IRISH SuDS: GUIDANCE ON APPLYING THE GDSDS SURFACE WATER DRAINAGE CRITERIA

Contents

Introduction

The GDSDS Criteria

Criterion 1: River Water Quality Protection

Criterion 1.1: Interception Storage

Criterion 1.2: Treatment Storage

Criterion 2: River Regime Protection

Criterion 3: Level of Service (Flooding) for the Site

Criterion 3.1: Minimum Level of Service Against Flooding

Criteria 3.2 and 3.3: Planned Temporary Flooding

Criterion 3.4: Protection Against Flooding of Other Sites

Criterion 4: River Flood Protection

Criteria 4.1 and 4.2: Volume of Stormwater Runoff

Applications of Drainage Criteria

1. High Density Small Infill Sites

2. Greenfield Development Draining to a Nearby Stream

3. Greenfield Development Near an Estuary

4. Development on Clay Soils, Contaminated Land or High Groundwater Levels

Introduction

The growth of Dublin automatically results in an increase in paved surface area which creates pressure on the environment and existing services due to the generation of increasing runoff and pollution. The GDSDS policy document on New Developments addresses this issue by producing a set of drainage criteria which aims to minimise the impact of urbanisation.

These criteria reflect the aim of replicating the runoff characteristics of greenfield runoff. In practice the exact hydraulic behaviour for any greenfield site can not be accurately be predicted. However the criteria provide a consistent approach to addressing the increase in both rate and volume of runoff as well as providing a mechanism for protecting the environment from the pollution that is washed off roads and buildings.

In producing a set of criteria which addresses these effects, it is unavoidable that a degree of increased complexity has resulted, making it more difficult for the drainage engineer to design an appropriate drainage system.

Certain elements of the criteria can be relaxed depending on the situation (the size, type and location of the development) and this guidance has been drawn up to assist drainage engineers to apply the criteria appropriately.

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The GDSDS Criteria

The following table comes from the GDSDS New Development policy document and summarises the criteria for the design of drainage systems. In principle these criteria should be applied to all sites, but certain practical aspects (throttle sizes for achieving low flow rates) and minimal consequences of non-compliance (draining to the estuary or coast) mean that these criteria can be relaxed in certain instances.

* 30 minutes is appropriate for assessing flooding where drainage is provided by a pipe based system. Longer duration events will need to be applied to drainage systems incorporating SuDS.

The following table details the GDSDS climate change criteria and these should also be applied when designing drainage systems.

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Criterion 1: River Water Quality Protection

Run-off from natural greenfield areas contributes a nominal amount of pollutant and sediment to rivers. For most rainfall events direct run-off to rivers does not take place with rainfall percolating into the ground. This water supports the base flow in the river days and weeks after the event has taken place.

By contrast urban run-off, when drained by pipe systems, results in run-off from virtually every rainfall event with high levels of pollution, particularly in the first part of the run-off, with little of the rainfall actually percolating into the ground. This results in virtually no support for base flows in rivers.

In practice, there are a number of practical constraints in applying these drainage criteria to replicate greenfield runoff characteristics. 5 to 10mm of rainfall run-off from an urban area, especially for a high density development, provides a considerable volume of runoff. Infiltration may be a problem for several reasons; the first being that the soil may be fairly impervious (clay), secondly groundwater levels may be high at certain times in the year and thirdly, wash off from certain surfaces, particularly roads, often contains high levels of polluted sediment which may result in blockage of infiltration units and pollute the groundwater.

The fact that it might be difficult to comply with these design criteria in all circumstances does not mean that these criteria are not valid. They should be applied wherever a reliable solution is possible. It should be noted that the issue of river pollution is particularly a problem in the summer when river flows are low and dilution is minimal. However this is the period in which infiltration units are most likely to be effective as the soil moisture deficit and the potential evaporation rate is high. Any interception will therefore be of some benefit.

Achieving zero runoff from the first 10mm of rainfall is often not practicable, and therefore emphasis is also needed on achieving some treatment of the stormwater run-off. This ensures that any runoff discharged to the river is of significantly better quality than direct runoff from a pipe network.

