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DESIGNInitial principles

2.1 2.1

Design of the appropriate system for aspecific project must always be precededby a detailed audit of the proposed site to establish:

z Existing factors and considerationsapplicable to the site

z Predicted factors relating to the site’s usefollowing the planned development, andthe parameters within which theinstallation is required to function

z The type of function or applicationsuggested by this audit.

The required design information to beestablished by this audit is summarised inthe Design Information Checklist inSection 2.2.

Successful design also requires anunderstanding of the design philosophybehind each of the Wavin StormwaterManagement systems.

Once the project criteria have beenestablished from the site audit, there arethen two key parts to the design procedure:

z Hydraulic design

z Structural design.

2.1.1 Wavin systems designphilosophy: AquaCell

The AquaCell unit has been designed tocombine high physical strength with goodthree-dimensional flow performance.

Hydraulic function

Water entering a box structure made up ofAquaCell units dissipates quickly. The unitdesign allows single point access for thethree main pipework systems:

z Inflow

z Outflow

z Air release.

The primary function of the AquaCell unit isfor the management of storm run-off fromimpermeable surfaces. This can be utilisedin three ways:

z Infiltration: water is collected in the unitsduring rainfall and allowed to drain awayby soaking into the surrounding groundover a substantial period of time after therain has stopped

z Attenuation: water is collected in the unitsduring rainfall and released at a reducedflow rate through a flow control deviceinto an appropriate outfall. This reducespeak flows in the watercourse andthereby minimises the risk of flooding

z Conveyance: units are used as analternative to conventional pipework,providing increased online storage andslower transfer rates into watercoursesand sewers. This gives the benefit ofattenuation and, if so designed,infiltration.

Structural function

The AquaCell units are designed tomaintain structural integrity, individually andin multi-unit assemblies, under both deadloads (e.g. covering and surrounding earth)and imposed loads (e.g. transientpedestrian and light vehicular traffic).

Independent testing

Hydraulic and structural testing of AquaCellunits was supervised by the University ofSalford, a well-respected independentauthority in this field.

Hydraulic testing was conducted in a flume– 13m long and 1.2m wide – at a variety ofwater depths, flow rates and gradients,using a series of AquaCell units. Thisserved to establish a fully validatedunderstanding of AquaCell’s hydraulicperformance capabilities.

Further testing determined the structuralcapacity of the system. This involved directloading and long term creep tests on singleAquaCell units.

NOTE: Detailed test methodology used in these independenttests is described and explained in Appendix 4. Designparameters established by the tests are included in Designprocedures (2.3 and 2.4)

2.1.2 Wavin systems designphilosophy: Garastor

Hydraulic function

The Garastor domestic attenuation systemwas developed in collaboration with BryantHomes. The complete system provides aninnovative method of creating a substantial,temporary water storage area in the voidspace under the floor slab of a residentialproperty garage (Fig. 2.6) or under a softlandscaped area (Fig. 2.8).

The key component in this system is theGarastor flow control, a plastic inspectionchamber with built-in flow control deviceand overflow.

During high intensity rainfall, water from theroof or hard standing areas is diverted viathe Garastor control unit into a void createdbetween the walls and beneath theconcrete floor of a domestic garage, or toan adjacent small reservoir created fromAquaCell units. As the storm subsides therainwater can be slowly released back intothe main drainage system.

The Garastor system has undergoneindependent assessment by the Universityof Edinburgh to test flow and dischargerates.

© Published by Wavin Plastics Limited 2002

2.2.1 General considerations

The following is a summary of theinformation which is normally required toestablish the design parameters for WavinStormwater Management systems. Thisincludes a thorough understanding of allthe physical and environmental attributes ofthe site, as well as the anticipated practicalrequirements and criteria for its futurefunctioning in times of heavy rainfall.

Where necessary, these are explained inmore detail in following pages.

2.2.2 General considerations

Infiltration or attenuation?

The full site audit will help determinewhether infiltration is required and/orpossible – or whether attenuation andtemporary storage is more appropriate.This may simply be dictated by the physicalattributes of the site (e.g. soil type andproperties). However, environmentalconsiderations may be particularlyinfluential (see Section 2.2.3).

Geotextile or geomembrane?

This in turn will establish whether apermeable geotextile or impermeablegeomembrane wrapping should bespecified for the AquaCell installation(s).

The choice of these materials is alsosubject to a number of physical andperformance criteria (see Section 2.4.2).

Bedding, backfill and cover fillspecification?

There are special considerations for thematerials to be used for bedding and fillingaround AquaCell assemblies. These aredescribed in detail in a later section whichincludes detailed design data for backfillspecification (see Section 2.4.3).

2.2.3 Environmental considerations

Stormwater quality

Using the AquaCell system to infiltratestormwater, or to temporarily store it onsite, can provide other environmentalbenefits besides reduced flood risk tostreams and rivers.

Description Information Sources

A. Existing factors

Topography Site survey or inspection

Area of catchment* Site survey

Hydrology of catchment Site inspection and observations

Soil type* Site investigation

Structural properties of soil Site investigation and laboratory testing– CBR, stiffness

Infiltration potential of soil* Site investigation

Contamination* Site investigation and desk research

Details of receiving water/ Environment Agency, Scottish Environment Protectionwatercourse/aquifer Agency or water and sewerage company

Environmental sensitivity Environment Agency, Scottish Environment Protectionof site Agency or water and sewerage company

Groundwater vulnerability Environment Agency, Scottish Environment Protectionand source protection status Agency or water and sewerage company

B. Predicted factors

Development type and Proposed development plansland use

Traffic loads Proposed development plans

Rainfall data* Meteorological Office or Wallingford Procedure

Discharge design criteria Environment Agency, Scottish Environment Protection– quantity Agency or water and sewerage company

Discharge design criteria Environment Agency, Scottish Environment Protection– quality Agency or water and sewerage company

Health and safety All affected parties

C. Planned function

Infiltration Conclusions from A and B audit/review

Attenuation Conclusions from A and B audit/review

*NOTE: For individual house soakaways, only the items in italics are required.

