Innovative Methods of Stormwater Control
Constructive management of stormwater serves to improve the aquifer rather than contribute to the problem of downstream runoff.
By E.W. Bob Boulware, P.E.
Achieving basic water security by harnessing the productive potential of stormwater, and limiting its destructive impacts, has been a constant struggle since the beginning of human society. Rapid change in the 21st century, in populations, economies, geopolitics and climate, has made achieving water security much more important than in the past. Recognizing the changing environment and growing international concern for water shortages will become a new challenge for engineers in order to avoid threatening the well-being of long-secure nations. Constructive management of stormwater will soon be seen as an asset to be taken advantage of rather than being a nuisance to be eliminated.
We have seen that the byproduct of progress commonly involves increased hardened surfaces—such as roofs, parking lots and walkways, which lead to increased runoff, flooding, and combined sewer overflow. The common solution is to install a bigger drainage pipe or increase the size of the sewage treatment plant to reduce sewage plant overflows into the waterways. A more constructive, and environmentally sustainable solution, is the use of passive stormwater management.
Passive stormwater management systems incorporate non-mechanical methods of collecting, cleaning and storing rainwater so that rainwater can be beneficially used or naturally absorbed into the landscape. The goal is to re-establish runoff to approximate the predevelopment environment. There are simple methods to accomplish this and also aide groundwater replenishment. These include:
- rain gardens and vegetative swales;
- drainage ditch check dams; and
- infiltration systems.
Each of these tools is relatively inexpensive and generally simple to design and build. While technically not producing water, passive stormwater management can be used to purposefully put water back into the aquifer for future use, rather than allowing it to flow to the waterways without benefit. Artificial groundwater recharge is designed to increase the natural replenishment or percolation of surface waters into the groundwater aquifers, which results in a corresponding increase in the amount of groundwater available for extraction. And by slowing stormwater runoff, flooding and sewage overflow to the watershed is diminished
Rain gardens are shallow retention swales, augmented with landscape, that combine the benefits of groundwater recharge with aesthetics. As can be seen in Figure 3, a side benefit of stormwater management is that trees are periodically watered and the bio-retention of the associated plants can serve to buffer pollutants before being absorbed into the aquifer.
Rain Garden integration with Tempe, Arizona street roundabout manages stormwater runoff while providing the aesthetic benefit of shade and sound abatement.
Heather Kinkade Photo
The bio-retention process provided by the plantings collects and filters stormwater through layers of mulch, soil and plant root systems. As part of this process, pollutants such as bacteria, nitrogen, phosphorus, heavy metals, oil and grease are retained, degraded and absorbed. Treated stormwater is then infiltrated into the ground as groundwater or, if infiltration is not appropriate, discharged into a traditional stormwater drainage system. Rain gardens may look similar to traditional landscaped areas, but they differ in design and function. Although rain gardens can be planted with a variety of perennials, grasses, shrubs and small trees, low maintenance native plants are preferred.
There are important considerations in the selection of sites for artificial recharge through rain garden techniques.
- The area should have gently sloping land without gullies or ridges.
- The aquifer being recharged should be unconfined, permeable and sufficiently thick to provide storage space.
- The surface soil should be permeable and have high infiltration rate.
- The unsaturated zone should be permeable and free from clay lenses.
- The water table should be deep enough to accommodate the recharged water so that there is no water logging.
- The aquifer material should have moderate hydraulic conductivity so that the recharged water is retained for sufficiently long periods in the aquifer and can be used when needed.
Rain gardens are a valuable addition to both residential and commercial sites because of their multi-faceted benefits: aesthetic, stormwater buffering and pollution remediation.
The purpose of a rain garden is to slow stormwater runoff so as to allow infiltration into the soil. A variation of this concept are check dams, which are counter to the common thought of removing water from the site as quickly as possible.
Check dams make the water take the slow way from the property by providing restrictions to flow, or integrated in to the landscape making them more of a linear rain garden. These are aimed at increasing the contact area and residence time of surface water over the soil to enhance the infiltration and to augment the ground water storage in phreatic aquifers. The downward movement of water is governed by a host of factors including vertical permeability of the soil, presence of grass or entrapped air in the soil zone and the presence or absence of limiting layers of low vertical permeability at various soil depths.
Infiltration happens naturally in undeveloped environments to various degrees according to the soils ability to absorb water. When the hardened surfaces of development occur, supplemental systems are necessary to revert infiltration back to pre-development conditions. Common methods of accomplishing this are the use of pervious surfaces and infiltration devices.
