Tropical Storm Allison and its impact on Harris County

Transcription

1 UNIVERSITY OF TEXAS AT AUSTIN CE 394K GIS in Water Resources Binbin Chen December 9, 2006 Tropical Storm Allison and its impact on Harris County Background Tropical Strom Allison Recovery Project Project purpose Data sources and methods Results and discussions Storm history and track Precipitation Harris County in the storm Stream Discharge Estimated flooding area Conclusions Acknowledgement References Background When Tropical Storm Allison suddenly formed 80 miles off the coast of Galveston, Texas, on Tuesday, June 5, 2001, no one expected that, five days later, it would go on record as one of the most devastating rain events in the history of the United States. Neither historical data nor weather forecasts could adequately predict this extraordinary storm that, before leaving Texas, would dump as much as 80 percent of the area's average annual rainfall over some Houston and Harris County neighborhoods, simultaneously affecting more than 2 million people. When the local rains finally eased, Allison had left Harris County, Texas, with 22 fatalities, 95,000 damaged automobiles and trucks, 73,000 damaged residences, 30,000 stranded residents in shelters, and over $5 billion in property damage in its wake. Simply put, everything about Allison was "off-the-charts." 1 I have experienced several severe storms in Houston and every time people would compare the storm to Allison in 2001, which had largely affected people s life in Houston and Harris County. So I want to use the tools I learned in GIS in Water Resources class to conduct a study of Tropical Storm Allison. Tropical Storm Allison Recovery Project The Federal Emergency Management Agency (FEMA) and the Harris County Flood Control District initiated the Tropical Storm Allison Recovery Project (TSARP), to develop technical products that will help the local community to recover from the devastating flooding and to understand their flood risks. TSARP has done detailed analysis work of Allison, including tropical storm Allison description, rainfall analysis, flood frequency analysis, individual 1 Tropical Storm Allison Recovery Project Website:

2 watershed analysis, rainfall pattern analysis and pre-development of Harris County watershed analysis. TSARP report provides great guideline and frame work for my study. Project purposes The purpose of this project is to using GIS tools comprehensively understand Tropical Storm Allison, the most devastating rain event in United States history, and its impact on Harris County and City of Houston, Texas. The study comprises several aspects of the storm event: the storm track, the rainfall intensity, the stream discharge changes in Harris County because of the storm, and flooding area in Harris County and Houston. I started the project with a broad ambition, but narrowed down during my working process. I found one of the most challenging parts of this project is collecting data from different sources. The data seem everywhere or nowhere. After obtaining data another challenging part is to understand the data, and bring data together to portray the picture of Storm Allison and its aftermath. The whole process of data collecting, interpretation, analyzing and visualizing gave me hands-on experience and better understanding the function and advantage of ArcGIS in water resource management as well, which is another important purpose of this project. Data sources and methods 1. Storm track data National Hurricane Center provides historical tropical cyclone track GIS data at The historical hurricane tracks can be found by Strom Name, ZIP code, Place Name, Climatology and Latitude/Longitude through Hurricane Viewer. The attributes of track data tell the information of hurricanes locations (longitude and Latitude), wind speeds (in KTS, Wind mph =1.15Wind KTS ), pressure, storm categories, etc. at different time. I used storm name search, located Allison in Atlantic storms category, and downloaded storm track data. Then I created a map showing Tropical Storm Allison s track during June 5 th and June 10 th near Harris County (map 1). 2. Precipitation data There re two types of rainfall data: rain data collected at rain gages and rainfall radar data. There re recording 150 rain gages located throughout Harris County operated by Harris County Office of Emergency Management (HCOEM) and the Harris County Flood Control District (HCFCD). Additionally there re 3 rain gages in Harris County operated by National Weather Service (NWS). But I couldn t get all the 150 rain gages data and the 3 NWS rain gages couldn t represent the whole picture of the rain event occurred in Harris County. So I used the rainfall radar data collected by NWS at WSR-88D radar site covering the Houston area for my study. National Climate Data Center (NCDC) provides NEXRAD Radar rainfall data at: After searching processed radar data from TACC (Texas Advanced Computing Center), I found they don t have data for June 2001 when storm Allison occurred. So I downloaded raw radar data from NCDC website, then used NEXRAD viewer to visualize the raw radar data and exported radar data format into ESRI ASCII Grid format through NEXRAD exporter. The radar data I used are radar Level III product Storm Total Precipitation (STP), which estimated storm total precipitation accumulation continuously starting from when a storm began. For Allison, the STP data are counted beginning from May 30, I downloaded data 2