Design information checklist

It may also reduce the impact of pollutantspresent in stormwater run-off if they areinfiltrated and filtered in the underlying soils.

However, as with any drainage system thedesigner must ensure that new risks arenot introduced. In each case, a site specificrisk assessment of the likely effects of theuse of the system on local water resourcesshould be undertaken by the client,particularly where infiltration is being used.

Roof water run-off

The Environment Agency’s GroundwaterProtection Policy identifies that installation ofsoakaways to discharge only roof water isacceptable in most situations. However, oncontaminated sites, care must be taken toensure that the water does not mobilise thecontaminants, and that the water can bedischarged below the base of contamination.

Surface water run-off

Surface water run-off can wash pollutantsinto watercourses or the soil. The natureand amount of pollution is dependent uponthe land use and human activities within acatchment. Similarly, the impact of surfacewater run-off is difficult to predict as it is notonly dependent upon the type and volumeof pollution transported, but also upon thenature and sensitivity of the receiving waters.

Hence the design and long termperformance of stormwater managementsystems needs to take into account thispotential variability of environmentalconditions over the operating life.

DESIGNDesign considerations

2.2


DESIGN Design considerations

2.2

Infiltration base

Many pollutants tend to adsorb to soilparticles when infiltrated to the ground. The evidence suggests that migration ofheavy metals does not extend to greaterthan about 500mm below the base ofsoakaways. Typically, therefore, the base of an infiltration system should be locatedat least 1m above the highest recordedgroundwater level below a site to minimisethe risk of pollutants being transported into groundwater.

High risk areas (stormwater hotspots)

These are areas where:

z Land use or activities have the potentialto generate highly contaminated run-off,or

z The groundwater is an important sourcefor drinking water abstraction.

Before embarking on any specific project,the Client should confirm that a StormwaterManagement System is suitable forinstallation in the location concerned.

Hotspot areas can be categorised intothree types:

z Resource based: areas providing avaluable resource that must be protected

z Use based: areas with activities whichare liable to generate pollutants ormaterial hazards

z Ground conditions based: wherepollutants are known to be alreadypresent.

Resource based hotspots

In these areas:

z Only roof drainage should be dischargedvia soakaway

z For highway and other drainage, fullytanked storage systems should be used,with outlet via pipes to surface watersewers or watercourses.

Hotspots defined by resource include:

z Groundwater inner source protectionzones.

Further advice on the use of infiltrationdrainage over aquifers is provided in theEnvironment Agency’s GroundwaterProtection Policy.

Use based hotspots

In these areas:

z Infiltration drainage systems should notnormally be used without a fullassessment of the risks andconsequences of both general day-to-day and major spillage

z The use of interceptors will normally berequired and a mechanism for closingthe outlet from the system provided

z Where the risk of pollution occurring isconsidered unacceptable, a fully tankedsystem should be used.

Hotspots defined by use include:

z Fuel stations

z Areas where hazardous/toxic materialsare stored, loaded or unloaded

z Vehicle or equipment service, cleaningand maintenance areas.

Ground conditions based hotspots

In these areas:

z Where contaminated soils are present,soakaways should not be used unlessthe water can be discharged below thebase of the contamination

z Where very minor ground contamination(non-mobile) exists, infiltration may bepossible if a risk assessment establishesthat any risks to groundwater areacceptable.

Groundwater pollution cannot be seen,and is difficult to put right once it hasoccurred. Therefore the precautionaryprinciple should be adopted whenassessing risks to groundwater anddeciding if to use infiltration drainage.

2.2.4 Health & Safety:Construction (Design andManagement) Regulations

Under the Construction (Design andManagement) Regulations 1994 (CDM),every project design must include a site-specific Health and Safety assessment.

Risks to construction workers, users andmaintenance workers need to be avoidedor minimised wherever possible. These aretypically associated with the design andconstruction of conventional drainage,such as working in excavations, excavationnear services, etc.

AquaCell units

However, there are no specific issuesrelating to the installation, use andmaintenance of the AquaCell unitsthemselves.

In many instances, the AquaCell systemcan help reduce the risks of injury duringconstruction. For example:

z The units are lightweight and easilymanhandled

Benefit: reduced risk of lifting injuries

z The system is also designed as a shallowattenuation or infiltration system

Benefit: reduced need for deep sewerexcavations.

Garastor

The Garastor system can use the housefoundations, walls and garage floor slab to form the storage space. A CDM riskassessment will always be required forthese elements.

There are no other specific health andsafety issues relating to the design andinstallation of the Garastor system.


2.3.1 Rainfall and run-off

Evaluating rainfall events

The most commonly used method forevaluating storm rainfall events in the UK isthe Wallingford Procedurev. This assessesthe total rainfall level of storms over definedtime periods – ranging from 5 minutes upto 48 hours in duration.

The procedure comprises use of a series of maps, factors and charts to enable thelevel of any given storm to be predictedbased on:

z Geographical location

z Storm return period

z Storm duration.

The output from the procedure is a depthof water in mm, which can then bemultiplied by the catchment area to assessthe size of infiltration/attenuation systems.

Storm duration - attenuation systems

In many attenuation systems the outflow isvery small in comparison with the overallstorage volume. In this case, a critical eventof 120 minutes (2 hours) is typically used.

Storm duration - infiltration systems

If the outflow is large, as can be the casewith infiltration systems in good soils, anumber of shorter durations must beevaluated to find the critical period whichleads to the largest storage volume.

To evaluate short duration rainfall eventsfalling on buildings, use British Standard BSEN 12056-3:2000

vi. This allows 2 – 10

minute events to be evaluated for a range ofreturn periods and locations. In the UK 2minutes is the commonly used concentrationperiod for roof drainage rainfall.

Return period

The return period chosen for either of theabove approaches can have a significantimpact on the volumes generated.

For underground drainage systems andsoakaways, 10 or 30 year events havecommonly been used in the past. However,ongoing climatic change and thepublication of Planning Policy GuidanceNote 25: Development And Flood Risk,have dictated a new approach. Many area

officers of the Environment Agency nowrequest allowance for a 100 year event.Others are asking for this to be increasedby a safety factor of 20%.

