Ten years of experience in combining ecology and navigation on Dutch waterways

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Dutch Nalional Paper 10 be presenled althe 29th PIANC Congress

Ten years of experience in combining ecology and

navigation on Dutch waterways

ir. R.EAM. Boeters, Rijkswaterstaat, Road and Hydraulic Engineering Division

ir. H. Havinga, Technical University Delft and Rijkswaterstaat, Directie Oost-Nederland

G. Litjens, World Wildlife Fund, Dutch Division,

ir. H.J. Verheij, Delft Hydraulics

1. Introduction

Waterways are made to sail on; to transport

people and goods by ships.

The first waterways were made centuries ago, by making rivers, natural watercourses, navigable. Afterwards, waterways were dug, canals, to enable ships to also put in at places by water which had been inaccessible up to then.

And certainly in the case of rivers made navigable, it is clear that offering a possibility to transport people and goods by ship is not the only function of a waterway. Rivers carry down water, ice and sediment and must be able to do this unhindered, so that floods will not occur. Rivers also play a vital part in moving all kinds of plant and animal life, which, by settling on the land, shape the

surrounding landscape and the nature there considerably.

Canals often have a soil-hydrological function, by means of the supply and discharge of fresh water. Be it to a lesser degree than rivers, canals and their banks can also be a corridor to plants and animals. These can find a station or habitat at suitable spots along the canal from which possibly the hinterland can be reached.

The water and the waterside are also attractive to people, who can recreate there in all kinds of ways by going swimming, windsurfing, walking, fishing, and skating in the winter.

In short, waterways have many functions and must offer room to a wide range of activities. To steer these in the right direction, an approach which makes it possible to do justice to the functions attributed to (a part of) the waterway is necessary in waterway management. A multifunctional approach in which the functions to be fulfilled are weighed carefully and are geared to each other well, is essential.

Such an approach makes a sustainable

management of the waterway and its surrounding areas possible.

Of course, what remains the first matter of importance with waterways or, more than that, what is a precondition is that sailing must be possible!

In this paper we will discuss the experience gained in the Netherlands with the multifunctional management of waterways. We will especially go into how the functions of rivers and canals for navigation and nature are combined.

For this we will briefly go back to the national paper for Seville 1994, "Waterways with room for nature", in which the essence of the

multifunctional approach has been outlined. Preserving the infrastructure of waterways in a sustainable manner is in the Netherlands also made possible by a special way of cooperation between responsible authorities and nature conservation organizations which we will discuss here.

By means of five cases we will present our experience with combining functions in waterway management, particular attention being paid to the combination of navigation and nature. The cases have been classified according to the Rivers and Canals watersystems. Before discussing expe-riences, we will give a short description of the characteristics of the systems and the possibilities in these to combine nature (development) and navigation.

We will conclude by going into what we have learnt the past ten years and will offer a view of what will become important in the future when combining the functions of waterways.

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2. Room is Made on the Dutch Waterways for Thriving Nature!

Can waterways, besides having a transport

func-tion, accommodate high-quality nature? Yes, indeed. The return of the f100dplain forest and the exuberantly blooming river and canal banks in the Netherlands can be called spectacular. Even though a loss of the nature values is observed in many places in the Netherlands, the river district has in a short time now shown the opposite! One of the reasons for this is that for some years now na-ture and environmental aims have been taken into account a great deal more in the management of waterways. Since the 'Ooievaar' (Stork) plan came out in 1986 as a blueprint for the original

nature in the Dutch river delta (see Waterways with Room for Nature, Boeters et al. 1994) the develop-ment in thinking about river ecosystems and the integration of nature functions with navigation, flood control, recreation and mineral extraction has moved rapidly.

The government has for example in the Third Policy Document on Water Management given a

clear field for an integral water management. In 1992, the Plan Levende Rivieren (Action Plan for Living Rivers, Overmars et al. 1992) of the WWF which is known all over the world by now, came out in the Netherlands. Rijkswaterstaat's plan

Oeverture (Action Plan Oeverture), dating from 1993, describes ecological target situations for the layout of the banks of branches of the Rhine (Rijkswaterstaat East-Netherlands, 1993). In 1994, the Handboek Natuurvriendelijke Oevers (Ecologically Sound Banks Handbook) appeared (CUR, 1994) in which the design approach as described in Waterways with Room for Nature is worked out and in which many examples can be found of ecologically sound banks along rivers and canals. Supported by the ideas and examples in these plans and in the Handbook, the managers have enthusiastically set to work on the integration of ecology in waterway management.

Organizations for the Preservation of Nature work together with the Government and Business

A number of pilot projects concerning nature development in and along waterways were started up as early as the eighties. The Duursche Waar-den along the IJssel (Fig. 2.1) is a project carried out by the Ministry of Agriculture, Nature Manage-ment and Fisheries and managed by the Dutch Forestry Commission. The Blauwe Kamer along the Lower Rhine (Fig. 2.1) is an initiative of the foundation Het Utrechts Landschap (the Utrecht Landscape) for which financing was borne by the national and provincial governments, the city of Wageningen and the WWF. Both projects provide a great deal of insight and experience in the fields of the

recovery of the river dynamics, and natural grazing on the river banks which formerly were quays. The Dutch division of the WWF (World Wildlife Fund) has since the early nineties been active in a number of nature development projects along large waterways in the river district, such as in the Gelderse Poort (e.g. Millingerwaard.) The WWF aims to double the current nature area in the Netherlands, which means an expansion of 2,000 km2 The WWF aims at wetlands in particular, be-cause these are very important internationally: the North Sea and its shallow Wadden Sea coast, the lakes, the marshlands of Holland and the large river delta of the Rhine, Meuse and Scheldt. By means of pilot projects, concrete experience is gained on two views that have appeared: one for the river district: Levende Rivieren (Living Rivers, 1992), and one for the coastal zone: Meegroeien met de Zee (Growing with the Sea, 1996). The organizations for the preservation of nature

Ten years of experience in combining ecology and navigation on Dutch waterways

have together built up an enormous number of sup-porters the past five years, with over 2 million pay-ing members/contributors out of a population of 15 million. With which pressure is exerted to gain a substantial expansion of the nature area. For ex-ample, Vereniging Natuurmonumenten (the Nether-lands Society for Nature and Environment) buys land itself, outside of the task with the government, in order to increase the pace of the restoration of nature. In the spring of 1997, the

national nature and environmental organizations together presented the prospectus Veters Los! (Laces Undone!) to the authorities and business, to integrate large infrastructure projects with nature. This not only yields more pUblic support, it also guarantees the livability of our country (over 400 inhabitants per square km).

Nearly all nature preservation organizations in the Netherlands cooperate closely with business, the government and fellow organizations. Attempts are made to combine economical or social interests, e.g. mineral extraction (clay, sand and gravel), drinking-water collection, navigation, river management and recreation, with objectives for high-quality nature. Only through such, often unconventional alliances, do new opportunities arise for nature in the Netherlands. As far as the WWF is concerned, that nature is open to and to be experienced by people. This increases the so-cial basis of the realization of ambitious projects. It gives business a greener image.

