· · · Repository Hydraulic Engineering Reports

Een groot gedeelte van de Nederlandse dijken is bekleed met een grasmat. Daar, waar men op een deel van de dijk een hard bekledingstype heeft aangebracht, is toch een belangrijk deel van het dijkoppervlak, zoals de kruin en het binnentalud, een grasmat. Het onderzoek en de ervaringen vanaf het midden van de jaren tachtig hebben geleerd, dat grasmatten een zeer hoge kwaliteit kunnen hebben, zowel qua erosiebestendigheid als qua natuur. Die hoge kwaliteit kan men goed bereiken met een daarop gericht graslandbeheer. Een grasmat is een even volwaardig bekledingstype als betonblokken of asfalt. De sterkte kan zelfs groter kan zijn dan die van sommige harde bekledingstypen. Deze brochure geeft de belangrijkste informatie over het gedrag, de aanleg en het onderhoud van grasmatten en verwijst voor details naar gespecialiseerde literatuur.

This report is the fourth in a series dealing with the use of natural grasses and bituminous and soil admixtures for surfacing farm dam bywash spillways. When the work reported here was completed, the test channels were demolished to make way for new construction, and no further tests of this nature are contemplated. The previous report (Yong and Stone, 1967) dealt with the scour resistance capacity for grasses having a dense even cover, tested at the peak of their growing season. This report covers winter tests with the grass dormant. Once they have established a good root system, the grases withstand high velocities amazingly well, even when dormant, and there is no need to reduce permissible design velocities as given previously for grasses subject to winter floods. Three channels were available for each grass type, so a test was also made on one channel of each grass of the extra protection that might be afforded by anchoring a layer of chicken wire on the spillway at or near the soil surface and letting the grass grow up through it. In fact, none of the spillways failed hydraulically, failure due to the structural inadequacy of the channels preceding scour failure in every case, and no definite conclusion could be reached as to the added benefit of the chicken wire.

Third report on use of grass on farm spillways; tests on slopes 1:10 to 1:2.5 (prototype scale tests). Permissible velocities suitable for design were given.

Investigation on the strength of grass roots in clay and sand subsoil with focus on shear strength.

Describes relation between wave impact and damage to grass layers.

Model studies formed a basis for investigating the hydraulics of bywash spillways for farm darns When constructed in natural earth such spillways are trapezoidal in cross section The surface of the spillway needs protection from scour and this protection is usually in the form of grass. These tests extended to variations in spillway geometry and variations in surface roughness. The effects of these variations on head discharge relationships were noted. The variation of discharge with head over the range of spillway surface lengths and the range of roughnesses tested was less than 10 pc. of the average discharge. The report contains recommendations for the design of new spillways and examples indicating the design procedure are given.

As part of a program of research aimed at improving the design and construction of farm dams, the Water Research Laboratory, has with the aid of a grant from the Water Research Foundation of Australia, undertaken a study of the protection afforded to spillways by various low cost surfacing techniques. These include vegetal cover by natural grasses as well as bituminous and soil admixtures. Preliminary study of available information was published as Water Research Laboratory Report No. 77. This study emphasized the necessity for tests of Australian grasses and products locally available under local conditions. The experimental program was divided into two parts. The first part, which is reported here, consisted of flume tests on numerous grasses and other surface treatments. The second part, which will be reported separately, consisted of more extensive testing of a selection of the more promising materials in long channels of continuously varying slope.