Reduction factors

Not all water runs off the surface.Accordingly, designers may reduce theoverall catchment areas to reflect this fact.For example:

z On hard surfaces, a quantity will beretained in pore spaces and on thesurface

z Even on metal roofs, a small amount willbe retained as droplets.

To allow for this, use the following reductionfactors:

z For hard surfacing: 10%

z For buildings: 5%.

2.3.2 Infiltration

Key factors checklist

When considering an infiltration drainagesystem there are a number of issues thatmust be investigated before embarkingupon detailed design:

z Is the soil type suitable for water to soakaway?

z Is the natural groundwater level belowthe base of the infiltration tank?

z Will the water quality cause pollution ofthe surrounding ground or anyunderlying aquifer? (e.g. chemicalpollution from an industrial site)

z Is there a suitable location for the tankaway from buildings or other structuresso that the water will not affect basementsand foundations? (Minimum distancefrom buildings: 5 metres – see Fig. 2.3)

z What are the consequences of the tankbecoming full due to rainfall rates abovethe design condition?

Site survey

These and other questions need to beaddressed by a thorough site survey whichis likely to involve:

z A geotechnical site investigation

z Construction of a trial pit.

This site test is exceptionally importantsince it proves the acceptability of the site.

Trial pit

The most commonly used method is setout in Building Research Establishment(BRE) Digest 365

vii. A typical procedure is

as follows:

z A pit is excavated down to the level ofthe proposed infiltration tank. The pitshould be between 0.3 and 1m wide and1 to 3m long

z The pit is filled with water to the invertlevel of the proposed inlet pipe

z The water level in the pit should be notedat regular intervals and the total timetaken for it to empty recorded

z This process should be repeated at leasttwice more

z The time taken to fall from 75% full to25% full is used in the calculation of a soilinfiltration coefficient, which is used in the design

z If this coefficient is less than 10-6 m/s, the site is likely to be deemed unsuitablefor infiltration

z If the area of the proposed tank is large(e.g. larger than a small single housedevelopment), further pits should be dugat 25m intervals.

As a guide a gravel will yield values in theregion 0.1 to 0.001, whilst silty clay or solidrock could be as low as 10-10 (Table 1).

Table 1 Typical infiltration rates

From Construction Industry Research and InformationAssociation (CIRIA) Project Report 21

viii

* Blocky fissured chalk where fissure flow is dominantExcludes putty chalk and Chalk Marl.

DESIGNDesign procedures: Hydraulic

2.3

Soil Type Typical Infiltration rate

Gravel 10-1 – 10-3 m/s

Sand 10-2 – 10-5 m/s

Silt 10-5 – 10-9 m/s

Clay < 10-9 m/s

Chalk* 10-3 – 10-5 m/s


System design

For the design of the system there are twoapproaches, either of which may be adopted:

z The Construction Industry Research andInformation Association (CIRIA) Report156 “Infiltration Drainage – Manual ofGood Practice”

ix

z BRE Digest 365 “Soakaway Design”.

A comparison of these design approachescan be found in Appendix 1.

Simplified Approximate Approach

Infiltration design to either the BRE or CIRIAmethods can be complex. Each requiresdetailed site infiltration rate informationwhich may not be available on very smallsites (e.g. a single house development).

The design parameters in Table 2 allow anestimate of the required tank size to bemade based on the area to be drained andsoil type. It assumes:

z 100% run-off

z A 1 in 100 year storm event of criticalduration

z UK Location

z Both vertical sides of structure availablefor infiltration (trench layout).

The above method is an approximationonly, and may not reflect accurately theinfiltration performance of a specific site.

For a more accurate approach, one of themore complex methods (BRE or CIRIA)should be used. These are compared inAppendix 1.

A worked example for the simple methodcan be found in Appendix 2.

Design volumes and areas

For users of the BRE 365 or CIRIA 156methods, two look-up tables showingvolumes and areas for trench or cuboidtype installations are given in this section(Tables 3 and 4).

Calculation principles

AquaCell units are 1m x 0.5m x 0.4m.Although the sides of an AquaCell box arenot totally open, in use they are wrapped in

a geotextile. This allows water to beadsorbed into contact with the wholefabric/soil interface.

Accordingly:

z The total side and base areas are used in calculations

z Storage volume is 95% of the total volume.

Linear trench configuration

Fig. 2.1 shows a typical linear trenchconfiguration.

Table 3 shows the volume and surface areaof a trench layout. These figures can bemultiplied by the trench length.

Cuboid configuration

Fig. 2.2 shows a typical cuboidconfiguration.

Table 4 gives details for some typical 3Dinstallations. These are site specific and the data is supplied as a guide to theexpected values.

DESIGN Design procedures: Hydraulic

2.3

Soil Type Maximum Impermeable Catchment No. of AquaCell units perArea per AquaCell unit (m2) 100m2 catchment area

Gravel 95.0 2

Sand 14.4 7

Chalk* 7.9 13

Silt 0.475 211

Clay Consult Wavin for project specific information

Number of units high Volume m3 Side area m2 Base area m2

1 0.19 0.8 0.5

2 0.38 1.6 0.5

3 0.57 2.4 0.5

Table 3 Volumetric data per linear m for a 1 unit (0.5m) wide trench configuration

Table 2 Design parameters for single house roof soakaway

* Blocky, fissured chalk, where fissure flow is dominant. EXCLUDES putty chalk and Chalk Marl.

Units long 2 wide (0.5m side) 4 wide (0.5m side) 8 wide (0.5m side)(1m side) Vol m3 Side m2 Base m3 Vol m3 Side m2 Base m2 Vol m3 Side m2 Base m2

1 0.76 3.2 1.0 1.52 4.8 2.0 3.04 8.0 4.0

2 1.52 4.8 2.0 3.04 6.4 4.0 6.08 9.6 8.0

4 3.04 8.0 4.0 6.08 9.6 8.0 12.16 12.8 16.0

8 6.08 14.4 8.0 12.16 16.0 16.0 24.32 19.2 32.0

10 7.60 17.6 10.0 15.20 19.2 20.0 30.40 22.4 40.0

100 76.00 161.6 100.0 152.00 163.2 200.0 304.00 166.4 400.0

Table 4 Volumetric data for 3D usage, 2 units high

Trench 40m long and 2 units deep

z Volume 0.38 x 40 = 15.2 m3

z Side areas of 1.6 x 40 = 64 m2

EXAMPLE

Fig.2.1: Typical linear trench configuration

L

W

H

Fig.2.2: Typical cuboid configuration


LW

H

Soakaways: minimum distances


2.3

Soakaways or reservoir tanks must be aMINIMUM of 5 metres from the nearestbuilding or other structures (Fig. 2.3).

