Hulscher, Suzanne J.M.H.

Knaapen, Michiel

Scholl, O.

Scholl, Olaf

English

Sand waves form a wavy pattern in the offshore sandy seabed. Since their crests reduce the navigability, it is important to know their evolution. A simple model is presented to estimate the recovery of sand wave amplitudes. This model is partially based on the similarity with sea ripples and partially on bathymetry measurements. No further information is needed. The model reproduces the data very well. Using this model, it is possible to evaluate different dredging strategies

University of Twente Research Information

Marine Sandwave Dynamics - 23 & 24 March 2000 - Lille, France93Regeneration of dredged sand wavesSuzanne J.M.H. HULSCHER1, Michiel A.F. KNAAPEN1, Olaf SCHOLL11Dep. of Civil Engineering, University of Twente, P.O.Box 217 7500 AE Enschede, The Netherlands.Fax.: (31)534894040Email :S.J.M.H.Hulscher@sms.utwente.nlAbstractSand waves form a wavy pattern in the offshore sandy seabed. Since their crests reduce the navigability, it isimportant to know their evolution. A simple model is presented to estimate the recovery of sand wave amplitudes.This model is partially based on the similarity with sea ripples and partially on bathymetry measurements.  Nofurther information is needed. The model reproduces the data very well. Using this model, it is possible toevaluate different dredging strategies.IntroductionThe seabed is usually not flat, but is covered with a variety of bedforms. These bedforms vary from the smallripples, which are often found sandy beaches, up to enormous tidal sandbank structures with wavelengths in theorder of kilometres and wave heights in the order of tens of meters.The nature of these features is not yet fully understood. Most authors agree that one of the most importantdriving forces for the occurrence of large-scale bedforms is the tidal movement. Another important factor is thediameter of the sediment grains on the sea bed.Sand waves are a separate category of bedforms on the continental shelf. Their wavelengths range from 100metres up to 1 kilometre. The height of a sand wave is related to its wavelength. It varies from 1 metre up to 10metres, although sand waves of 17 metres in height have been reported. Sand wave crests are usually orientatedperpendicular to the principal direction of the tidal current. Sand waves tend to migrate in the direction of theprincipal tidal current, although some, like the ones in the approach area to the Eurogeul in the Netherlands, donot seem to move at all. Since dredging works are executed all over the world for large sums of money, it isinteresting to model sand wave behaviour. When dredging the sand waves, one can vary parameters as dredgingdepth, spatial distribution (i.e. overall or only the crests) and the frequency. The latter is often determined by thenavigation depth. The dredging strategy could well be optimised if sand wave behaviour is understood better.2-The sand wave modelIn analogy to an earlier presented model for small ripples under sea waves (Blondeaux, 1990), a sand wave modelhas been derived. The sand wave model is a one-dimensional description of sand wave amplitude evolution. Themodel sand waves are presented as sinusoidal shaped, non-migrating waves:hHA x LAA A=∂∂τ= +cos( / )21 23πα α(1)Figure 1 Situation sketchh(x,t)xA (t=t1)A (t=t0)LHin which z b is the sand wave, with amplitude Aand wavelength L. x denotes the spatial co-ordinate and τ  themorphological time scale. The undisturbed water depth H is assumed to vary little on a long time scale. The ideaMarine Sandwave Dynamics - 23 & 24 March 2000 - Lille, France94behind the model is that sand waves with very small amplitudes tend to grow exponentially, which is consistentwith the work of Hulscher [1996].Therefore, the initial situation can give an exponential growth of sand wave amplitudes. Due to the non-linearterm, the sand waves tend to grow to some equilibrium height, which is dictated by local circumstances. Theequilibrium height is in the North Sea approximately 18% of the water depth.The scales of sand waves and ripples are different, but the processes involved are considered to be similar. Inorder to create an applicable model for a concrete situation, the various model coefficients need to be determinedfor that particular situation, which is done using measurement data.3-ResultsThe model parameters have been determined for sand waves in both the Bisanseto Sea in Japan (Katoh et al,1998), and the North Sea in the Netherlands. This has been done by means of data from the sea bottom in bothcases (Details are given in Scholl, 1999). After fitting of the parameters the model gives the growth as given infigures 2 and 3. The found parameters are in agreement with the theoretical values as given by (Hulscher, 1996).Figure 1: Growth of sand waves in the Bisanseto Sea. The model reproduces the data, but is very sensitive tooutliers (the fourth data point).0.0810.0820.0830.0840.0850.0860.0870.0880.0890.090.0910 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5Distance from 1990inτN ≈8.15yearsAmplitude-waterdepth ratio ModelModelObservations0.030.040.050.060.070 1 2 3 4 5Distance from March 1985 in τAmplitude-waterdepthratioall data used3th data point rejectedObservationMarine Sandwave Dynamics - 23 & 24 March 2000 - Lille, France95Figure 2: Growth of sand waves in the North Sea. 2 possible model runs vs. measurement data. Note that therehas been dredged before the final measurement.4-Application: Dredging strategiesBased on the conditions in the North Sea area, dredging strategies are tested using the model. The only selectioncriterion we considered is the time it takes for the sand waves to decrease the navigational depth to a referencedepth.The model is used to compare 2 strategies, in which the sand waves are topped off.(1) the sand is transported out of the area for commercial use.(2) the sand is redeposit in the troughs. It is assumed that the extracted sand causes a net transport into the area(See Katoh et al. 1998).For case (1) we find a period of recovery of almost 10 years. For case (2) this period is very sensitive for the netinflow of sand, see table 1.Sin  in cm/year-1 τ t in years1 2.33 192 1.1 93 0.81 6.64 0.66 5.45 0.56 4.66 0.49 47 0.44 3.6Table 1 Changes in the recovery time due to changes in net deposition of sand (Sin).5-Conclusions and recommendationsThe sand wave model presents an adequate estimate of complete trajectory of the sand wave growth. The simplemodel, combined with a parameter fitting procedure, makes the model useful for strategic decision making. Nophysical knowledge is required. However, to get an accurate parameter fit, long period data sets are necessary,which makes monitoring necessary.ReferenceBlondeaux, P., 1990. Sand ripples under sea waves. Part 1 Ripple formation, J. Fluid Mechanics, 218, pp. 1-17.Hulscher, S.J.M.H., 1996. Tidal-induced large-scale regular bed form patterns in a three-dimensional shallow watermodel. J. of Geophysical Research. 101, C9 , pp 20,727-20,744.Katoh, K., Kume, H., Kuroki, K., Hasegawa, J., 1998. The development of sand waves and the maintenance ofnavigation channels in the Bisanseto Sea, ASCE proceedings ICCE .Scholl, O., 1999. Estimating amplitude evolution of sandwaves. Master Thesis University of Twente, 99W-012/Mics-005.

Regeneration of dredged sand waves

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Regeneration of dredged sand waves

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