3 for different times: 06/05 13:00, 06/07 23:57, 06/08 16:00, 06/08 23:00, 06/09 01:00, 06/09 06:00, and 06/09 23:58. (I spent a lot of time finding appropriate times which could best represent the process of the rain event, as well as getting familiar with NEXRAD viewer and exporter) When I had data for all the times I needed, I converted ASCII format data into raster format using ArcToolbox ASCII to Raster function. Then I used ArcView Spatial Analysis Raster Calculator function to calculate rainfall amounts for periods from June 5 to June 9, from June 5 to June 7, and from June 8 afternoon 16:00 to June 9 morning 6:00. (Map 2, Map 3, Map 5) 3. Harris County watershed data I got Harris County watershed shapefile from where I worked before at Gulf Coast Institute. Harris County has 21 watersheds associated with their own major waterways. The watersheds were delineated by hand. 4. Harris County surface water data In order to get the surface water data of Harris County, I downloaded NHD (ftp://nhdftp.usgs.gov/subregions/) medial resolution (1:100,000) data of Unit 1204 which is for Houston region and NHDplus ( flowline attribute table (unit 12a). Then I cut Harris County flowlines out of the whole 1204 unit using ArcView select by location function, which was to select flowlines intersected with Harris County Watersheds. The NHDplus flowline attribute table was joined together with Harris County flowlines, so I could give the flowlines different thicknesses based on their meal annual flow MAFLOWU, as we did in Exercise USGS stream gages data I got the USGS stream gage Geodatabase from Exercise 5 Again, using select by location function I obtained all the 70 USGS stream gages inside Harris County watersheds. But some stream gages had stopped recording data for a while. I applied a selection of attribute DAYN > to these gages, which picked out the 44 USGS gages still being used to record stream discharge in Harris County. I loaded watersheds, NHD flowlines and NHDplus flowline attribute table and USGS stream gages of Harris County into one geodatabase and created a map of Harris County Hydrograph. (Map 6) 6. Stream gage time series data To find watersheds which had been attacked by Allison most intensively, rainfall map and Harris County Hydrograph were put together. (Map 7) Then 4 watersheds were picked to be studied in detail about the stream discharge during the period when Allison passed Harris County: Greens Bayou, White Oak Bayou, Brays Bayou and Sims Bayou. As we did in Exercise 6, I created a geodatabase named Individual_ArcHydro, applying Arc Hydro schema to the geodatabase, loading 4 watersheds and their flowlines, stream gages in the database, and linking them together. Then using weather downloader I got the time 3

4 series data for stream gages in the 4 watersheds for period from June 1 to June 12. The maps and graphs were made based on the time series data. (Map 8 Map 11) 7. Flooding area data I contacted TSARP members and they kindly sent me the estimated flooding area shapefile. So I was able to recreate the estimated flooding area map as seen in TSARP report and calculated areas for channel flooding and overland flooding. (Map 13) Then I was going to find out how different land uses in Houston and major roads were affected by the flooding. City of Houston releases a CD including various GIS data for the City every year, which provides me the needed land use and road data. Using select by location function I found lands inside Houston that are also in the flooding area and major roads in Harris County in the flooding area. Then I calculated areas for land uses of single family, multi family, commercial, industrial, office, and public and institutional, as well as the total length of flooded major roads in Harris County. (Map 14, Map 15) Results and discussions Storm history and track Map 1 4