Fig. 2.3 shows two alternative soakawayconfigurations: one designed using theCIRIA 156 method; the other using the BREmethod. The BRE method leads to a trenchtype layout, which can cause installationproblems on smaller constricted sites, butreduces the number of units required.

Soakaway layouts

The modular nature of the AquaCell systemmakes various trench layouts possible. Fig. 2.4 shows some suggested layouttypes (BRE-style) to allow trenches to beinstalled with greatest efficiency.

2.3.3 Attenuation

Key factors checklist

For an attenuation system, the key factorsto be determined are:

z What is the anticipated run-off volumefrom the site?

z What is the allowable discharge rate tothe appropriate outfall?

z By how much does run-off volumeexceed this limit?

z What storage volume is required to holdthe excess?

z How many AquaCell units are required toprovide this storage volume?

Fig. 2.3 Minimum distances for soakaways

Fig. 2.4 Typical BRE-style soakaway trench layouts



2.3

Calculation principles

Firstly, calculate the anticipated run-offvolume [A] from the site. This is normallybased upon a 2 hour storm of a returnperiod appropriate for the catchment (see

rainfall and run-off Section 2.3.1).

Establish the allowable discharge rate fromthe site to an appropriate outfall. This willnormally be set by the Environment Agencyor Planning Authorities.

Calculate the outflow volume [B] that canbe discharged at this rate over the twohour period (Table 5).

Subtract this allowable discharge (outflow)volume from the run-off volume [A–B]. Thisdefines the excess volume [C] which needsto be stored in AquaCell units constructedas an underground tank.

Calculate the number of AquaCell unitsneeded to contain this excess [C]. Table 6

provides a quick-reference calculator forthis purpose. The storage volume isactually equal to 95% of the externaldimensions of the tank to be created.

Outflow volume

Table 5 shows the outflow volume (in cubicmetres) for different discharge rates (litresper second) over a 2 hour storm. Thevolumes are cumulative, so volume lostfrom higher discharge rates can be easilycalculated (see EXAMPLES on the right). The required storage volume is obtained bysubtracting this figure from the calculatedrun-off total.

2.3.3.1 AquaCell system

Storage volumes

Table 6 gives storage volumes for a tankconsisting of a single layer of up to 180AquaCell units.

Volume is cumulative for each successivelayer. Accordingly, if the tank is two unitshigh, its capacity is simply double the singlelayer volume for the configuration used.

z 7 l/s outflow = 50.4 m3 volume over2 hours

z 14 l/s outflow = 7 l/s x 2 = 100.8 m3

volume over 2 hours

z 11 l/s outflow = 9 l/s + 2 l/s = 79.2 m3

volume over 2 hours

EXAMPLE

z Tank which is 4 AquaCell units wide, 6 units long, and 1 unit high (i.e. single layer)creates tank with 4.56 m3 volume

z Tank which is 4 AquaCell units wide, 6 units long, and 3 units high (i.e. triple layer)creates tank with 3 x 4.56 m3 volume = 13.68 m3

The single layer volumes have been calculated from the equation: Volume = 0.19 x number of boxes wide x number of boxes longA worked example for the simple method can be found in Appendix 2.

EXAMPLE

Discharge (l/s) Volume lost in 120 mins (m3 )

0 0.0

1 7.2

2 14.4

3 21.6

4 28.8

5 36.0

6 43.2

7 50.4

8 57.6

9 64.8

Number of units wide 1 2 3 4 5 6 7 8 9

1 0.19 0.38 0.57 0.76 0.95 1.14 1.33 1.52 1.71

2 0.38 0.76 1.14 1.52 1.90 2.28 2.66 3.04 3.42

3 0.57 1.14 1.71 2.28 2.85 3.42 3.99 4.56 5.13

4 0.76 1.52 2.28 3.04 3.80 4.56 5.32 6.08 6.84

5 0.95 1.90 2.85 3.80 4.75 5.70 6.65 7.60 8.55

6 1.14 2.28 3.42 4.56 5.70 6.84 7.98 9.12 10.26

7 1.33 2.66 3.99 5.32 6.65 7.98 9.31 10.64 11.97

8 1.52 3.04 4.56 6.08 7.60 9.12 10.64 12.16 13.68

9 1.71 3.42 5.13 6.84 8.55 10.26 11.97 13.68 15.39

10 1.90 3.80 5.70 7.60 9.50 11.40 13.30 15.20 17.10

11 2.09 4.18 6.27 8.36 10.45 12.54 14.63 16.72 18.81

12 2.28 4.56 6.84 9.12 11.40 13.68 15.96 18.24 20.52

13 2.47 4.94 7.41 9.88 12.35 14.82 17.29 19.76 22.23

14 2.66 5.32 7.98 10.64 13.30 15.96 18.62 21.28 23.94

15 2.85 5.70 8.55 11.40 14.25 17.10 19.95 22.80 25.65

16 3.04 6.08 9.12 12.16 15.20 18.24 21.28 24.32 27.36

17 3.23 6.46 9.69 12.92 16.15 19.38 22.61 25.84 29.07

18 3.42 6.84 10.26 13.68 17.10 20.52 23.94 27.36 30.78

19 3.61 7.22 10.83 14.44 18.05 21.66 25.27 28.88 32.49

20 3.80 7.60 11.40 15.20 19.00 22.80 26.60 30.40 34.20

Num

ber

of u

nits

long

Volume in m3

Table 5 Run-off volumes for a 2 hour storm

Table 6 Volume (m3) of tank created from the given number of single layer AquaCell units



2.3

Design software

Software for the design of the requirednumber of AquaCell units is available fromWavin. This enables the user to input:

z Allowable discharge from the development

z Storm duration (defaults to 2 hours).

