· · · Repository Hydraulic Engineering Reports

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.

Describes a relation between wave impact and clay strength.

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.

This is a progress report on the Homer Spit Beach Erosion Study. Information is presented regarding the rapid acceleration of the erosion processes due to the subsidence of the Homer Spit during the 27 March 1964 earthquake. The effect on existing groins both before and after the quake are discussed. Immediately after the quake, emergency measures were required to prevent wave and high water damage to existing structures on the Spit. The results concerning the effectiveness of these measures are presented for evaluation, along with the basic data gathered for the erosion study and some of the problems encountered.

The study area encompasses 18 miles of Oregon coastline known as Clatsop Beach, which lies between the Columbia River south jetty and Tillamook Head, south of Seaside, Oregon. The physical changes (including man-made changes such as jetty construction which was initiated in 1885) that have occurred in the area since 1792 are described and presented graphically. Theories based on an analysis of the very complex joint function of sand supply and incident wind, oceanographic, and estuarine forces are proposed as to the cause of erosion and accretion over the long-time span.

Erosion of banks and dikes subjected to water motion can be prevented, for instance by application of cellular concrete revetment blocks. This type of revetment combined the merits of a closed block revetment and a slope revetment consisting of loose materials. Advantages are high permeability, high stability against wave attack and low block weight. On top of this, cellular blocks are appealing because they allow the growth of vegetation. It is of vital importance that the granular material in the holes does not erode completely. Therefore physical model tests were carried out. The paper describes these tests and gives results.

Wave and sand movement conditions must be right for a coastal groin to produce the beneficial effects it was built to achieve. Some groins produce no shoreline improvement; some groins prove harmful; some that act beneficially also cause harm elsewhere, and disputes follow. Design data are sparse, so experience and judgment become important design skills to a greater degree than in most engineering problems. Court decisions are described which, from the engineering standpoint, seem opposite in effect where physical conditions were similar. Conclusion is made that the engineer who undertakes projects including seacoast groins needs legal counsel, special engineering knowledge and experience, and a healthy respect for the continuing acceptability of the client's seacoast groins to neighboring proprietors.

Scour is a natural phenomenon caused by the flow of water in rivers and streams. Scour occurs naturally as part of the morphological changes of rivers and as result of structures man-made. Several types of scour can be distinghuished. Experience has shown that due to sand or flow slides or micro instabilities at the end of the bed protection, the scour process can progressively damage the bed protection, leading eventually to the failure of the hydraulic structure for which the bed protection was meant. The length of the bed protection depends on the permissible amount of scour (maximum scour depth and the upstream scour slope) and the geotechnical structure of the soil involved (Pilarczyk, 1984). In the scope of the Dutch Delta works, a systematical investigation of time scale for two and three-dimensional local scour in loose sediments was conducted by Delft Hydraulics and the Department of Public Works (Rijkswaterstaat). From model experiments on different scale and bed materials, relations were derived in order to predict the steepness of the upstream scour slope (De Graauw and Pilarczyk, 1981 and De Graauw, 1983). Since the predictability of these relations is poor, especially for prototype conditions, a theoretical study concerning upstream scour slopes is carried out. In the present study a stability criterion is deduced which is based on the mass-balance equation and a stochastical bed-load predictor (Van Rijn, 1985) and fitted using approximately 250 clear-water scour experiments. In addition a empirical relation for undermining is discussed. The stability criterion for upstream scour slopes and the model relation for undermining are verified applying some prototype experiments (Delft Hydraulics, 1979 and De Graauw & Pilarczyk, 1981).

