Hydraulic (HEC-RAS) model of the Lower San Saba River between Harkeyville and San Saba, TX, USA

This model is a two-dimensional (2D) hydraulic model created in the Hydraulic Engineering Center’s River Analysis System (HEC-RAS). The model was created for a segment of the San Saba River between Harkeyville and San Saba, TX, USA. The model’s geometry is based on United States Geological Survey 3D Elevation Program data collected in 2018, and the channel bathymetry was burned in using cross-sectional data collected by Texas State University researchers in 2018. The model was calibrated using water surface elevation and velocity measurements taken during field data collection.

Methods
Available data:
Researchers from Texas State University collected depth, flow velocity, and wetted width data at 200 cross-sections spaced approximately 350 ft apart using the equipment listed in Table 1.

Table 1. Equipment used and their accuracy for Texas State University data collection. Table from Harris et al. (2023).

Parameter

Equipment

Unit Accuracy

Location

GPSMap 64 Handheld GPS

10-50 feet

Velocity

Hach Velocity Meter (Model FH950.1)

0.1 feet/second

Depth

An adjustable “ruler” stick with feet as units

0.1 feet

Wetted Width

Laser Technology Inc. TruPulse 360r

3 feet to nonideal (natural) target

Data was collected between June 4th and June 27th, 2018. During this time period, USGS gage 08146000 (San Saba, TX) recorded discharges ranging from 11.9 to 396 cfs, with an average discharge of 20 cfs.

USGS 3DEP 1 m resolution data collected between February 14th and April 22nd, 2018, was used to create the HEC-RAS terrain (Merrick-Surdex 2018). Discharge at USGS gage 08146000 ranged from 40.5 to 966 cfs during this time period. For much of the time period, the discharge was approximately 60 cfs.

Bathymetric areas:
The 3DEP data was imported as a terrain in HEC-RAS v.6.2, and field-collected cross sections were burned into the channel following methods from Harris et al. (2023). The 95 most upstream sites in the segment were associated with a single depth measurement in the center of the channel, whereas the remaining 105 cross sections were associated with three depth measurements collected in the center of the channel and on the left and right, although the position of measurements were not recorded. For cross sections that had three depth measurements, if the standard deviation of the depth exceeded 0.25 ft, all three measurements were used to delineate the cross section in HEC-RAS. For all other cross sections, a single depth was used to delineate the cross section (either the single available depth measurement or the average depth based on three measurements; Harris et al., 2023). A final bathymetric/topographic surface was generated following Harris et al. (2023) using inverse distance weighted interpolation with the field-collected cross sections to estimate channel bathymetry.

Landcover was delineated using aerial photography (USDA 2018) and associated Manning’s N roughness values were determined following Chow (1959) and Harris et al. (2023) (Table 2).

Table 2. Selected Manning’s N roughness values based on delineated landcover. Adapted from Harris et al. (2023).

Landcover Description

Chow 1959 Description, which has minimum/normal/maximum ranges (Manning's n Values (orst.edu))

Selected Roughness

Channel

(Main channel or Mountain Streams)

Channel

Sluggish reaches, weedy, deep pools (normal)

0.07

Channel2

clean, winding, some pools and shoals, some
weeds and more stones (maximum)

0.05

Channel3

Clean, straight, full stage, no rifts or deep pools
(minimum)

0.025

Cobbly3

No vegetation in channel, banks usually steep, trees and brush along banks submerged at high stages, bottom: gravel, cobbles, and few boulders (minimum)

0.03

Ineffective Sec2

Sluggish reaches, weedy, deep pools (maximum)

0.08

Ineffective Sec3

Very weedy reaches, deep pools, or floodways with heavy stand of timber and underbrush (normal)

0.1

Ineffective Sec4

Very weedy reaches, deep pools, or floodways with heavy stand of timber and underbrush (maximum)

0.15

Intermediate Zone

(Floodplains)

Grassy Floodway

Scattered brush/heavy weeds (maximum) or light
brush and trees in summer (between normal and maximum)

0.07

Floodplain

(Floodplains)

Dense Woody

Dense willows, summer straight (minimum) or
heavy stand of timber, downed trees, little
undergrowth (normal)

0.1

Dense Woody2

Dense willows, summer straight (normal)

0.2

Sparse Shrub

Light to dense brush (Various definitions, ranges
from minimum to maximum)

0.08

NoData

Scattered brush, heavy weeds (between normal
and maximum)

0.06

A 2-D HECRAS mesh was created following Harris et al. (2023) with a mesh size of 40 square feet and a breakline with cell size of 20 feet located in the center of the channel.