Pervious concrete. Parking is a common source of impervious surface that the use of pervious pavers will serve to buffer stormwater runoff. Water infiltrates between the pavers into a sand and rock base that serves to retain the immediate volume of water from a rain event and then allowing the retained water to infiltrate into the ground.
Infiltration Vaults. Infiltration vaults are another means to infiltrate water into the soil are. These underground chambers absorb the immediate volume of stormwater and then allow the water to be absorbed into the ground over time in time for the next rain event.
Factors that Affect Retention and Infiltration. Factors affecting natural infiltration are the amount of precipitation, the absorptive characteristics of the soil, degree of soil saturation, type and quantity of land cover, the slope of the land, and evapo-transpiration.
A simple calculation to remember is 620-gal of water result from 1-in of rain on 1000-ft² of impervious surface. Frequency and intensity data of rain events can be gathered from local weather data, the local Plumbing Code, or as indicated in the National Oceanic and Atmospheric Administration’s Rainwater Intensity Data for the location in question. Removal of the accumulated stormwater, ideally in time for the next rain event, is a function of soil absorptivity, plant transpiration and evaporation. Absorption is measured by the soil percolation rate measured in volume/time. Determining this can be simply done by a percolation test, which involves filling a hole with water and timing the volume over time the water is absorbed.
Transpiration is water lost through the metabolic (breathing) process of plants. Evaporation is water that leaves the soil as a function of the level of moisture in the soil and the relative humidity in the atmosphere. Transpiration and evaporation (evapotranspiration) is the loss of water from a vegetative surface through the combined processes of plant transpiration and soil evaporation. Water that stays near the surface is available for the plant roots to absorb as part of the plant transpiration process. This process varies according to climate and season. Several methods have been developed to estimate evapotranspiration that can best be quantified with the assistance of a local agricultural agent or master gardeners.
Stormwater Infiltration Case Study: Public Library - Menard, Texas
The pictures above are of the Menard, Texas Public Library and the use of rainwater runoff to change what otherwise would be a chronic soil erosion problem into constructive replenishment of the aquifer to support a flow garden. The library was built up on a caliche soil pad to raise it up above the flood plain. The noted downspout was in the front of the library and left as a finished job by the contractor. However the "planting bed" was surrounded by a concrete walkway on all the outside boarder and water from the downspout was left to cause erosion. The storm chambers seemed the most logical solution.
In the second picture, the caliche and soil was dug down 1-ft (0.33-m) below the sidewalk and 9-in (23-cm) of 0.75-in (19-mm) crushed rock was placed in the bottom as a storage area for excess rainfall and to support infiltration. The area receives about 20-in (50.8-cm) of rainfall annually and the downspout is supporting approximately 1,200-ft² (111-m²) of roof area. The crushed rock was leveled and then five storm chambers were placed on top with caps on each end. The storm chambers have no bottom to them and have drainage holes in rows along the side. Water is directed from the downspout into the top of one of the chambers and an overflow pop-up drain is added to the end chamber.
The storm chambers were then covered with geo-textile to prevent soil from falling into the storm chamber and top soil is added and larger boulders are added to the soil in place to allow planting.
The final picture is of native and adapted plants a year later covered the area with natural beauty. Plants have been added in cavities between boulders and on top of the chambers. About 3-in (7.6-cm) of mulch is added to keep soil in place and the ground cool and moist to aid plant health. To date no stormwater has left the site. Although drip irrigation has not been added, it would enhance the beauty in drought times.
The library has a total of four downspouts on the 4,800-ft² (446-m²) roof. Two are going to a 2,000-gal (7,600-l) above ground collection tank and used to irrigate native plants with gravity pressure drip irrigation and the fourth goes to a rain garden in the back of the library.
As water is recognized as being more and more limited, the importance of preserving this vital resource will become better known. And as a result, better means of managing will be utilized. The above examples are but a few methods that have been successfully used. For more information relating to this topic, you are invited to research sources listed below:
- Manual on Artificial Recharge of Ground Water, Government of India, Ministry of Water Resources Central Ground Water Board.
- Ontario Guidelines for Residential Rainwater Harvesting Systems, 2010 Handbook, Chapter 6.
- San Francisco Stormwater Design Guidelines.
- Guidelines for the design and construction of Stormwater Management Systems, New York City Department of Environmental Protection, July 2012.
- Alternative Water Sources and Waste Management Systems, by E.W. Bob Boulware, McGraw Hill.