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HomeMy WebLinkAboutFloodplain Analysis 11.01.2020c:oLoRADO ENGINEERING November 1, 2020 Mr. Mike Johnson c/o MWJ LLC Via Email only: mike@johnconinc.com RE: Job No. 1213 - Rifle Creek Property - Floodplain Analysis Dear Mr. Johnson: Colorado River Engineering, Inc. (CRE) has completed a floodplain analysis for the MWJ LLC property located along Rifle Creek, upstream of the Rifle Creek Golf Course in Garfield County. This study has been completed to analyze the development of a home site and creek crossing. Portions of the property fall within the FEMA mapped floodplain as part of the National Flood Insurance Program. County, State, and Federal regulations allow development within mapped floodplains; as long as the development occurs in fringe areas, can be flood proofed, and does not impact water levels of the stream system beyond acceptable limits. Our review shows that the house site is outside of the 100-year flood hazard boundary identified on the FEMA Flood Insurance Rate Map (FIRM). The analysis also shows that all work associated with the creek crossing can be completed outside of the calculated floodplain limits identified in our more detailed hydraulic modeling. In summary, the house is outside of the floodplain, and although the bridge crossing is within the mapped area, the work can be completed outside of the actual calculated floodplain extents and will not have any impact to the 100-year floodplain. It has been our experience and observation that bridge projects in Garfield County and other nearby counties do not typically require permitting or adjustments to FEMA mapping (e.g. LOMA's or LOMR's) to more accurately identify mapping based on updated engineering. This is likely due to bridges not being subject to any mandatory insurance requirements such as federally backed mortgages for homes within the National Flood Insurance Program. A floodplain permit or boundary adjustment does not appear to be warranted. We recommend this report be submitted with your building permit to Garfield County to document that the house is outside of the floodplain limits and the bridge does not impact the floodplain. Existing Conditions - Proposed Improvements The property (PIN 212718200027) consists of approximately 60 acres, as shown on the attached Figure 1. State Highway 325 and Rifle Creek pass through the property. The existing conditions in the area of the proposed home are illustrated on the attached Figure 2 (Sheet 2 from CRE Site Plan drawings). The Rifle Creek Canon Ditch (RCCD) and P.O. Box 1301 • Rifle, CO 81650 • Tel. 970-625-4933 • Fax 970-625-4564 COLORADO RIVE E N d 1 N E E R I N 6 INcOHPOIIA ri:r the West Lateral of the RCCD divert water from Rifle Creek and traverse the property. The proposed house site is situated along the valley floor at the base of the Grand Hogback. The home site was chosen based on our recommend siting to mitigate rockfall hazards. The property and the two irrigation ditch diversion points have historically been accessed from a dirt drive at the northern portion of the property. Previous analysis shows that a bridge crossing utilizing the existing drive entry could be accomplished but would require significant fill in the calculated