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Stormwater management concepts
By Earl Shaver and Sue Ira, who deliver the NZWETA Advanced Stormwater Design and Management course.
1. Introduction
This discussion presents a guide to the following stormwater management concepts of:
- site design,
- contamination control and treatment;
- common structural stormwater management practices and
- a summary of treatment mechanisms that they utilise.
2. Stormwater management concepts
2.1 Overview
Water is in a constant dynamic cycle between the land, water bodies and the atmosphere. Development alters the rate of water’s progress through the cycle, resulting in hydrological and water quality effects. The most effective forms of stormwater management try to redress this disruption by avoiding it as much as possible in the design phase. Where this is not possible, stormwater effects must be managed by constructed mitigated methods such as detention ponds and water quality treatment devices. Unfortunately, attempts at mitigation are usually only partially successful, as they control a limited proportion of contaminants and are restricted by technical and financial constraints.
Prevention is better than cure. Stormwater management solutions that fundamentally reduce the risk of stormwater effects are more successful as the potential effects are never generated. Even partial prevention is more useful than mitigation, in that the scale of mitigation required is reduced. Prevention is best achieved by integrating careful site design and contamination control measures.
The RMA outlines the multi- faceted, integrated approach to managing effects; Section 17 states: “Every person has a duty to avoid, remedy or mitigate any adverse effect on the environment arising from an activity...” In the context of stormwater management, the ‘avoid, remedy or mitigate’ concept matches three stormwater management concepts -
‘Avoid’ - Site design - practices which prevent stormwater becoming contaminated by reducing runoff or removing contaminant sources, e.g., use of non-zinc roofing materials, reduction of impervious area by porous paving.
‘Remedy’ - Contamination control
Source control - practices that contain contaminants or prevent them from contacting stormwater runoff, e.g. separation of stormwater and oil spills by bunding.
Management practices - work practices that avoid or reduce the potential for runoff to become contaminated, e.g. improved street sweeping practices, training staff in chemical handling procedures.
‘Mitigate’ - Treatment devices - constructed practices to reduce the quantity of contaminants in stormwater or retard the volume of flow e.g. constructed wetlands, detention ponds.
The purpose is to provide design guidance for treatment devices, and therefore primarily deals with the mitigative section of stormwater management tools. However, avoiding effects by careful site design and remedying effects by source control and management practices is a vital tool in the control of contaminants. Any one stormwater management tool, on it’s own, is unlikely to achieve the stormwater management objectives for any given development. For this reason it is necessary to consider the objectives early in the design process while competing demands can be carefully balanced and an integrated solution achieved. The need for, and size of, treatment devices is then minimised as is their installation and maintenance costs. The combination of a number of different tools or practices to achieve an overall stormwater management objective is called “The Treatment Train”.
Many of the effects of stormwater are, by themselves, very small. However when considered on a catchment basis, their cumulative effect is substantial - such as in the case of flooding due to gradual increases in upstream impervious areas. To manage these effects, we need to understand them on a catchment basis, where the effects are discernible, but prevent them on an individual site basis, where the physical changes to the hydrological cycle are made. This is the role of catchment management plans. They are a key tool for integrated stormwater management and are a range of the above approaches to achieve overall catchment objectives.
2.2 Site design
Site design, or runoff control practices, aim to fundamentally reduce the impact of development on the hydrological cycle by attempting to mimic pre-development rates of runoff, infiltration, and evapo-transpiration. To achieve this, we must carefully evaluate the components of a development proposal and identify how they will change the existing hydrological regime. Reduced infiltration, increased runoff and reduced evapotranspiration will result from the development. But, with careful design and control of construction processes, we can minimise the changes.
To manage the effect of development on runoff hydrographs, several defining rainfall events need to be considered to approximate predevelopment conditions as closely as possible to those post development. The 50%, 10% and 1% AEP events have been chosen for this purpose. The ARC considers that changes to the hydrological cycle are minimised by matching the pre and post development peak flow rates and minimising changes to the volume and duration for these events. This usually requires a mixture of site design practices and structural treatment practices.