Unit configurations are then changed untilthe required total storm flow is achieved.

Flow control

In the attenuation application, the AquaCellunits are wrapped with an impermeablegeomembrane. Water enters the tankthrough one or more inlet pipes andnormally exits through a single pipe to the outfall.

However, the outflow from the tank ofAquaCell units must be controlled tocomply with the discharge rate consentthat has been set for the site.

Outflow positioning and headcalculations

The invert level of the outflow pipe shouldbe flush with the bottom of the lowest unitto allow the tank to drain.

As the tank fills, a depth of water developson the upstream side of the outflowcontrol. For a tank which consists of 2layers of AquaCell units, this water depthwould rise to 0.8m when the units were full,creating a driving head to push the flowthrough the control device. For designpurposes, the head used in calculations istaken as that at the centre line of theoutflow device.

Flow control methods

There are four main methods to achievethis outflow control:

1. Orifice plate. A thin plate, with a smallsharp-edged hole in it, which is installedat the upstream end of a larger pipe. It isnormally positioned flush with the invertof the pipe. The hole is often circular butthis is not a requirement of the concept.

The required diameter [D] of the orifice(in millimetres) may be calculated usingthe following equation, in which Q = flowrate (m3/s) and H = head of water (m):

NOTE: This equation is valid only for small orifices withsharp edges. For the general equation from which this isderived, see Appendix 3.

The flow rate will vary with the head inthe tank, and so it is prudent to take only50% of the peak flow rate whencalculating the volumes discharged.

2. Garastor control chamber. A single sizeorifice and overflow system built into aplastic inspection chamber. This can beused with AquaCell units where very lowdischarge rates are required. See Figs. 2.7

and 2.9 for Garastor design charts.

3. Vortex controls. Devices which perform the function of orifices, but which aregeometrically designed to spin the flowthrough the hole leaving an air core.

The actual hole is much larger than the equivalent orifice, but achieves thesame degree of throttle. This means that the hole is less prone to blockage.These devices are designed to order bythe manufacturers.

4. Small pipe. For this option, the pipeline to the outfall is designed with a hydraulicgradient defined by the difference in levelbetween the top water level in the tankand the soffit of the pipe at its dischargepoint to the outfall (or into the firstmanhole, if this exists). The required pipe size is identified by using hydraulic pipe flow tables.

This option is normally only used forlarger flow rates because, for low flowsand short pipe runs, the pipe size canbecome small and hence prone toblockage by leaves and other debris thatmay enter the system.

Pipe friction will change over time, andthe flow rate will reduce, as aconsequence of the growth of a film ofbiological material on the pipe lining.

See Table 7 for comparative features and benefits of

these flow control options.

Maintenance access

For all flow control designs, it is sensible toincorporate access (via a manhole orsimilar) to the location of the pipe entry,orifice or vortex control. This will enableeasy removal of any blockage. The orificeitself may be protected by a debris screen(Fig. 2.5).

Orifice Plate Garastor Vortex-control Small Pipe

Cost low low high no extra cost

Usage any flow single low flow any flow large flow rates

Design simple design graph by manufacturer simplemethod calculations calculations

Risk of medium medium low highblockage

Adjustable fit new plate – No Purchase of new Nolow cost device – high cost

Table 7 Features and benefits of various flow control devices



2.3

Fig. 2.5 A Protected Orifice Plate

VENT COWL4S.302

PIPE BRACKET4S.082

UNEQUAL JUNCTION6UR.199

150mm OSMA ULTRARIB

SUPPORT BLOCK

110mm OSMADRAIN OVERFLOW PIPE cut to suit top water level

DEBRIS MESH

SUMP

DEBRIS MESH

WAVIN ORIFICE CONTROL UNIT

Min

. 300

mm



2.3

2.3.3.2 Garastor system

Application options

The Garastor control chamber applicationmay be designed to control attenuation inconjunction with:

z A storage void created beneath the floorof a garage (Fig. 2.6)

z A storage void (made up of AquaCellunits) under garden and driveway areas(Fig. 2.8).

Required storage volumes

The volume of the required storage facilitymay be influenced by consideration of:

z Area contributing to run-off

z Design basis in terms of length of stormreturn.

For each of the applications, an indicativechart is provided as a guide to requiredvolumes, correlating these factors at anumber of design levels (Figs. 2.7 and 2.9).

Fig. 2.6 Garastor: Typical system (garage void)

Fig. 2.7 Garastor: Design chart (garage void)

Garage void application


This chart is for indicative purposes only and is not for use in detailed design. Templates and data files, for use in detailed design of Garastor applications, are available from Wavin Plastics Limited. Curves created by WinDes drainage software (©Micro Drainage 2001).


2.3

Fig. 2.9 Garastor: Design chart (garden void)

Garden void application

2.3.4 Siltation management

Transportation of silt and debris

When stormwater passes over paved ornatural surfaces, it will pick up particles ofsand, silt and grit. These particles aretransported by the shear forces generatedby the velocity of the moving water. Theymay be carried in suspension, or moved byrolling and saltation along the surface overwhich the water is flowing.

The water will also move other debris. Thismay include leaves or rubbish which canbe transported by floating. The washing ofdebris from a surface normally happens atthe start of a storm and is often known as‘the first flush’.

Fig. 2.8 Garastor: Typical system (garden void)

This chart is for indicative purposes only and is not for use in detailed design. Templates and data files, for use in detailed design of Garastor applications, are available from Wavin Plastics Limited. Curves created by WinDes drainage software (©Micro Drainage 2001).



2.3

Build-up

If debris enters the AquaCell units (whetherin attenuation or infiltration applications),the still water within the unit will haveinsufficient velocity to keep the particlesmoving. This can lead to any of thefollowing undesirable consequences:

z Debris will be deposited in the AquaCellunits around the pipe entry

z Some of the void intended for waterstorage will begin to fill up

z Organic matter may start to decay

z Noxious gases may build up.

