Inspection and Maintenance Guidelines

Abstract

World-wide implementation of stormwater treatment practices is increasing. These are integral to Water Sensitive Urban Design (WSUD). In the United States, their implementation grew at a rapid rate in the late 1990s through the beginning of the millennium.

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World-wide implementation of stormwater treatment practices is increasing. These are integral to Water Sensitive Urban Design (WSUD). In the United States, their implementation grew at a rapid rate in the late 1990s through the beginning of the millennium. For instance, in the state of North Carolina, nearly 20,000 stormwater treatment practices are in place. In Asia, Singapore is also seeing a rapid uptake of stormwater treatment practices, as part of its Active Beautiful and Clean (ABC) Waters’ initiative.

Generally, much effort focused on design nuances for additional pollutant removal or better hydrologic performances. There is limited peer reviewed literature on maintenance issues, with a few notable exceptions (e.g., Asleson et al. 2009; Hunt et al. 2011; Wardynski and Hunt 2012).

The issue of maintenance, or a lack thereof, is generally recognized in the stormwater management industry worldwide. Practices that have been neglected do not mitigate runoff volumes or flows as well as intended (Brown and Hunt 2011).

3.1 Establishing Levels of Appearance

One principle that can be applied to determine maintenance requirements is establishing Levels of Appearance (LOA). LOA is a concept based on Levels of Service used in transportation structures such as roads and car parks, and this is equally applicable to water features.

LOA would specifically apply to aesthetic conditions. Two categories, Manicured and Rustic, have been established. The safety and hydraulic/hydrologic efficiency should not to be compromised and essentially would not vary between two LOAs, at all times.

The Manicured category includes highly visible practices, such as those in parks, in residential areas and along the roadway. Rustic practices are located in nature areas, or areas that mimic nature. They are expected to have a more natural appearance and therefore require less frequent upkeep.

The principal difference between the two levels of appearance is not so much the inspection and maintenance activities, but rather, the frequency of these tasks. As expected, Manicured LOA practices will necessarily be inspected more frequently (from daily to monthly), while Rustic LOA may only require visits quarterly to semi-annually.

Knowing the design intent of the practice can help establish an LOA. In general, a practice visible to the public will have a Manicured LOA, except when the setting makes park users expect a more Rustic look. On the other hand, a practice with little visibility by the public is more likely to have a maintenance regime aligned with a Rustic LOA.

For each practice, inspection and maintenance tasks are associated by their nominal frequencies. It should be noted that certain designs and systems may require more specific maintenance regimes. All Manicured LOA practices have a nominal long-term inspection frequency of once per month, but a more frequent inspection (such as weekly or daily) may be necessary, particularly immediately post-construction.

For cities with high rainfall distribution, the stormwater practices are designed to fill substantially with water on a frequent basis, and therefore require more frequent inspection.

It is also important to have in place a program to constantly train maintenance workers, in anticipation of turnover of personnel in maintenance positions.

The basis of maintenance for stormwater treatment practices is three-fold. Practices must be maintained sufficiently so that they are: (1) safe, (2) function hydraulically and meet water quality needs, and (3) aesthetic and litter free. The plant maintenance requirements will vary based on the LOA category.

The common practices, listed in general order of decreasing size are:

• (1) Constructed Stormwater Wetlands and Wet Ponds,

• (2) Sedimentation Basins,

• (3) Bioretention (Landscape Infiltration) and Bio-Swales,

• (4) Permeable Pavement,

• (5) Swales,

• (6) Infiltration Trenches, and

• (7) Sand Filters.

In the subsequent paragraphs, a description of each practice and the respective inspection and maintenance needs is outlined.

3.1.1 Constructed Stormwater Wetlands and Wet Ponds

The distinction between ponds and constructed wetlands has blurred, as most ponds now incorporate wetland zones along their perimeter. Inspection for vegetative debris, litter/rubbish, and vandalism (in some cities) in both the outlet or inlet structure is critical. If the outlet structure clogs with debris, the pond’s storage volume will decrease and plant survival becomes an issue.