floodplain limits due to minimal channel bank depths in the creek and elevated water levels from the diversion dams. A new access point is proposed from State Highway 325 that will utilize a bridge crossing the creek at an area with a deep incised channel and greater floodwater carrying capacity. Floodplain Mapping - FEMA According to the current FEMA Flood Insurance Rate Map (FIRM) Panel No. 0802050965B (attached), the proposed building site falls outside the Flood Hazard Zone A boundary. Preliminary floodplain mapping updates are also attached. The preliminary mapping is not a new study but merely provides improved background imagery and consistent elevation datums for multiple study areas across the county. Zone A delineates the floodplain boundary for the 100-year frequency event and has been superimposed by the surveyor on the site plan drawings, as shown on Figure 2. The FEMA Flood Insurance Rate Map for this area was completed by analyzing a few cross sections on the property and then extrapolating elevation onto topographic mapping. These can be considered "approximate" engineering methods due to the distance between cross sections with respect to the varying topographic transitions. The mapping also shows a Zone B, representing flooding hazards associated with the 500-year frequency event where shallow flooding of less than 1-foot in depth could occur. The house site does fall within Zone B. Zone B does not require the mandatory flood insurance for any federally backed mortgages unlike those that are associated with Zone A. Private lenders could require flood insurance. We recommend the home constructed in this zone be constructed without any basements and with the finished floor elevation constructed 2- feet above the existing grade. Floodplain Base Flood Elevation Determinations CRE has completed more detailed Hydraulic Analyses to develop Base Flood Elevations (BFE's) and floodplain limits for the 1% frequency flow occurrence (100-year flood event). 2 P.O. Box 1301 • Rifle, CO 81650 • Tel. 970-625-4933 • Fax 970-625-4564 COLOfADO WE ENGINEERING iNcoeroien rr.a Hydrology —100 Year flow amounts (Q1oo) Review of the FEMA Flood Insurance Study shows that the 100-year flood flow amounts for Rifle Creek, upstream of its confluence with Government Creek, is 2,070-cfs. Hydraulic Analyses Numerous cross sections and elevation points were surveyed along Rifle Creek and were utilized to develop topographic mapping (Figure 3). The survey included cross sections near the proposed bridge crossing and home site as well as areas downstream. The cross- section geometry was input into the US Army Corps of Engineers' HEC-RAS computer modeling software. This hydraulic model calculates water elevations based on cross section geometry, Manning's "n" friction coefficients, channel slope, and channel flow. Final inputs used for the analysis included a Manning's "n" coefficient of 0.045 for the overbank and channel areas. The Manning's "n" coefficients were chosen based on coefficients used in the FEMA Flood Insurance Study and our site observations. The attached printout summarizes the HEC-RAS computer model results. The Base Flood Elevations (BFE's) and the 100-year floodplain limits in the project area are shown on the attached Figure 3. The