Duncan Hastie, P.E. Dewberry

Transcription

1 FEMA s Riverine High Water Marks for Atlanta s September 2009 Flood Event Duncan Hastie, P.E. Dewberry

2 Background High Water Mark Recovery Performed Through FEMA s Hazard Mitigation Technical Assistance Program (HMTAP) In Response to Presidential Disaster Declaration #1858 Disaster initially declared for: Carol County Cobb County Cherokee County Douglas County Paulding County

3 Coordination with other Agencies United States Army Corps of Engineers Performed HWM flagging and provided this information to FEMA to be surveyed Cobb County Performed HWM flagging and provided this information to FEMA to be surveyed DeKalb County Performed HWM flagging and survey and provided this to FEMA Gwinnett County Performed HWM flagging and survey and provided this to FEMA Many HWMs were around spillways of Roller Compacted Concrete Dams which overtopped

4 Overview of the September Rainfall Event Heavy rains over Metropolitan Atlanta September 19-21, 2009; Many areas saw at least 0.2% annual chance (500-year) rainfall event of over 10 inches over 24 hours; National Weather Service in Peachtree City estimated that several counties in the greater Atlanta saw rainfall totals exceeding a 0.02% annual chance (5,000 year) rainfall event; USGS record west of Douglasville, GA indicated rainfall of total 16.7 inches over 24 hours; Douglas-Douglasville Water Authority indicated rainfall up to 20.5 inches over 24 hours was observed within the Dog River watershed.

5 USGS Precipitation Gages Provided Real Time Indication of the Magnitude and Severity of this Event TP40-1% Annual Chance Rainfall Depths in Metropolitan Atlanta Range from Approximately 7.5 to 8 inches over a 24 hours Period Approximately 2 Inches of Rain Fell in Preceding week saturating ti soils

6 Purpose of HWM Report Capture and document the maximum flood elevations observed at different locations throughout Carol, Cobb, Cherokee, Douglas and Paulding Counties during the unnamed September 21-24, 2009 flood event. 65 HWM along Sweetwater Creek through Paulding, Cobb and Douglas Counties; 66 HWM along Chattahoochee River in Carol, Douglas and Cobb Counties; Noonday Creek in Cobb and Cherokee Counties; Mill and Log Creeks in Paulding County; Sope and Willeo Creeks in Cobb County; Little River in Cherokee County; Anneewakee River in Douglas County.

7 Possible Uses of HWM Report Estimation of flood frequency; Assessment of the accuracy of the Flood Insurance Rate Maps; Calibration of models; Loses Avoided Studies; Prioritization of mitigation projects ; Preparation of benefit-cost analyses; Input to building performance assessments; Determination of depth of flooding for structures.

8 Methodology Pre-Field Deployment training i and planning Coordination with other Agencies (USGS, USACE) flagging HWMs Communication with Carol, Cobb, Cherokee, Douglas and Paulding Counties to determine heavily impacted areas and HWMs efforts already underway Flagging g and Survey Collection Indentifying the HWM Mud lines Debris lines Witness accounts on flooding

9 Flagging the HWM Utility paint Duct tape Wooden grade stakes Photographs of the HWM Geographic Coordinates of the HWM HWM location description and notes Field assessment of the HWM quality Three HWM flagged at each location to isolate erroneous points Field Survey of the HWM elevations

10 Surveying of the HWMs Survey crews deployed after HWMs have been flagged when evidence of high water would have likely perished Utilized Global Positioning Systems (GPS) and Virtual Referencing Stations Single man crews using robotics where GPS signal could be received Traditional survey used in covered areas where GPS signals could not be received well Lat-Long coordinates collected during flagging enabled surveyor to quickly locate flags and recover elevations

11 Quality Assurance of the collected HWM data Review of flagging field notes and photographs; Review of survey field data; Triple validation and expected elevation trends review (verifying that flagged elevations decreased in downstream direction); Receding flood waters often left multiple mud lines and debris lines which could be misleading when flagging HWMs. Quality assurance processes enabled identification of erroneous HWMs.

12 HWM Results Total of 151 HWMs collected after the September 2009 Flood Event in Carol, Cobb, Cherokee, Douglas and Paulding Counties. (additional 297 in Gwinnett County) Sample of the HWM Summary Table HWM_ID DATE OF FLAG COMPANY Flooding Source COUNTY HWM_QUAL Latitude Longitude HWM_ELEV (Feet NAVD 88) HUC12 Basin DF1_28A Dewberry Little River Cherokee Good DF1_28B Dewberry Little River Cherokee Good DF1_28C Dewberry Little River Cherokee Good DF1_30A Dewberry Little River Cherokee Moderate DF1_30B Dewberry Little River Cherokee Moderate DF1_30C Dewberry Little River Cherokee Moderate DF1_33B Dewberry Noonday Creek Cobb Good DF1_33C Dewberry Noonday Creek Cobb Moderate DF1_34B Dewberry Noonday Creek Cobb Good DF1_34C Dewberry Noonday Creek Cobb Good DF1_37B Dewberry Noonday Creek Cobb Poor DF1_37C Dewberry Noonday Creek Cobb Poor A10_NA USACE Noonday Creek Cherokee N/A A10A USACE Noonday Creek Cherokee N/A

