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HomeMy WebLinkAboutFDP202100003 Calculations 2021-11-23NYSTROM DRIVEWAY CROSSING HYDROLOGIC/HYDRAULIC CALCULATIONS FOR NYSTROM DRIVEWAY CROSSING October 1, 2021 LOCATION: Tax Map 7, Parcel 29A White Hall District, Albemarle County Virginia OWNER/DEVELOPER: Jonathan Nystrom 2417 Grove Avenue Richmond, Virginia 23220 PREPARED BY: Roudabush, Gale & Associates, Inc 999 Second St, SE Charlottesville, VA 22902 P: 434.977.0205 INDEX Summary 6 Background: Methodology: _ Requirements: Time of Concentration: CN values: Results: Cut/Fill Report: Appendices O��EALTN Op`�, O DUSTIN E. GREENE 2 � a Lic. No. 55833 Pp IV (/ /2,� �.<Iv �� IT/ONAL EN,L 2 3-4 5 6-7 3-9 10-11 12 Appendix A (Flood Development Permit Application):_ 13-14 Appendix B (No rise certification): 15 NYSTROM DRIVEWAY CROSSING Summary & Background: Roudabush, Gale & Associates is pleased to submit this Flood Study to Albemarle County for review. Our hydraulic & hydrologic calculations, plan and profile, cutifill computations will show that the flood development permit along with an Erosion and Sediment Control Agreement for single family residence can be approved for construction. The owner, Jonathan Nystrom, seeks to construct a vehicular access from Shiffletts Mill Road, across Buck Mountain Creek and Muddy Run Creek on 30.57 acres, TMP 7-29A. TMP 7-29A is located in the White Hall magisterial district approximately 1 mile west of Free Union Road. TMP 7-29A is zoned Rural Areas with a water protection ordinance buffers on both Buck Mountain Creek and Muddy Run. The elevations on the property range from 680' to 780' up to a summit making Shiffletts Mill Road the only access for this property. The vehicular access will consist of a 10' gravel driveway and (2) bridges for the use of the bulk of the property to a building site located adjacent to Muddy Run, well outside the 100 yr floodplain. The (2) bridge crossings span Buck Mountain Creek and Muddy Run. Buck Mountain Creeks watershed is approximately 5.1 miz in area and extends from the Appalachian Mountains to the west to the project site and extends an additional 11.5 miles to its confluence with the Rivanna River. Muddy Runs watershed is approximately 6.1 mi2 in area and extends from the Appalachian Mountains to the west to the project site and joins Buck Mountain Creek below the driveway crossing area. Joint permit applications will be provided by the owner under separate cover. Parcel 7-29A (parcel) is situated at the confluence of Muddy Run and Buck Mountain Creek. The 100-yr flood and 10-yr flood elevations were analyzed using the River Analysis System (HEC-RAS), v5.0.7 determine the feasibility of bridge placement on the property. Muddy Run 10yr Flow = 440 cfs, 100Yr Flow = 9,190 cfs Buck Mountain Creek 10yr Flow = 1,992 cfs, 100Yr Flow = 9,190 cfs NYSTROM DRIVEWAY CROSSING Methodology The distribution of flow was assigned to Buck Mountain Creek and Muddy Run due to the nature of the flow patterns upstream of Parcel 7-29A in Northern Albemarle County. When flood flows occur in this region, excess flow from Muddy Run spills across the floodplain into Buck Mountain Creek. For the 10-yr storm/flood, this flow pattern requires that the model be divided into two portions, since there is no continuous connection between the floodplains until downstream of the parcel boundary. River Stations 14+01 and 10+38 show the upstream conditions of Muddy Run while River Stations 4+44 and 1+15 show to total combined flows. 