The quality of stormwater runoff is an issue for frequent small events. This is due to the flush of debris and sediment from the catchment surface in the first part of the event together with any sediment deposits in the pipe network. This is compounded by the fact that this highly concentrated initial flow may enter the receiving water that is still flowing at base flow conditions, thus providing a minimum level of dilution. For large events, or during periods of high river flow, this water quality impact is much reduced, so the key period of concern is the summer months of low river flows and the small rainfall events which take place on a regular basis.

To remove the major proportion of the pollution, it is necessary to:
- Capture and treat the runoff from frequent, small events; and
- Capture and treat a proportion of the runoff (first flush) from larger events.

These processes are achieved by providing Interception storage and Treatment storage.

Criterion 1.1 Interception Storage

Good site design that does not allow runoff to pass to the river for small rainfall events (at least 5 mm, preferably 10 mm). This is likely to be through the use of infiltration techniques. These include the use of:
- Soakaways
- Infiltration trenches
- Pervious pavements
- Rainwater harvesting (though this is usually limited to roofs)

Criterion 1.2 Treatment Storage

The concept of Treatment storage is to provide a body of water in which dilution and partial treatment (by physical, chemical and biological means) of surface water runoff can take place. This is the volume of water that remains in a Retention pond during the dry weather periods between rainfall events. The amount of storage normally provided is defined in terms of the equivalent volume of runoff from a rainfall event of 10mm to 15mm. Paved areas served by SuDS units (if they are providing effective treatment) do not need to be considered in determining the treatment storage volume. However as sedimentation is the primary process by which treatment is achieved, it is important that the pond volume does not reduce to a level where efficient sedimentation is compromised.

This storage should not be confused with the concept of Interception. If no runoff takes place for small events, the most effective water quality protection is being achieved.

It should be stressed that drainage of a site should be designed using the treatment train concept using appropriate drainage mechanisms. Reliance on only a single pond prior to the outfall is not regarded as best practice in providing the best water quality protection for the receiving water.

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Criterion 2: River Regime Protection

Whatever the event, unchecked runoff through traditional pipe networks will discharge into receiving waters at rates that are an order of magnitude greater than that prior to development. This causes flashy flow in the river that is likely to cause scour and erosion that is likely to seriously affect the morphology and ecology of the stream. Attenuation storage is provided to limit the runoff from the site to prevent this occurring. The design principle is to limit the runoff for events of equivalent frequency of occurrence to the same peak rate of runoff as that which took place from the greenfield site.

In practice the actual rate of runoff needs to be appropriately low for the majority of events (river morphology), and not excessive for large events using the predicted greenfield runoff rates as a guide. Therefore the 1 and 100 year greenfield runoff rates are used for this purpose, with the 100 year event being used to define the maximum runoff rate from the site.

Criterion 2.1 (the discharge rate for the 1 year return period) also introduces the concept of having a minimum discharge rate of 2l/s/ha to limit the sizing of storage systems. In addition a lower threshold of 5l/s (a minimum orifice size of 75mm diameter, as stated in the policy document) should also be applied for small sites to avoid risks associated with blockage.

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Criterion 3: Level of Service (Flooding) for the Site

There are several mechanisms by which flooding can take place on site. These include:
- Flooding from the drainage (conveyance) system
- Flooding from the drainage (storage) system
- Flooding from the river
- Flooding from overland flows entering the site

In addition flooding can take place on properties below the site due to runoff from the site. This might be from the river, a drainage system or overland flow.

Criterion 3.1 Minimum Level of Service Against Flooding

Flooding should normally not occur at any location on the site for any event with a return period of less than 30 years. In some instances planned flooding in green areas, particularly floodplains, can be acceptable if explicitly planned.

Sites should take into account topography to maximise the benefits of low points for storage and avoid placing vulnerable structures and/or properties in these areas.

It should be remembered that steep open ground above a site can generate runoff from extreme events and flood protection should be provided against such an event occurring.