Four techniques for runoff control are outlined below:
2.3 Existing site features
A natural site contains an existing drainage network with features such as watercourses, depressions, floodplains, wetlands, vegetation and permeable areas that contribute to the current balance in the hydrological cycle. By identifying, preserving, and integrating these features with the development where appropriate, changes to the cycle are minimised. the residual changes are thus easier (and cheaper) to manage.
2.4 Reduce imperviousness
Impervious surfaces affect water cycle processes by reducing infiltration and increasing runoff. By reducing imperviousness, the overall percentage of hard surfaces can be reduced and the permeability of the required hard surfaces increased. Using pervious channels or infiltration practices at the start of the treatment train for onsite infiltration or to collect and transfer stormwater to a downstream treatment practice reduces the effective impervious area of the development. In either case, the amount of runoff is reduced, which will subsequently reduce the necessary volume of stormwater treatment devices on site.
Some methods to reduce impervious areas:
- Reduce road widths to suit actual traffic densities instead of generic minimum widths
- Make lots closer to the main roading network to minimise accessway lengths
- Use grass swales for drainage to reduce concentration times and encourage infiltration
- Use porous pavements, gravel or grass for low density accessways and parking areas
- Place footpaths on only one side of a street
- Reduce parking requirements to a minimum
2.5 Clustering/lot configuration
Subdivisions traditionally require significant amount of earthworks to produce flat sites with house lots of very similar sizes. Typically, each will have a house, front yard, back yard and separate access to the road. All streams, vegetation and site features are lost to maximise the number of lots. However, by clustering houses, as shown in Figure 3-1, together with smaller lot sizes, existing site features may become common recreational resources. Overall site imperviousness is then reduced and the existing stormwater channels are retained.
Some methods to change the lot configuration include using:
- Smaller lot densities with common recreational areas
- Duplex or terrace housing configurations instead of single family lots
- The same accessways to service multiple lots
2.6 Minimise site disturbance
Earthworks compaction produces high strength but high density soil with reduced permeability. Even when not sealed with impervious surfaces, this reduces infiltration and increases runoff. To prevent changes to the hydrological cycle, it is therefore very important to avoid earthworks on areas that are to be retained as permeable.
Existing vegetation also plays an important role in maximising infiltration and promoting evapotranspiration. Organic litter beneath trees and smaller vegetation acts a sponge by capturing rainfall and holding it while it slowly infiltrates into the ground. By analysing the existing topography and natural site features and carefully planning around them, it is possible to integrate the development with the environment and minimise the areas of vegetation and earthworks disturbance.
Some methods to minimise site disturbance include:
- Minimise bulk earthwork areas during construction
- Avoid earthworks on future permeable areas
- Maintain riparian margins of watercourses
- Maintain vegetated areas to promote long term infiltration
- Replant vegetation on slopes
2.7 Contamination control
Source control and management procedures attempt to reduce or avoid contaminants getting entrained in stormwater runoff. These practices assume that the contaminant source is necessary for the successful operation of the business or activity, and seek to control the release of contaminants or remove them before they come into contact with stormwater. For example, service stations inherently use trade oils and petrol as their main business activity, but, they are required to cover the service area and shut off stormwater pipes during tanker deliveries to prevent the discharge of petroleum products to the environment via stormwater drains.
The ARC advocates that businesses that handle chemicals or produce wastewater carry out an environmental self-audit to identify actual and potential contaminant sources. An action plan should then be developed to eliminate any actual pollution and minimise the risk of potential pollution.
2.8 Source control
Source control practices identify contaminant sources and construct physical works to prevent them coming into contact with stormwater. The classic example is the above ground storage tank with a bund constructed around the tank. The bund volume is slightly greater than the volume of the storage tank.
