CIRI uD anua 2015 · CIRI uD anua 2015 25FILTRATION: DESIGN METHODS IN Contents 25.1 General...

543Chapter 25: Infiltration: design methods542 Part E: Supporting guidance

CIRIA SuDS Manual 2015

25 INFILTRATION: DESIGN METHODS

Contents

25.1 General concepts 54325.2 Evaluating potential constraints to the use of infiltration 54425.3 Infiltration testing methods 54925.4 The impacts of siltation on infiltration system performance 55025.5 Reuse of existing soakaways 55125.6 Infiltration system hydraulic design 55125.7 Emptying time checks 55725.8 References 558

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25Chapter

25.1 GENERAL CONCEPTS

Infiltration systems allow surface water runoff to infiltrate into the ground over a period of time, thus reducing the volume of runoff during a rainfall event. Infiltration systems can deliver Interception for the upstream contributing catchment surface (Section 24.8) and can also help reduce the attenuation storage volume requirements required for the site (Section 24.9).

The use of infiltration to dispose of surface water runoff also has a number of other important benefits:

▪ It can help replenish aquifers local to the site through deep infiltration and/or act to support local river base flows and wetland systems via shallow infiltration processes.

▪ It can help support local soil moisture levels and vegetation. In urban areas this may reduce the adverse effects that trees can have on foundations by reducing the potential for shrinkage of soils.

For a soil to be suitable for infiltrating design runoff events, it should be:

▪ permeable, and

▪ unsaturated.

Also, it should be of sufficient thickness and extent to disperse the water effectively. Figure 25.1 is a schematic of a typical infiltration system.

There are a number of constraints to the use of infiltration. These should be fully evaluated for any site and any potential infiltration location, to ensure that risks are minimised (Section 25.2). The rate at which infiltration might occur – together with the design standard of service of the system and the contributing catchment area – will influence the area of the infiltration surface and the volume of temporary storage required. Guidance on establishing appropriate infiltration rates is set out in Sections 25.2 and 25.3.

For further detail on the hydraulic design of infiltration systems, see Bettess (1996).

Infiltration: design methodsThis chapter provides guidance on the suitability of using infiltration to dispose of surface water runoff, infiltration testing and design methods.

Water quantity design crieria are set out in Chapter 3.

Requirements for water quality management for groundwater protection are set out in Chapter 4.

Hydrology and hydraulic design methods and calculations are presented in Chapter 24.

Guidance on component sizing for water quantity and water quality management is provided in Chapters 11–23.

Methods for meeting water quality management requirements are presented in Chapter 26.



25.2 EVALUATING POTENTIAL CONSTRAINTS TO THE USE OF INFILTRATION

The following considerations should be fully evaluated before determining the extent to which infiltration can be used on a site:

▪ soil type and infiltration capacity

▪ groundwater level beneath the site

▪ risk of ground instability, subsidence or heave due to infiltration

▪ risk of slope instability or solifluction (the slow creep of saturated soils down slopes) due to infiltration

▪ risk of pollution from mobilising existing contaminants on the site due to infiltration

▪ risk of pollution from infiltrating polluted surface water runoff

▪ risk of groundwater flooding due to infiltration

▪ risk of groundwater leakage into the combined sewer due to infiltration

Each of these is discussed in more detail in the following subsections.

Infiltration surfaces can be at or near the ground surface and spread over a wide area (eg basin), or at a point location (eg soakaway). The risks posed by infiltrating water to nearby structures and slopes or groundwater become greater the higher the ratio of the contributing catchment area to infiltration surface area. For example, large-volume deep soakaways are more likely to cause adverse effects than small shallow basins or infiltration from pervious surfaces.

Preliminary information on whether a site may be suitable for infiltration can be obtained from:

▪ existing geological and hydrogeological studies and mapping for the site

▪ geohazard mapping (eg the British Geological Survey infiltration SuDS map (BGS, 2015))

▪ records of potential contamination at or beneath the site

▪ borehole records or groundwater observations relevant to the site

▪ aquifer designations at or near the site.

