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HomeMy WebLinkAboutDrainage Reports - 05/20/1991- PROPERTY OF FORT COLLINS uTn.i m FINAL DRAINAGE REPORT for SOMERVILLE P.U.D. FIRST REPLAT Fort Collins, Colorado prepared by Northern Engineering services, Inc. 420 South Howes, Suite 106 Fort Collins, Colorado (303) 221-4158 Northern Engineering Services, Inc. ' May 8, 1991 ' Mrs. Susan Hayes Stormwater Utility City of Fort Collins 235 Mathews P.O. Box 580 Fort Collins, Colorado 80522-0580 re: Somerville P.U.D., First Replat Fort Collins, Colorado Project No. 9108.00 Dear Susan: Northern Engineering is pleased to present this drainage study for your review. It; represents an addendum to an earlier for Somerville which was prepared by TEC in 1985 and approved by the City of Fort Collins. Your input during the preparation of this study has been very much appreciated. Should you have any questions or comments as you review this report, please feel free to contact me at your earliest convenience. Sincerely, ' NORTHERN ENGINEERING SERVICES, INC.: ' Michael F. Jones, E. 420 S. Howes • Suite 106 • Fort Collins, Colorado 80521 • (303) 2214158 ' Addendum to Final Drainage Report for SOMERVILLE P.U.D. Fort Collins, Colorado ' May, 1991 ' I. GENERAL ' 'This report summarizes the results of a stormwater runoff study conducted for the proposed replat of a portion of Somerville P.U.D. located in southwest Fort Collins. It represents an ' addendum to the "Final Drainage Report for Somerville P.U.D." which was prepared by TEC in September of 1985. A copy of that report is contained in the appendix to this report. ' Methods outlined in the City of Fort Collins' "Storm Drainage Design and Construction Standards" were used in the calculations for this study. Summary calculations and other supporting material ' are contained in the appendix to this report. The rational method was used to determine the modified runoff flows from the site. ' II. -SITE DESCRIPTION Somerville P.U.D. is an existing development on 2.56 acres of land in the northwest quarter of Section 21, Township 7 North, ' Range 69 West of the 6th P.M. in the city of Fort Collins, Colorado. It is bounded to the west by Somerville Drive, to the east by the Pleasant Valley and Lake Canal, and to the south by an undeveloped tract of land. The site slopes gently downward from the northwest to the southeast and was included in the Master Drainage Plan for the Fairbrooke Special Improvement District. (See Appendix.) Somerville P.U.D. Addendum to Final Drainage Report page 2 II. SITE DESCRIPTION (Continued) All utility and street improvements for the site are already in place. These improvements include a two -foot wide drainage pan which runs from the south end of Somerville Court to the Pleasant Valley and Lake Canal at the southeast corner of the site, where a concrete apron drops down the side of the canal. Rip rap erosion protection has been placed at the toe of the apron in the canal. Somerville P.U.D. has been partially -built out with duplex ' residential units existing on lots 1 through 8 and lots 17 and 18. The proposed replat would combine the eight remaining duplex lots into four single-family lots. The attached Drainage and Grading ' Plan shows how the proposed lots would be configured. Grading contours shown for the area to be replatted are for proposed final grading and do not necessarily represent existing grades. Stormwater runoff from the site currently flows undetained into the Pleasant