Answers to Frequently Asked Questions about Water Management and Green Infrastructure
The Greater New Orleans Water Collaborative includes architects, landscape architects, engineers, scientists and attorneys. Under the guidance of the Builders and Designer Working Group, these experts will field your questions about integrated urban water management. Every month or so, they take on a new topic. To submit a question, use our Ask a Pro form.
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Q: I’ve seen some news stories about subsidence in Greater New Orleans. Sounds pretty complicated. Can green infrastructure really reduce subsidence?
A: Subsidence is complicated and can be the result of a number of conditions. Here we will discuss localized subsidence that occurs at different rates in different areas of the New Orleans Region. To be clear, green infrastructure cannot stop or reverse subsidence , but green infrastructure can have positive impacts with regard to localized subsidence. In particular, it can minimize settlement of the upper soils layers , which can improve the stability of our homes, roads, and utilities.
In our region, and particularly south of Lake Pontchartrain, three general types of soils can be found: clay, sand, and organic. Even among these many variations occur, but the characteristics of each of the general soil types is comparable. Clay soils are low in porosity, and dry out and shrink when water is drained out of the subsurface. When water returns to clay soils, they swell back in volume. Sandy soils maintain their volume whether wet or dry and are highly permeable. Organic soils have various levels of porosity, but are more porous than clay soils. When water is drained out of the soils, the organic material is exposed to oxygen, resulting in oxidation of the organics. This is a chemical process, which means that organic soils do not return to their original volume when water is added back again.
Groundwater levels keep subsurface soils saturated and can be compromised in two ways. Impervious surfaces, such as rooftops, roads, and parking lots prevent rainwater infiltration, which would occur in natural conditions (e.g., a field, or swamp). The second cause is our drainage systems in New Orleans that drain and pump rainwater out of urban areas, which is necessary to prevent flooding in many instances, but these systems have also caused, to some degree, the depth of our groundwater to drop below natural levels. Subsidence is primarily caused by the loss of buoyancy of near-surface soils as groundwater lowers. This increases the weight of the near-surface soil and results in additional loading of the underlying soils (i.e., the near-surface soils become heavier). This new load compresses the underlying soils and results in subsidence. This is an inevitable consequence of area drainage and a constriction of our city existing at or below sea level.
Both conditions result in lowering the water table, even for only a period of time, which over many years has caused subsidence in New Orleans. Subsidence, consequently affects our walkways, roadways, underground utilities, building foundations, and other infrastructure.
We know that natural groundwater levels cannot return fully. However, green infrastructure can retard groundwater level declines. So it is important to allow as much rainwater as possible to infiltrate our soils, rather than pumping water into Lake Pontchartrain, and to also reduce the impervious surfaces we build at our homes and at our work places.
Q: My neighbors built a bioswale along the side of their property. They said it would solve a drainage problem. Can you explain what bioswales are?
A: Shallow channels with wide side slopes that use native plants to slow stormwater and filter out pollutants.
Bioswales are green infrastructure facilities whose primary function is conveyance of stormwater runoff. Bioswales are used as linear features to convey water from one to piece of green infrastructure to another, thus creating a stormwater treatment system. Bioswales can be used across a broad range of projects, from green spaces like parks, ball fields, and residential yards to paved areas such as plazas and roadways.
Urban bioswales are most often used along urban streets where long linear spaces are available and the conveyance of stormwater is needed. These bioswales may have hard edges along their length to fit and work in tight urban environments.
The width of urban bioswales is limited by the space available and the project design intent. Defined edges can be concrete, brick, modular pavers, and other hard materials. Along streets, curbs may define the bioswale. Small gaps in the curb allow runoff from the roadway to enter along the length of the bioswale.
Q: I have seen discussions of bioretention cells. What are they?
A: Land areas planted with water-loving plants.
Bioretention cells are designed to detain stormwater to allow for both infiltration and filtration. The name “bioretention” refers to the biological processes plants use to uptake and retain pollutants in their vascular system.