Infiltration on sites where there is storage of potential pollutants (eg industrial sites with chemical storage) is likely to require an environmental permit. The acceptability of infiltration at any site and the design of risk mitigation measures is set out in Chapter 4, Table 4.3 and Chapter 26.

Figure 25.1 Typical surface water management infiltration system



25.2.1 Infiltration capacity of the soils

The rate at which infiltration occurs depends on the properties of the soils and the underlying geology through which the water is discharged. The capacity of the soil to infiltrate water is given by the infiltration coefficient. This is the long-term infiltration rate into the soil divided by the area of infiltration. The infiltration rate is related to a soil’s permeability.

The permeability of a saturated soil, k, is its ability to transmit fluid under a hydraulic pressure gradient. It is often called the coefficient of permeability or hydraulic conductivity. Darcy’s law defines the flow per unit area under saturated conditions. Infiltration of water into soil above the water table will most likely be into partially saturated soils where the relationship between soil properties and flow is far more complex and is described by non-linear differential equations. Because of the difficulty in solving the equations, an empirical constant infiltration rate or coefficient, q, is used (derived from infiltration tests).

Where water is free-draining vertically in an unsaturated soil with a reasonably steady flow system it is reasonable to assume a unit hydraulic gradient (Watkins, 1995). Under these conditions the infiltration rate, q, is numerically equivalent to the soil coefficient of permeability, k.

Permeability or infiltration rate will be high for coarse-grained soils such as sands and gravels and low for fine soils such as silts and clays. However, it should be noted that in the UK, sand and gravel deposits often have a high silt or clay content, which will reduce their infiltration rate significantly.

Table 25.1 gives typical infiltration coefficients for different soil textures. Soil textures are defined by the proportion of different-sized particles as shown in Figure 25.2.

Figure 25.2 Soil texture classification (from LandIS, 2015)



Therefore, field tests should always be undertaken in order to determine infiltration coefficients for design purposes. Infiltration testing is described in Section 25.3. Any testing should be as extensive as possible and supported by evidence of wider soil characteristics, in order to avoid misrepresentation of relevant soil properties. Figure 25.3 illustrates an example of where local testing may not adequately characterise the soil horizons.

If infiltration is proposed at conceptual design stage and there are no infiltration test results available, alternative proposals for discharge should be provided. This will ensure that if infiltration tests show it is not possible, the site can still be effectively drained.

Infiltration viability should be given full consideration where rates of 10−6 m/s or greater exist on the site (subject to geotechnical and contamination considerations). Where rates are less than that, the soils can still usefully be used for Interception delivery, but disposal of significant volumes of runoff may not be cost-effective or appropriate, unless there is a large area of land available for this purpose.

It should be noted that Interception does not necessarily require any infiltration capacity, as it can also be delivered via green roofs, bioretention systems etc.

TABLE25.1

Typical infiltration coefficients based on soil texture (after Bettess, 1996)

Soil type/texture ISO 14688-1 description (after Blake, 2010)

Typical infiltration coefficients (m/s)

Good infiltration media ▪ gravel ▪ sand ▪ loamy sand ▪ sandy loam

Sandy GRAVELSlightly silty slightly clayey SANDSilty slightly clayey SANDSilty clayey SAND

3 × 10−4 – 3 × 10−2

1 × 10−5 – 5 × 10−5

1 × 10−4 – 3 × 10−5

1 × 10−7 – 1 × 10−5

Poor infiltration media ▪ loam ▪ silt loam ▪ chalk (structureless) ▪ sandy clay loam

