Content Objectives
Land Use and Impervious Surfaces
Impervious surfaces are any materials that prevent the infiltration of water into the soil and groundwater. Examples include roads, rooftops, driveways, parking lots, and sidewalks.6 The dominance of impervious surfaces in developed areas is a relatively recent development, emerging with the advent of interstate highway system and the suburban sprawl it enabled. Under pre-development conditions, natural surfaces are able to absorb and infiltrate rainfall. But when they are developed and replaced with impervious surfaces, the local water cycle is dramatically impacted,7 as depicted in Figure 1. The major impacts include reducing shallow and deep infiltration, reducing evapotranspiration, and significantly increasing the amount of surface water runoff.
Figure 1: Water cycle changes associated with urbanization.8
Historically, northern Delaware was mostly forest,9 with coastal wetlands bordering the Delaware River and its tributaries.10 However, as demonstrated in Figure 2A, almost all of this part of the state has been developed to some degree. The urban core of the city of Wilmington is nearly all high intensity developed, and suburban sprawl can be seen as the high degree of medium intensity developed land surrounding the city. Some pockets of forest and cultivated land are still present, but mostly as islands amidst heavily developed land. The floodplain of the Christina River (pictured in light blue running diagonally in the center of the image) and coastline along the Delaware River are covered by woody or herbaceous wetlands, though their extent has been significantly reduced over the past century.11
Figure 2: A: Land Cover in Northern Delaware Area.12 Data provided by National Land Cover Database through Esri and ArcGIS. B: Impervious Surface Cover in the Greater New Castle, Delaware Area. Data provided by National Land Cover Database through Esri.13 The northernmost blue marker denotes the neighborhood of Southbridge, the central marker denotes William Penn High School, and the southernmost marker denotes the Route 9 Corridor.
The relationship between land cover and impervious surfaces is driven home by comparing Figures 2A and 2B. The areas with the highest percentage of impervious surfaces are in the areas with the greatest amount of development. Major roadways can be seen as thin, interconnected strips of highly impervious surfaces, and major population centers or industrial areas are marked by spots of intense orange. Remaining areas of low impervious cover coincide partly with the “natural landscapes” bordering the Christina River and the coast of the Delaware River, as well as the limited forested and cultivated areas.
Stormwater Runoff and Gray Infrastructure
Stormwater is defined as the portion of precipitation that does not naturally percolate into the ground or evapotranspirate, but instead flows via overland flow, interflow, channels, or pipes, into a defined surface water channel or constructed infiltration facility.14 In this unit, stormwater and stormwater runoff are used interchangeably. There are several traditional methods of managing stormwater runoff, commonly referred to as gray infrastructure.
Gray infrastructure is designed to move stormwater away from the built environment, and has traditionally been deployed to collect and convey stormwater from impervious surfaces such as parking lots, roadways, and rooftops into a piped system that then discharges into a local surface water body such as a stream, river, or lake.15 Examples of gray infrastructure include curbs, gutters, grates, drains, subsurface piping, and collection systems. A combination of these specific pieces of infrastructure make up a storm sewer system, with the express purpose of rapidly clearing locations of stormwater.
Environmental Impacts of Gray Infrastructure and Stormwater Runoff
The rapid delivery of stormwater runoff to surface water bodies by gray infrastructure is problematic for several reasons. These problems include routine localized flooding, erosion of stream banks, alteration of stream/river hydrology, and transfer of surface pollutants into local surface water bodies.16 Routine flooding, erosion of stream banks, and alteration of stream/river hydrology have to do directly with the purpose of gray infrastructure. Recall that such systems are designed to rapidly move water away from the built environment. In periods of heavy rain, this system can become quickly over loaded, resulting in a backup of storm drains, which leads to localized flooding, especially in low-lying areas. Channeling stormwater from impervious surfaces into pipes greatly increases the velocity at which water enters streams, increasing its erosive potential and thus altering the stream/river hydrology. In areas where the stormwater and sewer systems are combined and routed to a treatment facility before discharge, an increase in stormwater flow can lead to a combined sewer overflow, where the volume of water is too great for the treatment facility and must be released untreated.17 This has significant environmental impacts, some of which are discussed below. In summary, the underground sewer systems so often employed to manage stormwater are problematic because they lead to the rapid release of polluted water into local surface waters,18 excessive flows into local surface waters,19 and backflow into streets and homes/businesses when the system is combined with sewage collection and overtaxed from a storm event.20