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Catastrophes or even narrow escapes are bles-sings in disguise: this way, the periods of high water in 1993 and 1995 on the Rhine and Meuse, which only just did not lead to floods, caused the carrying out of plans for nature development to move rapidly. Another example of a shock therapy was the disaster with the Swiss chemical concern Sandoz in 1986 which at one stroke killed all

biological life in the Rhine, but subsequently led to a stronger international cooperation for the cleaning up of the Rhine (het Rijn- en Noordzee-actie-programma [the Rhine and North Sea Action Programme]). The Rhine, formerly known as the sewer of Western-Europe is rapidly on its way to becoming a cleaner river. In record time, a large-scale ecological recovery is occurringI

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a. Biesbosch, nature and dike improvements b. Loevestein, clay dredging and nature c. Blauwe Kamer, hill-side and river d. Schipperswaard, clay dredging

e. Leeuwense VIIaard, secondary channels/dike improvements f Doorwerth, clay dredging and drinking-water collection g. Duursche VIIaarden

h. Meinerswijk, nature in the city i. Millingerwaard, river dunes j. Hochter Bampd

Figure 2.1. Overview of example projects in the Netherlands with large scale nature development.

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Dutch National Paper to be presented at the 291h PIANC Congress

3. Rivers

Functions of Rivers as Watersystems

In the area between the main levees, many functions 'compete for' a place. The following enumeration makes this very clear. When managing a river, therefore, making definite choices is a necessity. These choices will have to be based on a clear insight into the effects the measures linked to the functions have on the river. The function Flood Safety requires that the river water can be discharged safely in all circumstances (during large discharges or with ice formation). With the layout of a river, one must therefore aim at little hydraulic resistance and a flow section which together with the dikes can keep the chances of flooding within limits that are socially acceptable. The construction of dikes causes the sedimentation of sand and alluvium in the f1oodplain, resulting in the flood plain lying increasingly higher. As a result of this the water levels rise, causing the flood safety to decrease. Because of agricultural inter-ests and nature values, the flood plains were not dug off sufficiently in the past, as a result of which the dikes had to be raised all the time. The dike improvements carried out in the seventies were for that matter primarily inspired by stability consider-ations and increasing knowledge of the resistance elements in the river. The recent floods of 1993 and 1995 have led to a quicker

realization of dike improvements and a reconsider-ation of the layout of the river. An enlargement of the flow section is aimed at, so that also higher discharges than the design flood discharge (15,000 m3/s at Lobith) used up to now, can be carried down safely without more dike heigh-tenings hav-ing to be carried out. The project Ruimte voor de River (Room for the River) which was set up for this, removes hydraulic bottlenecks and lowers f1oodplains. With the re-layout of the river, due con-sideration is given to existing nature values and the possibilities to extend the nature values further by means of integration with nature development plans.

The river as a transport axis (function Main Transport Axis) is of enormous importance to the Netherlands. More than half of the international transport is carried by ship. The attraction of the in-land navigation lies in the low transport costs (lower than road and rail transport), the small burden to the environment and the high safety of this way of transport.

The function Agriculture used to be important in the river district due to factors as fertile soil, nearby wa-ter, river as the way of transport. The water quality, which has decreased rapidly in this century, in-creasing competition in which the dis-advantages of farming in the floodplain start to count heavily, and the increased significance of road transport

Ten years of experience In combining ecology and navIgation on DUlch waterways

have led to a decrease in the significance of agri-culture in the flood plains.

The Rhine and the Meuse supply a large amount of the water which, after storage and purification, be-comes available as drinking water. Industry also takes several cubic metres of water per second out of the river (function [Drinking] Water Supply). Function Raw Materials Supply: The river has from times immemorial been important for the clay sup-ply. Besides clay, very much sand has been taken from the flood plain the past century for the laying and construction of roads. The so-called scour holes (up to 25 m deepl) often have an negative ef-fect on the flow pattern. This is caused because by these holes with a very small hydraulic resistance extra water is carried down during high

water. The capacity of the river water to carry down sand is decreased because of this, as a

result of which siltations in the minor bed will occur which in the summer during low discharges will be an obstacle to navigation (limitation of the width and depth of the navigation channel).

The function Nature and Landscape has been un-der enormous pressure the last decades. The na-ture values between the main dikes have declined sharply during the past centuries due to the atten-tion paid to the safety, agriculture and navigaatten-tion functions. For ten years now, attempts have been made to increase the diversity of species by means of ecological recovery projects. This

demands a different layout of the river which basically is at odds with the layout for other interests. So this is a river-engineering challenge: the biotopes (specific area-layouts for nature target situations) must be so integrated in the remaining landscape elements that the other functions are not violated too much. With these (natural)

recovery projects the idea is assumed that the river is a part of the national ecological network of pro-tected areas. Important is the creation of regions of ecological value whose dimensions are sufficiently large, which are interconnected in ecological sense ('stepping stones' in green ribbons in the land-scape). Characteristic parts of the so-called nature development projects are: fJoodplain forests, secondary channels, and fish ladders at structures. Sloping, unprotected banks also are important for the distribution of flora and fauna (interaction between minor and major beds).

The river landscape, which is determined very strongly by human intervention, is one of the typically Dutch landscapes. The unobstructed view from the dike is not equalled by many other panoramas. Nevertheless, there are different ideas about the layout of the river area which vary between maintaining the present layout, a 'half open' layout of the landscape, and a 'small-scale'

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layout of the landscape for which Allier in France serves as a model.

The significance of the river to Recreation is very varied. Intensive forms of recreation such as camping and beach recreation are found, besides less intensive forms of recreational joint use such as walking, cycling, fishing and 'nature watching'. In the past years, nearly all of the functions mentioned have been badly affected by waste discharges. The unlimited use of the river as a sewer (discharge of effluents with heavy metals, pesticides, agricultural poisons, salt) leads to bad-quality river water worldwide. For the branches of the Rhine this abominable quality of the river water has this century led to phenomena as contami-nated sludge and polluted bottoms. The agricultural

Dulch Nalional Paper 10 be presenled allhe 29th PIANC Congress

products are also affected by this. Fortunately, the quality of the Rhine water has during the last few decades improved considerably through a coordinated approach (e.g. the Rijnactieprogram-ma [Rhine Action Programme] ); however, the inheritance in the form of contaminated soil and dredged material will cause problems for many years to come. Some soil is contaminated to such an extent that it may not be excavated nor trans-ported.