Criteria 3.2 and 3.3 Planned Temporary Flooding

For rare events up to the 100 year flooding can be allowed for, but should be planned. This should consider ponding locations, routing of flows, velocities and depths of flow and the likely effects associated with the aftermath of such situations (sediment and debris). This also applies to developments in or adjacent to floodplains where explicit consideration of floor levels must be made to ensure that internal flooding cannot take place.

The effect of river flooding is often severe and there is usually some degree of uncertainty with regard to the maximum flood level at any location for a particular return period. It is recommended that floor levels of all houses are at least 500mm above the predicted maximum 100 year flood level of all sources of flooding. This freeboard should be increased where there is significant uncertainty of the flood height and where predicted water levels are sensitive to the assumptions and parameters being used. Flood maps are being produced for rivers in Ireland, but these may not exist for all locations or may be very approximate in their estimation. It is therefore important to investigate the likely accuracy of this information or assess it specifically as part of the planning and design process.

In addition to floor levels of residential dwellings, other aspects such as access and the location of sensitive and important buildings (hospitals) should also be designed taking into account flood risk.

In considering the maximum water level, appropriate precautions must be taken in assuming the scenarios which might affect the level of flooding. These include:
- Current and proposed urban development upstream in the catchment.
- Throttles and other attenuation features upstream due to bridges and dams need to be specifically considered. These may be removed over the course of time or may fail suddenly in flooding conditions.
- Floodplain storage upstream might be reduced.

A position needs to be taken on all these issues in determining maximum river water levels at any site. These should be defined by the local authority for the area based upon the local area structure plan or agreed with them in the early stages of considering a planning application. The general position that should be taken is that man-made obstacles are likely to alter in time, but that natural water course characteristics will be preserved by all future development. The level of future development upstream and the runoff characteristics will depend on the local authority's views regarding future development and the level of enforcement of SuDS techniques in that catchment with its particular soil and topographic characteristics. This should not be limited to the structure plan horizon which is often 20 years; though a longer term view of future development in the catchment may be difficult to arrive at.

Occasionally river and sea defences result in embankments. The location of houses behind these defences is a risk which is a function of the water levels being restrained, the quality of the defence structure and the distance of the dwellings from the embankment. There is limited guidance available for this situation, but it is important to carry out a risk analysis where this circumstance arises.

Similarly this also applies to embankments of storage systems within the site. Although they should be designed to a higher standard than the 100 year event, consideration of the potential effects of failure need to be evaluated. The failure of a storage unit, particularly if it is embanked, can be dramatic, even if it is a relatively small storage system. Reservoir design standards may be appropriate to consider in certain circumstances. Failure of the structure is not the only thing to consider. Very extreme events, much larger than 100 years, can occur. The design of overflow structures should be for a 200 year event and still provide a freeboard of at least 200mm.

There are a number of less obvious aspects to consider related to storage ponds. These are:
- Hydraulic constraints to the pond outlet
- Overflow provision and risk of failure
- Hydraulic backwater effects of the pond

High water levels downstream of the pond/basin unit may affect the top water due to back water effect. This is a complex issue of joint probability (the river being high when the storage unit is full) and the relative levels of the water surfaces. A precautionary approach to the analysis should be assumed (possibly total dependency) to establish maximum storage water levels.

Similarly, flows into pond or basin which is full, may have a backwater issue for the inlet pipework serving the development which might result in local flooding upstream. This is only relevant for quite flat catchments, but generally this should not be too great a problem as the rainfall intensities in events where the storage is fairly full are generally not those which cause a pipe capacity problem.

Criterion 3.4 Protection Against Flooding of Other Sites

Flooding can be generated below the site from the stormwater runoff. Design of the drainage must take not presume that excess runoff can be allowed to runoff from the site and cause flooding to other properties.

It is useful to note that SuDS based systems, which are largely based on the provision of storage, and therefore are much less likely to cause downstream flooding of any consequence.