Very silty clayey SANDVery sandy clayey SILTN/AVery clayey silty SAND

1 × 10−7 – 5 × 10−6

1 × 10−7 – 1 × 10−5

3 × 10−8 – 3 × 10−6

3 × 10−10 – 3 × 10−7

Very poor infiltration media ▪ silty clay loam ▪ clay ▪ till

––Can be any texture of soil described above

1 × 10−8 – 1 × 10−6

< 3 × 10−8

3 × 10−9 – 3 × 10−6

Other ▪ rock* (note mass infiltration capacity will

depend on the type of rock and the extent and nature of discontinuities and any infill)

N/A 3 × 10−9 – 3 × 10−5

This classification is different from the description systems used by geotechnical engineers in site investigation reports, which should follow BS ISO 14688-1:2002. Further information on the properties of soil that are important in infiltration design are provided by Blake (2010). This paper also discusses how increased groundwater levels can reduce the infiltration rate of soils and provides a reduction coefficient to allow for this effect in design.

The figures in Table 25.1 provide a useful first indicator of the magnitude of the infiltration capacity, but the large ranges reported illustrate the significant influence of factors such as soil packing, soil structure, swelling clay content and the presence of bedding, jointing or other fissures in rock. Also, construction activities can severely affect infiltration rates if care is not taken to protect against compaction or blockage from fines. In some cases, infiltration of water into rock material can cause a reduction of infiltration capacity with time as the rock weathers. This is especially important with some fractured mudstones where initial high infiltration rates soon reduce as the rock softening and joints become infilled.



Figure 25.3 Local soil testing misrepresenting wider soil properties

25.2.2 Groundwater level

Groundwater levels should be investigated to ensure that the base of the proposed infiltration component is at least 1 m above the maximum anticipated groundwater level (taking account of seasonal variations in levels and any underlying trends). This should include assessment of relevant groundwater/borehole records, maps and on-site monitoring in wells. Guidance on the design of SuDS in areas with high groundwater is set out in Section 8.3. A 1 m separation distance ensures a depth of unsaturated soils to help ensure the infiltration performance of the component and protect underlying groundwater from contamination (Section 25.2.5).

25.2.3 Geohazards

Geotechnical advice should be taken and geotechnical properties of surrounding soils should be checked to ensure that the infiltration of water will not pose an unacceptable risk to the site and/or local area. It should be established that infiltration will not cause significant risk of instability (eg of retaining walls, slopes, solution features or loosely consolidated fill) or movement that could adversely affect any nearby buildings or other structures.

The potential risk of adverse effects from infiltrating water will depend on the volume of water being discharged along with the depth and plan area of the infiltration system. The smaller the area of the system in relation to the drained area, the greater the risk.

A geotechnical investigation is likely to be required to ensure that the ground conditions are suitable and to check the likely performance of the infiltration component. Figure 25.4 provides a decision tree for the assessment of geohazards for infiltration system. Further guidance on the design of SuDS in areas with unstable soils or backfill is set out in Sections 8.6 and 8.7.

Where infiltration is proposed closer than 5 m to the foundations of buildings or structures (except for a permeable pavement that does not take any extra impermeable catchment such as the roof) this assessment should be approved by a suitably qualified professional such as a registered ground engineering adviser. The BGS infiltration SuDS map (BGS, 2015) is a useful source of information. Advice on small-scale infiltration closer than 5 m to buildings is provided on www.susdrain.org

Infiltration near slopes also requires careful assessment of the impact of the moisture on slope stability. Over time the infiltration can cause an increase in moisture content of the soils below the slope and lead to instability. This is especially important near to slopes that are marginally stable. Such slopes can include manmade slopes (eg cuttings or embankments) or slopes that are or have been subject to solifluction. Some local authorities have solifluction maps showing areas where this may be an issue.



Guidance on the design of SuDS on sloping sites is set out in Section 8.4. Further information on geotechnical issues relating to infiltration can be found in Bettess (1996).

25.2.4 Site contamination

Infiltration can be used on many (but not all) contaminated sites. However, caution should be exercised when proposals include using infiltration methods on contaminated sites because they have the potential to cause pollution if the system is not carefully designed or managed. New pathways for pollutants to groundwater must not be created nor must contaminants be mobilised. Guidance on the design of SuDS for contaminated land sites is set out in Section 8.2.

