Жаңбыр бақшалары: су ағынын азайту және қоршаған ортаны жақсарту әдісі

Runoff reducing landscaping method

Rain gardens, also called bioretention facilities, are one of a variety of practices designed to increase rain runoff reabsorption by the soil. They can also be used to treat polluted stormwater runoff. Rain gardens are designed landscape sites that reduce the flow rate, total quantity, and pollutant load of runoff from impervious urban areas like roofs, driveways, walkways, parking lots, and compacted lawn areas. Rain gardens rely on plants and natural or engineered soil medium to retain stormwater and increase the lag time of infiltration, while remediating and filtering pollutants carried by urban runoff. Rain gardens provide a method to reuse and optimize any rain that falls, reducing or avoiding the need for additional irrigation. A benefit of planting rain gardens is the consequential decrease in ambient air and water temperature, a mitigation that is especially effective in urban areas containing an abundance of impervious surfaces that absorb heat in a phenomenon known as the heat island effect. Rain garden plantings commonly include wetland edge vegetation, such as wildflowers, sedges, rushes, ferns, shrubs and small trees. These plants take up nutrients and water that flow into the rain garden, and they release water vapor back to the atmosphere through the process of transpiration. Deep plant roots also create additional channels for stormwater to filter into the ground. Root systems enhance infiltration, maintain or even augment soil permeability, provide moisture redistribution, and sustain diverse microbial populations involved in biofiltration. Microbes help to break down organic compounds (including some pollutants) and remove nitrogen. Rain gardens are beneficial for many reasons; they improve water quality by filtering runoff, provide localized flood control, create aesthetic landscaping sites, and provide diverse planting opportunities. They also encourage wildlife and biodiversity, tie together buildings and their surrounding environments in integrated and environmentally advantageous ways. Rain gardens can improve water quality in nearby bodies of water and recharge depleted groundwater supply. Rain gardens also reduce the amount of polluted runoff that enters the storm sewer system, which discharges directly to surface waters and causes erosion, water pollution and flooding. Rain gardens also reduce energy consumption by decreasing the load on conventional stormwater infrastructure.

The first rain gardens were created to mimic the natural water retention areas that developed before urbanization occurred. The rain gardens for residential use were developed in 1990 in Prince George's County, Maryland, when Dick Brinker, a developer building a new housing subdivision had the idea to replace the traditional best management practices (BMP) pond with a bioretention area. He approached Larry Coffman, an environmental engineer and the county's Associate Director for Programs and Planning in the Department of Environmental Resources, with the idea. The result was the extensive use of rain gardens in Somerset, a residential subdivision which has a rain garden on each house's property. This system proved to be highly cost effective. Instead of a system of curbs, sidewalks, and gutters, which would have cost nearly $400,000, the planted drainage swales cost $100,000 to install. Redirected stormwater is often warmer than the groundwater normally feeding a stream, and has been linked to upset in some aquatic ecosystems primarily through the reduction of dissolved oxygen (DO). Stormwater runoff is also a source of a wide variety of pollutants washed off hard or compacted surfaces during rain events. These pollutants may include volatile organic compounds, pesticides, herbicides, hydrocarbons and trace metals.

Stormwater management occurs on a watershed scale to prevent downstream impacts on urban water quality. A watershed is maintained through the cyclical accumulation, storage, and flow of groundwater. Rainfall on impermeable surfaces accumulates surface runoff containing oil, bacteria, and sediment that eventually makes its way to streams and groundwater. Even rain gardens with small capacities for daily infiltration can create a positive cumulative impact on mitigating urban runoff. Increasing the number of permeable surfaces by designing rain gardens reduces the amount of polluted stormwater that reaches natural bodies of water and recharges groundwater at a higher rate. Additionally, adding a rain garden to a site that experiences excessive rainwater runoff mitigates the water quantity load on public stormwater systems. The bioretention approach to water treatment, and specifically rain gardens in this context, is two fold: to utilize the natural processes within landscapes and soils to transport, store, and filter stormwater before it becomes runoff, and to reduce the overall amount of impervious surface covering the ground that allow for contaminated urban runoff. Rain gardens perform most effectively when they interact with the greater system of stormwater control. This integrated approach to water treatment is called the "stormwater chain", which consists of all associated techniques to prevent surface run off, retain run off for infiltration or evaporation, detain run off and release it at a predetermined rate, and convey rainfall from where it lands to detention or retention facilities.

The concept of LID (low impact design) for stormwater management is based on bioretention: a landscape and water design practice that utilizes the chemical, biological, and physical properties of soils, microorganisms, and plants to control the quality and quantity of water flow within a site.

