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Home My WebLink AboutFlood Hazard Evaluation 6-10-2020 WEST SIDE FLATS MASTER SIDE PLAN - FLOOD HAZARD EVALUATION June 25, 2020 #180820 1 TABLE OF CONTENTS REPORT Introduction ..........................................................................................................................1 Hydrology ............................................................................................................................1 Hydraulics ............................................................................................................................4 Conclusion ...........................................................................................................................5 APPENDICES Appendix A: Contributing Areas Map Appendix B: AutoCAD SSA Output Files Appendix C: HEC-RAS Output Files Appendix D: 100-Year Floodplain Boundary Appendix E: 2006 Loyal Garden Subdivision – Design Report for Water, Sewer, and Pavement Improvements (TD&H Engineering) Appendix F: 2007 Norton Ranch Subdivision, Phase 1 – Flood Hazard Evaluation of Baxter Creek (Engineering, Inc.) Appendix G: 2009 Norton East Ranch Subdivision, Phase 1 – Stormwater Management Design Report (Sanderson Stewart Engineering) WEST SIDE FLATS MASTER SIDE PLAN - FLOOD HAZARD EVALUATION June 25, 2020 #180820 2 INTRODUCTION Overview The West Side Flats is a proposed residential development on Lot R-2 of the J & D Family Subdivision, Phase 2. The property is located in the southwest portion of the City of Bozeman. The development will consist of residential apartments in a series of 12-plex and 18-plex apartment buildings. Access improvements, dry and wet utilities, and landscaping/park improvements will be installed with the proposed development. Baxter Creek runs north/south along the eastern boundary line of the property. Since there is no official floodplain delineation for Baxter Creek, this flood hazard evaluation was prepared in accordance with Section 38.600.090 BMC. The purpose of this study was to delineate the 100- year floodplain for the portion of Baxter Creek that runs through the subject property. HYDROLOGY Background and Methodology Baxter Creek is a pseudo-natural stream that now receives a large portion of its flow from the Farmers Canal irrigation ditch south of Huffine Lane. The creek then runs north from Farmers Canal through the Loyal Garden Subdivision along the west side of Cottonwood Road and crosses under Huffine Lane. It then flows north through a vacant parcel along the west side of the Billion Auto Complex and crosses under Fallon Street, through the subject property, then under West Babcock, Durston Road, Annie Street, and West Oak Street before draining to undeveloped farmland north of Oak Street. No stream gauge data exists along Baxter Creek, so a combination of previous flood studies and hydrologic analysis were used to estimate the 100-year peak flows transmitted by Baxter Creek. AutoCAD Storm and Sanitary Analysis 2018 (SSA) was used to produce hydrographs for the 100-year storm event over the contributing area. The analysis is based on the Soil Conservation Service (SCS) hydrograph method variables that are outlined in this section. Contributing Areas The contributing area analyzed for this flood study was limited to the area between Huffine Lane and West Babcock Street, that drain to Baxter Creek. The drainage area south of Huffine Lane will be accounted for by using the 100-year peak flow developed from the 2006 Design Report for Loyal Garden Subdivision done by TD&H Engineering. Refer to the Peak Flows section of this report for a full summary of the flows used for this hydraulic analysis. Land use for the contributing area was determined via aerial imagery taken from City of Bozeman GIS department. The contributing area for this study was divided into three drainage areas. Drainage Area 1 accounts for the area between Huffine Lane and Fallon Street. Drainage Area 1 has a total area 1,570,879 sf, of which 712,163 sf was assumed to be commercial and 858,716 was assumed to be undeveloped. The area of the Billion Auto Complex lying east of Automotive Avenue between Fallon Street and West Babcock Street, and Common Open Space B were excluded WEST SIDE FLATS MASTER SIDE PLAN - FLOOD HAZARD EVALUATION June 25, 2020 #180820 3 from this analysis. Common Open Space B has a detention pond and outlet structure that discharges the pre-development runoff from this area into Baxter Creek just south of the 2 – 40” x 65” RCP culverts under West Babcock Street. Drainage Area 2 has a total area of 378,172 sf, of which 275,594 sf were assumed to be commercial and 102,578 sf were assumed to be undeveloped. Drainage Area 3 accounts for the subject property; this was assumed to be dense residential and has a total area 596,656 sf. A map showing the contributing area for this analysis is included in Appendix A of this report. SCS Curve Number For each drainage area the SCS runoff Curve Number (CN) was estimated using a weighted average approach. Approximately 45% of Drainage Area 1 was determined to be commercial while 55% remained undeveloped. Using standard CN values listed in the TR-55 Urban Hydrology for Small Watersheds Table 2-2a, Drainage Area 1 was given a composite CN of 79.43. Applying the same method to Drainage Area 2, which is 73% commercial and 27% undeveloped, generates a composite CN of 85.76. Drainage Area 3 was assumed to be 100% dense residential, which has a CN of 85. These CN values account for existing conditions around the subject property and the proposed development within the subject property. These values do not account for future development between Fallon Street and Huffine Lane. Time of Concentration A time of concentration (TOC) path was developed for each drainage area. Elevation contours from a 2017 LiDAR survey of Bozeman were combined with a site topographic survey conducted in 2018. This was the composite surface used in developing slopes for TOC calculations. For Drainage Area 1 the longest TOC path was calculated from the southwest corner of the drainage area, across a field and into Baxter Creek, with a portion of the path in the channel of Baxter Creek. The first 875 feet of the path through the undeveloped field was assumed to be sheet flow. Using a slope of 1.53%, a Manning’s “n” of 0.15 for short grass pasture, and the 2-year 24-hour rainfall of 1.20 inches yielded a TOC of 101.08 minutes for the first part of the path. The second 752 feet of the TOC path for Drainage Area 1 was from its entrance into Baxter Creek to the flow line of the Fallon Street culverts. Channel Flow was used for this portion, with an assumed “V” cross-section that yielded a cross-sectional area of 2 sq. ft. and a wetted perimeter of 4 feet. Using the slope of the channel yielded a TOC of 6.94 minutes for the second portion of the Drainage Area 1 path. Therefore, the total TOC for Drainage Area 1 was calculated to be 108.02 minutes. The same method for calculating TOC above was applied to Drainage Areas 2 & 3 excluding the channel flow portion. For Drainage Area 2 the first 633 feet of the path was calculated as sheet flow over a paved surface (n = 0.011) with a slope of 1.35%. The next 365 feet was calculated as sheet flow over short grass pasture (n=0.15) with a slope of 1.79%. Using