An assessment of the potential for deterioration in groundwater quality due to infiltration (eg due to the mobilisation of contamination) should be undertaken before detailed design. This should consider the spatial and vertical distribution of contamination in relation to the location of infiltration devices and also the nature of the contaminants and whether they are mobile. Details of any remediation or contamination “sealing” strategies – either previously undertaken or proposed as part of the development site design – should be carefully evaluated.

All contamination evaluation assessments should be undertaken by a qualified geo-environmental engineer or similarly qualified person and may require a site investigation with contamination testing. The BGS infiltration SuDS map (BGS, 2015) can provide useful preliminary information.

Figure 25.4 Decision guide for the use of infiltration systems



25.2.5 Groundwater pollution

If the surface water runoff is polluted, there is a risk that infiltration systems may introduce pollutants into the soil and ultimately into the groundwater. Checks should be undertaken to confirm that the soils beneath any proposed infiltration component are suitable to provide adequate protection to the underlying groundwater. The SuDS design should also ensure adequate treatment of the runoff before infiltration. Requirements for water quality management for groundwater protection are set out in Chapter 4. Methods for meeting these requirements are set out in Chapter 26.

Any requirements for environmental permits should be checked before conceptual design stage (Chapter 7).

25.2.6 Groundwater flooding

An assessment should be undertaken of the potential effect of infiltration on groundwater levels local to any infiltration component and the potential wider impact of multiple infiltration components within the site, with respect to groundwater flood risk. The use of infiltration for steep sites can increase the risk of springs developing lower down the slope in layered geology/steep topography.

25.2.7 Groundwater/combined sewer interaction

An assessment should be completed of the risk of groundwater leakage into any local foul or combined sewers owing to introducing infiltration drainage. The risk of water infiltrating to a sewer will depend on the area of the base of the infiltration system compared to the catchment – for example, infiltration over the wide area of a pervious pavement that is only managing water that falls directly on it will be a low risk. Other factors to consider are the depth of soil between the sewer and the base of the infiltration device, horizontal separation and the age and likely condition of the sewer.

25.3 INFILTRATION TESTING METHODS

Infiltration tests should be carried out in accordance with Bettess (1996), which is based on the design approach in BRE (1991). The test measures the rate at which water soaks away from the test pit and gives an infiltration rate in m/s or m/h. It is important that the test is carried out in accordance with the report and that the test pit is filled three times. Repeating the test in this way can reduce the measured infiltration rate by at least half an order of magnitude each time the test is repeated, and is likely to reflect realistic event conditions as shown by the example in Figure 25.5.

Figure 25.5 Example of reduction in infiltration rate with successive tests



In some cases it may not be possible to carry out tests in trial pits due to depth or access constraints. In this case, tests can be carried out in boreholes. The tests should follow the procedure in accordance with BS EN ISO 22282-2:2012. Falling head tests should still be repeated at least three times, as required in BRE (1991). Care should also be taken in the interpretation of the results, as a smaller volume of water is entering the ground during the test. Ideally, falling head tests should be repeated as many times as possible to increase the volume of water entering the ground.

One of the main risks to soakaway performance is inadequate infiltration testing, because of either time constraints at the planning stage or cost. If the water level in a test does not drop sufficiently quickly to do three tests in a day, it indicates low infiltration capacity and potential risks for long-term performance. However, the tests can be extended over two days using water-level loggers. If pits are left open overnight with water in them, then health and safety issues need to be addressed and as a minimum the areas will need to be securely fenced off.

The results of incomplete tests should not be extrapolated to obtain design values of infiltration rates. The head of water in the infiltration test should fall to less than 25% of the initial head of water. If this does not occur, the results should state that the infiltration rate cannot be determined. If other variations to the test method in Bettess (1996) are required, the test results sheet should clearly state what variations have been made to the test and why.

