Integrated Water Management and Green Infrastructure Retrofits in Urban Areas: Perspectives on Energy Savings, Water Quality Improvements and Economic Incentives
This research investigates various aspects of implementing integrated water management (IWM) measures in urban areas, with a specific focus on green infrastructure (GI) retrofits. Three major perspectives are examined in relation to water resources policy and management: energy savings benefits of IWM measures that reduce demand on potable water supplies and centralized wastewater treatment facilities; use of impervious area reduction as the key metric in determining the extent of GI retrofits possible relative to water quality goals in existing urban watersheds with aquatic life and biological impairments due to stormwater runoff and prioritization for GI retrofit experimentation at the catchment scale; and, efficacy of stormwater fee credits as an economic incentive for private commercial property owners to implement GI retrofits. The results of these investigations answer questions addressing knowledge gaps from these three perspectives and provide guidance for policy and management decisions regarding GI’s role in achieving sustainable urban water infrastructure goals. The first area of this research is based on the knowledge that water supply and wastewater treatment systems are energy intensive processes. Consequently, IWM measures that reduce potable water consumption and/or wastewater generation can potentially translate into significant energy savings. From this perspective, the energy savings associated with IWM measures of rainwater harvesting and gray water reuse are estimated both at national and local utility scales using published data. The results indicate that aggregate energy savings due to reduction in water and wastewater demand from widespread implementation of rainwater harvesting and gray water reuse can be large for water utilities. Although disaggregated savings at the household scale are small, the knowledge of potential energy and cost savings to individual consumers is important for water utilities and policy makers when considering how to promote and incentivize the sustainable use of water. Building on the concepts that stream health is related to extent of watershed impervious area and GI measures that remove runoff volume effectively reduce impervious area, the second area of this research identifies both the extent of impervious area reduction that GI retrofits can provide at the watershed scale and the relative contribution by property type. The extent of potential reduction in directly connected impervious area (DCIA) by GI retrofits is quantified within two impaired case study watersheds with different development characteristics in Mecklenburg County, North Carolina: Upper Little Sugar Creek (ULSC) which is dominated by commercial development and Six Mile Creek which is dominated by single-family residential development. The results indicate that GI retrofits are needed on all property types, public or private, to significantly impact aggregate DCIA reduction within the case study watersheds. Private commercial property plays a significant role in this regard providing almost 45% of the total DCIA reduction capability in ULSC and 35% in Six Mile Creek. Public property alone has the potential to provide approximately 35% of total DCIA reduction in both watersheds; however, the majority of this is from roadways and sidewalks with a small portion attributed to public owned commercial type development. The percentages of DCIA remaining in each watershed under maximum or moderate GI retrofit coverage scenarios do not appear to be particularly promising relative to a stream health threshold of 10% impervious area. However, in an adaptive management approach, actual measured improvements to water quality as a result of DCIA reduction will have greater meaning than magnitude of reduction or remaining DCIA percentage. The use of distributed stormwater controls is still mostly an unproven technology for urban stream restoration due to the limited number of watershed or catchment scale experiments of GI retrofits. A screening and prioritization scheme to select potential catchments for GI retrofit experimentation is developed using a multi-criteria decision analysis (MCDA) approach with a focus on DCIA reduction potential and applied to the two case study watersheds. Addition criteria are also considered and the overall prioritization goal is to identify catchments that will provide a manageable number and extent of GI retrofits such that measureable improvements in water quality can be potentially attained in a reasonable time horizon. The MCDA approach provides a framework to identify the best or few best catchment options within a priority watershed of interest to consider for further evaluation. The results provide decision makers and other stakeholders with information regarding the tradeoffs between different catchment options. Final catchment selection requires quantitative field evaluation and judgement calls as there are compromises to be made even when a few best catchment options are identified. Low participation rates in stormwater fee credit programs indicate that the benefits attributed to the credits are not being realized, most notably, the benefit of providing an incentive for private property owners to control stormwater on their sites. This is a problem if fee credits are to be used as an incentive to achieve the level of private property participation in GI retrofitting needed to impact stream quality improvements in impaired watersheds. In this third area of research an assessment is made of the economic value of various U.S. stormwater utility fee and credit structures, including the city of Charlotte’s existing and proposed programs, relative to GI investment value for both private commercial property owners and stormwater utilities. The results indicate that a stormwater fee and credit combination based on the cost of capital and fee credits to the stormwater utility and fee credits equal to the cost of annual maintenance to property owners can provide equitable incentives to both groups to invest in GI retrofits. These results are useful in addressing policy questions regarding the characteristics and role of equitable utility fee and credit programs in sustainable urban stormwater management.