Assessing the Risk of Flood

Community planners should begin assessing flood risk by coordinating with their local floodplain administrator, along with the appropriate staff at the Colorado Water Conservation Board (CWCB), on the identification of the best currently available data and tools for assessing flood risk, as well as the status of any updates or possible enhancements to those resources.

Communities with GIS resources available to support their planning efforts will be able to conduct more rapid and robust risk assessments, including overlay analysis, to quantify the exposure of people, parcels, buildings, critical facilities, and other community assets that are within Special Flood Hazard Areas (SFHAs) and other community assets that are within Special Flood Hazard Areas (SFHAs) or Fluvial Hazard Zones (FHZ). They may also use FEMA’s Hazus software to help estimate monetary losses based on a variety of scenario flood hazard events (this capability exists even for communities without their own digital flood risk data). However, while Hazus is a helpful loss estimation tool, it is not predictive of future events. Communities that do not have GIS capabilities or resources should consult the range of digital online mapping viewers available, including the Colorado Flood Decision Support System (DSS) or the FEMA Flood Map Service Center. These mapping tools allow for some higher-level spatial analysis and the creation of customizable and printable flood maps (FIRMettes) that can be helpful in assessing flood risk for local planning and regulatory measures. In some cases, however, communities may have on-the-ground information that is better than modeling, such as, for example, public works records of roads and bridges that consistently have issues in flood events.

Most communities in Colorado rely on their effective floodplain maps (i.e., Flood Insurance Rate Maps issued by FEMA that become effective on a particular date) as the official source of flood risk information for local planning and regulatory measures, which at a minimum includes adopting and enforcing the State’s Model Floodplain Damage Prevention Ordinance (Colorado Department of Natural Resources, 2012). All communities in Colorado with mapped flood hazard areas have access to either hard copy/static Flood Insurance Rate Maps (FIRMs) or Digital Flood Insurance Rate Maps (DFIRMs), along with the associated Flood Insurance Study from the FEMA Flood Map Service Center. Communities with GIS capability but without DFIRM datasets may still have access to digital Q3 flood layers, which provide some limited spatial data for GIS analysis and mapping purposes. Many communities would be well-served to add Fluvial Hazard Maps to their toolbox as these maps combined with floodplain maps provide a more holistic picture of risk related to floods in a stream corridor.

The delineation and updating of floodplain maps is generally performed by private engineering firms under contract with FEMA, which administers the nation’s flood hazard mapping program in coordination with CWCB. Based on strict guidelines, floodplain maps are created through the use of statistical information such as data for river flows, rainfall and topographic surveys, and hydrologic and hydraulic (H&H) analyses. Hydrologic modeling calculates the peak discharges of water at key locations in a watershed, while hydraulic modeling computes surface water velocities and elevations along with flood profiles and flood boundaries using input from the hydrologic models. FEMA offers technical guidelines and standards for flood risk analysis and mapping.

Fluvial Hazard Maps are considered best practice and not regulated by the Federal or State government. The Colorado Water Conservation Board provides information on the technical aspects of fluvial hazard mapping.

The primary gaps or weaknesses with these tools may include outdated and/or inaccurate map data. Even new DFIRMs may not reflect all flood hazards (e.g., urban drainage/stormwater flooding, fluvial erosion, etc.) or future conditions (e.g., future floodplains and base flood elevations that take into account projected watershed development, hydrologic changes, etc.). Also, aside from a few new Risk MAP projects, most flood risk databases do not include the non-regulatory information that may be useful for enhanced planning purposes (e.g., flood depth/velocity grids, areas of mitigation interest, and other flood risk assessment data). Another challenge for many communities is the lack of GIS capabilities or resources to help facilitate the spatial analysis and mapping of flood risk to support the implementation of additional planning tools or strategies.

The 2013 floods highlighted the need for better mapping (including floodplains, erosion zones, and debris flows), and in response the Colorado legislature provided funding in early 2015 to update natural hazard maps statewide. This update process is currently underway.

Communities that are interested in adjusting or improving the quality of their floodplain maps, or expanding on the accessible flood risk products associated with development, should coordinate closely with the CWCB. While flood risk studies and hazard mapping are often prohibitively expensive, a range of techniques is available to communities through higher regulatory standards encouraged under the National Flood Insurance Program (NFIP) and Community Rating System (CRS) that should be considered.

Fluvial Hazard Maps, conversely, should be relatively affordable for communities to produce and provide a wide variety of options, both regulatory and non-regulatory, for use. For example, if a community is interested in regulating new development to flood elevations that are expected to increase due to future growth and development, it may consider the use of future-conditions hydrology in the creation of its own regulatory floodplain maps. Such maps can account for future floodplain conditions and may be adopted by communities to enact more stringent development standards, but would not be linked to insurance rates and purchase requirements under the NFIP. In the absence of pursuing the development of future floodplain conditions maps (which is not part of FEMA’s flood hazard mapping program and may be costly), communities may consider adopting the 500-year (or 0.2-percent-annual-chance) flood zone as their regulatory floodplain versus the 100-year (or 1-percent-annual-chance flood zone) as currently required under the NFIP.