It is rare that sufficient tests are carried out on a site to allow statistical analysis. The worse-case infiltration rate value should be used (not the mean or any other value) unless a sound justification for doing otherwise is demonstrated.

Infiltration test results should always be provided together with trial pit records that include soil/rock descriptions of the materials in which the test has been completed in accordance with BS EN ISO 14688-1:2002 or BS EN ISO 14689-1:2003. The interpretation of the test results should be compared to the soil descriptions and any unusually high or low values assessed against the conceptual site ground model, and then confirmation should be provided that the measured infiltration rates are representative of the wider ground mass (eg the test has not been undertaken in a limited extent of sand within a mass of clay). The likely impact of water on the soil and the long-term infiltration rate should also be assessed.

The infiltration tests should be carried out at the location, depth and head of water that replicates the proposed design. For larger systems, the tests should provide sufficient coverage across the entire area to be occupied by the infiltration system. The test results sheet should state which stratum the results are appropriate to and any limitations in the test – for example, has the infiltration rate been estimated by assuming water only infiltrates into one particular stratum, such as a discrete layer of limestone?

25.4 THE IMPACTS OF SILTATION ON INFILTRATION SYSTEM PERFORMANCE

The soil surrounding an infiltration system can become blinded through ingress of silt, and the infiltration capacity reduced as a result. All infiltration system designs should therefore include appropriate pre-treatment (ie silt/sediment removal systems). Larger systems should also be monitored to check the extent or effects of long-term silt deposition if thought necessary – for example, by providing a monitoring well within the infiltration system that can be used to monitor the drop in water level after a rainfall event, or by providing access for visual inspection. Some blinding can be simply removed if there is access to the infiltration surface, but in other cases (eg deep soakaways) blinding can render the system useless over time. Even with upstream sediment protection it is likely that some silt will always collect in an infiltration device. The risk posed by silt depends on the relative difference in permeability between the silt and the surrounding soil, and on the design method used.

If the surrounding soil has similar permeability to the silt then there will be little effect. Also, if the design is based on the guidance in BRE (1991) then infiltration from the base is ignored and so any silt will also have negligible effect on infiltration rates (Figure 25.6).



There are many thousands of soakaways in the UK that have been working for over thirty years with no maintenance. This is probably because, in many cases, the infiltration capacity of the soil is relatively low in comparison with the accumulated silt, and so the reduction in capacity is marginal (Wilson and DeRosa, 2006).

The effects of siltation will be more noticeable when the infiltration rate of the soil is high in relation to that of the silt. Based on typical quoted gradings for silt, it is likely to have a permeability of around 1 × 10−6 m/s and, since many soakaways are designed using infiltration rates of around 1 × 10−5 to 1 × 10−6 m/s, the effects of the silt will be low. This points to the fact that care should be taken when assuming infiltration rates for design that are greater than 1 × 10−5 m/s, because, above this, the impact of silt on performance becomes more significant, especially where infiltration from the bottom of a systems is assumed (ie the design method in Bettess, 1996).

25.5 REUSE OF EXISTING SOAKAWAYS

Where sites are being redeveloped or extended, it may be possible to reuse existing soakaways. An approach for testing and assessing the capacity of existing soakaways is described by Chen et al (2008).

This method can be used to measure the performance and capacity of existing systems and examine whether the systems are suitable for reuse when design and construction details of the system are not available. Requirements for field observations and a procedure for a modified soil infiltration test performed within the system are proposed. The system’s working condition is measured by a performance indicator related to the time taken to empty the soakaway. This is then employed to evaluate the potential reuse capacity of the system.

25.6 INFILTRATION SYSTEM HYDRAULIC DESIGN

In most circumstances, the area over which infiltration is proposed will be considerably smaller than the impermeable area being drained. Except for the most permeable of soils, the inflow rate to the infiltration system will exceed the outflow rate (the product of the infiltration coefficient of the soil and the infiltration area). It is therefore necessary to store the water on site or in the infiltration unit to allow time for it to soak away.