calculated floodplain BFE's fall within the creek channel banks. The cross section near the proposed bridge (station 1+67) has a corresponding BFE of 5768.49-ft. I recommend that the bottom of the bridge have an minimum elevation of 5771-feet, which provides 2.5-feet of freeboard as a factor of safety. Summary of Results The hydraulic analysis shows that the improvements will not impact the Rifle Creek 100- yr floodplain. We recommend the proposed home's finished floor elevation be constructed 2-feet or higher than the existing ground elevation. If there are any questions or comments, please do not hesitate to call (970) 625-4933. Sincerely, Christopher Manera, P.E. 3 P.O. Box 1301 • Rifle, CO 81650 • Tel. 970-625-4933 • Fax 970-625-4564 eiPublic,net Garfield County, CO FIGURE 1 R210668:., RlP�E G•4A !SAND291) 34OU01-� 7 'l •.� �P RA:I -;RIF COURSE CGOLF Account Number R210472 Parcel Number 212718200027 Acres 60 Land SgFt 0 Tax Area 021 2019 Mill Levy 63.9580 Date created: 9/12/2020 Last Data Uploaded: 9/12/2020 2:05:43 AM Developed bytii. Schneider GEOSPATI A L Physical Address Owner Address 0 RIFLE KMLJ LLC PO BOX 1926 RIFLE CO 81650 Overview s' Legend ri Parcels Roads Parcel/Account Numbers Highways = Limited Access — Highway Major Road Local Road Minor Road Other Road Ramp Ferry Pedestrian Way Owner Name Lakes & Rivers — County Boundary Line 2019 Total Actual Value $298,000 Last 2 Sales 81650 Date Price 1/6/2017 $0 1/1/2017 $0 FIGURE 1 • Proposed Bridge I Crossing PO Box 1301 Rifle, CO 81650 Tel 970-625-4933 Fax 970-625-4564 rite Name: lKt JOHNSON PROPERTY FLOODPLAIN ANALYSIS c 0 a) W 50 Rifle1 Plan: Plan 06 9/12/2020 Alignment - (8) Alignment - (8) Legend 100 150 200 Main Channel Distance (ft) EG PF 1 WS PF 1 Crit PF 1 ■ Ground Rifle1 Plan: Plan 06 9/12/2020 RS = 2.82 5758- Legend EGPF1^ WS PF 1 Crit PF 1 Ground ■ Bank Sta 0 20 40 60 80 100 120 Station (ft) 1 Riflel Plan: Plan 06 9/12/2020 RS = 197.69 5780- Legend EG PF 1 WS PF 1 Ground • Bank Sta 0 20 40 60 80 100 120 Station (ft) 2 0 as as W 5778 5760 Rifle1 Plan: Plan 06 9/12/2020 RS = 187.76 045 0 Legend 20 40 60 80 100 120 Station (ft) -EG PF 1 WS PF 1 Ground • Bank Sta 3 5760 0 Rifle1 Plan: Plan 06 9/12/2020 RS = 167.79 o Legend EG PF 1 WS PF 1 Crit PF 1 Ground • Bank Sta 20 40 60 80 100 120 Station (ft) 4 Riflel Plan: Plan 06 9/12/2020 RS = 148.17 r 045 5776- 5774 5772 ■ ■ -_ 5770 5768 co w 5766- 5764- 5762- 5760 0 Legend EG PF 1 WS PF 1 —+— - Crit PF 1 Ground • Bank Sta 20 40 60 80 100 120 Station (ft) 5 Rifle1 Plan: Plan 06 9/12/2020 RS = 122.85 — — - 045 776 5768 0 a) W Legend EG PF 1 INS PF 1 Grit PF 1 Ground • Bank Sta 20 40 60 80 100 120 Station (ft) 6 w 5764 5756 .z Riflel Plan: Plan 06 9/12/2020 RS = 85.05 .045 Legend 0 20 40 60 80 100 120 Station (ft) EG PF 1 WS PF 1 Crit PF 1 Ground /. Bank Sta 7 Riflel Plan: Plan 06 9/12/2020 RS = 71.72 .045 5772 5770 5768 c 5764 a� w 5762 5760 5758 5r 35 Legend EG PF 1 WS PF 1 Crit PF 1 Ground ■ Bank Sta 20 40 60 80 100 120 Station (ft) 8 Riflel Plan: Plan 06 9/12/2020 RS = 46.09 Legend EG PF 1 WS PF 1 Crit PF 1 ■ Ground • Bank Sta 1 5756 0 20 40 60 80 100 120 Station (ft) 9 Plan: Plan 06 Alignment - (8) Alignment - (8) RS: 197.69 Profile: PF 1 E.G. Elev (ft) 5771.10 Element Left OB Channel Right OB Vel Head (ft) 0.40 