The US Wave Overtopping Simulator has to simulate overtopping discharges up to 2 cfs/s/ft for wave conditions of respectively 8 ft with a peak period of 14 s and 3 ft with a period of 6 s. This requires a Simulator which is in size about three times larger than the existing Dutch one. This report describes the theory of waves overtopping the crest of a levee, the design of the Simulator, how to operate it and possible ways to measure hydraulics during testing. We know a lot about wave overtopping over levees, but still there are discrepancies between various formulae. First the existing theory is given about wave overtopping discharge and individual wave overtopping volumes. This leads to the distributions of wave overtopping volumes that have to be simulated by the Simulator. Then flow velocities, flow depths and flow times or durations of overtopping wave volumes at the crest of a levee have been discussed, including re-analysis of existing work and some recent research. The conclusion is that using the equations in an integration, to calculate the wave overtopping volume, the volumes are much too large, indicating that at least flow depth andflow time predictions are too large. For this reason the Simulator will mainly be based on flow velocity and the given peak periods of the waves. Good experience is available with Simulators up to a size of about 6 m3/m width. The majority of all overtopping wave volumes will be limited to this size. It is for this reason that the US Simulator has been designed with an inner Simulator, comparable to the existing sizes, and an outer Simulator, giving the maximum capacity of 16 m3/m. The outer Simulator will only be used for the very large overtopping wave volumes. The mechanical design has been described and this design has been discussed during a visit to CSU, in order to start fabrication of the Simulator. The Dutch test set-ups at various locations have been described with their improvements every consecutive year. The operation of the Simulator by a steering file and PLC has been given, first by description of the Dutch Simulator and then by possible modifications and improvements for the US Simulator. Finally, experiences to measure flow depth and front velocity of overtopping waves have been described and possible ways of improvements, which will be performed in the Netherlands and tested in February/March 2010. If successful, these kind of measurements can also be developed at CSU.

A large part of the Netherlands’ dikes are covered with grass. Dikes with hard revetments also have grass cover on a significant part of the dike surface, such as the crown and inner slope. Research and experience since the mid-eighties have shown that grass coverings can be of high quality in terms of erosion resistance and encouragement of development of nature. This high quality can be achieved with suitable grassland management. Grass cover is a valuable type of revetment, just as cement blocks or asphalt. In fact, its strength can be even greater than some hard revetment types. This report gives the most important information about the behaviour, laying and maintenance of grass coverings and refers to specialized literature for details.

Guidance on the use of grass to stabilise surfaces subject to erosion by intermittent flow. Provides information on the erosion resistance and frictional resistance of grass. Includes recommendations on grass mixtures, etc. Extensive literature survey

With the aid of a grant from the Water Research Foundation of Australia, the Water Research Laboratory has undertaken an investigation into surface treatments for small farm dams Vegetal cover with natural gr asses and low cost soil and bitumen admixtures are under study. As a preliminary to laboratory and field testing, a survey of available information was made. The results of the survey are contained in this report. The experimental work, which is still in progress, has been divided into two phases. First tests have been made on the reaction of numerous grasses, admixtures and surface treatments when subjected to water flow in a flume. Secondly, tests are being conducted on a spillway of field dimensions for a selection of the more promising of the flume-tested materials. The research programme has been under the direction of various academic staff members from time to time Mr B. A. Cornish has been in charge of detailed experimental work.

Bij zeer veel zeedijken in Sleeswijk-Holstein en op Jutland breken de golven tijdens de maatgevende stormvloeden op grasmatten. Deze groene zeedijken blijken voldoende veilig te zijn. De aanleg wordt gekarakteriseerd door o.a. flauwe buitentaluds en een gedeeltelijke bezoding met zoute kwelderzoden. Bij een voldoende brede kwelder is een harde bekleding overbodig. In Nederland kunnen geheel of gedeeltelijk groene zeedijken onder vergelijkbare randvoorwaarden ook goed voldoen. Dat vraagt een hogere waardering van grasmatten en een andere aanleg en beheer. In het alsnog t.b.v. de advisering opgestelde reisverslag wordt ook ingegaan op de mogelijkheden en problemen in Nederland.

The Wave Overtopping Simulator was developed in 2006 and destructive tests have been performed in February and March of 2007, 2008 and 2009 and will probably be continued in 2010. The tests show the behaviour of various inner slopes of dikes, embankments or levees under simulation of wave overtopping, up to a mean overtopping discharge of 125 l/s per m. In 2010 a Technical Report on strength of inner slopes of dikes against wave overtopping will be written, leading to new guidelines for the required five-yearly safety assessment of flood defence assets in the Netherlands. This paper will give a mid-term review and first guidance, based on 3 years of destructive testing. Till summer 2009 15 sections of dikes at 5 different locations in the Netherlands have been tested. The paper will give guidance to practical engineers, based on observations and analysis from all the testing so far. It also discusses the possible modifications in safety assessment.

