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Home My WebLink About022 - Appendix J - Flood Hazard Evaluation FERGUSON FARM II - MAYNARD BORDER DITCH – FLOOD HAZARD EVALUATION January 2022 #170827 1 TABLE OF CONTENTS REPORT Introduction ..........................................................................................................................1 Hydrology ............................................................................................................................3 Hydraulics ............................................................................................................................5 Results ..................................................................................................................................9 APPENDICES Appendix A: Drainage Area Map Appendix B: Drainage Area Calculations Appendix C: HEC-RAS Geometry Maps Appendix D: HEC-RAS Output Files FERGUSON FARM II - MAYNARD BORDER DITCH – FLOOD HAZARD EVALUATION January 2022 #170827 2 INTRODUCTION The Ferguson Farm II Subdivision is a proposed 31-acre development that is located in the southwestern portion of the City of Bozeman in Gallatin County Montana. The proposed subdivision consists of 58 commercial lots and 14 open space lots and associated City water, sanitary sewer, stormwater, and road infrastructure improvements. Figure 1 below shows the subject property. Figure 1: Project Location Map The existing property is undeveloped Lot 5 of Minor Subdivision No. 295. The Maynard Border Ditch (MBD) runs north through the west side of existing Lot 5. The ditch is fed from a head gate at the confluence with the Farmers Canal, located approximately 1,900 feet south of the property. The headgate allows water to divert from Farmers Canal into the MBD. As such, the flows through the MBD are determined by the headgate and during high-flow events can be shut off. Downstream of the headgate is a Parshall flume, which is used by the canal company to determine the flow that is diverted into the MBD. The canal company typically leaves the gate operating at approximately 300-400 miner’s inches (7.5 cubic feet per second (cfs) – 10 cfs). The maximum possible flow is 752 miner’s inches (18.8 cfs). After the headgate, the MBD runs north through an agricultural lot to a 36-inch Corrugated Metal Pipe (CMP) culvert beneath U.S. Highway 191 (Huffine Lane). The culvert outlets into the portion of the MBD located on the subject property. The MBD then runs north, into a 32-inch PROJECT LOCATION FERGUSON FARM II - MAYNARD BORDER DITCH – FLOOD HAZARD EVALUATION January 2022 #170827 3 x 55-inch CMP culvert beneath an existing walking trail. It then runs 1,200 linear feet through the subject property and crosses beneath Fallon Street through a 32-inch by 55-inch concrete arch pipe culvert. Figure 2 below shows the MBD path from the confluence with the Farmers Canal through the property to the culvert beneath Fallon Street. Although not “naturally occurring”, this report addresses the estimated peak water surface elevations (WSEL) associated with the existing Maynard Border Ditch and the proposed realignment. The purpose of this report is to demonstrate that the anticipated WSELs under the existing and proposed conditions will not inundate existing infrastructure or the proposed development. Figure 2: Maynard Border Ditch Path HYDROLOGY There were two peak flows used to assess the MBD capacity in this analysis. A 2-dimensional hydraulic model of the MBD and Farmer’s Canal divergence was created in an effort to estimate the maximum possible flow from the Farmer’s Canal. The model assumed that the Farmer’s Canal was flowing at bank full levels and that the MBD headgate was removed. U.S. Army