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Cumulative impacts of residential rainwater harvesting on stormwater discharge through a peri-urban drainage network
Journal of Environmental Management 243 (2019) 127–136
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Research article
Cumulative impacts of residential rainwater harvesting on stormwater discharge through a peri-urban drainage network
T
Matthew J. Deitcha,∗, Shane T. Feirerb a b
Soil and Water Sciences Department, University of Florida IFAS West Florida Research and Education Center, Milton, United States Division of Agriculture and Natural Resources, University of California, Davis, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Rainwater harvesting Cumulative effects Stormwater management Spatial model Green infrastructure Peri-urban
Green infrastructure and techniques such as rainwater harvesting have been proposed as a means to reduce stormwater discharge in developed areas prone to floods. We examined the effects of rainwater harvesting on discharge cumulatively through the Perdido River drainage network in the US state of Florida, an area prone to routine rainfall-driven nuisance flooding. We considered scenarios where rainwater is stored in parcels with structures that use septic tanks (where tanks are retired and used as cisterns, volume approximately 5.7 cubic meters); and where a similar volume of water is stored at all developed parcels. To evaluate flow reduction through the drainage network, we modeled effects relative to a flow event with a 1.5-year recurrence interval using a spatial GIS-based cumulative-effects model. Our model predicted that retired septic tanks would reduce discharge by more than 10 percent in only a few areas in the study region, almost exclusively in headwater regions and where density of houses using septic tanks is high. Analysis of all developed parcels storing rainwater indicated that discharge in several areas would be reduced by more than 20 percent. Results indicate a spatially variable potential for rainwater harvesting to reduce routine storm discharge. Spatially continuous hydrologic tools such as the one we use here may be especially useful for managers seeking to prioritize limited resources at locations for maximum benefit.
1. Introduction Flooding in developed areas poses several threats to human wellbeing; these threats include risk to human health and the economic toll associated with loss of property and worker productivity (Moftakhari et al., 2018). Urbanization is a driver of flood risk: whereas features of an undeveloped catchment including floodplains, wetlands, and freely meandering stream channels provide high-flow attenuation to temper the effects of floods (Acreman and Holden, 2013), these features are often lost as catchments are urbanized (Walsh et al., 2005). Uncoordinated and disjointed development into forested and agricultural regions (which have greater capacity to mitigate stormwater flooding than developed regions with more impervious surfaces and altered soil characteristics) can further increase flooding where this development occurs, as well as in areas downstream that are already developed (Usinowicz et al., 2017; Muñoz et al., 2018). All of these conditions—increased development of floodplains and wetlands, expanded uncoordinated development, and loss of stormwater “sinks” in headwater areas—can exacerbate flooding associated with large infrequent storms as well as smaller more frequent rainfall events (Villarini et al.,
∗
2009). Novel methods for stormwater mitigation will be necessary to combat flooding of urbanized areas in the 21st Century (Barbosa et al., 2012). Traditional “grey” stormwater infrastructure may have limited effectiveness in controlling flooding because of changes in climate (leading to more frequent extreme events) and increased impervious surface in surrounding areas (Burns et al., 2012; Moore et al., 2016; Lopez-Cantu and Samaras, 2018). Stormwater channels designed to move water quickly from the landscape to receiving waters can also have adverse water quality effects, delivering higher loads of pollutants than non-channelized vegetated streams (McGrane, 2016; Meierdiercks et al., 2017). Benefits of green infrastructure such as vegetative swales, bioretention cells, permeable pavement, rainwater harvesting, and rain gardens have been documented at small scales (Zellner et al., 2016) as well as large (Ahiablame et al., 2013; Ahiablame and Shakya, 2016), though evaluations of effects are typically conducted at one or a few locations within a drainage network. Modeled multi-scale or continuous evaluations of change through the drainage network also may be useful for providing insights into the effects and variations caused by spatially distributed water infrastructure (Deitch et al., 2013; Joyce et al., 2017).
Corresponding author E-mail address: mdeitch@ufl.edu (M.J. Deitch).
https://doi.org/10.1016/j.jenvman.2019.05.018 Received 31 December 2018; Received in revised form 22 April 2019; Accepted 3 May 2019 Available online 13 May 2019 0301-4797/ © 2019 Elsevier Ltd. All rights reserved.
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infrastructure has the most potential to reduce flooding. The purpose of this research is to characterize the potential for residential rainwater harvesting—through the use of retired septic tanks as cisterns and more broadly where cisterns store water at all developed parcels—to reduce stormwater discharge throughout the Perdido River drainage network located in the state of Florida USA. Nuisance flooding is common in many peri-urban housing areas in this region, so evaluating the potential to reduce stormwater discharge over space can help to determine whether and where rainwater harvesting can ameliorate flooding impacts.
Rainwater harvesting has recently gained attention as a method for reducing peak storm flow as well as for providing water to meet agricultural and domestic needs (Walsh et al., 2014; Deitch and Dolman, 2017). Domestic rainwater harvesting has potential to reduce hydrologic input at its source, where it would otherwise result in efficient delivery of water off impervious rooftops to a stormwater conveyance or directly to a surface stream (Askarizadeh et al., 2015). Rainwater harvesting can have measurable benefits to stormwater reduction locally (Burns et al., 2015), but typical collection tank size often limits the capacity to meaningfully reduce stormwater runoff at a larger catchment scale (Petrucci et al., 2012). Rainwater harvesting where tanks are large (e.g., 20 cubic meters, or approximately 5000 US gallons) can meaningfully reduce peak flows under moderate flow conditions (Palla et al., 2017), but finding a suitable location for such a tank may limit feasibility in developed areas where parcel size is a constraint (Deitch and Dolman, 2017). In some peri-urban areas, infrastructure may already be available to facilitate domestic rainwater harvesting. Typically located at the edge (or periphery) of urban areas, peri-urban development is often not formally incorporated and may not be served by utilities in the same ways as development within municipal boundaries (Leker and MacDonald Gibson, 2018). For example, in the Panhandle region of northwest Florida (USA), many residential areas developed in the latter 20th Century located outside of municipal boundaries were not connected to centralized sewer systems. Rather, houses within these developments rely on onsite sewage treatment-and-disposal systems (septic systems) to treat household sewage. Properly maintained septic systems may be suitable and efficient methods for household sewage treatment in rural areas, but relatively large numbers of septic systems that have exceeded their functional lifespan concentrated in particular areas could be sources of nutrient loading to waterways downstream (Carey et al., 2013; Ye et al., 2017). Relatively large numbers of residences relying on septic systems are believed to be important sources of nonpoint pollutant loading in Northwest Florida panhandle watersheds (NFWMD, 2017a,b). To mitigate their impacts, state and local agencies in the Florida Panhandle are obtaining grant funding to connect houses to central sewer systems, lowering the initial homeowner cost and reducing nutrient inputs in the watershed. Retired septic tanks provide a reservoir volume that is comparable to residential rainwater tanks (Gomez and Teixeira, 2017). Residential septic tanks in the United States typically have a functional volume of 3.8–7.6 cubic meters (1000 to 2000 US gallons), with additional space above input and output baffles; if converted to rainwater cisterns, septic tanks can store an equivalent volume of rainwater. In 2013, the Florida state legislature adopted policies outlining the steps necessary for conversion of septic tanks to subsurface rainwater cisterns (FDOH, 2010), giving residences that formerly utilized onsite sewage treatment the potential to store a volume of water equivalent to the total capacity of their septic tank. Conversion of septic tanks to cisterns requires cleaning and sealing, disinfecting, and testing prior to use; costs for conversion may range from 1000 to 5000 US dollars (Favara, 2009; Shearer, 2010). Though the water-rich landscape of northwest Florida (Padowski and Jawitz, 2012) suggests that residential rainwater storage for domestic irrigation (e.g., lawn watering) may not be a priority, frequent nuisance flooding that occurs from overabundance of rainfall and insufficient infiltration is considered a primary concern in the region (NFWMD, 2017a,b). Spatial models can be especially useful for investigating the conditions for green infrastructure such as rainwater harvesting to reduce discharge magnitude. Based on the number and distribution of projects over space and quantitative details about their storage capacity, current water levels, and function, spatial models can quantify the changes in discharge cumulatively through the drainage network and under different conditions of discharge. Characterizing these potential changes over space may be especially useful for planning purposes, where goals may include identifying regions or stream reaches where green