Aan de orde komen waar een natuurtechnisch beheer gewenst is, waar met een landbouwkundig beheer kan worden volstaan, welke mogelijkheden deze gebruikswijzen hebben, welke moeilijkheden zich kunnen voordoen en wat men bij een goed gebruik dient te doen of dient na te laten. De adviezen voor het graslandbeheer hebben steeds als uitgangspunt dat er aan de veiligheid van de dijk niet mag worden getornd. De bijeengebrachte informatie is bestemd voor ontwerpers en beheerders van dijken, voor boeren en voor natuurbeschermers, kortom voor allen die op een of andere wijze met het rivierdijkgrasland te maken hebben. Nu de rivierdijkverzwaring op gang begint te komen, en op veel plaatsen de situatie drastisch zal veranderen, dient ook het gebruik van de dijkgrasmat alle aandacht te krijgen.

The effect of roots of grass and other vegetation on the strength of a clay and sand slope, as used in dikes and revetments. Focus on shear stress.

Old dikes are usually still constructed entirely from clay and since 1953 many much heavier dikes have been made from a core of sand with a covering. This construction puts heavier requirements on the covering, as it also does for grassland on a clay layer. These and other developments have led to the need for more insight into the strength of grassland as a dike covering and into the possibilities of managing it better. Regular maintenance is essential for grass coverings and furthermore they are relatively sensitive in their early years to damage; even somewhat older grass coverings sometimes show signs of unexpected damage. They require expenditure to put right and cause doubts about their strength under extreme circumstances. In addition, the Flood Defences Act requires an evaluation every five years of the existing primary flood water defences, of which the dike coverings form an important part. Dike managers have an inclination to put a hard covering on a dike because there is so little insight into the strength of grass coverings, thereby contributing to the reduction of herbage-rich grasslands. As a result, the other functions of dikes, such as the preservation of environment, landscape and culture-history values (abbreviated in Dutch as LNC-values), recreation and agriculture get less attention. Because dikes play a water defensive role before everything, other functions can only be considered if the required erosion resistance of the covering is sufficiently durable. Safety against water is the primary goal, for river, sea and lake dikes. This study has also been aimed at increasing insight into the strength of grassland as a dike covering and on measures that can increase the desired strength.

Erosional problems and damages include loss of land, imperilment to roadways, loss of recreational beach, loss of access to the lakes and structural damage to dwellings, boathouses, docks, and stairways: Some shoreline conditions along the Great Lakes are difficult to alter to control erosion, particularly in areas where there is no beach or where the water interfaces immediately with steep bluffs. However, there are numerous areas where erosion can be abated by attenuating wave action with mechanical barriers and then using terrestrial and aquatic vegetation to protect the shore against the reduced wave action and against surface runoff. There are three types of terrestrial plants that can be used to abate erosion: (a) pioneer plants, the species to first become established on new. substrates; (b) secondary plants, the species to first invade edaphically stable areas colonized by the pioneer plants; and (c) tertiary plants, the species poorly adapted to dynamic conditions and requiring areas previously stabilized by pioneer and secondary species. Most species used in terrestrial planting operations are the pioneer type. The pioneer initiates a development sequence (Cowles, 1899; Hack, 1941). It stabilizes the surface, provides lodging for windborne disseminules, shields seedlings from sun and wind, and prepares the way for natural invasion of other plant types (Daubenmire, 1968). Establishment of hydrophytes (submergent or emergent plants) is very difficult. They are highly restricted by currents and water level fluctuations. Emergent hydrophytes are limited to low-energy shores, w~ere.they modify less forceful waves; submergent aquatic plants establish ln even more protected areas. The restriction of submergent species to quiet waters limits their phytogeographical distribution, and their vulnerability to strong wave forces prohibits them from naturally colonizing the wave-swept littoral zones of lakes. The main purpose of this investigation was to determine if terrestrial and hydrophytic plants, either alone or in combination with structures, can be used to attenuate wave energy immediately offshore and to stabilize the areas adjacent to the waterline, thereby reducing the erosion rate along the Great Lakes shores.

