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Home My WebLink AboutLetter from Mtn. Cross Engineering regarding Detention Recommendations 07.30.2015ivilDUNTFAN CROSS • Civil and Environmental Consulling and Design July 30. 2015 Mr. Rich Camp Richard Camp Landscape Architecture P.O. Box 958 Carbondale, CO 81623 RE: Detention Recommendations for Lot ISI 1, Aspen Glen, Carbondale, CO Dear Rich: The purpose of this cot -respondence is to provide recommendations on detention concerning the above referenced lot. These recommendations are based on a review of the proposed grading plan dated December 2, 2014 provided by your office, a site visit on January 8, 2015, and the previous Drainage Recommendations dated January 09, 2015. The previous recommendations remain intact unless specifically modified by this correspondence. The methodology that was used is the Rational Method for estimating runoff and the FAA method for estimating detention volume. Percolation tests were performed by [IP Geotech; these were used to determine the drywelI infiltration rate for release of storm flows from the detention ponds. The calculated detention volume is 977 cubic feet. This volume has been sized to detain the 25 -year stomt in conformance with Garfield County standards. Calculations are attached. The following modifications should be made to the grading and drainage plan: 1. The two areas For detention should be combined into one and have two drywells installed, The dtywells inlet grate should be placed at 5998.5. A drywell detail is attached. 2_ There should be a 10" PVC pipe that connects the drywells so that they act as one unit equalizing storm flows between them. 3. To achieve the calculated detention volume of 977 cubic feet, the northeastern detention pond needs to be increased in size. This could be accomplished by raising the top of the pond by 12", offsetting the 5999 contour up to 6000 at a 3:1 slope. 4. The pond will need to have an emergency overflow that would safely convey storm flows exceeding the 25 -year event. Adjacent properties have an casement dedicated to drainage along the property lines, 7.5' each side for a 15' total width. Maintenance will be required periodically for the drainage system. At a minimum the following should be done hi -annually in the spring and fall: pipes, swales, and inlets should be cleaned and cleared of mud and debris: the drywells and grates should be cleaned of mud and debris: the infiltrative capacity of the drywell should be verified; all heat tape should be checked; and heat tape should be turned off in the spring and turned on in the faIl. 026 Y Grand Avenue, Glenwood Springs, Co 81601 P: 970.945.5544 F: 970.945.5558 www.mountaincross-eng.com Lot IS1 1, Aspen Glen Page 2 of 2 July, 2015 Thank you for the opportunity to provide this letter. We would be happy to help with any further recommendations or evaluations. If additional site information becomes available and/or the site design is modified, we reserve