A 12 cfs unsteady flow simulation was run as a “hot-start” to fill the modeled channel and subsequently used as the initial conditions for additional flows simulated for the segment. Because the discharge recorded at USGS gage 08146000 varied during the field sampling period, different sections of the segment were calibrated to different discharges to match field conditions at the time of data collection (Table 3).

Table 3. Discharges used to calibrate 2-D HEC-RAS model based on discharges recorded at USGS gage 08146000 during field collection dates in 2018.

Calibration discharge (cfs)

Average field discharge (cfs)

Range of field discharges (cfs)

Dates (2018)

Cross section

12

12.5

9.7-15.6

6/25; 6/27

29370-20044; 8279-467

16

15.9

13.5-17.4

6/21; 6/26

19597-8667

20

20

16.8-22.4

6/13-6/14; 6/20

49011-29699

26

26.5

21.1-30.3

6/12

56420-49340

34

33.8

30.3-36.5

6/11

63009-56840

40

40.3

20.6-114

6/4

72209-63766

Calibration was conducted in accordance with methods from Harris et al. (2023), with an initial channel roughness of 0.07 adjusted on a case-by-case basis throughout the segment based on comparisons of field-measured and modeled depth and velocity at cross sections. In addition, modeled channel widths were compared to aerial imagery for select discharges, and floodplain roughness was adjusted as needed in an attempt to match channel width from imagery (USDA, 2004-2018; Table 4).

Table 4. Average discharge recorded at USGS gage 08146000 on select dates when aerial imagery from the National Agricultural Inventory Program (NAIP) was available (USDA, 2004-2018), used for comparison of imagery channel width with modeled channel width.

Discharge (cfs)

Imagery

Imagery date

12.5

NAIP

August 16th, 2006

22

NAIP

July 12th, 2014

54

NAIP

July 31st, 2010

86

NAIP

August 3rd, 2016

241

NAIP

December 12th, 2004

1600

NAIP

October 26th, 2018

The final overall root mean-squared error of the model after calibration was 0.31 ft s-1 for velocity and 0.34 ft for depth. Error at individual cross sections was also recorded for reference purposes.

Summary of assumptions:
This HEC-RAS model has assumptions matching those of Harris et al. (2023). Discharge data from 2018 at USGS gage 08146000 (San Saba, TX) have been approved by USGS.

Usage notes:
HEC-RAS 6.2 is a free hydraulic analysis software available for download from the U.S. Army Corps of Engineers.

References:
Chow VT. Open-channel hydraulics: New York: McGraw-Hill; 1959.

Harris A, Wiest S, Cushway KC, Mitchell ZA, Schwalb AN. Hydraulic model (HEC-RAS) of the Upper San Saba River between For McKavett and Menard, TX [Dataset]. Dryad Data Repository; 2023. https://doi.org/10.5061/dryad.pc866t1tt.

Merrick-Surdex. Lidar Mapping Report. 2018. Prepared for United States Geological Survey contract G16PC0029.

Mitchell ZA. The role of life history strategies and drying events in shaping mussel communities: a multiscale approach [dissertation]. San Marcos (TX): Texas State University. 2020.

Mitchell ZA, Cottenie K, Schwalb AN. Trait-based and multi-scale approach provides insight on responses of freshwater mussels to environmental heterogeneity. Ecosphere. 2023; 14(7):e4533. https://doi.org/10.1002/ecs2.4533.

Mitchell ZA, Schwalb AN, Cottenie K. Trait-based and multi-scale approach provides insight on responses of freshwater mussels to environmental heterogeneity [Dataset]. Dryad Data Repository; 2023. https://doi.org/10.5061/dryad.msbcc2g3d.

United States Department of Agriculture (USDA). Texas NAIP Imagery, 2018. Web. 2022-03-09.