1. GIS FOR HYDROLOGIC DATA DEVELOPMENT FOR DESIGN OF HIGHWAY DRAINAGE FACILITIES
by Francisco Olivera and David Maidment Center for Research in Water Resources University of Texas at Austin
Transportation Research Board 77th Annual Research Meeting January 11-15, Session No TRB ID Number:

2. OBJECTIVE
To develop a user-friendly geographic information system (GIS) that calculates:
The main hydrologic parameters of the watershed.
The flood discharges for different return periods at any given location (according to the USGS/TxDOT regional equations).

3. HYDROLOGIC PARAMETERS OF THE WATERSHED
A: contributing drainage area
L: stream length
SL: stream slope
SH: watershed shape factor (SH = L2/A)
CN: average curve number

4. FLOOD DISCHARGES
According to the Regional Equations for Estimation of Peak-Streamflow Frequency for Natural Basins in Texas (USGS, 1997), flood discharges can be expressed as:
where a, b, c, and d are given parameters that depend on the hydrologic region, and T is the return period.

5. POTENTIAL EXTREME PEAK DISCHARGES
According to the Documented and Potential Extreme Peak Discharges ... in Texas (USGS, 1995), flood discharges can be expressed as:
where a and b are given parameters that depend on the hydrologic region.

6. DEM-Based Topographic Analysis

7. DIGITAL ELEVATION MODEL (DEM)

8. HYDROLOGIC (RASTER-)GIS FUNCTIONS

9. FLOW DIRECTION
Water flows to one of its neighbor cells according to the direction of the steepest descent.
Flow direction takes one out eight possible values.

10. FLOW ACCUMULATION
Flow accumulation is a measure of the drainage area in units of grid cells.

11. STREAM DEFINITION
All grid cells draining more than 250 cells (user-defined threshold) are part of the stream network.

12. STREAM SEGMENTATION
Stream segments (links) are the sections of a stream channel connecting two successive junctions, a junction and the outlet, or a junction and the drainage divide.

13. WATERSHED DELINEATION
All grid cells flowing towards a specific stream segment (link) constitute its watershed or drainage area.

14. DELINEATED STREAMS OF THE AUSTIN AREA
(Streams delineated from a 100-foot DEM)

15. Flood Discharge and Watershed Parameter ArcView Extension

16. WATERSHED AND STREAM DELINEATION
The watershed of a user-defined outlet, and its longest flow path are delineated automatically.

17. WATERSHED PARAMETERS AND FLOOD DISCHARGES
Watershed parameters and flood discharges for different return periods are calculated.

18. VELOCITY FIELD
where V: flow velocity S: terrain slope A: drainage area p, q, r: calibration parameters.

19. FLOW TIME AND ISOCHRONE LINES
Flow time in each grid cell is equal to the flow length divided by the flow velocity.
Changes in color correspond to isochrone lines.

20. Potential Extreme Peak Discharge Grid

21. POTENTIAL EXTREME PEAK DISCHRAGES
A grid of potential extreme peak discharges has been precomputed.
Grid values can be queried using the built-in ArcView query tool.

22. CONCLUSIONS
A GIS for calculating hydrologic parameters, such as design flows, has been developed.
The accuracy of the results depends on the resolution of the spatial data used as input.
This GIS is not bound to any specific spatial data and can be applied -- after minor modifications -- to any other geographic region of the country.
Equations derived for the National Flood Frequency (NFF) computer program can be included in the GIS and extend its applicability to the entire nation.