Prevention

To prevent this, it is recommended that asilt trap is planned for incorporation into thepipework at the inlet to the tank (Fig. 2.10).

To be effective, there must be amaintenance plan that ensures regularcleaning of the silt trap. Otherwise, if thetrap is full, any additional debris will simplypass into the tank.

Off-line construction

To reduce the risk of siltation further,AquaCell tanks may be constructed off-line.

In this case, the throttle arrangement forthe allowable discharge is located in amanhole chamber on the main surfacewater run-off pipe. This means that lowflows pass straight through the throttle andnever enter the tank. Much of ‘the firstflush’ debris is transported direct to theoutfall in this way.

When the flow exceeds the capacity of thethrottle, depths build up in the manhole andwater backs up into the tank. Heavy debristravelling along the invert of the main pipewill not be carried into the tank. In specialcircumstances scum boards can be usedto prevent the transfer of floating material.

2.3.5 Manifold design

Connection into an AquaCell unit is madeby removing a 150mm diameter break-outsection, allowing free discharge into the cell.

The capacity of this input pipe is limitedand may be insufficient for the anticipatedflow load. The flow load may therefore besplit between a number of 150mm inflowpipes from the adjacent manhole (Fig. 2.11).

Fig. 2.10 Typical silt trap design

Fig. 2.11 Typical manifold design

COVER & FRAME1.2m deep use 6D.935

Greater than 1.2m deep use 6D.939 with reduced opening of 350mm

SHAFT 500mm

150mm OSMA ULTRARIB

150mm OSMA ULTRARIB

WAVIN AQUACELL UNITS

150mm OSMA ULTRARIB

300mm OSMA ULTRARIB

750mm

1200mm

150mm side fill as dugmaterial with no particlesizes larger than 40mm

100mm min covering layer

100mm bed as per pipe bedding specification



2.3

Table 8 shows the maximum areas whichcan be drained according to the number ofinflow pipes provided. This has beencalculated to the following assumptions:

z Paved surfaces: 2 year, 3-5 minute event

z Eaves drained roofs: 1 year, 2 minuteevent

z Internal gutters: 500 year, 2 minute event.

2.3.6 Venting

The venting of air from box storagesystems is an important mechanism.

However, the diameter of these outlet pipesdoes not need to match that of the inletpipes because:

z The rapid inflow from a peak storm eventinto the storage void will be of shortduration and will fill only a small proportionof the volume required for a longer event

Number of Inlet PipesSurface type 1 2 3 4 5 6

Paved Area 1110 2220 3330 4440 5550 6660

Roof Area* 841 1682 2523 3364 4205 5046

Roof Area** 210 420 630 840 1050 1260

Drainable area in m2

* Roofs drained by eaves gutters, close (within 25m) to the attenuation site

** Roofs drained by internal gutters, close (within 25m) to the attenuation site (especially siphonic roof drainage)

Table 8 Multi-pipe Manifolds

z Air is much more compressible thanwater, so only part of the inflow volumeneeds to be vented during the stormevent

z A longer-term event which could fill thestorage void will flow in much more slowly,and will not require large venting solutions

z Air can flow in pipes with less resistancethan water, and so a similar size pipe willconvey more air than it would water.

Accordingly, It is therefore recommendedthat one vent pipe, 110mm in diameter, isprovided per 7,500 square metres ofimpermeable catchment area on a site.

VERMIN COWL4S.302

VENTILATION BOX4D.706

Fig. 2.12 Typical air vent design


FLANGE CONNECTOR6LB.104

150MM X 110MM REDUCER6UR.099

GEOMEMBRANE SEALED TO FLANGE

110mm OSMADRAIN PIPE

DESIGNDesign procedures: Structural

2.4

2.4.1 Structural testing

The independent testing of AquaCell unitsat Salford University included thedetermination of their structural capacity.Direct loading and long term creep testswere carried out on single units.

The results from these tests have beenused to generate the following structuraldesign parameters:

z Direct loading

z Deformation.NOTE: Detailed test methodology used in these independenttests is described and explained in Appendix 4.

Direct loading

Typical results are shown in Fig. 2.13.

Design parameters for the AquaCell units,as determined from the test results, aregiven in Table 9.

Deformation

Typical results are shown in Fig. 2.14. Fromthese, a long-term rate of deflection can bedetermined and long-term deformations forperiods up to 20 years estimated

x.

2.4.1.1 AquaCell: domestic soakaways

For small scale applications as soakawaysfor individual house roof drainage, theAquaCell system is typically located belowa garden (5m minimum from the building).In this situation there are no traffic loads.

Use Table 10 to determine design depths.

2.4.1.2 AquaCell: large scalestorage or infiltration

AquaCell units used for large scale storageor infiltration must be designed to carry allthe loads that will be applied. These loadswill include:

z Dead loads

z Imposed loads.

Fig. 2.13 Typical loading test results: vertical

Fig. 2.14 Typical creep test results

Vertical loading on top Lateral loading on sideface face

Ultimate compressive 560kN/m2 77.5kN/m2

strength at yield

Short term deflection 1mm per 97.0kN/m2 1mm per 7.0kN/m2

applied load applied load

Long term deflection Deflection (mm) = at up to 10 years, 20ºC 0.4705Ln (time in hours)10kN load

Table 9 Loading design parameters for AquaCell units

NOTE: Partial factors of safety as appropriate should be applied to these values for design (see Appendix 4, Section 3)

Maximum depth to base of units* 2.95m

Minimum cover depth cover units (to prevent accidental damage) 0.5m

Table 10 Design criteria for use of AquaCell system as soakaway for individual house

*Assumes a minimum value for the angle of shearing resistance of the surrounding soil of 29º. This should be confirmed from theresults of the site investigation. Groundwater must be at least 1m below base of units. No traffic loads.


DESIGN Design procedures: Structural

2.4

Dead loads

Dead loads are permanent loads applied tothe units. They include:

z The weight of fill placed over the top

z Lateral earth pressure loads acting onthe side of the system.

These factors determine the maximum depthof installation and maximum cover depth.

Imposed loads

Imposed loads are transient loads due tovehicle or pedestrian traffic andconstruction traffic.