Provision of sufficient storage capacity is essential for an infiltration system to meet the design standard of service. The purpose of hydraulic design is to select dimensions for the system that are sufficient to store and infiltrate the runoff from the design storm. Overflows or additional discharge points should be provided, if total infiltration cannot be relied on for all return period events, and exceedance flow management should always be considered (Section 24.12).

Figure 25.6 Effect of silt on performance of infiltration



The hydraulic design procedure is set out in Figure 25.7.

Infiltration devices are commonly designed for return periods up to 1:100 year, plus an allowance for climate change. Advice on suitable return periods for specific components is provided in the relevant technical component chapters.

One of the largest uncertainties in the design of infiltration systems is the infiltration coefficient, as this may reduce over time, particularly if effective pre-treatment is not included within the design, and/or system maintenance is poor. To account for this, a factor of safety is introduced into the design procedure that reduces the observed value of the infiltration coefficient. The factor used depends upon the consequences of failure and engineering judgement is therefore required as to the factor to be used. Factors are suggested in Table 25.2. It should be noted that the figures are not based on actual observations of performance loss.

The following sections describe the calculation methods for infiltration system sizing.

25.6.1 Plane infiltration systems

Plane infiltration systems are relatively thin, and cover a wide area. The side area is negligible compared to the base area. For a given rainfall event discharging to an infiltration system of a particular size, the hydraulic equations can be solved to give the maximum depth of water, hmax. The equation for hmax is given in Equation 25.1.

Figure 25.7 Hydraulic design process



TABLE25.2

Suggested factors of safety, F, for use in hydraulic design of infiltration systems (designed using Bettess (1996). Note: not relevant for BRE method)

Size of area to be drained

Consequences of failure

No damage or inconvenience

Minor damage to external areas or inconvenience (eg

surface water on car parking)

Damage to buildings or structures, or major

inconvenience (eg flooding of roads)

< 100 m2

100–1000 m2

> 1000 m2

1.51.51.5

235

101010

EQ.25.1

Determination of maximum depth of water for plane infiltration systems

where:

hmax = maximum head of water above base of infiltration component

R = ratio of the drained area to the infiltration area,

q = infiltration coefficient, from percolation test (m/h), adjusted by the appropriate factor of safetyi,D = intensity and duration of rainfall events with the required return period at the site location

(m/h, h)Ab = base area of infiltration system (m2)AD = area to be drained (m2)n = porosity of fill material (voids volume/total volume)

This may be obtained from laboratory tests, or else the guide values provided in the following table may be used. If a value of porosity greater than 0.3 is used, the material delivered to site should be tested to ensure that it meets the design requirement.

Material Porosity, n

geocellular systems 0.9–0.95

uniform gravel 0.3–0.4

graded sand or gravel 0.2–0.3

A perforated concrete ring soakaway may be installed in a square or rectangular plan excavation and the gap between the rings and the soil filled with clean stone. Under these circumstances an effective porosity, n’, applies.

where

r’ = radius of the ring sections (m)W = width of the excavation (m)L = length of the excavation (m)



The procedure set out in Equation 25.2 will ensure that surface water runoff will be able to infiltrate through the lower surface of the system into the soil at the required rate. For systems such as infiltration basins or bioretention systems which have a surface layer of topsoil or filter soil, the rate at which water can percolate through the surface may be the limiting factor (Chapter 13).

For an infiltration pavement, R = 1, step 3 is omitted and the maximum depth of water is given by:

For an infiltration pavement where no subgrade material is provided to allow short-term storage of water (ie open lattice blockwork), storage occurs on open ground above the infiltration surface. In this case R = 1, n = 1, steps 2 and 3 are omitted and the maximum depth of water is given by:

Alternatively, for an infiltration blanket, the maximum depth hmax may be fixed, and the designer may wish to know the base area of the infiltration system that will be required to ensure that the depth of water does not exceed hmax, in which case the procedure given in Equation 25.3 should be followed.