5 According to National Weather Service, Tropical Storm Allison began as a tropical wave leaving the coast of Africa on May 21. It reached the western Caribbean on May 29, and then moved into the Bay of Campeche on June 4 as an area of thunderstorms. On June 5 in the afternoon, the tropical storm actually formed 80 miles off the Galveston coast. Map 1 shows the slow and erratic process when Allison passed Harris County. After it formed on June 5, the storm made landfall to the east of Galveston on the evening of June 5. The Storm initially had higher winds (NWS reported the highest winds were northeast of the storm center, where sustained winds of 48 miles per hour and guts up to 61 miles per hour near the coast in western Jefferson County, 40 miles east of Houston, during the morning of June 6). When it passed inland, the storm weakened to a tropical depression. Allison crossed Galveston and Harris County during the morning of June 6, eventually reaching the furthest inland, 120 miles north of Houston. The heaviest rains occurred along and to the east of its path. Changes in upper level wind conditions during June 7 caused Allison to make a clockwise loop and move back toward the Harris County area. The center of Allison passed over Harris County the second time during the late afternoon and evening of June 8 and morning of June 9, which brought Harris County the most intensive rainfall. On June 10 th, Allison left Texas area and continued meandering eastward. The storm strengthened as it moved eastward. It then passed Louisiana, Mississippi, Alabama, and finally began to slow and came to a halt along the Atlantic coast near Wilmington, North Carolina on June 14 th. Precipitation It is the significant and in some cases, record amounts of rainfall Allison produced makes Allison the off-the-chart storm in the U.S. history. Map 2 shows the total rainfall amount during 1 pm on June 5 and 23 pm on June 9. Map 2 5

6 More than two-thirds of the Harris County received rainfall ranging from 10 to 25 inches. The east-central portion of Harris County had the highest amounts of rain. NEXRAD raster data has the resolution about 14-degree by 14-degree, (approximately 400-meter by 400-meter). It has the strength to see a whole picture of the rainfall, but lacks the consistency in estimating rainfall at a specific point. So although in Harris County there were some locations receiving rainfall more than 25 inches from Allison, they couldn t be shown in Map 2. Table 1 from TSARP report reveals locations with maximum rainfall during Tropical Storm Allison. The highest rainfall record from June 5 to June 9 is 38.8 inches. Table 1 2 The flood-producing rains generally occurred during three periods. The first heavy rain occurred during the afternoon and evening of June 5 in the southern part of Harris County. Typical rainfall amounts ranged from three to six inches, with amounts as high as 12 inches occurring along Clear Creek during the evening of June 5 3. The second period of heavy rain occurred on the morning of June 7 in the far south west part of Harris County and eastern Fort Bend County. Typical rainfall amounts ranged from three to five inches, with as much as 11 inches occurring in Fort Bend County over a six-hour period 4. Map 3 shows the total rainfall amount over Harris County from 1 pm June 5 through 23 pm June 7. As seen on the map, the eastern two-thirds of Harris County received 4 inches of rain, and the central of this area received as much as 14 inches. 2 TSA event analysis final report, volume 1, table TSA event analysis final report, volume 1, chapter TSA event analysis final report, volume 1, chapter 3.1 6

7 Map 3 The third and most intense rainfall period began late in the afternoon on June 8, continuing on to the early morning of June 9, as Allison retraced its path southward through Harris County. According to TSARP report, the most of the rain fell over roughly a ten hour period, with rainfall rates as high as five inches per hour. Map 4 below shows a time sequence of cumulative rainfall for different time from 4:00 pm on June 8 until 6:00 am on June 9. It shows the rain beginning with moderate rate during the early evening hours in eastern Harris County, followed by intense heavy rainfall in that same area, spreading also to the center and northwest part of Harris County in the early morning of June 9. Map5 shows the total rainfall for Harris County for the period from 4:00 pm on June 8 to 6:00 am on June 9. Heavy rain fell over 80% of Harris County, often affecting the same areas that had received the most rainfall during the June 5-7 period. Almost two-thirds of Harris County received over 12 inches of rain in 14 hours, with rainfall amounts exceeding 19 inches at several locations. The TSARP report also claimed that at two rain gages on Greens Bayou, over 25 inches were measured. 7