There must be sufficient cover fill to allowdistribution of point loads from wheels andthus prevent localised crushing of the units.The spread of load depends on the type ofmaterials overlying the units.

For well compacted type 1 sub-base, aload spread of 45º may be appropriate.

However, for poorly compacted selectedas dug fill, a distribution of 27º is moretypical.

Design limits

Use the design parameters and estimatedloads to determine:

z The maximum depth of installation

z Maximum and minimum cover depths

for AquaCell units.

The criteria provided in Tables 11 and 12 canbe used to design the AquaCell units forinstallation below lightly trafficked and non-trafficked areas. These design tables areonly applicable in temperate climateconditions such as the UK.

Full scale field trials of the AquaCell systemhave been undertaken in the UnitedKingdom which have confirmed the designapproach adopted and the performance ofthe AquaCell units.

Heavier loads

The AquaCell system can be used wheregreater loads are anticipated. These maybe areas trafficked by commercial andheavy goods vehicles, and includes allvehicles in excess of 2500kg gross weight.However, specific design advice should besought from Wavin for these situations.

Design information, required to carry out acomprehensive design for thesecircumstances, will include:

z Type of vehicle

z Maximum anticipated vehicle weight.

Maximum depth of installation (to base of units)

Typical soil type Typical angle With groundwater at 1m below Without groundwater (below baseof shearing ground level and units wrapped of units) – normal case

resistance, φ in geomembrane

Trafficked area Non-trafficked Trafficked area Non-trafficked(cars only) (cars only)

Stiff over consolidated 24º 1.65m 1.75m 2.35m 2.50mclay, eg London Clay

Normally consolidated 26º 1.70m 1.80m 2.50m 2.65msilty sandy clay, eg Alluvium, Made Ground

Loose sand 29º 1.80m 1.90m 2.85m 2.95mand gravel

Medium dense 33º 1.90m 2.00m 3.30m 3.45msand and gravel

Dense sand 38º 2.05m 2.15m 4.10m 4.25mand gravel

Table 11 Maximum installation depths (to base of units)

NOTES1. Horizontal ground surface is assumed2. Loosening of dense sand or softening of clay by water can occur during installation. The designer should allow for any such likely effects when choosing an appropriate value of φ.3. It is assumed that no shear planes or other weaknesses within the structure of the soil are present.4. The design is very sensitive to small changes in the assumed value of φ. The φ value assumed should therefore be confirmed by a chartered geotechnical engineer. In clay soils it may be possible to

utilise cohesion in some cases.5. Design table is only applicable for car parks or other areas trafficked only by cars or occasional refuse collection trucks or similar vehicles (typically one per week).

Location Minimum cover depth

Non-trafficked areas, eg landscaping 0.50m

Car parks, vehicle up to 2500kg gross mass, 0.60mAquaCell system up to three units wide in trench

Car parks, vehicle up to 2500kg gross mass, AquaCell 0.75msystem greater than three units wide

Table 12 Minimum cover depths over top of AquaCell units

Assumes 27º load distribution through fill material and overlying surface of asphalt or block paving, and trafficking by occasionalrefuse collection trucks or similar vehicles (typically one per week) is acceptable.



2.4

z Type of road or use (e.g. residential road,lorry park, industrial unit)

z Anticipated number of vehiclemovements

z Type of surfacing (e.g. block paving,asphalt or concrete)

z Soil conditions.

Flotation

When the units are wrapped ingeomembrane and placed below thegroundwater table, flotation may occur.

To prevent this, the weight of the soil overthe top of the units must be greater thanthe uplift force caused by the units’buoyancy in the water. This can beachieved with most fill types if the depth ofcover fill is equal to, or greater than, thedepth of penetration of the units below thegroundwater level.

Construction loads

Construction plant such as excavators canimpose significant loads on the AquaCellsystem. The following guidelines should be observed:

z Tracked excavators (not exceeding 21tonnes weight) should be used to placefill over the AquaCell units when thegeotextile or geomembrane wrappinghas been completed (see Section 2.4.2)

z At least 300mm of fill should be placedbefore the excavators or trucks deliveringthe backfill are allowed to traffic over theinstalled units

z Compaction plant used over theAquaCell units should not exceed2300kg/metre width. This will allow the compaction of Type 1 sub-base in 150mm layers over the units inaccordance with the Specification for Highway Works

xi

z All other construction plant should be prevented from trafficking over thesystem once it is installed and surfacingcompleted, unless a site specificassessment demonstrates that it is acceptable

z In particular cranes should not be used over, or place their outriggers over,the system.

Bearing capacity load and settlement

For lightly loaded applications (as detailedin this Manual) the bearing capacity of theunderlying soils should not typically beexceeded by the AquaCell System.Settlement of the underlying soils shouldtherefore be negligible.

However, on weak or compressible soils,such as very soft clay and silt or peat, thebearing capacity and settlementcharacteristics should be confirmed by ageotechnical engineer.

Infiltration below trafficked areas

Care should be taken when the AquaCellsystem is used for infiltration belowtrafficked areas and close to structures. It isimportant to ensure that the infiltratingwater will not soften the soils or cause lossof fines – and therefore cause settlement.

2.4.1.3 Garastor system:garage void

Structural design

When the Garastor system utilises asuspended floor slab to form a void belowgarages to store stormwater, thesuspended slab and associated walls andfoundations will require structural design.

This should be in accordance with therelevant design codes and currentprevailing Building Regulations.

Lining of void

To prevent softening of the soils below thefoundations or loss of fines leading tosettlement, the void beneath the garageshould be lined as follows:

1. Underneath the concrete base of thevoid area there should be a 1200gpolythene damp proof membrane.

2. Bitumastic paint should be applied to the walls of the void area and to theunderside of the reinforced garage floor beams.

2.4.1.4 Garastor system: garden void

This application involves the GarastorControl Chamber connected to AquaCellunits. Accordingly, structural design shouldfollow guidance given for AquaCell (see Section 2.4.1.1 and Section 2.4.1.2) as relevant to the proposed location,anticipated loading and required capacityof the storage void.