2. Methods 2.1. Project area The northwest Florida Panhandle region of the United States is among the wettest areas in North America. Average annual precipitation in northwest Florida exceeds 1600 mm, varying from 1000 mm to 2300 mm over the period 1975–2018. Hot summers and mild winters, combined with sandy beaches across the Gulf of Mexico barrier islands, make the Florida Panhandle a popular year-round tourist destination (Hollis, 2004). This has also led to substantial population growth in parts of the Florida Panhandle; for example, United States census data indicate that the population of the two westernmost counties in Florida (Santa Rosa County and Escambia County) have increased a combined 73 percent over the period 1975 to 2015. Population growth in these two counties has been especially high in unincorporated areas, more than doubling (from 190,000 to 399,000) between 1975 and 2015. Recent data compiled by the Florida Department of Health indicate that more than 40,000 parcels in Escambia and Santa Rosa Counties rely on septic tanks to treat sewage (NFWMD, 2017a,b). The portion of the Perdido River catchment in northwest Florida, which forms its western border with the state of Alabama, reflects typical development patterns in the Panhandle region: residential and commercial development is highest near the coast and diminishes with distance north. Within the 900 square km portion of the Perdido River catchment in Florida (representing 30 percent of the entire Perdido River catchment), approximately 80 percent of the total parcels are in the southern half of Escambia County (Fig. 1). None of the Perdido catchment within the state of Florida is within a municipal boundary. The relatively high number of parcels in this area that use septic tanks to treat waste is believed to contribute to adverse levels of nitrogen into Perdido Bay (NFWMD, 2017a,b). Frequent rainfall and growing development increase the susceptibility of the Perdido River catchment in northwest Florida to frequent nuisance flooding. Rainfall records for the nearby city of Pensacola, Florida indicate that daily rainfall exceeding 50 mm occurs an average once every 1.7 months; and rainfall exceeding 100 mm occurs, on average, once every 9 months. Large portions of land have been converted from forests on sandy soil with high capacity for infiltration to subdivisions with impervious roofs, driveways, wide streets, and less permeable compacted soil. These factors cause major changes to the capacity for rainwater to infiltrate into the ground (Dunne and Leopold, 1978). The landscape has low relief typical of the southern end of the Gulf Coastal Plain, resulting in water moving slowly over the surface through wetlands and stream channels. Temporary road closures are common in many Panhandle peri-urban areas following moderate and heavy rain events. 2.2. Analytical methods To evaluate the extent to which residential rainwater harvesting can reduce flood discharge, we created a GIS-based framework for Escambia County, Florida (which includes all of the portion of the Perdido River catchment in the state of Florida) that sums the number of potential rainwater harvesting sites through a flow accumulation 128
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Fig. 1. Locations of parcels that use onsite treatment (septic tanks) and centralized sewers to treat waste in the Perdido River/Perdido Bay catchment in the state of Florida USA.
rainwater onsite; and (2) all parcels using septic tanks as well as those identified by FDOH as connected to centralized sewer systems for waste treatment and disposal. The distinction of parcels as using septic, sewer, or neither (i.e., uninhabited) was based on a spatial dataset developed by FDOH in 2015 (updated in 2016), which identifies all properties in Florida believed or recognized as using septic tanks and those believed or recognized as using centralized sewer systems. Data were accessed from the Florida Department of Health data webpage (http://ww10. doh.state.fl.us/pub/bos/Inventory/
raster-derived drainage network. We developed the model in ESRI ArcMap by adding a component to the flow accumulation and stream channel delineation process that creates a running tabulation of identified features as flow accumulation increases (i.e., moving downstream) through the drainage network. For this project, we developed two scenarios of features to tabulate through the flow accumulation process: (1) parcels identified by the Florida Department of Health (FDOH) as using onsite sewage treatment and disposal systems (septic tanks), thus already having a tank that could be converted to store 129
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could be obtained from a driveway or other impervious surfaces within the parcel. These scenarios were included to examine whether a greater storage volume distributed through the catchment would increase the stormwater reduction compared to a commonly installed tank size. Scenarios 3 and 4 provide a larger subset of structures that could be connected to a rainwater collection device than those with septic tanks already installed. These scenarios were included to examine whether a greater (and often more dense) number of rainwater collection sites would further reduce stormwater, compared to only those sites that utilized septic tanks for wastewater treatment (and thus already had a tank that could be retrofitted for rainwater storage).
FloridaWaterManagementInventory/) on 01 February 2018. For each developed property polygon in Escambia County, we assigned a centroid point, distinguished as either using septic or on sewer; we then used these points as the input for the tabulation process. Using our framework, we calculated within the same attribute table the cumulative number of upstream parcels (either septic or septic plus sewer) and the total number of upstream grid cells (quantifying the catchment area) at every grid cell downstream of a parcel centroid. After the framework was developed to tabulate upstream parcels, we assigned a tank storage capacity for each parcel. Because the majority of parcels in the project area are single-family residential, we assumed a uniform volume of storage for all tanks in the storage area (which varied by two scenarios, described below). This allowed us to calculate the total upstream storage capacity of tanks for each grid cell as the product of tank volume and number of upstream tanks. We also assigned a discharge value for each grid cell in the drainage network using a linear scaling by catchment area within the same flow accumulation table as our DEM-based watershed area and the tabulated number of upstream parcels with rainwater storage. Discharge values were scaled from the flow magnitude corresponding to the mean daily flow with a 1.5-year recurrence interval, based on data from a historical streamflow gauge in the project area operated by the US Geological Survey from 1986 to 2016 (Eleven Mile Creek near Pensacola, tributary to the Perdido River, gauge number 02376115). Discharge for each cell was calculated as the 1.5-year mean daily discharge value multiplied by the ratio of upstream area at a given point to the upstream area of the Eleven Mile Creek streamflow gauge. This scaling technique offers a simple, straightforward means of characterizing daily discharge through the drainage network, and it allowed us to estimate a discharge value for each drainage network grid cell within our spatial framework (and thus at a comparable spatial scale as upstream storage capacity). The 1.5-year mean daily discharge (23.56 cubic meters, or 832 cubic feet, per second) from the 74.2 square kilometer Eleven Mile Creek watershed was also chosen because it scales to the approximate runoff that would fill a septic tank (total capacity approximately 5.67 cubic meters, or 1500 US gallons) from 185 square meters (2000 square feet) of area. We then estimated the potential for retired septic tanks to reduce stormwater discharge as the percentage of discharge at a given point that could be stored in upstream tanks:
Streamflow reduction (%) =
3. Results 3.1. Distribution of parcels with septic tanks and connected to sewer lines The Florida Department of Health has identified 12,526 parcels in the Florida portion of the Perdido River watershed believed to utilize onsite septic tanks for sewage treatment. Of these, more than 10,000 are in the southern portion of the project area. These tend to be concentrated in two watersheds: based on FDOH records, 7292 parcels with septic tanks (58% of those in the Florida Perdido region) are located in either the Eleven Mile Creek watershed or the Bayou Marcus watershed (Fig. 1; 125 and 61 square km, respectively). Similarly, the total number of developed parcels in the Florida portion of the Perdido watershed are also concentrated in the Eleven Mile Creek and Bayou Marcus Creek catchments (30,581 out of 47,536 total in the Perdido watershed). 3.2. Benefits to storm discharge 3.2.1. Scenario 1: Septic tanks, 5.67 cubic meters storage Retired septic tanks retrofitted to store rainwater have mostly negligible effects on mitigating the 1.5 year stormwater flow through most of the drainage network. Almost 90 percent of stream reaches with more than 10 septic tanks upstream would have an overall stormwater flow reduction of less than 2 percent (Fig. 2); the few areas within the Perdido basin with potential to benefit greater than 10 percent from rainwater harvesting in retired septic tanks are located in the more densely populated Eleven Mile and Bayou Marcus Creek catchments. Within the Eleven Mile and Bayou Marcus Creek catchments, rainwater harvesting has potential to reduce peak flows in some areas of dense housing by more than 10 percent (Fig. 2, inset), indicating some potential benefit for those neighborhoods where development is concentrated. However, these areas of peak flow reduction overall represent a small fraction of the total drainage network, and are exclusively in the headwaters of small streams (Table 1).