25.6.2 Three-dimensional infiltration systems

Three-dimensional infiltration systems are those that have a cuboid or trench shape, and the surface area of the sides is large compared to that of the base. For a given rainfall event discharging to an infiltration system of a particular size, the hydraulic equations can be solved to give the maximum depth of water, hmax. The approach used depends on whether the facility has vertical or sloping sides.

EQ.25.2

Procedure for design of plane infiltration systems

1 Obtain the infiltration coefficient, q, (m/h) by dividing the infiltration rate found from field tests by the appropriate factor of safety

2 Find the porosity of granular fill material

3 (i) Decide on the area to be drained, AD,(m2) and the infiltration surface area, Ab (m2)

(ii) Calculate the drainage ratio, R, where

4 (i) Select a storm duration, D (h)

(ii) Determine the corresponding rainfall intensity, i (m/h)

5 (i) Check whether q exceeds Ri. If so, the rate of infiltration exceeds the potential rate of runoff, in which case hmax = 0

(ii) Otherwise, calculate the value of hmax (m)

6 Repeat steps 4 and 5 for a range of rainfall durations, constructing a spreadsheet or table of results

7 Select the largest value of hmax



Vertical-sided structures

This procedure can be applied to soakaways and infiltration trenches. The maximum water depth hmax in the infiltration system is given in Equation 25.4 and the procedure in Equation 25.5.

These equations can be solved computationally, or graphically using Figure 25.8.

EQ.25.3

Procedure to determine the base area required for a given maximum depth

The equation for the base area Ab (m2) is given by:

1 Obtain the infiltration coefficient, q, by dividing the infiltration rate found from field tests by the appropriate factor of safety

where R is the ratio of the drained area to the infiltration area, R = AD/Ab

2 Find the porosity of granular fill material

3 (i) Decide on the area to be drained, AD (m2)

(ii) Decide on the maximum allowable water level, hmax (m)

4 (i) Select a storm duration, D (h)

(ii) Determine the corresponding rainfall intensity, i (m/h)

5 (i) Calculate AD.i.D, n.hmax, and q.D

(ii) Calculate Ab (m2)

6 Repeat steps 4 and 5 for a range of rainfall durations constructing a spreadsheet or a table of results

7 (i) Find the largest infiltration surface area required

(ii) If this area is unacceptably large then increase hmax or decrease AD and repeat from step 3

EQ.25.4

Determination of maximum depth of water for 3D infiltration systems

Where:

P = perimeter of the base of the infiltration system (m).



EQ.25.5

Procedure for design of 3D infiltration systems

1 Obtain the infiltration coefficient, q, by dividing the infiltration rate found from field tests by the appropriate factor of safety

2 Find the porosity of granular fill material; if the structure is open, n = 1, but if it is part-filled with gravel then the effective porosity, n’, is used

3 (i) Decide on the area to be drained, AD

(ii) Choose the type and shape of infiltration system, ie cylindrical soakaway, infiltration trench

4 (i) Select the proposed dimensions for the infiltration system, ie the radius of a cylindrical soakaway, the width and length of a rectangular plan system

(ii) Calculate the base area, Ab, and the perimeter, P, of the soakaway base from the proposed dimensions

(iii) Determine the value of b from

5 (i) Select a storm duration, D

(ii) Determine the corresponding rainstorm intensity, i

6 Determine the value of a from

7 Either calculate hmax or read off the value of hmax from Figure 25.8

8 Repeat steps 5 to 7 for a range of rainfall durations

9 (i) Find the largest value of hmax

(ii) If hmax is unacceptably high, return to step 4 and increase the dimensions

(iii) If hmax is still unacceptably high, either:

(a) return to step 3(i) and reduce the area drained to an individual system

or

(b) return to step 3(ii) and choose a different type of system



Sloping-sided structures

For sloping-sided structures, there is no simple analytical method for calculating the maximum water depth. A numerical procedure for calculating the depth is given in Bettess (1996). It is recommended that sloping-sided structures are approximated by a vertical-sided structure or that the method described in that publication is used.