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Criterion 4: River Flood Protection

Criteria 4.1 and 4.2 Volume of Stormwater Runoff

The total volume of runoff from extreme rainfall events of several hours can result in river flooding. Runoff volumes from a development site can be an order of magnitude greater than the greenfield runoff as well as being very much faster. It is therefore important to control the additional volume as well as the rate of runoff as floodplains have finite storage volumes, and even if the runoff is attenuated over the period that greenfield runoff occurs, by definition there must be greater depths of flooding. Control of the additional runoff volume from sites is therefore required in order to avoid an increase in river flooding.

There are three ways in which river flood protection can be provided. These are:
- Long terms storage
- Infiltration
- Extended attenuation storage

1. Long Term Storage
Volumetric control can be achieved by passing water from the attenuation storage system to an area that will drain very slowly, preferably by infiltration. This is termed Long Term Storage. As the design event is by definition a rare event, it may be possible to accommodate Long Term Storage within a park or football field with appropriate land drainage provision. However, to achieve the necessary volumes of long term stored runoff, the return period at which runoff will start to pass to such an area will need to take place for events significantly less than the 100 year event, probably of the order of a 10 year event. Flooding of the area must not occur too frequently, as the level of service, even for public open space, may be considered to be inadequate. This should be designed using detailed hydraulic modelling.

2. Infiltration
An alternative approach may be to manage an equivalent volume through infiltration, which comes into effect for all storm events. This is a simple design approach and provides a high level of environmental benefits (e.g. groundwater recharge). It should be noted that providing infiltration storage for the additional runoff from 60mm of rainfall (approximately the 100 year 6 hour event) can be quite significant.

3. Extended Attenuation Storage
It is possible that infiltration from a Long Term Storage basin cannot be provided at certain sites. In these situations it is recommended that Q_BAR or 2 l/s/ha is used as the limit of discharge for the attenuation storage control to ensure sufficient runoff is retained on site for extreme events. This will result in a less cost-effective solution as a result of the large storage volumes required.

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Applications of Drainage Criteria

1. High Density Small Infill Sites

Criteria that are likely to need to be applied are:
- criteria 1.1;
- criteria 3;
- criteria 4.3.

Small infill developments are usually found within existing residential or commercial areas. Re-development of these areas tends to be very high density with all surfaces being paved. Infiltration is often not a feasible option due to the ground being contaminated, high ground water levels, or inappropriate soil type. In many instances drainage can not be taken to a local water course and therefore connection to the local sewerage system will be required. Many existing sewers are already overloaded and therefore runoff needs to be controlled to as low a rate as possible.

Criterion 1.1 (Interception) should be provided if possible and this can only be provided in this situation in the form of green roofs or rainfall harvesting for either internal use (toilet flushing) or external use (garden watering / car washing). This form of stormwater management can be effective where there is a significant demand for toilet water relative to the roof catchment area. If criterion 1.1 is applied in the form of rainfall harvesting, design of the storage system should be based on stormwater management storage requirements and not standard rules for rainwater harvesting sizing, as insufficient provision is given to the spare storage needed to manage large rainfall events. Stormwater storage provision requires around 2 to 3 times the normal volume required to ensure supply. It should be noted that although Interception storage only addresses a small proportion of the design rainfall, its provision has benefits for any CSOs downstream, sewage treatment works and minimises the impact of pollution on the receiving river.

All criteria 3 (Level of Service) applies to all types of development.

As the existing sewerage system is expected to serve the re-development, criterion 4.3 which limits the runoff discharge from the site to the higher value of 2l/s/ha or Q_BAR, will normally be applied. However if the site is less than 2.5ha (which is quite likely), the discharge rate will be dictated by the minimum orifice size of 75mm which will control the outflow to around 5l/s.

Design of the Drainage System

The design of the drainage system is unlikely to be able to utilise surface based systems and limits. Green roofs provide a reasonable reduction in initial runoff from rainfall, but calculations will determine whether rainfall harvesting or green roofs is the more effective in terms of stormwater management. The two methods are generally not used together as runoff quality from green roofs is not suitable for subsequent internal use without treatment.