Bioretention controls the stormwater quantity through interception, infiltration, evaporation, and transpiration. Stormwater quality can be controlled by bioretention through settling, filtration, assimilation, adsorption, degradation, and decomposition. Even though natural water purification is based on the design of planted areas, the key components of bioremediation are the soil quality and microorganism activity. These features are supported by plants, which create secondary pore space to increase soil permeability, prevent soil compaction through complex root structure growth, provide habitats for the microorganisms on the surfaces of their roots, and transport oxygen to the soil. Rain gardens are at times confused with bioswales. Swales slope to a destination, while rain gardens are level; however, a bioswale may end with a rain garden as a part of a larger stormwater management system. Drainage ditches may be handled like bioswales and even include rain gardens in series, saving time and money on maintenance. Part of a garden that nearly always has standing water is a water garden, wetland, or pond, and not a rain garden. Rain gardens also differ from retention basins, where the water will infiltrate the ground at a much slower rate, within a day or two.

Collected water is filtered through the strata of soil or engineering growing soil, called substrate. After the soil reaches its saturation limit, excess water pools on the surface of the soil and eventually infiltrates the natural soil below. The bioretention soil mixture should typically contain 60% sand, 20% compost, and 20% topsoil. Soils with higher concentrations of compost have shown improved effects on filtering groundwater and rainwater. Non permeable soil needs to be removed and replaced periodically to generate maximum performance and efficiency if used in the bioretention system. The sandy soil (bioretention mixture) cannot be combined with a surrounding soil that has a lower sand content because the clay particles will settle in between the sand particles and form a concrete like substance that is not conducive to infiltration, according to a 1983 study. Compact lawn soil cannot harbor groundwater nearly as well as sandy soils, because the micropores within the soil are not sufficient for retaining substantial runoff levels. Rain gardens are often located near a building's roof drainpipe (with or without rainwater tanks). Most rain gardens are designed to be an endpoint of a building's or urban site's drainage system with a capacity to percolate all incoming water through a series of soil or gravel layers beneath the surface plantings. A French drain may be used to direct a portion of the rainwater to an overflow location for heavier rain events. If the bioretention site has additional runoff directed from downspouts leading from the roof of a building, or if the existing soil has a filtration rate faster than 5 inches per hour, the substrate of the rain garden should include a layer of gravel or sand beneath the topsoil to meet that increased infiltration load.

Typical rain garden plants are herbaceous perennials and grasses, which are chosen for their porous root structure and high growth rate. plants that can tolerate both saturated and dry soil are typically used for the rain garden. They need to be maintained for maximum efficiency, and be compatible with adjacent land uses. Native and adapted plants are commonly selected for rain gardens because they are more tolerant of the local climate, soil, and water conditions; have deep and variable root systems for enhanced water infiltration and drought tolerance; increase habitat value, diversity for local ecological communities, and overall sustainability once established. Vegetation with dense and uniform root structure depth helps to maintain consistent infiltration throughout the bioretention system. There can be trade offs associated with using native plants, including lack of availability for some species, late spring emergence, short blooming season, and relatively slow establishment. It is important to plant a wide variety of species so the rain garden is functional during all climatic conditions. It is likely that the garden will experience a gradient of moisture levels across its functional lifespan, so some drought tolerant plantings are desirable. There are four categories of a vegetative species’ moisture tolerance that can be considered when choosing plants for a rain garden. Wet soil is constantly full of water with long periods of pooling surface water; this category includes swamp and marsh sites. Moist soil is always slightly damp, and plants that thrive in this category can tolerate longer periods of flooding. Mesic soil is neither very wet nor very dry; plants that prefer this category can tolerate brief periods of flooding. The National Science Foundation, the United States Environmental Protection Agency, and a number of research institutions are presently studying the impact of augmenting rain gardens with materials capable of capture or chemical reduction of the pollutants to benign compounds. The primary challenge of rain garden design is predicting the types of pollutants and the acceptable loads of pollutants the rain garden's filtration system can process during high impact storm events. Contaminants may include organic material, such as animal waste and oil spills, as well as inorganic material, such as heavy metals and fertilizer nutrients. These pollutants are known to cause harmful over promotion of plant and algal growth if they seep into streams and rivers. The challenge of predicting pollutant loads is specifically acute when a rain event occurs after a longer dry period. The initial storm water is often highly contaminated with the accumulated pollutants from dry periods. Rain garden designers have previously focused on finding robust native plants and encouraging adequate biofiltration, but recently have begun augmenting filtration layers with media specifically suited to chemically reduce redox of incoming pollutant streams. Certain plant species are very effective at storing mineral nutrients, which are only released once the plant dies and decays. Other species can absorb heavy metal contaminants. Cutting back and entirely removing these plants at the end of the growth cycle completely removes these contaminants. This process of cleaning up polluted soils and stormwater is called phytoremediation. Melbourne Water's database of Water Sensitive Urban Design projects, including 57 case studies relating to rain gardens/bioretention systems. Melbourne Water is the Victorian State Government agency responsible for managing Melbourne's water supply catchments. Water By Design is a capacity building program that supports the uptake of Water Sensitive Urban Design, including rain gardens, in South East Queensland. It was established by the South East Queensland Healthy Waterways Partnership in 2005, as an integral component of the SEQ Healthy Waterways Strategy.