13 Surveyed High Water Marks

14 HWMs Reported Using the HUC12 Watershed Approach

15 HWMs Reported Using the HUC12 Watershed Approach. Cont

16 Details of Individual HWMs Documented in Certificates

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19 Comparison of High Water Marks with Effective FEMA data and USGS Gage Analysis Summary of USGS Gage Analysis for Streams Where HWMs Were Flagged USGS Gage Number USGS Gage Name Peak Flow (CFS) Annual Exceedance Probability CHATTAHOOCHEERIVERBELOW MORGANFALLS DAM, GA 30,900 1% CHATTAHOOCHEE RIVER AT ATLANTA, GA 40,900 1% to 0.5% CHATTAHOOCHEE RIVER AT GA ,300 1% to 0.5% CHATTAHOOCHEE RIVER NEAR WHITESBURG, GA 60,900 2% to 1% SWEETWATER CREEK NEAR AUSTELL, GA 31,500 Greater than 0.2% NOONDAY CREEK AT SHALLOWFORD ROAD, NR WOODSTOCK,GA 11,300 Greater than 0.2% NOONDAY CREEK AT HAWKINS STORE RD, NR WOODSTOCK,GA 12,200 Greater than 0.2% POWDER SPRINGS CREEK NEAR POWDER SPRINGS, GA 8,420 Greater than 0.2% SOPE CREEK NEAR MARIETTA, GA 9,400 2 % to 1%

20 Comparison of HWM Elevations to USGS Gage and FIS Profile Annual Exceedance Probabilities HWM_ID HWM Elevation (Feet NAVD 88) Flooding Source BFE Elevation from FIS (Feet NAVD 88) Annual Exceedance Probability of HWM Elevation Extracted from FIS Profiles Estimated Annual Exceedance Probability of HWM Based on Representative USGS Gage USGS Gage Number Used for Comparison USGS Gage Name Used for Comparison D3_15A Chattahoochee River % to 0.2% 1% CHATTAHOOCHEE RIVER BELOW MORGAN FALLS DAM, GA D3_04B Chattahoochee River % to 0.2% 1% CHATTAHOOCHEE RIVER BELOW MORGAN FALLS DAM, GA D3_14B Chattahoochee River 801 1% to 0.2% 1% CHATTAHOOCHEE RIVER BELOW MORGAN FALLS DAM, GA D3_14A Chattahoochee River % 1% CHATTAHOOCHEE RIVER BELOW MORGAN FALLS DAM, GA D3_13A Chattahoochee River % 1% to 0.5% CHATTAHOOCHEE RIVER AT ATLANTA, GA D3_13C Chattahoochee River % to 0.2% 1% to 0.5% CHATTAHOOCHEE RIVER AT ATLANTA, GA D3_12C Chattahoochee River % to 0.2% 1% to 0.5% CHATTAHOOCHEE RIVER AT ATLANTA, GA D3_10C Chattahoochee River % to 0.2% 1% to 0.5% CHATTAHOOCHEE RIVER AT ATLANTA, GA D3_22A Chattahoochee River % to 0.2% 1% to 0.5% CHATTAHOOCHEE RIVER AT ATLANTA, GA D3_09A Chattahoochee River 775 1% to 0.2% 1% to 0.5% CHATTAHOOCHEE RIVER AT ATLANTA, GA D3_07A Chattahoochee River % to 0.2% 1% to 0.5% CHATTAHOOCHEE RIVER AT GA 280 D3_06C Chattahoochee River % to 0.2% 1% to 0.5% CHATTAHOOCHEE RIVER AT GA 280 DF1_1C Chattahoochee River % to 2% 1% to 0.5% CHATTAHOOCHEE RIVER AT GA 280 DF1_2B Chattahoochee River % to 2% 1% to 0.5% CHATTAHOOCHEE RIVER AT GA 280 DF1_4A Chattahoochee River Greater than 0.2% 2% to 1% CHATTAHOOCHEE RIVER NEAR WHITESBURG, GA DF1_6A Chattahoochee River 746 Greater than 0.2% 2% to 1% CHATTAHOOCHEE RIVER NEAR WHITESBURG, GA DF1_9C Chattahoochee River 738 1% to 0.2% 2% to 1% CHATTAHOOCHEE RIVER NEAR WHITESBURG, GA DF1_10A Chattahoochee River Greater than 0.2% 2% to 1% CHATTAHOOCHEE RIVER NEAR WHITESBURG, GA

21 Comparison of High Water Marks with Effective FEMA data and USGS Gage Analysis, cont Chattahoochee River (Carol, Cobb, Douglas, and Fulton Counties) Indicated that generally the HWMs and estimated flood frequency were generally in agreement with the FEMA flood profiles although there were sections where differences occurred. Sweetwater Creek (Cobb, Douglas and Paulding Counties) All HWMs agreed with the USGS conclusion which indicated that the event was significantly greater than a 0.2% annual chance (100-year) flooding event. Elevations were typically feet higher than the FEMA BFE Powder Springs Creek (Cobb County) All HWMs indicated that the flood was significantly greater than a 0.2% annual chance flood. Elevations were typically 4-6 feet higher than the FEMA BFE Noonday Creek (Cobb County) Indicated that generally the HWMs and estimated flood frequency were generally in agreement with the FEMA flood profiles although there were sections where notable differences occurred.

22 Questions Duncan J. Hastie, P.E. Dewberry 2835 Brandywine Road, Suite 100 Atlanta, GA Tel: (678)