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Appendix ACottonwood Branch LOMR – Bridge Improvements (Sandy Lake Rd. & Freeport Pkwy.) 2 Model Descriptions  Effective – Since no digital floodplain model was made available by FEMA, Effective floodplain elevations were obtained from the un-encroached, 100-year water surface elevations listed in the HEC-2 floodway output table. These values are tabulated in Table 3.  Duplicate Effective – model was created by importing the HEC-2 Effective floodway model (discussed above) into HEC-RAS and making minimal revisions to allow the model to run. These changes consisted of: 1. Adjusting bounding bridge sections to be 1’ off of the structure, and 2. Removal of the floodway encroachments. The effective model code appears to contain eight (8) cross sections (sections 32816 to 34822) that are common with Denton Creek, the receiving stream for Cottonwood Branch. These sections were imported as cross-sections 1 through 8 in HEC-RAS in order to maintain continuity in stream stationing, thereby allowing the software to identify the sections in their appropriate location. The effective model utilized a starting 100-year WSEL of 464.39 as the downstream boundary condition (at cross section 32816). This value does not appear to match the WSEL shown in the effective Denton Creek flood profiles at Station 328+16. This value was revised in subsequent “Pre-Project” runs (discussed below). The resulting 100-year profile matches the effective FIS profile within 0.00 feet and 0.21 feet at the downstream and upstream limits, respectively, as indicated in Table 3. A comparison of the 100-year WSELs from the Effective Model and Duplicate Effective model are given in the ”Duplicate Effective” section of Appendix B. The differences in water surface elevations are attributed to changes necessary to execute HEC-2 models in HEC-RAS (namely reach length revisions), as well as differences in the hydraulic computation engines between both platforms, particularly at bridges and culverts.  Pre-Project – The Pre-Project Floodplain model was created by adding and updating topographic definition and hydraulic data in the Duplicate Effective Model. Updates were made to cross sections 9430 through 9590 (for Sandy Lake Road) and 12420 through 12957 (for Freeport Parkway) to reflect design ground survey conducted by TNP, as well as newer aerial topographic data in the overbanks. Reach lengths, Manning’s “n” values, and contraction/expansion coefficients were also changed in this model in order to reflect the present floodplain conditions. The changes to hydraulic parameters between the (Duplicate) Effective and Pre-Project model are documented in Table 2. The original HEC-2 model provided by FEMA only contains peak discharge information for the 100-year event. Peak discharges for the 10-, 50-, and 500-year event were added using the information provided in the FIS Study. The effective model uses a known 100-year WSEL of 464.39 at cross-section 32816 (equivalent to cross section 1 in the Pre-Project HEC-RAS model) as the downstream boundary condition. This value does not match the WSELs shown on the Denton Creek flood profiles in the Dallas County FIS, which appears to be approximately 461.50. The starting water surface elevations were revised to match those shown in the current effective FIS profiles at Denton Creek river station 328+16. A copy of the pertinent Denton Creek stream profile is provided in Appendix A. Modeling indicates that the change in boundary condition is far enough downstream that its impacts are not felt through the revision reach. However, it should be noted that FEMA typically Cottonwood Branch LOMR – Bridge Improvements (Sandy Lake Rd. & Freeport Pkwy.) 3 requires the “normal depth” boundary conditions at the downstream end of a stream. This revision should be considered if the revision reach were to be extended to Denton Creek. This model represents the pre-project condition and is used to evaluate the actual impact of the project. Pre-Project floodplain limits at both crossings are shown on Exhibits 2A and 3A. Pre-Project WSELs are tabulated in Table 3.  