The 12,000 rain garden campaign for Puget Sound is coordinating efforts to build 12,000 rain gardens in the Puget Sound Basin of Western Washington by 2016. The 12,000 rain gardens website provides information and resources for the general public, landscape professionals, municipal staff, and decision makers. By providing access to the best current guidance, easy to use materials, and a network of trained "Rain Garden Mentor" Master Gardeners, this campaign seeks to capture and cleanse over 200 Million gallons of polluted runoff each year, and thereby significantly improve Puget Sound's water quality. Maplewood, Minnesota has implemented a policy of encouraging residents to install rain gardens. Many neighborhoods had swales added to each property, but installation of a garden at the swale was voluntary. The project was a partnership between the City of Maplewood, University of Minnesota Department of Landscape Architecture, and the Ramsey Washington Metro Watershed District. A focus group was held with residents and published so that other communities could use it as a resource when planning their own rain garden projects. Some local governmental organizations offer local grants for residents to install raingardens. In Dakota County, Minnesota, the Dakota County Soil and Water Conservation District offers $250 grants and technical assistance through their Landscaping for Clean Water program to encourage residents to install residential raingardens. In Seattle, a prototype project, used to develop a plan for the entire city, was constructed in 2003. Called SEA Street, for Street Edge Alternatives, it was a drastic facelift of a residential street. The street was changed from a typical linear path to a gentle curve, narrowed, with large rain gardens placed along most of the length of the street. The street has 11% less impervious surface than a regular street. There are 100 evergreen trees and 1100 shrubs along this 3 block stretch of road, and a 2 year study found that the amount of stormwater which leaves the street has been reduced by 99%. 10,000 Rain Gardens is a public initiative in the Kansas City, Missouri metro area. Property owners are encouraged to create rain gardens, with an eventual goal of 10,000 individual gardens. The West Michigan Environmental Action Council has established Rain Gardens of West Michigan as an outreach water quality program in Grand Rapids, Michigan. Also in Michigan, the Southeastern Oakland County Water Authority has published a pamphlet to encourage residents to add a rain garden to their landscapes in order to improve the water quality in the River Rouge watershed. In Washtenaw County, homeowners can volunteer for the Water Resources Commissioner's Rain Garden program, in which volunteers are annually selected for free professional landscape design. The homeowners build the gardens themselves as well as pay for landscaping material. Photos of the gardens as well as design documents and drainage calculations are available online. The Washtenaw County Water Resource Commissioner's office also offers yearly in person and online Master Rain Gardener classes to help guide those interested in the rain garden design, building, and upkeep process. Portland, Oregon, has established a Clean River Rewards program, to encourage residents to disconnect downspouts from the city's combined sewer system and create rain gardens. Workshops, discounts on storm water bills, and web resources are offered. In Delaware, several rain gardens have been created through the work of the University of Delaware Water Resources Agency, and environmental organizations, such as the Appoquinimink River Association. In New Jersey, the Rutgers Cooperative Extension Water Resources Program has installed over 125 demonstration rain gardens in suburban and urban areas. The Water Resources Program has begun to focus on using rain gardens as green infrastructure in urban areas, such as Camden and Newark to help prevent localized flooding, combined sewer overflows, and to improve water quality. The Water Resources Program has also revised and produced a rain garden manual in collaboration with The Native Plant Society of New Jersey. According to the Massachusetts Department of Environmental Protection, rain gardens may remove 90% of total suspended solids, 50% of nitrogen, and 90% of phosphorus. Much of the runoff from the University of Maryland campus, a member of the Anacostia Watershed Restoration Partnership, ends up in the Anacostia River feeding into the Chesapeake Bay. This research finds rain gardens to be a very effective method of water capture and filtration, encouraging others in the Chesapeake Bay Watershed to implement rain gardens. Davis' research showed that rain gardens aid in the capturing and bio degradation of pollutants such as suspended solids, bacteria, metals, oil, and grease. Water quality analyzed at the University of Maryland showed a significant increase in water clarity after rain garden filtration. There is a rain garden at the Center for Young Children (CYC) at University of Maryland designed by students from the Department of Plant Science and Landscape Agriculture. The rain garden allows teachers at the CYC to educate future students on sustainability.

At the University of Technology in Xi'an, China, a rain garden was built to observe and study over four years. This study showed that over four years, there were 28 large storm events in Xi'an. Within these 28 storms, the rain garden was able to retain the rainfall from a majority of the storms. Only 5 of these storms caused the rain garden to overflow. Rain Gardens in this sub humid loess region of Xi'an China, are Low Impact Developments (LID).