The transfer of surface pollutants into local surface water bodies is a substantial threat to stream, river, and pond/lake ecosystems: any pollutant on an impervious surface can potentially be washed away into a local creek or other water body during normal rain events. Such pollutants include oxygen depletors, pathogens, metals, nutrients, and still others that impact pH, turbidity, and total dissolved solids.21 Oxygen depletors include ammonia and other constituents that increase the biochemical oxygen demand (BOD) of a water body, such as the organic compounds in sewage. Enteric pathogens can be released and re indicated by the presence of fecal coliform. Common metals in stormwater runoff include lead, mercury, and cadmium. Nitrogen and phosphorous are the primary nutrients carried by stormwater runoff and can cause eutrophication, a process that leads to oxygen and pH fluctuations and can lead to the release of toxins by blooming algae. Stormwater runoff can also directly impact the turbidity, and salinity or total dissolved solids in a surface water body.22
Green Infrastructure
A changing stormwater management paradigm has begun to emerge in the last fifteen years as engineers and urban planners have turned to green infrastructure to help manage stormwater in developed areas.23 This changing paradigm attempts to recognize the value of stormwater in urban areas instead of treating it as a problem to be discharged to adjacent water bodies, and has resulted in the development of such things as constructed urban wetlands, vegetated swales, and bioretention basins.24 Other examples of green infrastructure include green roofs, rain gardens, and permeable pavements.25
In stark contrast to gray infrastructure, green infrastructure is designed to mimic nature and capture rainwater where it falls, with the chief goal of reducing, slowing, and filtering stormwater runoff into local waterways.26 The most commonly deployed examples of green infrastructure are retention and detention facilities, which are used in conjunction with the gray infrastructure described above. Retention facilities are designed to hold stormwater and do not contain an outlet. Water leaves through evapotranspiration or through infiltration into the soil. Detention facilities are designed to temporarily hold water before it is released at a slow rate, with little infiltration into the surrounding soil.27 These work well in suburban settings where impervious cover percentages are lower and less connected than in urban settings. In urban areas, where space is limited and retention/detention facilities are not feasible, the use of green roofs, vegetated swales, rain gardens, and impervious concrete have gained popularity.28 Where geography permits, constructed wetlands have shown significant promise for retaining and treating stormwater runoff.29 According to the USEPA, using such techniques has the effect of reducing localized flooding, improved community aesthetics, increased community socialization, increased property values and job opportunities, decreased economic and community burden associated with flooding, and diverse, location specific environmental, social, and economic benefits.30
Constructed Wetlands
Constructed wetlands (Figure 3A) are those that are created in areas that were not previously wetlands, typically to absorb and filter stormwater runoff before its discharge into a surface water body.31 At their simplest, they are artificial, shallow, and extensively vegetated water bodies designed to absorb and treat stormwater. Ancillary benefits provided by constructed wetlands include improving community aesthetics, offering recreational activities such as birding, and ensuring the availability of water for re-use.32
A typical constructed wetland consists of three zones: an inlet zone, a macrophyte zone, and a high flow bypass channel. The inlet zone is the area of the wetland designed to take in flow, and typically by placing a sedimentation pond upstream of the desired wetland area. This zone acts as the first step in the treatment of any pollutants in incoming stormwater as coarse particles settle out. Water then moves into the macrophyte zone, a shallow area with extensive emergent vegetation. The specific type of vegetation in this zone is dictated by the water depth. In general, a constructed wetland contains four vegetation zones: shallow marsh vegetation, marsh vegetation, deep marsh vegetation, and submerged vegetation. The combination of these different vegetation types stabilizes the marsh soil, slows down flow, and promotes UV exposure in open water areas. The flow of water from the inlet zone into the macrophyte zone is carefully controlled with a bypass channel so as to prevent scour of marsh vegetation and soil.33
Figure 3: Select Green Infrastructure Example. A) Constructed Wetland. B) Vegetated Swale. C) Bioretention Basin. D) Rain Garden.
