City of Katy Flood Protection Study (Meeting 3 of 3) October 23, 2017
Phasing of Meetings April Meeting May Meeting October Review of Tax Day Storm Event and Immediate Actions Taken By City Review of Coordination with Various Agencies Presentation of Results
Presentation Objectives Summary of Hurricane Harvey Review of the existing condition analysis Tax Day 100-Year event Review of short-term drainage improvements Review of long-term drainage improvements Emergency response measures Recommendations
Tax Day Flood (April 18, 2016) 125 Residential Structures Flooded Rainfall Amount (in 24 hours) Frequency 4.1 2-YR 5.8 5-YR 7.1 10-YR 9.0 25-YR 10.6 50-YR 12.4 100-YR 15.2 250-YR 17.7 500-YR
Hurricane Harvey (August 25-29, 2016) 685 Residential Structures Flooded Rainfall Amount (in 24 hours) Frequency 4.1 2-YR 5.8 5-YR 7.1 10-YR 9.0 25-YR 10.6 50-YR 12.4 100-YR 15.2 250-YR 17.7 500-YR
Hurricane Harvey Rainfall Return Frequency
Harvey versus Tax Day at US 90 Gage Rainfall (inches) 35 30 25 20 15 10 5 0 Cumulative Rainfall (inches) 0 20 40 60 80 100 120 Time (hr) US 90 Gage Stream Elevation Tax Day - 125 Homes Harvey - 685 Homes Tax Day Harvey Stream Elevation (ft) 142 140 138 136 134 132 130 128 126 124 122 0 20 40 60 80 100 120 140 160 Time (hours) Tax Day Storm Harvey
Watershed Model Development Determine the existing condition drainage areas Areas of undetermined flow direction Review previous reports and analysis Create rainfall-runoff model for Tax Day Develop dynamic unsteady hydraulic model 1D Simulation of CIB channel 2D Simulation of overbank areas and Cypress Creek Overflow Calibrate Tax Event Day results to high water marks and local observation of flooding patterns
Overall Watersheds
Cane Island Branch: Watershed Map
Hydraulic Model Stream (Cypress Creek, Cane Island Branch, and Willow Fork) simulated using 1D Cross Sections Overbank areas simulated using 2D 2D allows simulation of movement of water in multiple directions
Cypress Creek Overflow Broad wide floodplain Shallow in nature Undefined overland flowpaths Begins around a 10 - year event Difficult to conventionally control as floodwaters cannot be easily collected
Cypress Creek Overflow Third Reservoir Cypress Creek Overflow Partner with other political and stakeholder groups City hosted meeting with US Representative Michael McCaul and other local leaders to discuss path forward Plan 5 proposed by HCFCD estimated to cost $369 million Anticipated to expand size of proposed reservoir for additional protection to Barker and Addicks
Short-Term Drainage Projects Update
Expanded City Regional Detention Pond Simulated expanded detention pond on city property Reviewed modifications to existing weir to improve performance Reviewed channel cleanout from Regional Pond to Morton Road (modified Manning s n-value from 0.08 to 0.04 to reflect a grass lined channel) Review of pond drain time Proposed pond detention volume approx. 60 ac-ft
Expanded City Regional Detention Pond OPTION Channel Clean Out Only (From Regional Pond to Morton) Existing Pond Modification (Lower Weir Elevation by approx. 4 feet) Existing Pond Modification with Channel Clearing Proposed Regional Pond with upstream weir Proposed Regional Pond, Inline Structure, Channel Clearing Proposed Regional Pond, Inline Structure, No Channel Cleanout Combination Existing Weir Adjustment, Regional Pond, Inline Structure, No Channel Cleanout RESULT (as measured at Morton Road) Rise of 0.21-feet Reduction of 0.1-feet Rise of 0.15-feet Reduction of 0.1-feet Reduction of 0.18-feet Reduction of 0.3-feet Reduction of 0.4-feet
Regional Pond Drain Time HCFCD Drain Time Requirement 1 day (24 hours) but can have up to 3 days (72 hours) Reports of extended drain time Shorter drain time reduces risk from back to back storms 3000-GPM pump station to drain pond in 3-days cost approximately $1M Drain Time (days) Existing Volume Proposed Volume 29.1 ac-ft 38.2 ac-ft 1 6575 8651 3 2192 2884 5 1315 1730 7 939 1236 Flow rates in GPM
Long Term Improvements Option Channel improvements expanded railroad and US 90 bridge Upstream regional detention (north of Pitts Road) Shared detention with Cane Island Branch development Key Assumptions All alternatives investigated for the 100-year storm event Assumes Cypress Creek Overflow is cutoff from Cane Island Branch Existing condition landuse
Channel Improvements Expand bridge under railroad and US 90 Channel improvements to increase depth and conveyance along creek Detention pond to mitigate impact due to improved conveyance Complex interactions with multiple agencies to make project viable (Railroad, City, TxDOT, HCFCD, FBCDD, USACE)
Channel Improvements Results Location Δ 100 Year Undetained Channel Improvements Δ 100 Year Detained Channel Improvements Pitts Road 0.00-0.02 Clay Road 0.00-0.02 Morton Road 0.00-0.01 Franz Road -0.28-0.35 10th Street -2.13-2.23 Highway 90-0.87-1.31 Interstate 10 0.40-1.06 Willow Fork 0.09-0.01
