Effect of Urbanization on Runoff in the Whiteoak Bayou Watershed in Houston, Texas

1. AWRA GIS in Water Resources Houston, May 8-10, 2006 Effect of Urbanization on Runoff in the Whiteoak Bayou Watershed in Houston, Texas Francisco Olivera Texas A&M University Department of Civil Engineering Burren DeFee University of Arkansas at Monticello School of Forest Resources

2. Whiteoak Bayou Watershed • The watershed is located in northwest Houston in Texas. • Area = 223 km2.

3. Whiteoak Bayou Watershed • Precipitation and flow have increased significantly over time.

4. Whiteoak Bayou Watershed • Cumulative annual runoff.

5. Whiteoak Bayou Watershed • Peak 12-hour precipitation depths and peak flows have significantly increased over time.

6. Whiteoak Bayou Watershed • Cumulative annual peak flow.

7. Whiteoak Bayou Watershed • Parcel data was obtained from the Harris County Appraisal District. • Data included: parcel digital map, parcel area, building square footage, number of floors and year of construction.

8. Whiteoak Bayou Watershed • Developed parcels and roads.

9. Whiteoak Bayou Watershed • One point per developed parcel. 1950 1960 1970 1980 1990 2000

10. Whiteoak Bayou Watershed • Percent developed area and percent impervious cover

11. Whiteoak Bayou Watershed • Developing new parcels can change the spatial configuration of the developed areas.

12. Whiteoak Bayou Watershed • Edge density and mean shape index.

13. Whiteoak Bayou Watershed • Number and mean area of the developed patches.

14. Whiteoak Bayou Watershed • Observed and predicted annual runoff values. • Break point: 1972 – 1973.

15. Whiteoak Bayou Watershed • Annual runoff assuming pre-1972 conditions.

16. Whiteoak Bayou Watershed • Observed and predicted annual peak flows.

17. Whiteoak Bayou Watershed • Peak flows assuming impervious remains lower than 10%.

18. Whiteoak Bayou Watershed • Annual runoff depth for the period 1973 – 2000: • Accounting for increasing urbanization level • Average: 603 mm. Range: 310 mm – 1030 mm • Assuming 1972 urbanization level (i.e., 10% impervious cover) • Average: 340 mm. Range: 50 mm – 600 mm • Annual runoff increment because of urbanization is 77%. • Total annual runoff increment is 146%.

19. Whiteoak Bayou Watershed • Peak flows for the period 1973 – 2000: • Accounting for increasing urbanization level • Average: 299 m3/s. Range: 108 m3/s – 711 m3/s • Assuming 1972 urbanization level (i.e., 10% impervious cover) • Average: 226 m3/s. Range: 57 m3/s – 434 m3/s • Peak flow increment because of urbanization is 32%. • Total peak flow increment is 159%

20. Whiteoak Bayou Watershed • Peak flows tend to increase less than annual runoff depths because a significant part of the additional flow takes place during small events. • It was observed that the 12-hour precipitation increased much more over time than the annual precipitation. • Urbanization below a critical threshold value appears not to affect annual rainfall depth, after which it increases proportional to the developed area. That threshold value took place in 1972-1973 in the Whiteoak Bayou watershed, when the watershed reached a 10% impervious cover. • The variation in the number of developed patches can be interpreted as a measure of the connectivity of the impervious cover and of its capacity to convey flow. The number of patches reached a maximum value in 1972.

21. Whiteoak Bayou Watershed • Future work: • Apply same methodology to other cities in the world: • Portland, Oregon, an example of a “planned” city as opposed to Houston, Texas. • Madrid, in collaboration with the Castilla – La Mancha University in Spain. • Consider non-linear regressions.