Cumulative upstream storage capacity Estimated discharge × 100
A streamflow reduction value was calculated for each 30-by-30 m grid cell downstream of a developed property centroid (defining the DEM-based drainage network) in the Perdido catchment in Florida. We examined the potential for rainwater storage to reduce stormflow runoff under four scenarios:
3.2.2. Scenario 2: Septic tanks, 11.3 cubic meters storage If retired septic tanks could store a volume of 11.3 cubic meters, potential for these tanks to mitigate the 1.5-year flow event is still overall low. Approximately three-quarters of stream reaches with more than 10 septic tanks upstream will have an overall flow reduction of less than 2 percent, and 90 percent have flow reduction less than 4 percent (Fig. 3). Within the more heavily developed Bayou Marcus and Eleven Mile Creek catchments, the potential for retired septic tanks to reduce the 1.5-year flow event by more than 10 percent expands into more areas of concentrated development and benefits those areas more than if the tanks only stored 5.67 cubic meters. However, the amount of drainage network where the 1.5-year flow event is reduced by more than 15 percent is still very low and only occurs in headwater areas of relatively dense development (Fig. 3, inset).
1. Storage at all parcels currently using septic tanks, tank capacity of 5.67 cubic meters (1500 US gallons); 2. Storage at all parcels currently using septic tanks, tank capacity of 11.3 cubic meters (3000 gallons); 3. Storage at all parcels using septic tanks and connected to sewer lines, tank capacity of 5.67 cubic meters; 4. Storage at all parcels using septic tanks and connected to sewer lines, tank capacity of 11.3 cubic meters. In each case, model simulations assumed tank storage was zero at the onset, to maximize storage capacity. Scenarios 2 and 4 consider the potential for stormwater reduction if storage tank size is greater than typical septic tank size. Filling a tank with a capacity of 11.3 cubic meters from the 1.5 year event would likely require a surface catchment area greater than the roof of a residential structure; additional water
3.2.3. Scenario 3: All developed parcels, 5.67 cubic meters storage If all parcels that utilize septic tanks and are on central sewer lines were to store 5.67 cubic meters of runoff, the potential for these tanks 130
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Fig. 2. Reduction of 1.5-year discharge by rainwater harvesting through the Perdido drainage network, where water would be stored in retired septic tanks, with tank volume 5.7 cubic m.
developed parcels upstream would have an overall flow reduction of more than 10 percent, and 4.4 percent have flow reduction greater than 15 percent. The potential for rainwater harvesting to reduce flow is greater within the Bayou Marcus and Eleven Mile Creek drainage networks: almost 20 percent of the drainage network with more than 10 septic tanks upstream would have a flow reduction of more than 10 percent,
to reduce the 1.5-year flow event is more significant and more widespread. Within the Perdido catchment project area, rainwater harvesting has little capacity to reduce streamflow among streams in the northern portion of the area; but can provide flow reductions locally through many neighborhoods, as well as cumulatively in many of the larger stream channels in the southern portion of the area (Fig. 4). Overall, 9.6 percent of the drainage network with more than 10 131
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Table 1 Average upstream catchment area (hectares) for stream segments with peak flow reduction ranging from less than 1 percent to more than 20 percent, within the entire Perdido catchment project area and the Eleven Mile/Bayou Marcus Creek subcatchments, according to Scenario 1, Scenario 2, Scenario 3, and Scenario 4. Percent flood. reduction:. Scenario Scenario Scenario Scenario Scenario Scenario Scenario Scenario
1 (5.7 m3), entire project area 1, Eleven Mile and Bayou Marcus 2 (11.3 m3), entire project area 2, Eleven Mile and Bayou Marcus 3 (5.7m3), entire project area 3, Eleven Mile and Bayou Marcus 4 (11.3 m3), entire project area 4, Eleven Mile and Bayou Marcus
catchments catchments catchments catchments
< 1%
1–2%
2–4%
4–6%
6–8%
8–10%
10–15%
15–20%
> 20%
15,990 1430 22,070 314 27,570 197 36.820 160
603 569 1221 1850 978 890 473 205
73 90 603 569 378 595 978 890
23 26 93 118 620 523 434 710
9.1 9.5 33 35 37 46 310 480
31 6.5 32 36 22 20 593 389
4.1 4.1 9.2 9.6 14 14 276 371
3.2 3.0 24 6.8 5.3 3.8 19 20
0.81 1.08 3.3 3.9 2.5 1.9 9.5 9.7
reduce peak flows such as the 1.5-year event through nearly all the project drainage network, even if tanks are empty at the onset. The small reduction in peak flow from catching rain in retired septic tanks is a result of two related factors: the number of tanks relative to the total project area, and the tank volume. The project area has more than 12,000 septic tanks, but they would store a relatively small amount of water compared to catchment discharge under our modeled conditions. We selected a discharge value of the 1.5-year peak daily discharge because it occurs frequently and it corresponds to a high tank efficiency (where the volume stored in tanks approximately equals the volume of runoff from a typical residential rooftop); in the project area, residences that use septic tanks are generally not high enough in number or in sufficient density for rainwater harvesting to reduce peak flows. Larger tanks may have potential to store more runoff, but larger tank volumes alone may not lead to reduced peak flows in our project areas because rainwater harvesting is limited by the harvested surface size. Additional rainfall will produce additional runoff through the rest of the catchment as well, offsetting any benefit of harvesting rainwater into a larger tank. A tank with greater volume may have benefit to reducing peak flows, but only if its catchment area were large as well (Palla et al., 2017). This research also underscores the importance of sufficient tank volume for mitigating peak discharge. Commercially available residential rainwater harvesting tanks are commonly 0.2–0.4 cubic meters, fitting next to a residence as one or a small series of tanks below a residential downspout (Jennings et al., 2012). Such tanks may provide adequate water for small-scale landscaping in places where rain occurs often enough to re-fill tanks as needed (Gao et al., 2016). However, larger volumes of water must be stored to reduce stream discharge during events of sufficient magnitude to cause flooding. Larger storage capacity would also be needed to support non-potable household use (Palla et al., 2011), which may equal one or more cubic meters a few times per week. One or a few commercially available rainwater tanks may provide some light watering needs, but are unlikely to cause a measurable reduction of peak event discharge. Additionally, the utility of rainwater tanks for reducing peak flows will diminish if tanks are not empty before the next event (Amos et al., 2018); the capacity for stormwater mitigation will be less in proportion with unavailable volume. In our project area, an increased quantity of 5-cubic meter rainwater tanks may have greater benefit to reduce peak discharge in our project area than larger tank volumes. We found a greater potential for rainwater harvesting to reduce peak discharge in areas where houses do not use septic tanks to treat wastewater—especially in residential subdivisions where the houses are more densely placed—than areas where houses rely on septic tanks. In our modeled scenario, this greater density of houses would lead to a greater number of rainwater tanks per area, thus reducing peak flow more than in those areas where houses utilize septic tanks. Several peri-urban areas in the Bayou Marcus and Eleven Mile Creek watersheds that do not use septic tanks could have reduced stormwater volume through rainwater harvesting, and our results show the cumulative effects of water storage from large neighborhoods has potential to reduce peak discharge in larger streams as
and 3.3 percent could have a reduction of more than 20 percent (Fig. 4, inset). This reduction in discharge is still mostly in headwaters; the average upstream catchment area among reaches with more than 10 percent flow reduction is 9.7 ha (Table 1), whereas the average upstream catchment area among reaches with 4–6 percent flow reduction is more than 500 ha. 3.2.4. Scenario 4: All parcels, 11.3 cubic meters storage If all developed parcels (those using septic tanks plus sewer lines) were to store 11.3 cubic meters of runoff, the potential for these tanks to reduce the 1.5-year flow event increases overall and extends to more neighborhoods within the catchment. Rainwater harvesting still has little capacity to reduce streamflow among streams in the northern portion of the area; but flow reduction may be substantial in developed areas in the southern portion (Fig. 5). Within the Perdido study area, 23 percent of the drainage network with more than 10 developed parcels upstream would have a flow reduction of more than 10 percent, and 14.8 percent of the drainage network have flow reduction greater than 15 percent. The potential for rainwater harvesting to reduce the 1.5-year flow within the Bayou Marcus and Eleven Mile Creek drainage networks exceeds 20 percent in some areas, especially in neighborhoods of concentrated development (Fig. 5, inset). Within the Bayou Marcus and Eleven Mile subcatchments, 44 percent of drainage network with more than 10 developed parcels upstream has potential for rainwater storage to reduce peak flow by more than 10 percent; and 20 percent of the drainage network has potential for storage to reduce peak flows more than 20 percent. In this scenario, areas with peak flow reduction more than 10 percent are no longer concentrated in headwaters; the average upstream catchment area having flow reduction more than 10 percent is 188 ha (Table 1). Almost two kilometers of stream length with upstream catchment area greater than 1 square km have flow reduction of more than 20 percent. 4. Discussion Analytical tools that provide spatially variable output have particular value for catchment- and regional-scale planning (Herrmann and Osinski, 1999; Deitch et al., 2016). By displaying modeled conditions through an entire region (here, continuously through a drainage network), spatially comprehensive output provides the foundation for managers to identify locations within the region where impairment may be greatest or where restoration may be most beneficial. Our spatial output indicates those peri-urban residential areas that are most likely to benefit from residential rainwater harvesting, as well as whether rainwater harvesting in these areas can have cumulative benefits to stormwater farther downstream. This case study of stormwater management in the Perdido, Florida catchment provides several important insights about the potential for dispersed, spatially distributed low-impact development (LID) to reduce peak flows in our project area. Re-use of existing infrastructure (retired septic tanks with volume of 5.7 cubic meters) is unlikely to noticeably 132
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Fig. 3. Reduction of 1.5-year discharge by rainwater harvesting through the Perdido drainage network, where water would be stored in retired septic tanks, with tank volume 11.3 cubic m.