the 2-year 24-hour rainfall of 1.20 inches the total TOC for Drainage Area 2 was found to be 57.26 minutes. For Drainage Area 3 the path was a single 886-foot segment across short grass pasture (n=0.15). Applying the same rainfall as before the TOC for Drainage Area 3 was found to be 106.30 minutes. Drainage Areas 2 & 3 do not include a channel flow portion because their peak flows were assumed to enter Baxter Creek at positions in the hydraulic model. The Peak Flows and Hydraulics sections of this report summarizes the assumptions used. A map showing TOC paths WEST SIDE FLATS MASTER SIDE PLAN - FLOOD HAZARD EVALUATION June 25, 2020 #180820 4 for all drainage areas can be found in Appendix A of this report. Time of concentration calculations can be found in the SSA output calculations in Appendix B of this report. These TOC estimates and the following flow projections are considered to be conservative as they do not account for the holding time in the stormwater facilities in the Billion Auto Complex or the proposed Sundry Station development. Instead this model assumes that the runoff produced over these drainage areas sheet flow and deposit directly into Baxter Creek. 100-Year Peak Flows As stated in the previous section, AutoCAD SSA was used to produce the hydrographs for the 100-year storm events over each drainage area. To produce the hydrographs an SCS Type II 24- hour rainfall curve was used. Using the 100-year return period for the storm a rainfall depth of 2.80 inches was determined. Applying the rainfall depth and the parameters outlined above produced peak runoff volumes for each drainage area. The peak runoff for Drainage Areas 1, 2, & 3 were calculated to be 13.12 cfs, 7.32 cfs, and 6.98 cfs respectively. The output file with the runoff hydrographs are included in Appendix B of this report. To account for the contributing area south of Huffine Lane, the 100-year storm flow from the 2006 TD&H report was used (see Appendix E for the 2006 report). TD&H calculated the peak flow that would cross Huffine Lane as 106.2 cfs. This flow was combined with the peak runoff produced from Drainage Area 1 and rounded up for contingency for a total of 124 cfs that will be used as the initial input to the hydraulic model. Drainage Areas 2 & 3 deposit into Baxter Creek at a secondary point along the stream segment through the subject property. The peak flows for these two drainage areas were combined and rounded to 19 cfs. Therefore, the 100-year flows that were used in the hydraulic model were 124 cfs at the initial point, and 143 cfs at the convergence point of Drainage Areas 2 & 3. A full summary of the hydraulic analysis is provided in the Hydraulics section of this report. It should be noted that a similar flood hazard evaluation on Baxter Creek was done by Engineering, Inc. for Norton Ranch Subdivision, Phase 1 in 2007. This report estimated the peak flow for this portion Baxter Creek as 232 cfs. It is our opinion that this flow is an over-estimation for the Baxter Creek 100-year storm flow through this property. It is our opinion that the flows used in the 2006 report done by TD&H more accurately reflect the real-world conditions. The 2007 Engineering, Inc. report is included in Appendix F of this report. HYDRAULICS Methodology The hydraulic modeling of the portion of Baxter Creek that runs through the subject property was completed in HEC-RAS Version 5.0.5. The stream model was an approximately 1,200-foot segment of Baxter Creek. The upstream (beginning) station 12+62 is located at the outlet of the Fallon Street culvert. The downstream (end) station 0+00 is located at the inlet of the West Babcock culverts. Cross-sectional data was input into the program for 8 cross sections through the site. Similar to the hydrograph development, elevation data for the cross-sections was gathered from the combined 2017 Bozeman LiDAR and 2018 site topographic survey. The Manning’s roughness coefficients for the model were determined from aerial photography and WEST SIDE FLATS MASTER SIDE PLAN - FLOOD HAZARD EVALUATION June 25, 2020 #180820 5 site survey done in August, 2018. The channel through the entire site was modeled with an n=0.045, while the left and right overbank values were modeled as n=0.05. The culvert crossings beneath Fallon Street and West Babcock Street were excluded from the analysis. Each crossing has two 40” x 65” RCP culverts. Based on the Norton East Ranch Subdivision, Phase 1 Design Report prepared by Engineering Inc. (now Sanderson Stewart), each 40” x 65” RCP culvert has a design capacity of 150 cfs, for a total capacity of 300 cfs at each street. Therefore, it can reasonably be assumed that the each street crossing can adequately transmit the 100-year flows of 124 cfs and 143 cfs respectively, including the detention pond outflow from Common Open Space B. Modeling Steady state flow analysis was used to determine the 100-year flood elevations in the creek. At the upstream station 124 cfs was input into the model. As discussed above, the contributing flow from Drainage Areas 2 & 3 was added downstream at station 4+67.71 where the total flow was modeled as 143 cfs. Cross-sectional data and stream profile outputs can be found in Appendix C of this report. The 100-year floodplain boundary is shown on the map overlay included in Appendix D of this report. CONCLUSION Summary Based on the results provided from the HEC-RAS output the 100-year floodplain boundary of Baxter Creek will not affect the proposed development within the subject property. As depicted on the map in Appendix D, the 100-year floodplain lies entirely within the current wetland and watercourse setbacks which will remain undeveloped. Therefore, all proposed buildings and stormwater facilities are well outside of the 100-year floodplain, and no flooding risk is anticipated to the proposed development. Limitations This flood hazard evaluation was prepared for the West Side Flats project only and should not be used for planning of any other future developments along Baxter Creek. This report assumes that future developments along Baxter Creek will be subject to City of Bozeman design standards to ensure no increase in flood risk to downstream developments. Any modification to the stream itself shall be analyzed independently to ensure no increase to the floodplain elevations for existing developments. APPENDIX A CONTRIBUTING AREAS MAP APPENDIX B AUTOCAD SSA OUTPUT FILES Project Description Baxter Creek.SPF Project Options CFS Elevation SCS TR-20 SCS TR-55 Kinematic Wave YES NO Analysis Options May 02, 2019 00:00:00 May 04, 2019 00:00:00 May 02, 2019 00:00:00 0 days 0 01:00:00 days hh:mm:ss 0 00:05:00 days hh:mm:ss 0 00:05:00 days hh:mm:ss 30 seconds Number of Elements Qty 1 3 3 0 3 0 0 0 0 0 0 0 0 0 0 0 0 Rainfall Details SN Rain Gage Data Data Source Rainfall Rain State County Return Rainfall Rainfall ID Source ID Type Units Period Depth Distribution (years)(inches) 1 Rain Gage-01 Time Series TS-01 Cumulative inches Montana Gallatin 100 2.80 SCS Type II 24-hr Outlets ..................................................... Pollutants ......................................................... Land Uses ........................................................ Links.................................................................. Channels ................................................. Pipes ........................................................ Pumps ..................................................... Orifices .................................................... Weirs ....................................................... Nodes................................................................ Junctions ................................................. Outfalls .................................................... Flow Diversions ....................................... Inlets ........................................................ Storage Nodes ......................................... Runoff (Dry Weather) Time Step ..................... Runoff (Wet Weather) Time Step .................... Reporting Time Step ........................................ Routing Time Step ........................................... Rain Gages ...................................................... Subbasins......................................................... Enable Overflow Ponding at Nodes ................. Skip Steady State Analysis Time Periods ........ Start Analysis On ............................................. End Analysis On ............................................... Start Reporting On ........................................... Antecedent Dry Days ....................................... File Name ......................................................... Flow Units ........................................................ Elevation Type ................................................. Hydrology Method ............................................ Time of Concentration (TOC) Method ............. Link Routing Method ........................................ Subbasin Summary SN Subbasin Area Weighted Total Total Total Peak Time of ID Curve Rainfall Runoff Runoff Runoff Concentration Number Volume (ft²)(in)(in)(ac-in)(cfs)(days hh:mm:ss) 1 DA1 1570879.02 79.43 2.80 1.07 38.55 13.12 0 01:48:01 2 DA2 378172.02 85.76 2.80 1.48 12.81 7.32 0 00:57:15 3 DA3 596656.00 85.00 2.80 1.42 19.48 6.98 0 01:46:18 Node Summary SN Element Element Invert Ground/Rim Initial Surcharge Ponded Peak Max HGL Max Min Time of Total Total Time ID Type Elevation (Max)Water Elevation Area Inflow Elevation Surcharge Freeboard Peak Flooded Flooded Elevation Elevation Attained Depth Attained Flooding Volume Attained Occurrence (ft)(ft)(ft)(ft)(ft²)(cfs)(ft)(ft)(ft)(days hh:mm)(ac-in)(min) 1 Out-02 Outfall 0.00 0.00 0.00 2 Out-03 Outfall 0.00 0.00 0.00 3 Out-04 Outfall 0.00 0.00 0.00 Subbasin Hydrology Subbasin : DA1 Input Data Area (ft²) .........................................................................1570879.02 Weighted Curve Number ...............................................79.43 Rain Gage ID .................................................................Rain Gage-01 Composite Curve Number Area Soil Curve Soil/Surface Description (ft²)Group Number Undeveloped 858716.01 B 69.00 Billion 712163.01 B 92.00 Composite Area & Weighted CN 1570879.02 79.43 Time of Concentration TOC Method : SCS TR-55 Sheet Flow Equation : Tc = (0.007 * ((n * Lf)^0.8)) / ((P^0.5) * (Sf^0.4)) Where : Tc = Time of Concentration (hr) n = Manning's roughness Lf = Flow Length (ft) P = 2 yr, 24 hr Rainfall (inches) Sf = Slope (ft/ft) Shallow Concentrated Flow Equation : V = 16.1345 * (Sf^0.5) (unpaved surface) V = 20.3282 * (Sf^0.5) (paved surface) V = 15.0 * (Sf^0.5) (grassed waterway surface) V = 10.0 * (Sf^0.5) (nearly bare & untilled surface) V = 9.0 * (Sf^0.5) (cultivated straight rows surface) V = 7.0 * (Sf^0.5) (short grass pasture surface) V = 5.0 * (Sf^0.5) (woodland surface) V = 2.5 * (Sf^0.5) (forest w/heavy litter surface) Tc = (Lf / V) / (3600 sec/hr) Where: Tc = Time of Concentration (hr) Lf = Flow Length (ft) V = Velocity (ft/sec) Sf = Slope (ft/ft) Channel Flow Equation : V = (1.49 * (R^(2/3)) * (Sf^0.5)) / n R = Aq / Wp Tc = (Lf / V) / (3600 sec/hr) Where : Tc = Time of Concentration (hr) Lf = Flow Length (ft) R = Hydraulic Radius (ft) Aq = Flow Area (ft²) Wp = Wetted Perimeter (ft) V = Velocity (ft/sec) Sf = Slope (ft/ft) n = Manning's roughness Subarea Subarea Subarea Sheet Flow Computations A B C Manning's Roughness :0.15 0.00 0.00 Flow Length (ft) :876 0.00 0.00 Slope (%) :1.53 0.00 0.00 2 yr, 24 hr Rainfall (in) :1.20 0.00 0.00 Velocity (ft/sec) :0.14 0.00 0.00 Computed Flow Time (min) :101.08 0.00 0.00 Subarea Subarea Subarea Channel Flow Computations A B C Manning's Roughness :.045 0.00 0.00 Flow Length (ft) :752 0.00 0.00 Channel Slope (%) :0.75 0.00 0.00 Cross Section Area (ft²) :2 0.00 0.00 Wetted Perimeter (ft) :4 0.00 0.00 Velocity (ft/sec) :1.81 0.00 0.00 Computed Flow Time (min) :6.94 0.00 0.00 Total TOC (min) ..................108.02 Subbasin Runoff Results Total Rainfall (in) ............................................................2.80 Total Runoff (in) .............................................................1.07 Peak Runoff (cfs) ...........................................................13.12 Weighted Curve Number ...............................................79.43 Time of Concentration (days hh:mm:ss) ........................0 01:48:01 Subbasin : DA1 Subbasin : DA2 Input Data Area (ft²) .........................................................................378172.02 Weighted Curve Number ...............................................85.76 Rain Gage ID .................................................................Rain Gage-01 Composite Curve Number Area Soil Curve Soil/Surface Description (ft²)Group Number -275594.01 B 92.00 -102578.01 B 69.00 Composite Area & Weighted CN 378172.02 85.76 Time of Concentration Subarea Subarea Subarea Sheet Flow Computations A B C Manning's Roughness :.011 .15 0.00 Flow Length (ft) :633 365 0.00 Slope (%) :1.35 1.79 0.00 2 yr, 24 hr Rainfall (in) :1.20 1.20 0.00 Velocity (ft/sec) :1.04 0.13 0.00 Computed Flow Time (min) :10.13 47.12 0.00 Total TOC (min) ..................57.26 Subbasin Runoff Results Total Rainfall (in) ............................................................2.80 Total Runoff (in) .............................................................1.48 Peak Runoff (cfs) ...........................................................7.32 Weighted Curve Number ...............................................85.76 Time of Concentration (days hh:mm:ss) ........................0 00:57:16 Subbasin : DA2 Subbasin : DA3 Input Data Area (ft²) .........................................................................596656.00 Weighted Curve Number ...............................................85.00 Rain Gage ID .................................................................Rain Gage-01 Composite Curve Number Area Soil Curve Soil/Surface Description (ft²)Group Number -596656.00 -85.00 Composite Area & Weighted CN 596656.00 85.00 Time of Concentration Subarea Subarea Subarea Sheet Flow Computations A B C Manning's Roughness :.15 0.00 0.00 Flow Length (ft) :886 0.00 0.00 Slope (%) :1.38 0.00 0.00 2 yr, 24 hr Rainfall (in) :1.20 0.00 0.00 Velocity (ft/sec) :0.14 0.00 0.00 Computed Flow Time (min) :106.30 0.00 0.00 Total TOC (min) ..................106.30 Subbasin Runoff Results Total Rainfall (in) ............................................................2.80 Total Runoff (in) .............................................................1.42 Peak Runoff (cfs) ...........................................................6.98 Weighted Curve Number ...............................................85.00 Time of Concentration (days hh:mm:ss) ........................0 01:46:18 Subbasin : DA3 APPENDIX C HEC-RAS OUTPUT FILES HEC-RAS Plan: (COMBINED SURFACES) River: Baxter Creek Reach: Baxter Creek CL Profile: 100 YEAR STORM Reach River Sta Profile Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width Froude # Chl (cfs) (ft) (ft) (ft) (ft) (ft/ft) (ft/s) (sq ft) (ft) Baxter Creek CL 1262.07 100 YEAR STORM 124.00 4811.56 4813.51 4813.27 4813.76 0.009976 4.77 37.22 57.22 0.64 Baxter Creek CL 1191.15* 100 YEAR STORM 124.00 4810.78 4812.75 4813.05 0.010225 4.87 34.23 39.02 0.64 Baxter Creek CL 1120.23 100 YEAR STORM 124.00 4810.00 4811.81 4811.64 4812.21 0.013758 5.33 27.50 26.36 0.74 Baxter Creek CL 1027.50* 100 YEAR STORM 124.00 4808.67 4810.61 4810.99 0.012521 5.24 28.13 26.06 0.71 Baxter Creek CL 934.78* 100 YEAR STORM 124.00 4807.33 4809.32 4809.15 4809.75 0.014430 5.55 26.24 24.35 0.75 Baxter Creek CL 842.05 100 YEAR STORM 124.00 4806.00 4808.20 4808.57 0.011270 5.17 28.72 25.28 0.67 Baxter Creek CL 763.85* 100 YEAR STORM 124.00 4805.00 4807.29 4807.68 0.011492 5.36 27.76 23.47 0.68 Baxter Creek CL 685.65 100 YEAR STORM 124.00 4804.00 4806.43 4806.09 4806.82 0.010597 5.37 28.05 22.45 0.66 Baxter Creek CL 607.46* 100 YEAR STORM 124.00 4803.06 4805.23 4805.16 4805.77 0.017305 6.28 23.60 20.81 0.83 Baxter Creek CL 529.26 100 YEAR STORM 124.00 4802.13 4804.71 4804.97 0.006797 4.54 34.81 26.38 0.54 Baxter Creek CL 467.71* 100 YEAR STORM 143.00 4801.67 4804.12 4804.47 0.009738 5.27 35.39 29.48 0.64 Baxter Creek CL 406.16 100 YEAR STORM 143.00 4801.20 4803.54 4803.86 0.009904 5.19 37.54 35.50 0.64 Baxter Creek CL 334.57* 100 YEAR STORM 143.00 4800.47 4802.74 4803.10 0.011288 5.36 34.14 31.04 0.68 Baxter Creek CL 262.97* 100 YEAR STORM 143.00 4799.73 4802.13 4801.72 4802.42 0.008217 4.73 38.01 32.02 0.59 Baxter Creek CL 191.38 100 YEAR STORM 143.00 4799.00 4800.95 4800.95 4801.52 0.021199 6.37 25.75 24.83 0.90 Baxter Creek CL 95.69* 100 YEAR STORM 143.00 4798.21 4800.07 4799.65 4800.27 0.007834 3.72 42.72 40.52 0.55 Baxter Creek CL 0 100 YEAR STORM 143.00 4797.43 4798.61 4798.61 4798.97 0.029322 4.86 30.59 45.87 0.96 0 200 400 600 800 1000 1200 14004796 4798 4800 4802 4804 4806 4808 4810 4812 4814 BaxterCreek Plan: BaxterCreek 5/16/2019 Main Channel Distance (ft) Ele v a t i o n ( f t ) Legend EG 100 YEAR STORM WS 100 YEAR STORM Crit 100 YEAR STORM Ground Baxter Creek Baxter Creek CL 0 100 200 300 4004811 4812 4813 4814 4815 4816 4817 BaxterCreek Plan: BaxterCreek 5/16/2019 XS STA 12+62 Station (ft) Ele v a t i o n ( f t ) Legend EG 100 YEAR STORM WS 100 YEAR STORM Crit 100 YEAR STORM Ground Bank Sta .05 .045 .05 0 100 200 300 4004810 4811 4812 4813 4814 4815 4816 BaxterCreek Plan: BaxterCreek 5/16/2019 XS STA 11+20 Station (ft) Ele v a t i o n ( f t ) Legend EG 100 YEAR STORM WS 100 YEAR STORM Crit 100 YEAR STORM Ground Bank Sta .05 .045 .05 0 100 200 300 4004806 4807 4808 4809 4810 4811 BaxterCreek Plan: BaxterCreek 5/16/2019 XS STA 8+42 Station (ft) Ele v a t i o n ( f t ) Legend EG 100 YEAR STORM WS 100 YEAR STORM Ground Bank Sta .05 .045 .05 0 100 200 300 4004804 4805 4806 4807 4808 4809 4810 BaxterCreek Plan: BaxterCreek 5/16/2019 XS STA 6+85 Station (ft) Ele v a t i o n ( f t ) Legend EG 100 YEAR STORM WS 100 YEAR STORM Crit 100 YEAR STORM Ground Bank Sta .05 .045 .05 0 100 200 300 4004802 4803 4804 4805 4806 4807 4808 4809 BaxterCreek Plan: BaxterCreek 5/16/2019 XS STA 5+29 Station (ft) Ele v a t i o n ( f t ) Legend EG 100 YEAR STORM WS 100 YEAR STORM Ground Bank Sta .05 .045 .05 0 100 200 300 4004801 4802 4803 4804 4805 4806 4807 4808 BaxterCreek Plan: BaxterCreek 5/16/2019 XS STA 4+06 Station (ft) Ele v a t i o n ( f t ) Legend EG 100 YEAR STORM WS 100 YEAR STORM Ground Bank Sta .05 .045 .05 0 100 200 300 4004799 4800 4801 4802 4803 4804 4805 4806 BaxterCreek Plan: BaxterCreek 5/16/2019 XS STA 1+91 Station (ft) Ele v a t i o n ( f t ) Legend EG 100 YEAR STORM WS 100 YEAR STORM Crit 100 YEAR STORM Ground Bank Sta .05 .045 .05 0 100 200 300 4004797 4798 4799 4800 4801 4802 4803 BaxterCreek Plan: BaxterCreek 5/16/2019 XS STA 0+00 Station (ft) Ele v a t i o n ( f t ) Legend EG 100 YEAR STORM WS 100 YEAR STORM Crit 100 YEAR STORM Ground Bank Sta .05 .045 .05 APPENDIX D 100-YEAR FLOODPLAIN BOUNDARY APPENDIX E 2006 LOYAL GARDEN SUBDIVISION DESIGN REPORT FOR WATER, SEWER, AND PAVEMENT IMPROVEMENTS (TD&H ENGINEERING) APPENDIX F 2007 NORTON RANCH SUBDIVISION, PHASE 1 FLOOD HAZARD EVALUATION OF BAXTER CREEK (ENGINEERING, INC.) t\ n w a s S K 0 E N G I N E E R I N O ^ I N C . C o n s u l t i n g E n g i n e e r s a n d L a n d S u r v e y o r s S e p t e m b e r 1 0 , 2 0 0 7 S u s a n S t o d o k , P E P r o j e c t E n g t a e e r C i t y o f B o z e m a n 2 0 E a s t O l i v e B o z e m a n , M T 5 9 7 1 5 R e f e r e n c e : C e r d f i c a r i o n o f B a x t e r C r e e k F l o o d p l a i a B o u n d a r y N o r t o n R a n c h S u b d i v i s i o n , P h a s e I E . I . N o . B O Z - 0 7 0 0 4 D e a r S u s a n : I n a c c o r d a n c e - w i t h C h a p t e r 1 8 . 5 8 ( F l o o d p l a m R e g u l a t i o n s ) o f t h e C i t y o f B o z e m a n U n i f i e d D e v e l o p m e n t O t d i n a n c e ( U D O ) , E n g m e e r i n g , I n c . c o m p l e t e d a F l o o d H a . z a . t d E v a l u a t i o n o f B a x t e r C r e e k f o r t h e p r o p o s e d N o r t o n R a n c h S u b d i v i s i o n , P h a s e I . T h i s l e t t e r i s t o c e t t i f y t h a t d i e f l o o d p l a i a b o u n d a r y o f B a x t e r C r e e k , a s d e l i n e a t e d o n E x h i b i t 2 o f t h e F l o o d H a z a r d E v a l u a t i o n , h a s b e e n m a p p e d u s i n g a p p r o p r i a t e e n g i n e e r i n g m e t h o d s f o r h y d t o l o g y a n d h y d r a u l i c m o d e l i n g a n d t h e r e f o r e s h o u l d p r o t e c t a g a i n s t d a m a g e t o t h e p r o p o s e d N o r t o n R a n c h S u b d i v i s i o n , P h a s e I , b y t h e 1 0 0 - y e a r f l o o d o f B a x t e r C r e e k . R e f e r t o t h e a t t a c h e d F l o o d H a z a r d E v a l u a t i o n o f B a x t e r C r e e k f o r N o r t o n R a n c h S u b d i v i s i o n , P h a s e I , d a t e d S e p t e r a b e t 1 0 , 2 0 0 7 , f o r m o r e i n f o r m a t i o n . S i n c e r e l y , ^ % ^ - 7 J ^ H e i d n e r , P E P r o j e c t E n g i n e e r J H / s k w E n c . P : B O Z _ 0 7 0 0 4 _ S t o d o l a _ L t r _ 0 9 1 0 0 7 ' J ; 1 3 0 0 N o r t h T r Q n s t e c h W o y • B i l ! i n ( ) s , M T 5 9 1 0 2 • P h o n e ( 4 0 6 1 6 5 6 - 5 2 5 5 • f m i . W M M - M h J . u ^ p n n i n r . n m ( n o ^ J M / ^ ^ D . J E F F R & ) : ^ t i E i D N E ? ^ c - • o . \ " , ^ < ^ ' x < 3 ^ T w ^ { . n w a s n E N O I N E E R I N O , I N C . C o n s u l t i n g E n g i n e e r s o n d L a n d S u r v e y o r s S e p t e m b e r 1 0 , 2 0 0 7 E . I . N o . B O Z - 0 7 0 0 4 F L O O D H A Z A R D E V A L U A T I O N O F B A X T E R C R E E K F O R N O R T O N R A N C H S U B D I V I S I O N , P H A S E I I N T R O D U C T I O N I n a c c o r d a n c e w i t h C h a p t e r 1 8 . 