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HomeMy WebLinkAboutDrainage Reports - 03/23/2006TST 748 Whalers Way, Bldg, D Fort Collins, CO 80525 970.226.0557 metro 303.595.9103 fax 970.226.0204 TST, INC. CONSULTING ENGINEERS January 24, 2006 Mr. Basil Hamdan City of Fort Collins Utilities P.O. Box 580 Fort Collins CO 80522 final Approved Report Re: Lot #1 Harmony Market P.U.D., Eleventh Filin rate _NNW% Drainage Addendum Letter III Project No. 0833.0018.00 Dear Mr. Harridan: This drainage addendum letter addresses proposed revisions to the approved utility plans for Harmony Market P.U.D. Eleventh Filing dated September 1995. The Final Drainage and Erosion Control Study for Harmony Market P.U.D. Eleventh Filing dated September 15, 1995 (RBD, Inc. Engineering Consultants) was also referenced. The Harmony Market P.U.D. Eleventh Filing appears to have been constructed in general accordance with the approved utility plans. The revisions that are being proposed include the addition of a 6,095 square -foot commercial building in the northwest comer of the site, that is currently parking, and the addition of 12 parking stalls at the south end of the site between existing detention pond V and the existing building. Please refer to the attached site plan, drainage exhibit and utility plans for Lot #1 Harmony Market P.U.D., Eleventh Filing. We feel that the addition of a commercial building within the previously approved and constructed Harmony Market P.U.D. Eleventh Filing should not be held to the new rainfall criteria. This building is being proposed in an already impervious area where parking lot detention ponds and drainage already exist. It would be cost prohibitive to redesign and construct the existing detention pond to the new rainfall criteria. The proposed commercial building is located in the northwest comer of the Harmony Market P.U.D. Eleventh Filing. The site is currently being used for parking for the adjacent commercial businesses. In referring to the Final Drainage and Erosion Control Study for Harmony Market P.U.D. Eleventh Filing dated September 15, 1995 (RBD, Inc. Engineering Consultants), the proposed building is located within existing sub -basin 2. Runoff from existing sub -basin 2 historically travels to existing detention pond 'E' which is a parking lot detention pond consisting of 2.82 acre-feet. The proposed building does not alter the existing drainage pattern for the existing basin or existing detention pond. The info@tstinc.com www.tstinc.com r L J proposed building does, however, encroach slightly on the existing 100-year water surface elevation (w.s.el.) of existing detention pond 'E'. Please refer to the drainage exhibit included with this letter for a graphical depiction of the proposed building encroachment on the existing 100-year w.s.el. An attempt has been made to minimize the effect of the proposed building on the existing 100- year w.s.el. by decreasing the slope of the asphalt around the building to a minimum slope of 1.00%. This decrease in slope resulted in 384 square feet of pond surface area and 170 cubic feet of volume being removed from existing detention pond 'E'. This results in a net loss to the existing detention pond 'E' surface area and volume of 0.28% and 0.14%, respectively. We feel that the encroachment on existing detention pond 'E' and the subsequent minimization of this encroachment on the pond will not negatively affect the intended function of this pond. The proposed addition of 12 parking stalls is located at the south end of the Harmony Market P.U.D. Eleventh Filing between existing detention pond 'G' and the existing building. The site currently has an existing gazebo, garden and grass area leading to existing detention pond 