), the 'environmental utilisztion space' (Opschoor, 1990) and the model of the spatial organization (Kleefmann, 1990), drawing on scientific literature and documents produced by the Dutch government. The same procedure was followed for the dominant concepts of steering: the classic model, the network model and the market model (Koppenjan et al. 1993) in relation to the adressees: the 'active citizen' in spatial planning, the 'target group' in environmental planning and the 'user of the land' in (regional) water management. A model based on these analyses was built for assessing and designing of integrated planning practices.Model for assessment and designThe relation between Dutch government and society has changed in the last decade and forms of network steering are becoming more important. As a result integrated planning practices can only succeed when there is a need to cope with diffuse pollution and a change in behaviour. Three changes have come about a) private actors are participating in the planning process, b) the existing concepts of development and steering are being transformed into wholly of partically new concepts and c) the plan is now a joint product containing a vision of how the region should develop. These aspects are elaborated in Table 11. In short, the planning process ought to have the signature of two styles: planning as learning process and/or planning as exchange process. Planning as policy analysis and planning as reform are less suitable.Table II. Aspects for assessing and designing planning practicesSource: this thesis table 9.3CasesA wide range of cases is needed in order to analyse the state of the art of integrated planning practices in the Netherlands. The cases were selected according to the following criteria. The plans had to express the willingness of government (national, provincial or local) to construct integrated planning practices. The selection had to contain plans of all three tiers government. The plans had to embody a relation between at least two concepts of development and a change in the concept of steering, compared with the pre- plan situation. Both regular and non-regular plans had to be included. This resulted in the following cases being selected (Table Ill): The National Memorandum on Environmental Policy II (Ministry of Housing, Spatial Planning and Environment, 1993); The Management Plan for the Waters of national importance (Ministry of Transport, Public Works and Water Management, 1992); especially for the IJsselmeergebied (Ministry of Transport, Public Works and Water Management, 1991), North Sea (Ministry of Traffic and Water Management, 1991) and the Rijntakken (Ministry of Transport, Public Work and Water Management, 1994); The Ammonia Reduction Plans for Southeast Friesland (forthcoming) and the Gelder Valley (1994); The coordination and integration between the Water Management Plan, the Environmental Policy Plan and the Spatial Plan of Gelderland Province (Provincie Gelderland, 1996abc); The vision for the Regge river basin (Goossensen et al., 1997).In cases one, two and three, seven plans were analysed and assessed. Cases four and five differ because the researcher participated in the planning processes. As a result the aspect of the design of integrated planning practices received more emphasis; an insider's assessment.Table III Cases of integrated planning practicesResultsThe results of the assessment have been compiled in Table IV. This overview shows that some planning practices can be classified as integrated: the Ammonia Reduction Plans and the vision for the Regge river basin. In the Ammonia Reduction Plans a combination of spatial zoning and a bulb (marketable emission rights) for the reduction of ammonia creates a new balance between agricultural development and protection of the nature conservation areas. The vision on the Regge river basin combines spatial hydrological zoning and the water management area defined as a bulb in which the problems of eutrophication and the fall in hydraulic head can be solved. In both planning practices, the participation of private actors is important in the discussion about the vision and the relation between this joint vision and the opportunities for fitting this in with the evryday practices of the private actors.Conclusions and discussion1. The three planning systems, current in the Netherlands today each have their own signature. In the last decade the development task has changed as a result of new insights into the relation between the components of the physical environment. The task of steering has changed too, as a result of a changing relation between Dutch government and society. The planning systems have reacted differently to these changes. The system of spatial planning is evolving from planning as policy analysis into planning as a learning process; environmental planning is evolving from planning as reform to planning as exchange process; water management is evolving from planning as reform to planning as policy analysis. The modernisation of the systems is proceeding in different directions.2. Interesting experiments are going on in Dutch planning practice in the countryside, where private and public actors are looking for a practical definition of sustainable development. It can be concluded that integrated planning practices can only succeed when public and private actors are able to achieve a creative combination of elements of the concepts of development and the concepts of steering and when this combination is based on clear relation between the steering and development task.3. The distinction between the planning task, the development task and the steering task and the combination in different styles of planning turns out to be a promising analytical tool for analysing planning practices with regard to the physical environment. But despite this there is a need for elaboration in more detail.