Corps. of Engineers (US ACoE) hydraulic modeling computer software HEC-RAS (version 6.0.0) was used to assess the divergence. The 2D HEC-RAS model map begins approximately 100-feet upstream of the MBD divergence and follows the MBD approximately 100-feet MBD HEADGATE DIVERGENCE HUFFINE LANE CROSSING FALLON STREET CROSSING FERGUSON FARM II - MAYNARD BORDER DITCH – FLOOD HAZARD EVALUATION January 2022 #170827 4 downstream. The model used City of Bozeman LiDAR data for elevation information. The LiDAR surface was modified to remove and replace the MBD headgate with a straight graded channel. The upstream boundary condition was set to a stage hydrograph to model the bank full water surface elevation. The downstream boundary conditions were set to normal depth (MBD Slope = 0.0364, Farmer’s Canal Slope =0.006) based on elevations obtained from LiDAR data. See Figure 3 below for the geometry of the divergence model. The model produced an estimated flow through the MBD of 26.34 cfs. Figure 3: Maynard Border Ditch -Farmer’s Canal Divergence Geometry In addition to the flow provided from the Farmers Canal, there is a small contributing area south of the Huffine Lane culvert crossing. The contributing area was determined to be approximately 39.68 acres, of which 23.91 acres were determined to be medium-density residential, 14.28 acres were open land, and 1.48 acres were water. This yielded a composite C of 0.32 for the contributing area, the limits of the drainage area can be seen on the map provided in Appendix A. A time of concentration (TOC) path was developed for the drainage area. Elevation information derived from City of Bozeman LiDAR data was utilized for estimating flow path slopes. The longest flow path was determined to begin at the southern end of the Kountz Trailer Park, flow north through the trailer park and into a ditch along Huffine Lane. The path then flows from the ditch into the MBD. The TOC for the drainage area was determined to be 21.7 minutes. Time of concentration calculations are included in Appendix B. Per the City of Bozeman Design Standards and Specification Policy (2017) the rational method was used to estimate the runoff generated from the contributing area. In addition to the runoff parameters discussed previously, the 100-year rainfall intensity-duration curve was used to MODIFIED TERRAIN DATA (MBD HEADGATE REMOVED) UPSTREAM STAGE HYDROGRAPH BC DOWNSTREAM NORMAL SLOPE BC (SLOPE = 0.006) DOWNSTREAM NORMAL SLOPE BC (SLOPE = 0.0364) FERGUSON FARM II - MAYNARD BORDER DITCH – FLOOD HAZARD EVALUATION January 2022 #170827 5 calculate the peak flow. The peak runoff value was determined to be 25.03 cfs. Drainage area calculations are included in Appendix B. The flow from the Farmers Canal gate (26.34 cfs) was combined with the 100-year peak flow calculated for the contributing area (25.03 cfs) for use in the hydraulic analysis. Because the flow from Farmers Canal is operating during the ‘dry-season’, it can be reasonably assumed that the 100-year rainfall event will likely occur during lower operating flows. It should also be noted that during any high flow event, the gate can be shut off and eliminate any contributing flow from the Farmers Canal. As such, using 51.37 cubic feet per second as a design flow can be considered a conservative value for assessing the MBD hydraulics. The existing channel has a berm along the banks which prevents any additional runoff from the subject site from entering the MBD. The proposed channel will have a 12-inch berm along both banks, in order to mimic the existing channel cross section. Proposed site topography drains to the northern and western boundaries of the site, and does not require the use of sag curves in the road