Navigation and Nature Development

From the Middle Ages onward, man has been working at the Rhine to make the added functions be shown to advantage. In the Rhine, the first so-called regulation works consisted of river-bend cut-offs, the construction of groynes (also for land reclamation) and guide levees to force the flow in a minor bed of limited width, and the cutting off of secondary channels. These works were intended for the increase of the flow velocities in the main channels, as a result of which the formation of sandbanks was prevented. In the winter periods, ice dams could form where these shallows were sit-uated which cut off the flow profile and so caused great floods, also when the discharges were low. Above-mentioned measures, meant for the func-tions Flood Safety and Agriculture turned out to of-fer great advantages to navigation: a larger naviga-ble depth and width became availanaviga-ble. After the first regulations, therefore, there followed another two trainings in the 19th and 20th centuries espe-cially for shipping, limiting the minor bed to one main channel and aiming at keeping the navigable depth along the Rhine as constant as possible. These river training works consisted of constructing groynes at regular intervals which resulted in the flow being 'imposed' better and it being kept further from the erodible bank. It still could happen that the bank eroded too much, e.g. as a result of naviga-tion. Loss of (agricultural) land was the result, and the navigable depth was limited locally because of the sand that ends up in the fairway. In that case, the banks between the groynes were also 'secured' with stone.

In order to increase the economical attraction of the most ecologically sound way of transport, transport by water, an enlargement of the load draught and a broadening of the fairway is required. For the Waal, the Waal project (Havinga, 1996), which was

Ten years of experience in combining ecology and naVigation on Dutch waterways 5

brought into being for this, aims at enlarging the available navigation profile of this river from a cur-rent (aimed at) profile of 170 m wide and 2.5 m deep in relation to the waterlevel that is exceeded 95% of the time (defined as OLR), to a profile of 170 m wide and 2.8 m deep in relation to OLR. In this project, the interest of navigation is the main point, but the improvements for navigation must be realized within the strict preconditions of the exist-ing river system.

Complying with the conditions for ecological recovery and the demands of the other interests in particular, means that working very cautiously is the only way to prevent a clear curtailment of one or more interests. The traditional chief measures of narrowing and river-bend cutting-off have water-level lowering effects on the upper course of the Waal. The changes in discharge over the branches of the Rhine caused by this, and the erosion which is striding back in the direction of Germany are, in view of the choices with respect to the use of the river system and the agreements made with Ger-many, not acceptable. Moreover, hardly any public support can be gained in the Netherlands for river-bend cutting-off, because of the large intervention in the landscape and the interests of nature. Enlarging the navigation profile must therefore be realized in a different way.

For the increase in navigation possibilities on the river, locally effective additional measures are therefore resorted to nowadays. For example, the closing off of a groyne field with wooden poles, the decrease in the exchange of water between the mi-nor and the major beds, dredging, and various forms of river bend improvement without the river bend being cut off (filling up the outside bend, bendway weirs, bottom vanes). These measures increase the ecological possibilities because they have only slight hydraulic effects and because of

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DUlch Nallonal Paper 10 be presenled al Ihe 29th PIANC Con9ress

the possibilities of local ecological development. In nature development projects often one or more

of the following elements are included, see figure 3.1 (Silva and Kok, 1996).

Inside~

Ma·or bed

the dike

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Floo~~

.

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7 Minor bed

1. Narrowing

2. Lowering of groynes

3. Dredging

4. Dumping back the sediment

5. Solid layer

6. Ecologically sound banks

7. Removing summer embankments

Removal of existing hard structures (bank protec-tions in particular) in order to increase the migration of plants and animals from minor bed to major bed and vice versa. A gentle slope is part of this. For na-vigation there is a danger of the bank crumbling and the released soil leading to shallows in the fairway.

More vegetation in the form of brushwoods and floodplain forests. Although various forms of vegeta-tion density are found, vegetavegeta-tion basically means a rise in the water level.

8. Secondary channel

9. Lowering of the flood plain

10 Development of vegetation

11. Removing flood-free land

12. Dike improvement

13. Dike shifting

14. Dike heightening

Secondary channels and "strangen" (secondary channels with one connection with the river). These elements cause a drop in the water level. However, secondary channels have an adverse effect on navi-gation because of the drawoff of water from the mi-nor bed, resulting in silting in the mimi-nor bed.

An increase of the flood frequency in the floodplain. Here, too, there is the danger of shallows forming in the minor bed because of this.

Figure 3.1. Elements of nature development projects.

These elements are in principle threatening to the functions Flood Safety, Navigation and Agriculture. Moreover, there is the fear that the stability will disappear from the Dutch Rhine system, which has just been reached by the application of many river works from the category chief measures (training and river-bend cutting-off). Due to the circumstance that even improvement measures for navigation can no longer be taken with the large-scale chief

measureseither, small-scale measures may be de-veloped that are favourable to both functions and, in addition, are no threat to the stability of the river system.

An example of a conflict of interests is (re)opening secondary channels. The adverse effects of the secondary channels, shoaling of the fairway, and

Ten years of expenenceIncombiningecotoQrand navlgallon on Dutcnwater....ays

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possible extended bending of the river in the direc-tion of the main dike, can be removed in a simple manner. The removal of the silting in the fairway can easily be included in the dredging programme for navigation, which as a small-scale measure counts as the alternative to further training. The ex-tended bending can be counteracted by allowing vegetation to grow at the right places. The positive effects are an increase in the nature values, aquatic environment in the secondary channel and the vegetation, and a larger flow section during high waters.

However, the larger natural dynamics complicates river management. In this example, the develop-ments in minor and major beds should be watched more closely than in the situation without

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secondary channels and vegetation. The following questions must be answered constantly for the pur-pose of sound river management:

Is the combination of secondary channels and floodplain forest vegetation water-level neutral?

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What about the stability of secondary chan-nels and "strangen"? Is there no risk to the dikes?

Should preventive dredging for navigation take place?

Scope for nature development, set alongside preconditions set by navigation In the above it has been illustrated that although

ecology and navigation are basically opposite interests, measures can be found which have the intended effect for one function and at the same time can counteract the adverse effects on the other function. It is therefore required of the river engineer that he designs measures which may pro-mote the different interests. For which it is, how-ever, of importance to assume a division of func-tions between minor and major beds.

When designing solutions to improve the circum-stances for ecological recovery and navigation, the following basic principles apply:

the values of the present layout should be pre-served at least, or be reinforced;

the existing interests should be observed; solutions should chiefly be sought in the "reserves" of the system;

quick reaction to deteriorating circumstances. In the following example, it is described how the specific requirements of the functions Nature and Navigation can be combined in a direction of a so-lution. In the example, it is indicated what an inte-gral solution can look like in practice. It concerns a fictitious working out of a real situation. A map of the situation is represented in figure 3.2. Four situ-ations are described. The present situation speaks for itself. The works that realize the nature and navigation target situations cause too much raising, which must be neutralized. This can be done without integration with the nature plans, re-sulting in the nature target situation certainly not being reached, or with integration, as a result of which more advantage will be gained for nature. The description of the elaboration follows the num-bering on this map.

1) Present Situation

The Gendtse Waard is situated in the Ooypolder area along the Waal and falls within the land-use project Ooypolder. The entire floodplain is sur-rounded by an embankment. There are two access roads to the brickworks of which only the northern route is free of high water.