Other examples include:
- Physical control structures such as bunding, spill containment
- Covering stockpiles of soil, waste products
- Directing washwater to sanitary sewer
- Covering “dirty” work areas such as truck washes or oil changing bays
2.9 Management practices
Numerous procedures can be designated as management practices, from council initiatives to regularly remove gutter dusts before they get entrained in stormwater to industrial protocols for handling chemicals. The common factor is that there is a process to be followed that minimises the risk of contaminant transfer to stormwater.
Council initiatives include:
- Street vacuuming
- Education initiatives
- Recycling
Industry initiatives include:
- Refuelling procedures
- Chemical handling procedures
- Staff training re proper disposal areas for wastes, chemicals etc.
- Proper storage for chemicals, fuel etc. i.e. not outside, forgotten
2.10 Treatment
Treatment practices attempt a difficult task; the removal of contaminants entrained in stormwater flows. Significant proportions of contaminants are dissolved in stormwater, and many others are attached to fine particles of silt which do not easily settle. Removing these contaminants needs a complex combination of processes such as sedimentation, adsorption, and filtration. When site size constraints and limited financial resources to implement treatment are also considered, the complete removal of contaminants from stormwater is basically not achievable. This is why the ARC emphasizes that opportunities for good site design practices and contamination control must be incorporated as a necessary precursor to effective treatment practices. This will produce a better overall result for treating the effects of stormwater.
The time of installation and the maintenance of treatment devices are important issues. Much of the impact of development occurs in the early stages of construction when the significant changes occur to the hydrological regime and large quantities of sediment are discharged during earthworks. The early installation of stormwater management facilities is the best defence to these changes and also provides a backup to earthworks controls. After development, stormwater devices require ongoing maintenance to ensure that inlets and outlets are not blocked and the full treatment volume is available to remove contaminants.
The remainder of this section provides an introduction to the different types of treatment practices.
(a) Sedimentation
Most particles suspended in stormwater are less than 120 um diameter. Coarser fractions, above 120 um tend to remain in gutters or get caught in catchpits. However, contaminants attach to particles less than 20 um in disproportionately high numbers, meaning that effective removal devices must target these very small clay particles.
Sediment coarser than medium silt (approx. 20 um) settles rapidly, but much longer settling times are required for finer particles to settle. Particles less than 10 um tend not to settle discretely according to Stokes Law but must flocculate before settling. The particle shape, density, water viscosity, electrostatic forces and flow characteristics affect settling rates.
The proportion of sediment and contaminated sediment removed can be improved by the following measures:
- longer detention times
- larger surface area for settling
- promoting laminar flow and reducing turbulence
- promotion of coagulation
(b) Aerobic and anaerobic decomposition
Microorganisms reduce soluble BOD (biological oxygen demand) and break down nutrients and organic compounds by aerobic and anaerobic oxidation. Once the aerobic microorganisms have taken up contaminants they die, and settle to the bottom of ponds where further anaerobic oxidation may take place. In anaerobic conditions, microorganisms can remove nitrogen by de-nitrification. This is an importance process in constructed wetland function.
Aerobic:
Organic matter + bacteria + O 2 => new cells + CO 2 + NH 3 + H 2 O
Anaerobic:
Organic matter + bacteria => new cells + alcohols/acids + bacteria =>
new cells + CH 4 + H 2 S + NH 3 + CO 2 + H 2 O
(c) Filtration and adsorption to filter material
As sediment particles pass through a filter bed or through soil, they may be removed in the following filtration processes:
- settling into crevices
- enmeshment in interstices (sieving)
- impingement onto filter particles followed by sticking onto particles (by electrostatic or other bonding)
Adsorption is the accumulation of dissolved substances on the surface of a media such as plants or filters. Dissolved substances can also be removed by adsorption to filter material and biological uptake by microorganisms living among the filter material.
(d) Biological uptake
Plants take up nutrients or metals from stormwater via absorption processes. However they may also re-release them to the water column when they die and decay.
