Sand bypassing using the shore parallel trap

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The Institution of Engineers Australla Porto and Harbours Conference Melbourne 28-30 August 1990

Sand Bypassing Using the Shore

Parallel Trap

R. BRINDLEY

D. FOSTER Department of Marine & Harbours, WA Unisearch Ltd, Tasmania K. BLAKE A. THOMAS Clough Engineering Group, WA Slurry Systems Pty Ltd, NSW

breakers differ little in fluid potion

less botton

1. INTRODUCTION tend to be less effective in Littoral drift of sand along coastlines can result breakers. The most intense local fluid motions are transporting sediment than plunging

sand bars may cause navigational produced boilan deposee front face begine co

steepen. When the wave reaches a

ing conventional dredges, In instances where the the crest. The crest of the it breaks by curling over at

prevents cost effective falling jet that scours a trough in tie bottom.

dredging one of the more attractive methods 1s a entrance. The lictoral drift sand is intercepted pumpe buried in a sand trap located updrift of To a lesser degree unbroken vaves can also suspend sand with increasing vich their vater depth. Studies (e.g. suspending capability reducing Gordan

entrance for diacharge on the downdrift beach, The trap, picked and Roy. (2)) have shown that appreciable sand

submarine pipeline across goveRent can occur out to depths of 20 metres.

Hoverer littoral drift of sand suspended by

sand then resumes its longshore movedent. unbroken vaves 18 of little significance

regards formation of sand bars across entrances.

2. LITTORAL DRIFT

2.1 Suspension of Sand

reaches a vater depth equal to about 1.3 times the an angle other than normal. This striking the coast at wave height (1). Breaking results in a dissipation ware action mainly in the of wave energy by the generation the transport of sediment lifted littoral drift sand intercepts an entrance much of off the bottom by the turbulent water. The waximum

the deep water of the

approximated by solitary wave theory to be entrance no longer break and so lose most of their 8 (H+d) the surf zone will still contiaue Non breaking waves beyond max sand will not be

deposited but vill continue past the entrance.

height of the wave gravitational acceleration, depth of water. is formed. This then causes the the entrance

Subatituting dal. 3H at the breaking point equation waves to break allowing them to suspend sand again 1 becomes litcoral drift process continues past the

- Г2.3 8 н (2) entrance. This 18 process by which

max entrance. If this process, vith its resulting sand

artiticial means of

The table below shows the maximum bypassing the sand must be employed. calculated using equation 2 for various heights. 3. SHORE NORMAL TRAP Velocity (a/s) Ware Fleighe (o) 3.4 4.т 8.2 3 9.5. 10.6 system is Probably the most the Nerang River entrance system on successful fixed sand bypass the Even for waves of moderste heisht the predicted Gold Coast, Queensland. The sand trap in this velocities are substantial and dore than 300 system is normal to the shore. A jetty| metre length of the trap with spans the jet pumps sufficient to suspend sand. located on the jetty pylons at 30 metre spacing. The above predictions derive from solitary wave Sand accumulated in the trap is lifted by the jet theory. Of the four types of breakers (spilling, pumps into a sloping flume running the length of plunging, collapsing and surging) spilling the jetty. The sand flows by gravity down the breakers most closely resemble solitary waves. fluse into a sump on the shore, This surp feeds

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into a centrifugal pump laid beneath the entrance to pumps the sand in the fillet, The further the trap is away from the breakvater the greater the potential storage through a pipeline side of the entrance. The sand is the trap and the breakvater the further the fillet volume. Hovever the greater the distance betveen and resumes its littoral drift along the beach, picked up by wave action vill need to extend offshore to present the this means the formation of a sand bar is avoided. required reverse angle of incidence to and generate the necessary Ieverse littoral drift. the vayes The shore norsal trap at Nerang the whole of the vas designed to There is then the possibility around the tip of the breakwater into the of sand migrating littoral surf zone. When the trap entrance. refilled principally fron the sides by littoral Another consideration is that some of the sand

needed to form the fillet is permanently lost from

4. SHORE PARALLEL TRAP downdrift beaches the coastal process. This to a certain extent. loss vill deplete The 4.1 General positioning of the trap is therefore a compromise between peak storm sand storage volume and the littoral drift of ultimate concern there is a much practice a trap located about 100 to 200 metres avoidance of t00 greater permanent sand loss. In greater oscillating movement of from the breakwater is ideal,