2.4.2 Geotextiles andgeomembranes

Geotextiles: function

A geotextile is wrapped around the AquaCellsystem in infiltration applications. Thisfiltration layer has the following functions:

z Preventing clogging of the soil whichsurrounds the units with silt that is present in run-off

z Preventing soil entering the units

z Protection around geomembrane instorage applications.

Geotextiles: properties andperformance

The physical properties and performance ofavailable geotextiles vary widely. There canalso be marked differences in such attributesas UV resistance, durability and robustnessduring installation.

Accordingly, the selection of an appropriategeotextile for a specific AquaCell infiltrationinstallation deserves very carefulconsideration. This should be done withparticular reference to the surrounding soilproperties and required performance.

Important aspects to consider are:

z Pore size: should be designed andspecified to assist infiltration and preventmigration of fine soil particles

z Permeability and breakthrough head: thegeotextile should not limit flow of water inthe system, and should have a similar orgreater permeability than the surroundingmaterials

z Puncture resistance: the geotextile mustbe able to resist the punching stressescaused by loading on sharp points ofcontact

z Tensile strength: the geotextile shouldhave sufficient strength to resist theimposed forces from traffic or otherloading.

Geotextile should be selected according tospecific site conditions. However, typically a300g non-woven material will be suitable formost situations. Unless either thesurrounding soil characteristics exhibit a highdegree of fines /low infiltration capacityand/or there is risk of damage from in-ground contaminants, when specialistadvice should be sought.


DESIGN Design procedures: Structural

2.4

Geomembranes: function

A geomembrane is wrapped around theAquaCell system in attenuation/storageapplications where infiltration is notpossible or permitted. This impermeablelayer can have the following functions:

z Preventing release of attenuated/storedwater to surrounding ground

z Preventing inflow of pollutants fromcontam- inated subsoil into the storagereservoir

z Lining of Garastor system storage voidbeneath a garage.

Geomembranes: properties andperformance

The importance of specification andselection of the right impermeablegeomembrane for AquaCell installationscannot be understated. Its integrity andsuitability are key to successfulperformance as required.

It is crucial that the specified material will:

z Withstand the rigours of installation

z Resist puncture

z Resist multi-axial elongation stress andstrains associated with settlement

z Resist environmental stress cracking

z Resist damage from in-groundcontaminants

z Remain intact for the full design life of theAquaCell installation.

Geomembranes less than 1mm in thicknessare unlikely to meet these criteria, and are notrecommended for use with the AquaCellsystem.

Geomembranes: sealing

The long term integrity of the joints isequally as critical as the selection of thegeomembrane. To ensure totalimpermeability:

z Joints between adjacent sheets ofimpermeable geomembranes should be sealed correctly using proprietarywelding techniques

IMPORTANT NOTE: Jointing with tape is not recommendedas this places reliance on the mechanical properties of thetape to maintain the integrity of the system.

z The integrity of joints should bedemonstrated by non-destructive testing.

Advice on seam testing is given in CIRIAReport 124

xii.

2.4.3 Backfill and beddingspecification andplacement

Non-trafficked areas

In areas where the AquaCell system is notsubject to traffic loads, such as belowpedestrian areas, gardens or landscaping,the system can be backfilled with selectedas dug material.

This material should NOT contain:

z Pieces of rock or gravel above 75mm in diameter

z Any other sharp materials (e.g. metal)which could pierce the units, or causeuneven loading and cause them to fail.

Selected fill material should be compactedto 90% maximum dry density. A minimumof 300mm working space will be requiredaround the units to allow compaction.

Trafficked areas

When the AquaCell system is installedbelow car parks and other trafficked areas,the use of well compacted backfill andcover fill is particularly important.

In these areas, a well compacted granularmaterial should be used:

z Recommendation: Type 1 or Type 2 sub-base as defined in the Specificationfor Highway Works (see Table 14)

For backfill to the sides of the AquaCellsystem:

z Recommendation: Type 1 or Type 2 sub-base, or Type 6P (side fill only)selected granular material.

The material should be compacted inaccordance with Table 6/1 or 8/1 of theSpecification for Highway Works and takinginto account the recommended maximumroller weight (see Construction loads 2.4.1.2).

The precise material used will depend onthe overlying pavement design and, ifappropriate, the requirements of theadopting authority.

Percentages passing by mass

Grading Sieve size Sub base Type Sub base Type Type 6P selected1 2 granular fill

75mm 100 100 100

37.5mm 85–100 85–100 –

20mm 60–100 60–100 –

10mm 40–70 40–100 –

5mm 25–45 25–85 –

600 micron 8–22 8–45 –

75 micron 0–10 0–10 –

63 micron – – <15

Other requirements

Ten percent fines value 50kN 50kN 30kN(BS 812: Part 111)

Soundness value >65 >65 n/a(BS 812: Part 121)

Material passing 425 Non-plastic Plasticity index n/amicron sieve <6

Uniformity coefficient n/a n/a >5

Table 14 Backfill specification for use under trafficked areas



2.4

Preventing uneven loads

For placement between the AquaCell unitsand the backfill:

z Recommendation: 100mm of coarsesand or a geotextile protective fleece.NOTE: this is NOT the same type as that required forinfiltration in AquaCell soakaway applications, as describedin 2.4.1.1

This is designed to prevent angular materialsimposing uneven loads that could causefailure of the units.

Typical backfill details are shown in Fig. 2.15.

Specification sheets for the WavinStormwater Management Systems are provided in Appendix 5.

Asphalt

Sub-base

min 0.6m trench cover for up to 3 units wide.0.75m cover for greater widths. Type 1 orType 2 sub-base or Class 6P (side fill only) selected granular backfill

For storage applications the AquaCell units are wrapped in geomembrane (impermeable) and then wrapped in a protective geotextile (as shown). For soakaway applications the AquaCell units are wrapped in a geotextile (permeable) only

Excavation trimmed to smooth level base and sides battered back at safe slope angle

Pre-formed socket

Coarse sand or non angular granular material base

Suitable impermeable geomembrane

Suitable protective geotextile layer

AquaCell System

Fig. 2.15 Backfill details

DESIGN Specification

2.5


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