5 8 ( F l o o d p l a i a R e g u l a t i o n s ) o f t h e C i t y o f B o z e m a n U n i f i e d D e v e l o p m e n t O r d i n a n c e ( U D O ) , E n g i n e e r m g , I n c . c o m p l e t e d a F l o o d H a z a r d E v a l u a t i o n f o t B a x t e r C r e e k i n r e l a t i o n t o t h e p r o p o s e d N o r t o n R a n c h S u b d i v i s i o n , P h a s e I . T h e p u r p o s e o f t h i s F l o o d H a z a r d E v a l u a t i o n i s t o e s t i n i a t e w h e r e t h e 1 0 0 - y e a r f l o o d p l a m o f B a x t e r C r e e k i s l o c a t e d i n r e l a t i o n t o t h e p r o p o s e d N o r t o n R a n c h S u b d i v i s i o n , P h a s e I . S i n c e t h e d r a i n a g e a r e a o f B a x t e r C r e e k i s l e s s t h a n 2 5 s q u a r e a i d e s , a n d n o o f f i c i a l f l o o d p l a i n o r f l o o d w a y d e l i n e a t i o n s t u d y h a s b e e n m a d e f o r B a x t e r C r e e k , t h e l e q u i t e m e n t s o u t l i n e d i n S e c t i o n 1 8 . 5 8 . 0 9 0 . B . 2 . c & d o f t h e C i t y o f B o z e m a n U D O ( O c t o b e r 1 , 2 0 0 5 ) w e r e f o l l o w e d f o r t h i s F l o o d H a z a r d E v a l u a t i o n . T h e f o l l o w i n g i s s u m m a r y o f t h e h y d i o l o g y a n d h y d c a u l i c m o d e l i n g p e r f o r m e d f o r B a x t e r C r e e k t a r e l a t i o n t o t h e p r o p o s e d N o r t o n R a n c h S u b d i v i s i o n , P h a s e I . H Y D R O L O G Y B a x t e r C r e e k i s n a t u r a l s t r e a m t h a t f l o w s a p p r o x i m a t e l y 5 0 0 f e e t e a s t o f t h e p r o p o s e d N o r t o n R ^ n c h S u b d i v i s i o n , P h a s e I , a n d o r i g m a t e s n o r t h o f B l a c k w o o d R o a d a p p r o x i r a a t e l y 2 . 5 m i l e s s o u t h e a s t o f t h e p r o p o s e d s i t e . S i n c e t h e r e i s n o k n o w n s t r e a m g a g e d a t a a v a i l a b l e f o r B a x t e r C r e e k , t h e 1 0 0 - y e a r p e a k f l o w w a s e s t i m a t e d u s i t l g t h e S o i l C o n s e r v a t i o n S e r v i c e ( S C S ) h y d t o g r a p h m e t h o d . T h e i n p u t p a r a m e t e r s n e e d e d f o r t h e S C S h y d c o g i - a p h m e t h o d i n c l u d e : d r a m a g e a r e a , r a m f a U d a t a , S C S t u n o f f c u r v e n u m b e r , a n d t i m e o f c o n c e n t r a t i o n . D R A I N A G E A R E A T h e B a x t e r C r e e k d r a i n a g e a i - e a w a s d e l b e a t e d f r o m i t s o r i g i a a t B l a c k w ^ o o d R o a d s o u t h t o a p p r o x i t n a t e l y 5 0 0 f e e t d o w n s t r e a m f r o t n t h e p r o p o s e d N o r t o n R a n c h S u b d i v i s i o n , P h a s e I , u s i n g t h e U S G S B o z e r a a n Q u a d r a n g l e m a p a s s h o w n o n t h e a t t a c h e d E x h i b i t 1 . T h e d r a i n a g e a r e a f o r t h e a b o v e d e s c r i b e d p o r t i o n o f B a x t e r C r e e k w a s e s t i m a t e d t o b e a p p r o x i t n a t e l y 7 0 0 a c r e s . 1 3 0 0 N o r t h T r a n s t e c h W a v • B i l l i n o s . M T 5 9 1 0 7 • P h n n B M n ^ U ^ - v ? ^ C n , , I A ! \ i \ L i L n o Z 7 - n 0 R A I N F A L L D A T A T h e r a i n f a l l d a t a w a s o b t a i n e d f r o m t h e R a i n f a U I n t e n s i t y - D u c a r i o n C u r v e s ( I D F ) f r o m F i g u r e s 1 - 2 a n d 1 - 3 o f t h e C i t y o f B o z e m a n D e s i g n S t a n d a r d s a n d S p e c i f i c a r i o n s P o l i c y . F r o m t h e p o w e r c u r v e e q u a t i o n p r o v i d e d f o r d i e 1 0 0 - y e a r f r e q u e n c y a n d i n p u t t i n g 2 4 h o u r s a s t h e d u r a t i o n , t h e 1 0 0 - y e a r , 2 4 - h o i u : p r e c i p i t a t i o n w a s d e t e r m i n e d t o b e 2 . 8 8 i n c h e s . S C S R U N O F F C U R V E N U M B E R T h e S C S r u n o f f c u r y e n u m b e r ( C N ) f o r t h e B a x t e r C r e e k d r a i n a g e a r e a w a s d e t e r m i n e d u s i n g a w e i g h t e d a v e r a g e o f t h e h y d r o l o g i c s o i l g r o u p s a n d l a n d u s e s w i t h i n t h e d r a i n a g e a r e a . H y d r o l o g i c s o i l g r o u p d a t a f o r t h e d r a i n a g e a r e a w a s o b t a i n e d f r o m t h e N a t u r a l R e s o u r c e s C o n s e r y a r i o n S e r v i c e ( N R C S ) W e b S o i l S u r v T e y . T h e s o i l g r o u p s w i t h i n t h e d r a i n a g e a r e a r a n g e d f r o m B t o D , w i t h B h a v i n g a l o w e r r u n o f f p o t e n t i a l t h a n D . T h e s o i l g r o u p p e r c e n t a g e s w e r e c a l c u l a t e d a s f o l l o w s : B = 3 3 p e r c e n t , C = 6 0 p e r c e n t , a n d D = 7 p e r c e n t . T h e l a n d u s e d a t a w a s b a s e d o n a s i t e o b s e r y a d o n o n J u l y 1 8 , 2 0 0 7 , a n d t h e m o s t r e c e n t s a t e l l i t e i m a g e r y o f t h e d r a i n a g e a r e a a s o f S e p t e m b e r 1 0 , 2 0 0 7 . A p p r o x i m a t e l y 8 0 p e r c e n t o f t h e d r a i n a g e a r e a w a s e s t i m a t e d t o b e p a s t u r e o r h a y c r o p p i o d u c d o n i n f a i r t o g o o d c o n d i d o n , 1 7 p e r c e n t w a s e s t i m a t e d t o b e r e s i d e n t i a l d e v e l o p m e n t w i t h 3 8 p e r c e n t i m p e r y i o u s n e s s , a n d 3 p e r c e n t w a s e s t i m a t e d t o b e c o m m e r c i a l w i t h 8 5 p e r c e n t i m p e r y i o u s n e s s . U s i n g T a b l e 2 - 2 o f T R - 5 5 U r b a n H y d r o l o g y f o r S m a U W a t e r s h e d s ( 1 9 8 6 ) f o r t h e a b o v e m e n t i o n e d h y d r o l o g i c s o i l g r o u p s a n d l a n d u s e s , a w e i g h t e d c u r v e n u m b e r o f 7 5 w a s c a l c u l a t e d . T h i s i s t h e e s t i t n a t e d S C S r u n o f f c u r y e n u m b e r f o r t h e B a x t e r C r e e k d r a i n a g e a r e a u n d e r e x i s t i n g c o n d i t i o n s a n d d o e s n o t t a k e i n t o a c c o u n t f u t u r e d e v e l o p m e n t . T I M E O F C O N C E N T R A T I O N T i m e o f c o n c e n t r a t i o n w a s e s t i m a t e d f r o m w h e r e B a x t e r C r e e k o r i g i n a t e s a t B l a c k w o o d R o a d t o a p p r o x i m a t e l y 5 0 0 f e e t d o w n s t r e a m o f t h e p r o p o s e d N o r t o n R a n c h S u b d i v i s i o n , P h a s e I . T h e f i r s t 3 0 0 f e e t o f t h e d r a i n a g e w a s e s t i m a t e d t o b e s h e e t f l o w , a n d M a n n i n g ' s k i n e m a t i c s e q u a t i o n f r o m T R - 5 5 U r b a n H y d r o l o g } r f o r S m a U W a t e r s h e d s ( 1 9 8 6 ) w a s u s e d t o c a l c u l a t e t h e r i m e o f t r a v e l . A s s u m i n g a M a n n i n g ' s " n " v a l u e o f 0 . 1 5 ( s h o r t g r a s s p a s t u r e ) , a l e n g t h o f 3 0 0 f e e t , a 2 - y e a r , 2 4 - h o u r r a i n f a U o f 1 . 2 8 i n c h e s , a n d a s l o p e o f 1 . 4 3 p e r c e n t , a t r a v e l t i m e o f 4 2 . 7 m i n u t e s w a s c a l c i i l a t e d f o r s h e e t f l o w . F r o m t h a t p o i n t , s h a l l o w c o n c e n t r a t e d f l o w w a s e s t i m a t e d f o r 7 0 0 f e e t d o w n s t r e a m t o t h e s t a r t o f t h e B a x t e r C r e e k c h a n n e l a s i d e n t i f i e d o n t h e U S G S B o z e m . a n Q u a d r a n g l e m a p . T i m e o f t r a v e l f o r s h a l l o w c o n c e n t r a t e d f l o w w a s d e t e r m i n e d t o b e 5 . 