1 dimensional modeling calculates the parameters of flow at cross -sections perpendicular to the direction of flow. Cross Section 1+15 is the closest station to the parcel boundary, therefore that location was used in the distribution of flow. The parameters are shown in Table 1 below. Reach River Profile Flowrate Water Surf. Velocity Flow Station Elevation Area (fts/5) (ft) (ft/5) (ftZ) Muddy Run 1401 10 YR 1742.8 723.11 8.77 198.72 Muddy Run 1038 10 YR 1742.8 718.93 3.97 438.99 Buck Mtn 775 10 YR 743.6 719.17 5.95 124.97 Creek Buck Mtn 392 10 YR 743.6 716.08 2.85 260.91 Creek Buck Mtn 444 10 YR 2431.7 711.31 5.47 444.55 Creek Buck Mtn 115 10 YR 2431.7 706 5.22 465.84 Creek Table 1: Flow Characteristics and Parameters from Part 1 Consider Equation 1 below, where flowrate is Q in ft3/s, V is velocity in ft/s, and A is cross - sectional area. At river station 1+15, the velocity is 5.22 ft/s. The cross -sectional area of Muddy Run at station 1+15 is approximately 85 ft2. Equation 1 calculates an approximate flow through Muddy Run of 440 ft3/sec. Given that the total flowrate at station 1+15 is 2,432 cfs, then the flow in Buck Mountain Creek is 1,992 cfs. Q=V*A Equation 1: Continuity of Flow VGIN sourced topography has been used to develop cross sections and design of the driveway profile. Site visits, within the past year, have been made to determine any discrepancies from the topography data. The Hydrologic Modeling System (HEC-HMS), v4.4.1, was used to perform hydrologic analyses on both of these creeks to generate peak flows and hydrographs. NYSTROM DRIVEWAY CROSSING Curve number values were determined from the Soil Conservation Service Curve Number Methodology. The design curve numbers used in this study were based on the Runoff Curve Number Tables found in the National Engineering Handbook (NEH), Part 630.09. These numbers were used in combination with the different hydrologic soil complexes determined from Natural Resources and Conservation. Service (NRCS) soil survey data (from the Web Soil Survey website) and land use classifications as verified with aerial imagery. The NRCS soil survey data provided the hydrologic soil group designations and the percentage of each soil group within the respective components of each watershed. Some soil units shown in the soil survey consisted of two hydrologic groups. For these, the more conservative soil type was assumed. These curve numbers were then used to develop time of concentration values in accordance with NRCS TR-55 method. NYSTROM DRIVEWAY CROSSING Reauirements: Section 17-604 of Albemarle County's the water protection ordinance allows for stream crossings accessing a single-family residence. Stream crossings of perennial streams shall be sized to pass the ten (10) year storm without backing water onto upstream properties. Bridges must (a) leave the stream bed and streambank, undisturbed, or (b) allow the stream to return to natural, stabilized condition upon completion. In lieu of submitting a formal erosion and sediment control plan for development of this single-family residence the owner is submitting an agreement with disturbance that will be less than 1 acre for the entire driveway, house and drain field construction. A building permit may satisfy the requirements of Section 17-406(A) for any mitigation plan pertaining to a development of one single-family detached dwelling at the discretion of the administrator. Section 30.3.11 Stream crossings for a driveway serving only one single-family dwelling and pedestrian trails, including, but not limited to, pedestrian and multi -use paths that are within county -owned or operated parks and greenways, and any footbridges necessary to cross tributary streams, watercourses and swales, that: (i) meet the applicable requirements of sections 17-406 and 17-604; (ii) demonstrate, in a floodplain impact plan, to the floodplain administrator's satisfaction, that construction of the crossing will have no impact on the elevations or limits of the floodplain; and (iii) will serve one dwelling unit that could not be accessed by any other means. Under the encroachment standards of Sec. 30.3.13 we are satisfying Section A-1 which states there will be no increase in the water surface elevation of the base flood. RGA has demonstrated this by showing that the cut/fill has been offset for the bridges and the crossings. A no -rise certification has been provided. Worksheet 3: Time of Concentration (Tc) or travel time (Tt) Project Nystrom Driveway Crossing IBy Alex Flint Dae 1/7/21 Location Buck Mountain Creek Check one: Z Present ❑ Developed Dustin Greene 1 2/18/21 Check one: ❑ T; ❑x Tt through subarea Notes: Space for as many as two segments per flow type can be used for each worksheet. Include a map, schematic, or description of flow segments. Segment ID 1. Surface description (table 3-1)................................... 