profile at the ditch crossings. As such, a majority of the 100-year storm runoff from the property will drain to existing city infrastructure in Fallon Street and Resort Drive. There is an approximately 80’ wide corridor around the realigned ditch that is expected to drain into the MBD. The area includes open spaces, sidewalk, and some roadway along the ditch corridor. The total area draining to the MBD was found to be 2.20 acres with a composite C factor of 0.53. This produced an estimated runoff value from the developments of 6.17 cfs. This runoff value was added to the hydraulic model downstream of the Huffine Lane outlet. HYDRAULICS The proposed development plans to relocate the MBD approximately 20 feet east from its present location, with transitions at the north and south ends. There are also 6 culvert crossings planned which are for 2 streets, an alley, parking lot and 2 walking trails. In addition to the culvert crossings there are 5 proposed pedestrian bridges. HEC-RAS (version 6.0.0) was used to assess the existing conditions and proposed realignment. Physical parameters for the crossings, upstream and downstream reaches, and computed peak discharges are input into the program to develop a number of hydraulic variables associated with the crossings. It was assumed that the hydraulic opening for all of the crossings were operating unobstructed. The geometry for the existing and proposed alternatives were based on a combination of the field survey and LiDAR data. A 2D HEC-RAS model was selected for use in this analysis. The 2D model requires users to create a computational mesh over the model area, in which cells are assigned elevation and manning’s roughness values based on terrain and land cover data. The mesh spacing was reduced in areas where more detail was desired, specifically at the crossing structures and within the stream channel. HEC-RAS will route a design flow through the mesh using physically derived equations selected by the user. The more detailed full momentum (Saint-Venant) equation was selected for use in the 2D analysis, in order to capture changes in momentum (energy losses) associated with constriction of flow through the culvert crossings as well as changes in the flow characteristics during high-flow scenarios. FERGUSON FARM II - MAYNARD BORDER DITCH – FLOOD HAZARD EVALUATION January 2022 #170827 6 The 2D HEC-RAS model map begins approximately 350-feet upstream of the Huffine Lane crossing structure and follows the channel north through the confluence of Fallon Street and Slough Creek Drive crossings. The model terminates approximately 350-feet downstream of the Slough Creek Drive crossing. The upstream boundary condition was set to a synthetic hydrograph that utilized the peak values obtained from the hydrologic analysis. The hydrographs linearly increased for 2-hours to the peak runoff value, the peak value was held constant for 1- hour, then decreased linearly for 2-hours back down to zero. The downstream boundary condition was set to normal depth (Slope = 0.0129) based on elevations obtained from LiDAR data. Appendix C includes diagrams of the existing and proposed condition geometry, identifying the boundary conditions, meshes spacing, and crossing locations associated with each model. Manning’s “n” coefficients have a significant effect on the hydraulic computations in the software as they are used to represent the energy loss through the channel and overbanks, based primarily on surface roughness. The “n” values for the channel were selected from Open Channel Hydraulics (Chow, 1959). The channel “n” value was determined to be 0.03 based on orthoimagery and site observations. The overbank and surrounding areas “n” values were also