De mogelijkheden van een gedifferentieerd risicobeleid bij het bestrijdingsmiddelengebruik zijn onderzocht. Er is behoefte aan een eenduidige, inzichtelijke en verifieerbare indeling van alle waterlopen in Nederland. Daartoe is Nederland verdeeld in landschapsregio's met een regiospecifieke structuur en geometrie van het afwateringsstelsel. Voor elke regio is het oppervlaktewaterstelsel geanalyseerd met het Waterstaatkundig Informatiesysteem en de digitale topografische kaart van Nederland, schaal 1 : 10 000. Er is gekeken naar het afwateringssysteem, de afwateringsintensiteit, en dimensies van de meest voorkomende waterloopklassen. Een differentiatie van waterlopen naar fysische kenmerken is bijna onmogelijk, indien geen rekening wordt gehouden metde landschapsregio's. Differentiatie is wel mogelijk op basis van een juridische status.

De mogelijkheden van een gedifferentieerd risicobeleid bij het bestrijdingsmiddelengebruik zijn onderzocht. Er is behoefte aan een eenduidige, inzichtelijke en verifieerbare indeling van alle waterlopen in Nederland. Daartoe is Nederland verdeeld in landschapsregio's met een regiospecifieke structuur en geometrie van het afwateringsstelsel. Voor elke regio is het oppervlaktewaterstelsel geanalyseerd met het Waterstaatkundig Informatiesysteem en de digitale topografische kaart van Nederland, schaal 1 : 10 000. Er is gekeken naar het afwateringssysteem, de afwateringsintensiteit, en dimensies van de meest voorkomende waterloopklassen. Een differentiatie van waterlopen naar fysische kenmerken is bijna onmogelijk, indien geen rekening wordt gehouden metde landschapsregio's. Differentiatie is wel mogelijk op basis van een juridische status.

In many parts of the Netherlands a dense network of watercourses is necessary to drain the land. A related aspect is thechanging transport capacity of the water courses due to thegrowth of aquatic plants in the growing season. In turn thisphenomenon requires aquatic weeds to be controlled by waterboards in order to maintain the water course's required capacity.The procedure outlined in this paper can be used to determinethe frequency of weed control based on hydrological andhydraulic conditions. The drainage from groundwater gives anexpected variation in discharge over the growing season. Thepermissible flow rate is governed by both the growth of weedsand the permissible water levels in the water course. Weedcontrol is necessary during the growing season when theexpected discharge is more than the permissible flow rate. Thisprocedure has been incorporated in the model MWW(Maintenance of Water courses by Water boards). An examplehow to use MWW is given.

Maaionderhoud in waterlopen is nodig om de watertransportfunctie ervan te waarborgen. Het benodigde maaionderhoud in waterlopen is vanuit hydrologisch oogpunt berekend. De belangrijkste aspecten hierbij zijn: verwachte afvoer, dimensies van de waterloop, drooglegging, stromingsweerstand, en groei van water- en oevervegetatie. Voor het berekenen van tijdstip en frequentie van onderhoud zijn de verwachte afvoeren berekend met een eendimensionaal grondwatermodel. Als de verwachte afvoer groter is dan het toelaatbare debiet, is onderhoud nodig. Het onderhoud is berekend voor twee waterlopen in Twente. De methode biedt de waterschappen de mogelijkheid om het maaibeheer te verantwoorden of te verminderen.

Maaionderhoud in waterlopen is nodig om de watertransportfunctie ervan te waarborgen. Het benodigde maaionderhoud in waterlopen is vanuit hydrologisch oogpunt berekend. De belangrijkste aspecten hierbij zijn: verwachte afvoer, dimensies van de waterloop, drooglegging, stromingsweerstand, en groei van water- en oevervegetatie. Voor het berekenen van tijdstip en frequentie van onderhoud zijn de verwachte afvoeren berekend met een eendimensionaal grondwatermodel. Als de verwachte afvoer groter is dan het toelaatbare debiet, is onderhoud nodig. Het onderhoud is berekend voor twee waterlopen in Twente. De methode biedt de waterschappen de mogelijkheid om het maaibeheer te verantwoorden of te verminderen.