2. Manning's roughness coefficient, n (table 3-1) .......... 3. Flow length, L (total L t 300 ft)................................. ft 4. Two-year 24-hour rainfall, P2.................................. in 5. Land slope, s ........................................................ IVft 6. Tt = 0.007 (nQ 0 8 Compute Tt ....---- hr P2 0.5 50.4 Segment ID 7. Surface description (paved or unpaved) ..................... 8_ Flow length, L...........................................................ft 9. Watercourse slope, s............................................ fVft 10, Average velocity, V (figure 3-1) ............................. ft(s 11. Tt= L Compute Tt... ........ hr 3600 V Segment ID 12. Cross sectional flow area, a ................................. ft2 13. Wetted perimeter, pw.............................................. ft 14. Hydraulic radius, r= a Compute r ......................... ft 15 Channel slope, s ...... ow. ........ ft/ft .................................... 16. Manning's roughness coefficient, n ............................ 17. V = 1.49 r 2/3 s 1/2 Compute V ................ft/s 18. Roovn -tength, L.......................................................... ft 19. Tt = L Compute Tt .............. hr 3600 V 20. Watershed or subarea Tc or Tt (add Tt in steps 6, 11, and 19) ............................. AB Woods, light 0.4 100 3.65 0.03 0.28 + BC Unpaved 4,253 0.35 1.4 CD UMN I = []0..4386 .... Hr 1 (330-tT-Tk-35, Second Ed., June 1986) D-3 Worksheet 3: Time of Concentration (Te.) or travel time (T+) Project Nystrom Driveway Crossing By Alex Flint Date 1/7/21 Locadon Muddy Creek Checkad i Date Dustin Greene 2/1812, Check one: © Present ❑ Developed Check one: ❑ Tc ©Tt through subarea (Votes: Space for as many as two segments per flow type can be used for each worksheet. Include a map, schematic, or description of flow segments. Segment ID 1. Surface description (table 3-1)................................... 2. Manning's roughness coefficient, n (table 3-1) .......... 3. Flow length, L (total L t 300 ft)................................. It 4. Two-year 24-hour rainfall, P2....................... I.......... in 5. Land slope, Is ........................................................ f ift 6. Tt = 0.007 (nQ o.a P2 o.s so.4 ComputeTt.._...... hr Segment ID 7. Surface description (paved or unpaved) ............. B. Flow length, L...................................................... .... .ft 9. Watercourse slope, s............................................ fVft 10. Average velocity, V (figure 3-1) ............................. f /s 11. Tt = L Compute Tt ........... hr 3600 V Segment ID 12. Cross sectional flow area, a ................................. ft2 13. Wetted perimeter, pw.............................................. ft a 14. Hydraulic radius, r= - Compute r ......................... It 15 Channel slope, s pw ... fult 16. Manning's roughness coefficient, n ............................ 17. V = 1.49 r V3 s 12 Compute V ................fVs n 18. Ftow-tength, L.......................................................... It 19. Tt = 60 V Compute Tt .............. hr AB Woods, light 0.4 100 3.65 0.24 0.12 + BC Unpaved 4,206 0.31 1.4 0.83 + 20. Watershed or subarea Tc or Tt (add Tt In steps 6, 11, and 19) CD 2.5 0.072 0.04 18.39 17, 651 0.27 + = 0.12 _I = ...... H r[:O�q27 1 The thousandth significant digit (210-V1-TR-55, Second Ed., Jun, 1986) for all three Tt add up to more D_3 Worksheet 2: Runoff curve number and runoff Project BY Date Nystrom Driveway Crossing Alex Flint 1/7/21 Location Checked ! Date Buck Mountain Creek Dustin Greene 2/24121 Check one: ❑ Present ❑ Developed Soil name and hydrologic group (appendix A) Group A Group B Group A m# Cover description ;cover type, treatment. and hydrologic ccndit:on; percer, impervious; unconrecled//connected impervious area ratio; Use only one CN source per line CN (weighted) = total product = total area Wooded Wooded Pasture Pasture Frequency ..... ... ........... .......... .......... Rainfall, P (24-hour) .......................... Runoff, Q........................................... (Use P and CN with table 2-1, figure 2-1, or equations 2-3 and 2-4) yr in in CN J Area Product Of CN x area ❑acres ci to =v m 5 ❑ mi2 1 3D 25.5 765 55 I- I 49.5 1930.5 i 39 8.5 467.5 61 16.5 1006.5 Totals » 100 J 4825.5 Use CN» 48.3=48 Storm #1 1 Storm #2 1 Storm #3 D 2 (210-17-T&35, Second FA., June 1986) Worksheet 2: Runoff curve number and runoff Project Nystrom Driveway Crossing By Alex Flint Date 1/7/21 Location Muddy Run Checked Dustin Greene Date 2l24/21 Check one: ❑ Present ❑ Developed Soil name Cover description CN Area Product and 0( hydrologic CN x area group '' v ❑acres ;cover type,. trealmm, and hyorolog;c condition; percent d2 ❑mil (appendix A) impervious; unconnectedhonrecied impervious area ra'io) H LL 139 1 tL Group A Wooded 30 I 9.2 276 Group B Wooded Wooded 55 65.4 3597 Group C 70 l 0.3 21 Group A Pasture 39 3.1 120.9 Group B Pasture 61 21.8 1329.8 Group C Pasture 77 0.1 7.7 1 --T J Use only one CN source per line Totals » 99•9 5352.4 CN (weighted) = total product = _ Use CN total area Runoff2. Storm #1 Storm 42 Storm 93 Frequency ................................................. yr Rainfall, P (24-hour)......_.......................... in Runoff, Q.................................................. in (Use P and CN with table 2-1, figure 2.1, or equations 2-3 and 2.4) D-2 Second Ed -dune 1936) NYSTROM DRIVEWAY CROSSING Results: The flood plain development (bridging the perennial stream) for driveway access on TMP 7-29A does not impact the 100yr WSE. The installation of the bridge, the approach ramp, driveway gravel and bridge volumes will be offset by the volume of earth removal along the same cross section path, keeping both cross -sections and volumes equal before construction as well as after construction. The plans and forms are submitted concurrently with this certification. The predeveloped and post -developed WSE is unaffected for river station 7+89 for Buck Mountain Creek and 5+91 for Muddy run. The upstream property is free of modeled inundation for the 10yr post -developed condition. Stream Designation River Sta 10 yr (cfs) W.S. Elev Buck Mountain Creek Pre -developed 6+65 1,992 696.46' Buck Mountain Creek Post -developed 6+65 1,992 696.53' Buck Mountain Creek Pre -developed 7+89 1,992 698.09' Buck Mountain Creek Post -developed 7+89 1,992 698.09' Muddy Run Pre -developed 4+93 440 697.49' Muddy Run Post -developed 4+93 440 697.59' Muddy Run Pre -developed 5+91 440 700.15' Muddy Run Post -developed 5+91 440 700.15' The Cut/Fill Analysis shows that we have a net excess of 51.22 cuyd of Cut. Bridge length = 35', Bridge width = 10', Bridge depth = 1.5' Bridge Volume = 19.4 cuyd 2 Bridge Volume = 38.8 cuyd Cut/Fill Report in 100yr floodplain = 51.22 excess cut 51.22 cuyd > 38.8 cuyd Fill volume has been offset by the cut volume including the volume of the bridges. Therefore, there will be no rise in the 100 yr floodplain for this project. NYSTROM DRIVEWAY CROSSING Bridge Deck Elevation Check Stream Designation River Sta 10 yr Ids) W.S. Elev Buck Mountain Creek Post -developed 3+33 1,992 694.32' Buck Mountain Creek Post -developed 2+45 1,992 692.04' Buck Mountain Bridge Crossing Elevation average 1,992 693.18' Bridge Deck Buck Mountain Creek = 693.50' > 693.18' OK Muddy Run Post -developed i 2+79 440 695.19' Muddy Run Post -developed 2+20 440 693.73' Muddy Run Bride Crossing Elevation average 440 694.46' Bridge Deck Muddy Run = 694.60' > 694.46' OK Page 1 of 1 d ut Uil! i+'Cp rt Generated: 2021-03-04 10:37:47 By user: DGreene Q:\RGA\TMPROJ\20.1962 - Nystrom Driveway Drawing: Crossing\DWG\Q:\RGA\TMPROJ\20.1962 - Nystrom Driveway Crossing\DWG\DRIVEWAY PLAN-20210122.dwa 1. „iurn, Sm. :man Name Type Cut Fill 2d Area Cut Fill Net Factor Factor (acres) (Cu. Yd.) (Cu. Yd.) (Cu. Yd.) EW 20210217 full 1.000 1150 0.571 165.71 114.49* 51.22* 1ot �l> 2d Area (acres) Cut (Cu. Yd.) Fill (Cu. Yd.) Net (Cu. Yd.) Total 0.571 165.71 114.49* 51.22* * Value adjusted by cut or fill factor other than 1.0 ris.////.NT----RJ-..---- -IA__. ll_._.T 11 1- ..-....