determined using the same methods. The existing crossings beneath Huffine Lane, Fallon Street, and Slough Creek Drive are summarized in Tables 1, 2, and 3 respectively. The geometric data is based on a combination of City of Bozeman GIS information and field survey data. These three culverts remained unchanged in the proposed conditions model. Table 4 summarizes the existing culvert beneath the walking path on the subject property, this culvert was revised in the proposed conditions model as discussed in the next section. Table 1: Huffine Lane Crossing Geometry Stream Crossing Structure Type Circular CMP Diameter (ft) 3 Hydraulic Opening (sq. ft.) 7.1 Length Parallel to Flow (ft.) 121.5 Upstream Invert Elev. (ft.) 4841.32 Downstream Invert Elev. (ft.) 4840.55 Table 2: Fallon Street Crossing Geometry Stream Crossing Structure Type RCP Arch Pipe Culvert Span (ft.) 4.83 Rise (ft.) 3 Hydraulic Opening (sq. ft.) 11.1 Length Parallel to Flow (ft.) 103 Upstream Invert Elev. (ft.) 4822.27 Downstream Invert Elev. (ft.) 4821.74 FERGUSON FARM II - MAYNARD BORDER DITCH – FLOOD HAZARD EVALUATION January 2022 #170827 7 Table 3: Slough Creek Drive Crossing Geometry Stream Crossing Structure Type RCP Arch Pipe Culvert Span (ft.) 4.83 Rise (ft.) 3 Hydraulic Opening (sq. ft.) 11.1 Length Parallel to Flow (ft.) 67.4 Upstream Invert Elev. (ft.) 4820.26 Downstream Invert Elev. (ft.) 4819.64 Table 4: Walking Trail Crossing Geometry Stream Crossing Structure Type RCP Arch Pipe Culvert Span (ft.) 3.27 Rise (ft.) 2.7 Hydraulic Opening (sq. ft.) 11.1 Length Parallel to Flow (ft.) 28.2 Upstream Invert Elev. (ft.) 4839.68 Downstream Invert Elev. (ft.) 4839.20 In addition to the road and parking area crossings there are five proposed pedestrian bridge crossings. The bridge crossings are in the conceptual design phase, as such, proposed span and heights are unknown at this time. To assess the proposed conditions hydraulics, two proposed models were created. The first model assumed no encroachment from the bridges; the bridge abutments were located outside of the channel. Tables 5-10 summarize the geometries used in the no encroachment version of the proposed conditions model. A figure detailing the layout of the proposed model crossings is included in Appendix C. Table 5: Proposed Walking Trail 1 Crossing Geometry Stream Crossing Structure Type Concrete Box Culvert Span (ft.) 6 Rise (ft.) 2 Hydraulic Opening (sq. ft.) 12 Length Parallel to Flow (ft.) 37 Estimated Slope (ft./ft.) 0.0143 Table 6: Proposed Alley Crossing Geometry Stream Crossing Structure Type Concrete Box Culvert Span (ft.) 6 Rise (ft.) 3 Hydraulic Opening (sq. ft.) 18 Length Parallel to Flow (ft.) 75 Estimated Slope (ft./ft.) 0.0092 FERGUSON FARM II - MAYNARD BORDER DITCH – FLOOD HAZARD EVALUATION January 2022 #170827 8 Table 7: Proposed Valley Commons Drive Crossing Geometry Stream Crossing Structure Type Concrete Box Culvert Span (ft.) 6 Rise (ft.) 3 Hydraulic Opening (sq. ft.) 18 Length Parallel to Flow (ft.) 65.7 Estimated Slope (ft./ft.) 0.0139 Table 8: Proposed Field Street Crossing Geometry Stream Crossing Structure Type Concrete Box Culvert Span (ft.) 6 Rise (ft.) 3 Hydraulic Opening (sq. ft.) 18 Length Parallel to Flow (ft.) 69.7 Estimated Slope (ft./ft.) 0.0131 Table 9: Proposed Parking Area Crossing Geometry Stream Crossing Structure Type Concrete Box Culvert Span (ft.) 6 Rise (ft.) 3 Hydraulic Opening (sq. ft.) 18 Length Parallel to Flow (ft.) 61.9 Estimated Slope (ft./ft.) 0.0120 Table 10: Proposed Walking Trail 2 Crossing Geometry Stream Crossing Structure Type Concrete Box Culvert Span (ft.) 6 Rise (ft.) 3 Hydraulic Opening (sq. ft.) 18 Length Parallel to Flow (ft.) 19.3 Estimated Slope (ft./ft.) 0.0218 The second proposed