RAINWATER CATCHMENT AS A MEANS OF STORMWATER MANAGEMENT. E. W. Bob Boulware, P.E.,M.B.A Vessela Monta, MSCE

Transcription

1 RAINWATER CATCHMENT AS A MEANS OF STORMWATER MANAGEMENT E. W. Bob Boulware, P.E.,M.B.A Vessela Monta, MSCE Abstract: Flooding and combined sewer overflows are a serious and growing issue. Rather than applying the traditional expensive and disruptive solutions of installing larger pipes, this paper proposes an alternate solution of managing stormwater that serves to better manage stormwater as a valuable asset, rather than a nuisance to be gotten rid of as quickly as possible. This paper proposes solutions that are often proven to be cheaper, and less expensive for buffering the immediate stormwater volume, while serving a dual purpose of making non-utility provided water available for alternative applications. The systems illustrated in this paper include soakaway pits, rain gardens, and using rainwater catchment with a metered discharge of stored water. Benefits of these proposed solutions are improved stormwater management, constructive use of the managed water volume for irrigation and aquifer replenishment, and a viable source for non-potable use. Introduction: Flooding and combined sewer overflows are a serious and growing issue. Rather than applying the traditional expensive and disruptive solutions of installing larger pipes, this paper explores alternative opportunities that treat stormwater as an asset to be constructively utilized. As many municipalities struggle to solve stormwater issues, dual purpose rainwater collection is a cost effective option for consideration. The proposed solutions are often proven to be cheaper, and less expensive for buffering the immediate stormwater volume, while serving a dual purpose of making nonutility provided water available for alternative applications. This paper shows how to design such systems which includes How to size a rain garden or soakaway pit How to size a cistern stormwater capture volume The methodology needed to size a metering orifice needed to release the stormwater volume over a determined time.

2 Discussion: 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, in order to reduce sewage plant overflows into the waterways. A more constructive, and environmentally sustainable solution, is the use of passive stormwater management. Figure 1: Unmanaged stormwater leads to flooding and combined sewer overflows 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 (Figure 2) by allowing it to take the l o n g w a y to the storm sewer. Simple methods to accomplish this, and also aide groundwater replenishment: Bioswales, Rain gardens, Drainage ditch check dams, and Metered discharge of rainwater collection detention volume Each of these tools are relatively inexpensive and generally simple to design and build. While technically not producing water, the beneficial management of stormwater runoff can be used to Figure 2: Runoff Response Curve : ( basics) 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 Page 2 of 10

3 Rain Gardens: Rain gardens are shallow retention swales, augmented with landscape, which 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, all while providing the aesthetic benefit of shade and sound abatement The bio-remediation 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. Biologically treated stormwater then infiltrates into the ground, or if infiltration capacity is not sufficient, is allowed to overflow into a traditional stormwater drainage system. Figure 3: Rain Garden integration with Tempe, Arizona street roundabout (Heather Kinkade Photo 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. Important considerations in the selection of sites for artificial recharge through rain garden techniques include: 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. Figure 4: Illustration of an Integrated Rainwater Catchment with a raingarden and overflow to a percolation tank (Courtesy the Aquascape Company - 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. Page 3 of 10

4 As can be seen, rain gardens are a valuable addition to both residential and commercial sites because of their multi-faceted benefits: aesthetic, stormwater buffering and pollution remediation. Figure 4 shows an ideal installation by integrating a rainwater collection that stores a volume of rainwater for beneficial use, while allowing groundwater infiltration before overflowing to the stormwater sewer Check Dams: 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, as seen in Figure 5, or integrated in to the landscape making them more of a linear rain garden (Figure 6). Figure 6: Check dams aide infiltration by slowing stormwater runoff (Heather Kinkade Photo - Figure 5: Linear Rain Garden integrated into stormwater retention ditch. (Heather Kinkade Photo The objective of a check dam is to increase 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 i 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. Check dams, generally are constructed for soil conservation, can be considered mini-or micropercolation tanks from which water is not directly drawn for irrigation but is allowed to percolate into subsurface strata, thus augmenting the groundwater. 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. Page 4 of 10

