All topographic data used in this study are referenced vertically to NAVD 88 and horizontally to the North American Datum of 1983. The terrain data for the Blue River Basin in Missouri were obtained from a digital elevation model (DEM) derived from light detection and ranging (lidar) data collected from April 10, 2020, through November 27, 2020, and processed by NV5 Geospatial (U.S. Geological Survey, 2023a). The lidar data used in developing the DEM had a horizontal-accuracy root-mean-square error (RMSE) of 0.47 ft and a nonvegetated vertical accuracy of 0.26 ft RMSE. The terrain data for the part of the upper Blue River Basin in Kansas were collected between January 17, 2018, and March 22, 2019, and processed by the Atlantic Group, LLC (now WGI Geospatial; U.S. Geological Survey, 2023a) in accordance with USGS V1.2 QL2 data accuracy specifications (Heidemann, 2012). The generated DEM had a vertical accuracy of 0.25 ft RMSE. Because of uncertainty issues with this DEM along flight paths within the BR5 terrain region (David Nail, U.S. Geological Survey, written commun., 2024), the 2012 DEM (AIMS, 2012) used in the previous flood-inundation-mapping effort (Heimann and others, 2015) was substituted for this reach. The lidar used in this dataset had a vertical accuracy of 0.314 ft RMSE.

Because lidar data cannot provide ground elevations below the water surface of a stream, the lidar-derived DEM data used in the hydraulic models were augmented with channel and bridge survey data collected by USGS field crews between December 2021 and May 2023. Channel water depths in the thalweg were acquired using an acoustic Doppler current profiler, and water-surface elevations were obtained using global positioning system (GPS) with real-time kinematic technology or static observations. Water depths were subtracted from the interpolated water-surface elevations to obtain channel depths. Additional bathymetric data were acquired by tape-down measurements from bridge decks. A differential GPS with real-time kinematic technology was used to obtain reference elevations for the bridge decks from which channel-bed elevations were derived. A Level-IV survey procedure (Rydlund and Densmore, 2012) was used to acquire reference-point elevations. The bathymetric data were incorporated into the HEC–RAS model by merging the bathymetry DEM with the terrain DEM (Kostynick and others, 2026).

Estimates of first-floor elevations of commercial and residential structures within the Blue River floodplain were determined by a combination of field surveys and estimates from the model DEM. A total of 1,334 structures were identified within the 0.2-percent AEP floodplain area (as determined from Heimann and others, 2015). Elevations from a differential GPS field survey of 187 points that included building first-floor elevation points, lowest adjacent-grade elevation points, and flat-surface elevation points (road intersections) were compared to the DEM elevation at each point location. The mean absolute difference between survey-point elevations and corresponding DEM elevations was 0.18 ft (Kostynick and others, 2026). When the first floor of a structure was above the adjacent grade, an offset was applied either using a measured tape-down, for field survey-derived elevations, or estimated from Google Earth Pro version 7.3 Street View (Google, 2024), for DEM-derived first-floor elevations.

The study models contain 91 hydraulic structures, including 25 four-lane road bridges; 20 railroad bridges; 17 two-lane road bridges; 8 six-lane road bridges; 6 pedestrian bridges; 4 three-lane road bridges; 2 one-lane road bridges; 8 weirs; and 1 culvert, representing conditions as of June 2023. These structures have the potential to affect water-surface elevations during floods along the study reaches. Bridge-geometry data were obtained from previous Blue River studies (Heimann and others, 2015, 2020), field surveys completed by USGS personnel, and bridge plans obtained from the Missouri Department of Transportation (Bryan Hartnagel, Missouri Department of Transportation, written commun., 2023). The three levee areas within the study area (East Bottoms, Swope Park Industrial, and Bannister-Dodson; fig. 1) were incorporated into the model via the existing terrain or by using terrain modifications if the levees were constructed after the lidar data were collected.

Hydraulic analyses require the estimation of energy losses resulting from frictional resistance exerted by a channel on flow. These energy losses are quantified by Manning’s roughness coefficient (n value). Initial (precalibration) n values were selected using the National Land Cover Database land cover classifications (U.S. Geological Survey, 2024a) field observations, and tabulated estimates of n values (Chow, 1959).