a regional level (Deitch and Dolman, 2017). Because rainwater harvesting participation approaching 100 percent in is unlikely, especially in places that are already developed, it may be more useful to consider the benefits of several LID elements working together to reduce peak flows in a region. Rainwater harvesting in conjunction with rain gardens, grassy swales, and other LID elements can cause hydrologic benefits including reduced flood frequency and improved water quality (Bedan and Clausen, 2009; Brown et al., 2012). The greatest benefit to reducing stormwater discharge in developed areas may occur through implementation of green infrastructure at the
well. Our rainwater harvesting evaluation also highlights the challenge of using small spatially distributed projects to mitigate impacts cumulatively. Flooding cannot be mitigated through small-scale pilot or demonstration rainwater storage projects; rather, our results corroborate other studies that point to the need for widespread participation among buildings to appreciably mitigate peak discharge (Palla et al., 2017). While pilot projects may be critical to increasing participation among residents in a region, sufficient participation in small-scale water conservation projects to have measurable outcomes may not be realistic at 133
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Fig. 4. Reduction of 1.5-year discharge by rainwater harvesting through the Perdido drainage network, where water would be stored at all developed parcels, with tank volume 5.7 cubic m.
economic benefits: residents in many places along Florida's coast pay a higher price for water used for irrigation because of threats from saltwater intrusion into freshwater aquifers. Opportunities for water supply cost savings can increase interest in rainwater storage (Coombes et al., 2002): mandating water storage is not feasible, but incorporating water storage through onsite cisterns could provide benefits to stormwater mitigation at a parcel-scale as well as a development-scale.
outset of development. Local policies in the US state of Florida require stormwater mitigation for new development, which often takes the form of dry retention basins or wet ponds. Rainwater cisterns and other green infrastructure could provide a reduction of water from primary sources before reaching a retention basin. Green infrastructure has added benefits of increasing property values as well (Boatwright et al., 2014; Mell et al., 2016). Using water stored in cisterns can have other 134
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Fig. 5. Reduction of 1.5-year discharge by rainwater harvesting through the Perdido drainage network, where water would be stored at all developed parcels, with tank volume 11.3 cubic m.
5. Conclusions
drainage network. Tools that explore how LID can mitigate impacts of development such as flooding throughout a region can provide manager with the ability to prioritize resources at locations where benefits will be appreciable. The results of our study indicate that residential rainwater harvesting has potential to play a significant role in stormwater mitigation, especially in places where housing is dense, but only if participation is
Green infrastructure and low-impact development will play a critical role in sustaining stream ecosystem services and community resilience in the 21st Century. Growing populations in urban areas and surrounding periphery will place new pressures on streams locally where development occurs, as well as cumulatively through the 135
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comprehensive among residences and tanks store sufficient water volumes. In contrast, rural areas where housing density is lower are unlikely to see benefit of reduced peak flows through rainwater harvesting. Having nearly complete participation in a retrofitted rainwater harvesting program may be less realistic than implementing rainwater harvesting at the onset of development. In places prone to flooding, this type of planning in conjunction with other LID practices could help to make urban areas more sustainable and resilient in the future.
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The role of land surface versus drainage network characteristics in controlling water quality and quantity in a small urban watershed. Hydrol. Process. 31, 4384–4397. Mell, I.C., Henneberry, J., Hehl-Lange, S., Keskin, B., 2016. To Green or Not to Green: Establishing the Economic Value of Green Infrastructure Investments in the Wicker, vol. 18. Urban For. Urban Gree., Sheffield, pp. 257–267. Moftakhari, H.R., AghaKouchak, A., Sanders, B.F., Allaire, M., Matthew, R.A., 2018. What is nuisance flooding? Defining and monitoring an emerging challenge. Water Resour. Res. 54, 4218–4227. Moore, T.L., Gulliver, J.S., Stack, L., Simpson, M.H., 2016. Stormwater management and climate change: vulnerability and capacity for adaptation in urban and suburban contexts. Clim. Change 138, 491–504. Muñoz, L.A., Olivera, F., Giglio, M., Berke, P., 2018. The impact of urbanization on the streamflows and the 100-year floodplain extent of the Sims Bayou in Houston, Texas. Int. J. River Basin Manag. 16, 61–69. NFWMD Northwest Florida Water Management District, 2017a. Perdido River and Bay Surface Water Information and Management Plan. Program Development Series 1707. Northwest Florida Water Management District, Mariana, Florida, USA. NFWMD Northwest Florida Water Management District, 2017b. Pensacola Bay System Surface Water Information and Management Plan. Program Development Series 1706. Northwest Florida Water Management District, Mariana, Florida, USA. Padowski, J.C., Jawitz, J.W., 2012. Water availability and vulnerability of 225 large cities in the United States: urban water availability and vulnerability. Water Resour. Res. 48. Palla, A., Gnecco, I., Lanza, L.G., 2011. Non-dimensional design parameters and performance assessment of rainwater harvesting systems. J. Hydrol. 401 (1–2), 65–76. Palla, A., Gnecco, I., La Barbera, P., 2017. The impact of domestic rainwater harvesting systems in storm water runoff mitigation at the urban block scale. J. Environ. Manag. 191, 297–305. Petrucci, G., Deroubaix, J.-F., de Gouvello, B., Deutsch, J.-C., Bompard, P., Tassin, B., 2012. Rainwater harvesting to control stormwater runoff in suburban areas. An experimental case-study. Urban Water J. 9, 45–55. Shearer, L., 2010. POLL: Septic-To-Cistern Conversion Banks Rainwater for Irrigation. Marco Eagle. 02 February 2010. http://archive.naplesnews.com/community/ marco-eagle/poll-septic-to-cistern-conversion-banks-rainwater-for-irrigation-ep395953754-334172601.html/, Accessed date: 6 March 2019. Usinowicz, J., Qiu, J., Kamarainen, A., 2017. Flashiness and flooding of two lakes in the upper Midwest during a century of urbanization and climate change. Ecosystems 20, 601–615. Villarini, G., Smith, J.A., Serinaldi, F., Bales, J., Bates, P.D., Krajewski, W.F., 2009. Flood frequency analysis for nonstationary annual peak records in an urban drainage basin. Adv. Water Resour. 32, 1255–1266. https://doi.org/10.1016/j.advwatres.2009.05. 003. Walsh, C.J., Roy, A.H., Feminella, J.W., Cottingham, P.D., Groffman, P.M., Morgan, R.P., 2005. The urban stream syndrome: current knowledge and the search for a cure. J. N. Am. Benthol. Soc. 24, 706–723. Walsh, T.C., Pomeroy, C.A., Burian, S.J., 2014. Hydrologic modeling analysis of a passive, residential rainwater harvesting program in an urbanized, semi-arid watershed. J. Hydrol. 508, 240–253. Ye, M., Sun, H., Hallas, K., 2017. Numerical estimation of nitrogen load from septic systems to surface water bodies in St. Lucie River and Estuary Basin, Florida. Environ. Earth Sci. 76, 32. Zellner, M., Massey, D., Minor, E., Gonzalez-Meler, M., 2016. Exploring the effects of green infrastructure placement on neighborhood-level flooding via spatially explicit simulations. Comput. Environ. Urban Syst. 59, 116–128.