8 Map 4 Map 5 8

9 Because of its proximity to the Gulf of Mexico, Harris County has experienced many tropical storms and other storms that have produced severe, high-intensity, flood-producing rains. A comparison with previous historically significant rains will put Allison s rainfall in perspective. Table 2 from TSARP report compares Allison with other 8 previous storm event since 1899 about the data, location, maximum measured rainfall, the typical rainfall intensity, and the approximate area of rainfall coverage. It can be seen that Allison s rainfall ranks fourth among all the storms in terms of maximum point rainfall, with 38 inches measure along Greens Bayou. While Allison produced rainfall rate averaging as much as 2.4 inches per hour, significantly higher than other storms except Claudette storm in 1979 at Alvin Texas. It should be noted that the rainfall rates shown in Table 2 are averages for the entire storm durations. Rainfall intensities for Tropical Storm Allison as well as the other storms considered were significantly higher for selected shorter time periods 5. The table also indicates that for Allison the 24-inch area of rainfall is the largest of all the storms. All the three factors in the table the total rainfall, the average rainfall intensity, and the rainfall area explicate Allison as one of the greatest rain-producers in Texas history. Table TSA event analysis final report, volume 1, chapter TSA event analysis final report, volume 1, table 3.6 9

10 Harris County in the storm Harris County has 21 major watershed that each drain into a major waterways. 7 Along those waterways, there re 70 USGS stream gages, among which 44 gages have still been recording stream discharge data. Map 6 shows the 21 watershed in Harris County with NHD flowlines and USGS stream gages. Map 7 has the watersheds and the flowlines of Harris County and the total Allison rainfall from June 5 to June 9 together, which gives a clear image of which watersheds received the most severe rainfall during the period when Allison passed Harris County. The central and south-central watersheds in Harris County had the most intensive rain, including Greens Bayou, Hunting Bayou, White Oak Bayou, Brays Bayou, Sims Bayou, and Clear Creek. The entire Greens Bayou watershed was covered by the highest rainfall. Rain gage data also shows one location on Greens Bayou had the highest total rainfall record (38.8 inches, see table 1). So in next section I will exam the stream discharge observed at USGS stream gages in Greens Bayou watershed, White Oak Bayou watershed, Brays Bayou watershed, and Sims Bayou watershed to see how the heave rain affected the stream runoff in these watersheds. 7 Harris County Flood Control District website, 10

11 Map 6 Map 7 11

12 Stream discharge Map 8 Greens Bayou received the highest point rainfall amounts and generally the highest average watershed-wide rainfall in the County. In the 5 days, over 20 inches rain covered almost the entire watershed. At one gage 21 inches rain was recorded in six hours 8. The heavy rain had affected the stream discharge in Greens Bayou watershed largely. There re 5 USGS gages in Greens Bayou watershed. But Greens Bayou at Cutten Rd near Houston station doesn t have data for period from June 1 to June 12; Halls Bayou at Houston gage only has data before June 8. The other three stream gages all recorded the maximum stream discharge on June 9, which were much higher than 1% exceedance flow. TSARP found that the station on Greens Bayou at Ley Road had the largest observed flow at any of the stream flow stations in Harris County for this event. 8 TSA event analysis final report, volume 1, chapter

