STORMWATER AND GROUNDWATER MANAGEMENT: AN APPROACH TO ENVIRONMENTAL PROTECTION

1.0  INTRODUCTION

The Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), released in 2001, predicted a global average surface temperature rise of 1.4 to 5.8°C (2.5 to 10.4°F) over the period 1990 to 2100 compared to a previous assessment of 1.0 to 3.5°C (1.8 to 6.3°F) for the same period. The report also for the first time identified human activity as the primary cause for global warming.

The report noted that “As the CO₂ concentration of the atmosphere increases, ocean and land will take up a decreasing fraction of anthropogenic O₂ emissions. By 2100, carbon cycle models project atmospheric CO₂ concentrations of 540 to 970 ppm (90 to 250% above the concentration of 280 ppm in the year 1750). As part of its conclusions, the report noted that changing land use could influence atmospheric CO₂ concentration. “Hypothetically, if all of the carbon released by historical land-use changes could be restored to the terrestrial biosphere over the course of the century, CO₂ concentration would be reduced by 40 to 70 ppm.

The above report no doubt poses a challenge to all stakeholders to evolve means of protecting the environment from further harmful human activities.

The objective of this technical paper is two fold. First it seeks to draw the engineers’ attention as designers, builders and developers away from conventional stormwater and groundwater management systems and present us with the rapidly developing sustainable systems that impact positively on the environment. Second, this paper seeks to draw government’s attention to the need for a policy framework that compels land owners and developers to rise up to the requirements of sustainable stormwater and groundwater management systems, if we are not to pay lip service to environmental protection.

It cannot be overemphasized that stormwater and groundwater management are fundamental considerations in the planning and design of urban development. Unfortunately, these are often treated as subsidiary issues that are not addressed until the final stages of the planning and design process.  By considering stormwater and groundwater management at the initial design phase it is possible to ensure viable and sustainable solutions that are compatible with other design objectives for the site and the global environmental interest.

2.0  STORMWATER MANAGEMENT

Conventional drainage practices aim at collecting and conveying stormwater to the street drainage system with a minimum of nuisance, danger or damage. However, this causes rapid and concentrated stormwater discharges that contribute to increased flooding, erosion and sedimentation, and reduced stormwater quality. These problems can be mitigated by alternative measures that delay stormwater discharges and reduce the total volume of stormwater discharged.

The drainage engineer should have a number of questions that are addressed by the proposed drainage design. A non-exhaustive list of questions includes:

• What are the normal operating and maintenance requirements of the design?
• What are the risks of failure of the proposed design and the consequences in terms of impact?
• How effective will the system be in treating the stormwater?
• What are the social / aesthetic benefits of the proposed design?
• What are the environmental benefits / protection of the proposed design?

A careful consideration of the above questions will ensure the design of a sustainable drainage system.

2.0.1 Sustainability

In terms of drainage, sustainability can be interpreted as:

• Drainage systems should utilize natural resources.
• Drainage systems should aim to replicate the natural characteristics of rainfall runoff for any site;
• The environmental impact of man should be minimized.

The concept of sustainability results in a move away from traditional drainage methods, and the use of recommended SuDS (Sustainable Urban Drainage Systems) to provide hydraulic, water quality and environmental benefits.

The design of the drainage system should try and replicate, in a general way, the same rainfall runoff characteristics for the pre-development condition of the site. At pre-development the runoff is much slower, less polluted and has virtually no runoff from ordinary rainfall events. The use of SuDS, particularly components which encourage infiltration, will enable this principle to be achieved.

The design of drainage systems needs to minimize water pollution and maximize environmental benefits. SuDS units are designed to address stormwater water quality as well as providing hydraulic conveyance.

2.0.2 Level of Service

• Flood protection should be provided to a minimum level of service;
• No negative aesthetic effects;
• Social benefits;
• Safety.

The principal objective of drainage is to provide protection from flooding due to rainfall on an area. The level of service provided is a function of society’s expectations as well as the cost-benefit of the system based on the damage consequences due to flooding.

2.0.3 Cost-effectiveness

• Principles of whole life costing (WLC) should be applied.

Drainage design should aim to provide the most cost-effective solution, particularly in terms of

maintenance requirements.

2.0.4 Other design issues

There are a few other issues which influence the generic design criteria discussed above. These

are:

• Size of development;

• Environmental issues influencing storage design;

• Density of development;

• Location of development;

• Extending urban catchments.

2.1 CONVENTIONAL APPROACHES

Conventional drainage practices generally involve rapid discharge of stormwater from the site to a public drainage system. The main objective is to collect and convey stormwater to the street drainage system. Roof runoff is discharged via small diameter pipes (usually 100 mm diameter), and surface stormwater is usually conveyed by overland flow. The public drainage system usually consists of a system of gutters, streets, pipes, culverts and channels.

Where the site slopes towards the street, roof runoff and overland flow are drained directly to the street drainage system (see Figure 1). Where the site slopes away from the street, these are

connected to the street drainage system or a receiving waterway via a drainage easement at the rear of the property (see Figure 2). The drainage easement generally incorporates a drainage pipe, and may also include a table drain on the ground surface for the collection of overland flow.

