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Home My WebLink About20100035 Ver 1_More Info Received_20100128v MCKIM&CREED January 28, 2010 Betsy Pearce, CFM Environmental Engineer / Consultant Wake County Environmental Services 336 Fayetteville St / PO Box 550 Raleigh, NC 27602 aTF tD - n35 c h,Dt?- 1 LOS-) E N G I N E E R S SURVEYORS P L A N N E R S 5658-0002 RE: Additional Analyses for Bend Areas Research Triangle Park Pedestrian Trail System (Phases 14-16) Wake County Project b: S2334 Research Triangle Park, Wake County, North Carolina Dear Ms. Pearce: Per our January 25, 2010 phone conversation, you indicated that Wake County would like to see additional analyses for 2 areas within Phase 15 of the project where the proposed trail makes 90-degree turns. These areas of interest, labeled Area A and Area B, are shown on the attached Drainage Area Maps. Photographs of existing conditions in these areas are also attached. It is our understanding that Wake County is concerned about the potential for erosive velocities at locations where the trail makes a turn at the downstream end of runs where longitudinal flows along the proposed asphalt surface may occur. Therefore, we have performed additional analyses to look at this concern. Because the trails are being installed on grade in these areas and do not impact Venture IV B u i l d i n g existing drainage patterns, we looked at the impact that the material change would have on velocities at the points of interest. The areas of potential Suite 500 longitudinal sheet flow (versus sheet flow across the width of the trail) were 1 730 Varsity Drive analyzed to determine if the resulting velocities would be erosive at the points of interest. The computed sheet flow velocities resulting from installation of the Raleigh, NC 27606 asphalt trail are approximately 2.3 ft/s for Area A and 2.0 ft/s for Area B, which are non-erosive for the areas receiving the flows. Velocity calculations are attached. 919.233.8091 Fax 919.233.8031 www.mckimcreed.com Ms. Pearce January 28, 2010 Page 2 of 2 In addition, the drainage areas to the points of interest were delineated and 10- year peak discharges for proposed conditions were computed to provide additional information on the flows at the points of interest. The 10-yr peak discharge is approximately 0.57 cfs for Area A and 1.2 cfs for Area B. Peak discharge calculations are attached. Based on the computed velocities and 10-yr peak discharges, there does not appear to be a need for erosion protection or dispersion of concentrated discharge (i.e. level spreaders) in these areas. For reference, we have also included a photograph of an area of the existing trail system where a sharp bend occurs at the lower end of a run of trail. There does not appear to be any issues associated with this configuration, which is similar to the areas of concern identified by Wake County for the proposed project. We hope you find this information satisfactory. Please contact us with any questions or comments. Sincerely, °°eo?N GARS `°6P, McKIM & CREED, P.A. ?' ?'? 27??0 l e a Y 0303E18 Kathleen P. Gray Staff Engineer ?? ?p @0A je v! McIQM&CREED i I f t j ?' i L f !1 ! AREA A- EXISTING CONDITIONS 1 1 097 MKO M-977 AREA B- EXISTING CONDITIONS \V / LL / / O O C \ i Z? _ Q w i i 7 ? ? 