The following is a summary of data utilized for developing a bathymetric terrain for 2D hydraulic modeling using HEC-RAS. Data used for model calibration and validation is also discussed.

 

Available Data

Cross-section elevation data were collected by the United States Army Corps of Engineers (USACE) Kansas City District at approximately 200-foot to 1000-foot increments at the confluence of the Big Blue River and the Kansas River near Manhattan, Kansas. The following equipment was used by two complete surveying teams:

• Ohmex SonarMite single beam echo sounder SFX @ 200khz, 

• Ohmex SonarMite single beam echo sounder DFX @ 28kHz & 200kHZ, 

• Trimble R12i 0096 & 0098, 

• Trimble R8 1984 & 6282

 

The cross-section elevation data were collected by boat and supplemented by hand-carried, pole-mounted Trimbles on April 10 to 14, 2023. The USGS gage on the Big Blue River near Manhattan, KS (06887000) had an average discharge of 425 cfs during the field collection time period (Figure 1). A USGS gage downstream of the confluence, Kansas River at Wamego, KS (06887500) shows an average discharge of 780 cfs at the same time period (Figure 2).

       

Figure 1 (Refer to supplemental information file). USGS gage Big Blue R NR Manhattan, KS – 06887000 discharge data for the week of April 11, 2023 – April 15, 2023. The average flow was taken as 425 cfs.

       

Figure 2 (Refer to supplemental information file). USGS gage Kansas River at Wamego, KS (06887500) discharge data for the week of April 11, 2023 – April 15, 2023. The average flow was taken as 780 cfs. Wamego, KS is downstream of the Big Blue River and Kansas River confluence and represents combined flow for both tributaries.

 

Figure 3 (Refer to supplemental information file). Map of bathymetric cross-sections collected in April 2023 near Manhattan, KS. Arrows show flow direction. Inset is the data collection location relative to the state of Kansas.

 

The field data collection featured 56 cross-sections. HEC-RAS 6.3.1 was utilized to create a bathymetric surface by interpolating 1-D cross-sections, while a 1-m resolution USGS 3DEP terrain (2015) was used for the floodplain and surrounding areas. A more recent USGS 3DEP (2018) data was available but featured higher stream flow than the 2015 data collection and therefore, more of the channel was submerged. Overall, the difference between 2015 and 2018 had a mean deviation of ~0.04 feet, with a majority of the differences in the channel ranging between +/-0.5 feet. Islands in this reach are unvegetated and prone to movement, and therefore the exact channel form is uncertain. However, it is assumed that relative island areas are consistent throughout the reach, and 2015 LiDAR was used to delineate the most island area as possible. 

 

To build the bathymetric terrain, a similar process as what was discussed in Harris et al. (2023), field collected data were imported into ArcGIS Pro 3.0 as a point shapefile. To preserve georeferencing, the point shapefile was segmented into groups of 3-4 cross-sections and these cross-sections were interpolated into mini-surfaces using the Inverse Distance Weighted (IDW) spatial analysis tool. These mini-surfaces were brought into HEC-RAS and cross-sections were drawn to intersect with these field surveyed locations. The 1-D cross-sections were then used to create a TIFF for the entire channel area. The 1D interpolation captures the channel centerline between measured cross-sections but meanders and channel widening may not be covered by the interpolated channel. The channel raster was broken into its component objects or “exploded”, in ArcGIS Pro using the Raster to Point tool. The points were then interpolated using the Inverse-Distance-Weighted interpolation tool (IDW). This creates a terrain that covers meanders and channel expansion while maintaining fidelity to the original channel raster.

 

Areas where the terrain was inundated at the time of LiDAR data collection are “flat” and referred to as a hydro-flattened surface. The Slope tool in ArcMap was used to delineate these hydro-flattened areas and a shapefile tracing unsubmerged islands was used. The IDW surface was clipped to the hydro-flattened extents and then mosaicked with the original 3DEP terrain to create a seamless bathymetric and topographic surface. 

 

The field data collected in April 2023 (Figure 3) required supplemental information to cover a fish monitoring instance upstream of the bridge at Pillsbury Drive/177. In September 2021, the USACE Kansas City District collected sediment samples with XY-georeference and depth measurements. The LiDAR hydro-flattened surface was used to estimate the energy grade slope from the new cross-section to the recent field monitoring extents. The model scenario or “plan” on the April 2023 extents was run at a similar flow as was occurring in September 2021. The combination of water surface elevation at that flow (780 cfs), the energy grade slope in the 3DEP data and field measured depth in 2021 were used to estimate the elevation at the channel bed. 

 

Land cover was delineated using the Multi-Resolution Land Characteristic (MRLC) Consortium’s 2019 National Land Cover Data (NLCD) (MRLC 2016). Fifteen types of landcover were identified for this study area by the NLCD: Hay-Pasture, Shrub-Scrub, Developed Low Intensity, Developed Medium Intensity, Cultivated Crops, Deciduous Forest, Herbaceous, Develop Open Space, Developed High Intensity, Woody Wetlands, Emergent Herbaceous Wetland, Open Water, Mixed Forest, Barren Land, and Evergreen Forest. Manning’s n values were selected based on a range of n values along with a “Suggested Initial n” provided by Krest Engineers (2021) (Table 1). 