conditions model accounted for a 6-foot-wide span in the pedestrian bridges. The bridges were modeled using the culvert routine and assumed a 6-foot by 4-foot box culvert to match the anticipated hydraulic opening. The inverts of the crossing were set to match channel elevations in the model. It should be noted that because there was approximately 2-3 feet of freeboard available in the culvert during the hydraulic analysis a larger rise in the bridge would likely have no effect on the ditch’s hydraulic capacity. A figure showing the layout of the encroachment in the proposed conditions model is included in Appendix C. FERGUSON FARM II - MAYNARD BORDER DITCH – FLOOD HAZARD EVALUATION January 2022 #170827 9 RESULTS Tables 11-13 summarize the culvert hydraulics observed in the existing and proposed condition models. The proposed and existing ditch layouts were assessed on their ability to convey the calculated peak flows. The results from the hydraulic analysis are outlined in Table 16. The cross-sections referenced in the table correspond to those depicted on the geometry layouts included in Appendix C. Additional output information from HEC-RAS for the existing and proposed conditions can be found in Appendix D. Table 11: Existing Model Culvert Hydraulic Summary Description Huffine Lane Crossing Walking Trail 1 Crossing Fallon Street Crossing Slough Creek Drive Crossing Upstream Water Surface Elevation at Opening (ft.) 4844.81 4842.09 4824.55 4822.84 Headwater (ft.) 3.49 2.41 2.35 2.46 Embankment Low Point Elevation (ft.) 4846.19 4841.82* 4825.73 4823.85 Velocity In Culvert (ft/s) 3.43 2.65 2.25 1.91 *Existing crossing is a crest curve, the observed low point was beyond the headwater extents. Table 12A: Proposed Model with No Bridge Encroachment Culvert Hydraulic Summary Description Huffine Lane Crossing Walking Trail 1 Crossing Alley Crossing Valley Commons Drive Crossing Upstream Water Surface Elevation at Opening (ft.) 4844.79 4841.58 4839.59 4834.65 Headwater (ft.) 3.47 2.11 1.93 2.11 Embankment Low Point Elevation (ft.) 4846.19 4841.81 4841.34 4836.04 Velocity In Culvert (ft/s) 2.81 2.61 1.50 1.37 FERGUSON FARM II - MAYNARD BORDER DITCH – FLOOD HAZARD EVALUATION January 2022 #170827 10 Table 12B: Proposed Model with No Bridge Encroachment Culvert Hydraulic Summary Description Field Street Crossing Parking Area Crossing Walking Trail 2 Crossing Fallon Street Crossing Slough Creek Drive Crossing Upstream Water Surface Elevation at Opening (ft.) 4829.57 4826.18 4825.38 4824.97 4822.94 Headwater (ft.) 2.22 2.27 2.61 3.10 2.56 Embankment Low Point Elevation (ft.) 4830.66 4827.42 4826.57 4825.73 4823.85 Velocity In Culvert (ft/s) 2.09 1.90 1.37 2.14 1.82 Table 13A: Proposed Model with Bridge Encroachment Culvert Hydraulic Summary Description Huffine Lane Crossing Walking Trail 1 Crossing Alley Crossing Valley Commons Drive Crossing Upstream Water Surface Elevation at Opening (ft.) 4844.79 4841.58 4839.57 4834.65 Headwater (ft.) 3.47 2.11 1.91 2.11 Embankment Low Point Elevation (ft.) 4846.19 4841.81 4841.34 4836.04 Velocity In Culvert (ft/s) 2.81 2.61 1.24 1.33 Table 13B: Proposed Model with Bridge Encroachment Culvert Hydraulic Summary Description Field Street Crossing Parking Area Crossing Walking Trail 2 Crossing Fallon Street Crossing Slough Creek Drive Crossing Upstream Water Surface Elevation at Opening (ft.) 4829.52 4826.18 4825.26 4824.77 4822.94 Headwater (ft.) 2.17 2.27 2.49 2.56 2.56 Embankment Low Point Elevation (ft.) 4830.66 4827.42 4826.57 4825.73 4823.85 Velocity In Culvert (ft/s) 2.51 1.67 1.39 2.15 1.82 Based on these results and those presented in Table 16 below, it is anticipated that the proposed channel realignment will not adversely affect the observed WSELs