1 \ 1 ?a4SN' dals,sino?? \1• 1 i J W J w oI?Q w Q s 115 SJ / LLr U) r- Y W \ I Q C?? / CIO C7 j? W Q O z I L 1 " m _ 4 ~ m 2 ' v L` 1 4 n\? \ ?/ W W H Z Z \ \ I \ r \c~i?1 co w0 , w Q i= w \ 1 ? 6-W Z - N W l W w I W a J gfi = Z O tl 9E O0 / / y ~Q p O IL Z_ i LL J 00 v N a. Q :J H J O W it 1 ?• M O 2 U 4. m c?> w ?E m? J >M W 0 z QQ O z A > Q> I- z J Z d Z I Q W o C9 > _ Q Q HI U U U Q OJZ>000. a? _ -j p ?W Q I ?I Q?>boC?1 U 5> a I0 0OJ>? f it W 4LUu\ d t- Z W 0 D W D O ;E -i U- O J W I- 0 Q M Q Q W - O / T/ I (.) -> W I- /U Q Z > I- O?LL J > W ?D > > $ sue- a'? > > > ggoy, , ? i "N Project: RTP Pedestrian Trail System Phases 14-16 MC Project No.: 05658-0002 Prepared by: - KPG Date Prepared: January 28, 2010 _ Checked by: DNH --- Date Checked: January 28, 2010 Area: A n: Phase 15 Trail Bend- Near Louis Stephens Drive Sheet Flow (ref: NRCS TR-55) Surface description (Table 3-1) Grass _ Manning's roughness coeff., n (Table 3-1) 0.24 Flow length, L (Total < 300 ft) 100 ft Two-year 24-hour rainfall, P 3.42 in Land slope, S 0.065 ft/ft Travel time, Tt 0.144 hr Velocity 0.19 ft/s Sheet Flow (ref: NRCS TR-55) Surface description (Table 3-1) Asphalt Manning's roughness coeff., n (Table 3-1) 0.011 Flow length, L (Total < 300 ft) 100 ft Two-rear 24-hour rainfall, P 3.42 in Land slope, S 0.065 ft/ft Travel time, Tt 0.012 hr Velocity 2.28 ft/s Allowable V61oci 4.0 ft/s Grass, 5-10% -- ref: NCESCPDM, Table 8.05a Project: RTP Pedestrian Trail System Phases 14-16 MC Project No.: 05658-0002 Prepared by: KPG Date Prepared: January 27, 2010 Checked by: DNH Date Checked: January 28, 2010 Area: B - - Description: Phase 15 Trail Bend- at Wetland Divider Berm Near Kit Creek Road Sheet Flow (ref: NRCS TR-55) Surface description (Table 3-1) Gravel and Bare Soil Manning's roughness coeff., n (Table 3-1) 0.011 Flow length, L (Total < 300 ft) 180 ft Two-year 24-hour rainfall, P 3.42 in --- i•, = 0.007 („??nA Land slope, S 5 0.033 ft/ft Travel time, Tt rz 0.025 hr Velocity - I - 1.96 fvs PwwiJ7tt .s t.. ef..?.. a..?A Sheet Flow (ref: NRCS TR-55) Surface description (Table 3-1) Asphalt Mannin 's roughness coeff., n (Table-3-1) 0.011 Flow length, L (Total < 300 ft) 180 ft Two-year 24-hour rainfall, P 3.42 in Land slope, S fr, 0.007 0.033 ft/ft _ Travel time Tt _ 0 025 hr , . Velocity 1.96 ftfs Allowable Velocity 5.0 ft/s fine gravel/stiff clay (ref: NCESCPDM, Table 8.0 CALCULATION I/ Proj No. _D` (9,?_X Project /??Ac? t.r 1_r. at. Client p 61 vi W _S Date ?,d Ar?c? Des. B Subject -[`?<_ ] - f vv GP & By Ch k. By - _ N _H 1 a U- 1?' F C+ ?i i . / (i (r( C? 1 L ?1t-t:??i Ci1^ f' -Q v. , 19 , , , A I - , N I I I ? ? ZI s cAe? - ?I > ?_0 0 C) ? i 4, 1 1& , I L --- ---- J I- I -- - I? g - - -- - j , y I , I ?Qli L I, I Tl? F ;01°_23 A -1- -- (A j- ' C 4 -g p H r ?? y o ?t , I Fs d 2 3 B _ l r ?,T _ a : 9 A L q>e AQ) , - -- --- , ( ! WPB ?1 P ' - ; , : --- ------- --------- - - : n , a I 6 n ? I I I 'r ?? YVQIc?C? ` {u?? .,_nl'c/_-?!s?cVl 110.n-V?l ? I I 41k. ` '• } } r ? } r 1 t ? F ? yA 1 •, , r 4 ! '• ??.a.> ?F ? N,v.!;` ll11 Y ?ll WUI:! tl? LSJ ri r •,•Y ,y?3+}'l - ^?l,' ? • ?' .l . it t , 1-EXISTING TRAIL WITH SHARP BENDS ON KIT CREEK ROAD 1 "'r.- fit:'.. 1 .. "?aa?%Y.i?t?i'f?CE? ,.• ? ' r ?tA, 1r 6 X11 Ile ?? :.I:', •1(?. 11•• ['? t LY r? .:t .. Y 2-SAME AS ABOVE CLOSER TO BEND AREA (AREA IS SIMILAR TO AREAS OF CONCERN)