2a) Nature Target Situation

In this floodplain, a clearing is already being carried out. Outside the embankment (in the southeast), there are some desandings and inside the embank-ments some declayings. The present land-use plan

Ten years of experience In combining ecology and navigation on Dutch waterways 7

departs from a nature-oriented declaying of the area inside the embankments and from

dynamic nature on the south-eastern side. A young river dune has developed here through the years. The plan, as presented by the Government Service for Land and Water Use shows rises in the water level ('head water') when the discharges are high. This is not acceptable.

2b) Waterway Target Situation

Between 1994 and July 1996 a river-bend improve-ment with bendway weirs was carried out at the river bend Erlecom. Bendway weirs consist of riprap and it is their purpose to reduce the flow area in the outside bend, but allow ships to sail over them.

With these bendway weirs, the roughness of the bottom in the outside river-bend is also increased. Both effects lead to a deepening of the inside river bend so that a wider navigation channel is

obtained.

An adverse effect of this measure is also head water.

3) Compensation of Bendway Weir Field without Integration with Nature Development ('independent compensation')

Head water caused by the bendway weirs in the minor bed can be compensated by lowering the embankments, so that the flood plain will sooner flow along and have less resistance at high water, causing the water levels to rise less high. A better flow is also attained by locally lowering the flood-plain two meters and levelling the river dune.

4) Compensation of Bendway Weir Field and Real-ization of Nature Development

By adapting the nature target situation ('more dy-namics') and increasing the water component in re-lation to the original plan, the nature development plan may provide so much compensation that there will be enough room for both nature development and fairway improvement. The largest advantage particularly lies in a better flow of the flood plain at high discharges, because of a more upstream in-flow opening of the floodplain and a better dis-charge by the floodplain.

This example illustrates that there are technical possibilities to realize plans by adapting them. After the works have been carried out, the new

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Dutch National Paper 10 be presented at the 29th PIANC Congress

situation must be managed actively because of the nature elements.

emoving

embankment

4) Compensation: Waal project

with nature development

River dun

2) Desired situation: nature

and waterway target situation

3) Compensation: Waal project

without nature development

1) current situation

Figure 3.2. Gendtse Waard, four layout options.

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Dutch National Paper to be presented at the 29th PIANC Congress

4. River Case Studies

Description of the Situation The situation in the river branches has changed

greatly during the past decades. Were the flood-plains until 20 years ago almost completely owned and used by the agrarian sector, by now the nature area is growing considerably, particulary in the wake of the economical factors mentioned in

section 2, such as mineral extraction and flood con-trol. Below, and by means of three projects that were carried out: Millingerwaard (Waal), Leeuwense Waard (Waal) and the Grensmaas, recent developments will be examined.

Millingerwaard

(Location: left bank of the Waal, near the bifurcation point Waal and Pannerdens CanaL)

Figure 4.1 The bend of Erlecom, just downstream of the Millingerwaard. Sand was deposited on the river banks by the floods of Christmas 1993.

Navigation:

The Millingerwaard (700 ha.) lies on the main trans-port axis, the Waal (Rotterdam <>Ruhr Area) in which the Ministry of Transport, Public Works and Water Management is modernizing the navigation route (cost Dfl. 450 million), ready in 2004. One of the remarkable techniques of the

water management with which the morphological dynamics are almost kept intact is the construction of the approximately 55 bendway weirs in the bend of Erlecom (finished in 1996). The construction of bottom vanes in the bend of Millingen is planned. Nature:

The flood plains of the Waal are part of the

National ecological network of protected areas, with its broad flood plains with old river beds, f100dplain forest complexes and alluvial banks/beaches. The WWF and the Dutch Forestry Commission have since 1991 and in cooperation with two private clearing companies and the Ooijpolder Commission for Land Use been managing a model site for nature development, the surface area of

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which has up to now been approximately 160 ha. The use has so far been limited to the implementa-tion of clearing concessions granted years ago with a partial duty to recultivate to agricultural land, which will be changed into a nature finishing in co-operation with the province of Gelderland. Galloway cows and Konik horses in very low

densities grazing all the year round is how it is managed. In 1994, the beaver was reintroduced. As for diversity of species, the Millingerwaard belongs to the top ten of regions of ecological value in the Netherlands. This appears to be linked with the existence of a natural gradient of high, seldom flooding, river dunes which erode by the wind (see figure 4.2) to fully-grown, soft-wood flood plain forest and accreted river courses with marsh vegetations. There are over 100 species of summer birds!

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Dutch National Paper 10 be presented at the 29tn PIANC Congress

Figure 4.2 The Millinger dune after the summer of 1995.

Accessibility

The model site is entirely open to the walking and cycling public, which is why large numbers come to visit it. Estimations vary from 50,000 to 100,000 people a year.

Mineral Extraction.

Clay dredging that follows the relief is an important activity; limited sand dredging serves to create room for the river.

Flood Control.

A structural increase of the discharge capacity

(recommendation 2001) takes place by implemen-ting the land-use plan with for instance a flowing secondary channel and a lowering of the summer dike. Compensation and improvement of the river dikes is done in the scope of the Deltawet Grate Rivieren (Delta Law for the Big Rivers; ready in 2000) by clay dredging and local sand dredging.

Agricultural Interest:

Declining, connected withlfollowed by disconti-nuation of farms or scaling up the cattle farms inside the dikes (Land Use Ooijpolder).

Figure 4.3

Leeuwense Waard

(Location: left bank of the Waal, upstream of the Prins Willem Alexander bridge at Tiel)

Leeuwense Waard, March 1994. The photo was taken before the construction of the flowing secondary channel.

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Navigation:

The Leeuwense waard is also situated on the main transport axis. the Waal. No specific measures for the improvement of the navigation channel are taken here, but measures are mainly carried out for maintenance work (dredging). Upstream of the Leeuwense waard lies a large sand-dredging pond, which affects the navigation channel negatively by immobilization of the flow and subsequent silting. For which it is currently being examined in an Environmental Impact Study if shoaling can take place with nautical dredged materials.

DUlch Nallonal Paper 10 be presented at the 291h PIANC Congress

Nature:

The Leeuwense waard (70 ha.) is, as a nature development area, part of the larger Waaier van Geulen (Fan of Channels) project. Since 1994, a test has been in progress with the construction of a permanently flowing secondary channel. Construc-tion was completed in the spring of 1997, in the wake of clay dredging for dike improvement and the ceramic industry.

Figure 4.4 The secondary channel in the Leeuwense Waard, summer 1997, as seen from the dike.