Problems with conventional practice

The majority of stormwater runoff in urban area is from impervious surfaces such as roofs, paved areas and roads. Except in the case of major storms, little or no runoff occurs from pervious surfaces such as lawns, gardens and landscaped areas. Urbanization has dramatically increased the area of impervious surfaces. This in turn has resulted in increased peak discharges and greater volumes of runoff per storm.

The direct discharge of roof water and overland flow to the street drainage system under conventional drainage practices causes rapid and concentrated discharges of stormwater. This contributes to increased flooding, erosion and sedimentation, and reduced stormwater quality. These problems can be reduced by measures that delay stormwater discharges and reduce the total volume of stormwater discharged.

The alternative

Alternative stormwater management measures, when used in conjunction with conventional

practices, have many cost, aesthetic and environmental benefits. For example, roof runoff can be managed using rainwater tanks and filtration/ infiltration trenches. Paved areas can be minimized or replaced with porous paving. A variety of landscape measures and practices can also be applied. These measures reduce the volume of stormwater runoff and the rate at which it is discharged. Figure 3 shows how these measures can be combined on a typical residential lot.

2.2 SUSTAINABLE DRAINAGE SYSTEMS

A good drainage strategy provides a balance between social, economic and environmental requirements. Sustainable Urban Drainage Systems (SUDs) are methods that take account of quantity, quality and amenity issues. It is an alternative to more traditional (conventional) drainage systems in terms of planning, design and management. The application of SUDs aims to reduce pollution and flood risk, whilst also improving the urban environment for those who live and work in it. SUDs are more sustainable than conventional drainage systems because they:-

• Manage the runoff from new urbanized areas and their impact on the existing network, thereby reducing the risk, frequency and size of floods
• Protect or improve river and ground water quality
• Contribute to the environmental setting and the needs of the local community
• Provide a habitat for wild life in urban watercourses
• Encourage natural groundwater recharge (where appropriate)

They do this by:

• Dealing with storm runoff close to the area of the rainfall
• Managing potential flooding at its source now and in the future
• Protecting water resources from direct pollution (such as accidental spillage) and diffusing any pollution.

SUDs is an alternative approach to conventional drainage design and implementation,

because it attempts to deal with drainage in a more natural way. Elements of SUDs that should be introduced into the surface water design and/or developed by the purchasers of individual site plots are:-

• Swales
• Passive treatment systems such as detention basins, retention ponds and wetlands
• Porous pavements for roads and car parks
• Infiltration basins and storage tanks

3.0 GROUNDWATER MANAGEMENT

Engineers worldwide have developed sustainable solutions that protect the environment and costs less.  This concept is termed WaterSmart. In a  WaterSmart Development is one in which the dwelling and its surrounding land are designed and used so as to minimize harmful impacts on the natural water cycle. The development responds to natural site features, takes advantage of natures own water supply (rain), uses water efficiently and helps maintain the quality of water in our rivers and streams.

By incorporating WaterSmart measures in the design of new developments, it is possible to help:

• Reduce flood risks in urban areas
• Prevent erosion of waterways, slopes and banks
• Improve water quality in streams and groundwater
• Make more efficient use of water resources
• Reduce the cost of providing and maintaining water infrastructure
• Protect and restore aquatic and riparian ecosystems and habitats
• Protect the scenic, landscape and recreational values of streams

The WaterSmart Development emphases on-site collection, treatment and utilization of water flows as part of an integrated ‘treatment chain’. Elements in the chain may include;

• Reuse of roof water for hot water systems, toilet flushing or irrigation
• Reuse of runoff or wastewater for irrigation
• Infiltration of stormwater to underground aquifers
• Specially designed and maintained landscaping for cleansing runoff and conserving water.

The WaterSmart Development requires sensitive responses and gives considerations to site characteristics such as soil type, slope, water table, rainfall, and the scale and density of development.

SITE PLANNING

The first rule is to understand the site. Before deciding where to place buildings, driveways and other structures, the wise designer identifies the opportunities and constraints of the site and integrates them into a ‘whole site’ approach. This allows existing problems to be dealt with and helps ensure that the final design is in tune with the site’s topography, climate, soils, vegetation and water. Such a design is said to respond positively to site constraints and opportunities.

RAINWATER TANKS

There is currently an enormous resurgence of interest in using rainwater tanks due to their many economic and environmental benefits. Rainwater collected from roofs and stored in tanks is an excellent water source for hot water systems, toilets, washing machines and garden irrigation. Benefits of rainwater tanks include reduced mains water demand, reduced water supply infrastructure costs, improved environmental flows downstream of water supply dams, and reduced concentration of stormflows in urban streams.

INFILTRATION DEVICES

Infiltration devices allow percolation of stormwater to the surrounding soil, whilst also providing temporary storage of storm runoff. This can have many benefits, including reduced concentration of stormflows, less reliance on piped drainage and increased groundwater recharge. 