Valley and Lake Canal. As part of the Fairbrooke Master Drainage Report (1983), excess runoff from this site, along with that from other areas of Fairbrooke, is to be discharged from the canal by means of a side -outlet spillway further downstream. See the appendix of this report, "Final Drainage Report for Somerville P.U.D.". Basin boundaries within the project were changed somewhat when a new grading plan was approved in 1986. Apparently, no changes were made to the original drainage study when the grading was changed. This study includes the revised boundaries for the entire site to reflect this change. III. RUNOFF CALCULATIONS Current City of Fort Collins design standards require stormwater runoff to be calculated using a slightly different method than was used in the 1985 drainage study for this site. Therefore, although the actual runoff from the site will be reduced slightly, the calculated runoff value for the site appears higher than reported in 1985. This is due largely to the method of determining the Runoff Coefficient, or "C",'to be used in the Rational Method calculations. The 1985 study used an over-all C which was based on the zoning classification, while the current study uses a value which is calculated from the proposed site characteristics. In the case of Basin A, the 1985 study used a Somerville P.U.D. Addendum to Final Drainage Report page 3 III. RUNOFF CALCULATIONS (Continued) value of 0.45 for the basin C, while the current study determined the value to be 0.57. Since the Rational Method equation is a simple linear relationship, the increase in calculated runoff is proportional to the increase in C, all other factors being equal.The Runoff Coefficient for Basin B was calculated to be only slightly higher than the value used in the earlier study, so the previous and current runoff values are much closer than for Basin A. The 1985 drainage study calculated a 2-year historic runoff of 1.02 cubic feet per second (cfs) for the entire site and it determined that the 100-year flows from Basins A and B would be 2.54 cfs and 3.27 cfs, respectively. For consistency, stormwater runoff from the project site was evaluated for both existing and proposed configurations, based on the revised basin boundaries and on current City design standards. It was found that the flows from Basin A would drop from 1.9 cfs to 1.8 cfs for the 2-year storm event, while the 100-year storm runoff would drop from 7.4 cfs to 7.2 cfs. Flows from Basin B would remain unchanged at 0.5 cfs and 1.8 cfs for the 2- and 100- year events, respectively. Runoff calculations and results are summarized in Table A. The Fairbrooke Master Drainage Plan allows for a spill of 66 cfs out of the Canal at the downstream spillway. This is based on a simple sum of contributing flows and does not allow for channel attenuation and is, therefore, conservative. Based on proportional areas, the flow allotment for the Somerville site is 1.7 cfs for the 2-year and 6.8 cfs for the 100-year events. Since the cumulative flows from Basins A and B exceed the planned amount, a revised depth of flow was calculated for the downstream spillway to take into account the additional 2.1 cfs. Using the same method as the original study, the elevation at the weir crest was found to increase by approximately 0.01 foot to accommodate this increased flow. More realistically, the peak flows, which were simply summed to calculate the additional peak at the weir, would actually attenuate by the time they reach the spillway, so these calculations are somewhat conservative. ' Somerville P.U.D. Addendum to Final Drainage Report ' page 4 IV. SUMMARY Essentially all of the stormwater runoff from the site.is to ' flow undetained from Somerville P.U.D. into the Pleasant Valley and Lake Canal. With the proposed site revisions, the minor storm runoff will drop by 0.1 cfs, while the major storm flow will decrease by 0.2 cfs. 