Acknowledgements This research was supported by United States Department of Agriculture Hatch Grant number FLA-WFC-005577. The framework used for assessment in this research was developed by United States Environmental Protection Agency STAR Cumulative Effects Award R829803. The authors are also grateful for support from the University of Florida Institute for Food and Agricultural Sciences (IFAS) and the UF IFAS West Florida Research and Education Center. References Acreman, M., Holden, J., 2013. How wetlands affect floods. Wetlands 33, 773–786. Ahiablame, L., Shakya, R., 2016. Modeling flood reduction effects of low impact development at a watershed scale. J. Environ. Manag. 171, 81–91. Ahiablame, L.M., Engel, B.A., Chaubey, I., 2013. Effectiveness of low impact development practices in two urbanized watersheds: Retrofitting with rain barrel/cistern and porous pavement. J. Environ. Manag. 119, 151–161. Amos, C.C., Rahman, A., Karim, F., Gathenya, J.M., 2018. A scoping review of roof harvested rainwater usage in urban agriculture: Australia and Kenya in focus. J. Clean. Prod. 202, 174–190. Askarizadeh, A., Rippy, M.A., Fletcher, T.D., Feldman, D.L., Peng, J., Bowler, P., Mehring, A.S., Winfrey, B.K., Vrugt, J.A., AghaKouchak, A., Jiang, S.C., Sanders, B.F., Levin, L.A., Taylor, S., Grant, S.B., 2015. From rain tanks to catchments: use of low-impact development to address hydrologic symptoms of the urban stream syndrome. Environ. Sci. Technol. 49, 11264–11280. Barbosa, A.E., Fernandes, J.N., David, L.M., 2012. Key issues for sustainable urban stormwater management. Water Res. 46, 6787–6798. Bedan, E.S., Clausen, J.C., 2009. Stormwater runoff quality and quantity from traditional and low impact development watersheds 1. J. Am. Water Resour. Assoc. 45, 998–1008. Boatwright, J., Stephenson, K., Boyle, K., Nienow, S., 2014. Subdivision infrastructure affecting storm water runoff and residential property values. J. Water Res. Pl. ASCE 140, 524–532. Brown, R.A., Line, D.E., Hunt, W.F., 2012. LID treatment train: pervious concrete with subsurface storage in series with bioretention and care with seasonal high water tables. J. Environ. Eng. 138, 689–697. https://doi.org/10.1061/(ASCE)EE.19437870.0000506. Burns, M.J., Fletcher, T.D., Duncan, H.P., Hatt, B.E., Ladson, A.R., Walsh, C.J., 2015. The performance of rainwater tanks for stormwater retention and water supply at the household scale: an empirical study: rainwater tanks for stormwater retention and water supply. Hydrol. Process. 29, 152–160. Burns, M.J., Fletcher, T.D., Walsh, C.J., Ladson, A.R., Hatt, B.E., 2012. Hydrologic shortcomings of conventional urban stormwater management and opportunities for reform. Landsc. Urban Plann. 105, 230–240. Carey, R.O., Hochmuth, G.J., Martinez, C.J., Boyer, T.H., Dukes, M.D., Toor, G.S., Cisar, J.L., 2013. Evaluating nutrient impacts in urban watersheds: Challenges and research opportunities. Environ. Pollut. 173, 138–149. Coombes, P.J., Frost, A., Kuczera, G., O'Loughlin, G., Lees, S., 2002. Rainwater tank options for stormwater management in the upper Parramatta River catchment. In: Barton, A.C.T. (Ed.), Water Challenge: Balancing the Risks: Hydrology and Water Resources Symposium. Institution of Engineers, Australia, pp. 474–482 2002. Deitch, M., Dolman, B., 2017. Restoring summer base flow under a decentralized water management regime: Constraints, opportunities, and outcomes in MediterraneanClimate California. Water 9, 29. Deitch, M.J., Merenlender, A.M., Feirer, S., 2013. Cumulative effects of small reservoirs on streamflow in northern coastal California catchments. Water Resour. Manag. 15, 5101–5118. Deitch, M.J., van Docto, M., Feirer, S.T., 2016. A spatially explicit framework for assessing the effects of weather and water rights on streamflow. Appl. Geogr. 67, 14–26. Dumit Gómez, Y., Teixeira, L.G., 2017. Residential rainwater harvesting: effects of incentive policies and water consumption over economic feasibility. Resour. Conserv. Recycl. 127, 56–67. Dunne, T., Leopold, L.B., 1978. Water in Environmental Planning. W.H. Freeman and Company, New York. Favara, C., 2009. Septic to Cistern: Savings from a Rainy Day. Naples Daily News 14 April
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Contents lists available at ScienceDirect
Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Research article
Cumulative impacts of residential rainwater harvesting on stormwater discharge through a peri-urban drainage network
T
Matthew J. Deitcha,∗, Shane T. Feirerb a b
Soil and Water Sciences Department, University of Florida IFAS West Florida Research and Education Center, Milton, United States Division of Agriculture and Natural Resources, University of California, Davis, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Rainwater harvesting Cumulative effects Stormwater management Spatial model Green infrastructure Peri-urban
Green infrastructure and techniques such as rainwater harvesting have been proposed as a means to reduce stormwater discharge in developed areas prone to floods. We examined the effects of rainwater harvesting on discharge cumulatively through the Perdido River drainage network in the US state of Florida, an area prone to routine rainfall-driven nuisance flooding. We considered scenarios where rainwater is stored in parcels with structures that use septic tanks (where tanks are retired and used as cisterns, volume approximately 5.7 cubic meters); and where a similar volume of water is stored at all developed parcels. To evaluate flow reduction through the drainage network, we modeled effects relative to a flow event with a 1.5-year recurrence interval using a spatial GIS-based cumulative-effects model. Our model predicted that retired septic tanks would reduce discharge by more than 10 percent in only a few areas in the study region, almost exclusively in headwater regions and where density of houses using septic tanks is high. Analysis of all developed parcels storing rainwater indicated that discharge in several areas would be reduced by more than 20 percent. Results indicate a spatially variable potential for rainwater harvesting to reduce routine storm discharge. Spatially continuous hydrologic tools such as the one we use here may be especially useful for managers seeking to prioritize limited resources at locations for maximum benefit.
1. Introduction Flooding in developed areas poses several threats to human wellbeing; these threats include risk to human health and the economic toll associated with loss of property and worker productivity (Moftakhari et al., 2018). Urbanization is a driver of flood risk: whereas features of an undeveloped catchment including floodplains, wetlands, and freely meandering stream channels provide high-flow attenuation to temper the effects of floods (Acreman and Holden, 2013), these features are often lost as catchments are urbanized (Walsh et al., 2005). Uncoordinated and disjointed development into forested and agricultural regions (which have greater capacity to mitigate stormwater flooding than developed regions with more impervious surfaces and altered soil characteristics) can further increase flooding where this development occurs, as well as in areas downstream that are already developed (Usinowicz et al., 2017; Muñoz et al., 2018). All of these conditions—increased development of floodplains and wetlands, expanded uncoordinated development, and loss of stormwater “sinks” in headwater areas—can exacerbate flooding associated with large infrequent storms as well as smaller more frequent rainfall events (Villarini et al.,
∗
2009). Novel methods for stormwater mitigation will be necessary to combat flooding of urbanized areas in the 21st Century (Barbosa et al., 2012). Traditional “grey” stormwater infrastructure may have limited effectiveness in controlling flooding because of changes in climate (leading to more frequent extreme events) and increased impervious surface in surrounding areas (Burns et al., 2012; Moore et al., 2016; Lopez-Cantu and Samaras, 2018). Stormwater channels designed to move water quickly from the landscape to receiving waters can also have adverse water quality effects, delivering higher loads of pollutants than non-channelized vegetated streams (McGrane, 2016; Meierdiercks et al., 2017). Benefits of green infrastructure such as vegetative swales, bioretention cells, permeable pavement, rainwater harvesting, and rain gardens have been documented at small scales (Zellner et al., 2016) as well as large (Ahiablame et al., 2013; Ahiablame and Shakya, 2016), though evaluations of effects are typically conducted at one or a few locations within a drainage network. Modeled multi-scale or continuous evaluations of change through the drainage network also may be useful for providing insights into the effects and variations caused by spatially distributed water infrastructure (Deitch et al., 2013; Joyce et al., 2017).
Corresponding author E-mail address: mdeitch@ufl.edu (M.J. Deitch).
https://doi.org/10.1016/j.jenvman.2019.05.018 Received 31 December 2018; Received in revised form 22 April 2019; Accepted 3 May 2019 Available online 13 May 2019 0301-4797/ © 2019 Elsevier Ltd. All rights reserved.
