Hydrologic Data

The study area includes three streamgages (fig. 1; table 1). Stage is measured every 15 minutes, transmitted hourly by a satellite radio, and made available through the USGS National Water Information System database (U.S. Geological Survey, 2023b). Stage data from these streamgages are referenced to a local datum but can be converted to water-surface elevations referenced to the North American Vertical Datum of 1988 (NAVD 88) by adding the NAVD 88 streamgage datum value in table 1.

The peak flows used in the model simulations for the Irvington reach were taken from the current stage-streamflow relation (rating number 4, effective February 28, 2021) and corresponded to the target stages at the Little Papillion Creek at Irvington, Nebr. (USGS station 06610750) streamgage (table 2). For peak flows that were greater than the current stage-streamflow rating (greater than 6,720 cubic feet per second [ft³/s]), the HEC–RAS model calibrated for the current study was used to extend the rating. Flows ranging from 4,300 to 17,800 ft³/s were used as input into the hydraulic model to produce water-surface profiles. No major tributaries join Little Papillion Creek within the Irvington reach; therefore, the gage-derived streamflows were not adjusted for tributary inflows but were held constant throughout the study reach for a given profile.

The Ak-Sar-Ben reach peak flow data were obtained from a combination of the Little Papillion Creek at Ak-Sar-Ben at Omaha, Nebr. (USGS station 06610765; rating number 7, effective May 21, 2020) and the Big Papillion Creek at Q Street at Omaha, Nebr. (USGS station 06610770; rating number 9, effective November 27, 2022) to create peak flow scenarios that could occur in the reach (table 3). Peak flows input into the model on the Big Papillion Creek above the confluence were calculated by subtracting the target flow at the Little Papillion Creek at Ak-Sar-Ben at Omaha, Nebr. (USGS station 06610765) from the flow at Big Papillion Creek at Q Street at Omaha, Nebr. (USGS station 06610770). If this calculation resulted in a negative or unrealistically low streamflow value, that scenario was not used to create a water surface profile or inundation map. For peak flows that were greater than the current stage-streamflow rating (greater than 7,800 [USGS station 06610765] and 13,100 [USGS station 06610770] ft³/s), the calibrated HEC–RAS model developed for the current study was used to extend the rating. Flows ranging from 6,600 to 34,900 ft³/s were used as input into the hydraulic model to produce water-surface profiles. Cole Creek joins the Little Papillion Creek above the Little Papillion Creek at Ak-Sar-Ben at Omaha, Nebr. (USGS station 06610765) streamgage. However, the upstream limit of the Ak-Sar-Ben reach model was positioned so that the Cole Creek inflow would be included in the streamflow accounted for at the Little Papillion Creek at Ak-Sar-Ben at Omaha, Nebr. (USGS station 06610765) streamgage; therefore, the gage-derived streamflow was not adjusted for tributary inflows but were held constant throughout the study reach for a given profile.

Topographic Data

All topographic data used in this study are referenced vertically to NAVD 88 and horizontally to the North American Datum of 1983 (NAD 83). The digital elevation model (DEM) that was used in the study was derived from light detection and ranging (lidar) data, which were collected during December 2016 by Woolpert, Inc., of Dayton, Ohio (U.S. Geological Survey, 2020). The original lidar data vertical accuracy was 0.50 ft at a 95-percent confidence level based on a root mean square error (RMSE) of 0.26 ft for the “open terrain” land-cover category. By these criteria, the lidar data supported production of 2-ft contours (Dewberry, 2012). The final DEM had a 10-ft grid-cell size and a vertical accuracy of plus or minus 1 ft (U.S. Geological Survey, 2020).

Hydraulic Structures

A total of 34 crossings (railroad, roadway, and pedestrian bridges) have the potential to affect water-surface elevations during floods along the stream reaches. Levees line portions of the banks of the Ak-Sar-Ben reach, as described in the “Study Area Description” section (fig. 1) and were represented as lateral structures in the hydraulic model. In some cases, lateral structures on the Ak-Sar-Ben reach needed to be realigned to the high ground to provide a more realistic result and to minimize issues with stability. In addition, some portions of the levee tiebacks (originally modeled as break lines) were modeled as weirs. These changes allow the model to accurately represent the best alignment and surveyed elevation, instead of relying solely on the DEM representation of these structures. Survey data available on the National Levee Database indicated georeferenced elevation points and were used to represent tie backs (U.S. Army Corps of Engineers, 2023). This choice was made at the Thompson Creek and Copper Creek tributaries. For inundation mapping purposes, the landward side of the levee is shown as an “area of uncertainty” because there is uncertainty as to how this area would be inundated in the event of the levee overtopping or failing.