the right to revise or modify the above recommendations. Feel free to call if any of the above needs clarification or if } ou have any questions or comments. Sincerely. Mountain Cross Engine C`fiiis Hale, PE ring.. Inc. V345 Mountain Cross Engineering. Inc. Civil and Environmental Consulting and Design 826'/7 Grand Avenue, Glenwood Springs, CO 81601 P: 970.945.5544 F: 970.945.5558 www.mountaincross•eng.com 24 DIAM. CAST IRON OR ALUMINUM FRAME AND NEENAH R --2560—E10 DOMED GRATE INLET UD CONE SECTION OR SOLID CONC. UD RIM FLUSH W/ FINISHED GRADE. GRADE RINGS AS NECESSARY FINISHED FLOOR OR CRAWL SPACE 10' MIN INSTALL COARSE FILTER FABRIC AROUND ALL SIDES AND OVER THE SCREENED ROCK 1-1 BACKFILL AROUND THE PERFORATED MANHOLE WITH A MINIMUM OF 12' THICK OF 1 -1/2 - SCREENED ROCK '1 SET THE MANHOLE LEVEL ON 3--6- OF SCREENED ROCK DRYWELL DETAIL NOT TO SCA E Ip nsw 04/o2/t9 >L list m amts IMPVIR dab ad. On Mt demon are Drywell Aspen Glen, IS -11 MOUNTAIN CROSS ENGINEERING, INC. Civil and Environmental Consulting and Design 826112 Grand Avenue Glenwood Springs, CO 81601 ph 970.945.5544 !x 970 915.5558 www.mountaincross-eng.c ,m INTENSITY -DURATION- FREQUENCY CURVES 6 wl oximingi ..is11MII OWNmNmenW:1Q1a1Q :•RQ■K..111:u... INIR.i1■ a`■a■,Na■N�lr� Q=u,lli. ■11#,.1.,•.....•■1■.a„w.wOM Irma ■1 1 .a11Kr8 w111131 sua[ aIMMIIIlilMMwMiCw�riiiilsr,i,m■,tlMIwaaa 11.ulIrwa■ Erna ,Qa1■1■N1 iQilm11l..... 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Kl.■■■d■1■Q.N.wwNl.Nia NQ. a■N.NNN11NNi.wNN WNW Imams I1 IME SIMliNslrSam 11f111r1r111111ems i 20 30 40 (TIME MiNUTES) 50 60 GL ENWOOD SPRINGS , COLORADO RAINFALL INTENSITY (IN/HR) MOUNTAIN CROSS ENGINEERING, INC. maim &Mit: um:Mal ComvlWq awd aal7n OM 1R 114.11 Ola.+aW *Ow.CDMel a+guarwaa 1anaawaus+.hm-memos Drainage Area : El Surface description: Existing restaurant, motel buildings, and parking w/ grass open space PROPERTY GENERAL SURFACE DATA Total Drainage Area: 21,943 sq. ft. 0.50 ac. c2,,,;:sk-,,^r✓r zke .?.....,;-,-4c._-lhJ„y it ,-, :... -:. _,Y. ...?5=�-":'6,- -"zT^ ; -l..^.ay�r� yr 1: Kx . RUNOFF COEFFICIENT sq. ft. or C Land Use est. % acres factors Building Roof 0 0.00 0.95 Scrub oak grass understory 0 0.00 0.2 Hardscape and Paving 0 0.00 0.95 Landscaped turf and planters 0 0.00 0.3 Pine and Aspen 0% 0.00 0.2 Sage and Grasses 100% 0.50 0.4 TOTAL BASIN ACERAGE 0.50 Wel• htedC: 0.400 V� 4.4:'. -.G!y -.. 1+4.4 ... 'lam '_Y,�$ri ..% .NrIRIZ55.1 :i�?'LL.{�.k.FF. v. -: YK'''F, :..414.:_ C T TIME OF CONCENTRATION Overland Flow Time Length Slope C Tc Upstream Elevation 6007.0 170 0.04 0.40 10 59 Dnstream Elevation 6000.5 Channel Flow Time Length Slope K Tc Upstream Elevation 6000.5 0.0 0.10 7.000 0.00 Dnstream Elevation 6000.5 Total 10.59 min rig. - ':V =Eer `z' :.:.- +'1i` .,r 'Z'-'- �'.1.a.4;)-45,� r. w r:. ,G1i�0Sc,,; 1'.' '-'r-.- -' :RUNOFF RUNOFF Intensity Flowrate (in/hr) (cfs) 2 -yr Storm 1.5 0.30 10 -yr Storm 2.7 0.54 25 -yr Storm 3.2 0.64 100 -yr Storm 4.3 0.87 V y.r;+... '...-::5 Al' .