At present, well over half of the area is being managed as a nature area with Galloway cows and Konik horses grazing all the year round. Young f100dplain forests, clay pits and the per-manently flowing water leads to a varied river environment with bird species such as the corncrake, the com-mon sandpiper and the bluethroat. Rijkswater-staat's Institute for Inland Water

Management and Wastewater Treatment is carry-ing out the monitorcarry-ing both above and below the

T en years of expenence In combining ecology and nal/Igauon on Dutch waterways

water surface. Thus it was found that shortly after construction of the channel, well over 23 species of fish, of which a number like to live in flowing water, had already found the small secondary channel. Moreover, after the flood of 1995 it was found that dozens of species of invertebrate animals, which had disappeared from the Netherlands, had settled here once again.

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Figure 4.5 Excavation works at the clay dredging for dike improvement, May 1996.

Accessibility:

The model site is entirely open to the walking public. The flood plain is also well-known to all pri-mary schools in the area.

Mineral Extraction:

Clay dredging that follows the relief is an important activity and one of the important ways of financing the projects. Permits are, if necessary, oriented to-wards a nature destination.

Flood Control:

A structural increase of the discharge capacity, as is laid down in the line of policy Ruimte voor de

Rijntakken (Room for the Branches of the Rhine), is obtained by means of clay dredging. A part of which has already been carried out in connection with the dike improvement (Deltawet Grote Rivieren [Delta Law for the Big Rivers], ready in 1996) by realizing the land-use plan with, for instance, a flowing secondary channel.

Agricultural Interest:

Declining, mainly because of the concentration of farms inside the dikes.

Grensmaas

A gravel river on the Belgian border, situated roughly between Maastricht and Stevensweert, see figure 2.1.

Navigation.

Takes place on the lateral canal, the Juliana Canal. Attempts in the 19th entury to make the Grens-maas navigable ended in failure, with considerable consequences for the ecology and the hydrology of the river.

Nature:

At present, the minor bed is narrow and very dy-namic, which offers little room for nature values. The elevated major bed is still largely in agricultural use. A plan is being prepared to combine large-scale, superficial gravel dredging with nature development. This project has become even more

actual after the floods of 1993 and 1995 because of the favourable effect of superficial mineral extrac-tion on the flood discharge.

Since 1991, a number of model sites for nature development on both sides of the river are being managed by Dutch and Flemish organizations for the preservation of nature, in order to familiarize the population of Limburg with and involve them in the changes in the Grensmaas area (Hochter Bampd, Kerkeweerd, Koningssteen, for instance).

(13)

Dutch National Paper 10 be presenled al the 29th PIANC Congress .,.} Neerharen Hocht Castle

_..

' ~ ». low :"dam

11

Parking

o

Entrance Pedestrians Itteren

Figure 4.6 Hochter Bampd, wedged between the Grensmaas and the Zuid-Willemsvaart.

Accessibility:

The above-mentioned model sites are open to the public.

Mineral Extraction:

An important function to improve safety and nature in the area. Expectations are that the Grensmaas project will supply the last Dutch gravel production for the coming ten years. There will be a gradual switchover to secondary building materials and import to replace it.

Flood Control:

13

There are no dikes along the bed of the Grens-maas. As an emergency measure, a number of protecting embankments were built in 1995 and 1996 around nuclei of buildings on the major bed. A further increase in building is not permitted.

Agricultural Function:

Gradually declining, staying in line with the realization of the project.

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Dutch National Paper to be presented at the 29thPlANe Congress

5. Canals

Characteristics and Functions of Canals in the Netherlands

Canals are watercourses that have been dug. In the Netherlands, the canals cut across sandy, clay and peat soils. The water level in canals which do not have an open connection with the sea or estuaries is practically constant. The canals in the Netherlands that are connected to the IJsselmeer have a summer and winter level. In the winter the level is approximately 0.20 metres lower than in the summer.

The canals in the Netherlands were originally made to make transportation by water possible. For this, a large number of kilometres were dug in the nineteenth and the beginning of the twentieth century, at the time suitable for small (sailing) ships and tow barges. After World War 11, a large number of canals were broadened to be able to

offermotorized cargo ships passageway.

Most canals in the Netherlands also play a part in the water control. They carry down water that precipitated in their 'basin'. In the western part of the Netherlands, brackish groundwater is found. The purpose of the canals there can also be the supply of fresh water, intended for agricultural uses.

The Dutch love water and water-bound activities. As soon as it is at all possible, they are on the water in all sorts of pleasure boats, fishing at the waterside, sunbathing on the banks or taking a dip in the water. In the winter, when the water is frozen over, a national 'fever' takes possession of the country, because skating is what must be done now. This fever goes so far that in Friesland, a northern province, canals are blocked for navi-gation, to enable thousands to skate the

"Elfstedentocht", a day race in which people skate past eleven Frisian towns, over a distance of over 200 kilometres.

In the Netherlands, areas and routes have been designated which together constitute the

Ecologische Hoofd Structuur (National Ecologi-cal Network of Protected Areas). This ecologiEcologi-cal infrastructure offers nature the chance to develop permanently there. Many canals are an important part of this infrastructure, as corridors between regions of nature/ecological value. There is room along the banks of canals, be it often to a limited extent, for the settlement of water and bank plants and the fauna which feels at home in them.

Navigation and the Development of Nature

For navigation in canals it is a requirement that there is a guaranteed navigation profile with sharply defined boundaries (banks and bottoms) and no obstacles to obstruct passage. To realize this, CEMT (Conference Europeenne des Ministres de Transport) directives are used which, depending on the type of cargo ship that can navigate the canals, state the minimum required profile dimen-sions. The navigation profiles know standard shapes: trapezium-shaped, bin-shaped or a combi-nation of these. The ships on the canal produce a complex and violent water motion, consisting of waves and current. These may cause high loads

on the banks.

Nature benefits from a gradual transition from wa-ter to land, where there are limited wawa-ter dy-namics. Under such conditions a zone may form in which water plants grow (to a maximum depth of approximately 1.5 metres below canal level, when the water is sufficiently clear), a zone may form which is occupied by bank plants (of 1 metre below canal level to about 0.30 metres above it), which gradually changes into grassland situated higher up and possibly marshwood beyond it (CUR, 1994).

Figure 5.1. Natural zoning of canal banks.

\,li. ",

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In the open water zone there is room for macro-fauna and fish, who also use the zones with water and bank plants for reproduction and foraging. Am-phibians also use these for reproduction. Birds nest in the zone with bank plants, or find their food there. Insects feel at home there too, just like in the neighbouring herbaceous grassland. Small mam-mals use the different zones as hunting grounds or as a corridor through which they can move,

protected by the vegetation.

On the face of it, navigation and nature do not appear to be combinable. The ships produce far too much dynamics and clouding of the water to make the sketched development of nature possible.

Water and bank plants there may be obstacles to navigation or cause shoals through land accretion, making passage no longer possible. Moreover, they do not provide sharply defined boundaries of the navigation profile.

Nevertheless, combining is possible by

construc-Dutch National Paper to be presented at the 29th PIANC Congress

ting a defence on the canal side with behind it a marshlike zone which gradually merges into the real bankvia a gentle slope.