Aquatic weed control, carried out by the water boards in the Netherlands, is required to maintain sufficient discharge capacity of the surface water system. Weed control affects the conditions of both surface water and groundwater. The physically based model MOGROW was developed to simulate the flow of water in the unsaturated zone, the saturated zone and the water courses in an integrated manner. The model was verified with field data on groundwater levels, discharges, moisture storage in the root zone and evapotranspiration in the Hupselse Beek catchment.The time and frequency of weed control during summer were determined using data on weed growth, flow resistance and discharges. This study was carried out in the Poelsbeek and Bolscherbeek catchments located in the east of the Netherlands. Weed obstruction was measured in nine water courses of these catchments. The boundary roughness coefficientκ M was calculated from field data and laboratory experiments. Theκ M varied for the unobstructed part of the cross-section between 30-34 m 1/3.s -1. A numerical model was used to determine the velocity distribution in a cross-section of a water course. This calculated velocity distribution resulted also inκ M values of the same order. The minimally required weed control was estimated using the data on weed obstruction, flow resistance and estimates of expected discharges. These expected discharges for the summer period were calculated for three water table fluctuation classes, using a one-dimensional groundwater model. Comparing the expected discharges and the permissible flow rates for a water course, gives the required periods of weed control.Weed control strategies were evaluated for the Poelsbeek and Bolscherbeek area using the model MOGROW. Results of the modelling experiments are the occurrence of high surface water levels, high groundwater levels and high stream flow velocities. In a cost-benefit analysis the cost of weed control and the damage for pasture (in terms of financial losses) were considered. Seven simulation years with varying wet conditions in summer were selected. The financial loss resulting from one clearing at the end of summer, was too high for only one of these years. The multi-objective decision method ELECTRE II enabled the ecological impact to be incorporated in the evaluation. The method identified weed control strategies that are most favourable for agriculture or nature conservation.

Aquatic weed control, carried out by the water boards in the Netherlands, is required to maintain sufficient discharge capacity of the surface water system. Weed control affects the conditions of both surface water and groundwater. The physically based model MOGROW was developed to simulate the flow of water in the unsaturated zone, the saturated zone and the water courses in an integrated manner. The model was verified with field data on groundwater levels, discharges, moisture storage in the root zone and evapotranspiration in the Hupselse Beek catchment.The time and frequency of weed control during summer were determined using data on weed growth, flow resistance and discharges. This study was carried out in the Poelsbeek and Bolscherbeek catchments located in the east of the Netherlands. Weed obstruction was measured in nine water courses of these catchments. The boundary roughness coefficientκ M was calculated from field data and laboratory experiments. Theκ M varied for the unobstructed part of the cross-section between 30-34 m 1/3.s -1. A numerical model was used to determine the velocity distribution in a cross-section of a water course. This calculated velocity distribution resulted also inκ M values of the same order. The minimally required weed control was estimated using the data on weed obstruction, flow resistance and estimates of expected discharges. These expected discharges for the summer period were calculated for three water table fluctuation classes, using a one-dimensional groundwater model. Comparing the expected discharges and the permissible flow rates for a water course, gives the required periods of weed control.Weed control strategies were evaluated for the Poelsbeek and Bolscherbeek area using the model MOGROW. Results of the modelling experiments are the occurrence of high surface water levels, high groundwater levels and high stream flow velocities. In a cost-benefit analysis the cost of weed control and the damage for pasture (in terms of financial losses) were considered. Seven simulation years with varying wet conditions in summer were selected. The financial loss resulting from one clearing at the end of summer, was too high for only one of these years. The multi-objective decision method ELECTRE II enabled the ecological impact to be incorporated in the evaluation. The method identified weed control strategies that are most favourable for agriculture or nature conservation.

Achtereenvolgens worden besproken: de gevolgen van eutrofiering voor een aquatische levensgemeenschap, de effecten van de huidige beheersmethoden en een model, dat de belangrijkste interacties van de levensgemeenschap beschrijft

Korte weergave van drie projecten (Wageningen, Arnhem, Zwolle) met als centraal thema: de landschaps-architectonische relatie van de stad met de rivier waaraan zij ligt

Verslag van een studie, waarbij werd uitgegaan van een aantal vraagstellingen, nl. welke graslandtypen op de Nederlandse zeedijken voorkomen en hoe de botanische samenstelling is; wat de relatie tussen het beheer en de botanische samenstelling van de aangetroffen graslandtypen is; en welke geomorfologische en/of bodemkundige factoren dragen bij tot de verscheidenheid aan typen

Een overzicht wordt gegeven van in 1987 lopende onderzoeksprojecten met vermelding van contactpersoenen en publikaties van de laatste 5 jaar

Achtereenvolgens worden besproken: de gevolgen van eutrofiering voor een aquatische levensgemeenschap, de effecten van de huidige beheersmethoden en een model, dat de belangrijkste interacties van de levensgemeenschap beschrijft

Verslag van het opschonen van 11 wijken (een wijk is een zijvaart in veenderijen, oorspronkelijk gegraven voor het vervoer van turf naar het hoofddiep) in het waterschap De Veenmarken, waarbij verschillende machines en werkmethoden bij het opschonen en verwerken van uitkomende specie zijn beproefd, waar tevens is gekeken naar de reaktie van het gewas op de verwerking en waar tenslotte onderzoek is verricht naar effekten op de waterhuishouding