Wt. n-Val. 0.045 W.S. Elev (ft) 5770.69 Reach Len. (ft) 9.93 9.93 9.93 Crit W.S. (ft) Flow Area (sq ft) 405.98 E.G. Slope (ft/ft) 0.002194 Area (sq ft) 405.98 Q Total (cfs) 2070.00 Flow (cfs) 2070.00 Top Width (ft) 59.76 Top Width (ft) 59.76 Vel Total (ft/s) 5.10 Avg. Vel. (ft/s) 5.10 Max Chl Dpth (ft) 9.87 Hydr. Depth (ft) 6.79 Conv. Total (cfs) 44193.7 Conv. (cfs) 44193.7 Length Wtd. (ft) 9.93 Wetted Per. (ft) 67.83 Min Ch El (ft) 5760.82 Shear (Ib/sq ft) 0.82 Alpha 1.00 Stream Power (lb/ft s) 4.18 Frctn Loss (ft) 0.03 Cum Volume (acre-ft) 0.92 C & E Loss (ft) 0.03 Cum SA (acres) 0.23 Plan: Plan 06 Alignment - (8) Alignment - (8) RS: 187.76 Profile: PF 1 E.G. Elev (ft) 5771.04 Element Left OB Channel Right OB Vel Head (ft) 0.71 Wt. n-Val. 0.045 W.S. Elev (ft) 5770.33 Reach Len. (ft) 19.97 19.97 19.97 Crit W.S. (ft) Flow Area (sq ft) 307.05 E.G. Slope (ft/ft) 0.004665 Area (sq ft) 307.05 Q Total (cfs) 2070.00 Flow (cfs) 2070.00 Top Width (ft) 51.53 Top Width (ft) 51.53 Vel Total (ft/s) 6.74 Avg. Vel. (ft/s) 6,74 Max Chl Dpth (ft) 9.47 Hydr. Depth (ft) 5.96 Conv. Total (cfs) 30308.7 Conv. (cfs) 30308.7 Length Wtd. (ft) 19.97 Wetted Per. (ft) 59.41 Min Ch El (ft) 5760.86 Shear (lb/sq ft) 1.51 Alpha 1.00 Stream Power (lb/ft s) 10.15 Frctn Loss (ft) 0.17 Cum Volume (acre-ft) 0.84 C & E Loss (ft) 0.15 Cum SA (acres) 0.22 Plan: Plan 06 Alignment (8) Alignment-(8) IRS: 167.79 Profile: PF 1 E.G. Elev (ft) 5770.72 Element Left OB Channel Right OB Vel Head (ft) 2.18 Wt. n-Val. 0.045 W.S. Elev (ft) 5768.54 Reach Len. (ft) 19.62 19.62 19.62 Crit W.S. (ft) 5768.38 Flow Area (sq ft) 174.97 E.G. Slope (ft/ft) 0.020662 Area (sq ft) 174.97 Q Total (cfs) 2070.00 Flow (cfs) 2070.00 Top. Width (ft) 37.29 Top Width (ft) 37.29 Vel Total (ft/s) 11.83 Avg. Vel. (ft/s) 11.83 Max Chl Dpth (ft) 7.99 Hydr. Depth (ft) 4.69 Conv. Total (cfs) 14400.7 Conv. (cfs) 14400.7 Length Wtd. (ft) 19.62 Wetted Per. (ft) 44.47 Min Ch El (ft) 5760.55 Shear (lb/sq ft) 5.08 Alpha 1.00 Stream Power (lb/ft s) 60.05 Frctn Loss (ft) 0.35 Cum Volume (acre-ft) 0.73 C & E Loss (ft) 112 Cum SA (acres) 0.20 Plan: Plan 06 Alignment - (8) Alignment - (8) RS: 148.17 Profile: PF 1 E.G. Elev (ft) 5770.25 Element Left OB Channel Right OB Vel Head (ft) 1.76 Wt. n-Val. 0.045 W.S. Elev (ft) 5768.49 Reach Len. (ft) 25.32 25.32 25,32 Crit W.S. (ft) 5767.87 Flow Area (sq ft) 194.43 E.G. Slope (ft/ft) 0.015257 Area (sq ft) 194.43 Q Total (cfs) 2070.00 Flow (cfs) 2070.00 Top Width (ft) 40.42 Top Width (ft) 40,42 Vel Total (ft/s) 10.65 Avg. Vel. (ft/s) 10.65 Max Chl Dpth (ft) 8.44 Hydr. Depth (ft) 4.81 Conv. Total (cfs) 16758.6 Conv. (cfs) 16758.6 Length Wtd. (ft) 25.32 Wetted Per. _(ft) 46.10 Min Ch El (ft) 5760.05 Shear (Ib/sq ft) 4.02 Alpha 1.00 Stream Power (lb/ft s) 42.77 Frctn Loss (ft) 0.45 Cum Volume (acre-ft) 0.64 C & E Loss (ft) 0.06 Cum SA (acres) 0.18 Plan: Plan 06 Alignment -(8) Alignment -.