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infrastructure has the most potential to reduce flooding. The purpose of this research is to characterize the potential for residential rainwater harvesting—through the use of retired septic tanks as cisterns and more broadly where cisterns store water at all developed parcels—to reduce stormwater discharge throughout the Perdido River drainage network located in the state of Florida USA. Nuisance flooding is common in many peri-urban housing areas in this region, so evaluating the potential to reduce stormwater discharge over space can help to determine whether and where rainwater harvesting can ameliorate flooding impacts.
Rainwater harvesting has recently gained attention as a method for reducing peak storm flow as well as for providing water to meet agricultural and domestic needs (Walsh et al., 2014; Deitch and Dolman, 2017). Domestic rainwater harvesting has potential to reduce hydrologic input at its source, where it would otherwise result in efficient delivery of water off impervious rooftops to a stormwater conveyance or directly to a surface stream (Askarizadeh et al., 2015). Rainwater harvesting can have measurable benefits to stormwater reduction locally (Burns et al., 2015), but typical collection tank size often limits the capacity to meaningfully reduce stormwater runoff at a larger catchment scale (Petrucci et al., 2012). Rainwater harvesting where tanks are large (e.g., 20 cubic meters, or approximately 5000 US gallons) can meaningfully reduce peak flows under moderate flow conditions (Palla et al., 2017), but finding a suitable location for such a tank may limit feasibility in developed areas where parcel size is a constraint (Deitch and Dolman, 2017). In some peri-urban areas, infrastructure may already be available to facilitate domestic rainwater harvesting. Typically located at the edge (or periphery) of urban areas, peri-urban development is often not formally incorporated and may not be served by utilities in the same ways as development within municipal boundaries (Leker and MacDonald Gibson, 2018). For example, in the Panhandle region of northwest Florida (USA), many residential areas developed in the latter 20th Century located outside of municipal boundaries were not connected to centralized sewer systems. Rather, houses within these developments rely on onsite sewage treatment-and-disposal systems (septic systems) to treat household sewage. Properly maintained septic systems may be suitable and efficient methods for household sewage treatment in rural areas, but relatively large numbers of septic systems that have exceeded their functional lifespan concentrated in particular areas could be sources of nutrient loading to waterways downstream (Carey et al., 2013; Ye et al., 2017). Relatively large numbers of residences relying on septic systems are believed to be important sources of nonpoint pollutant loading in Northwest Florida panhandle watersheds (NFWMD, 2017a,b). To mitigate their impacts, state and local agencies in the Florida Panhandle are obtaining grant funding to connect houses to central sewer systems, lowering the initial homeowner cost and reducing nutrient inputs in the watershed. Retired septic tanks provide a reservoir volume that is comparable to residential rainwater tanks (Gomez and Teixeira, 2017). Residential septic tanks in the United States typically have a functional volume of 3.8–7.6 cubic meters (1000 to 2000 US gallons), with additional space above input and output baffles; if converted to rainwater cisterns, septic tanks can store an equivalent volume of rainwater. In 2013, the Florida state legislature adopted policies outlining the steps necessary for conversion of septic tanks to subsurface rainwater cisterns (FDOH, 2010), giving residences that formerly utilized onsite sewage treatment the potential to store a volume of water equivalent to the total capacity of their septic tank. Conversion of septic tanks to cisterns requires cleaning and sealing, disinfecting, and testing prior to use; costs for conversion may range from 1000 to 5000 US dollars (Favara, 2009; Shearer, 2010). Though the water-rich landscape of northwest Florida (Padowski and Jawitz, 2012) suggests that residential rainwater storage for domestic irrigation (e.g., lawn watering) may not be a priority, frequent nuisance flooding that occurs from overabundance of rainfall and insufficient infiltration is considered a primary concern in the region (NFWMD, 2017a,b). Spatial models can be especially useful for investigating the conditions for green infrastructure such as rainwater harvesting to reduce discharge magnitude. Based on the number and distribution of projects over space and quantitative details about their storage capacity, current water levels, and function, spatial models can quantify the changes in discharge cumulatively through the drainage network and under different conditions of discharge. Characterizing these potential changes over space may be especially useful for planning purposes, where goals may include identifying regions or stream reaches where green
2. Methods 2.1. Project area The northwest Florida Panhandle region of the United States is among the wettest areas in North America. Average annual precipitation in northwest Florida exceeds 1600 mm, varying from 1000 mm to 2300 mm over the period 1975–2018. Hot summers and mild winters, combined with sandy beaches across the Gulf of Mexico barrier islands, make the Florida Panhandle a popular year-round tourist destination (Hollis, 2004). This has also led to substantial population growth in parts of the Florida Panhandle; for example, United States census data indicate that the population of the two westernmost counties in Florida (Santa Rosa County and Escambia County) have increased a combined 73 percent over the period 1975 to 2015. Population growth in these two counties has been especially high in unincorporated areas, more than doubling (from 190,000 to 399,000) between 1975 and 2015. Recent data compiled by the Florida Department of Health indicate that more than 40,000 parcels in Escambia and Santa Rosa Counties rely on septic tanks to treat sewage (NFWMD, 2017a,b). The portion of the Perdido River catchment in northwest Florida, which forms its western border with the state of Alabama, reflects typical development patterns in the Panhandle region: residential and commercial development is highest near the coast and diminishes with distance north. Within the 900 square km portion of the Perdido River catchment in Florida (representing 30 percent of the entire Perdido River catchment), approximately 80 percent of the total parcels are in the southern half of Escambia County (Fig. 1). None of the Perdido catchment within the state of Florida is within a municipal boundary. The relatively high number of parcels in this area that use septic tanks to treat waste is believed to contribute to adverse levels of nitrogen into Perdido Bay (NFWMD, 2017a,b). Frequent rainfall and growing development increase the susceptibility of the Perdido River catchment in northwest Florida to frequent nuisance flooding. Rainfall records for the nearby city of Pensacola, Florida indicate that daily rainfall exceeding 50 mm occurs an average once every 1.7 months; and rainfall exceeding 100 mm occurs, on average, once every 9 months. Large portions of land have been converted from forests on sandy soil with high capacity for infiltration to subdivisions with impervious roofs, driveways, wide streets, and less permeable compacted soil. These factors cause major changes to the capacity for rainwater to infiltrate into the ground (Dunne and Leopold, 1978). The landscape has low relief typical of the southern end of the Gulf Coastal Plain, resulting in water moving slowly over the surface through wetlands and stream channels. Temporary road closures are common in many Panhandle peri-urban areas following moderate and heavy rain events. 2.2. Analytical methods To evaluate the extent to which residential rainwater harvesting can reduce flood discharge, we created a GIS-based framework for Escambia County, Florida (which includes all of the portion of the Perdido River catchment in the state of Florida) that sums the number of potential rainwater harvesting sites through a flow accumulation 128
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Fig. 1. Locations of parcels that use onsite treatment (septic tanks) and centralized sewers to treat waste in the Perdido River/Perdido Bay catchment in the state of Florida USA.
rainwater onsite; and (2) all parcels using septic tanks as well as those identified by FDOH as connected to centralized sewer systems for waste treatment and disposal. The distinction of parcels as using septic, sewer, or neither (i.e., uninhabited) was based on a spatial dataset developed by FDOH in 2015 (updated in 2016), which identifies all properties in Florida believed or recognized as using septic tanks and those believed or recognized as using centralized sewer systems. Data were accessed from the Florida Department of Health data webpage (http://ww10. doh.state.fl.us/pub/bos/Inventory/
raster-derived drainage network. We developed the model in ESRI ArcMap by adding a component to the flow accumulation and stream channel delineation process that creates a running tabulation of identified features as flow accumulation increases (i.e., moving downstream) through the drainage network. For this project, we developed two scenarios of features to tabulate through the flow accumulation process: (1) parcels identified by the Florida Department of Health (FDOH) as using onsite sewage treatment and disposal systems (septic tanks), thus already having a tank that could be converted to store 129
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could be obtained from a driveway or other impervious surfaces within the parcel. These scenarios were included to examine whether a greater storage volume distributed through the catchment would increase the stormwater reduction compared to a commonly installed tank size. Scenarios 3 and 4 provide a larger subset of structures that could be connected to a rainwater collection device than those with septic tanks already installed. These scenarios were included to examine whether a greater (and often more dense) number of rainwater collection sites would further reduce stormwater, compared to only those sites that utilized septic tanks for wastewater treatment (and thus already had a tank that could be retrofitted for rainwater storage).
