Performance of Houston's Drainage Systems During Hurricane Harvey
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Hurricane Harvey was a game changer on how we approach flood risk management and storm water management. It was unprecedented and unimaginable.
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Performance of Houston's Drainage Systems During Hurricane Harvey
- 1. STORM WATER SOLUTIONS KEYNOTE WEBINAR Performance of Houston's Drainage Systems During Hurricane Harvey Andres A. Salazar, Ph.D., P.E., D.WRE April 19, 2018
- 2. Houston’s Tough Drainage Conditions
- 3. Topography Average slope across Harris County is 4.5 feet per mile In this part of Houston, average slope is 0.4 feet per 1,000 feet
- 4. Soil Map Houston soils are predominantly clays with low infiltration potential. Saturate and produce runoff quickly.
- 5. Rainfall High moisture near the Gulf of Mexico. Houston’s climate is influenced by tropical activity.
- 6. Urban Environment Large urban development over the years increases runoff potential. Storm water detention policy did not start until the 1980s.
- 7. Houston Population Growth First FIRM maps in the 1980s. Houston’s population was over 1.5 million at that time. Urban development inadvertently happened in flood zones. 0.0 0.5 1.0 1.5 2.0 2.5 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2017 Millions Houston
- 8. Harris County Population Growth Houston’s population is approximately 50% of the population of Harris County. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2017 Millions Houston Harris County
- 10. Average Rainfall Annual average is 47 to 48 inches. Hurricane Harvey had an average of 35 inches in Houston and recorded close to 50 inches in southeast Harris County. 0 10 20 30 40 50 60 70 80 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 (Inches)
- 11. Intensity-Duration- Frequency City of Houston’s and Harris County underground drainage systems are designed for the 2-year rainfall event. Additional runoff is to be conveyed over the surface. 13.2 8.9 2.1 4.4 3.1 1.1 0 2 4 6 8 10 12 14 24-hour 6-hour 15-minute 24-hour 6-hour 15-minute Inches 2-YEAR EVENT 100-YEAR EVENT
- 13. Aging Infrastructure From the population growth mentioned earlier, it could be concluded that about 50% of Houston’s systems were build before the 1970s. This example is dated July 1958.
- 14. Aging Infrastructure This drawing shows a 120-inch monolithic concrete storm sewer with a slope of 0.1%. The drainage area of this storm sewer is more than 600 acres. 120-inch pipe is not adequate for the 2- year storm.
- 15. Development in Floodplains 24% of Harris County is inside a regulatory 100-year floodplain. Development in flood prone areas happened prior to floodplain regulations.
- 17. Two-Year Criteria Maintain H.G.L. below the gutter line
- 18. 100-Year Criteria Keep water below the right-of-way elevation. Figure from City of Houston TP 101
- 19. 100-Year Criteria Street Flood Video This video is an example of a street performing according to the City’s criteria during Harvey.
- 20. Detention Criteria Provide 0.5 ac-ft./acre of added impervious cover. Restrict flow to 0.5 cfs/acre (25% full) and to 2 cfs/acre (75% full) Harris Co requires min 0.55 ac-ft/acre for less than 50 acres.