'G'. In referring to the Final Drainage and Erosion Control Study for Harmony Market P.U.D. Eleventh Filing dated September 15, 1995 (RBD, Inc. Engineering Consultants), the proposed additional parking is located within existing sub -basin 1. Runoff from existing sub -basin 1 historically travels to existing detention pond 'G' which is a grass detention pond consisting of 0.16 acre-feet. The proposed additional parking does not, alter the existing drainage pattern for the existing basin or existing detention pond. The proposed 12 parking stalls are graded at 2.00% to 3.00% in the direction of existing detention pond 'G' outlet. The proposed parking stalls will consist of asphalt with a 1' concrete edge surrounding the asphalt. The runoff will sheet flow south across the proposed parking and directly into existing detention pond 'G'. A portion of existing detention pond 'G' will be slightly regraded so that the runoff can effectively flow to the existing outlet. The existing designed 100-year w.s.el. of existing detention pond 'G' from the Final Drainage and Erosion Control Study for Harmony Market P.U.D. Eleventh Filing dated September 15, 1995 (RBD, Inc. Engineering Consultants) is 4987.10, however, the existing calculated 100-year w.s.el. of existing detention pond 'G' from a recent on -site survey is approximately 4987.62. The emergency overflow elevation for existing detention pond 'G' is 4988.00. The proposed addition of 12 parking stalls will encroach on the existing 100-year w.s.el. with a maximum depth in the parking stalls at 0.52'. Also, in Section IV, Part C of the Final Drainage and Erosion Control Study for Harmony Market P.U.D. Eleventh Filing dated September 15, 1995 (RBD, Inc. Engineering Consultants), the additional parking impervious has been accounted for so that no additional storage volume is required. To minimize sediment transport during construction activities, temporary straw bales will need to be installed at the outlet of existing detention pond 'G' and around the area inlet in pond 'E'. To minimize sediment transport after construction activities, a water quality outlet structure will be constructed only on the existing detention pond 'G' outlet. In discussions with the City, it was decided that a water quality structure in the existing parking lot of existing detention pond 'E' would be costly and impractical. For construction of the water quality outlet structure on detention pond 'G' outlet, the flared -end -section will be removed. The water quality outlet structure will be cast -in -place on the end of the existing 18" R.C.P. pipe and constructed per the details on the utility plans. All supportive material for the design of the water quality outlet structure is included with this drainage addendum letter. Included with this drainage addendum letter are the erosion control effectiveness calculations and the erosion control cost estimate. We look forward to your review and will gladly answer any questions you may have. Respectfully, TST, INC. CO TING ENGINEERS r s.Mike Jacob , P.E.z MRJ/mgf Enclosures �I � Irb n oil I IT 1, i �P ! .lFFFTT FT .,, I"PAMM MA1Mom01i P.U.A. 9WI/R WAM901 PJAD. N u� m socilom r p I REV" I D. so"" , II 11 N _I I n �r F I 1 4 I II i Y uac==o=oo=c=. N � 777 r-. I �_——_-----—______________________________ w—W- --_-------------- —rmk� ------- �� ------- u `— 11 ♦ \ r—� I I--------1 \t} mll I II �♦♦♦ ` ss I I I 9 9 9 �® O \\ \\\�\ ss on'�°odoen°w RMRo q A A N A '^ n n n . —I ' II n t 11 II I I N xARMOfl' mm ROf0a1MI a�eoe� N M.R.J. S.A.S. N.R.J. I�I re err. �i s� o nn my a rr� ROT M SC" 1/24/-W Rw. 1 OF 226 S.F. ADDED TO EXISTING DETENTION POND 'E' AREA i I�a \ ` AJ EXISTING DETENTIb1 POND 'E' 100-YR W.S.EL. = 4987.00 A 1355 S.F.