Scour is the lowering of the sea or river-bed as a result of non-equilibrium sediment transport conditions and can be divided into several categories (Breusers and Raudkivi, 1991). Local scour, which may occur at the base of a structure because of the affected flow pattern, can severely endanger the stability of this structure. Many varieties of localscour systems downstream from hydraulic structures exist, each with its own particular geometry and hence local scour mechanism. Local scour is superimposed on general and constriction scour. The prediction of local-scour holes that develop downstream from hydraulic structures plays an important role in their design. Excessive local scour can progressively undermine the foundation of a structure. Because complete protection against scour is too expensive generally, the maximum scour depth and the upstream slope of the scour hole have to be predicted to minimize the risk of failure. In 1961 a systematical research with respect to scour holes started at Delft Hydraulics within the scope of the Dutch Delta works. After the catastrophic flood disaster in 1953 the Delta plan was made to protect the Rhine-Meuse-Scheldt delta for future disasters. Dams with large scale sluices were planned in some estuaries. The severe scour expected necessitated a better understanding of the scour process. To find detailed information about the physical processes playing a role in scour many experiments were carried out, in which various parameters of the f l ow and the scoured material were varied. From the results of experiments in flumes with all difficulties of scale effects and limitations in instrumentation some empirical relations were obtained, which describe the erosion process as function of time and place (Prins, 1963 and Breusers, 1966, 1967). In these empirical relations a not well defined turbulence coefficient was introduced. Up to now this coefficient was related to the geometry upstream of the scour hole, which relation was based on trial and error. Based on theoretical grounds an analytical relation for the depth-averaged turbulence intensity is derived. This relation, which implies a modification of the turbulence coefficient in the Breusers scour formula, is verified using approximately 300 experiments. The modified scour formula yields results that compare reasonably well to measured and computed developments of a scour hole in case of a uniform flow upstream of the scour hole corresponding with a large protected bed area. The computations were based on the two-dimensional Navier-Stokes and convection-diffusion equations (Hoffmans, 1992) . The present paper aims at extension of the domain of application of the scour formula to non-uniform flow conditions upstream.

Design, construction and calibration of wave overtopping simulator

Overview of procedures to protect the coastline of Florida.

De TAW Leidraad Duinafslag van 1984 is vervangen door dit Technisch Rapport Duinafslag 2006 (TRDA2006). De hoofdreden daarvoor is dat er indertijd bij het opstellen van de Leidraad 1984 niet voldoende rekening is gehouden met het effect dat de golfperiode heeft op de mate van duinafslag. Metingen van golfkarakteristieken hebben inmiddels laten zien dat er tijdens zware stormvloedomstandigheden langere golfperioden kunnen voorkomen (Tp ≈ 16 - 20 s) dan waar indertijd rekening mee werd gehouden (Tp = 12 s). Met de procedures die in het TRDA2006 worden beschreven, wordt wel met het effect van deze langere golfperioden rekening gehouden. Daarnaast zijn er enkele andere nieuwe inzichten in het TRDA2006 verwerkt.

Vooral gestimuleerd door het ontwerp van de Oosterschelde stormvloedkering krijgt de zogenaamde probabilistische ontwerpfilosofie in toenemende mate aandacht in de waterbouwkundige ontwerppraktijk. (Bij een probabilistisch ontwerp wordt op basis van risico-analyses en waarschijnlijkheidsberekeningen de veiligheid van het ontwerp beoordeeld.) Vooral in ontwerpgevallen waarbij een relatief groot aantal bepalende factoren in het spel is, is een probabilistische aanpak de aangewezen weg om tot een verantwoord ontwerp te komen. Het duinafslagproces tijdens een hoge stormvloed is daarvan een voorbeeld. Er zal blijken dat uiteindelijk 7 invloeden van belang worden geacht de mate van duinafslag te bepalen. Hoewel dus een probabilistische aanpak van hetduinontwerp voor de hand ligt, worden daarmee echter ook extra complicaties geintroducerd. Essentieel bij een probabilistische ontwerpmethode is immers dat vooraf een toelaatbare bezwijkkans kan worden aangegeyen. In de waterkeringspraktijk en de waterkeringspolitiek is de toelaatbare bezwijkkans van een waterkering echter nog geen ingeburgerd begrip. In dit verband is voor een ad hoc oplossing gekozen om toch tot een uitspraak omtrent een aanvaardbare bezwijkkans te komen als randvoorwaarde bij de berekeningen.

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.