Openings in the defence connect the bank with the canal. The defence and the openings in it must be dimensioned in such a way that there are limited water dynamics in the marsh zone, also called the wet strip, and the gentle slope can remain unpro-tected. For this purpose, the defence must largely reduce the water motion caused by ships; to

re-move it completely is not necessary. The openings

in the defence make water exchange between the wet strip and the canal possible, by making the drop of the waterlevel affect the canal to a limited

degree during the passage of ships. The wet strip has an unprotected bottom, in which water and bank plants can root. The bottom merges into the slope, on which bank plants and, higher up, grasses, brushwood herbs and bushes can grow.

Figure 5.2. Bank in which the functions Nature and Navigation are combined well.

Depending on the available space, the defence can consist of a protected slope with a low dam as the crest, a vertical construction or a combination of these. The protection constitutes a boundary of the waterway which is clearly visible to navigation. By means of the slope of the defence or through

open-Ten years of experience in combining ecology and navigation on Dutch waterways

ings in the vertical construction, animals thathave

fallen into the water and fish can reach the wet strip.

Depending on the nature aimed at in the wet strip, this can be built deep or shallow, with a bowl-shaped, bin-shaped or stepped profile.

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Dutch National Paper to be presented atlhe 29th PIANC Congress

l . ,

Figure 5.3. Possible profile forms of the wet strip.

A bowl-shaped profile provides the most gradual transition and the best chance of all possible ecosystems developing, provided that the slope has a slight gradient (slighter than 1:4) and the deepest point is situated at least 1.5 metres below canal level.

A bin-shaped profile is suitable when the emphasis is on the development of a single ecosystem. For example the zone with open water and water plants, in which case the wet strip should be at least 1.5 metres deep over a width of a number of metres. When the development of reed vegetation in particular must be stimulated, a depth of approximately 0.75 metres over a few metres width is suitable.

A stepped profile may be suitable to be conducive to more than one ecosystem, for example open wa-ter (with or without wawa-ter plants) and bank plants, by building two levels.

Water exchange between the wet strip and the canal is arrived at by using the drop in water level caused by passing ships. When openings have been made in the defences, a hydraulic gradient will form between the canal and the wet strip when a ship passes. Depending on the discharge qualities of the opening, which can consist of a hole or a lower part in a sheet piling, a pipe or a lower part in a riprap dam, a quantity of water will flow

Ten years of experience In combining ecology and navigation on Dutch waterways 16

from the wet strip to the canal during the time the ship passes. When the ship has passed, the same quantity of water (but not the same water!) flows back again.

This way, navigation can cause the water in the wet strip to be refreshed regularly, as a result of which no processes harmful to nature can occur, like al-gal growth. The disadvantage of this way of water exchange is the fact that besides water from the canal, alluvial particles are brought to the wet strip. In low dynamic conditions, these particles may eas-ily settle in the wet strip and so cause accretion of the bottom there.

For the dimensioning of (fore)bank protections and for the calculation of the degree of water exchange between fairway and wet strip, Rijkswaterstaat has developed two computer programs, DIPRO and PLONS respectively. DIPRO (Dlmensioning PRO-tections) calculates water motions on waterways caused by ships and the hydraulic load on banks caused by them. This is done using a large number of calculation rules, which have been obtained by years of laboratory and prototype research. In PLONS, the process of water exchange is modelled using the one-dimensional partial differential equations, that describe non-stationary flow in open channels, combined with suitable discharge-head relations for the applied apertures or pipes (de Visser and Boeters, 1997).

(17)

6. Canal Case Studies

In the South-east of the Netherlands lies a system

of canals dug in the nineteenth and early twentieth centuries. These canals were made to connect industrial centres in the South, in order to be able to transport raw materials and semimanufactured and finished products to and from these centres by water. The canals cut mainly through sandy grounds and bottoms that are composed of clayey sand.

During broadenings and diversions of the canals in the eighties and the early nineties, ecologically sound banks were constructed at several sites and monitored intensively after construction.

Experience with two sites that are indicated on the above map will be treated below.

ZUid-Willemsvaart

Figure 6.1. Plan of the canals in the South-east of the Netherlands.

17

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Wilhelmina Canal Test Sections

At a broadening of a canal for the purpose of

navi-gation, Rijkswaterstaat erected a steel sheet piling in 1983 over a distance of 3 kilometres and behind this, in 7 places, appromately 100-metre-long test sections with wet strips approximately 3.5 metres wide on the water line. The purpose of the test sec-tions was to see if by the realization of wet strips behind the sheet pilings the banks would develop sufficient value for nature, comparable with the ecological value present before broadening. The old canal banks had a varied structure with a relatively large number of plant and animal species. Shallow bank zones functioned as

spawning grounds for fish and nest sites for various bird species.

The broadened section is situated along a new dike, over which a cycle path runs. Behind the dike,

the rivulet the Donge runs through low-lying grass-land with an agricultural destination.

The chief purpose of the construction of banks with wet strips was to test and assess in biological-ecological sense seven variants of wet strips. For that purpose, intensive monitoring took place in the years after construction (1983) and several evaluation reports have come out (Rijkswaterstaat Noord-Brabant and Rijksinstituut voor Natuurbe-heer, 1989; Bureau Waardenburg, 1995). So, obtaining a specific ecological zoning or developing one or more ecosystems was not aimed at in the layout.

Realized layout

Two of the seven test sections are treated here of which the cross section is depicted below.

5.55+datum

5.15+datum

4.45+datum

3.50+datum

~

----6.50+datum

3.45m~-

--

.

I ~

L ....__ _

'\

'.?J

_ .

-...

--______:;.-.... •••••••

1'6

-I-t---"""'I....__ · ••••••• ..••••• •••••••

jAnchor, each 7.10 m

I

1.35 m

I

Gap, bottom at

4.75+datum

Figure 6.2. Cross-section of the two test sections along the Wilhelminakanaal.

Both test sections are located at the south side of the canal and are 106 metres long. Test section 1 has been cut off from the canal since construction until 1991. In 1991, the section was connected with the canal by means of two openings of 0.70 m x 0.48 m around the water line. Until 1987, test sec-tion 2 was connected with the canal by means of

two openings of 0.35 m x 0.48 m. In 1987, three more openings of the same size were put in. In both test sections reed was planted out. The ends of test section 2 are equipped with a clay lining. The diagram below gives an overview.

(19)

Dutch Nalianal Paper 10 be presented allhe 291h PIANC Congress

R

:Section-1j

R

H

R

~H;

~)~:

~'H:

I

H

I

'-' - ' - '

; Section

21 I

I

~

R

,,"---.', / t--i \ Gap,

0.70

m x

0.48

m ", / permanently closed '. '

..

