Cost-Effective Planning for a Resilient and Sustainable Wastewater Collection and Transmission System for the City of Houston

Purpose: The main purpose of this presentation is to show how the City of Houston is making their wastewater collection and transmission system (WCTS) more resilient to future growth and more frequent flooding events due to climate change through the planning efforts during two programs. Benefits: This presentation will demonstrate the how the City of Houston is planning to make their wastewater system more resilient to future growth of the service area and to the impact of more frequent flooding events due to climate change. This planning work is being done during two programs: Hurricane Harvey disaster mitigation under FEMA's Public Assistance funding program and the City's Wastewater Consent Decree. The presentation will illustrate the City's approach to planning capacity improvements and flood mitigation measures to provide its customers with the target level of service, reliability and sustainability. Abstract: Introduction In August 2017, Hurricane Harvey made landfall and stalled over southeastern Texas for one week, dropping more than 60 inches of rainfall in some areas. This resulted in catastrophic flooding throughout the City of Houston, including many of the City's thirty-eight (38) wastewater treatment plants, resulting in hundreds of millions of dollars in damages, as well as environmental and public health impacts for weeks after the hurricane. To mitigate the damages to wastewater facilities induced by Hurricane Harvey and increase the resilience of the wastewater system against future flood events, the City of Houston, Houston Public Works (HPW) sought financial assistance offered through the Federal Emergency Management Agency's (FEMA) Public Assistance funding program. To secure this funding, HPW procured four consultant teams to provide planning, preliminary engineering and FEMA funding application assistance (Figure 1). The general concept to provide greater future resilience was to either harden or eliminate the flood prone WWTPs and convey the consolidated flows to a more resilient WWTP in the area, which would be expanded to accommodate the additional flows. Additionally, flooded lift stations were also consolidated, where possible, making the system not only more resilient against flooding, but also reducing O&M costs and greenhouse gas emissions. FEMA Program Approach To develop a consistent approach in developing the preliminary engineering reports (PERs) and FEMA funding applications, expert focus groups, referred to as Tiger teams were formed. The Tiger teams were led by industry experts in the areas of hydraulic modeling, tunneling, wastewater treatment and FEMA funding. These Tiger teams worked with the four consultant teams to develop defensible methodologies to be used for this project. A methodology was developed to project future wastewater flows 50 years into the future to meet Texas Commission on Environmental Quality (TCEQ) requirements. In addition, since the flow consolidation and conveyance alternatives included tunnels, a methodology was developed to project the 100-year future flows to ensure the tunnels have sufficient capacity throughout anticipated service life. The future flow projections also allow the City to plan for future treatment plant capacity expansions (Figure 2). Mitigation alternatives were developed using the framework presented in Table 1. Alternatives were formulated to either harden flooded WWTPs or consolidate flows from the flooded WWTPs and convey them to the expanded WWTP. The methodology used to optimize the management of peak wet weather flows into the expanded WWTPs is presented in Figure 3. Peak flow management strategies (i.e., storage) could be applied upstream at the flooded WWTP, or downstream, just prior to entering the expanded WWTP. The optimization identified the least-cost combination of peak flow treatment capacity and storage facilities in each of the project areas. The cost effectiveness of the proposed solutions had to meet the criteria of FEMA's benefit-cost analysis (BCA) methodology. To meet the BCA criteria, WWTP consolidation and expansion alternatives were developed and the costs estimated. The FEMA team quantified the benefits for each alternative. This was an iterative process until an acceptable benefit-cost ratio was obtained. Houston SSO Consent Decree Approach With respect to the HPW's Consent Decree planning, a heightened focus has been placed on the reduction of rainfall-derived infiltration and inflow (RDII). RDII reduction address the root cause of wet weather sanitary sewer overflows (SSOs). Improving a collection system to convey peak wet weather flows is often a viable solution. However, the collection system is constructed on flat slopes due to Houston's topography. Building gravity sewers capable of conveying flows with a wet weather peaking factor of four or more can create operational and maintenance issues, since dry weather flows may not produce self-scouring velocities. Houston's Consent Decree states that 'if the remedial action to address the capacity restraint is to upsize the Sewer Segment, the new Sewer Segment shall be designed to prevent sewer asset surcharge during a five-year, six-hour rain event, except in situations where the remedial action design to prevent the surcharge during a five-year , six-hour rain event would be detrimental to the operation of the WCTS, in which case the remedial alternatives analysis shall include alternative measures that will be taken to correct the capacity constraint.' Based on this requirement, HPW is aggressively exploring remedial measures alternatives to reduce RDII and will consider private-side measures in addition to actions on the public-side of the collection system. HPW is aiming to reduce the wet weather peaking factors in the WCTS to a target value of four or less. This will provide a system that is resilient against dry and wet weather SSOs and is less costly to operate and maintain. The planned studies include field characterization efforts and pilot projects. HPW recognizes that implementation of RDII reduction projects in service areas included in the FEMA program may require that the FEMA flood hazard mitigation projects be downsized. Status of Completion The PERs and FEMA funding applications were submitted to FEMA in February 2021. HPW entered the Consent Decree on April 1, 2021, and capacity remedial measures planning is in progress. Per the Consent Decree, all system improvements must be constructed by April 1, 2036. Conclusion: The City of Houston's wastewater system planning will yield a system that will not only provide the target level of service into the future but will also provide greater resiliency against climate-induced flooding, while being cost-effective for its customers. The long-term plan consists of hardening or eliminating flood-prone WWTPs and lift stations and conveying flows from eliminated facilities to expanded and optimized WWTPs. Addressing the root cause of wet weather SSOs through RDII reduction efforts is a priority of Houston's Consent Decree program and will help to reduce the size and cost of wastewater system improvements for the benefit of its customers and provide a system that is more efficient to operate and maintain, while also reducing dry weather SSOs.