8 m i n u t e s f r o m F i g u r e 3 - 1 o f T R - 5 5 U r b a n H y d r o l o g y f o r S m a U W a t e r s h e d s ( 1 9 8 6 ) u s i n g t h e c u r v e f o r a n u n p a v e d s u r f a c e . C h a n n e l f l o w w a s e s t i m a t e d f o r t h e r e m a i n i n g 1 3 , 7 7 0 f e e t f r o m w h e r e U S G S i d e n t i f i e s t h e s t a r t o f B a x t e r C r e e k d o w n s t r e a i n t o a p p r o x i m a t e l y 5 0 0 f e e t n o r t h o f t h e p r o p o s e d N o r t o n R a n c h S u b d i v i s i o n , P h a s e I . T h e c h a n n e l v e l o c i t y w a s e s t i m a t e d u s i n g a n i t e r a d v e p r o c e s s w i t h t h e h y d r a u l i c m o d e l d i s c u s s e d b e l o w t o d e t e r m i n e a n a v e r a g e c h a n n e l v e l o c i t y . A n a v e r a g e c h a n n e l v e l o c i t y o f 5 . 7 4 f t / s w a s e s t u n a t e d a n d t h e t i m e o f t r a v e l f o r t h e c h a n n e l f l o w w a s c a l c u l a t e d t o b e 4 0 m i n u t e s ( 1 3 , 7 7 0 f t / ( 6 0 x 5 . 7 4 f t / s ) ) . T h e t o t a l t i m e o f c o n c e n t r a t i o n w a s t h e n c a l c u l a t e d a s t h e s u m o f t h e t r a v e l t i m e s f o r s h e e t f l o w , s h a l l o w c o n c e n t r a t e d f l o w , a n d c h a n n e l f l o w , w h i c h e q u a t e s t o a p p r o x i m a t e l y 8 8 . 5 m i n u t e s o r 1 . 4 8 h o u r s f o r t h e B a x t e r C r e e k d r a i n a g e a r e a . P : B O X _ 0 7 0 0 4 _ F l o o d _ I ] a 2 a r d _ I i v a l 2 ( 0 9 / 1 0 / 2 0 0 7 ) J D I l / s k w n n P E A K F L O W A N A L Y S I S T h e a b o v e d e s c r i b e d S C S h y d r o g i - a p h p a r a m e t e r s w e r e i n p u t i n t o t h e S C S T R - 2 0 c o m p u t e r p r o g r a m i n A u t o C A D t o g e n e r a t e a p e a k f l o w h y d r o g c a p h ( s e e a t t a c h e d o u t p u t ) . T h e 1 0 0 - y e a r , 2 4 - h o u r p e a k f l o w w a s e s t i m a t e d a t 2 2 3 c u b i c f e e t p e r s e c o n d ( c f s ) f o r B a x t e r C r e e k f o r t h e a b o v e d e s c r i b e d d r a i n a g e a r e a r e l a d v e t o d i e p r o p o s e d N o r t o n R a n c h S u b d i v i s i o n , P h a s e I . T h i s e s t i m a t e d p e a k f l o w w a s c o m p a r e d t o o t h e r m e t h o d s o f p e a k f l o w c a l c u l a t i o n t o v e r i f y t h e a c c u r a c y o f t h e r e s u l t s . T h e U n i t e d S t a t e s G e o l o g i c a l S u r v e y ( U S G S ) r e g r e s s i o n e q u a t i o n f o r t h e U p p e r Y e U o w s t o n e - C e n t r a l M o u n t a i n R e g i o n u s i t i g b a s i n a n d c l i n i a d c c h a r a c t e r i s t i c s e s t i m a t e d t h e 1 0 0 - y e a r p e a k f l o w t o b e a p p r o x i m a t e l y 1 9 3 - _ c f s . T h e r a t i o n a l m e d i o d c a l c i i l a t e d t h e 1 0 0 - y e a r p e a k f l o w t o b e a p p r o x i m a t e l y l _ 8 6 , c f s . T h e s e e s r i n i a t e d p e a k f l o w s w e r e a l s o c o m p a r e d t o t h e p e a k f l o w s e s t i m a t e d b y T h o m a s , D e a n , a n d H o s k i n s , I n c . ( T D & H ) f o r t h e B a x t e r C r e e k c u l v e r t c r o s s i n g a t H u f f i n e L a n e f o r t h e L o y a l G a r d e n S u b d i v i s i o n ( A p p e n d b r - S e c t i o n 8 , J u n e 2 0 , 2 0 0 6 ) . T h e c u l v e r t a t H u f f i n e L a n e w a s d e s i g n e d a s a 5 4 - i n c h d i a m e t e r c u l v e r t , a n d d i e 1 0 0 - y e a r w a t e r s u r f a c e e l e v a t i o n w a s c a l c u l a t e d a s 4 , 8 1 3 . 3 7 f e e t . B a s e d o n T D & H ' s c u l v e r t a n a l y s i s r e s u l t s , a 1 0 0 - y e a r p e a k f l o w o f a p p r o x i m a t e l y 1 1 0 c f s a t P I u f f i n e L a n e w a s d e t e r m i n e d . A p p l y i n g a s c a l e f a c t o r o f 1 . 2 t o a c c o u n t f o r t h e a d d i t i o n a l d r a i n a g e a r e a n o r t h o f H u f f i n e L a n e c o n t r i b u t i n g t o t h e p r o p o s e d N o r t o n R a n c h S u b d i v i s i o n , P h a s e I , a 1 0 0 - y e a r p e a k f l o w o f 1 3 2 c f s w a s e s t i m a t e d f o r t h e B a x t e r C r e e k d r a i n a g e a r e a u s i n g T D & H ' s c a l c u l a t i o n s . T h e r e f o r e , t h e p e a k f l o w o f 2 2 3 c f s u s i n g t h e S C S h y d r o g r a p h m e t h o d i s t h e h i g h e s t f l o w p r e d i c t e d b y s e v e r a l e s t u n a t e s f o r t h e 1 0 0 - y e a r p e a k o f B a x t e r C r e e k a n d w f f l b e u t i l i z e d f o r h y d r a u l i c m o d e l i n g o f B a x t e r C r e e k a s d i s c u s s e d b e l o w . H Y D R A U L I C M O D E L I N G T h e h y d r a u l i c m o d e l i n g o f B a x t e r C r e e k w a s p e r f o r m e d u s i n g H E C - R A S V e r s i o n 3 . 1 . 2 ( A p r i l 2 0 0 4 ) , t h e U n i t e d S t a t e s A r m y C o r p s o f E n g i n e e r s ( U S A G E ) r i v e r a n a l y s i s s o f t w a r e f o r o n e - d i m e n s i o n a l h y d r a u l i c m o d e U n g . C r o s s s e c t i o n d a t a f o r B a x t e r C r e e k c o n s i s t e d o f e i g h t s u r v e y e d c r o s s s e c t i o n s a n d f o u r i n t e r m e d i a t e i n t e r p o l a t e d c r o s s s e c t i o n s , w h i c h w e r e i n p u t i n t o H E C - R A S . T h e u p s t r e a m c r o s s s e c t i o n ( X S - 8 ) i s l o c a t e d a p p r o x i m a t e l y 3 8 0 f e e t s o u t h o f F a U o n S t r e e t a n d t h e d o w n s t r e a m c r o s s s e c t i o n ( X S - 1 ) i s l o c a t e d a p p r o x i m a t e l y 5 0 0 f e e t n o r t h o f W e s t B a b c o c k S t r e e t . T h e M a n n i n g ' s r o u g h n e s s c o e f f i c i e n t s ( " n " v a l u e s ) f o r B a x t e r C r e e k w e r e e s t i m a t e d b a s e d o n s i t e p i c t u r e s t a k e n o n J u l y 1 8 , 2 0 0 7 , a t e a c h c r o s s s e c t i o n a n d u s i n g T a b l e 5 - 6 o f C h o w ' s O p e n C h a n n e l H y d r a i i l i c s ( 1 9 5 9 ) b o o k . T h e " n " v a l u e s w e r e e s r i m a t e d t o b e 0 . 0 4 5 f o r t h e c h a n n e l a n d 0 . 0 5 f o r t h e o v e r b a n k s f o r c r o s s s e c t i o n s 4 t h r o u g h 8 . C r o s s s e c t i o n s 1 t l i r o u g h 3 i n c l u d e d s o m e t r e e s a l o n g d i e l e f t o v e r b a n k ( w e s t ) a n d a n " n " v a l u e o f 0 . 1 2 w a s e s t i m a t e d f o r t h e l e f t o v e r b a n k f o r c r o s s s e c t i o n s 1 t h r o u g h 3 , w h i l e t h e m a i n c h a n n e l a n d r i g h t o v e r b a n k m a i n t a i n e d " n " v a l u e s o f 0 . 0 4 5 a n d 0 . 