To provide the remaining attenuation storage, either a high voids storage systems or a permeable pavement will be needed. The latter is preferred as it provides effective treatment to the runoff, though this may be largely irrelevant depending on the receiving sewer. If infiltration is not acceptable, these units will need to be lined.

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2. Greenfield Development Draining to a Nearby Stream

Criteria that are likely to need to be applied are:
- criteria 1;
- criteria 2;
- criteria 3;
- criteria 4.1.

The size of a greenfield development is immaterial as the cumulative effects of lots of developments must be assumed in most cases. Thus the argument that an individual development is small relative to the size of the watercourse should not be used. Unless particular circumstances allow otherwise, the expectation must be to design the drainage system so as to minimise the impact of the development on the receiving water course.

The soil type may be suitable for infiltration, and as long as the groundwater is more than 2m below the ground in winter, emphasis should be placed on the use of infiltration. It should be noted that the storage volumes needed to meet the GDSDS criteria increase dramatically for permeable soils (type 1 and 2) and therefore where it possible to use infiltration it is important to do so. If reliance is placed on provision of attenuation, the land-take and cost of construction of providing storage will be significant.

Criterion 1.1 (Interception) is particularly important for protecting a stream, though it is important to recognise that provision of 5mm of rainfall interception should be provided for runoff from all hard surfaces. As the use of soakaways for roads has maintenance and ground water pollution issues, the use of pervious pavements, swales and detention basins should be preferred. It is important to recognise the high level of benefit being provided by interception of all the runoff in terms of stream protection in terms of both water quality and morphologically.

Criterion 1.2 (treatment) would normally be applied unless it could be shown that at least 10mm of rainfall was intercepted and that discharge to the stream only takes place for larger events. The treatment volume may be reduced from 15mm of rainfall from all paved surfaces depending on those paved areas which are disconnected or receiving some degree of treatment. It is important to recognise that sedimentation is the critical feature of the treatment process and therefore explicit consideration of velocities in the pond is needed if the treatment volume is significantly reduced.

Criteria 2.1 and 2.2 (river regime) are applicable to minimise the hydraulic change in terms of rate of runoff on the water course. Many site developments will be too small to apply the 1 year greenfield rate of runoff as the throttle size will be less than 75mm diameter to achieve this. Also permeable soil areas may have 1 year flow rate values below 2l/s/ha. Thus in many situations the controlling throttle requirements will be dictated by the minimum limits criteria and result in smaller storage volumes than would otherwise be calculated in complying with greenfield conditions.

All criteria 3 (Level of Service) apply to all types of development. Particular consideration should be given to land use and protection from runoff from areas uphill of the site.

Criteria 4 (River flooding) is likely to be implemented as a combination of criterion 4.1 and 4.2. Where the soil type is permeable, at least 5mm of Interception storage may already have been provided. Similarly paved areas served by soakaways may only contribute a small proportion of runoff. However for sites with pervious surfaces, the Long Term storage element is fairly large and therefore it is likely that a significant residual volume still needs to be provided. This could be provided by careful design of the retention pond storage by providing a non-return flood area which acts by infiltration. This is likely to be a dual purpose area as it should only come into effect less frequently than once in 5 years.

Design of the Drainage System

The design of the drainage system will maximise the use of infiltration systems; infiltration trenches or soakaways for roof runoff, and swales, detention basins and permeable pavements for road runoff.
A retention pond, preferably in two or three parts, will be provided as treatment and additional attenuation, depending on the design of the upstream SuDS components used. If the roads drain directly to the pond, it is important to maximise the sedimentation efficiency of the pond and the operation and maintenance requirements of removing silt and vegetation. As the silt may well be characterised as being Hazardous, (particularly for heavily trafficked roads, commercial and industrial developments) emphasis should be placed on minimising the volume of silt likely to be deposited in the pond.
Big areas of public open space in small developments are rarely available for locating a large pond. It is important to recognise that effective use of other forms of SuDS across the site will minimise the size of a pond.