A simple way to detect clogging is to determine whether the normal pool (water’s surface) is abnormally high; this can often be accomplished by visual observation. Several other items should also be inspected:

• (1) presence of odours indicating an acute accumulation of pollution,

• (2) erosion in forebay/sedimentation zone or spillway

• (3) animal damage, particularly from larger species like monitor lizards, and

• (4) eutrophication, commonly identified by an abnormal amount of algae (possibly an algal mat) in the pond or wetland.

Certain maintenance activities can be performed during inspection. They include:

• (1) skimming of floating or trapped litter/rubbish and vegetative debris and

• (2) removal of invasive vegetative species

Another maintenance task is mowing the wet pond’s banks and perimeter, as ponds often have manicured grass up to the water’s edge. The frequency of mowing is dependent upon the required aesthetic appeal and safety issues pertaining to the wet pond and ranges from at least once per month (Manicured LOA) to as few as four times per year (Rustic LOA).

Of the monthly maintenance tasks for Manicured LOAs, the more important with respect to safeguarding the wetland or ponds’ function are making sure the outlet is not clogged and verifying that there are no burrows or tunnels made by animals that artificially lower the water level inside the wet pond. If this maintenance is not performed, the hydraulic and water quality function of the wetland or wet pond will be compromised and a consequent high probability that the designed-for vegetation (vital to pollutant removal and aesthetics) will not survive.

Due to the design that includes an extensive coverage of emergent vegetation, stormwater wetlands run a higher risk of having their outlets clog than ponds.

Maintaining desirable plant species is also important for mosquito control. A few invasive species in the United States—particularly Typha spp. (cattails) and Phragmites australis (common reed), and Salix nigra (black willow) have been observed to form monocultures that may become habitats for mosquitoes (Greenway et al. 2003; Hunt et al. 2006).

Skimming rubbish from the pond will prevent future clogging of the drawdown outlet as well as improve the pond’s aesthetics and eliminate a likely mosquito habitat.

Certain inspection tasks can be performed much less frequently. These include:

• (1) examining root intrusion into inlet or outlet pipes,

• (2) assuring that the vegetated zones are, in fact, vegetated,

• (3) determining the depth of sediment in the forebay,

• (4) verifying dam health, and

• (5) confirming the inlet rip rap apron is stable.

These inspection tasks may lead to the following maintenance tasks:

• (1) clear inlet and outlet channels as well as internal drainage structures of roots,

• (2) verify that emergency drawdown valves and appurtenances function, and

• (3) sediment removal.

The frequency of tasks varies according to the configuration of the pond, watershed stability, and watershed composition. All of these maintenance tasks are essential to ensure both safety and the water quality and hydraulic function of wet ponds and wetlands.

The forebay should be excavated when sediment occupies 50 % or more of the forebay volume. Because the forebay in wetlands and wet ponds tend to be 1–1.5 m deep, a good rule of thumb is that if the average sediment level is within 0.5 m of the surface, the accumulated solids in the forebay should be excavated (dredged). For some cities, a second trigger mechanism for dredging could be whether any sediment or rubbish is visible at the water’s surface (normal pool) or above in the forebay. The required frequency of forebay dredging varies from once per 6 months in developing, unstable catchments to being extremely rare (never or only once in the 20–30 year life of a wetland or wet pond) when sited in a stable catchment. If the forebay fills with sediment and is not removed, the remainder of the pond or wetland will begin to “silt in,” essentially filling portions of the practice that are not designed for sediment capture. The main consequences of a silted-in stormwater practice are degraded pollutant removal capabilities and a degraded appearance. Excess sediment in the body of the wetland or wet pond will change the wetland plant population by creating areas elevated higher than as designed.

Constructed wetlands and wet ponds are designed to maintain a specific volume of water downstream of the forebay. This volume is to be preserved for stormwater runoff, thus, sediment accumulation in these areas is not desirable.

It is essential that wetland drawdown outlets are inspected for clogging. Clogged outlets may increase the water elevation inside the wetland for extended periods, potentially killing much of the desired vegetation. This will dramatically decrease the stormwater wetland’s ability to remove certain dissolved pollutants, particularly some nitrogen species.