- 21. Hurricane Harvey
- 22. Credit: Andy Yung, Chief Hydrologist, WALTER P MOORE
- 24. Return Period Calculation 2-day and 4-day rainfall exceeded by far 1,000-year event. A probability distribution puts the 4- day storm anywhere between 2,000 and 50,000 year event for the southeast end of the County. 24
- 25. Return Period Calculation 2-day and 4-day were close to the Probable Maximum Precipitation. 25
- 26. Harvey Rainfall (% PMP) It was close to 95% of the PMP. Close to the maximum rainfall that the atmosphere can generate. 51% PMP 95% PMP
- 27. Harvey Rainfall in 15-Minute Intervals 100-Year Storm=1.1” . Harvey in general did not exceed the 15- minute, 100-year storm. Brays Bayou at Harris Gully 0 0.2 0.4 0.6 0.8 1 1.2 1.4 15-MINUTE, 4 DAY PRECIPITATIONPRECIPITATION IN 15-MINUTE INTERVALS, AUG 26-29 Aug 26 Aug 30Aug 27 Aug 28 Aug 29
- 28. Harvey Rainfall in 3-Hour Intervals 2-Year = 2.6” 100-Year Storm=6.7” Harvey, for the most part, did not exceed the 3-hour, 100-year storm. Brays Bayou at Harris Gully 0 1 2 3 4 5 6 7 '08-2603:00 '08-2609:00 '08-2615:00 '08-2621:00 '08-2703:00 '08-2709:00 '08-2715:00 '08-2721:00 '08-2803:00 '08-2809:00 '08-2815:00 '08-2821:00 '08-2903:00 '08-2909:00 '08-2915:00 '08-2921:00 '08-3003:00 3-HOUR, 4 DAY PRECIPITATIONPRECIPITATION IN 3-HOUR INTERVALS, AUG 26-29
- 29. Harvey Rainfall in 6-Hour Intervals 2-Year = 3.1” 100-Year Storm=8.9” Harvey, in general, did not exceed the 6- hour, 100-year storm. 0 1 2 3 4 5 6 7 8 '08-2606:00 '08-2612:00 '08-2618:00 '08-2700:00 '08-2706:00 '08-2712:00 '08-2718:00 '08-2800:00 '08-2806:00 '08-2812:00 '08-2818:00 '08-2900:00 '08-2906:00 '08-2912:00 '08-2918:00 '08-3000:00 6-HOUR, 4 DAY PRECIPITATION Brays Bayou at Harris Gully PRECIPITATION IN 6-HOUR INTERVALS, AUG 26-29
- 30. Harvey Rainfall in 12-Hour Intervals 2-Year = 3.7” 100-Year Storm=10.8” In several gages, Harvey exceeded 12- hour, 100-year storm. 0 2 4 6 8 10 12 8/26/201712:00 8/27/20170:00 8/27/201712:00 8/28/20170:00 8/28/201712:00 8/29/20170:00 8/29/201712:00 8/30/20170:00 8/30/201712:00 8/26/2017 0:008/26/2017 12:008/27/2017 0:008/27/2017 12:008/28/2017 0:008/28/2017 12:008/29/2017 0:008/29/2017 12:008/30/2017 0:00 12-HOUR, 4-DAY PRECIPITATION Brays Bayou at Harris Gully PRECIPITATION IN 12-HOUR INTERVALS, AUG 26-29
- 31. Harvey Daily Precipitation 2-Year = 4.4” 100-Year Storm=13.2” In most gages, 500- year storm was exceeded for duration longer than 1 day. Because full bayous have higher conveyance, water elevation peaked at a frequency lower than the rainfall. 0 2 4 6 8 10 12 14 16 18 8/26/20180:00 8/27/20180:00 8/28/20180:00 8/29/20180:00 24-HOUR, 4-DAY PRECIPITATION Brays Bayou at Harris Gully DAILY PRECIPITATION, AUG 26-29
- 32. Harvey Sequence of Events Aug 25 • Storm onset. Ground saturates. Aug 26 • Bayous fill, some to the top of bank. Ponds fill Late 8/26 to early 8/27 • High intensity bands cause floods
- 33. Harvey Sequence of Events By Saturday afternoon and evening, storm sewers carried just a small portion of the runoff. Areas relied on overland flow.
- 34. Harvey Sequence of Events Rainfall continued Monday and Tuesday aggravating flood conditions.
- 35. What caused flooding? Lots or rain. Rain over 4 days contributed more more than 30 inches, which would inevitable cause problems.
- 36. What caused flooding? Homes built before the 1980s were located lower than current criteria.
