Assessing Climate Change's Impact on the Size of CSO Facilities in King County (WA)

Introduction and Purpose King County's 2015 Strategic Climate Adaptation Plan (SCAP) contained a variety of actions that have, or will, reduce the county's greenhouse gas emissions and help King County (KC) prepare for the impacts of climate change. To address one priority action, a study was implemented in collaboration with the University of Washington Climate Impacts Group (UWCIG) 'to assess climate change impacts on local rainfall patterns.' The results of that research were then to be used by KC to assess potential impacts on the size of required CSO control facilities, so that this information can be incorporated into future wastewater capital planning efforts. Recent research indicates that future heavy rain events will be more intense in the Pacific Northwest. The purpose of this Climate Change Impact Analysis is to understand the impact of climate change on CSO facility control volumes and peak overflow rates necessary to achieve and/or maintain compliance with the current Washington State CSO regulatory compliance criteria. This paper presents the methodology and results of the analysis and may be useful for other utilities working to incorporate climate impacts into their capital planning. King County Wastewater Treatment Division (KCWTD) is the wholesale wastewater conveyance and treatment provider for 34 component agencies including the City of Seattle. King County has 39 permitted CSO locations and the City of Seattle has 99 of their own locations. King County is obligated by a federal and state consent decree to control its CSO outfalls, meeting CSO regulations such that there will be no more than one CSO event per year, in any 20-year period, by the year 2030. Methodology King County's approved CSO long-term control plan has a combination of storage and treatment alternatives recommended for the outfalls not currently in 'control.' For simplicity of communication, the impacts of climate change in this paper are presented as the volume of CSO storage that would be required to achieve control, meeting the Washington state CSO regulatory standard. Thus, the 1-year control volume, over the worst 20-year period, is used to assess the climate change impact by the end of this century. The UWCIG used the results of two Global Climate Models (GCMs) and downscaled them using a Regional Climate Model (RCM) to produce rainfall for the period 1970 -- 2099. The two GCM models selected were: ACCESS 1-0 and GFDL-CM3. Two representative concentration pathway (RCP) scenarios -- RCP 4.5 and RCP 8.5 -- were selected from the scenarios used by the Intergovernmental Panel on Climate Change for the Fifth Assessment Report (IPCC, 2014). The RCPs represent the concentration of greenhouse gasses in the atmosphere resulting from different levels of greenhouse gas emissions. In the RCP 4.5 projection, greenhouse gas concentrations peak before 2100 and emissions decline substantially by the end of the 21st century. RCP 8.5 is the scenario in which emissions are not stabilized in the 21st century, instead continuing to grow throughout the 21st century. The intent in this analysis was to select a future climate condition representing a high climate change scenario and one on the lower side. The grid size for the GCMs was approximately 108 kilometers. Nested RCMs were run for a grid sizes of 36 km and 12 km, using the GCM model output as boundary conditions. The rainfall characteristics were evaluated for 2070 -- 2099 and compared to the characteristics for 1970 -- 1999. The change in annual rainfall amounted to an increase of 5% for the lower emissions scenario and an increase of 6% for the higher emissions projection. The change in rainfall intensities for larger, less frequent events was significantly higher. The 2-year (1-hour duration) rainfall increased by 21% and 32% in the lower and higher emissions scenarios, respectively. Rainfall also varied by season, with less rainfall falling in the summer months (June – August) and more falling during the other months. To determine sizing of CSO facilities, King County simulates historic rainfall with calibrated hydrologic and hydraulic computer models using rain data from 17 rain gauges spread throughout the City of Seattle. The MIKE Urban Software (DHI) was used for both hydrologic and hydraulic simulation of the collection and conveyance system. For climate simulations predicting future conditions, the historical rainfall was adjusted to conditions near the end of this century as reflected in the RCM results. The historical rainfall was factored to maintain the variability of rainfall across the service area and to keep the rainfall record in a minute-by-minute time scale (the RCM output is hourly). Model simulation results (without climate impacts) have been generated by KCWTD for the period from 1978 through 2018 (41 years) to estimate the CSO control volumes and peak flowrates that would be necessary to maintain CSO locations to an average of one event per year over any 20-year period during the historical record. For comparison, the same years were simulated using the factored rainfall associated with the two climate change scenarios representing climate at the end of this century. The resulting CSO control volumes were tabulated for each CSO location to show the potential impacts of climate change on control volumes. Summary of Results This research was conducted on thirty-five of King County's CSO outfalls. The results show that climate change will likely have a substantial impact on the size of CSO control facilities. Of the 18 outfalls that were simulated as 'in control' using historical rainfall, 11 would require additional projects not currently in KCWTD's long-term control plan. The total volume of storage that would be required to keep these 11 sites in control with climate projections is estimated between 2.2 and 4 million gallons (MG) for the low and high greenhouse gas emissions, respectively. Of the 27 outfalls currently not in control, additional storage (or equivalent) on the order of 64 -- 83 MG would be required to bring these outfalls into control and accommodate the impacts of climate change by the end of the century in the climate scenarios. This additional required storage volume would increase the total storage required in the KCWTD CSO program by 43 -- 55%, at a significantly higher cost than would be required without climate change. Quantifying the potential impact of climate change will help project teams and management develop strategies to adaptively manage the risks and uncertainties in sizing CSO facilities in a changing climate. The methodology and analysis can provide other agencies with an example of how Global Climate Model results can be used in long-term planning for CSO.