Arlington County's Flood Risk Assessment Management Plan: A Blueprint For Living With Water In A Changing Climate

Introduction
Arlington County, Virginia, is developing a Stormwater Program Risk Assessment Management Plan (RAMP). The paper and presentation will summarize how the RAMP team conducted flood modeling and risk assessment considering climate change for urban interior flooding, riverine flooding and coastal flooding. The RAMP flood implementation strategy will inform and prioritize strategic investments for mitigating risk, based on economic, environmental and social equity criteria and quantified project benefits and co-benefits with a measurable return-on-investment methodology. The presentation will illustrate modeling visualization tools used to communicate and analyze risk and consequential impacts in support of a long-term resilience strategy.
Methodology
The RAMP project includes the following key tasks: data collection; climate projections and scenario development; Interior and riverine flood modeling using Flood Modeller FAST, PCSWMM, and HEC-RAS; coastal storm surge modeling using MIKE 21; vulnerability assessments; risk assessments; mitigation alternatives analysis; project prioritization; and support for public outreach and grant writing.
The flood modeling was conducted based on both current conditions and projected future conditions in 2040 and 2070, with changes in rainfall intensity-duration-frequency (IDF) and sea level rise. Jacobs developed the IDF projections using SimCLIM, which collates the latest global climate model data. Sea level rise projections were compiled from the most recent projections available from NOAA for the Chesapeake Bay and Washington DC (see Figure 1). The interior flood modeling was conducted in two steps, with initial screening of 2D overland flow for extreme events with FAST without the pipe network. These modeling results, coupled with historical flood complaint records, were used to select priority watersheds for detailed 1D/2D modeling with PCSWMM of storm drain pipes and surface overland flow, which is necessary to identify overland flow paths and facilities at risk of flood damage and repetitive losses. Refinement of 2D model extents and modeling approach allows for efficient 2D hydraulic modeling, with a balance between model resolution and runtime (see Figure 2).  
Preliminary Results
The flood modeling results were used to conduct a qualitative vulnerability assessment to determine which critical facilities (including public and private utilities), vulnerable groups and environmental resources are most at risk, based on current and projected flooding (see Figure 3).
The watersheds with the highest number of vulnerable areas, assets and resources will then be selected for detailed risk assessments using HAZUS and also site-by-site assessments of selected public facilities to assess individual assets at risk based on projected depth of flooding. To facilitate analysis and communication of results, the Jacobs team developed a GIS viewer tool for all the flood modeling results to assist in model validation and subsequent analysis of vulnerability, risk and alternatives. The RAMP analysis will inform the prioritization of strategic investments and policy guidelines for mitigating flood risk under current and future climate conditions. Work on the plan is ongoing and should be complete by April 2022.