- 37. What caused flooding? Lack of conveyance or relief. Places with little or no overland flow relief were vulnerable: • Depressed highways • Around sag inlets • Homes below the crown of roadway
- 38. What caused flooding? Accumulation of runoff over large drainage areas. Four days of high, intense rain: • Exceeded historical peaks • Exceeded design storage of ponds and reservoirs
- 39. What systems fared well? Systems that had adequate conveyance and relief: Examples: • Sims Bayou (Recently Improved to 100-year capacity) • New neighborhoods outside bayous’ flood zones built per criteria
- 40. Closing Remarks Harvey was back-to-back-to back 100- year/500-year storms for durations between 12 and 24 hours. That sequence exceeds by far any design event.
- 41. Closing Remarks No system or criteria would have prevented flooding during Harvey.
- 42. Closing Remarks Systems with overland relief and conveyance fared better that those without.
- 43. Closing Remarks There is no direct correlation between the return period of rainfall and the return period of the water level in Bayous. Many bayous did not reach historical records of prior floods. Others did not reach the 500-year level.
- 44. Possible Solutions 1. Add conveyance. 2. Add storage. 3. Elevate structures. 4. Remove structures from flood zones.
- 45. Questions?
Notes de l'éditeur
- Hurricane Harvey was a game changer on how we approach flood risk management and storm water management. It was unprecedented and unimaginable. Nowhere else in the United States we have had (since record are kept) a storm that big and that long. It covered 30,000 sq-miles with an average precipitation of 30 inches in 4 days, One gage near Port Arthur recorded a total of 60 inches. The storm as you saw in the news, caused extensive flooding. The latest estimate from HCFCD is 123,000 homes were flooded, only in Harris County. To put in context, that is 8% of the total residences. Counting adjacent counties, we might be talking about 200,000 homes. Etimated loss close or exceeding $100 Billion. But in the aftermath of the storm, there were many questions about how Houston has approached urban development. Is there anything Houston could have or should have done to mitigate the damage and be better prepared for such storm? One specific question was about how we handle storm were in Houston. Do we design streets to intentionally flood? Should have development been allowed in flood prone zones? Outline of the presentation: *Challenges in Houston *History of the development *Design criteria *What happened during Harvey: What work and what did not? THIS IS A PRESENTATION BY WALTER P MOORE. Based on our experience in the work in the City. ANY OPINION IN THIS PRESENTATION DOES NOT NECESSARILY MEAN AN ENDORSEMENT BY THE CITY OF HOUSTON OR HARRIS COUNTY.
- For this presentation, it is important to understand the conditions that Houston has to deal with to manage storm water. So let’s start with reviewing the basics of the geographical context of Houston related to storm water management.
- This also means that the flow path of the overland flow does not necessarily follow the alignment of the streams.
- From this map, you can see the blue-gray regions, which identify clay soils. In addition, the green areas on this map and the light brown shading in the areas are considered poorly draining soils (the Natural Resources Conservation Service classifies these soils as “Type D,” or soils having very low infiltration/high runoff potential). So, impermeable soils cover about two-thirds to three-quarters of the County, indicating that even if development were sparse, we would still see significant runoff from storm events. This is supported by the events of the 1935 flood which devastated downtown Houston at a time where there was no real development outside of Loop 610 (in the area where the rain centered).
- In summary, we have: Flat topography, soils that do not infiltrate water, high rainfall amount and urban development. What else can you ask to create a more difficult condition for urban drainage?
- 40,000 at the beginning of last century to 2.3 million today. 1950’s to 1900’s are the decades with most aggressive growth. By 1980, population was more 1.5 million. Then, nore than 2/3 of urban development happened before the establishement of the NFIP, or any guidelines regarding to development in flood zones.
- THERE IS ALSO THE REQUIREMENT FOR FLOODPLAIN DEVELOPMENT: 12 inches above 100year. Post Harvey, it will be 2 feet above the 500-year.
- L-Moment extension
- L-Moment extension
- ** In Most gages **
- ** In all gages**
- Added detention would have not been effective during Harvey. Detention fills and has limited capacity.
- Conveyance was proven to be effective Storage would reduce conveyance needed. However, storage has a limit and eventually conveyance is needed for emergency overflows. Elevate is the action taken by the City. May be not cost effective and manadates could be stricter than required