FloridaWaterManagementInventory/) on 01 February 2018. For each developed property polygon in Escambia County, we assigned a centroid point, distinguished as either using septic or on sewer; we then used these points as the input for the tabulation process. Using our framework, we calculated within the same attribute table the cumulative number of upstream parcels (either septic or septic plus sewer) and the total number of upstream grid cells (quantifying the catchment area) at every grid cell downstream of a parcel centroid. After the framework was developed to tabulate upstream parcels, we assigned a tank storage capacity for each parcel. Because the majority of parcels in the project area are single-family residential, we assumed a uniform volume of storage for all tanks in the storage area (which varied by two scenarios, described below). This allowed us to calculate the total upstream storage capacity of tanks for each grid cell as the product of tank volume and number of upstream tanks. We also assigned a discharge value for each grid cell in the drainage network using a linear scaling by catchment area within the same flow accumulation table as our DEM-based watershed area and the tabulated number of upstream parcels with rainwater storage. Discharge values were scaled from the flow magnitude corresponding to the mean daily flow with a 1.5-year recurrence interval, based on data from a historical streamflow gauge in the project area operated by the US Geological Survey from 1986 to 2016 (Eleven Mile Creek near Pensacola, tributary to the Perdido River, gauge number 02376115). Discharge for each cell was calculated as the 1.5-year mean daily discharge value multiplied by the ratio of upstream area at a given point to the upstream area of the Eleven Mile Creek streamflow gauge. This scaling technique offers a simple, straightforward means of characterizing daily discharge through the drainage network, and it allowed us to estimate a discharge value for each drainage network grid cell within our spatial framework (and thus at a comparable spatial scale as upstream storage capacity). The 1.5-year mean daily discharge (23.56 cubic meters, or 832 cubic feet, per second) from the 74.2 square kilometer Eleven Mile Creek watershed was also chosen because it scales to the approximate runoff that would fill a septic tank (total capacity approximately 5.67 cubic meters, or 1500 US gallons) from 185 square meters (2000 square feet) of area. We then estimated the potential for retired septic tanks to reduce stormwater discharge as the percentage of discharge at a given point that could be stored in upstream tanks:
Streamflow reduction (%) =
3. Results 3.1. Distribution of parcels with septic tanks and connected to sewer lines The Florida Department of Health has identified 12,526 parcels in the Florida portion of the Perdido River watershed believed to utilize onsite septic tanks for sewage treatment. Of these, more than 10,000 are in the southern portion of the project area. These tend to be concentrated in two watersheds: based on FDOH records, 7292 parcels with septic tanks (58% of those in the Florida Perdido region) are located in either the Eleven Mile Creek watershed or the Bayou Marcus watershed (Fig. 1; 125 and 61 square km, respectively). Similarly, the total number of developed parcels in the Florida portion of the Perdido watershed are also concentrated in the Eleven Mile Creek and Bayou Marcus Creek catchments (30,581 out of 47,536 total in the Perdido watershed). 3.2. Benefits to storm discharge 3.2.1. Scenario 1: Septic tanks, 5.67 cubic meters storage Retired septic tanks retrofitted to store rainwater have mostly negligible effects on mitigating the 1.5 year stormwater flow through most of the drainage network. Almost 90 percent of stream reaches with more than 10 septic tanks upstream would have an overall stormwater flow reduction of less than 2 percent (Fig. 2); the few areas within the Perdido basin with potential to benefit greater than 10 percent from rainwater harvesting in retired septic tanks are located in the more densely populated Eleven Mile and Bayou Marcus Creek catchments. Within the Eleven Mile and Bayou Marcus Creek catchments, rainwater harvesting has potential to reduce peak flows in some areas of dense housing by more than 10 percent (Fig. 2, inset), indicating some potential benefit for those neighborhoods where development is concentrated. However, these areas of peak flow reduction overall represent a small fraction of the total drainage network, and are exclusively in the headwaters of small streams (Table 1).
Cumulative upstream storage capacity Estimated discharge × 100
A streamflow reduction value was calculated for each 30-by-30 m grid cell downstream of a developed property centroid (defining the DEM-based drainage network) in the Perdido catchment in Florida. We examined the potential for rainwater storage to reduce stormflow runoff under four scenarios:
3.2.2. Scenario 2: Septic tanks, 11.3 cubic meters storage If retired septic tanks could store a volume of 11.3 cubic meters, potential for these tanks to mitigate the 1.5-year flow event is still overall low. Approximately three-quarters of stream reaches with more than 10 septic tanks upstream will have an overall flow reduction of less than 2 percent, and 90 percent have flow reduction less than 4 percent (Fig. 3). Within the more heavily developed Bayou Marcus and Eleven Mile Creek catchments, the potential for retired septic tanks to reduce the 1.5-year flow event by more than 10 percent expands into more areas of concentrated development and benefits those areas more than if the tanks only stored 5.67 cubic meters. However, the amount of drainage network where the 1.5-year flow event is reduced by more than 15 percent is still very low and only occurs in headwater areas of relatively dense development (Fig. 3, inset).
1. Storage at all parcels currently using septic tanks, tank capacity of 5.67 cubic meters (1500 US gallons); 2. Storage at all parcels currently using septic tanks, tank capacity of 11.3 cubic meters (3000 gallons); 3. Storage at all parcels using septic tanks and connected to sewer lines, tank capacity of 5.67 cubic meters; 4. Storage at all parcels using septic tanks and connected to sewer lines, tank capacity of 11.3 cubic meters. In each case, model simulations assumed tank storage was zero at the onset, to maximize storage capacity. Scenarios 2 and 4 consider the potential for stormwater reduction if storage tank size is greater than typical septic tank size. Filling a tank with a capacity of 11.3 cubic meters from the 1.5 year event would likely require a surface catchment area greater than the roof of a residential structure; additional water
3.2.3. Scenario 3: All developed parcels, 5.67 cubic meters storage If all parcels that utilize septic tanks and are on central sewer lines were to store 5.67 cubic meters of runoff, the potential for these tanks 130
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Fig. 2. Reduction of 1.5-year discharge by rainwater harvesting through the Perdido drainage network, where water would be stored in retired septic tanks, with tank volume 5.7 cubic m.
