Which Hazard Modeling Tool Should Texas Use for Flood Planning?

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Dawit Zeweldi

Stormwater Engineer

The newly created framework that institutes flood planning across the state of Texas requires the development of Regional and State Flood Plans. Two of the primary Guidance Principles for these plans are preservation of life and property, and identification of strategies and projects that will reduce both current and future flooding risks. A key factor in developing Regional and State Flood Plans under this new process will involve adopting the right tools to evaluate potential project benefits.

This comparison examines the strengths and potential applications of three options in light of the State Flood Plan requirements:

  • HAZUS-Multi-Hazard
  • HEC-Flood Damage Reduction Analysis
  • HEC-Flood Impact Analysis

HAZUS-MH (HAZUS-Multi-Hazard)

Purpose: Hazus-MH version 4.3 is a free ArcGIS extension that facilitates a risk-based approach to mitigation planning. It identifies and visually displays hazards and vulnerabilities. It calculates potential damages, economic losses and mitigation benefits. Decision makers use Hazus-MH tools to allocate resources effectively and efficiently during emergency response and recovery. Application of this tool also helps prioritize mitigation measures to reduce future losses.

Capability: Hazus can be applied to models for natural disasters that include earthquake, flood, tsunami and hurricane wind hazards using nationwide GIS database and user-supplied specific information.

Input/Output: Hazus has three levels of analysis approach that depend on the level of accuracy sought from the analysis. That, in turn, depends on the data input into the model. The model input can be aggregate data (default), site-specific or hazard-specific. The model analysis quantifies the output into three different categories as direct physical damage, induced damage and direct loss. Model results can be presented in the form of maps, tables and graphs to provide simplicity in communicating and interpreting model outputs.

Scale of Analysis: Hazus can be used for mitigation plans at city, county or statewide spatial scales, including tribal lands with multiple areas that are not contiguous.

Application to the Texas State Flood Plan: The State Flood Plan process requires that flood risk analyses be performed to determine the location, magnitude and frequency of flooding, and to identify who and what might be harmed by floods (31 TAC, Chapter 361, Subchapter C). It also requires identifying vulnerabilities of communities and critical facilities. HAZUS-MH can provide a robust platform to perform these analyses at varying scales for both existing and future conditions.

Source: The latest version 4.3 of the Hazus-MH model can be downloaded for free from FEMA’s MSC portal.

HEC-FDA (HEC-Flood Damage Reduction Analysis)

Purpose: HEC-FDA version 1.4 is a stand-alone free software used to formulate and evaluate flood damage reduction plans. The program performs economic (flood inundation damage analysis) and hydrologic engineering performance computations for plan evaluations. HEC-FDA uses risk analysis method that follows U.S. Army Corps of Engineers Policy regulations (ER 1105-2-100 and ER 1105-2-101), and the process applies the Monte Carlo simulation approach.

Capability: HEC-FDA provides capability to perform an integrated hydrologic engineering and economic analysis during the formulation and evaluation of flood damage reduction plans. It allows plan formulation and evaluation for flood damage reduction studies. The files associated with a study contain information on plan, analysis years, streams, damage reaches, damage categories, structures, etc. The files are stored as xBASE-formatted database files. The first study plan is always without-project condition, and additional plans may contain alternatives such as levees, reservoirs, channels, nonstructural measures, etc. Various plans can be compared after analysis.

Input/Output: HEC-FDA requires two types of model input parameters that include (1) hydraulic and hydrologic data such as water surface profiles, analysis year, frequency flows, stage-discharge functions and levee features, and (2) economics data such as damage categories, structure occupancy types, structure inventory data and stage-damage functions. The software uses a graphical user interface (GUI) for model input and output through dialogue boxes. Model computed results include expected annual damage, equivalent annual damage and project performance by plan and analysis year. A summary of model outputs can be presented in tables and graphs by plans, years, streams and damage reaches.

Scale of Analysis: The scale of study includes streams and damage reaches to be analyzed as part of the study area. Only one study is open at a time.

Application to the Texas State Flood Plan: Regional Flood Planning Groups will be required to perform Flood Management Evaluations (FME) for specific areas to determine the feasibility of each potential Flood Management Strategy (FMS) and Flood Mitigation Project (FMP) (31 TAC, Chapter 361, Subchapter C, §361.38). Evaluation of FMSs and FMPs will require detailed hydrologic and hydraulic modeling to quantify the reduced impacts from floods and the associated benefits and costs. HEC-FDA provides the necessary tools to prepare the required quantitative reporting of the estimated project benefits, reductions of flood impacts and benefit-cost ratio for FMPs while explicitly incorporating the uncertainty of future flood events.

Source: The latest version 1.4.2 of the HEC-FDA model can be downloaded for free from USACE website.

HEC-FIA (HEC-Flood Impact Analysis)

Purpose: HEC-FIA version 2.2 is a stand-alone free software that provides the capability to estimate the impacts associated with flood events and the benefits attributed to flood control projects. HEC-FIA is used in developing annual reports of U.S. Army Corps of Engineers projects’ benefit achievement and in developing consequence estimates to support risk assessment for dam safety.

Capability: HEC-FIA is designed to facilitate assessment of disaster impacts after a flood event. It is also capable of providing analysis results of flood impacts in real time to assist in decision making and response in a flood emergency. HEC-FIA is designed to analyze the consequences from a single specific event to calculate damages to structures and contents, agricultural losses and estimates of potential loss of life. Its capability includes computing annual benefits across the full range of potential flood events and performing dam and levee failure scenario analysis to support consequence estimates to determine the risk posed or prevented by Corps projects.

Input/Output: HEC-FIA uses hydraulic model outputs in a gridded data (ex.: depth and arrival-time grids) and/or HEC-DSS stage hydrographs (single continuous or forecasted hydrographs) as its input parameter. Output of the model quantifies consequences of the flood damage and its economic impacts, including structural damage and content losses, agricultural losses and area inundated, number of structures inundated, expected life losses and project benefits. Results are presented in graphical plots, geo-referenced maps, tables and reports. The analysis outputs also provide a report that details damage prevented by flood control projects identified for the study area.

Scale of Analysis: The results of the analysis can be presented at several possible spatial scales, including impact area, Corps district or county, community, study region, sub-basin, township, congressional district or flood control district.

Application to the Texas State Flood Plan: Similar to HEC-FDA, HEC-FIA can provide the necessary tools to perform benefit/cost analyses and determine the economic feasibility of a potential FMP. In addition, HEC-FDA can be used to analyze the risks associated with a potential catastrophic failure of a proposed dam or levee as part of the FMP assessment requirements set forth in 31 TAC, Chapter 361, Subchapter C, §361.38.

Source: The HEC-FIA model is developed by HEC in collaboration with the Risk Management Center (RMC) and Engineering Research and Design Center (ERDC). The latest version 2.2 of the HEC-FIA model can be downloaded for free from the USACE website.

For more information, contact me at dawit.zeweldi@freese.com.

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Dawit Zeweldi, PhD, PE, PH, CFM, D.WRE, is a Stormwater Engineer in Dallas.