Pollutant removal in constructed wetlands occurs in several stages: settling, vegetation uptake, adsorption, filtration, and biological decomposition. Wetland-specific vegetation promotes a high degree of settling because it reduces water flow. Plants take up a great deal of the excess N and P contained in stormwater, filter other pollutants through their roots, stems, and leaves, and promote the growth of biofilms (thin films of bacteria colonies) that can also assimilate dissolved nutrients from the water.34
In addition to acting as a natural filter for stormwater, constructed wetlands are proven to be effective at controlling stormwater flows via infiltration, evaporation, and retention. Infiltration is the smallest contributor to reducing stormwater flow since wetlands areas are characterized by saturated soils. But effective wetland design can dramatically increase the time stormwater spends in the wetland, promoting evaporation and slowing release into local surface water bodies.35
Vegetated Swales and Bioretention Basins
Vegetated swales (Figure 3B) are excavated trenches that are filled with layers of porous media to create a shallow channel and covered with vegetation on the slopes and/or the top layer. The primary purpose of channelized swale design is to disconnect impervious surfaces from downstream surface water bodies. In this way, it slows down stormwater and acts as a filter for any pollution contained in it. A typical vegetated swale has a layer of gravel in which a drain pipe is placed, overlaid by layers of sand and soil, and topped with small rocks and vegetation. A bypass is often installed so as not to overtax the system during high flow events. The parabolic or trapezoidal shape of the swale allows for coarse and medium sized sediments to settle out, while the vegetation and microbes in the soil remove finer materials and associated pollutants through filtration, infiltration, adsorption, and biological uptake. Vegetated swales are highly effective for areas such as road medians, parking lots, and parks or recreation areas where flow is low and infrequent. They are green alternatives to traditional curb and gutter installations.36
Bioretention basins (Figure 3C) are similar to vegetated swales in their use of vegetation. However these basins have a critical difference to the vegetated swales: they are designed to pool water on the surface and then promote infiltration and percolation through filter media. The vegetation slows down the stormwater, enhances infiltration and maintains filter media porosity. These media typically consist of three layers: filter media layer, transition layer, and drainage layer. The filter media layers usually coarse sand or fine gravel, with grain size ranging from one to five millimeters. Below the filter media, water moves through the transition layers (which is designed to prevent downward migration of filter media) into the drainage layer, where it enters a perforated pipe connected to a stormwater drain. In some cases the drainage layers sits atop an impervious barrier to ensure that water enters the drainage system. Similar to vegetated swales and constructed wetlands, bioretention basins limit stormwater runoff and act as natural filters for any pollutants it contains. The primary method for limiting stormwater runoff is by retention by vegetation and soil moisture. Because of this, they are also limited to smaller scale usage like vegetated swales.37
Green Roofs
Green roofs are structures that are covered with growing media and vegetation that enable rainfall infiltration and evapotranspiration of stored water.38 They consist of a waterproof membrane at their base, a growing medium, and vegetation. Water that falls on the roof infiltrates into the soil where it is held until it is either taking up by plants or evaporates. Overflow drains ensure that the roof doesn’t become overloaded in high precipitation events. Effectively designed green roofs significantly reduce the amount of stormwater runoff that would otherwise runoff an impervious roof surface, even reducing stormflow by up 65% and increasing the time it takes for water to flow from the surface to a sewer by up to three hours.39 Additional environmental benefits include providing habitat for local wildlife and increasing biodiversity, limiting the effects of the urban heat island, reducing energy costs, improving air quality in the surrounding area, and increasing the longevity of roof structures.40 It is important to note that not all buildings are suitable for green roofs. Structures with a green roof must be able to withstand the extra load brought about by the soil, vegetation, and water.41
Rain Gardens
Rain gardens (Figure 3D) function similar to vegetated swales and bioretention basins in that they are designed to slow the movement of stormwater and provide natural filtration of contaminants.42 They are formally defined as shallow vegetated basins that collect and absorb runoff from rooftops, sidewalks, and streets. Rain gardens collect runoff in depressions called ponding areas. These depressions are typically planted with vegetation that thrive in saturated soils. A mulch layer promotes retention of stormwater without scouring plants and washing away soil. Beneath the mulch layer is a soil mixture of sand and organic matter that is designed to promote infiltration and subsurface flow. Rain gardens mimic natural hydrology by promoting evapotranspiration in the ponding area and infiltration through the mulch and soil layers.43 These features are becoming increasingly popular with homeowners because they offer stormwater management while increasing the aesthetic appeal of a landscape. Parking lots and urban parks are also excellent locations for rain gardens.44
Permeable Pavements
Permeable pavements are another green infrastructure technique used to catch rainwater where it falls instead of moving it through a piped system. Unlike traditional impermeable pavements, permeable pavements behave more like natural systems that allow for the infiltration and possible treatment rainwater.45 A permeable pavement system typically includes an engineered porous urban surface composed of pavers, concrete, or asphalt and an underlying stone reservoir. The permeable layer allows for surface water to infiltrate instead of runoff, and the stone reservoir allows the water to slowly infiltrate into adjacent soil or to discharge via a drain system.46 Such systems have proven effective in reestablishing more natural hydrologic balances by reducing runoff volume and slowly releasing water into the ground. They also reduce concentrations of some pollutants by physically trapping them in pore space in the pavement or soil below and biodegrading contaminants via the bacteria and plants living in the system. Additionally, because of the above positive controls on stormwater, permeable pavement systems can reduce the need for larger regional stormwater management systems.47
Modelling Stormwater Runoff
Rudimentary stormwater runoff models can be generated and used by high school students to study how land cover and other factors impact the flow of stormwater in different areas. Figure 4 provides a visual demonstration and associated mathematical expression of a basic stormwater runoff model based on six simple variables.