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3. Greenfield Development near an Estuary

Criteria that are likely to need to be applied are:
- criteria 1;
- criteria 3;

A development on the coast or an estuary is discharging into waters where high levels of dilution are being provided and the hydraulic effects of additional or rapid runoff will have minimal impact. Issues which are more likely to be of concern relate to flood risk due to the drainage system within the site (due to backwater effects) and flooding from the sea or estuary if the development is on low lying land. In this situation groundwater levels are often close to the surface.

Criterion 1.1 (Interception) The application of criteria 1 (River Water Quality protection) will generally be required, though its application may be relaxed due to the high levels of dilution available, it is important to apply the principle of treatment at source.

As groundwater levels are almost certainly too high to allow the use of infiltration, the only way to achieve some degree of Interception is to use green roofs or rainwater harvesting. However this means that surface water from roads (which is by far the most contaminated runoff) will not be addressed by Interception.

Criterion 1.2 (treatment) with the likelihood that Interception storage cannot be provided, greater emphasis in needed in providing effective treatment in the form of a pond or other facility. The degree of importance associated with the provision of effective treatment will be influenced by the opportunity for dilution in the receiving waters and the ecological status of the local environment.

Criteria 2.1 and 2.2 (river regime) The hydraulic impact of a development, whatever size, is of no consequence when discharging to an estuary or the coast. Therefore there is no need for Criteria 2 River Regime Protection to be applied. However it should be noted that the jetting effect of a high rate of discharge may be an issue with regards to erosion and sediment entraining effects.

All criteria 3 (Level of Service) apply to all types of development. Particular consideration should be given to low lying land and depressions being saturated and backwater effects of high receiving water levels. Detailed design of the drainage system will need to consider the joint probability issues of high receiving waters and rainfall return period. In addition developments within 100m of embankments must consider the risks associated with these units breaching as a result of extreme conditions.

Criteria 4 (River flooding) As with criteria 2, the hydraulic impact of discharges from the site will have no consequence on water levels in the receiving system.

Design of the Drainage System

The design of the drainage system by an estuary or the sea will therefore focus on water quality treatment requirements and ensure an adequate level of service against flooding. This latter aspect can be a complex issue for low lying areas where drainage by gravity is being used. Back-up systems involving pumps can provide some form of insurance. However infrequent use of such units may mean that in circumstances when they are needed to operate, they are found to be defective. In general, it should be questioned whether the development is sustainable that needs to rely on a pumped solution for the stormwater system to operate.

The GDSDS criteria on climate change for both sea level rise and rainfall increase needs to be taken in to account. The design horizon for the development should also be explicitly considered. Although a specific development may have a life expectation of 50 years, the fact that an urban development is proposed at that location, possibly implies that the area will continue to be used beyond the 50 years design horizon.

Where a development is protected by an embankment that has a significant head of water behind it during extreme conditions, the minimum distance from the embankment should be based on current science and a breach risk assessment.

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4. Development on Clay Soils, Contaminated Land or High Groundwater Levels

This situation could be found at any location (by a small stream, by the coast or estuary, or within an urban development), the latter being the most likely. Therefore guidance relating to these locations should also be considered. The key issue in this situation is that solutions involving infiltration cannot be applied. This reduces the options available and requires space for the provision of attenuation storage.

Criteria that may need to be applied are:
- criteria 1;
- criteria 2;
- criteria 3;
- criteria 4.

Criteria 1 (1.1 Interception and 1.2 Treatment) will normally apply unless discharge is into a combined sewer.

Criteria 2 (River regime protection) will only apply when the surface water is being discharged to a river.

Criteria 3 (Level of service) always applies to any development.

Criteria 4 (Flood protection) applies to all situations, but in the case of surface water being discharged to sewers, the rate of flow will usually be severely limited to minimise impact downstream.

Design of the Drainage System

The design of the drainage system for clay soils, contaminated land and high groundwater levels will not involve the use of infiltration. This means that storage systems will be an important element of the drainage design. Where the water is discharged to sewerage systems, it is likely the high voids geo-cellular systems will be the most appropriate solution.

Mobilisation of contaminants must be avoided, which means that liners must be sealed and demonstrated as such. There have been a number of sites where linings to pervious pavements appear to have been ineffective.

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