AsBuilt – The following paragraphs describe the scope of both projects. Sandy Lake Rd.: This project will replace the existing bridge with a 150-foot, three-span bridge. The main channel will remain in place and be straddled by two sets of piers. Grading will consist of benching/excavation in the right overbank under the bridge, and transitioning back to the natural channel within 25 feet upstream and 60 feet downstream. The as-built low chord of the new bridge will be at an elevation of approximately 482.59 feet, approximate 1.7 feet higher than the existing bridge. Given the proposed abrupt transition grading, channel bank armoring using gabions is proposed in select locations. Disturbed areas will be re-vegetated as needed. Freeport Pkwy.: This project will replace the existing bridge with a 180-foot, three-span bridge to be supported by two rows of 30-inch piers. The main channel will remain in place. Grading and benching improvements are proposed in the left overbank, which will increase conveyance capacity through the structure. The proposed low chord of the new bridge will be at an elevation of approximately 494.4 feet, approximately 2 feet higher than the existing bridge. Rock riprap and gabions are proposed in order to armor the benched overbank of the channel through the bridge. At present, there is a “bowl” or low-lying depression in the ineffective flow area in the left overbank, just downstream of the bridge. This area is located within the MAPMOD 100-year floodplain. The City proposes to place fill in this area to reclaim a portion of the property from the floodplain. The fill is proposed to be retained by a gabion wall and obstructed flow area is proposed to be mitigated by the proposed bridge transition grading. Modeling Parameters  Reach Length – Reach lengths were measured for sections 24700, 24660, and 24620. The reach length for section 24585 equals the original lengths for section 24700 minus the added sections. This approach maintains the original length of stream.  Mannings ‘n’ – Manning’s roughness coefficients were carried over from the effective model and were applied to the new sections at corresponding horizontal locations. These values were also confirmed with visual observations to reflect actual field conditions.  Contraction/Expansion – Contraction/expansion coefficients were not increased above 0.3/0.5 (respectively) at the bridge since the bridge clear spans the conveyance area. Cottonwood Branch LOMR – Bridge Improvements (Sandy Lake Rd. & Freeport Pkwy.) 4 Table 2: Effective and Pre-Project Hydraulic Parameter Comparison Comparison (FEMA Effective Discharges) Effective Floodway – The digital floodway model obtained from FEMA was executed using the HEC-2 software. The resulting output contains “negative surcharges” in some locations. That is, the 100-year WSEL is reduced when floodway encroachments are included. These results render the effective floodway out of compliance with current National Flood Insurance Program (NFIP) standards. Furthermore, the floodway widths contained within this model do not match the floodway widths currently shown on the FEMA DFIRM flood maps, within the project reach. Effective floodplain/floodway limits (obtained from the FEMA DFIRM data) are provided on the FIRM map in Appendix A. Duplicate Effective Floodway – Using the duplicate effective floodplain model as a base, the duplicate effective floodway model was created by inserting the effective model encroachment stations listed in the HEC-2 input. Again, these encroachment widths do not match the floodway widths shown on the FEMA DFIRM map in the project reach. There appear to be four (4) cross sections in the HEC-2 model that do not have floodway data (Cross sections 32816, 11380, 11880, and 12420). The floodway encroachments for these sections were determined by measuring the effective floodway widths from the FEMA DFIRM data. Similar to the Effective Floodway model, the Duplicate Effective Floodway model also shows negative surcharge (cross sections 9590, 10120, 12675, 19794, 19944, 20069). Pre-Project Floodway – Proposed Floodway – This model uses Pre-Project as a base, but includes the proposed improvements at both crossings as discussed above. The Post-Project Conditions model reflects the proposed bridge and associated grading improvements. At both crossings, the proposed structures are wider than those already in place. As a result, the Pre-Project bridge face cross sections had to be removed (cross sections 9494, 9526, 12595, and 12661) in order to accommodate wider bridges in the model. LOB Chan ROB C E LOB Chan ROB C E -- - - - - -9233 1233 0.065 0.05 0.06 0.1 0.3 9430 1430 0.065 0.05 0.06 0.1 0.3 9475 242 0.065 0.05 0.06 0.1 0.3 9494 64 0.02 0.02 0.02 0.3 0.5 9494 19 0.06 0.05 0.07 0.3 0.5 9526 32 0.02 0.02 0.02 0.3 0.5 9526 32 0.06 0.05 0.07 0.3 0.5 9590 64 0.06 0.05 0.07 0.1 0.3 9590 64 0.06 0.05 0.07 0.3 0.5 -- - - - - -9890 300 0.06 0.05 0.07 0.1 0.3 10120 530 0.06 0.05 0.075 0.1 0.3 10120 228 0.06 0.05 0.075 0.1 0.3 10130 10 0.06 0.05 0.075 0.1 0.3 10130 10 0.06 0.05 0.075 0.1 0.3 12420 540 0.06 0.05 0.075 0.1 0.3 12420 540 0.035 0.05 0.06 0.1 0.3 -- - - - - -12539 117 0.035 0.05 0.06 0.1 0.3 12540 120 0.06 0.05 0.075 0.1 0.3 12567* 30 0.035 0.05 0.06 0.1 0.3 12585 45 0.02 0.02 0.02 0.3 0.5 12595 30 0.035 0.05 0.06 0.3 0.5 12615 30 0.02 0.02 0.02 0.3 0.5 12661* 66 0.055 0.05 0.06 0.3 0.5 12675 60 0.06 0.05 0.07 0.3 0.5 12707* 34 0.055 0.05 0.06 0.1 0.3 -- - - - - -12868 163 0.055 0.05 0.06 0.1 0.3 -- - - - - -12957 87 0.055 0.05 0.06 0.1 0.3CrossingEffective Model Pre-Project Model Manning's "n"Contraction/Expansion Effective XS D/S Reach Length Revised Existing XS D/S Reach Length Manning's "n"Contraction/Expansion Sandy Lake Road Freeport Parkway Freeport ParkwaySandy Lake Rd.Freeport Pkwy.Sandy Lake Road Cottonwood Branch LOMR – Bridge Improvements (Sandy Lake Rd. & Freeport Pkwy.) 5 Modeling indicates that the combination of the proposed taller and wider bridge structures, coupled with the grading improvements, would result in lowered 100-year WSELs upstream of both bridges. Proposed conditions WSELs are tabulated in Table 3. Table 3: 100-Year WSEL Comparison (FIS Flows)1 (#####) – Parenthetical notation indicates equivalent section in the effective FEMA model 1 – See Appendix B for complete flood profile data 2 – Per FEMA LOMR MT-2 instructions, tie-in must be within 0.5 feet of effective flood elevations Ultimate Conditions Considerations Per City criteria, both bridges are designed to pass the Ultimate Conditions, 100-year discharges, as determined by the “City-Wide Storm Water Management Study for the City of Coppell (Halff Associates; Jan 1991).” With the City’s consent, the bridge at Freeport Parkway was designed to have less than 1-foot of freeboard. Starting boundary conditions for the Ultimate Conditions model were obtained from the modeling associated with the “City Wide Storm Water Management Study for the City of Coppell (Halff Associates, January 1991).” The modeling indicates that post-project, 