Figure 4: Simple Stormwater Runoff Model for Student Use48
In this rudimentary model, surface runoff flow (Qf) is equal to precipitation (P) minus losses to soil moisture (ΔSm), groundwater (ΔGw), evaporation and transpiration (ET), and absorption by vegetation (V). Such models offer an opportunity for students to isolate variables and determine the impact of specific management practices, which are discussed in detail below. For instance, in an area that has 100% impervious surface cover, is essentially equal to P. As impervious surface cover decreases, so does surface runoff flow. Water is absorbed into the soil, some of which will infiltrate and reach the groundwater. Some will be absorbed by plants, and some will be lost to evaporation and transpiration. Using simple math along with climate data and simple approximations for infiltration and vegetation, students can easily determine the impact of reducing impervious surface cover and/or using one or more of the green infrastructure techniques presented below. For instance, if impervious surface cover is reduced to 50%, then Qf is equal to 0.5P – [0.5P – (ΔSm + ΔGw + ET + V)].49 Other more complex variations can be introduced, like parceling out the watershed area into regions with different soil types that influence losses to soil moisture and groundwater, specifying vegetation types and amounts, and using local climate variation to modify evapotranspiration rates over seasons and time.
Stormwater Flooding in Delaware
Of critical importance to this unit is how stormwater runoff impacts my school community. Specifically, this unit focuses on the Route 9 corridor between New Castle and Wilmington, Delaware. Just south of New Castle, Route 9 is known as River Road (see Figures 2A and 2B above). River Road is bordered by the Delaware River to the east and marshland to the west. The areas around the road used to be marshland as well but were long ago filled in and developed. The proximity to the river and marsh is a frequent source of flooding on this specific stretch of road. In addition to frustrating residents and businesses along Route 9, this routine flooding poses a significant public safety threat because the road is designated as an emergency evacuation route. The primary existing flood control method involves the use of curbs and storm sewers designed to channel water away from the road into an adjacent wetland. This corridor has yet to receive any local, state, or federal funding for the construction of any green infrastructure, though it seems to be a prime candidate for bioretention basins, vegetated swales, and possibly an expansion of existing wetlands, which are most effective when they are present throughout the drainage system.
To the north in Southbridge, a similar confluence of river and marshland poses a significant problem during rain events and at high tide. Here, residents have developed their own methods of dealing with the chronic flooding, such as covering their basement floors with gravel and having elevated storage spaces. More formal measures have been taken by the city, including frequently de-clogging storm sewer lines and clearing a large drainage ditch of debris.50 However, given the specter of rising sea levels due to climate change compounded with the already existing flooding issues, Southbridge is in need of a major overhaul to its stormwater management system. A major step towards more effective management of stormwater was proposed in 2014, when the Department of Natural Resources and Environmental Control proposed the construction of an urban wetland to act as a sponge during storms and high tide events. The project, known as the South Wilmington Wetlands Park, recently broke ground in the summer of 2019 and is expected to provide significant flood relief for the neighborhood.51 The project’s chief goal is to reduce flooding in the neighborhood by rerouting stormwater from the streets into the wetland.52 Part of this involves separating the antiquated combined sewer system into stormwater and sewer pipes. The stormwater pipes will convey stormwater off the street into the wetland through a subsurface system. In addition to mitigating the flood risk from surface runoff, the wetland will also serve to increase regions resiliency against extreme high tides, storm surge, and rising sea levels. The wetland will also partially restore habitat for a variety of fish, aquatic, and wetland species. It will also provide recreational opportunities and green space to a neighborhood sorely lacking in those attributes.53 However, without further deployment of green infrastructure that reduces the amount of stormwater running off into the wetland and adjacent river, this feature may have a limited impact on ameliorating the problem of routine flooding in the area.
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