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H i0 H 10 .., 'p IA mm nm L9m n m m 110 m ELM E m cm E V 0 C, ii E L E L S L E L 0 q- n'+ 6 ti 0'- L L L L '+ 0 '+ 0 4 0 y. 0 ''+C'+C4 C'+C r i - L C 7 7 L C L 7 7 L OILO L 0)L-0 C y C i' L4 0) � r Iq C.0 m 0 c.0 N 0 UI N 0- H In VI CL In X a m •. P' i H a 0 aH n) 0 n. X L X L IL L CL L L L CL L T m m 19 L 0 L Q' L N L 2+ m 71m Tm JIm 7, Q' 1 4' I W I N I TO _nn 710 TG IOIOIO10 N.N• N- N. if If I! 11 II 1! if 11 mli Un m� U"0 *-� -+ � �+ fd C•1 fJ fJ Ul W F O 2 APPENDIX DESCRIPTION PAGE Calculations ......................................... I References ........................................... 4 1985 Drainage Report ............................... 8 :: �2�a�NQS� T2;eY.IoHS 4'ZZ-nl ' i I � �JI(..71N4S � DI`t�%Z� l p•. �5 1Qc. C �'. : li$. I I + ! !J 1 1 ' � G=, •C (�,3s)(•9s�+; (r3��(,95� t C,71� �,Z��'� � {�) ,�}$, I � L� I I JLoh3C.5 G I 1 r,r I� i 4Ic c= i yy-I I Cr7 f lrw'Alls, III ' Ii�i G44 !:.!jl!III a.N i:, .'!III ► ,j ! u Pnac�1��NII Tli., r C1`^aMNIK41 I p�j..lpc 2A ! 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I I � 1 ; I I I I i I I 1 1 i t I No Text LOVELAND STORM DRAINAGE CRITERIA MANUAL FIGURE 602 TRAVEL TIME VELOCITY FOR RATIONAL METHOD so 30 h 2O z w U D: w a 10 w 0oil NOR O N 5 w ¢ 3 O U 2 w H 1 5 2 3 5 10 20 VELOCITY IN FEET PER SECOND WRC. ENG, REFERENCE: "Urban Hydrology. For Small Watersheds" Technical Release No. 55, USDA, SCS Jan. 1975. iiiiiio III �IJ�I/I ��������� —b41— k 01+L0L •TRAPP• 51 ENTERt 02 'b ,• 52 3 03 PROMPT 53 / 04 STO 00 54 RCL 06 05 •Z 55 X<3Y 06 PROMPT 56 YtX 07 STO 01 57 ENTERT 08 •N ?• 58 RCL 04 09 PROMPT 59 SORT 10 STO 02 60 * 11 •S ?• 61 1.486 12 PROMPT 62 * 13 STO 04 63 RCL 02 14*L8L 01 64 / 15 •Y ?• 65 RCL 05 16 PROMPT 66 * ;Tip m 67 STO 97 an E_; 68 'b: 01 69 ARCL 00 9 x 70 XEO 03 RCL H 71 Z: . 72 ARCL 01 Fri 01 73 XEO 03 x 74 •N: STO 95 75 ARCL 02 RCL 03 76 XEO 0.3 27 RCL 01 77 •S: 29 * 78 ARCL 04 29 RCL w 79 XEO 03 30 + 80 'Y: 31 RCL 03 81 ARCL 03 32 * 82 XEO 03 33 STO 06 83 •O' 34 RCL 01 84 ARCL 07 35 Xt2 85 XEO 03 36 1 86 RCL 07 37 + 87 RCL 05 38 SORT R8 / 39 RCL 03 89 •V • 40 * 98 ARCL X 41 2 91 XEO 03 42 * 92 APV 43 RCL 00 93 ,TO 01 44 + ='4H.Pl 03 l' SNTERt 41 : rl_ 06 4 Ir;)y 97 Sliip 98 RTt1 49 cl 4 99 END 50 2 z'.Iy C,j �N cog '., IF j l 5Y /lva r4 n. 1 r.4 c, l 1 A 2135'/� A : (L-i v') .a W TZ n u a'! MWIN q, ior' ,�,FROW CREST!O -' AZ LAY TO GRAK r IJP�Ifl K ki V LAIC ro� ; ATYP a q'y F IGN PEf C1 i1 Z'�41 'Z.A' p'. ♦ E:, F 3j4 fir,. tq "IV q . OKI: All YI�... F FORT"i'5OLL-W. Mx� T 0" Ix f,:.ti 72 0 ;4 k I LE V '�tU 0 3 7�- oq 7, 75 I. pk kj .4 m't-trJ FINAL DRAINAGE REPORT FOR SOMERVILLE P.U.D. CITY OF FORT COLLINS LARIMER COUNTY COLORADO September, 1985 Prepared By: TEC, The Engineering Co. 1630 So. College Ave. Ft. Collins, Colorado 80525 September 6, 1985 Re: Somerville P.U.D. Drainage Report 85-010 City of Fort Collins 300 LaPorte Ave. Ft. Collins, CO 80522 Attn: Ms. Bonnie Tripoli Dear Bonni: Enclosed please find the Stormwater Drainage Report for the Somerville P.U.D. for your review and approval. Should you