1. Flooding in the Coastal Plain National Flood Workshop October 26, 2010 Houston, Texas Thomas Mountz, P.E., D.WRE, CFM Espey Consultants, Inc. 4801 Southwest Parkway Parkway 2, Suite 150 Austin, Texas 78735

2. Lower Rio Grande Valley NFIP Program losses from ’78-’04 - $9.6 Million NFIP Program losses from ’78-’04 - $9.6 Million Fourth fastest growing metropolitan area in the State: Growth = Development Pressure Fourth fastest growing metropolitan area in the State: Growth = Development Pressure Federal disasters in 1984, 1991, 2008, 2010 Federal disasters in 1984, 1991, 2008, 2010 FLAT - no natural streams FLAT - no natural streams Arid region with significant Arid region with significant flooding potential from flooding potential from Rio Grande, hurricane/ Rio Grande, hurricane/ tropical storm and other tropical storm and other rainfall events. rainfall events.

3. How Flat is Flat? Slope = 1 foot per mile or less

4. Hurricane Dolly 10-16 inches of rainfall in 24 hours 10-16 inches of rainfall in 24 hours Two weeks of heavy rains followed Hurricane Dolly Two weeks of heavy rains followed Hurricane Dolly Some areas were flooded for months Some areas were flooded for months

5. Cameron County Storm Drainage Study Focus on Unincorporated Areas Focus on Unincorporated Areas Focus on flooding from Hurricane Dolly Focus on flooding from Hurricane Dolly Lake Tio Cano had extensive damage Lake Tio Cano had extensive damage Problem Area Unincorporated Area Incorporated Area

6. Tio Cano Lake 171 Parcels at or below elev. 46 ft 171 Parcels at or below elev. 46 ft Appraised value in excess of $8 Million Appraised value in excess of $8 Million More than 2.5 miles of streets at or below elev. 46 ft More than 2.5 miles of streets at or below elev. 46 ft No Natural Outlet No Natural Outlet FM-506 closed for more than one month (raised by TxDOT in 2009) FM-506 closed for more than one month (raised by TxDOT in 2009)

7. Flatland Runoff Methodology HEC-HMS Hydrology Model HEC-HMS Hydrology Model Calculated using Kerby-Kirpich method for Time of Concentration Calculated using Kerby-Kirpich method for Time of Concentration Kirby-Kirpich recommended by Roussel et al., 2005 Study of 92 gauged Texas watersheds and 1,600 storm events Kirby-Kirpich recommended by Roussel et al., 2005 Study of 92 gauged Texas watersheds and 1,600 storm events Peak Rate Factor Reduction as suggested by recent USACE studies Peak Rate Factor Reduction as suggested by recent USACE studies Modified Puls & Reservoir Routing due to extensive overbank storage Modified Puls & Reservoir Routing due to extensive overbank storage Source: NEH 4, Fang et al., 2005

8. Kerby – Kirpich Method Kerby Overland Flow Equation Kerby Overland Flow Equation L = Length in feet L = Length in feet S = Slope (ft/ft)(not to exceed 1,200 feet) S = Slope (ft/ft)(not to exceed 1,200 feet) N = Retardance Coefficient (dimensionless) N = Retardance Coefficient (dimensionless) Kirpich Shallow Concentrated /Channel Flow Equation Kirpich Shallow Concentrated /Channel Flow Equation L = Length in feet L = Length in feet S = Slope (ft/ft) S = Slope (ft/ft)

9. Calibration

10. Proposed Tio Cano Lake Outlet Outlet Channel Approx. 3.8 mile channel Approx. 3.8 mile channel 12-20 ft BW, 2.5:1 SS 12-20 ft BW, 2.5:1 SS 10 ft water depth 10 ft water depth 70 ft weir length at channel inlet 70 ft weir length at channel inlet 28 ft max cut 28 ft max cut

11. Questions, Discussion Thank you for this opportunity! Thank you for this opportunity! Thomas W. Mountz, P.E., D.WRE, CFM Thomas W. Mountz, P.E., D.WRE, CFM tmountz@espeyconsultants.com tmountz@espeyconsultants.com (512) 326-5659 (512) 326-5659

13. Lake Tio Cano “Dry” lake bed located in sump area between La Feria and Santa Rosa “Dry” lake bed located in sump area between La Feria and Santa Rosa Much of the lake area included in National wildlife Refuge Much of the lake area included in National wildlife Refuge No Natural Outlet No Natural Outlet Pumped outlet to Wilson Canal & trucks for several months Pumped outlet to Wilson Canal & trucks for several months Hurricane Dolly reached approx. Elevation 46 (100-YR FIRM). Hurricane Dolly reached approx. Elevation 46 (100-YR FIRM).