within the Maynard Border Ditch. The results also suggest that the existing and proposed channels, as well as proposed culvert crossings, provide sufficient capacity to convey the design storm flows discussed earlier FERGUSON FARM II - MAYNARD BORDER DITCH – FLOOD HAZARD EVALUATION January 2022 #170827 11 in this report. Additionally, it is anticipated that the encroachment of proposed bridge abutments within the channel side slopes will cause a negligible effect on the WSELs within the MBD. Table 16: Channel Hydraulic Analysis Summary Description EC WSE (1) (ft) PC1 WSE (2) (ft) PC2 WSE (3) (ft) EC WV (4) (ft/s) PC 1 WV (5) (ft/s) PC 2 WV (6) (ft/s) XS 1 4845.43 4845.51 4845.51 3.041 2.755 2.754 XS 2 4844.82 4844.82 4844.82 2.307 2.153 2.152 XS 3 4844.81 4844.79 4844.79 2.915 2.204 2.204 XS 4 4842.13 4841.81 4841.81 3.265 3.748 3.749 XS 5 4842.09 4841.58 4841.58 1.862 2.497 2.497 XS 6 4840.32 4840.00 4840.00 3.340 3.327 3.340 XS 7 4839.37 4839.59 4839.57 3.349 1.343 1.356 XS 8 4838.19 4838.00 4838.21 3.672 2.926 2.423 XS 9 4837.88 4837.09 4837.92 3.727 4.233 0.858 XS 10 4837.68 4836.79 4837.24 4.385 4.175 2.347 XS 11 4836.22 4835.79 4836.86 4.653 4.254 0.844 XS 12 4835.86 4835.52 4835.85 4.882 4.210 2.773 XS 13 4834.88 4834.83 4835.61 4.236 3.153 1.750 XS 14 4834.53 4834.71 4834.87 3.802 2.913 1.934 XS 15 4833.98 4834.65 4834.65 4.540 2.016 1.997 XS 16 4833.22 4832.79 4832.84 3.790 3.318 3.170 XS 17 4832.20 4831.44 4832.32 3.945 4.238 2.176 XS 18 4831.94 4831.18 4831.76 4.539 4.398 2.775 XS 19 4831.29 4830.86 4831.53 4.074 4.239 1.820 XS 20 4831.23 4830.45 4830.78 2.977 4.335 3.090 XS 21 4830.03 4829.88 4830.17 3.409 3.373 2.506 XS 22 4829.80 4829.60 4829.69 3.356 2.641 1.879 XS 23 4829.33 4829.57 4829.52 4.230 2.447 1.046 XS 24 4828.16 4827.71 4827.71 4.852 3.672 3.554 XS 25 4826.68 4826.18 4826.18 3.380 2.526 2.528 XS 26 4824.98 4825.32 4825.19 4.477 2.109 2.128 XS 27 N/A(7) 4825.38 4825.26 N/A(7) 1.924 2.024 XS 28 N/A(7) 4825.02 4824.83 N/A(7) 1.509 1.572 XS 29 4824.56 4825.31 4824.77 2.257 1.509 1.534 XS 30 4823.13 4823.30 4823.30 2.031 1.998 1.996 XS 31 4822.84 4822.94 4822.94 1.169 1.221 1.222 XS 32 4820.57 4820.75 4820.75 3.993 3.653 3.653 XS 33 4816.03 4816.30 4816.30 4.749 4.211 4.211 (1)Existing Conditions Water Surface Elevation (2)Proposed Conditions with No Bridge Encroachment Water Surface Elevation (3)Proposed Conditions with Bridge Encroachment Water Surface Elevation (4)Existing Conditions Water Velocities (5)Proposed Conditions with No Bridge Encroachment Water Velocities (6)Proposed Conditions with Bridge Encroachment Water Velocities (7)XS 26 and XS 27 parallel the existing channel, no WSE or velocity could be obtained. APPENDIX A Drainage Area Map APPENDIX B Drainage Area Calculations USER INPUT GIS REFERENCED VALUE COMPUTED VALUE Surface Description Grass-Average Cover Manning's Roughness Coefficient 0.4 Ref: Table 15-1, NEH 630 Chapter 15 Flow Length 30 ft Two Year 24-hour Rainfall 1.2 in Ref: NOAA ATLAS-2 Land Slope 0.123 ft/ft Average Velocity 0.077 ft/s Sheet Flow Tc 0.1079 hr Ref: Eq. 15-8, NEH 630 Chapter 15 Surface Description Paved Flow Length 1408.5 ft Watercourse Slope 0.0134 ft/ft Average Velocity 2.353 ft/s Ref: Table 15-3 V=20.328(S)^0.5 S = Slope Shallow Concentrated Flow Tc 0.1663 hr Surface Description Short Grass Pasture Flow Length 105.77 ft Watercourse Slope 0.01494 ft/ft Average Velocity 0.851 ft/s Ref: Table 15-3 V=6.962(S)^0.5 S = Slope Shallow Concentrated Flow Tc 0.0345 hr Surface Description Upstream Channel 1 Open channel flow calculation based on approximate bankfull flow. Dimensions estimated from LIDAR DEM elevation data. Cross-Sectional Area 6.0 sq. ft. Wetted Perimeter 9.32 ft Hydraulic Radius 0.6 ft Channel Slope 0.0066 