5 Pervious Pavement: 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 allows the retained water to infiltrate into the ground. (Figure 7) Figure 7: Pervious parking pavers at Indiana Nature Conservancy, Indianapolis, Indiana ( EWB Photo) 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. An example of how this was used to solve a storm runoff problem, yet doing so in an aesthetic manner, can be seen in the attached case study. 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 evapotranspiration. Figure 8: Typical infiltration vault section Page 5 of 10

6 Passive Rainwater Release Management for controlling Stormwater Runoff. The concept of Passive Rainwater Release Management is using the concept of rainwater catchment that is coupled with the associated metered release of a designated volume of the rainwater. As seen in Figure 9, there is a portion of the rainwater storage cistern that is dedicated for stormwater detention. This volume corresponds to a designated amount mandated for on-site stormwater detention, that is released thru a sized orifice in a time frame that ideally approximates the time between the next rain event. This collected rainwater, or Detention Volume, can be used for beneficial use of rainwater ii while also serving to buffer stormwater runoff. The cost of such a system often can be offset by eliminating, or significantly reducing, the cost of a retention pond or other municipally required stormwater management device. Figure 9: Rainwater Storage tank with Detention Volume Component Design Methodology: A simple calculation to remember is 620 gallons of water result from 1 of rain on a 1000 square feet 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 NOAA 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. A simple calculation to remember is gallons of water result from 1 of rain on a 1000 square feet of surface. Frequency and intensity data of rain events can be gathered from local weather data, the local Plumbing Code, or as indicated in the NOAA 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, and is often dictated by local authorities. Example 1: Sizing and Absorption Field or Rain Garden: 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. For more detailed information on how to perform a perc test, contact your local agricultural agent iii. Example percolation rates, which is the amount that the soil can typically absorb, can be seen in Table 1 Soil Type Steady State Infiltration Rate (Inches / Hour) Sands >.80 Sandy and Silty soils Loams Clayey soils Sodic clayey soils <0.04 Table 1: Steady infiltration rates for general soil texture groups in a very deeply wetted soil (Hillel, 1982) Page 6 of 10

7 Calculate the size of an infiltration bed to absorb.25 rain event for a 10,000 sf building roof in a 72 hour period. Assume average sandy soil (0.60 in / hr). Water Volume to be infiltrated =.623 Gallon / inch *.25 * 10,000 sf = 1,558 Gallons = 208 ft 3 Infiltration area (ft 2 ) = [Water Volume to be infiltrated (ft 3 )] / [infiltration rate (in / hr) * time (hrs.)] = 208 ft 3 / [.6 in / hr. * 1/12 ft / in * 72 hrs. = 57.8 ft 2 To improve the performance of a infiltration bed, plants can be added that will absorb water as part of their metabolic process. This absorption of water as part of the growing process is called transpiration. Water that stays near the surface is available for the plant roots to absorb as part of the plant growing process and thereby aids in the removal of water. 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 - ET) is the loss of water from a vegetative surface through the combined processes of plant transpiration and soil evaporation. This process varies according to climate and season. Several methods have been developed to estimate ET that can best be quantified with the assistance of a local agricultural agent or master gardeners. Example 2: Sizing a Passive Rainwater Release System: Step1: Determine detention water volume to be infiltrated: Assume example 10,000 sf roof surface,.25 of rain, to be distributed over 24 hours. Column of water above the Controlled Discharge outlet is 2 feet. Step 2: Determine drawdown rate Q = ( V * 7.48 ft 3 / gal ) / (3600 sec/hr * t (hrs)) = [1558 gallons * 7.48 ft 3 / gal] / [3600 sec / hrs * 24 hrs] =.0024 ft3 / second where Q = average drawdown rate (ft 3 /second), and t = time to release full detention volume (hrs.). Step 3: Calculate area of Controlled Discharge orifice A = 144 Q g d = [144 * 0024 ft3 / second] / [.623 * (2 * 32.2 * 2 ft) 0.5 ] = 0.03ft 2 where A = cross-sectional area of PDD orifice (in 2 ), g = gravitational constant (32.2ft/s 2 ). d. = water height above outlet (ft) Page 7 of 10