'"

-

-" " \, Gap, 0.70 m x 0.48 m

:\ t--i ,: open since

1991

':H:'

Gap,

0.35

m x

0.48

m '-~ open since

1987

H

Gap,

0,35

m x

0.48

m open since

1983

' - l

' - - '

Figure 6.3, Overview of the layout of the test sections. DIPRO and PLaNS were used to make

calcula-tions of the water mocalcula-tions on the canal and in the wet strips which were caused by navigation, as well

as the water exchange between wet strips and canal. The calculation results can be found in the table.

table Calculation results for the sections along the Wilhelmina Canal

Hydraulic load Size Remarks

Return flow velocity 0,95m/s as a result of the passage of loaded class IV ship

Transerval sternwave/waterlevel

0.40

m idem depression

Interference peak

0.25

m as a result of the passage of a high-powered, fast motor vessel

(pleasure yacht, tugboat)

Water exchange compared to the original volume

Test section 1 7% from 1991

Test section 2, 2 openings 4% from 1983 - 1987 Test sections 2,

5

openings 11%

I

from 1987

Ten years of experience incombiningecology and navIgation on Dutch waterways 19

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Evaluation

To be able to assess the test sections, matters such as development of vegetation, settling of (macro)fauna, existence of amphibians and fish,

water quality and bottom level were monitored every year. The photographs show the develop-ments through the years.

Figure 6.4 Photographs of the test sections in 1983, 1986 and 1996.

The results of biological monitoring through 1987 are found in the following table.

table Results of biological monitoring of Wilhelmina Canal sections

,

I

I _{test section 1} _{test section 2}

I water plants

-

-bank plants - + avifauna ++ + insects + + aquatic macrofauna + +

Explanation of the symbols: insignificant

+ some significance

++ fairly valuable

20

+++ ++++

valuable very valuable

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The developments in sections 1 and 2 are strongly influenced by the planting out of reed. Its growth has dominated to such an extent that the develop-ment of other plants, especially water plants, lags far behind. An additional factor for water plants practically lacking in section 1, is the presence of thread algae in the summer months in this cut-off section. In section 2 the absence of a valuable water plant zone is also caused by the accretion of oxygen-poor and contaminated sediment. The reason for the bank plant zone in section 2 being of some significance, is due to its open character, which makes the existence of other varieties possible.

In the cut-off test section 1, the circumstances have turned out to be more favourable, for summer birds in particular. An investigation into the presence of fish has shown that even in the cut-off test section 1 some species existed, probably planted out by anglers. In the open section 2, many more species and numbers were found. It therefore seems that the open section makes a contribution to the preservation of the fish stock in the canal.

In 1995, the two test sections were evaluated once more, after the monitoring investigation in the pe-riod 1991-1995. Conclusions are that in both sec-tions a homogeneous reed vegetation grew, which

Du.ch Na"onal Paper 10 be presenled a"he 291h PIANC Congress

near the openings in the sheet piling became more open as a result of dying back reed stems.

In both sections, willow thicket has also come up. In test section 2 large numbers of larvae of the Common Toad (Buro buro) were found. In the same section the macrofauna has grown, probably due to the lesser occupation by the reed. The water depth in section 2 has decreased considerably.

In 1997, the initial differences between the two test sections have practically disappeared. As of 1996, all openings in section 1 were opened.

In both wet strips, more than 100 toad larvae were found (Common toad); many young fish were also found. The thickness of the mud layer measured approximately 0.2-0.3 m, at a water depth of 0.5-0.6 m. During the activities, 6 specimens of the reed warbler were observed in the reed vegetation. The open water zone is practically absent, only in front of the openings is the reed less dense. Because of the dense reed vegetation, a large part of the water is shaded. At present, the wet strip provides spawning and shelter opportunities for fish and toads. In the future, the reed vegetation will only increase, as a result of which land accretion will ultimately occur. The ecological significance for water organisms will disappear due to this.

Functioning as construction, bottom elevation, bottom quality

The steel sheet piling functioned well as bank protection. The fairly large height above the canal level has largely prevented overtopping and no erosion of the slope behind has been observed. In section 2, no erosion occurred behind the openings either. Nevertheless, the continuous sheet piling remains a considerable barrier to animals that have fallen into the water.

Much sediment has entered the test sections

through the openings in the sheet piling around the water line which has resulted in the bottom of the wet strips being raised considerably at a rate of approximately 3 to 4.5 centimetres a year. The bot-tom alluvium turned out to be of bad quality. To let the strips function well in ecological respect. me-chanical removal of this contaminated alluvium will be necessary in the long term.

Test sections Wessem-Nederweert Canal

At the regarded location situated along the Wessem-Nederweert Canal, the banks were pro-tected by concrete sheet pilings, which rose 0.35 metres above the average canal level. The bank behind it started at the same level and consisted of tufts of reed alternating with thicket. On the land side, this vegetation was and is bounded by a road, behind which lies a wood. The concrete sheet pil-ing caused an abrupt transition from water to land, where there was no room for water plants, a variety of bank plants and herbaceous grassland. More-over, the sheet-piling protection constituted a bar-rier to all kinds of fauna, both water- and land-bound.

For navigation, the bank protection was

Ten years of exoertence In comOlnlng ecology and navigation on Dutch walel"\....ays 21

satisfactory, were it not that the canal profile there was due to be enlarged. This would mean that sec-tions of the canal could be upgraded from CEMT class 11 to class IV, to admit ships with maximum dimensions of 85 m x 9.5 m x 2.8 m (length x width x draught). To prepare the proposed profile en-largement, the manager of the canal, the Regional Direction Limburg of Rijkswaterstaat, decided to construct several test sections. These were to be used to test versions of bank protections which might offer room for ecological developments and which also were already tailored to the hydraulic circumstances that go together with a class IV canal.

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Dutch National Paper to be presented al the 29th PIANC Congress

Objective

The objective with the re-layout of the described bank was twofold. Firstly, a gradual transition from water to land was aimed at, in which zones with open water, marsh plants and herbaceous vegeta-tion could develop, despite the hydraulic loads caused by passing ships belonging to CEMT class IV.

The objective for ecological development is in this case described in an open way. The development of specific species is not aimed at.

In the open water zone, water plants should grow and fish should find an opportunity to spawn. The zone with marsh plants had to offer room for a varied vegetation in which birds could nest. Here,

too, fish had to be able to drop their spawn. The herbaceous vegetation was to attract insects in par-ticular.

The second objective of the re-layout was to deter-mine which layout variant would qualify most for the future broadening of the canal. For this

purpose, four test sections were laid out. Two of which with a porous stone sheet dam as partition from the canal, the other two with an impermeable stone dam in which pipes were installed. There was no open connection with the canal, which the other two did have.

Realized layout

In the illustrations below, the cross sections are drawn of the test sections realized in 1991.

0.75 m

I I

IArmourstone

5/40

kgl

5.5m

5.5 m

.

. 5==1

11.0m

~

.~

I

Figure 6.5. Cross sections of the test sections along the Wessem-Nederweert Canal.

Ten years of experience in combining ecology and navigation on Dutch waterways

22

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In the plan, the position of the test sections in rela-tion to each other is drawn, and the manner of

layout is indicated.

Canal Wessem - Nederweert

-Permeable dam

Impermeable dam and pipes

~, ...