-REMOVED, 971 S.F. ADDED C�cTOTAL = 384 S.F. REMOVED * EXISTING POND 'E' %RF`A = 138,731 S.F. * % CHANGE _ ,28% FXIgTING nFTFNTInN PnNn 'F' 1 W-Yp�- W C FI = 4QR7 nn TOTAL = 170 C.F. REMOVED EXISTING POND 'E' VOLUME = 122,840 C.F. �?.82 AC -FT) * % CHANGE = 0.14% 4990 1 \ \\-1355 S.F. REMOVED FROM EXISTING DETENTION POND 'E' AREA 745 S.F. ADDED TO EXISTING DETENTION POND 'E' AREA M.R.J. SAS N.R.J. M / \ a omoolaoo NOT to s 1 244/06 1 OF . , Design Procedure Form: Extended Detention Basin (EDB) - Sedimentation Facility Sheet 1 of 3 Designer: MRJ Company: TST Date: January 19,2006 Project: Lot #1 Harmony Market P.U.D., Eleventh Filing Location: Detention Pond G - sub -basin 1 1. Basin Storage Volume - I, = 49.10 % A) Tributary Area's Imperviousness Ratio (i = I, / 100) 1 = 0.49 B) Contributing Watershed Area (Area) Area = 1.08 acres C) Water Quality Capture Volume (WQCV) WQCV = 0.20 watershed Inches (WQCV =1.0 . (0.91 . I' - 1.1912 + 0.78 • 1)) - D) Design Volume: Vol = (WQCV / 12) ' Area • 1.2 Vol = 0.022 acre-feet 2. Outlet Works A) Outlet Type (Check One) x Orifice Plate Perforated Riser Pipe Other: B) Depth at Outlet Above Lowest Perforation (H) H = 1.13 feet C) Required Maximum Outlet Area per Row, (A,) A, = 0.11 square inches D) Perforation Dimensions (enter one only): 1) Circular Perforation Diameter OR D = 0.375 Inches, OR ii) 2" Height Rectangular Perforation Width W = inches E) Number of Columns (nc, See Table 6a-1 For Maximum) nc = 1 number F) Actual Design Outlet Area per Row (A,) Ao = 0.11 square inches G) Number of Rows (nr) nr = 3 number H) Total Outlet Area (A„) A,r = 0.37 square inches water quality pond g.xls, EDB 212/2006, 4:31 PM TST TST, INC. CONSULTING ENGINEERS 748 Whalers Way, Bldg, D Fort Collins, CO 80525 970.226.0557 metro 303.595.9103 fax 970.226.0204 January 24, 2006 Mr. Basil Hamdan City of Fort Collins Utilities P.O. Box 580 Fort Collins, CO 80522 Re: Lot #1 Harmony Market P.U.D., Eleventh Filing Erosion Control Cost Estimate Project No. 0833.0018.00 Dear Mr. Harridan: This portion of the drainage addendum letter is to satisfy the City of Fort Collins requirements for erosion control security deposit for Lot #1 Harmony Market P.U.D., Eleventh Filing. There will be approximately 0.55 acres disturbed within this project. Using the city criteria of $650.00 per acre for sites less than 10 acres, and a 150% contingency, the total obligation for a security deposit would be: (0.55) x ($650.00 per acre) x (150% contingency) = $536.25 A cost breakdown of the erosion control measures that will need to be installed is listed below: Sod (4800 s.f. @ $0.40 per s.f.) = $1920 Straw Bale Barriers (2 @ $200.00 each) = $ 400 Water Quality Outlet Structure = 2500 TOTAL x 150% CONTINGENCY = $7230 Therefore, the larger amount, or the amount of security deposit obligation should be in the amount of $7230.00. Please call with any questions regarding this estimate. Respectfully, TST, INC. CONSUL - Mike Jaco s , P.E. ENGINEERS info@tstinc.com I . www.tstinc.com RAINFALL PERFORMANCE STANDARD EVALUATION MARCH 1991 8-14 DESIGN CR81'ERIA EFFECTIVERESS CALCULATIONS PROJECT: /-Ora / YAR AAj9lj,� f�/a. A X Z 7- P, LB , D, gl pow7w FCC tArr STANDARD FORM B r COMPLETED BY: j0p-T DATE: r`L % 0(, Erosion Control C-Factor P-Factor Method Value Value 'Comment SOD ;'R�I'. i!Zv I. 6'!'P=' W. Pr ot! A*e, P' IIJ& MAJOR PS SUB AREA BASIN ($) BASIN (Ac) CALCULATIONS 77-4 y /.C)t A(P.FS ()rV9 A?iZG OAft Sots �"jy"si.fiE.7' :. ,jCs�a .0 l.��+" !?.=`-ef`� S :. ®, Kr•/d4C�°�� 0501 X v,' 5 t)/l, 4)S x 0r1'6NX p.SO 0 N0 15® $APp- Se) L = �7@/, x i,�a Aep.t-, a 0.AeAc5, ASpr.;A -`r a x /,'rO AcRe•S /,09 A4"5 w+A C• r'A�c.TOt a C /,®DX 0.41 1 ©,ui oER3 A 0•v4 r lrird'c tl�i p-c.'+y�Pi Axuk MARCH 1991 8-15 DESIGN CRffM:UA