have any questions regarding this report, please do not hesitate to call. Sincerely, 0�orz=�' OV C H. Meslj r.E. encl. THE ENGINEERING CO. 1630 SOUTH COLLEGE FT. COLLINS, CO 80525 PHONE: (303) 484-7477 STORM DRAINAGE REPORT ' SOMERVILLE P.U.D. FORT COLLINS, COLORADO General The purpose of this report is to outline the methods of conveyance and disposal of stormwater runoff for the proposed Somerville P.U.D. the project consists of 9 duplex lots situated on 2.56 acres. This report was prepared in combination with the "Preliminary Design Report - Diversion of Storm Water Runoff Through Irrigation Canals From Mulberry St. To Spring Creek, Ft. ' Collins, Colorado" by Resource Consultants, July 1980, and the "Final Drainage Report for Fairbiooke S.I.D." by Parsons and Associates, Nov. 1983. Site Description The Somerville P.U.D. is located in Section 21, Township 7N, Range 69W of the 6th P.M., City of Ft. Collins, Larimer Co., Colorado, consisting of 2.56 ' acres. In general the site is bounded on the north and east by the Pleasant Valley and Lake Canal, on the west by Somerville Dr. and on the south by unplatted land. ' The site is presently undeveloped and is used primarily for agricultural purposes. The existing drainage pattern is to the northeast at a slope of approximately 1 to 3% and drains in to the Pleasant Valley and Lake Canal. Design Criteria The design criteria for this study was based upon the Denver Regional Council of Governments "Urban Storm Drainage Manual". The rational method can be used to evaluate flows for the site for the 2 year and 100 year for both the developed and undeveloped conditions. The rational method relates runoff to rainfall intensities by the formula Q - CiACf where: ' Q = Maximum peak runoff (cfs) C - Runoff coefficient (in./hr.) 0.45 for developed lots ' 0.95 for asphalt paving 0.20 for unimproved areas 0.20 for lawns with flat slopes (less than 2%) ' 0.25 for lawns with average slopes (22 to 7%) 0.35 for lawns with steep slopes (greater than 7%) L = Average rainfall intensity Intensities are based on intensity curves shown in Exhibit A. ' Cf Frequency Factor 1.00 for a 2 to 10 year storm 1.25 for a 100 year storm 1 Proposed Storm Drainage System ' The overall drainage plan for the Somerville P.U.D. is shown of Exhibit B with the two and one hundred year flow routes. With the grading plan it is intended to convey all storm water from the site into the Pleasant Valley and 'Lake Canal. It is understood that the canal cannot handle undetained runoff from new developed areas. The Final Design Report for Fairbrooke S.I.D. has proposed using a 66 cfs side outlet spillway downstream from Somerville P.U.D. It is proposed to allow the Peak flows from Somerville to flow into the canal, then released those flows at the side outlet downstream. The location of the outlet structure is shown on Exhibit B. Basin A consists of 1.18 acres which include Somerville Ct. and the lots on the west and south side of the P.U.D. Drainage is generally off of the .lots onto Somerville Ct. and collected at a low spot in the southeast corner of the Cul-de-sac. From there water is released through a curb cut and flows through a concrete lined swale'into the canal. Basin B consists of 0.97 acres on the east side of the P.U.D. Drainage from ' these lots is generally to the east into the canal by sheet flow and through swales created by the grading between the buildings. ' A copy of the calculations to arrive at the 2 year historical 100 year developed