4.5 Trap Utilisation

to and recede from the beach. Just as the normal Sand sovement 9 greatest in the near shore zone the shore parallel component, so the his has been illustrated by experience at leran rate of oscillating sand jet puaps are required to

littoral drift pump the majority of the sand outer jet

coaponent. This suggests the possibility of a trap pumps operated less frequently. The parallel trap means whereby sand enters such a trap; direct feed is greatest. The size of the trap can therefore be and reverse littoral drift feed. 4.2 Direct Trap Feeding Any sand which passes the trap accumulates against

the breakwater and 1s returned to the trap by the

trap orlentated parallei to the shore reverse littoral drift process. and situated close to sand moves along the coast it follows a

5.0 MODEL STUDIES POR DAWESVILLE PROJECT

course moving in gone of the moving sand will be fed directly into The Department of Marine and Harbours (DMH), directly in this manner will depend on the length is considering construction of trap, the rate of littoral drift and the an artificial channel between Inlet near Mandurah to Improve flushing of the should be noted that

each particular grain of sand does not necessarily estuary (see Figure 1). There is an estimated nett littoral drift of 80,000 cubic metres

per year along this coastline. To avoid formation of a sand bar across the entrance to this proposed

results in a local decrease channel it is intended to construct a fixed sand

Orignally a syaten similar to the

compensated for either by migration of suspended successful Nerang Installation was proposed and Engineering, Slurry Systems and Unisearch by additional suspension of a system, During this study discussiona 4.3 Reverse 11ttoral Drift Feed resulted in the idea of the parallel trap. trap size and other conditions S,2 Hydraulic Model sode of the littoral drift sand may not be trapped the trap and accumulate To demonstrate the effectiveness of the parallel against the breakvater as shown in eventually trap basin model studies were conducted. The model vas constructed at the Coastal and Hydraulic sufficiently striking the fillet are Engineering Laboratory at Florest Park, the assistance of the DMH. The operation of the Wa with reversed. This Dodel vas carried out by DH staff under the

direction of Unisearch Ltd.

4.4 Location of Trap The model was a 1:100 scale representation of the

Dawesville beach and the proposed southern

The fillet storage and reverse littoral breakvater. The model basin vas rectangular in drift process means capacity plan vith a 1 in 2 slope energy absorption beach

have to match the peak around its perineter Loreed vich 12 em crushed

stora littoral drift systen can be blue metal. designed for a lesser transfer rate.

The irregular sea state of the prototype was

positioning of the trap in relation to the stmplified and generated as uniform regular waves breakwater dictates the storage volume avallable by l1 metre vide wave paddle driven by a

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• MANDURAN

BREAKWATER-

N

INDIAN OCEAN

IAL CON SHORE NORMAL TRAP CHANNEL

PF EL INLET

BREAKWAIER == •PARALLEL

-AREA REMOVED TRAP

CHANNEL WAVE DIRECTION BY TRAP

-SANDY

DAWESVETE FORESHORE

HARVEY ESTUARY FIGURE 2. DAWESVILLE CHANNEL SAND BYPASS MODEL

0.6 cubie metres per hour.

FIGURE 1. OAWESVILLE CHANNEL LOCALITY PLAN trap In operating the model the rate of f1111ng vas observed for various trap orientatione of the

and for Incident waves with measured perameters. The 100 metre (prototype) trap was tested in both the shore normal and shore parallel orientations.

waves then propogated the reproduced bathymetry of the srea 5.4 Model Scaling

under investigation. Wave guides vere installed

The horizontal scale was selected to be 1:100. The

limit scale effects included a wire approximate vertical scale vas selected on the mesh wave filter in front of the paddle to reduce following basis. In the model onshore/offshore ware reflection from the movement of sand was observed to be limited to a fins attached perpendicular vater depth of 30 tus, Measurements of prototype suppress the development of edge waves beach profiles undertaken by DHH on sandy beaches

along this section of the coast indicate that significant sand movement takes place shoreward of

The bed material used in the the 4 metre depth contour. This results in a having a median grain size of 0,4 am. The trap was vertical scale of 1 in 133 and this was adopted modelled with a perspex box for prototype lengths for this study, The volume scale is therefore 1.33 of 50 and 100 metres. Sand was redoved from x 10'6. trap manually.