Energy-Loss Factors

Hydraulic analyses require the estimation of energy losses that result from frictional resistance exerted by a channel on streamflow. These energy losses are quantified by the Manning’s roughness coefficient (n value; Barnes, 1967). As part of the calibration process, the initial n values were adjusted until the differences between simulated and observed water-surface elevations at the streamgage within the study reach were minimized.

The initial (precalibration) n values for the Irvington reach were selected based on field observations and high-resolution aerial photographs. The final n values were 0.033 for the main channel and ranged from 0.05 to 0.06 for the overbank areas simulated in this analysis.

Initial n values for the Ak-Sar-Ben overbank flow areas were assigned based on the 2016 National Land Cover Dataset (Dewitz, 2019). For the main channel areas, n values were selected based on field observations and high-resolution aerial photographs. The final n values were 0.02–0.03 for the main channel and ranged from 0.04 to 0.08 for the overbank areas.

Hydraulic Model

HEC–RAS was used for the analysis of both study reaches. The Irvington reach was modeled using the one-dimensional steady-state flow computation option. The Ak-Sar-Ben reach was modeled using a one-dimensional/two-dimensional unsteady-state flow computation option because of the flow dynamics at the confluence and the levee storage area system. For both modeled reaches, flow data consisted of the flow regime, boundary conditions, and flows that produced water-surface elevations at the streamgage that matched target water-surface elevations for the streamgages in the reach. The models assumed a subcritical (tranquil) flow regime. The downstream boundary condition was defined as normal depth (depth of flow in a channel when the slope of the water surface and channel bottom is the same and the water depth remains constant), and these values were set to a water slope of 0.001 ft of vertical change for every foot of horizontal distance for the Irvington reach and 0.00134 ft for the Ak-Sar-Ben reach. The calculation of normal depth was estimated from the slope of the channel bottom of the four farthest downstream cross sections in the model. The HEC–RAS model for the Irvington reach was calibrated to the current (2022; rating number 4) stage-streamflow relation at the Little Papillion Creek at Irvington, Nebr. (USGS station 06610750) by the method described in the “Energy-Loss Factors” section. Differences between observed and simulated water-surface elevations for the four simulated calibration flows at Little Papillion Creek at Irvington, Nebr. (USGS station 06610750) streamgage were less than or equal to 0.21 ft (table 4), with an RMSE of 0.20 ft.

The Ak-Sar-Ben reach HEC–RAS model was calibrated to the current (2022) stage-streamflow relation at the Little Papillion Creek at Ak-Sar-Ben at Omaha, Nebr. (USGS station 06610765) and Big Papillion Creek at Q Street at Omaha, Nebr. (USGS station 06610770) streamgages. Because the rating for the Little Papillion Creek at Ak-Sar-Ben at Omaha, Nebr. (USGS station 06610765) streamgage had a maximum stage of 17.5 ft, the calibration included two additional runs at 1-ft intervals below the range of stages used to create the inundation maps for the reach to include a few more data targets for calibration. Differences between observed and simulated water-surface elevations for the four simulated calibration flows at the Little Papillion Creek at Ak-Sar-Ben at Omaha, Nebr. (USGS station 06610765) streamgage were less than or equal to 0.11 ft (table 5) and had an RMSE of 0.04 ft. Similarly, all the target stages used to create the inundation maps for the Big Papillion Creek at Q Street at Omaha, Nebr. (USGS station 06610770) streamgage were larger than the maximum stage of the rating curve. Therefore, the highest four stages of the rating curve were used for calibration. The modeled differences at the Big Papillion Creek at Q Street at Omaha, Nebr. (USGS station 06610770) streamgage were less than or equal to 0.30 ft (table 5) and had an RMSE of 0.18 ft for the four simulated calibration flows.

Development of Water-Surface Profiles

For the Irvington reach, the calibrated hydraulic model was used to generate water-surface profiles for a total of 11 flood stages at 1-ft intervals between 15 and 26 ft (excluding the 24-ft stage) as referenced to the local streamgage datum of the Little Papillion Creek at Irvington, Nebr. (USGS station 06610750). The 24-ft stage could not be modeled because the simulated streamflow at that stage was in a transition zone between over-bridge weir flow and pressure-flow state, which caused the elevation of the stage at this point to rise above the 24-ft stage. Streamflows corresponding to the various stages were obtained from the current (2022) stage-streamflow relation for the streamgage Little Papillion Creek at Irvington, Nebr. (USGS station 06610750; table 2).