25.7 EMPTYING TIME CHECKS

The hydraulic equations in Section 25.6 take both storage and infiltration into account and, if the infiltration rate is low, will ensure that the system incorporates sufficient storage. However, if infiltration is too low, there is the possibility that the system will not have emptied before the next rainfall event starts.

The infiltration component should discharge from full to half-full within a reasonable time so that the risk of it not being able to manage a subsequent rainfall event is minimised. Where components are designed to manage the 1:10 year or 1:30 year event, it is usual to specify that half emptying occurs within 24 hours. If components are designed to infiltrate events greater than the 1:30 year event, designing to half empty in 24 hours can result in very large storage requirements and, with agreement from the drainage approving body, it may be appropriate to allow longer half emptying times. This decision should be based on an assessment of the performance of the system and the consequences of consecutive rainfall events occurring.

Where emptying times are found to be too long, extra storage may be required (see Equation 25.6).

Figure 25.8 Graph to determine maximum depth for 3D infiltration systems (from Bettess, 1996)



25.8 REFERENCES

BETTESS, R (1996) Infiltration drainage – manual of good practice, R156, CIRIA, London, UK (ISBN: 978-0-86017-457-8). Go to: www.ciria.org

BGS (2015) Infiltration SuDS maps, British Geological Society, London, UK. Go to: http://tinyurl.com/oruu25r

BLAKE, J (2010) “Adjusting soil infiltration coefficients for groundwater level”, Proceedings of the ICE – Water Management, vol 163, 5, Institution of Civil Engineers, London, UK, pp 239–245

BRE (1991) Soakaway design, BRE Digest 365, Building Research Establishment, Bracknell, UK (ISN: 0-85125-502-7)

CHEN, H, STEVENSON, M and LI, C (2008) “Assessment of existing soakaways for reuse”, Proceedings of the ICE – Water Management, vol 161, 3, Institution of Civil Engineers, London, UK, pp 141–149

LANDIS (2015) Tools and utilities: Soil texture triangle, LandIS, Cranfield University, Cranfield, UK. Go to: www.landis.org.uk/services/tools.cfm

SUSDRAIN (fact sheets): http://www.susdrain.org/resources/factsheets.html

WATKINS, D (1995) Infiltration drainage – literature review, PR21, CIRIA, London, UK (ISBN: 978-0-86017-821-7). Go to: www.ciria.org

WILSON, S and DEROSA, D (2006) “Siltation in SUDS – myth and reality”. In: Proc CIWEM WaPUG 2006, spring conference, Coventry, UK. Go to: http://tinyurl.com/o62fpgh

EQ.25.6

Equations to calculate the time to empty an infiltration system

1 Time for half-emptying a plane infiltration system:

If the time for half-emptying is stipulated to be less than 24 hours and q is measured in m/h, then an acceptable infiltration coefficient is determined by:

2 Time for half-emptying a 3D infiltration system.

If the time for half-emptying is stipulated to be less than 24 hours and q is measured in m/h, then an acceptable infiltration coefficient is given by:



STATUTES

British Standards

BS EN ISO 14688-1:2002+A1:2013 Geotechnical investigation and testing – identification and classification of soil. Part 1: identification and description

BS EN ISO 14689-1:2003 Geotechnical investigation and testing. Identification and classification of rock. Part 1: Identification and description

BS EN ISO 22282-2:2012 Geotechnical investigation and testing. Geohydraulic testing. Water permeability tests in a borehole using open systems

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CIRI uD anua 2015 · CIRI uD anua 2015 25FILTRATION: DESIGN METHODS IN Contents 25.1 General...

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