The geomorphological tide scale for operation of

The model is shown in Figure 2. the dodel was eatinated as follows. The model time

required to reproduce an average annual littoral

5.3 Model Operation drift of 80,000 cubic metres can be calculated

from:

Con elon ere noreced into toe bodel with the simulate a northerly littoral drift Model Rate(cu.r/h) x Volume scale x Model Time (h)

following model characteristics: - 80,000 cu./y Prototype Wave height 20 ma The sodel time so' calculated to reproduce 1 yest Wave period 2 secs prototype is 1 hour. updrift degrees. This represents typical Slug flows of sand movement in the model were south vest swell direccion in suaner at reproduced by varying the interval beteen sand Dawesville. accumulation and sand bypassing. For example the

design criteria requires the plant to be able to

As the breakwater acts as a total littoral drift handle a slug of 30,000 cum in a 7 day period. If barrier the longshore transport rate in the model there is no sand:bypassing during such an event can be determined by measuring the volune change prototype beach changes would be represented i: with time of the at the breakwater. The 0.4 hours in the model. longshore transport to the north ao obtained vas

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S.S Results With a parallel trap the whole of the trap 13

closer to the shore. It is possible to use land

A number of tests were conducted. In one test the baged methods of construction for the jetty if the equivaleat of several years sand supply VaS fillet of sand which forms behind the breakwater allowed to accumulate against the breakwater is alloved to advance seaward prior Co burying the trap. Sand was then redoved from the construction. This obriates the need for an trap intermittently as it filled or near filled. expensive asrine construction Plant. Once the The accumulated sand against the breakwater was bypass system has been completed it can be rapidly transported to the trap and remored. operated to draw the beach back to the desired

alignment.

Overall conclusions reached vere:

In situations where the trap must be excavated in

(a) Wave action was capeble of supplying sand to rock, such as at Dawesville, the parallel trap has the trap at rate greatly in excess of the average advantages since It 13 cheaper to excavate in annual littoral drift rate of 80,000 cubic netres shallow water close to the shore. Once again land per year. based excavation and construction methods may be

utilised.

b) Sand slugs resultin; from storms could be llowed to accumulate agains! the breakwater for subsequent removal, 7.0 APPLICATION TO DAWESVILLE (c) Under normal operation there was little or no The proposed Dawesville sand bypass System will chance of the trap being "sanded 1e" (access lost use long parallel ocean). Should this occur however the approximately 35 metres offshore. The trap vill be system could be cleared by artificially opening a excavated in rock to a depth of 7 detres. small cut to connect the trap to the ocean. - (d) Both the 50 and 100 aetre equivalent trap 8.0 CONCLUSIONS lengths perforned satisfactorily with lovering of efficiency as the length is reduced. only a small Sand bypassing using a fixed jet pump installation

has been considered with attention given to both

(e) The prototype trap length could be reduced, the shore parallel and shore normal sand trep

required configurations. The shore parallel trap is shown

although further testing. minimum length possible required to have advantages over the trap orientated normal to the shore, Model tests conducted for the

proposed sand bypass system at Dawesville, Western

(f) The tests clearly indicated that the shore Australia confirmed the suitability of the shore

hydraulically more parallel trap.

efficient for vave transport of sand to the trap 8.0 REFERENCES than a shore normal systen. 6.0 ECONOMIC ADVANTAGES 1. Munk, Application to Surf Problems W.H. "The Solitary Wave Annals Theory and its of. the New is that The major offehore economic advantage of the parallel trep structures minimised. York Accadeny of Sciences, Vol. 51, 1949, pP 376- 462. a 500 metre long ahore normal trap configuration at Nerang involves jetty exposed coastline 2. Gordon, A,D. Nevcastle Bight", and Roy, P.S. "Sand Movements in

subject to regular cyclones. There is considerable Coastal and Ocean Engineering, Melbourne, 1977, PP Proc. 3rd Australasian Cont. on

cost in such a structure. 64-69.

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