13 Map 9 Map 10 13

14 Map 11 There re 2 stream gages in White oak Bayou watershed; 4 in Brays Bayou watershed; and 3 in Sims Bayou watershed, among which 2 gages in Brays Bayou watershed and 1 gage in Sims Bayou watershed don t have records for period from June 1 to June 12. All the other stream gages records of stream discharge from June 1 to June 12 are shown in Map 9, 10, and11. 5 gages out of 6 had the maximum average daily stream discharge on June 9. Although Brays Bayou at Houston station didn t have the maximum average daily discharge at that day, it reached peak flow as high as 33,000 cfs at some moment on June 9. All the 6 stream gages data show that the stream discharge on June 9 surpassed their 1% eceedance flow because of the intense rainfall Allison brought. Out of the 44 USGS stream gages in Harris County, 17 9 exceeded the previous historical peak flood level when Allison passed through Harris County. Map 12 and Table 3 shows the locations and information of those stream gages. 9 TSA event analysis final report, volume 1, Executive Summary 14

15 Map 12 Table TSA event analysis final report, volume 1, Executive Summary, Table ES-1 15

16 Estimated flooding area There re two main formation of flooding 11 : 1. Local drainage system being overwhelmed. The storm sewer and roadside ditches in Houston and Harris County are typical designed to handle a rainfall rate about 1-2 inches per hour. Allison drenched large areas of Houston area with 3 to 5 inches per hour for several hours. 2. Flooding along creeks and bayous. Generally, flooding begins along Harris County s creeks and bayous when there s 4-6 inches of rainfall in several hours. Channels that have been significantly enlarged can handle 6-8 inches of rainfall in several hours. Map 13 Using data collected at HCOEM rain gages, USGS stream gages and field surveys which were performed after the flood to identify high water marks at bridges crossing the major streams and in neighborhoods where flooding occurred, the area that flooded was outlined in the TSARP. Using their data I created Map 13 showing the estimated extent of flooding for the entire Harris County. As seen in the map, flooding occurred along all the major streams in the central portion 11 Tropical Storm Allison Public Report 16

17 of Harris County. Flooding also occurred at areas away from the major streams in the central area, which resulted from overland sheet flow that exceeded the capacity of the existing storm sewers and street storage areas. The total flooded area covered approximately acres, with 42% of the area being flooded from local storm sewer and street drainage overflow. Map 14 Inside City of Houston, the flooding area covered various land uses: acres of single family, 6721 acres of multi family, 7667 acres of commercial land, 1425 acres of office, acres of industrial land, and acres of public and institutional land, as shown in Map 14. There re approximately 1197-mile major roads in Harris County were flood because of Allison, including north I-45, east I-10, north US-59, more than half of I-610 and several segments of Beltway 8. Map 15 shows the flooded major roads in Harris County. 17

18 Map 15 Conclusions Through the study, it is explicitly shown by all the figures and maps that Tropical Storm Allison was the most damaging tropical storm in Harris County. Harris County received massive rainfall from Tropical Storm Allison, with the highest rainfall amount of 38 inches. More than two-thirds of Harris County had rainfall amount as high as 10 inches and above. Streams in Harris County were greatly affected by Tropical Storm Allison, 17 out of 44 stations reached record peak flow on June 19, Many stream gages showed stream discharge exceeding 1% exceedance flow (100 years). A large amount of lands and roads in Harris County were flooded. The result of the project also proves that ArcGIS has the powerful function to integrate different source data, visualize and analyze data, and the clearly depicted picture of the storm event will help people understand flooding and flood risk, as well as better prepare for the next Allison. 18

19 Acknowledgement Dr. Maidment David Cedric Nishesh Mehta Reference Exercise 5, NHDPlus and Flow Networks, Prepared by Tyler L. Jantzen and David R. Maidment Exercise 6, Building an Arc Hydro with Time Series Geodatabase, Prepared by Tyler L. Jantzen, David R. Maidment and Ernest To Lecture Spatial Analysis Using Grid, by David G. Tarboton NEXRAD Viewer Tutorial & Help, National Weather Service, Tropical Storm Allison Synopsis, Harris County Flood Control District website, Tropical Storm Allison Recovery Project website, Tropical Storm Allison Event Analysis Final Technical Report, Volume 1, 2, 3 Tropical Strom Allison Public Report, Off the charts 19