,.

... ~·t " .l! " i:"-"

11 .""

1l'

-

11

I/J

~JI;

'"

_I

,...

"

Section 1 Section

2 \

Section

3

Section 4

I

50 m

Figure 6.6. Plan of the test sections along the Wessem-Nederweert Canal.

These forebank constructions are dimensioned using the PC program DIPRO, with which ship-induced water motions on canals and the accompa-nying protection constructions can be

calculated. The slope protection and the height of

the forebank protections are based on hydraulic loads which are indicative in the situation after broadening to a class IV canal. The result of the calculations is given in the table below.

table Calculation results for the sections along the Wessem-Nederweert Canal

Hydraulic load Size

i

Remarks

!

-Return flow velocity 1.19 m/s as a result of the passage

of loaded class IV ship

Transversal sternwave/wa- 0.50 m idem

terlevel depression

Interference peak 0.45 m as a result of the passage

of highpowered, fast motor vessel (pleasure yacht, tugboat)

-Dimensions

I

Crest level of dam 0.5 m above canal level no overtopping possible

Slope gradients 1:2 steep gradients to save

space

Rip-rap grading 5 - 40 kg

23

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Test sections 1 and 2 have a permeable, riprap dam without extra openings to the canal. Water exchange can take place through the openings between the stones. Test sections 3 and 4 have an impermeable dam, in which a pipe with a diameter ofDAD metre is installed every 25 metres, by

means of which water exchange between the wet strip and the canal can take place. This pipe was assumed to be large enough for fish to swim in and out through.

The degree of water exchange was calculated in

the design phase using the PC model PLONS. The calculation results are given in the table. These apply to the passage of the largest possible passing loaded motor vessel. The given volumes of exchanged water were during design considered suitable to keep the water in the wet strip of satisfactory quality, without too much dynamics oc-curring in the wet strip during the exchange process.

The slopes on the land side of the wet strips were not protected.

table Rates of water exchange per passage of loaded motor vessel Type of forebank

pro-I

Exchanged volume- Exchanged volume

per-tection percentage with respect to centage with respect to the

the volume of wet strip, class volume of the wet strip,

I

II navigation class IV navigation

porous stone dam 19 35

compact dam with pipes 6 11

Evaluation

Figure 6.7. Photographs of the test sections in the period 1991 - 1997.

Ten years of experience in combining ecology and navigation on Dutch waterv/ays

24

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The four test sections were monitored extensively during the period 1991-1994 (Mosterdijk and Moller Pillot. 1994). The development of flora, the exis-tence of (macro)fauna and the quality of the water and bottom were followed meticulously. The level of the bottom of the wet strips was measured regu-larly. In 1992, a hydraulic measuring campaign was conducted to measure the degree of water ex-change and the occurring flow velocities

DUICh National Paper la be presenteo at the 29th PIANC Congress

(Rijkswaterstaat, Dienst Weg- en Water-bouwkunde.1994).

In 1997. bottom samples were taken and various recordings were made in connection with research into the bottom quality of wet strips and the existence of amphibians.

The photographs on page 24 show the progress of the development of the plants during the years af-ter construction.

Testing against the objectives - ecological targets

Whether the objectives for the ecological funciion of the bank were attained can be deduced well from the table below. In the table. the existence and significance of the plants and animals are

represented for the two types of test sections. in comparison with the original bank construction. just south of the test sections.

table Results of biological moniioring of test sections along the Wessem-Nederweert Canal

sections with sections with original bank pro-porous:dam, with- pipes', ifnperme- tection

out pipes able dam

water plants

I

-

I

+

I

-bank plants +++ +++/++++

I

+ fish fauna

I

+

I

+++/++

-avifauna (+)

I

(+)

I

-flower-visiting in- +++/++ +++/++ + sects aquatic macro- - - -fauna moisture-loving +++ +++ -bottom fauna

*Indicative of herbaceous vegetation Explanation of the symbols:

insignificant + some significance ++ fairly valuable

Ten years of expenence In combining ecology ana navIgation on DUICh waterways

25

+++ ++++ (+) valuable very valuable

some significance in the future

(26)

Dutch National Paper to be presented at the 29lh PlANe Congress

The table shows favourable ecological develop-ment mainly in the sections with pipes, only macro-fauna lagging behind. The cause of this is the sedimentation of alluvium on the bottom.

Compared with the original protection, the ecologi-cal values of the test sections are much higher. In 1997, the differences between these two types of construction can still be clearly seen. In the test sections with the pipes, a 15-20 % covering of water plants was found (including three kinds of pondweed species). Furthermore, larvae of the Brown frog, an adult Green frog and both young and old fish were found. The visibility in the water in

the test sections with pipes was> 0.8 m in the test section with most water plants and 0.2 m in the other. An alluvium layer of 40 cm was found, the depth was 80 cm.

In the test sections with a permeable dam, hardly any water plants were found « 1%); in one test section 20% of the water surface was covered by reed, in the other 80%. No amphibians or fish were observed. In both test sections the visibility was merely 0.2 m, the water had a milky colour. An alluvium layer of 0.25 m was found: the depth was 0.60 m.

Testing against the objectives - functioning of the constructions

Both protection constructions are most satisfactory as protection against the attack of water motions caused by ships. No damage to the protection itself and behind it was detected. During the hydraulic measuring campaign in 1992 it was determined that per ship's passage the amount of water ex-changed at the porous dam was approximately 20% of the existing volume. At the dam with pipes this is approximately 10%. On the basis of soundings, it was established that the thickness of the settled alluvium layer in the sections with a porous dam is one and a half times that in the sections with the compact dam with pipes. The turbidity of the water in the sections with a porous dam is also larger than in the sections with

the compact dam with pipes.

This means that the porous dam does not perform as well as the compact dam with pipes. For in the sections with a porous dam, the possibilities for the preservation of the zone with open water and water plants is quickly undone through accretion and clouding. In the sections with the compact dam with pipes, those possibilities are still there, but it needs to be said that here, too, the degree of accretion is a point of concern in this respect.

The compact dam with pipes gives fish the opportunity to swim in and out and appears to be a place of settlement for valuable vegetation, two valuable characteristics lacking in the porous dam.

Choice of variant for broadening

When it has to be decided which variant is most suitable for the broadening of the canal (one of the original objectives), the choice falls on the variant with the compact dam and pipes. The broadening has been put on a back burner, though, so that the choice is no longer under discussion. For the existing test sections this in fact means that a construction was built that was too high and too heavy, calculated for the conditions which apply in

Ten years of expe(lence In comolnlng ecology and navigation on Dutch waterways

26

the broadened situation. If for the current circum-stances an ecologically sound bank had to be designed again, a lighter protection is sufficient and a dam which rises less high above canalleve!. Fur-thermore, one pipe per 50 metres instead of 25 metres will suffice for the water exchange, resulting in a reduction in the degree of accretion, while preserving a sufficient degree of dynamics in the wet strip.