The wetland and wet pond environment is a convenient place for some animals to live, especially monitor lizards. While these large lizards are skittish and avoid human contact, when they bed down in a wetland (or wetland-like fringes of a pond) they can damage vegetation. Maintenance personnel should also be wary of pythons and species of poisonous spiders that can live in wetlands and wet ponds.

A summary of the most likely maintenance needs and their triggers is presented in Table 3.1.

Table 3.1 Common wetland and wet pond maintenance needs and drivers

3.1.2 Sedimentation Basins

Sedimentation basins are related to wet ponds and wetlands, as often the latter two practices were initially designed to be sedimentation basins during construction. The purpose of sedimentation basins is to capture sediment generated by upstream construction. Thus, the main maintenance requirement for sedimentation basins is to remove collected sediment and other rubbish and debris. As sediment collects, the basin becomes more unsightly.

Most sedimentation basins take on a Rustic LOA, as they are associated with catchments under construction. However, if a sedimentation basin is located in an urban area that is frequented by people, its LOA may be raised to Manicured. By design, sedimentation basins are less attractive than other WSUL water features.

The ease of maintenance is dictated by the sedimentation basin design. To facilitate maintenance, designers should allow for access by excavators/diggers to all parts of the basin. If access to the basin is restricted, so that only a part of the basin can be reached by the bucket of excavators, maintenance cost in terms of personnel time increases dramatically.

The sediment depth should be checked frequently. A monthly regime is recommended. The designer should establish a threshold, which when met, triggers excavation of captured sediment. Thresholds could be:

• (1) exposure of (or visible) sediment across a 10 % footprint of the sedimentation basins, or

• (2) the average depth of sediment is within 0.5 m of the surface.

Setting stricter thresholds could be necessary if digger access is limited and sediment build-up needs to be minimal. If sediment from the basin is unable to be excavated using a digger, then small bobcats or crews with shovels and wheelbarrows are needed.

Clogged outlet structures and sediment accumulation are important concerns with sedimentation basins. Because they are usually constructed in catchments with active construction, the likelihood of rubbish accumulation inside the sedimentation basin is therefore high. If the outlet structure clogs, and water levels increase inside the basin, upstream flooding may occur. Like wet ponds, as sediment occupation increases, the volume available to capture stormwater diminishes. Inlet inspection and internal erosion are also concerns.

Common maintenance needs and triggers are listed in Table 3.2.

Table 3.2 Common sedimentation basin maintenance needs and triggers

3.1.3 Bioretention Systems and Bio-swales

Bioretention systems, also termed biofiltration and rain gardens, are growing in popularity and usage in cities like Singapore. Locations for this practice include alongside roadways, in parks, adjacent to car parks, and in residential areas. Bioretention systems are expected to nearly always have a Manicured LOA, due to their locations that have easy public access.

Much of the maintenance needs of bioretention cells are aesthetic in nature. Considering that bioretention cells are sometimes used in lieu of standard medians in car parks or in place other landscape features, the maintenance requirements, while more costly per hectare treated than wet ponds and constructed stormwater wetlands, are actually very similar to standard landscape maintenance costs. Many maintenance tasks which are aesthetic also improve both safety and hydrologic and water quality performance.

Like any landscape feature, bioretention cells must be pruned and initially mulched, watered and limed. Some bioretention cells are grassed and require mowing. Because vegetation is an important investment and is essential to the aesthetic appeal of bioretention systems, vegetation needs to be established as quickly as possible. In some cases, the need for rapid establishment could require the bioretention system to be limed. This need may be established through conducting a soil test. Where soil alkalinity is a problem, ammonium sulfate may be added to the grounds. This may help establish vegetation within bioretention system.

Vegetation may need to be spot-fertilized to ensure rapid growth. If plant establishment occurs near a drought period, watering the plants every 2 to 3 days until a rainy period recommences is recommended. The frequency of these tasks varies based on the age of the bioretention cell. Newer bioretention cells require more frequent maintenance (weekly), while cells with well-established vegetation may only require monthly visits.

Bioretention outlets are prone to clogging. It is imperative that outlets are checked on a monthly basis to ensure they are free of litter and debris. This is particularly important if a grate is used on the highflow bypass structure. Nearly every bioretention cell has an overflow, so if it cannot be found, the location should be checked using the engineering design plans.