flows is included in Appendix A. This illustrates the discharges projected for the site. ' Historically the site does not present any major runoff problem as the drainage area is small and resulting discharges are also small. The release point into th canal will be protected by rip -rap to prevent erosion problems. It is believed that the small contribution resulting from this site can easily be handled by the canal with the addition of the side outlet spillway. Calculations for the side discharge spillway are included in APPENDIX A (page ' 2). 2 APPENDIX A CALCULATIONS Historic Flows - 2 Year A = 2.15 Ac. D1 = 300 ft. S1 = 5120 - 5113 = 2.33% 300 C. = 0.25 Cf = 1.00 T= 1.87(1.1 - CC )D1/2 1.87(1.1 - 0.25(1.0))3001/2 c S1/3 f 2.330.33 = 20.8 min I 1.90 in./hr. Q2 = CCfIA = 0.25(1.0)(1.9)(2.15) = 1.02 cfs Basin A - Developed Flows A 1.18 Ac. D1 300 ft. S1 = 5118.5 - 5115.5 = 1.00% 300 C 0.45 Ci 1.25 Tc1 = 1.87(1.1 - CC,)D1/2 1.87(1.1 - 0.45(1.25))3001/2 S1 3 1.00.33 = 17.41min D2 120 ft. S2 = 5115.5 - 5112 2.922 120 C = 0.45 Ci = Tf 1.25 = 1.87(1.1 - CC )D1/2 = 1.87(1.1 - 0.45(1.25))1201/2 c2 f S1 3 2.920.33 = 7.73min Tctot ' 25.14 min. I 4:8 in./hr. 100 Q100 ' CCfIA = 0.45(1.0)(4.8)(1.18) = 2.54 cfs Basin B - Developed Flows A = 0.97 Ac. D1 = 125 ft. S1 = 5118 - 5112 = 4.80% 125 C = 0.45 1.87(1.1 - 0.4511.25))1251/2 4.80.33 ' APPENDIX A (Page 2) Side Outlet Weir Calculations Basic equation for a sharp crested side overflow weir ' Q 3.32 10.83h1.67 Ref. ASCE MOP -No. 37 Page 113 eq. 38 ' Proposed structure is actually a broad crested weir. Therefore, c factor must be corrected. ' Ref. "Handbook of Hydraulics" by King & Brater Page 5-24 c = 3.087 Solving for h h = Q 1/1.67 h 0.75 ft. 3.087 1�3 Page 5-46 c = 2.70 ' h = 0 1/1.67 h = 0.81 ft. 2.70 ' Top of Ditch Bank = 5111.80 field mess. Top of Exst. Earth spillway approx. 65 ft. east of bike path bridge = 5110.30 field meas. Normal High Waterline = 5110.20 ' (Ref. Resource Consultants' report on Diversion of Storm -Water Runoff, etc. ' Therefore, set crest of spillway at 5110.30 (same as existing). ' Therefore, maximum waterline @ 66 cfs. is 5111.05 to 5111.11 depending on which coefficient is used. Note: Resource consultants report recommended setting ' the crest at 1 ft. below top of bank which would be 0.50 higher than the existing earth spillway. ul aW z i z m W x w vnoH V3d S3HON1-AIISN31N1 [ I • VM ti u V u.1 L CL ltJ 8." 1 •fx ,µjl O NN rM fM Nf V LL 40 N .-•0 W < 1 ty � 4 =RS I i• t t 1 • C % ' LL LL J I � • V Y 7�[ � � kt 1y N YI T t• � [ 1 v I x YI 1 1arekL zco t Y V •+' N NOTES: TOPOGRAPHY SHORN ON LOTS 0 THRU 12 IS FOR PROPOSED FINISH GRADE All street, liters Sewer, eenitery Honest mE water line construction dll conform to Cl ty of Fort Co l line Standards u d specifications current at date or ex c tlon or the development agreement pertaining to this do Clinical. Any detruauon occurrly e three rears after the xecution or the envelopment agreement shall require re-examl}requireiof the plane by the Director of engineering, who may that they be Made to conform to standards aM specifications current at that time. Street pnvinR .mall not begin until the Nulls report and pavement design are approved by the Director or FnNinnnr fng and suhYrede (inspection tests taken and approved. The slre, type, location, am NgE!6 r all known underground utilities are approximate when `hown on the drawings. It .nail be the responsibility of tire contractor r fry the existence and location of all underground