(e) Biofiltration
A variation to the filtration mechanism is to use plants as the filter media. Contaminants adhere to plant surfaces or are absorbed into vegetation. This mechanism is a combination of filtering, reduced settling time and adhesion.
(f) Precipitation
Colloidal particles may, under the right physical-chemical conditions, flocculate and settle out, enabling sedimentation devices to sometimes remove apparently dissolved trace metals. The precipitation process may be slow, requiring large detention times, but may be assisted by mechanical flocculation or chemical additives.
3. Typical stormwater management practices
3.1 Water quality ponds
Ponds detain stormwater inflows to allow suspended solids to settle. There are two main types; wet ponds and detention ponds. Wet ponds have a permanent pool with very slow flow through the pond. Detention ponds have a temporary pool formed by capturing and releasing stormwater at a slow rate. Sedimentation is promoted by slow flows that give longer detention times and minimise turbulence. Aerobic decomposition and adsorption of contaminants on to plants provide secondary treatment benefits by removing some nutrients and further sediment.
3.2 Wetlands
Wetlands detain flows to allow sediments to settle, but also remove a significant proportion of contaminants by adhesion to vegetation and aerobic decomposition. Vegetation is an integral component of the wetland system and assists each of the treatment mechanisms. It reduces velocities and turbulence, provides significant surface area for silt adhesion and reduces dissolved metals and nutrients through biological uptake. Wetlands also have the potential to provide hydrological benefits in a similar fashion to detention ponds.
3.3 Detention practices
Detention ponds and tanks intercept stormwater flows, store it and release it at a reduced rate. Their volume is determined according to flood routing principles for a range of rainfall events.
Their primary function is to reducing flooding and erosion of the downstream channel, but they also contribute to water quality by retaining water, thereby giving silt particles some opportunity to settle out of suspension.
3.4 Filtration
Sand, topsoil or compost are filter media that can remove contaminants when stormwater is passed through them. Coarse sediment particles are generally removed by sedimentation and then silt (and attached contaminants) are removed by sieving and adhesion to filter media. Underdrains collect water at the base of the filter media and discharge to the outlet. Filters generally only service a small catchment area and therefore only give limited hydrological benefit from flow attenuation on a catchment basis.
3.5 Infiltration
Infiltration practices collect and hold water below ground for disposal to the groundwater table. Sediments are removed by filtering in the stone reservoir or by in situ soils adjacent to the excavation where the stormwater is stored. Practices include infiltration trenches, soakage pits and porous block pavements. Soils must be permeable enough to disperse stormwater in a reasonable time and ensure the practice is ready to receive further inflow. Consequently, infiltration practices are more often used in areas with volcanic soils. Infiltration practices can have significant hydrological benefits by assisting groundwater recharge.
3.6 Rain gardens
Rain gardens are a combination of an infiltration and filtration device. Water is directed to a local hollow where it soaks into a organic filter medium such as topsoil or compost. Some water soaks into the ground while the remainder is collected and piped to the stormwater drainage system.
3.7 Biofiltration
Passing stormwater through vegetation removes sediment particles by adhesion to the plants and organic material as it filters through them. Dense vegetation, low water velocity and a long exposure time through the vegetation are required to ensure reasonable effectiveness. Biofiltration practices may have multiple benefits by reducing impermeable area, assisting groundwater recharge and increasing hydrological response times.
Vegetative swales are well suited to collecting and treating non-point source flows from long impermeable surfaces such as roads and carparks.
3.8 Vegetative filters
Vegetative filter strips are another biofiltration practice. They rely on distributed flow to produce a thin layer of water passing through the vegetation to ensure reasonable treatment. They are generally only used in conjunction with another stormwater treatment practice (both upstream and down).
The above text covers material and themes from the Advanced Stormwater Design and Management course to be delivered by Earl Shaver and Sue Ira in Palmerston North and Christchurch in October 2007.
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