- 2.r. -.gay,_>' of mw NOTES: - Soils are hydrologic soil group B - Conveyance coefficient K is based on "lawn" or vegetation lined channel Drainage Area . P1 Surface description. Proposed improvements with apartment buildings, parking, and landscaping PROPERTY GENERAL SURFACE DATA Total Drainage Area: 21.943 sq ft. 0.50 ac. V x .: '' # ,4 RUNOFF COEFFICIENT sq. ft. or C Proposed Land Use est. % acres factors Building Roof 3820 0.09 0.95 Scrub oak grass understory 0 0.00 0.2 Hardscape and Paving 2057 0-05 0-95 Landscaped turf and planters 16.066 0.37 0.3 Pine and Aspen 0% 0.00 0.2 Sage and Grasses 0% 0.00 0,4 TOTAL BASIN ACERAGE 0.50 Weighted C: 0.474 TIME OF CONCENTRATION Overland Flow Time Length Upstream Elevation 6003.25 65 Dnstrearn Elevation 6000.50 Channel Flow Time Length Upstream Elevation 6000.50 0.0 Dnstrearn Elevation 6000.50 Slope 0.04 Slope 1.00 Total C 0.47 K 20.000 5.66 min Tc 5.66 Tc 0.00 RUNOFF 2 -yr Storm 10 -yr Storm 25 -yr Storm 100 -yr Storm Intensity (inlhr) 2.0 3.4 4.1 5.5 Flowrate (cfs) 0.47 0.80 0.98 1.30 NOTES: - Soils are hydrologic soil group B - Conveyance coefficient Kis based on paved channel MOUNTAIN CROSS DETENTION CALCULATIONS for Area 1 (ENGINEERING, INC- Aspen Glen I3-11, Garfield County, CO U antl imtfalMMnW CanaullNlp ana Ilisi OM 1R Oland Ammo alsrmaw1 Spkgs, CO.1p1 ....'1"...." ".'''..".......*.'.......... 04/01/15 Area = Tc = Release rate = C= 0.474 0.5 acres 5.66 minutes 0.05 cfs = 1.43 miniin. perc. rate with 4' dia. drywell 25 -yr Detention. Duration ' I Release Volume Inflow Volume Volume Difference (min.) (in./hr.) (113) (113) (113) 5 4.40 16 315 299 10 3.25 23 466 443 15 _ 2.75 31 591 560 20 2.40 38 688 650 25 2.15 46 771 725 30 1.95 53 839 785 35 1.75 61 878 817 40 1.60 68 918 849 45 1.45 76 936 860 50 1.35 83 968 884 55 1.30 91 1025 934 60 1.25 98 1075 977 Maximum Volume Difference = Required Detention = Release rate -- C = 0.474 0.05 cfs 977 2 -yr Detention Duration (min.) I (in./hr.) Release Volume 013) Inflow Volume (13) Volume Difference (13) 5 2.00 16 143 127 10 1.55 23 222 199 15 1.30 31 280 249 20 1.15 38 330 291 25 1.00 46 358 312 30 0.90 53 387 334 35 0.80 61 401 340 40 0.75 68 430 362 45 0.70 76 452 376 50 0.65 83 466 383 55 0.60 91 473 382 60 0.55 98 473 _ 375 Maximum Volume Difference = Cubic Feet 383 Cubic Feet 6000 456.00 5999 94.00 275 1 275 5998.5 2.00 46 9.5 24 299 Cubic Feet 6000 585.00 5999.0 293.00 439 1.0 439 Cubic Feet 5996.5 2.00 147.5 0.5 73 75 512.75 Channel Report Hydraflow Express Extension for Autodesk® AutoCAD® CM13D0 by Autodesk, fnc. Wednesday, Apr 1 2915 10 inch Pipe, 100 -yr Flow Circular Highlighted Diameter (ft) = 0.83 Depth (ft) = 0.47 Q (cfs) = 1.300 Area (sqft) = 0.32 Invert Elev (ft) = 5999.00 Velocity (Efts) = 4.11 Slope (%) = 1.00 Wetted Perim (ft) = 1.42 N -Value = 0.013 Crit Depth, Yc (ft) = 0.51 Top Width (ft) = 0.82 Calculations EGL (ft) = 0.73 Compute by: Known Q Known 0 (cfs) = 1.30 Elev (ft) 6000.00 5999.75 5999.50 5999.25 5999.00 Section 5998.75 0 1 Reach (ft)