las colonias subdivision september 2010 flood study · •programmed for fy12-13 design •$17.5m...
TRANSCRIPT
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
Las Colonias Subdivision
Margaret Kelly Ditch
Saratoga Downs Ditch
Airport Ditch
Tot
Total Contributing Drainage Area Approximately 17,172 Acres
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
Saratoga Bridge – Downstream Face
September 2010 Event Summary
Looking North from the Saratoga Bridge
September 2010 Event Summary
Las Colinas Subdivision
September 2010 Event Summary
• 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
NEXRAD Radar Rainfall Analysis
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
Return Frequency (years)
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 < 5 < 5 < 5
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
• 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
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)
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)
Impact of Proposed CIP Projects
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
• 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
Current & Future Actions