nUlrrLles In the area or the work. Before cenmencing new Construction. the contractor shall be eeponmible for :eling all underground utu ittee and shall be responsible for mll unknown uoderyrrood utilities. The contractor Still off Northern Engineering Services, Inc. or any Incorrect or unfnrasen site conditionm rifler to nonmana sent of work. The enoinmer who ham prepared these piano. by execuuion and/orandr goal hereof, does hereby strife responsibility to the City, as a beneficiary or said enginsar m work, for any errors and onisslans contained theme piano and approval ,if these in plane by life Director of Engineering shall not relieve the engineer who ham prepared these Plana of all much responalhn fly. When an existing asphalt street moat be cut. the street must be. restored to a rendition equal to or better then Its ICY q midi ,onn.0 1. ion. fife existing street condition shall be ,I nnume nl ad fly Lhe Ti Ly Can a t ruc 1. l oil Inapec t or he rare any Bute are made. listening .hall ha done In conformance w1 In the Clly of Fort Collins Street Repair Standards. The finished Patch anon bland mmaathly Into the existing sumo.. All large p•teneo shell be paved with asphalt lay -dawn .chi a In streets where more than o e out is mada, n overlay of the entire .treat°idth, Including the patched else, may be require°. The determination of need for nmpiate overlay .hail be man" by the Director of Engineering. Prior to Commencement of coinstrisition. vine contractor sh ti contest all utilltitlix and minrt the City Forester to schembute a slemselonapaction for any trot removal requiring a permit. flogel Prior to commencement or any construction, the fontractor shell give the city Engineering Department twenty- our au hours advance notice. Prior to uoem foement of any construction that will effect any traffic osigns of any type. the contractor shall tnntaet the city of Fort Collins Streets Division, mite will emporarily remove or rel°oeke the sign at no cost to the hantractor; however, if the contractor moves the traffic sign imself. the contractor will be charged for the labor. materiels and equipment to reinstall the sign sa needed. Ali damaged existing curb, gutter and sidewalk shall be repaired prior to acceptance or completed Improvements. These documents are Instruments of Service provided by Northern Engineering Services, Inc. and ehmll not be reproduced or in any way used by anyone without prior written approval by Northern Engineering Services. Northern Engineering services. Ina. in not responsible for the exietenc , give. type or location or any buried utilities shown on these plane, her for other unfurseen site conditions, rho city of Fort Caning shall not maintain drainage facilities outside of the street Right at ray yS o.e• Main 0.06. / 2- / 2--0- Drainage Pan Section (NTS) TBM "X- N.W.;COR. \\// BRIDGE ABUTMENT 5118.40 � SCALE: r = 30' APA N�♦ I �VI I i W i= �I I� III III A— News list j � \ ti TFm20.2 .,mac 115 FGNWo TF•AJ Drainage Basin Runoff Table Basin Design Area 02 cm Point (AU (cfs) CieS) A 1 1.45 1.8 T2 B'. 2 0.42 0.5 1.8 LEGEND B Based Designation ® Design Point Bass Boundary Property Line \- Flow Direction Exlst. Conc.1 To°beeclothes (See Defoll lJ NORTHERN ENGINEERING SERVICES Project: Print Date: — heat 9108.00 NRYE01�1 SOMERVI LE P.U.D. Ist REPEAT 420 SOUTH HONES SUITE 106. FT. COLLINS. COLORADO 80521 Scale: rao" et net mar: PFJ Checked D NRN GRADI G % DRAINAGE PLAN L (303) 221-4150 Orof,an�n, roan Prnnnreo- nAy k2q Sheen