Modern infiltration devices are designed and constructed so as to minimize clogging by silt material, and can be designed to overflow to landscaped areas or the street drainage system when their capacity is exceeded during major storms. A number of pollutant removal mechanisms operate within these devices, giving significant water quality benefits. Extensive research has shown that infiltration is a very practical option for stormwater and groundwater management.

PAVING

In present developments, paved (or ‘impervious’) surfaces such as roads, driveways and courtyards cover a very significant area. These surfaces have many harmful impacts on the water cycle. They contribute to increased peak and total stormwater discharges, increased downstream flooding, streambank erosion, sewer surcharges and the need for expensive infrastructure to mitigate nuisance flooding. Paved areas also reduce the volume of rainwater that infiltrates to the subsoil.

The impacts of paved surfaces can be reduced by:

• Limiting the area of paved surfaces
• Directing stormwater runoff from paved surfaces to landscaped areas, gardens and lawns rather than to the street drainage system.
• Using porous paving systems.

LANDSCAPE MEASURES

A wide variety of landscape measures can be used to manage stormwater flows, utilize stormwater within the site and minimize supplementary watering of landscaping. The careful design and placement of landscape measures can have many benefits for the water cycle, including reduced peak stormwater discharges, increased groundwater recharge, reduced erosion and sedimentation, increased retention of soil moisture and lower water costs. This is in addition to likely aesthetic and ecological benefits. Landscape measures include but are not limited to:

• Rock and gravel basins
• Vegetated filter strips
• Soak areas
• Wind and sun protection
• Plant selection
• Efficient irrigation

WASTEWATER REUSE

The majority of water used for indoor domestic purposes is discharged after use as ‘wastewater’. Wastewater can be collected by a reticulated sewage system and treated at a conventional wastewater treatment plant. Alternatively, it can be collected, treated and re-used on-site, thereby promoting more efficient water use. This has many significant economic and environmental benefits for the community. However, on-site reuse of domestic wastewater is subject to various restrictions due to concerns about effluent quality, maintenance and health issues.

GROUNDWATER

Groundwater extracted from bores can be an important water source for domestic use. Many urban areas occur over a suitable aquifer. Groundwater quality varies from place to place, and may be unsuitable for domestic purposes. For example, groundwater can be saline or be contaminated by human activities. Where groundwater quality is unsuitable, artificial recharge of stormwater into the quifer can often be used to produce suitable water supplies. This process is known as aquifer storage and recovery.

4.0  RECOMMENDATIONS FOR A STORMWATER AND GROUNDWATER MANAGEMENT POLICY ON ENVIRONMENTAL PROTECTION FOR NEW DEVELOPMENTS.

Stormwater runoff from intense storm events can pose serious risks to life and property. It is essential that the design of overland flow paths, on-site detention storages and other stormwater management measures meet relevant safety criteria for pedestrians, vehicles and property damage. Similarly poor considerations of groundwater management issues have negative impacts on the environment as discussed in the preceding paragraphs. It is therefore pertinent that all stakeholders in urbanization form part of the contemporary trend towards more ‘sustainable’ solutions that protect the environment.

Environmental impact of urban stormwater run-off is characterized by the high levels of sediment and other pollutants, both particulate and dissolved, together with the volume and rate of flow of the run-off causing flooding and erosion in the receiving water. The government in conjunction with the Nigerian Society of Engineers should develop appropriate design criteria to address these various effects for private and public developments.

The design criteria shall be developed to enable urban run-off (run-off from new developments) to more closely replicate the greenfield (predevelopment) condition in protecting river water quality and reducing other negative environmental impacts.

Appropriate design criteria to help protect the environment through stormwater and groundwater management include:

• No run-off shall pass directly from any property to public drainage channels for rainfall depths of 10mm and up to 25mm.
• Infiltration drainage techniques shall be designed for every new development ;
• Use of stormwater treatment techniques shall be mandatory for all developments covering a land area of more than 3500 sq.m.

In practice, there are a number of practical constraints in applying these criteria. 10mm of rainfall run-off from an urban area, especially with 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 washoff from certain surfaces, particularly roads, often contains high levels of polluted sediment and, depending upon the maintenance regime, will usually result in blockage of infiltration units over a period of time.

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. Where it is not possible to store and dispose of 10 mm of rainfall, it might be possible to intercept runoff from 5 mm, which will still provide considerable benefits.

Achieving zero runoff from the first 5mm or 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 above measures will also help in aquifer storage and recovery thereby reducing the high cost of water and drainage infrastructures.

References

Akintola, F.O. (1978). The Hydrological Consequences of Urbanization: A Case of Ibadan

City in Sada and Oguntoyinbo (eds). Urbanization processes and problems in Nigeria.

Ibadan, Ibadan University Press.

Argue, J.R. (2002). On-site Retention of Stormwater: Introduction and Design Procedures.

Urban Water Resources Centre, University of South Australia, Adelaide.

Carleton, M.G. & Ing (1992). ‘Stormflow reduction using site infiltration-detention’, Proc. International Symposium on Urban Stormwater Management, Sydney, The Institution of Engineers, Australia.

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