Some bioretention cells in temperate climates have been found to clog due to sediment accumulation (Brown and Hunt 2011). Dense and uniform root depth helps to maintain good infiltration throughout the cell, as has been shown in Australia (Bratieres et al. 2008; Lucas and Greenway 2008).

When bioretention cells are surrounded by landscapes under active construction, they are at high risk of clogging by sediment. In these bioretention cells, it is important to inspect the bowl for water collection and to identify any wet spots in the media, particularly if it has not rained in at least 2 days. If surface clogging is observed, the top layer (25–100 mm) of fill soil needs to be removed. Because clogging occurs most frequently at the top of the soil column, the bioretention cell rarely needs to be completely excavated.

While not included as part of bioretention maintenance, protecting the perimeter of the bioretention cell from construction-generated sediment has a dramatic impact on the severity of clogging and the type of maintenance required resuscitating the bioretention cell’s infiltration rate.

To prevent premature clogging of bioretention cells due to sediment accumulation, designers are recommended to incorporate pre-treatment. The most commonly used pre-treatment in temperate climates in descending order are:

• (1) gravel verge (thin strip) with sod surrounding the perimeter,

• (2) grass swale, and

• (3) forebays.

Installing sod on the down slopes of the pavement provides an immediate layer of pre-treatment before runoff enters the bioretention cell proper. The sod is part of a grassed filter strip. The minimum width required for the sod filter strip is 1 m, with 1.5 m recommended. In addition to trapping pollutants before they reach the bioretention cell media, the sod immediately stabilizes the perimeter of the bioretention cell, preventing “internal” erosion from occurring. In the tropics, cowgrass (Axonopus compressus) and Nabhali (Cyanotis cristata) most are resilient turf grasses often considered. The larger the grass filter strip, the more important mowing becomes as part of its maintenance regime. The grassed filter strip functions best when it is not level with the pavement. A 50–75 mm drop from the pavement edge to the filter strip is recommended. This prevents preferential flow through the filter strip and reduces the risk of rill erosion.

Like those of wetlands and wet ponds, bioretention forebays capture larger sediment particles and litter/debris. They should be checked regularly and noticeable accumulations of sediment and litter should be removed as needed. The frequency of this maintenance task is associated with the cell’s LOA, but is expected to range from a few times a year to once every 3–5 years.

Another practice that is becoming common in cities is the bio-swale. Bio-swales provide similar pollutant treatment mechanisms as bioretention cells, and they also filter water through a permeable media. The two principal differences between bio-swales and bioretention are

• (1) bioretention is designed to pond water, while bio-swales are not and

• (2) bio-swales convey water, while bioretention cells pond water.

Outlet structures in bio-swales need not become water collection points, and are therefore nearly flush with ground level. Checking outlet structures will be paramount to bio-swale success.

Like bioretention cells, most bio-swales are expected to be Manicured LOA, thereby requiring weekly to monthly inspection and simple maintenance. All of the maintenance tasks reviewed for bioretention is applicable to bio-swales.

Table 3.3 summarises the most likely maintenance needs and their triggers.

Table 3.3 Common maintenance needs and triggers for bioretention and bio-swales

3.1.4 Sand Filters

The sand filter comprises two main chambers: a sedimentation chamber, where gross solids and some sediment collect, and a sand chamber which filters some pollutants which remain in the runoff. Sand filter maintenance is relatively simple, but somewhat laborious and expensive. Both chambers require inspection and maintenance on regular intervals. Because sand chambers are usually hidden from the public, they are likely to require a Rustic LOA.

On a monthly to quarterly basis, the sand chamber needs to be checked to see if the top layer of sand has become clogged with fine particles, which limits the infiltration rate through the soil. This clogging is identifiable by a thin (few millimetres in thickness) black or dark brown/gray layer resting on the sand surface. This is referred to as the smutzdecke, German for “dirty floor”. If this layer of smutzdecke is observed at a given site, it should be broken up using a hard garden rake. Typically only the top 25–50 m forms a crust, therefore the depth the rake needs to penetrate is not substantial. The broken up smutzdecke does not need to be removed regularly, because it contains microbes that facilitate the breakdown of petroleum products. The frequency of this maintenance task varies, but is expected to occur between quarterly to annually.

