Las Colonias Subdivision September 2010 Flood Study Curtis Beitel, P.E., CFM Scott Muchard, P.E. Project Engineer William Badini, CFM Senior Meteorologist
Location Map
Background Los Colonias Subdivision Constructed in 1986 Construction Standards for Floodplain Management essentially unchanged Build finish floor at the Base Flood Elevation Subdivision is currently located partially in FEMA Zone A & B flood zones Zone A is 1% chance (100 year) floodplain Zone B is 0.2% chance (500 year) floodplain
Background Effective FIRM = July 18, 1985 Zone A4 BFEs 22 to 24
La Volla Creek Watershed Tot Total Contributing Drainage Area Approximately 17,172 Acres Margaret Kelly Ditch Airport Ditch Saratoga Downs Ditch Las Colonias Subdivision
September 2010 Event Summary Monthly Precipitation reached record levels: Continued rainfall recharged reservoirs and saturated ground surfaces Subjected the City to extreme flooding during rain events the weekend of September 18th, 2010 Two Tropical Depressions Sept. 6 thru 8th, Tropical Storm Hermine Sept. 14 thru 18, Tropical Storm Karl September 18, 2010 - Water Surface Elevations (WSE) Los Colonias Subdivision El. 27.8 (at high point) Greenwood WWTP El. 27.x Saratoga Bridge El. 28 La Volla Creek / Oso Creek Convergence El. 25
September 2010 Event Summary Greenwood Waste Water Treatment Plant
September 2010 Event Summary Saratoga Bridge Downstream Face
September 2010 Event Summary Looking North from the Saratoga Bridge
September 2010 Event Summary Las Colinas Subdivision
NEXRAD Radar Rainfall Analysis Utilized both Corpus Christi and Brownsville radars Corpus Christi NEXRAD site is in watershed
NEXRAD Radar Rainfall Analysis Reviewed Atmospheric Conditions Estimated Maximum Precipitation Rates Related to Reflectivity to Develop Grid
NEXRAD Radar Rainfall Analysis GIS polygons Basin Average 15-minute Rainfall Depths Compared to Drainage Criteria Manual Rainfall Depths
Rainfall Frequency Analysis Duration Las Colonias (OCB-200) (inches) Oso Creek U/S of La Volla Creek (inches) Oso Creek D/S of La Volla Creek (inches) 15 Minute 0.72 0.60 0.64 30 Minute 1.29 1.14 1.28 60 Minute 2.30 2.09 2.47 2 Hour 4.41 3.75 4.50 3 Hour 5.92 4.97 5.92 6 Hour 7.65 6.68 7.90 12 Hour 8.04 7.07 8.29 Duration Las Colonias (OCB-200) (inches) Return Frequency (years) Oso Creek U/S of La Volla Creek (inches) Oso Creek D/S of La Volla Creek (inches) 15 Minute < 5 < 5 < 5 6 hour duration Oso Creek above La Volla 40-year frequency La Volla Creek 80-year frequency Oso Creek below La Volla 90-year frequency 60 Minute < 5 < 5 < 5 2 Hour 30 14 33 3 Hour 68 29 68 6 Hour 79 43 90 12 Hour 40 23 45
Hydrology Basin Hydrology Oso Creek HEC-HMS model from Draft Storm Water Master Plan New rainfall gauge for each 15-minute Basin Average Rainfall Same loss rates Same unit hydrograph parameters
Hydrology Resulting Peak Flows 5,435 cfs in La Volla Creek at Saratoga FEMA 1992 Flood Insurance Study Peak Discharges 4,359 cfs = 100-year at Saratoga 6,150 cfs = 500-year at Saratoga
Hydraulics Draft SWMP HEC-RAS model of La Volla Creek Included 2004 TxDOT bridge configuration at Saratoga Boulevard Adjusted roughness coefficients
Hydraulics Resulting Water Surface Elevations 28.1 at Greenwood WWTP pump house 28.5 upstream of Saratoga bridge 28.5 to 28.7 in Las Colonias subdivision Natural ground at 23 to 24
Impact of Proposed CIP Projects 1. Channel Clearing and Stabilization Project Upstream of Saratoga to Airport Ditch Vegetation clearing only If in place in September 2010 28.1 at Greenwood WWTP pump house (0.0 ft reduction) 28.3 upstream of Saratoga bridge (0.2 ft reduction) 28.3 in Las Colonias subdivision (0.4 ft reduction) Future Overbank / floodplain excavation (100 ft bottom width)
Impact of Proposed CIP Projects 2. Drainage Channel Excavation La Volla Creek Airport Ditch down to Oso Creek (150 ft bottom width) Programmed for FY12-13 Design $17.5M Project Construction Estimate If in place in September 2010 23.9 at Greenwood WWTP pump house (4.2 ft reduction) 23.9 upstream of Saratoga bridge (4.6 ft reduction) 23.9 to 24.0 in Las Colonias subdivision (4.7 ft reduction)
Status & Future Actions Presented at December 14, 2010 City Council meeting Subdivision Ideas Committee Citizens, City staff, City Council member Recognized community relief contributions Event Summary Engineering Analysis Less than 3 months after flood event occurred Future Actions: Storm Water Master Plan and Drainage Criteria Adoption Storm Water Operation and Capital Improvement Revenue Floodplain Management and Drainage Design Review
Current & Future Actions La Volla Creek Detention Improvements HMGP Grant Application pending La Volla Creek Drainage Channel Excavation project (Saratoga Bridge to Oso Creek) Currently in preliminary design Oso Creek Basin Drainage Enhancement Study Alternatives to relieve several hydraulic choke points Study is underway TxDOT Bridge Replacement
Questions Curtis Beitel, PE, CFM HDR Engineering, Inc. Curtis.Beitel@hdrinc.com 817-333-2817