developed parcels upstream would have an overall flow reduction of more than 10 percent, and 4.4 percent have flow reduction greater than 15 percent. The potential for rainwater harvesting to reduce flow is greater within the Bayou Marcus and Eleven Mile Creek drainage networks: almost 20 percent of the drainage network with more than 10 septic tanks upstream would have a flow reduction of more than 10 percent,
to reduce the 1.5-year flow event is more significant and more widespread. Within the Perdido catchment project area, rainwater harvesting has little capacity to reduce streamflow among streams in the northern portion of the area; but can provide flow reductions locally through many neighborhoods, as well as cumulatively in many of the larger stream channels in the southern portion of the area (Fig. 4). Overall, 9.6 percent of the drainage network with more than 10 131
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Table 1 Average upstream catchment area (hectares) for stream segments with peak flow reduction ranging from less than 1 percent to more than 20 percent, within the entire Perdido catchment project area and the Eleven Mile/Bayou Marcus Creek subcatchments, according to Scenario 1, Scenario 2, Scenario 3, and Scenario 4. Percent flood. reduction:. Scenario Scenario Scenario Scenario Scenario Scenario Scenario Scenario
1 (5.7 m3), entire project area 1, Eleven Mile and Bayou Marcus 2 (11.3 m3), entire project area 2, Eleven Mile and Bayou Marcus 3 (5.7m3), entire project area 3, Eleven Mile and Bayou Marcus 4 (11.3 m3), entire project area 4, Eleven Mile and Bayou Marcus
catchments catchments catchments catchments
< 1%
1–2%
2–4%
4–6%
6–8%
8–10%
10–15%
15–20%
> 20%
15,990 1430 22,070 314 27,570 197 36.820 160
603 569 1221 1850 978 890 473 205
73 90 603 569 378 595 978 890
23 26 93 118 620 523 434 710
9.1 9.5 33 35 37 46 310 480
31 6.5 32 36 22 20 593 389
4.1 4.1 9.2 9.6 14 14 276 371
3.2 3.0 24 6.8 5.3 3.8 19 20
0.81 1.08 3.3 3.9 2.5 1.9 9.5 9.7
reduce peak flows such as the 1.5-year event through nearly all the project drainage network, even if tanks are empty at the onset. The small reduction in peak flow from catching rain in retired septic tanks is a result of two related factors: the number of tanks relative to the total project area, and the tank volume. The project area has more than 12,000 septic tanks, but they would store a relatively small amount of water compared to catchment discharge under our modeled conditions. We selected a discharge value of the 1.5-year peak daily discharge because it occurs frequently and it corresponds to a high tank efficiency (where the volume stored in tanks approximately equals the volume of runoff from a typical residential rooftop); in the project area, residences that use septic tanks are generally not high enough in number or in sufficient density for rainwater harvesting to reduce peak flows. Larger tanks may have potential to store more runoff, but larger tank volumes alone may not lead to reduced peak flows in our project areas because rainwater harvesting is limited by the harvested surface size. Additional rainfall will produce additional runoff through the rest of the catchment as well, offsetting any benefit of harvesting rainwater into a larger tank. A tank with greater volume may have benefit to reducing peak flows, but only if its catchment area were large as well (Palla et al., 2017). This research also underscores the importance of sufficient tank volume for mitigating peak discharge. Commercially available residential rainwater harvesting tanks are commonly 0.2–0.4 cubic meters, fitting next to a residence as one or a small series of tanks below a residential downspout (Jennings et al., 2012). Such tanks may provide adequate water for small-scale landscaping in places where rain occurs often enough to re-fill tanks as needed (Gao et al., 2016). However, larger volumes of water must be stored to reduce stream discharge during events of sufficient magnitude to cause flooding. Larger storage capacity would also be needed to support non-potable household use (Palla et al., 2011), which may equal one or more cubic meters a few times per week. One or a few commercially available rainwater tanks may provide some light watering needs, but are unlikely to cause a measurable reduction of peak event discharge. Additionally, the utility of rainwater tanks for reducing peak flows will diminish if tanks are not empty before the next event (Amos et al., 2018); the capacity for stormwater mitigation will be less in proportion with unavailable volume. In our project area, an increased quantity of 5-cubic meter rainwater tanks may have greater benefit to reduce peak discharge in our project area than larger tank volumes. We found a greater potential for rainwater harvesting to reduce peak discharge in areas where houses do not use septic tanks to treat wastewater—especially in residential subdivisions where the houses are more densely placed—than areas where houses rely on septic tanks. In our modeled scenario, this greater density of houses would lead to a greater number of rainwater tanks per area, thus reducing peak flow more than in those areas where houses utilize septic tanks. Several peri-urban areas in the Bayou Marcus and Eleven Mile Creek watersheds that do not use septic tanks could have reduced stormwater volume through rainwater harvesting, and our results show the cumulative effects of water storage from large neighborhoods has potential to reduce peak discharge in larger streams as
and 3.3 percent could have a reduction of more than 20 percent (Fig. 4, inset). This reduction in discharge is still mostly in headwaters; the average upstream catchment area among reaches with more than 10 percent flow reduction is 9.7 ha (Table 1), whereas the average upstream catchment area among reaches with 4–6 percent flow reduction is more than 500 ha. 3.2.4. Scenario 4: All parcels, 11.3 cubic meters storage If all developed parcels (those using septic tanks plus sewer lines) were to store 11.3 cubic meters of runoff, the potential for these tanks to reduce the 1.5-year flow event increases overall and extends to more neighborhoods within the catchment. Rainwater harvesting still has little capacity to reduce streamflow among streams in the northern portion of the area; but flow reduction may be substantial in developed areas in the southern portion (Fig. 5). Within the Perdido study area, 23 percent of the drainage network with more than 10 developed parcels upstream would have a flow reduction of more than 10 percent, and 14.8 percent of the drainage network have flow reduction greater than 15 percent. The potential for rainwater harvesting to reduce the 1.5-year flow within the Bayou Marcus and Eleven Mile Creek drainage networks exceeds 20 percent in some areas, especially in neighborhoods of concentrated development (Fig. 5, inset). Within the Bayou Marcus and Eleven Mile subcatchments, 44 percent of drainage network with more than 10 developed parcels upstream has potential for rainwater storage to reduce peak flow by more than 10 percent; and 20 percent of the drainage network has potential for storage to reduce peak flows more than 20 percent. In this scenario, areas with peak flow reduction more than 10 percent are no longer concentrated in headwaters; the average upstream catchment area having flow reduction more than 10 percent is 188 ha (Table 1). Almost two kilometers of stream length with upstream catchment area greater than 1 square km have flow reduction of more than 20 percent. 4. Discussion Analytical tools that provide spatially variable output have particular value for catchment- and regional-scale planning (Herrmann and Osinski, 1999; Deitch et al., 2016). By displaying modeled conditions through an entire region (here, continuously through a drainage network), spatially comprehensive output provides the foundation for managers to identify locations within the region where impairment may be greatest or where restoration may be most beneficial. Our spatial output indicates those peri-urban residential areas that are most likely to benefit from residential rainwater harvesting, as well as whether rainwater harvesting in these areas can have cumulative benefits to stormwater farther downstream. This case study of stormwater management in the Perdido, Florida catchment provides several important insights about the potential for dispersed, spatially distributed low-impact development (LID) to reduce peak flows in our project area. Re-use of existing infrastructure (retired septic tanks with volume of 5.7 cubic meters) is unlikely to noticeably 132
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Fig. 3. Reduction of 1.5-year discharge by rainwater harvesting through the Perdido drainage network, where water would be stored in retired septic tanks, with tank volume 11.3 cubic m.
a regional level (Deitch and Dolman, 2017). Because rainwater harvesting participation approaching 100 percent in is unlikely, especially in places that are already developed, it may be more useful to consider the benefits of several LID elements working together to reduce peak flows in a region. Rainwater harvesting in conjunction with rain gardens, grassy swales, and other LID elements can cause hydrologic benefits including reduced flood frequency and improved water quality (Bedan and Clausen, 2009; Brown et al., 2012). The greatest benefit to reducing stormwater discharge in developed areas may occur through implementation of green infrastructure at the
well. Our rainwater harvesting evaluation also highlights the challenge of using small spatially distributed projects to mitigate impacts cumulatively. Flooding cannot be mitigated through small-scale pilot or demonstration rainwater storage projects; rather, our results corroborate other studies that point to the need for widespread participation among buildings to appreciably mitigate peak discharge (Palla et al., 2017). While pilot projects may be critical to increasing participation among residents in a region, sufficient participation in small-scale water conservation projects to have measurable outcomes may not be realistic at 133
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Fig. 4. Reduction of 1.5-year discharge by rainwater harvesting through the Perdido drainage network, where water would be stored at all developed parcels, with tank volume 5.7 cubic m.
economic benefits: residents in many places along Florida's coast pay a higher price for water used for irrigation because of threats from saltwater intrusion into freshwater aquifers. Opportunities for water supply cost savings can increase interest in rainwater storage (Coombes et al., 2002): mandating water storage is not feasible, but incorporating water storage through onsite cisterns could provide benefits to stormwater mitigation at a parcel-scale as well as a development-scale.
outset of development. Local policies in the US state of Florida require stormwater mitigation for new development, which often takes the form of dry retention basins or wet ponds. Rainwater cisterns and other green infrastructure could provide a reduction of water from primary sources before reaching a retention basin. Green infrastructure has added benefits of increasing property values as well (Boatwright et al., 2014; Mell et al., 2016). Using water stored in cisterns can have other 134
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Fig. 5. Reduction of 1.5-year discharge by rainwater harvesting through the Perdido drainage network, where water would be stored at all developed parcels, with tank volume 11.3 cubic m.
5. Conclusions
drainage network. Tools that explore how LID can mitigate impacts of development such as flooding throughout a region can provide manager with the ability to prioritize resources at locations where benefits will be appreciable. The results of our study indicate that residential rainwater harvesting has potential to play a significant role in stormwater mitigation, especially in places where housing is dense, but only if participation is
Green infrastructure and low-impact development will play a critical role in sustaining stream ecosystem services and community resilience in the 21st Century. Growing populations in urban areas and surrounding periphery will place new pressures on streams locally where development occurs, as well as cumulatively through the 135
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comprehensive among residences and tanks store sufficient water volumes. In contrast, rural areas where housing density is lower are unlikely to see benefit of reduced peak flows through rainwater harvesting. Having nearly complete participation in a retrofitted rainwater harvesting program may be less realistic than implementing rainwater harvesting at the onset of development. In places prone to flooding, this type of planning in conjunction with other LID practices could help to make urban areas more sustainable and resilient in the future.
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