(8) RS: 122.85 Profile: PF 1 E.G. Elev (ft) 5769.73 Element Left OB Channel Right OB Vel Head (ft) 2.35 Wt. n-Val. 0.045 W.S. Elev (ft) 5767.38 Reach Len. (ft) 37.80 37.80 37.80 Crit W.S. (ft) 5767.38 Flow Area (sq ft) 168.27 E.G. Slope (ft/ft) 0.021475 Area (sq ft) 168.27 Q Total (cfs) 2070,00 Flow (cfs) 2070.00 Top Width (ft) 35,83 Top Width (ft) 35.83 Vel Total (ft/s) 12.30 Avg. Vel. (ft/s) 12.30 Max Chl Dpth (ft) 8.74 Hydr. Depth (ft) 4.70 Conv. Total (cfs) 14125.6 Conv. (cfs) 14125.6 Length Wtd. (ft) 37.80 Wetted Per. (ft) 41.51 Min Ch El (ft) 5758.64 Shear (Ib/sq ft) 5.43 Alpha 1.00 Stream Power (lb/ft s) 66.85 Frctn Loss (ft) 0.82 Cum Volume (acre-ft) 0.54 C & E Loss (ft) 0.17 Cum SA (acres) 0.16 Plan: Plan 06 Alignment - (8) Alignment - (8) RS: 85.05 Profile: PF 1 E.G. Elev (ft) 5766.55 Element Left OB Channel Right OB Vel Head (ft) 1.78 Wt. n-Val. 0.045 W.S. Elev (ft) 5764.77 Reach Len. (ft) 13.33 13.33 13.33 Crit W.S. (ft) 5764.77 Flow Area (sq ft) 193.37 E.G. Slope (ft/ft) 0.021874 Area (sq ft) 193.37 Q Total (cfs) 2070.00 Flow (cfs) 2070.00 Top Width (ft) 54.97 Top Width (ft) 54.97 Vel Total (ft/s) 10.70 Avg. Vel. (ft/s) 10.70 Max Chl Dpth (ft) 7.17 Hydr. Depth (ft) 3.52 Conv. Total (cfs) 13996.1 Conv. (cfs) 13996.1 Length Wtd. (ft) 13.33 Wetted Per. (ft) 59.58 Min Ch El (ft) 5757.60 Shear (Ib/sq ft) 4.43 Alpha 1.00 Stream Power (lb/ft s) 47.44 Frctn Loss (ft) 0.29 Cum Volume (acre-ft) 0.38 C & E Loss (ft) 0,01 Cum SA (acres) 0.12 Plan: Plan 06 Alignment - (8) Alignment - (8) RS: 71.72 Profile: PF 1 E.G. Elev (ft) 5764.83 Element Left OB Channel Right OB Vel Head (ft) 1.74 Wt. n-Val. 0.045 W.S. Elev (ft) 5763.09 Reach Len. (ft) 25.63 25.63 25.63 Crit W.S. (ft) 5763.09 Flow Area (sq ft) 195.70 E.G. Slope (ft/ft) 0.021426 Area (sq ft) 195.70 Q Total (cfs) 2070.00 Flow (cfs) 2070.00 Top Width (ft) 56.35 Top Width (ft) 56.35 Vel Total (ft/s) 10.58 Avg. Vel. (ft/s) 10.58 Max Chl Dpth (ft) 5.89 Hydr. Depth (ft) 3.47 Conv. Total (cfs) 14141.6 Conv. (cfs) 14141.6 Length Wtd. (ft) 25.63 Wetted Per. (ft) 60.45 Min Ch El (ft) 5757.20 Shear (lb/sq ft) 4.33 Alpha 1.00 Stream Power (Ib/ft s) 45.80 Frctn Loss (ft) 0.56 Cum Volume (acre-ft) 0.32 C & E Loss (ft) 0.04 Cum SA (acres) 0.10 Plan: Plan 06 Alignment - (8) Alignment - (8) RS: 46.09 Profile: PF 1 E.G. Elev (ft) 5763.76 Element Left OB Channel Right OB Vel Head (ft) 1.61 Wt. n-Val. 0.045 W.S. Elev (ft) 5762.15 Reach Len. (ft) 43.27 43.27 43.27 Crit W.S. (ft) 5762.15 Flow Area (sq ft) _ 203.21 E.G. Slope (ft/ft) 0.022057 Area (sq ft) 203.21 Q Total (cfs) 2070.00 Flow (cfs) 2070.00 Top Width (ft) 64.65 Top Width (ft) 64.65 Vel Total (ft/s) 10.19 Avg. Vel. (ft/s) 10.19 Max Chl Dpth (ft) 5.62 Hydr. Depth (ft) 3.14 Conv. Total (cfs) 13937.9 Conv. (cfs) 13937.9 Length Wtd. (ft) 43.27 Wetted Per. (ft) 67.88 Min Ch El (ft) 5756.53 Shear (lb/sq ft) 4.12 Alpha 1.00 Stream Power (lb/ft s) 41.99 Frctn Loss (ft) 0.91 Cum Volume (acre-ft) 0.20 C & E Loss (ft) 0.03 Cum SA (acres) 0.06 Plan: Plan 06 Alignment - (8) Alignment - (8) RS: 2.82 Profile: PF 1 E.G. Elev (ft) 5761.91 Element Left OB Channel Right OB Vel Head (ft) 1.53 Wt. n-Val. 0.045 W.S. Elev (ft) 5760.38 Reach Len. (ft) Crit W.S. (ft) 5760.31 Flow Area (sq ft) 208.93 E.G. Slope (ft/ft) 0.020003 Area (sq ft) 208.93 Q Total (cfs) 2070.00 Flow (cfs) 2070.00 Top Width (ft) 65.43 Top Width (ft) 65.43 Vel Total (ft/s) 9.91 Avg. Vel. (ft/s) 9.91 Max Chl Dpth (ft) 5.08 Hydr. Depth (ft) 3.19 Conv. Total (cfs) _14636.2 Conv. (cfs) 14636.2 Length Wtd. (ft) Wetted Per. (ft) 67.61 Min Ch El (ft) 5755.30 Shear (Ib/sq ft) 3.86 Alpha 1.00 Stream Power (lb/ft s) 38.23 Frctn Loss (ft) Cum Volume (acre-ft) C & E Loss (ft) Cum SA (acres)