1. Calculation of River Stages in Bastrop, Lee and Milam Counties Overlying the Carrizo-Wilcox Aquifer
Shiva Niazi
4/29/99
CE 394K2

2. Background Bureau of Economic Geology Project outline:
City of San Antonio wants to purchase groundwater produced from depressurizing lignite mines near Austin
Groundwater modeling of Carrizo-Wilcox aquifer in Bastrop, Lee and Milam counties to assess impacts on private wells and surface water

3. Objective
To calculate river stages in Colorado R., Brazos R. and Yegua Cr. using Manning’s equation

4. Data Runoff Data (CRWR) River Reach 1, Rf1 (CRWR - EPA)
90 m. DEM (USGS)
Flow of rivers upstream of model domain (USGS)
Width of rivers (Bureau of Econ. Geology)

5. Domain Carrizo-Wilcox Aquifer Bastrop, Lee and Milam Counties
Underlying HUC8
Subwatersheds
Brazos River
Colorado River
Area of Interest

6. MODFLOW River Package
QR= CR (h-HR)
h i,j,1
HR i,j

7. Riverbed Conductance, CRw L L
CR= wLKR/l
l, KR

8. Flow Accumulation Grid
NPS Analysis/
Accumulated Flow
CRWR
Pre-Pro
DEM
Flow Dir. Grid +
Accumulated Flow Grid
Runoff Grid

9. USGS Gages - Upstream Flow
Average daily flowrate data used starting from 1974
Brazos cfs
Little R cfs
Colorado cfs
Brazos
Little R.
Colorado

10. Manning’s Equation Q (cfs) = (1.49/n)*A*(R2/3*Sf 1/2) Assumptions:
Sf = So
Rectangular Conduit and y << w
R = A/ P = w*y/(w + 2y) = y
n = 0.04 (winding rivers)
y = (Q*n/(1.49*w*So 1/2)) 3/5

11. Segmented Principal Rivers into Reaches
Brazos R.
Colorado River 2 4 1 3 5
Yegua R. 6
Colorado R.

12. Calculations
So was calculated using the difference between upstream and downstream DEMs divided by the reach length
For slopes < or = 0, avg. slope = used
For each segment, Qmidpoint used to solve for ymidpoint
h = Avg. DEM + ymidpoint

13. Hydraulic Heads in Segments of the Colorado River

14. Hydraulic Heads in Segments of the Brazos River

15. Results River Stages low ! Colorado : 3 ft. - 6 ft.
Brazos : 5 ft ft.
Yegua - seasonal river, width yet to be determined

16. Conclusions
Upstream USGS flowrates used may underestimate true flowrates into the watershed:
25 years of data may neglect recent urban development of area
Gages are not directly at inlet of domain
DEMs are inconsistent with direction of flow
Slope calculations are highly sensitive to accuracy of DEM
Future work includes averaging more recent USGS gage data and looking at DOQs for river width