 Table 1.  A table representing a range of Manning’s n values, a suggested Manning’s n value, and percent imperviousness for each NLCD land cover type. (Krest Engineers, 2021)

 

The 2D HEC-RAS mesh was set to 40-feet square, with breaklines to orient cell edges along areas of steep elevation change or to support model convergence. Boundary conditions were placed at three locations in the 2D flow area: the inflow of the Big Blue River (boundary condition type: flow hydrograph), the upstream end of the Kanas River (flow hydrograph), and the downstream end of the Kanas River (normal depth). An energy grade slope was given as 0.0005 ft/ft for the Big Blue River and 0.0003 ft/ft for the Kansas River. Advanced time step control adjustments were implemented using Courant’s Criterion, with a minimum Courant of 0.75 and a maximum of 3. 

 

The suggested value from Krest Engineers (2021) was the initial Manning’s n used for each land cover type (Table 1). The hydraulic model was then run, and the Manning’s n was changed to better conform to water surface elevations observed during field data collection.  Flows corresponding to the field collection dates were 415 cfs for the Big Blue River and 360 cfs for the Kansas River. These streamflows were determined by cross-referencing the field collection dates (April 10 to 14, 2023) to continuous monitoring data available from USGS at gages Big Blue R NR Manhattan, KS (06887000) and Kansas R at Fort Riley, KS (06879100).

The 2D model simulation results were compared to the field-measured water surface elevations at each channel cross-section with the ArcGIS Zonal Statistics as Table tool. Model improvement was determined by calculating the Root Mean Square Error (RMSE) of the simulated water surface elevation to the field observed water surface elevation, and the Manning’s n values resulting in the lowest error were selected. Following calibration, the model has overall RMSE of 0.29 ft for depth. The final Manning’s n values used for all the following simulations are included in Table 2. 

 

Table 2. The selected Manning’s n per Landcover classification after calibration

 

Apart from the calibration simulations, further simulations were conducted to match additional fish data collection from July 17 – 21, 2023 and October 2- 6, 2023. USGS gages, Big Blue R NR Manhattan, KS (06887000) and Kansas R at Fort Riley, KS (06879100), were used to find the discharge rates (in cfs) during those fish sampling periods. While discharge was consistent throughout the weeks for some gages (Figures 4 and 7), others showed differences greater than 10% or 100 cfs (Figures 5 and 6). The gages that showed significant differences were divided into two sub-simulations for the lower and higher flows during that week. 

 

Table 3. A table representing each scenario ran for the week of July 17, 2023 – July 21, 2023 with HEC RAS descriptions and USGS streamflow data. 

 

Table 4. A table representing each scenario ran for the week of October 2, 2023 – October 6, 2023 with associated HEC RAS descriptions and USGS streamflow data.

 

Figure 4 (Refer to supplemental information file). USGS gage Big Blue R NR Manhattan, KS – 06887000 discharge data for the week of July 17, 2023 – July 21, 2023. The average flow was taken as 500 cfs. 

 

Figure 5 (Refer to supplemental information file). USGS gage Kansas R at Fort Riley, KS - 06879100 discharge data for the week of July 17, 2023 – July 21, 2023. Because the differences in high and low flows were significant (> 100 cfs) the lowest flows and highest flows were averaged for two sub-simulations of 330 cfs and 490 cfs. 

 

Figure 6 (Refer to supplemental information file). Big Blue R NR Manhattan, KS – 06887000 discharge data for the week of October 2, 2023 – October 6, 2023. The differences in high and low flows were considered not significant (<100 cfs), the substantial increase in discharge on October 4th resulted in the lowest flow and peak flow being averaged for two sub-simulations of 570 cfs and 660 cfs.

 

Figure 7 (Refer to supplemental information file). USGS gage Kansas R at Fort Riley, KS - 06879100 discharge data for the week of October 2, 2023 – October 6, 2023. The average flow was taken as 200 cfs. 

 

Summary of Assumptions:

1. The accuracy of discharge measurements from river gages has a margin of error up to 10%. The data used for 2023 near the Manhattan, KS gages at Big Blue River and Kansas River has been approved by USGS.

2. The accuracy and reliability of field-measurements are subject the accuracy of tools used for field measurement.

3. The data provided in 1D cross-section dimensions are minimal laterally, but there was a rather high data resolution longitudinally in the channel. River area between 1D cross-sections is interpolated. Bank to measured depth contours and measured depth to measured depth contours are uncertain. 

4. Unmeasured pools and widening locations have assumed depths that conform to channel energy grade. Actual bathymetries are most uncertain in these locations relative to other channel areas, as channel slopes cannot be assumed to be constant in backwater areas.

5. Bridge structures were not included into the model.

6. Groundwater contributions and losses were not incorporated into the model.

7. The model does not incorporate sediment transport, so changes in islands configurations or scour/deposition is not simulated.

 

Usage Notes: HEC-RAS 6.3.1 is a free hydraulic analysis software available from US Army Corps of Engineers. 

 

Reference: 

Krest Engineers, LLC. 2021. Manning’s n (Roughness Coefficient) for HEC-RAS 2D Modeling. Written January 10 2021 on RASHMS.com. https://rashms.com/blog/mannings-n-roughness-coefficient-for-hec-ras-2d-modeling/.

 

Harris, Aubrey, Samantha R. Wiest, Kiara Cushway, Zachary Mitchell, Astrid Schwalb. 2023. Hydraulic model (HEC-RAS) of the Upper San Saba River between Fort McKavett and Menard, TX [Dataset]. Dryad. https://doi.org/10.5061/dryad.pc866t1tt

 

Multi-Resolution Land Characteristic (MRLC) Consortium: National Land Cover Database. 2016. 2019 National Land Cover Data. https://www.mrlc.gov/data 

US Geological Survey, 2020-03-30, USGS one meter KS South Central AOI 2 Manhattan 2015: USGS.