However, when the sedimentation chamber needs to emptied of the accumulated debris, which occurs much less frequently than breaking up the smutzdecke, it is advisable to then excavate the top layer of sand (perhaps to a depth of 75–100 mm) and transport this material away with the spoils from the sedimentation basin.

When the material is taken from the sand chamber it must be replaced. The type of sand needed to “top off” the sand chamber should be specified in the design plans. If the design plans are not available, a clean sample of the existing sand media can be found at a depth of 20–30 cm in the current sand media chamber. Taking a sample at this depth and analyzing it will allow the proper sand media needed to “top off” the sand chamber to be identified. It is possible the sample will need to be send to an engineering laboratory.

The depth of gross solids in the sedimentation chamber should be inspected each site visit; one can use a shovel or a crowbar. If the sedimentation chamber is at least one-half full of detritus and trash, then the accumulated debris should be removed. Additionally, if a stench arises from the sedimentation chamber, it is likely a good time to remove the accumulated solids, even if the depth of solids is not yet 50 % of capacity. Removal of debris can be accomplished with a vac-truck or a small excavator.

The process for permanently disposing sand filter pollutant material is currently under debate. In some countries, the disposal practice is to deposit the accumulated pollutant in a lined landfill. In future years, it may be necessary to have the polluted media tested to determine the toxicity of its contents before landfill disposal is allowed.

Sand filters are often accessible to vehicle traffic and as a result have heavy grates overlaying them. Removing and replacing metal grates is an additional expenditure of time and effort. Another unique sand filter maintenance need is to remove vegetation and/or trash blocking the path of water from the sedimentation chamber to the sand chamber, which impedes water passage. If a sand filter still has collected a pond of water for more than 12 h after a rain event, it is likely clogged.

Table 3.4 summarise the likely sand filter maintenance needs and triggers.

Table 3.4 Common sand filter maintenance needs and triggers for sand filters

3.1.5 Swales

Swales are used primarily to convey water, but also provide treatment of solids and pollutants associated with sediment. Swales are related to bio-swales, in that they both convey water, but bio-swales have a specialized fill media and tend to be vegetated with a larger variety of species. Most swales are predominantly turfgrass.

Swales are maintained similarly to their surrounding landscape. Mowing and weeding at regular intervals ensures a clean appearance. Care must be taken to avoid mowing when the swale is wet, otherwise rutting may occur. Also, mower blades must be sufficiently high so that the turfgrass is not scalped or cut too low. It is recommended to set the height of the mower at least 70 mm high, with 100 mm preferable, for turfgrass health.

Because swales convey runoff, they also transport litter and other solids. Depending upon the surrounding land use, it may be necessary to remove litter weekly.

Occasionally, the swale will receive flows exceeding their capacity. This results in either overtopping of the swale’s banks or erosion within the swale. Internal swale erosion is most likely apparent after a very intense rain event and should be repaired shortly after discovered. Eroding swales can “migrate” causing a more expensive repair to be needed with time. The simplest fix is to reline the toe of the swale with rocks or perhaps use a turf reinforcement mat that allows grass growth but provides erosion-resistant structure.

When swales are constructed in flat topography, or they are built as small cascades, they run the risk of pooling water. Water pooling in swales facilitates mosquito survival. While wetlands and wet ponds also have standing water, they also provide habitat and attract mosquito predators. Bio-swales and bioretention cells both drain water through a media, reducing the likelihood of mosquito presence. Swales, however, neither attract mosquito predators with their vegetation, nor do they have a mechanism to dewater.

There are two basic solutions to the small pools within swales:

• (1) to drain these pools, by changing the plan form of the swale (increasing its slope) or

• (2) ensure pools are identified and provide mosquito treatment within them.

Biocides are available in many parts of the world; they are called “mosquito dunks” in the USA.

Sometimes swales will intersect high water tables or have water backed into them from nearby waterways. There is very little that can be done in this situation, as the turf will die. Sometimes these wetter swales can be transformed into linear wetlands. Mosquito issues will always be of concern here, particularly in the rainy season. Depending on the water depth, lining the bottom with larger rocks might be a feasible and more attractive.