Bioretention cells are used in street curb extensions or bulb outs, plazas, and other urban open spaces. Because their size and shape can vary substantially, cells are well-suited to fit in most locations.
Bioretention cell size is determined by the volume of stormwater it must manage from the catchment area. Length and width can vary substantially and are determined by site limitations and the project’s design intent. Depth can vary based upon amount of water, void space of aggregate specified, above-ground storage depth, and site groundwater table.
Inflow of stormwater can be accomplished by way of overland flow, gaps in hard edges such as curbs, and pipes that connect to another green infrastructure facility or the storm drain system. Bioretention cells must have an outlet to the storm drainage system to provide for overflow during intense storms and to gradually empty the cell. The cell’s outlet pipe connects the storm drain system at an existing catch basin or manhole; if none is available a new manhole must be constructed. Edge conditions can vary substantially based on adjacent land cover and the project’s design intent.
Q: Doesn’t our high water table prevent water absorption?
A: No. But it does help determine what type of green infrastructure to use.
The water table is high in some areas, particularly when the river level is high, and often lower close to pumps and canals. Everywhere, the water table fluctuates by season due to varying amounts of rainfall. The underlying soil type may also be a factor.
Our region’s approach to stormwater has been to drain it away into canals and pump it into Lake Pontchartrain. Canal water levels are kept low. As a result, our clay soils shrink and our organic soils decompose, causing subsidence. The clearest evidence that our water table is not as high as we often think, or perhaps as it once was, is where subsidence has occurred.
Subsidence and runoff keep water from entering the ground. Green infrastructure acts as a counterbalance and contributes to the enrichment of depleted water tables.
Soil borings and infiltration tests are performed before designing or implementing green infrastructure. These are required by the City of New Orleans Comprehensive Zoning Ordinance, Article 23, and are considered industry best practices. After these tests have established the depth of the water table and soil type, green infrastructure can be designed accordingly. A high water table doesn’t mean green infrastructure isn’t an option, but it does help determine the best installation for a particular site.
The beauty of green infrastructure is that it’s not just one thing or one solution. It can be an urban forest or constructed wetlands to provide wildlife habitat. It can also be a streetside rain garden planted with attractive native plants that help filter and absorb the captured rain water. Because New Orleans only receives rain approximately 54 times a year, rain gardens will be dry the majority of the year. In an area of New Orleans, where the water table is high, this would be a benefit for the plants in the rain gardens allowing their root systems to continually drink up.
Q: Won’t canals, rain gardens and other green infrastructure breed more mosquitoes?
A: If designed correctly, these features actually prevent mosquitoes.
Mosquitoes can live in a variety of habitats where water is present. The mosquito life cycle typically takes 7 days for development from larvae to adults. The insects can lay eggs in nearly any amount of water; as small as a bottle cap. However, few mosquito species can procreate in moving water, and fish species present in natural water bodies often consume mosquito eggs or larvae. Thus, Bayou St. John and canals in which water is kept circulating are not mosquito breeding grounds.
Green Infrastructure, if properly designed, detains water temporarily and prevents standing water for more than 72 hours. Persistent standing water may indicate the need for a green infrastructure solution, such as a rain garden or bioswale.
The New Orleans Mosquito, Termite, and Rodent Control Board monitors mosquito populations throughout the City. Their testing at green infrastructure sites installed in multiple neighborhoods shows no increase in mosquito breeding occurs in properly designed and installed green infrastructure.
At your own home or business, ensure standing water drains within 48 to 72 hours; that buckets, garbage cans, lids, and other containers located outside are covered or turned upside down; and that your property drains well. Low spots that collect water can be turned into rain gardens. These gently sloped landscape features collect rainwater and allow it to filter into the soil or be absorbed by plants. A landscape architect can help you test and amend the soil, select appropriate plants, and install overflow drainage if necessary, ensuring that standing water will not result.