ft/ft Manning's Roughness Coefficient 0.027 Assumed based on visual observations of nearby streams. Average Velocity 3.34 ft/s Ref: Eq. 15-10, NEH 630 Chapter 15 Flow Length 651 ft Open Channel Flow Tc 0.0541 hr Flow Length 2,195 ft Flow Length 0.42 mi Time of Concentration 21.766 min Time of Concentration 0.36 hr Total Huffine Lane Culvert - Time of Concentration Legend Sheet Flow Shallow Concentrated Flow 1 Open Channel Flow 1 Shallow Concentrated Flow 2 Huffine Culvert 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2)C * Area Med Residential 0.35 1041559 364546 Water 0.90 64567 58110 Open Land 0.2 622175 124435 Total 1728301 547091 A = Area (acres) 39.6763 C = Weighted C Factor 0.32 Tc (see calculation sheet) (minutes) 21.76 2. Calculate Rainfall Intensity (Duration = Pre-Development Tc) i =1.01x-0.67 (100-yr Storm, Fig. I-3, COB Design Standards) x = storm duration (hrs) 0.36 (Tc) i = rainfall intensity (in./hr.) 1.99 3. Calculate Runoff Rate (Pre-Development) Q = CiA C = Rational Method Runoff Coefficient 0.32 (open land) i = rainfall intensity (in./hr.) 1.99 (calculated above) A = Area (acres) 39.68 (calculated above) Q = Runoff Rate (cfs) 25.03 FERGUSON FARM II DITCH RUNOFF 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft 2)C * Area Hardscape 0.95 41579 39500 Landscape 0.20 54127 10825 Total 95706 50325 A = Area (acres)2.1971 C = Weighted C Factor 0.53 2. Tc (Time to Concentration) Tc Total =5.0 (5 minute minimum) 3. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.53 (calculated above) I =1.01x-0.67 (100-yr Storm)5.34 (100-yr storm) A = area (acres) 2.20 (calculated above) Q = REQUIRED GUTTER CAPACITY (cfs)6.17 (assuming no carry flow) APPENDIX C HEC-RAS Geometry Maps APPENDIX D HEC-RAS Output Files 1. Calculate Area and Weighted C Factor Contributing Area C Area (ft2) C * Area Open Land 0.2 1069301 213860 Hardscape 0.95 0 0 Total 1069301 213860 A = Area (acres) 24.55 C = Weighted C Factor 0.20 2. Calculate Required Volume Q = CIA V=7200Q C = Weighted C Factor 0.20 I = intensity (in/hr) 0.41 (10 yr, 2hr storm) A = Area (acres) 24.55 Q = RUNOFF (cfs) 2.01 V = REQUIRED VOL (ft3) 14493 3. Calculate Tc (Pre-Development) Tc Overland Flow Tc = 1.87 (1.1-CCf)D1/2/S1/3 S = Slope of Basin (%) 1.26% Storm C = Rational Method Runoff Coefficient 0.2 Return (yrs) Cf Cf = Frequency Adjustment Factor 1.25 2 to 10 1 D = Length of Basin (ft) 1665 11 to 25 1.1 26 to 50 1.2 Tc (Pre-Development) (minutes) 60 51 to 100 1.25 2. Calculate Flow (Rational Formula) Q = CIA C = Weighted C Factor 0.20 (calculated above) I = 1.01 Tc-0.67 (in/hr) 1.01 (100-yr storm) A = area (acres) 24.55 (calculated above) Q = Flow (cfs) 4.96 (assuming no carry flow) Crossing Type Dim Wetted Perimeter (ft)Hydraulic Opening (ft)Hydraulic radius (ft)Slope (ft/ft)Mannings nMax Hydraulic Capacity (cfs) EX. Huffine Lane Crossing Circular CMP 3' Dia.9.42 7.1 0.75 0.00633 0.013 53.48EX. Fallon Street Crossing RCP Arch Pipe Culvert 4.83'x3'=48" Equiv.12.56 11.1 0.88 0.0051 0.013 83.45EX. Slough Creek Drive Crossing RCP Arch Pipe Culvert 4.83'x3'=48" Equiv.12.56 11.1 0.88 0.0091 0.013 111.47EX. Walking Trail CrossingRCP Arch Pipe Culvert 4.25'x2.6'=42" Equiv.10.99 8.7 0.79 0.014 0.013 100.69Proposed Walking Trail 1 Crossing Concrete Box Culvert 6'x2'10 12 1.2 0.0143 0.013 185.23Proposed Alley Crossing Concrete Box Culvert 6'x3'12 18 1.5 0.0092 0.013 258.60Proposed Valley Commons Drive CrossingConcrete Box Culvert 6'x3'12 18 1.5 0.0139 0.013 317.87Proposed Field Street CrossingConcrete Box Culvert 6'x3'12 18 1.5 0.0131 0.013 308.59Proposed Parking Area Crossing Concrete Box Culvert 6'x3'12 18 1.5 0.012 0.013 295.35Proposed Walking Trail 2 Crossing Concrete Box Culvert 6'x3'12 18 1.5 0.0218 0.013 398.08