100-year ultimate WSEL’s are lower than in Pre-Project conditions at both locations. Table 4 provides a comparison of Pre-Project and Post-Project 100-year WSELs at both locations. Effective (HEC-2) (EFF) Duplicate Effective (DE) Corrective Effective (CE) Pre- Project (PRE) Post- Project (PP) Tie-In Comparison2 (RE-EFF) Project Impact Comparison (PP-PRE) (ft)(ft)(ft)(ft)(ft)(ft)(ft) 9233 --476.75 476.75 0.00 9430(9475)477.20 477.25 478.31 477.69 1.11 -0.62 9494 477.50 477.52 477.58 -0.08 - (9526)479.40 479.33 481.62 -2.22 - (9590)481.90 481.92 483.29 477.85 1.39 -5.44 9890 --483.43 480.06 -3.37 (10120)481.90 481.83 483.65 482.05 1.75 -1.60 (10130)482.70 482.49 483.67 482.11 0.97 -1.56 (12420)490.40 490.51 490.51 490.51 0.11 0.00 12539 --491.25 491.06 -0.19 12567 (12540)491.10 491.15 491.37 491.40 0.27 0.03 12595 (12585)490.90 491.00 491.19 -0.29 - 12661 (12615)491.00 491.04 491.97 -0.97 - 12707 (12675)491.70 491.76 492.30 491.50 0.60 -0.80 12868 --492.23 491.48 -0.75 12957 --492.28 491.80 -0.48Freeport ParkwayCross Section CrossingFREEPORT PARKWAY BRIDGE SANDY LAKE ROAD BRIDGE Cottonwood Branch LOMR – Bridge Improvements (Sandy Lake Rd. & Freeport Pkwy.) 6 Table 4: 100-Year WSEL Comparison (Ultimate Flows)1 (#####) – Parenthetical notation indicates equivalent section in the effective FEMA model 1 – See Appendix B for complete flood profile data 2 – Per FEMA LOMR MT-2 instructions, tie-in must be within 0.5 feet of effective flood elevations Exhibit 2B and 3B provide comparisons of the pre-project and proposed 100-year ultimate floodplains. Conclusion Based on the analyses, approval of a Letter of Map Revision by FEMA is requested. Hydraulic modeling indicates that construction of the proposed improvements at Sandy Lake Road and Freeport Parkway will have no adverse impacts on the regulatory floodplain or the ultimate conditions floodplain. At both locations, 100-year flood elevations will be reduced as a result of the wider and taller bridge structures proposed. Furthermore, changes in width and alignment of the effective floodway in the vicinity of both projects were necessary in order to make the effective floodway compliant with current FEMA standards and regulations (i.e. less than one foot of increase, and no negative surcharge upon encroachment). Results indicate that the proposed improvements do not adversely impact the floodway. Pre- Project (PRE) Post- Project (PP) Project Impact Comparison (PP-PRE) (ft)(ft)(ft) 9233 477.84 477.84 0.00 (9475)478.62 478.45 -0.17 (9494)478.66 -- (9526)484.58 -- (9590)485.12 478.61 -6.51 9890 485.21 481.37 -3.84 (10120)485.36 482.52 -2.84 (10130)485.37 482.60 -2.77 (12420)491.71 491.71 0.00 12539 492.64 492.28 -0.36 12567 (12540)492.76 492.82 0.06 12595 (12585)492.67 -- 12661 (12615)494.31 -- 12707 (12675)494.61 492.95 -1.66 12868 494.57 493.06 -1.51 12957 494.64 493.33 -1.31Freeport ParkwayFREEPORT PARKWAY BRIDGECrossingCross Section SANDY LAKE ROAD BRIDGE Freeport Parkway (Study Vicinity) Sandy Lake Road(Study Vicinity)Cottonwood BranchFREEPORT PARKWAYSANDY LAKE HALIFAXALLEY COPPELLOAKBENDBEAL BRANCHWOODCOTTONWOODOAK GROVE TIMBERVIEW EDINBURGH POST OAK FOREST RIDGE NEWPORTMANCHESTERGREENWICH A U T U M N W O O D WINDHAMKINGSTONHAZ ELWOOD RIPPLEWOODHARTFORDDARTMOUTH A L L E Y A LLE Y N 0 300 600150 1 inch = 600 feet Coppell, TexasSandy Lake & FreeportEXHIBIT 1: LOCATION MAP April 2017 Legend FEMA Streams/Shorline D ent o n Cre e k C o tto n w o o d B ra n c h Esri, HERE, DeLorme, MapmyIndia, © OpenStreetMap contributors, and the GISuser community N 0 1,000 2,000500 1 inch = 2,000 feet REVISED FLOODWAY 100-YEAR FLOODPLAIN 500-YEAR FLOODPLAIN Exhibit 4:Annotated FIRM0400200 1 inch = 400 feet LIMITS OF REVISION tnp 2" WATER METER1" IRRIGATIONMETER