0 5 r e s p e c t i v e l y . A s t e a d y s t a t e f l o w m o d e l w a s d e v e l o p e d i n H E C - R A S a f t e r i n p u t t i n g t h e a b o v e p a r a m e t e r s , a n d w a t e r s u r f a c e e l e v a t i o n s w e r e c o m p u t e d a t e a c h c r o s s s e c t i o n f o r t h e e s t i i n a t e d 1 0 0 - y e a r p e a k f l o w o f 2 2 3 c f s . T h e r e s u l t s o f t h i s h y d r a u l i c m o d e l a r e i n c l u d e d a s a n a t t a c h m e n t t o t h i s F l o o d H a z a r d E v a l u a t i o n . T h e w a t e r s u r f a c e e l e v a t i o n s w e r e m a p p e d o n p l a n v i e w a t e a c h c r o s s s e c t i o n l o c a t i o n a s s h o w n o n E x h i b i t 2 , t o s h o w t h e a p p r o x i m a t e f l o o d p l a i n b o u n d a r y o f t h e 1 0 0 - y e a r s t o r m i n P : B 0 7 _ 0 7 0 0 4 _ F I o o d _ J I a / . a r d _ R v ! i l 3 ( 0 9 / 1 0 / 2 0 n 7 ) J U l l / s k w n n r e l a t i o n t o t h e p r o p o s e d N o r t o n R a n c h S u b d i v i s i o n , P h a s e I . T h e r e s u l t s i n d i c a t e t h a t t h e 1 0 0 - y e a r f l o o d p l a i n b o u n d a r y o f B a x t e r C r e e k s h o u l d n o t i n u n d a t e l o t s w i t h i n t h e p r o p o s e d s u b d i v i s i o n . I t s h o u l d b e n o t e d t h a t a l t h o u g h t h e r e s u l t s o f t h e h y d r a u l i c m o d e l i n g d o n o t i n d i c a t e f l o o d i n g w i t h i n t h e p r o p o s e d N o r t o n R a n c h S u b d i v i s i o n , P h a s e I , f o r t h e 1 0 0 - y e a r p e a k f l o w f r o m B a x t e r C r e e k , d i e p o t e n t i a l s f c i U e x i s t s f o r s t o r m e v e n t s g r e a t e r t h a n t h e 1 0 0 - y e a r s t o r m . I t i s r e c o m m e n d e d t h a t h o m e s i t e s w i t h i n t h e p r o p o s e d s u b d i v i s i o n b e c o n s t r u c t e d w i t h a f i n i s h e d f l o o r e l e v a t i o n a m i n i m u m o f 1 8 i n c h e s a b o v e t h e t o p o f c u r b , w h i c h w i U p r o v i d e a n a d d i t i o n a l f a c t o r o f s a f e t y a g a i n s t f l o o d i n g . S U M M A R Y T h e r e s u l t s o u t l i n e d h e r e i n a n d t h e a t t a c h e d e x h i b i t s d e m o n s t r a t e t h a t t h e 1 0 0 - y e a r p e a k r u n o f f f r o m B a x t e r C r e e k u n d e r e x i s t i n g c o n d i t i o n s s h o u l d n o t a d v e r s e l y a f f e c t t h e p r o p o s e d N o r t o n R a n c h S u b d i v i s i o n , P h a s e I , b a s e d o n t h e c r i t e r i a o u t l i n e d m S e c t i o n 1 8 . 5 8 . 0 9 0 . B . 2 . C o f t h e C i t y o f B o 2 e m a n U D O . T h i s F l o o d H a 2 a r d E v a l u a d o n h a s b e e n p r e p a r e d f o r t h e p r o p o s e d N o r t o n R a n c h S u b d i v i s i o n , P h a s e I , a n d i s n o t i n t e n d e d f o r f u t u r e p l a n n i n g o f d e v e l o p m e n t s a l o n g B a x t e r C r e e k . A n y f u t u r e d e v e l o p m e n t s a l o n g B a x t e r C r e e k s h a U b e f u r t h e r a n a l y z e d t o e n s u r e p r o t e c t i o n a g a i n s t f l o o d i n g a n d s h a U b e r e q u i r e d t o h a v e t h e n e c e s s a r y d e t e n t i o n f a c i l i t i e s t o e n s u r e t h a t t h e 1 0 0 - y e a r p e a k f l o w o f B a x t e r C r e e k i s n o t i n c r e a s e d a s a r e s u l t o f a d d i t i o n a l i m p e n ^ i o u s a r e a . F u r t h e r m o r e , a n y o b s t r u c d o n s s u c h a s c u l v e r t s , s t r e e t e m b a n k m e n t s , e t c . , p l a c e d i n t h e d e l i n e a t e d f l o o d p l a i n o f B a x t e r C r e e k s h a l l b e a n a l y z e d t o e n s u r e t h a t t h e b a s e f l o o d e l e v a t i o n i s n o t i n c r e a s e d b y m o r e t h a n o n e h a l f f o o t a s r e q u i r e d i n C h a p t e r 1 8 . 5 8 ( F l o o d p l a i n R e g u l a t i o n s ) o f t h e C i t y o f B o 2 e m a n U D O . R E F E R E N C E S 1 . C h o w , V e n T e ( 1 9 5 9 ) . O p e n C h a n n e l H y d r a u l i c s . N e w Y o r k : M c G r a w - H i l l B o o k C o m p a n y , I n c . 2 . C i t y E n g i n e e r i n g D h n s i o a . ( 2 0 0 5 ) . D e s i g n S t a n d a r d s a n d S p e c i f i c a t i o n s P o l i c y . B o 2 e m a n , M T : A u t h o r . 3 . U i u t e d S t a t e s D e p a r t n i e n t o f A g r i c u l t u r e . N a t u r a l R e s o u r c e s C o n s e r v a t i o n S e r v i c e . C o n s e n ^ a r i o n E n g i n e e r i n g D i v i s i o n . ( 1 9 8 6 ) . U r b a n H y d r o l o g y f o r S m a U W a t e r s h e d s : T R - 5 5 . W a s h i n g t o n , D C : A u t h o r . P : B O Z _ ( ) 7 0 0 4 J ' l o o d _ I I a z a r c l _ E v a I 4 ( 0 9 / 1 0 / 2 0 0 7 ) J D I - I / s k w : 1 " - £ % f - \.~v. \ \ PQL< \ x, • r Km \ ^ Q \ s- o - I - h - I M ^ ^ / -\\-i ^ ( \ J ^ • '-—~1?=i ' Q 0 \ < - ^ i^--^^^. ^ ^ r ^ . ^ ! 1 r- ^ "*r^ \ » • " • r 4^e^M l i A \ ^ 0 J I M A T E D I T E I ^ C R E ^ [ 3 | R 4 L N A ^ . E A B t A I ! _...—l,_...l- » , 1 1 w IQ • t e . s ^ s % » f 7 a: / / Q ^ ^) \ 50 ^ y • > < \ . n g k . ^ K K . ' i n K < c \ s ^ c w . 0s < / Y M ' L sLU • ^ . 0 t ^ \ I LL ^ y ^ T C . \ I-K ^ ) •f?cbNs ^ - ^ ' 0 ^ \ . . - ' ^(~^\ / - ^ r ......nj r \ irr^ • f N W O O D R D C O T T C ^ » • b ' . • h - ^ Q B'. Q w a : \ ^ f e C - l m o r \ \ ' • 5 / 0 } 0 1 ^ \ > - v ( D ^ 3"; u ^s&» ^ = ) '.; \ \ L i J \ m m ^ w \ w < C D I C 4 ) & k <", 3 : x < C L L d //; i •I '\' li 'Vaw^i iti\ s % • • ysfe < ^ ^ ) U J 0 ^ - ! Q : L d ^ ( 7 ) < K \ u ..:.-. ^ ^ L ^ Q u e Q : c ^ . 1 c o < G ^ 3 2 < ^ ' s w < x ^ < w. L d Q : ^ s ^ ^ I m \ N Q f c D o n a ^ : u J 9^0 V ^ [ D 0 z Q i C D < 0 <:•q- - ^ - 1 1 • I Q : tVJ p " - CD \ \ " l a I . 00CO % L O 1 ^ , inr--. 5 5 ' C ^ J 1 0 <r 0 3 t £ ^ » s ^ ^ > 0 . A , O T \ • O T * G O O C H H } i C " ^ . . . — • • . • . ^ c o C D ^ fo- f ? ° < ^ - ^ ^ ^ \ I R D \ : Q : I < r X-. 0 x ^ " ^ • " • " ^ l i - " : . 1 ! ou ^ L d \ . < • ^ M . - 4 • * > « a m ( r l \ \ v , \ 4 k - ^ . - . — - ^ - - . . * . X ^ . - " - . X < ^ ^ ! ) 1 ' ^ ^ ^ - ^ J ? ^ s \ ^ \ ' A \ s? ' ' • » k\.. \ \® - ,---^^.,.''"" v~~"^?..-—^.=^':^..es ^ ^ \ • \ 3?--! ? . ? : C A D . O A I E : S / 1 0 / 0 7 K E V I S I O N S : _ J D H ^ - I : " » • \ \ ^ • s S f : ' < \ r ^ A P P R O V E D S T : '!>... ^ •^ O U A I I H A S S U R A N C E : — I ^ * . •; : T ^ • ' - . , ^ ' - t r r - ~ \ t & ^;^_.. ^ v & T J U F - - S S H O - H L [ : N O f ? T O N _ F L O O O _ E X H P R O J E C I N O . : B 0 2 . o 7 o o < S H E E 1 _ _ O F _ c ^ " y ^ • C T 0 y ? - — \ ) - i 300015001500 ! - \ . 7500 \ i \ . " \ J A ^ ^ J y " \ < » 1 ; ^ h - ^ / < \ . / / 1500'SCALE: 1 n X S - 1 w ^ ® 1 z ^ — - X S - 2 8 S @ , : X S ^ s ; r . . . : r : . . ; . . . ...,^ ...,. ,. .. ^...; _..._.. . . . . . . . , J , r-WES'T^ABGOG K - e ^ ^ • • w < : • ^ - f r . . ! ^ S ; » A K < c w ] _,, , . . B L O C K 1 2 ' BLOtK ).. - X S - 4 3 § s ^ ir- A P P R O X I M A T E 1 0 0 - Y E A R F L O O D P L A I N B O U N D A R Y O F B A X T E R C R E E K ~l!:- • ' • : ' • | ; y B : i i d e K - 4 - ^ : ^ W ! ! 1 • s - X S - 5 .,-£li.-irs.^ ^ j i ! iU-L.Sffi . ' . ifiIBTE E B l lip^.jif a ^ . l - X S - 5 . 5 I ! ' ' • V • • - B - f c e C K y " ; f l . S U R V E Y E D C R O S S S E C T I O N ; ( X S ) L O C A T I O N A N D N U M B E R ( T Y P . ) ^ Q ' a : < - 1 - 1 f t 1 ) I D O L X S - 6 1 0 : Q u i 0 0 a •*;.i m V ) 0 ^ < ..-»•"^~: ^ I L J Q - 0 C d Q : < 2 u - X S - 6 . 3 3 0 > ! , j ! { j [ B L O C K ! l | ^1' L O Q : 0 s - - 0 0 2 |[ B17X''K'; k 7 ^•L. .f ^... ^ ^ . . 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B a x t e r C r e e k D r a ~ i n a g e a r e a . . . . . . . . . . . . . . . . . . . 7 0 0 . 0 0 0 0 a c R u n o f f c u r v e n u m b e r , C N . . . . . . . . . 7 5 T i m e o f c o n c e n t r a t i o n , T c . . . . . . . 8 8 . 5 0 0 0 m i n D l m e n s i o n l e s s H y d r o g r a p h . . . . . . . . s c s d - i m R a i n f a l 1 . . . . . . . . . . . . . . . . . . . . . . . . 2 . 8 8 0 0 i n D i s t r i b u t i o n C u r v e . . . . . . . . . . . . . . t r 2 0 t 2 : T y p e 2 , 2 4 h r s D u r a t i o n . . . . . . . . . . . . . . . . . . . . . . . . 1 4 4 0 . 0 0 0 0 m i n A n t e c e n d e n t M o i s t u r e C o n d i t i o n . . T y p e I I T i m e i n c r e m e n t , T p . . . . . . . . . . . . . . 6 . 