The Ak-Sar-Ben reach hydraulic model was used to generate water-surface profiles for a total of 63 flood stage scenarios. The flood stage scenarios corresponded to combinations of stage data from the Little Papillion Creek at Ak-Sar-Ben at Omaha, Nebr. (USGS station 06610765) and Big Papillion Creek at Q Street at Omaha, Nebr. (USGS station 06610770) streamgages to represent the possible flood and backwater conditions from the confluence of Little and Big Papillion Creeks. Little Papillion Creek was represented by stage values in 1-ft stage intervals that ranged from 16.5 to 28.5 ft, as referenced to the local datum of Little Papillion Creek at Ak-Sar-Ben at Omaha, Nebr. (USGS station 06610765) streamgage. Big Papillion Creek below the confluence was represented by stage values in 1-ft stage intervals that ranged from 33 to 39 ft, as referenced to the local datum of the Big Papillion Creek at Q Street at Omaha, Nebr. (USGS station 06610770) streamgage. Streamflows corresponding to the various stages were obtained from the current (2022) stage-streamflow relation for the streamgages (table 3). For Big Papillion Creek above the confluence, flows were calculated by subtracting the flow at the Little Papillion Creek at Ak-Sar-Ben at Omaha, Nebr. (USGS station 06610765) streamgage from the flow at Big Papillion Creek at Q Street at Omaha, Nebr. (USGS station 06610770) streamgage. If the calculation of the flow for Big Papillion Creek above the confluence resulted in a negative or unrealistic flow, a water-surface profile was not created for that scenario. The flood stage scenario combinations used to create water surface profiles and 63 inundation maps for the Ak-Sar-Ben reach are shown in figure 2.

Flood-Inundation-Map Delivery

A FIM Program website (U.S. Geological Survey, 2023a) has been established at https://www.usgs.gov/mission-areas/water-resources/science/flood-inundation-mapping-fim-program to make USGS flood-inundation study information available to the public. The website links to a mapping application called the “USGS FIM mapper” that provides map libraries and detailed information on flood extents and depths for modeled sites. The mapping application enables the production of customized FIMs from the map library for the Irvington and Ak-Sar-Ben reaches. A link on this website connects to the USGS National Water Information System database (U.S. Geological Survey, 2023b), which provides the current stage and streamflow at the Little Papillion Creek at Irvington, Nebr. (USGS station 06610750), Little Papillion Creek at Ak-Sar-Ben at Omaha, Nebr. (USGS station 06610765), and Big Papillion Creek at Q Street at Omaha, Nebr. (USGS station 06610770) streamgages to which the inundation maps are referenced. The estimated FIMs are shown in sufficient detail so that preparations for flooding and decisions for emergency response can be completed 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. Bridge surfaces are shown as noninundated up to the lowest flood stage that intersects the lowest structural chord of the bridge. A shaded building should not be interpreted to mean that the structure is completely submerged; rather, bare earth surfaces near the building are inundated. In these instances, the water depth (as indicated in the mapping application by holding the cursor over an inundated area) near the building would be an estimate of the water level inside the structure, unless flood-proofing measures had been implemented.

Disclaimer for Flood-Inundation Maps

The FIMs 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.

Uncertainties and Limitations Regarding Use of Flood-Inundation Maps

Although the FIMs 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 along the stream reaches were estimated by hydraulic modeling, assuming unobstructed flow, and using 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 April 2023. Unique meteorological factors (timing and distribution of precipitation) may cause actual streamflows along the model reach to vary from those assumed during a flood, which may lead to deviations in the water-surface elevations 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 vary with the accuracy of the DEM used to simulate the land surface.

If this series of FIMs will be used in conjunction with NWS river forecasts, the user should be aware of additional uncertainties that may be inherent or factored into NWS forecast procedures. The NWS 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 proceeds downstream, and (3) predict the flow and stage (and water-surface elevation) for the stream at a given location (Advanced Hydrologic Prediction Service forecast point) throughout the forecast period (every 6 hours and 3–5 days out in many locations). For more information on Advanced Hydrologic Prediction Service forecasts, please refer to https://water.weather.gov/ahps/pcpn_and_river_forecasting.pdf.

A couple other sources of uncertainty relevant to the study reach are the levee system and the extension of ratings by the model. Breached levees can produce variable flood extents and water depths on the landward side of the levee. These areas of uncertainty are shown as a shaded green color on the flood-inundation mapping web application and depict what the flooding extents would be if there was no levee at all. No levee breach analysis was done as part of the study; levees were assumed to perform until overtopped. Another source is the extension of streamflow ratings beyond the observed and measured streamflows that are available. Although the guidelines and methods were followed for proper model extension, data to validate/verify/calibrate the model at these high flows are unavailable; therefore, the resulting maps at the uncalibrated flows have more uncertainty. Additional uncertainties and limitations pertinent to this study may be described elsewhere in this report.