As part of the calibration process, the initial n values were adjusted until the differences between simulated and observed water-surface elevations at the reference streamgage within each map reach were minimized. The final n values ranged from 0.02 to 0.056 for the main channel and from 0.030 to 0.16 for the land-cover features in overbank areas, except for building footprints (refer to specified model roughness characteristics in Kostynick and others, 2026). The floodplains in the highly developed areas were simulated by representing buildings as separate calibration regions assigned an n=1.0, whereas the remaining adjacent-floodplain matrix consisting of streets, parking lots, developed open areas, and grassland was assigned a separate n value depending on the predominant land cover. The lowest channel roughness coefficients were placed in downstream reaches of the mapping area with concrete channels and fine-material substrate, and the highest were placed in sinuous, large substrate upstream reaches with vegetated banks. The lowest roughness coefficients on the floodplain were placed in open-water bodies, parking lots, and roadways. The highest were placed in calibration regions representing timber and buildings.

The HEC–RAS analyses for this study were done by using four two-dimensional, hydraulic models with two map reaches within each model (map reach combinations within the models included BR1 and BR2, BR3 and IC1, BR4 and BR5, and BC1 and BC2; table 2). The models were run using a quasi-unsteady flow condition, because model-input data consisted of a “stair-stepped” temporally varied flow regime over a range of flows necessary to simulate the range of flood-inundation map stages. The range of streamflows used in the models is discussed in the “Hydrologic Data” section and specified in table 2. The models were used to generate water-surface profiles at target elevations that coincided with 1-ft increments of stage, referenced to the local streamgage datum. Subcritical (tranquil) flow conditions were assumed for the simulations. Generally normal depth, based on an estimated average water-surface slope downstream of the model extent, was used as a downstream boundary condition. For the model used to simulate reaches BR1 and BR2, normal depth was used as the downstream boundary condition for the model calibration scenario because it corresponds to a stage at the Missouri River at Kansas City streamgage of about 10 ft—a stage well below the NWS flood stage of 32 ft (National Water Prediction Service, 2025d). Additional scenarios were developed for the BR1 and BR2 model to represent the effects of backwater from the Missouri River at high-flow conditions (Missouri River at Kansas City stages of 33, 37, 41, 45, 49, 53, 57, 61, and 65 ft; table 2). In these scenarios, the downstream boundary condition of the lower BR1 and BR2 model near the junction with the Missouri River was set to varying Missouri River water-surface elevations, representing a range of Missouri River flood conditions. The water-surface at the mouth of the Blue River for a range of streamflows (20- to 0.2-percent AEP; U.S. Army Corps of Engineers, 2023) at the Missouri River at Kansas City streamgage was determined using the relation between the water-surface elevation at the Missouri River at Kansas City streamgage and the water-surface elevation at the mouth of the Blue River defined in figure 9 in Heimann and others (2015). More detailed information regarding the hydraulic models used in this study, including the model run files and outputs, can be obtained from Kostynick and others (2026).

Developed USGS stage–streamflow ratings at the streamgages were used in calibrating the models. Model calibration for each map reach was accomplished by adjusting n values until the results of the hydraulic computations closely agreed with the observed water-surface elevations for given flows.

The hydraulic model used for representing reach BR1 was calibrated to the stage–streamflow rating at the USGS streamgage Blue River at 12th Street (station number 06893590; hereafter referred to as 12th Street; fig. 1). The model was calibrated at the reference streamgage within map reach BR1 using a normal depth downstream boundary condition, excluding the hydraulic effects of the Missouri River. The simulated water-surface-elevation–streamflow rating at the streamgage location in the map reach was compared to rating number 4.1 at the 12th Street streamgage over a range of stages (20.0–32.0 ft) bounding the highest streamflow measurement recorded at 21.61 ft. The differences between the simulated values and the USGS rating at 1-ft stage increments ranged from −0.49 to 0.16 ft, with a mean absolute difference of 0.19 ft (table 3).

The calibrated hydraulic models were used to generate from 15 to 169 water-surface profiles, depending on the map reach (table 2; refer to Kostynick and others (2026) for detailed information on model output for each map reach), for stages at 1-ft intervals that were referenced to a USGS streamgage within each reach. The water-surface profiles for two Blue River reaches BR1 and BR2 included the effects of backwater from the Missouri River (refer to “model_outputs” folder for the BR1_BR2 model in Kostynick and others [2026]). A total of 155 water-surface profiles were generated for map reach BR1; these included a series of 3 to 23 Blue River reach profiles under 10 backwater conditions from the Missouri River referenced to the Missouri River at Kansas City, Mo., streamgage. A total of 169 profiles, including 7 to 20 profiles under 10 backwater conditions from the Missouri River, were developed for Blue River reach BR2. Streamflows corresponding to the various target stages for each map reach were obtained from the stage–streamflow rating curve at the reference streamgage. Streamflows were then adjusted throughout the reaches to account for tributary inflows.