8 Step 4: Determine diameter of orifice dia.= 4 A π = [ (4 * 0.03ft 2 ).5] / ] =.19 or about 3/16 inch where dia. = diameter of orifice (in), Stormwater Infiltration Case Study: Public Library, Menard, Texas iv The pictures 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 v 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. pop-up drain is added to the end chamber. In the second picture, the caliche and soil was dug down 1foot (.33 m) below the sidewalk and 9 inches (23 cm) of 3/4 inch (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 inches (50.8 cm) of rainfall annually and the downspout is supporting approximately 1200 square foot (111 square meters) of roof area. The crushed rock was leveled and then 5 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 Page 8 of 10

9 The storm chambers were then covered with geotextile 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 inches (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 end result was a serious drainage and erosion issue was turned into a significant asset thru creative application of stormwater management. Conclusion: Flooding, and its destructive impacts, has been a constant struggle since the beginning of human society. More recently, combined sewer overflows have become a serious and growing issue that endangers our sources of potable water. Water security is a new term being mentioned more frequently, often associated with the term food security, not only in foreign lands, but starting in this country as well. Rapid change in the 21st century, in populations, economies, geopolitics and climate, has made maintaining water security much more important than in the past. As water sources are recognized as being increasingly limited, the importance of knowing how to manage this vital resource will become essential to maintaining our way of life for future generations. For more information from relating to this topic, you are invited to research sources listed in the bibliography vi Page 9 of 10

10 i Phreatic soil is defined as soil below water level in which all the pores and inter-granular spaces are full of water. ii Per ARCSA / ASPE / ANSI Standard 63: Rainwater Catchment Design and Installation Standard iii A source from the University of Minnesota relating to how to perform a percolation test can be seen at iv Project Design, photography and project narrative by Billy Kniffen, Menard Texas, who is retired Water Resource Specialist from the Texas A& M Agri-Life Extension in the Biological and Agricultural Engineering Department, and a past Board Member of the American Rainwater Catchment Systems Association (ARCSA.org). Although retired, he continues heading educational efforts regarding rainwater and stormwater management projects in the United States and abroad. v Caliche: A layer of clay or sand containing minerals such as sodium nitrate and sodium chloride, found in dry regions. vi Bibliography: For additional information refer to the following 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 Alternative Water Sources and Waste Management Systems, by E. W. Bob Boulware PE, McGraw Hill Author Biography Mr. Boulware is founder and president of Design-Aire Engineering, an Indianapolis based mechanical-electrical consulting firm specializing in off grid and resource efficient mechanical, electrical and plumbing systems. Mr. Boulware is past national president of the American Rainwater Catchment Systems Association (ARCSA.org), author of Alternative Water Sources and Waste Management Systems (McGraw-Hill), is a board member and Vice President of the International Rain Harvesting Alliance (IRHA-h2o.org) in Geneva Switzerland, and presently sits on national committees that are developing the new green plumbing codes that reflect the new urgency of water shortages. Contact Information; Design-Aire Engineering, 220 North College Avenue, Indianapolis, Indiana 46202, Tel ; bboulware@daengineering.com, website: Vessela Monta is a Civil Engineer with a Master of Science degree from the Higher Institute for Civil Engineering and Architecture in Sofia, Bulgaria. In 2002, Ms. Monta became a founding member and Executive Director of the International Rainwater Harvesting Alliance IRHA, based in Geneva, Switzerland. IRHA is a non-governmental organization active in the international promotion of Rainwater Harvesting and the knowledge sharing linked to its beneficial implementation. Since 2002, she has been involved in Rainwater Harvesting activities: project implementation, training, awareness raising, networking and advocacy. Ms. Monta is the creator of the Blue Schools programme, which integrates potable water from rainwater harvesting, composting toilets for sanitation, and compost fertilizer as a profit center to support the schools. Today, more than 56 Blue Schools in the world are the beneficiary of this program. She has authored various articles on the topics of sustainable school construction techniques, rainwater harvesting, and climate change effects. Contact Information: International Rainwater Harvesting Alliance IRHA, International Environment House II, Ch. de Balexert , Châtelaine Switzerland, Tel: , vessela@irha-h2o.org, web-site: Page 10 of 10