Dit rapport bevat verscheidene tabellen met overschrijdingskrommen of overschrijdingsduren en drukgradienten met betrekking tot graserosie in deltagootproeven. Parameters en karakteristieken van de grond worden besproken in meerdere scenario's.

Er is een analyse van de waarnemingen en metingen met betrekking tot ontgronding uitgevoerd voor de 1:1 modelproef in 1992 in de Deltagoot met golven van Hs 0.75 m tot 1.35 m op een grastalud van een dijk. De gemeten ontgronding is veel geringer dan tijdens vergelijkbare proeven op klei met een bodemstructuur, maar zonder zodelaag. Pas na vele uren belasten met 1.35 m golven kan de zodelaag plaatselijk bezweken beschouwd worden. Een gat in de zodelaag dat tussen 3 en 9 uren golven met 1.35 m golven ontstond lijkt niet sterk uit te breiden. Tijdens proeven met 1.35 m golven is er een daling van het oppervlak door onder andere veranderingein in de waterstand in de opstelling. Er blijkt ook een onregelmatige permanent vervormingen van het talud op te zijn getreden als gevolg van verschuiving van de grond tijdens de proef, hetgeen gepaard ging met veranderingen in het patroon van de signalen van waterspanningsmeters en hetgeen wijst op aanpassing van de structuur in de grond tijdens vervorming bij golfbelasting. De gemeten waterspanning in het talud is niet hydrostatisch, vanwege de opbouw en inrichting van de modeldijk in de Deltagoot en het daarmee samenhangende stromingspatroon door de graszode en onderlaag. Tot ongeveer 15 minuten na het beginnen van golfproeven stijgt de waterspanning in het talud, evenals na verhoging van de waterstand in de proefopstelling. Het patroon van het signaal van bijna alle opnemers reflecteert de waterdrukken op het talud, echter de ampitude en gemeten drukvariatie verschilt sterk tussen de opnemers als gevolg van lokale verschillen in directe omgeving van de opnemers, drukopnemers die in verbinding met grotere porien hebben een hoge amplitude. Waterspanningsmetingen geven een goede indruk van de variatie in waterspanningen in het talud tijdens golfaanval. De uit de metingen afgeleide gradienten in druk zijn zodanig hoog en frequent gedurende 0.5 tot 1 s aanwezig dat bij golven van 1.35 m de zodegrond zeker opgetild kan worden. Uit modellering van de effecten van de golfbelasting op het talud blijkt dat de elasticiteit en sterkte van de intacte zode voldoende zijn om bezwijken door golven van 1.35 m te weerstaan. De grond onder de zode blijkt wel plastisch te worden door belasting, wat geen directe consequenties voor ontgronding heeft zolang de zode instand blijft. De effecten van de verandering in de tijd van de waterdrukken over het talud domineren de in de ondergrond opgeroepen waterspanningen. Het effect van samendrukbaarheid van water met lucht lijkt niet zeer sterk beneden de bovenste centimeters. Voor de bovenste millimeters kunnen door dit effect de hogerfrequente fluctuaties van turbulentie en dergelijke de oppervlakkige ontgronding beinvloeden. Ontgronding tot golven van tenminste 0.75 m wordt gedomineerd door het verwijderen van (bijna) losliggende gronddeeltjes aan het oppervlak en de daarbij behorende ontgrondingssnelheid bedraagt minder dan 1 mm per uur bij 0.75 m golven. Bij golven van 1.35 m treedt zodanige vervorming van de zodelaag op dat de dunne wortels die de zode-aggregaatjes bijeenhouden kunnen gaan bezwijken waardoor er snellere ontgronding kan optreden, 2.5 tot 5 mm per uur is gemeten. Bij hogere golven kan de zodelaag als geheel gaan bezwijken, scheuren, echter hiervoor zijn geen directe waarnemingen of berekeningsresultaten beschikbaar. Er is waargenomen dat de zode bij 1.35 m golven is bezweken ruim beneden de stilwaterlijn waar grotere uitwaarts gerichte verhangen optreden. De verschillende ontgrondingsmechanismen zijn vereenigd in een zeer beknopt model voor het evalueren van de bezwijkduur van graszoden van dijktaluds bij golfaanval, waarin gegevens van andere en veldwaarnemingen zijn verwerkt.