The time of travel between upstream-downstream streamgage pairs were analyzed using the 2010 to 2025, 15-year period of observed stage record at the Blue River and tributary reference streamgages used in this study. Peaks in observed stage above the defined or estimated NWS Action Stage were synchronized, the values between the upstream-downstream pairs were compared, and the lag time was determined. The lag times were stratified into stage categories matching the defined or estimated NWS flood categories (Action Stage, Minor Flood Stage, Moderate Flood Stage, Major Flood Stage; National Weather Service, 2024) at the streamgage, and the median travel time and the range of observed travel times were tabulated (refer to “Travel_Time_Analysis” in Kostynick and others (2026). Sometimes, observed peaks during the analysis period within a specific flood category were not available; therefore, the time-of-travel estimate from the previous flood category was used.

A Flood Inundation Mapping Program website (U.S. Geological Survey, 2023b) has been established to make USGS flood-inundation study information available to the public. The website links to a mapping application that presents map libraries and provides detailed information on flood extents, water depths, water velocities, water-surface elevations, and travel times for modeled sites. The mapping application enables the production of customized flood-inundation maps from the map libraries (Kostynick and others, 2026). A link on the mapping application website connects to USGS Water Data for the Nation (U.S. Geological Survey, 2023d), which presents the current stage at the USGS streamgages to which the inundation maps are referenced. Additionally, a link is provided on the mapping application to the USGS WaterAlert application (U.S. Geological Survey, 2024b), which notifies the user when a specified stage threshold at a streamgage is reached. The estimated flood-inundation maps are displayed in sufficient detail so that preparations for flooding and decisions for emergency response can be performed efficiently. Depending on the flood magnitude, roadways are shown as shaded (inundated and likely impassable) or not shaded (dry and passable) to facilitate emergency planning and use. Bridges are shaded—that is, shown as inundated—when the flood stage exceeds the elevation of the bridge deck or both approaches. A shaded building should not be interpreted to mean that the structure is completely submerged, but, rather, that bare-earth surfaces near the building are inundated. In this application, the water depth (displayed when the user hovers the cursor over an inundated area) at a building footprint would provide an estimate of the water level above the first floor of the structure, unless flood-proofing measures had been implemented.

The flood-inundation maps should not be used for navigation, regulatory, permitting, or other legal purposes. The USGS provides these maps “as-is” for a quick reference, emergency planning tool, but assumes no legal liability or responsibility resulting from the use of this information.

Although the flood-inundation, velocity, and water-surface-elevation maps represent the boundaries of inundated areas with a distinct line, some uncertainty is associated with these maps. The flood boundaries shown were estimated based on water stages and streamflows at selected USGS streamgages. Water-surface elevations and velocities along the stream reaches were estimated by quasi-unsteady-state hydraulic modeling, which assumed unobstructed flow, and used streamflows and hydrologic conditions anticipated at the USGS streamgage(s). The hydraulic model reflects the land-cover characteristics and any bridge, dam, levee, or other hydraulic structures existing as of July 2023. For the three leveed areas in the study area, the areas behind levees are shown in green on the USGS flood-inundation mapping application unless the lowest elevation of the levee was exceeded. Unique meteorological factors (timing and distribution of precipitation) may cause actual streamflows along the modeled reach to vary from those assumed during a flood, which may lead to deviations in the water-surface elevations, velocities, time of travel, and inundation boundaries shown. Additional areas may be flooded because of unanticipated conditions, such as changes in the streambed elevation or roughness, backwater into major tributaries along a main-stem river, or backwater from localized debris or ice jams. The accuracy of the floodwater extent portrayed on these maps will also vary with the accuracy of the DEM used to simulate the land surface. In developing the map libraries, the model scenarios exceed the highest streamflow measurement recorded at the streamgage location; therefore, flood-inundation maps at stages above that highest streamflow measurement reflect increased uncertainty.

If this series of flood-inundation maps will be used in conjunction with NWPS river forecasts, the user should be aware of additional uncertainties that may be inherent in, or factored into, NWPS forecast procedures. The NWPS uses forecast models to estimate the quantity and timing of water flowing through selected stream reaches in the United States. These forecast models (1) estimate the amount of runoff generated by precipitation and snowmelt, (2) simulate the movement of floodwater as it moves downstream, and (3) predict the flow and stage (and water-surface elevation) for the stream at a given location (NWPS forecast point) throughout the forecast period (every 6 hours for a forecast window of 3 to 5 days in many locations). For more information on NWPS forecasts and products, please refer to https://water.noaa.gov.

The water depth above the first floor of a structure was estimated from the DEM elevation adjacent to the building or from the DEM estimate plus an offset if the first floor was above the adjacent grade. Any structure with a first floor above the adjacent grade will, therefore, have additional uncertainty in water depths compared to structures at ground level with first-floor elevations derived from the DEM alone.