A summary of the most common swale maintenance needs and triggers is presented in Table 3.5.

Table 3.5 Common maintenance needs and triggers for swales

3.1.6 Infiltration Trenches

Infiltration trenches resemble swales and bio-swales. However, they are not designed to have vegetated cover. As its name implies, they are specifically intended to infiltrate runoff. Infiltration trenches may have either a Manicured or Rustic LOA, depending upon their location.

The primary maintenance concern for infiltration trenches is sediment collection. The gravel trench will clog with substantial sediment accumulation. The infiltration trench’s catchment should be inspected regularly. Sediment may migrate deeper into the gravel column, but not be visible at the gravel surface, so digging into trench, perhaps to a depth of 150 mm, is important to detect long-term clogging sediment.

Visiting infiltration trenches during storm events is a good way to determine if the trench is working. Infiltration rates of non-clogged gravel can easily exceed 1,000 mm/h, which exceeds precipitation + runoff rates into the infiltration trench. If during a rain event, runoff appears to flow right over the infiltration trench, it is likely clogged.

If the trench is inspected outside of a rain event, other signs of clogging besides exposed sediment include moss and other vegetation growing on the surface, as well as collected water. The repair may be costly, as the gravel trench will need to be removed, accumulated sediment excavated from the exposed trench, and clean gravel replaced in the newly-formed cavity.

When infiltration trenches are constructed in flat landscapes or other locations where the water table emerges at or is near the surface, the infiltration trench will not work (lack of infiltration). There is nothing that can really be repaired by maintenance personnel; the issue is a design and site-specific problem.

A summary of common infiltration trench maintenance and triggers is found in Table 3.6.

Table 3.6 Common infiltration trench maintenance needs and triggers for infiltration trenches

3.1.7 Permeable Pavements

Permeable pavements are widely adopted in North America and Europe. When designed and installed correctly, permeable pavements effectively reduce runoff volumes and pollutant loads. Because permeable pavement is most likely used in car parks, it is nearly always going to have a Manicured LOA.

Permeable pavement allows water to pass through the surface and be stored in an underlying gravel storage layer. Water then drains from the gravel storage zone or infiltrates into the in situ soil.

Functionally, permeable pavement must remain permeable. As the surface of permeable pavement clogs, the pavement will begin to generate runoff. Main causes of clogging are vegetation/detritus and sediment accumulation in the permeable gaps of the pavement. Therefore, the principal maintenance tasks associated with permeable pavement focus on prevention of complete surface clogging.

Permeable pavement should be inspected during heavy rain events. Because by design, permeable pavement is expected to have infiltration rates exceeding 1,000 mm/h. Even the most intense rain should fail to generate runoff. Anywhere that runoff is observed to occur is therefore clogged.

There are areas most susceptible to clogging:

• (1) landscape-hardscape interface (particularly including pavement located under tree canopies),

• (2) impermeable pavement-permeable pavement interface, and

• (3) along the paths of “dirty” vehicles (such as rubbish collecting trucks).

Thus, these are the prime locations where runoff generation may occur and these should be inspected most frequently. If the surface of the permeable pavement has either turned dark in colour or has vegetation growing in it, then the pavement is likely clogged. Either is a sign of sediment and/or organics collection at the surface.

If clogging is observed, the pavement should be maintained. In many parts of the world, this is done with a street sweeper. The general type of sweeper used depends upon the permeable pavement type and the severity of clogging (Table 3.7).

Table 3.7 Comparison of street sweepers for permeable pavement

If a clogged permeable pavement application is unreachable by a street sweeper or vacuum truck, the surface can be painstakingly unclogged using a portable industrial strength vacuum cleaner, referred to as a “shop vac” in North America. To successfully clean permeable pavement using a “shop vac,” suction must be hand applied to every joint that is clogged. Often multiple passes are needed.

Ideally, vegetation and detritus that could potentially clog permeable pavement is removed before it does so. This may be accomplished by air blowing detritus off the pavement. This is clearly preventative maintenance. The frequency at which air blowing should occur is dependent upon rates of leaf litter collection on the surface of permeable pavement.