0 0 0 0 m - i n C o m p u t e d R e s u l t s : P e a k d i s c h a r g e , q p . . . . . . . . . . . . . . 2 2 2 . 4 1 9 1 c f s P e a k T i m e , T p . . . . . . . . . . . . . . . . . . . 7 7 6 . 2 3 3 2 m i n P e a k r a t e f a c t o r . . . . . . . . . . . . . . . . 4 8 4 C o n s t a n t , K . . . . . . . . . . . . . . . . . . . . . 0 . 7 5 0 0 R u n o f f V o l u m e . . . . . . . . . . . . . . . . . . . 0 . 8 8 3 0 i " n 6 2 3 . 2 9 0 4 c f s - h r s 5 1 . 5 0 8 7 a c f t P a g e 1 0 n B a x t e r C r e e k S C S H y d r o g r a p h P e a k F l o w 2 2 2 . 4 1 9 1 c f s - T i m e 7 7 6 . 2 5 5 2 m i n 2 2 2 . 0 1 1 7 7 . 6 1 h ^ u < u - 1 - 1 0 I - ? 0 1 3 3 . 2 1 8 8 . 8 1 h 4 4 . 4 1 h 0 . 0 1 6 6 6 . 0 0 I I 8 7 4 . 8 0 1 0 8 5 . 6 0 1 2 9 2 . 4 0 1 5 0 1 . 2 0 1 7 1 0 . 0 0 T i m e - m i n 0 0 ? ? u . L L s a . Q - Q - g > § ^ c o s 5 2 u % § 0 L U 8 s \ \ \ § ^ ^ \ v • T O c § \ ^ 1 0 u g ^ c 5 S . s C t : u c £ S 3 S N ^ 0 ) Q 5 ^ ^ - < B z § E \ l i t C L 5 - t : ( D ^ 0 K 2 c 1 U u \ ^ Y S ) £ £ 8 v 0 m m ^ ! ^ \ ' ' 0 ^ s s 0 s s ? ^ ? ? ^ & • i t ( U ) u o i i e A 9 | 3 n 0 c D i ^ l i ^ i T l n i - t r i - ^ l r - j l c o l ( D ] S . I ( p i t ^ | c O | r > . i ( D i ( D d j o j o | 5 ' o j o j o | 5 j 5 | o | 3 ! o u » ^ 2 I — ^ ! n | o • c r ! i ~ - f c ? s C T ) g r ~ ? . l 1 0 • f l - I 0 " I c o ; c o o o ! o o ! ^ d c o i ? s s s I ? ) s ( ^ 0 g & c n g g S I S 8 g c o K i ^ t > ~ ' < 3 - C O r > - c d : i c r i ( D o i 1 ^ w r i & ^ s 3 S I S 3 s 1 0 C D U - l C D 1 ^ - ( D < t 0 - s • s E o j ^ l s i n i c o ! i r i s s c o i § i K ! § 0 0 1 I ~ ~ . • < t C N I • E i r i u - i 1 0 c d i n 1 0 s c £ • 5 > n l S ! § s i s ^ I S l i t I s ~ - § S ! F : s a 1 0 s s s . s i s C M I 1 0 • ^ 5 0 0 O l i C M s ^ I s ^ - 1 § ° ° 0 y ~ ° ! q 0 0 ° £ 0 d i d | d 0 0 0 0 0 0 0 0 C 3 U J i s s s § s f e 3 ; g f e s T ~ c o r > - • f l : c d 0 4 d r o 8 C N g g l s U J § S i g 0 g 9 ? ! ? ? ? 0 0 ? ! ^ ^ 1 • d - ^ f ! • y • q - 0 L U r - T ^ — i - { - _ r ^ T ? s I c o f e c o 5 T - [ 1 0 i n M C M c d c i ^ g & s § ? ? i £ ? ^ ^ 0 0 • » • ^ T • f l - • • E 0 T - o - 1 s n s ; s n s n r M 0 F : n 0 r ~ < ° 0 ) c ° c ° - 1 5 c d n 0 1 c d C D ^ s § s s § s s s u . U J 5 £ 9 ? 9 ! ? 9 9 ! ? ; D . S l " t • d - i • < t • » f ^ § s C L w , S I S j ^ i s i f e 3 f e 5 s f e § s ^ " U J 1 0 n N 0 ) d c o C M s s g X : g g § s ^ u 0 0 $ ? ? ? ^ • < f • ^ • t ! • ^ • ^ s . s 2 Q : € § § s § § S l g § § § § § s ' I n 2 r i r i c o r i r i n s s a a s Q : 9 C M r j C M S ! ^ ! r s j • § 1 ° M N N M C M C M C M c s 0 t t : L U ^ C C L : s s £ a . i ^ l y - 1 o - i a . 1 a - i a . 1 a . y . 1 y - [ y . 1 y . | y . a - l ( L I a - 1 D - l Q - s s a . ' S \ 3 - c w s a s s ! o h - I S s : i d ( D 1 0 0 0 h ~ ( 0 u i l t l n i C M a : w ^ T T V V T 7 < ^ £ £ ^ ^ € 5 ° ? s ^ ^ u u s s s B : 0 ( 0 s s s ( 0 g a > < u < u ( 2 L U a : a : ( K Q ; Q : a : 1 £ C K Q : 0 : 1 0 : 3 : 0 0 B a x t e r C r e e k _ N o r t o n R a n c h F i n a l R S = 8 F l o w = 2 2 3 c f s 0 5 ? t ^ - . 0 4 5 ^ ^ » . 0 5 4 8 2 2 - 1 4 8 2 1 - 1 4 8 2 0 - 1 g 4 8 1 9 s < D 4 8 1 8 ^ s - n ; ^ s 4 8 1 7 - 1 4 8 1 6 - 1 4 8 1 5 2 0 4 0 6 0 8 0 1 0 0 0 S t a t i o n ( f t ) B a x t e r C r e e k N o r t o n R a n c h F i n a l R S = 7 . 5 * F l o w = 2 2 3 c f s 4 8 1 3 0 1 0 0 S t a t i o n ( f t ) B a x t e r C r e e k N o r t o n R a n c h F i n a l R S = 7 F l o w = 2 2 3 c f s - ^ . 0 4 5 - ^ — . 0 5 - L e g e n d E G P F 1 W S P F 1 - » • G r o u n d B a n k S t a K . 0 5 i ^ - . 0 4 5 ^ . 0 5 ? 4 8 1 8 - 1 4 8 1 7 - 1 g 4 8 1 6 - 1 s % s 4 8 1 5 - 1 U J 4 8 1 4 - 1 2 0 4 0 6 0 8 0 L e g e n d E G P F 1 W S P F 1 G r o u n d B a n k S t a 1 2 0 0 5 4 8 1 6 . 0 - 1 L e g e n d 4 8 1 5 . 5 - j E G P F 1 W S P F 1 4 8 1 5 . 0 - 1 g C r i t P F 1 4 8 1 4 . 5 - 1 i > : g % G r o u n d B a n k S t a f t T S 4 8 1 4 . 0 d E i % T ? 4 8 1 3 . 5 d 4 8 1 3 . 0 ^ 4 8 1 2 . 5 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 0 S t a t i o n ( f t ) 0 n 4 8 1 3 . 5 - ] 4 8 1 3 . 0 - ) 4 8 1 2 . 5 - 1 g 4 8 1 2 . 0 - 1 • I 4 8 1 1 . 5 ^ i 5 4 8 1 1 . 0 - 1 4 8 1 0 . 5 - 1 4 8 1 0 . 0 - 1 4 8 0 9 . 5 - 1 0 0 4 8 1 1 . 5 - ) 4 8 1 1 . 0 - 1 4 8 1 0 . 5 - 1 g . 4 8 1 0 . 0 - j g 4 8 0 9 . 5 - j g 4 8 0 9 . 0 - j 4 8 0 8 . 5 - ) 4 8 0 8 . 0 - 1 4 8 0 7 . 5 - 1 4 8 0 7 . 0 - - 2 0 0 4 8 0 4 - 3 0 0 B a x t e r C r e e k . N o r t o n R a n c h F i n a l R S = 6 . 6 6 6 6 6 * F l o w = 2 2 3 c f s 0 5 . 0 5 0 4 5 - 5 0 T 0 T 5 0 T 1 0 0 S t a t i o n ( f t ) B a x t e r C r e e k N o r t o n R a n c h F i n a l R S = 6 . 3 3 3 3 3 * F l o w = 2 2 3 c f s - . 0 5 - . 0 5 ) 0 4 5 . . . • " T T - 1 5 0 T 0 - 1 0 0 - 5 0 0 5 0 S t a t i o n ( f t ) B a x t e r C r e e k N o r t o n R a n c h F i n a l R S = 6 F l o w = 2 2 3 c f s - . 0 5 - T 1 0 0 - 2 0 0 - 1 0 0 1 0 0 S t a t i o n ( f t ) L e g e n d E G P F 1 W S P F 1 C r i t P F 1 G r o u n d • B a n k S t a ^ 1 5 0 L e g e n d E G P F 1 W S P F 1 C r i t P F 1 G r o u n d B a n k S t a ^ 1 5 0 . 0 5 4 8 0 9 - ] L e g e n d 4 5 E G P F 1 4 8 0 8 - 1 i W S P F 1 s ' - e G r i t P F 1 4 8 0 7 - 1 § % G r o u n d B a n k S t a s 4 8 0 6 - 1 U J 4 8 0 5 - 1 0 1 2 0 0 0 n 4 8 0 8 - ] 4 8 0 7 - 1 4 8 0 6 H I 4 8 0 5 | I 1 s 4 8 0 4 ^ 4 8 0 3 - 1 < - 4 8 0 2 - - 1 5 0 B a x t e r C r e e k N o r t o n R a n c h F i n a l R S = 5 . 5 * F l o w = 2 2 3 c f s . 0 5 - 0 0 0 4 5 - — . - — - ^ ^ - T - 1 0 0 T - 5 0 T 0 5 0 T 1 0 0 S t a t i o n ( f t ) B a x t e r C r e e k N o r t o n R a n c h F i n a l R S = 5 F l o w = 2 2 3 c f s - * f ^ . 0 4 5 - ^ 2 0 1 6 0 S t a t i o n ( f t ) B a x t e r C r e e k N o r t o n R a n c h F i n a l R S = 4 F l o w = 2 2 3 c f s - ^ . 0 4 5 ^ 5 0 1 0 0 S t a t i o n ( f t ) L e g e n d E G P F 1 W S P F 1 G r o u n d B a n k S t a - I 1 5 0 . 0 5 . 0 5 4 8 0 7 - 1 L e g e n d 4 8 0 6 - 1 E G P F 1 4 8 0 5 1 W S P F 1 e G r o u n d • B a n k S t a 4 8 0 4 - 1 g % i 4 8 0 3 - j U J 4 8 0 2 - 1 4 8 0 1 - 1 4 8 0 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 8 0 0 5 . 0 5 4 8 0 4 H L e g e n d 4 8 0 3 E G P F 1 W S P F 1 4 8 0 2 - 1 C r i t P F 1 s • S 4 8 0 1 - 1 G r o u n d B a n k S t a ( 0 s i 5 4 8 0 0 - 1 4 7 9 9 - 1 4 7 9 8 1 5 0 2 0 0 0 0 I 0 0 0 B a x t e r C r e e k N o r t o n R a n c h F i n a l R S = 3 F l o w = 2 2 3 c f s . 1 2 » < . 0 5 4 8 0 2 - 1 0 4 5 4 8 0 1 - 1 4 8 0 0 - 1 g § f 4 7 9 9 ^ s t i r E D m 4 7 9 8 - 1 . ^ - ' 1 4 7 9 7 - 1 4 7 9 6 4 0 6 0 8 0 1 0 0 L e g e n d E G P F 1 W S P F 1 G r i t P F 1 G r o u n d - & - L e v e e B a n k S t a 2 0 1 2 0 1 4 0 1 6 0 S t a t i o n ( f t ) B a x t e r C r e e k _ N o r t o n R a n c h F i n a l R S = 2 F l o w = 2 2 3 c f s < 1 2 < . 0 5 i 4 7 9 9 - 1 0 L e g e n d 4 5 4 7 9 8 - 1 E G P F 1 W S P F 1 4 7 9 7 d g G r o u n d B a n k S t a 4 7 9 6 - 1 g • • s 4 7 9 5 ^ s i u 4 7 9 4 - 1 4 7 9 3 - 1 4 7 9 2 5 0 1 0 0 1 5 0 1 2 0 0 2 5 0 . 1 2 - S t a t i o n ( f t ) B a x t e r C r e e k N o r t o n R a n c h F i n a l R S = 1 F l o w = 2 2 3 c f s ^ - . 0 4 5 - ^ — . 0 5 - ^ 4 7 9 5 - 1 L e g e n d 4 7 9 4 ^ E G P F 1 W S P F 1 4 7 9 3 - 1 g C r i t P F 1 g 4 7 9 2 - 1 r o G r o u n d • B a n k S t a a s f f l 4 7 9 1 - 1 • s * : y * 4 7 9 0 - 1 4 7 8 9 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 1 6 0 1 8 0 S t a t i o n ( f t ) APPENDIX G 2009 NORTON EAST RANCH SUBDIVISION PHASE 1 STORMWATER MANAGEMENT DESIGN REPORT (SANDERSON STEWART)