Upstream Regional Detention Regional detention pond to reduce peak water surface elevations within the City of Katy Evaluate range of Level of Service 1000, 1500, and 2000 acre-foot ponds
Upstream Regional Detention Δ 100 Year 500 ac-ft Pitts Detention Δ 100 Year 1000 ac-ft Pitts Detention Δ 100 Year 1500 ac-ft Pitts Detention Δ 100 Year 2000 ac-ft Pitts Detention Location Pitts Road -0.44-0.89-1.28-1.62 Clay Road -0.5-0.75-1.27-1.71 Morton Road -0.11-0.42-0.89-0.9 Franz Road -1.32-1.74-1.77-1.76 10th Street -0.63-0.6-0.65-0.63 Highway 90-0.27-0.25-0.27-0.27 Interstate 10-0.05-0.04-0.05-0.05 Willow Fork -0.01-0.01-0.01-0.01 1,500 ac-ft Regional Pond
Shared Detention Pond with CIB Development Over excavate bottom of Cane Island Branch Detention Pond for regional volume Approx. 100 ac-ft of additional volume available Requires a 23,000 GPM Pump Station to drain pond in 24 hours. Additional Coordination and analysis required with developer and developer engineer Use side-weir to allow high flows to enter detention pond
Upstream Development Developed conditions upstream of Katy were analyzed Utilized a landuse assumption of small lot residential, this assumes a percent impervious value of 50%. Updated hydrology for developed conditions Simulated developed conditions without any detention volume Determined detention volume and rate to mitigate development
Upstream Development - Landuse
Upstream Development No Detention Input developed hydrology into HEC-RAS model Increase of water surface elevations of 1.4-feet within the City of Katy if no detention is provided Need to ensure upstream development detains to proper rate
Development Criteria Compute detention rate for range of landuse assumptions 40, 60, 80 % impervious and detention rate Peak flow rate limited to existing conditions which is approximately 0.5 cfs/acre based on existing condition hydrology Utilize Harris County Criteria Supplemental Criteria for development Flow (cfs) Development % Impervious Upstream Pitts Road 9000 8000 7000 6000 5000 4000 3000 2000 1000 0 100-Year Δ WSEL (No Improvements) Required Detention Rate (ac-ft/ac) 40 1.14 0.54 60 1.43 0.60 80 1.65 0.67 0 10 20 30 40 Time (hr)
Development Criteria Harris County Criteria Supplemental Criteria for all development in Cane Island Branch watershed Min. detention rate of 0.55 ac-ft/ac Additional retention rate of 0.17 ac-ft/for residential and 0.32 ac-ft/ac for commercial Total rate of 0.72 to 0.87 ac-ft/ac for development Compensatory Floodplain Fill Mitigation Volume at 1:1 ratio Require developments to use City of Katy HEC-RAS 2D model (or approved alternative) to show floodplain fill mitigation volume is provided. Sheetflow Impacts Require developments to utilize the HEC-RAS 2D model to show no impact to the surrounding sheetflow on adjacent properties due to placement of fill. Partner with Brookshire-Katy Drainage District to create and enforce development regulations within the watershed
Emergency Response Inundation mapping for various rainfall amounts to allow for advanced planning of emergency response Identification of flooding zones based on predicted rainfall amounts Neighborhood level of service identified City can better plan resources based on predicted rainfall Develop map of road depths based on rainfall amounts Provide depth maps for roadways based on range of rainfall amounts Allow for emergency personnel to plan and react to predicate conditions
Emergency Response Inundation Mapping
Recommendations Continue with planned short term projects to improve local rainfall capacity within the city limits. Work with TxDOT and Union Pacific on possible expansion of the Railroad and US 90. Construct expanded city pond with improvements to the existing weir structure. Increase pump station size to provide further resiliency to the system. Coordinate with Cane Island Branch Engineer on possible opportunities to expand proposed detention pond for additional flood reduction. Continue to advocate for the third reservoir to protect from the Cypress Creek Overflow. Review localized drainage systems for opportunities to improve conveyance and stormwater collection within the city. Pine Forest Fortuna/Patna 1st Street Partner with Brookshire-Katy Drainage District to enforce consistent development criteria within the CIB watershed.
Flood Reduction Strategy Cypress Creek Third Reservoir (Plan 5, etc..) Cane Island Branch Creek Large scale regional improvements $$$$$$ channel improvements, regional detention City detention pond expansion Potentially expand CIB development pond Develop criteria and partner with BKDD to ensure responsible development upstream Local Drainage Pine Forest drainage improvements Fortuna/Patna drainage review 1st St. street and drainage improvements Morton Road drainage improvements Downtown storm sewer improvements
Pine Forest Drainage Review City authorized Costello, Inc. to look at alternatives to improve local drainage conditions within the Pine Forest subdivision. Planned workshop November 7 th with City Council to discuss the findings