Permeable pavement, like any pavement, is at risk to stain due to vehicle leaks. Assuming a Manicured LOA is needed, these stains need to be removed. Biodegradable detergents can be applied to oil and grease stains and then water blasted from the pavement surface. This has anecdotally removed 85–90 % of the stain, with the remainder resembling something like a water mark. If a Rustic LOA is acceptable, and provided the permeable pavement has a darker hue, it may be reasonable to forego stain removal.

Table 3.8 highlights the most common permeable pavement maintenance needs and triggers.

Table 3.8 Common permeable pavement maintenance needs and triggers

3.2 Mowing and Turf Grass

Nearly every vegetated practice can be partially comprised of turf grass. Swales are turf grassed-lined long their batter slopes and, if small enough, their “channel” bottom. Bioretention employs turf grass as a short filter strip that water passes through from a paved surface. The perimeter of ponds and wetlands are often ringed by turf as part of their desired appearance. This turf grass is often intended to be a final gross solids filter for runoff prior to entering the pond or wetland.

As such, after keeping practices litter free, mowing turf grass is the most unilateral and common maintenance practice. In general, the higher the shoot (or blade) of the grass is, the deeper the root. Deeper rooted grasses are better able to tolerate weather extremes, namely droughts. Ideally then, mowing regimens and recommendations should reflect the desire for taller, but still aesthetically pleasing, turf grasses.

Cow grass, a commonly used in the tropics, forms a very dense mat. It also has a dwarf variety. Table 3.9 establishes targeted mowing cut heights for cow grass with respect to location in the landscape. Mowing or cutting is site- and technique-specific.

Table 3.9 Guide to mowing height and frequency

It should be noted that, particularly in bioretention and swales, the underlying soils are prone to be wetter than those in surrounding landscapes. This is due to runoff passing over this turf. Because the underlying soils tend to be wetter, mower rutting is more likely. Maintenance personnel will need to be mindful of rutting and either use different equipment or wait for a slightly drier condition before mowing (if possible).

3.3 Removing Unwanted Invasive Species

Wet ponds with aquatic shelves, stormwater wetlands, poorly draining bioretention cells, and even swales can become overgrown with invasive species. A few most common invasive species are Eichhomia crassipes and Hydrilla verticillata. Cattails (Typha angustifolia spp.). Cattails are tolerant of a variety of environmental conditions and can sequester pollutants (Wang et al. 1993). From a function standpoint, cattails may be considered a good species of plant to have in any of stormwater treatment practices. However, a practice that is overgrown with cattails is not a diverse ecosystem. Ecosystem diversity is critical for mosquito control. Rafts of cattails provide a safe environment for mosquito larvae to mature to adulthood.

Removing cattails can be tricky. Cattails grow by rhizomes and also spread by seed. If a piece of rhizome is left in the soil after cattail removal, the stand will re-establish. One means of mass cattail removal is by way of a digger. This is recommended when a wet pond or wetland is completely overgrown by cattails.

If a stormwater practice, particularly a wetland or wet pond, has a diversity of vegetation, but cattails are beginning to colonize, broadcast spraying of herbicides must be avoided, as broadcast spraying will kill not only the cattails, but other desirable vegetation as well.

The frequency of cattail removal somewhat varies. Factors influencing the need to “wipe” cattails are the density at which the wetland or wet pond is planted with desirable species and the maturity of the practice. It has been observed in temperate and sub-tropical climates that during the first 1–2 years, cattails should be removed twice annually. As the wetland matures and desirable species begin to firmly establish, the maintenance frequency is reduced to once annually at the most. The amount of time needed to remove unwanted vegetation varies, but a well-maintained mature wetland will likely require between 20 and 30 min per 0.10 ha of impacted wetland (or wetland shelf within a pond) per visit.

Simple plant removal, like weeding, may sufficiently control invasive and other undesirable species if performed at a very high frequency—such as daily visits. It is critical that maintenance personnel be trained in the appearance of undesirable species early in their life cycle. This technique of weed control is labour intensive, particularly initially. As the practice matures, the frequency decreases.