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Home My WebLink AboutGrading Plans - 04/25/2016FORBES ENGINEERING Full Service Civil & Structural Engineering April 10, 2016 Dan Mogen City of Fort Collins Stormwater Engineering 700 Wood Street Fort Collins, CO 80521 BEG AT PT 50 FT W OF SE COR LOT 10, BLK 61, FTC, TH N 115 FT, W 50 FT, S 115 FT, E 50 FT TPOB B1508397 Grading plan & drainage memo accepted 4/25/2016 DM RE: �rading & Drainage Memo for site improvements at 524 West Mountain Avenue Fort Collins, CO 80521 Mr. Mogen, This letter and attached exhibits are to serve as a drainage memo & grading plan for the above referenced single family residential property. The property is located along the north side of West Mountain Avenue, two doors east of South Whitcomb Street. The surrounding neighborhood is existing residential lots. The original home was approximately a 950 square foot single family residence with a 320 square foot detached garage (the amount of concrete walks and patio are unknown and left out of the run-off calculations). The proposed site improvements include the demolition of the existing residence & garage and the construction of a new home with a modified building footprint, a full basement and attached garage. The foot print has been increased to 2240 square feet, an increase in roof area of approximately 970 square feet. The original finished floor elevation was set at 5006.95 (1988 datum) and the proposed finished floor elevation is set at 5008.0. Site improvements will not include any additional concrete for walks or drives other than what was originally on site. The existing spot elevations for the proposed grading & drainage plan are based on survey data provided by Stewart & Associates Land Surveying, Inc. dated January 15, 2016. The lot contains 5756 square feet of surface area; of this approximately 4000 square feet of the surface run-off historically sheet flowed towards the north and f�om there traveled east along the rear lot line exiting at the northeast corner of the property. This will remain the same other than now the newly constructed home will have gutters and downspouts to direct the majority of the home (from approximately the mid-point of the lot) towards the south and out onto Mountain Avenue. The amount of increase surface run-off directed to the north has been minimized by using a sloping gutter system that limits the amount of discharge to the north. The 2 year and 100 year run-off values have been calculated as being the same at 0.04 cfs and 0.17 cfs respectively (see attached calculations). The side yards have been sloped to allow drainage to flow either north to the rear lot line or to the south and into the street gutter system, The original site contained a low spot on the west lot line (at the location of the proposed window well), this area has been raised approximately 6" to allow for the drainage to function as needed. The fill will eliminate the sump condition and the added grading will not adversely affect the neighboring residence to the west as the west lot is elevated by approximately 18" above the proposed finish grade. With the increase in building footprint comes an increase in percent impervious for the site, this has been calculated at 25.1% for the existing conditions and 40.9% for the developed conditions (see attached calculation sheet). The risk of flooding is non-existent as the site is outside of any flood plain and the finish floor of the home has been set at 18" above the existing detached sidewalk. The garage floor elevation has been established at an elevation of 5007.4 which is approximately one foot above the detached sidewalk allowing for an approximate grade of 3% towards the south. It is my conclusion that with observance to the site grading and the finish floor elevation, no adverse affects to the site or to the surrounding properties are anticipated from the proposed improvement of this site. Sincerely, -- �� _ ��, , �._ x Kevin Forbes, P.E. #35265 ; �� y�z�� r , 10. �:�,:�� , � 64o S89'42'41 "E 50.02' � � ro, '?J DS � �, n• � 06.90 ui � N LOW SPOT RAISED TO APPROX. 7.1 °s s 2 � J W 3 w 3 � O N p 1� Z 3 z � O J d� � W � c9 7 �o� '�J 06.95 �ryi --- oy. + � � NEW HOME � � CONSTRUCTION � � TOF 0 7.8 I W i FF O 8.0 I Q . � � � � � I GF O 7.4 COVERED PORCH 0 9 06. + 9h o`�� 05.90 � 06.39 n �t � b 0 � PROPERTY INFORMATION OWNER: JOHN H& JOAN M SPURGIN ADDRESS: 524 WEST MOUNTAIN AVENUE FORT COLLINS, CO 50521 PARCEL NUMBER: 97114-09-024 BENCH MARK CITY OF FORT COLLINS BENCH MARK ACBM 1-12 EIEVATION = 5004.12 (1988 DATUM) THIS PROPERTY IS NOT LOCATED IN A FEMA OR CITY OF FORT COWNS DEFlNE� FLOOD PLAIN NOTES: EXISTING FF EIEVATION = 5006.95 � �S 06.43 DESIGN FF ELEVATION = 5007.20 � MODIFIED FF ELEVATION = 5008.0 EXISTING GROUND ELEVATIONS WERE SURi/EYED BY STEWART & ASSOCIATES, INC. ON JANUARY 15, 2016 N89'43'26"W I 50.00' I v 06J3 � x 06.17 DETACHED WALK 06.37 05.91 W. Mountain Avenue GRADING AND DRAINAGE PLAN SCALE 1' = 20' LEGEND: PROPERTY LINE BUILDING WALL EXISIING FENCES —x —x x —x — CONCRETE WALKS EXISIING GROUND SHOT x 06.43 PROPOSED FINISHED GRADE (O�,y� DOWN SPOUTS ❑ DS FOR B ES EN GI N EER I N G cuENT• BILL GURSKI PREPARED BY KEVIN FORBES 2908 GHERLY STREE� PROJECT� 425 W MOUNTAIN AIiE DR.4WN: DATE Ff. Co/lins, Colo�ado 80524 FT COLUNS, CO KEF 3-14-16 Tele: (970) 295-4874 Futt �ce tarc�a�Nc, DRAf1/MG AND DE5lGN SHEET TITLE.• PROPOSED GR.4DING PR�1 � OWG � cm[,/srnucnirra„ Fourvw� UVYD � AIUN/ClPAL PtANN/NG & DRAINAGE PLAN 16-2495 2495—GR North Run—off Calculations Hi�toric Run—off Areas A (Landecape) A�(Conc) Ax(Roofs) Sq F`t Sq Ft Sq Ft 3365 0 635 C = �•24 Adjusted Composite Runoff Coefficients Area = 0.092 Acres AG(Gravel) AG(Pavement) Sq Ft Sq Ft 0 0 C 2 to C lo � ioo 0.24 0.29 A.� (Total) Sq Ft � (0.092) Time of Concentration T� = TI + TT — Initial Flow Time TI = [(0.395)(1.1—05)(L)1/z��s��/s C�= 0.24, L= 100 ft, Drop = 1 ft, S= 0.01 ft/ft TI = 15.8 Minutes Use T �— 16 �inutes Kational Equation (Q=CIA) C2 = .24 I2 = 1.8 � ioo = .29 Iioo = 6.3 (Ah) = 0.092 Acres Q2 = �C2��I2��A� = 0.04 CF'S Qioo = (�ioo )(Iioo )(A) = 0.17 cFs Propased Run—off Areas A (Landscape) A�(Conc) A$(Roofa) Sq Ft Sq Ft Sq Ft 1951 0 605 C = �•30 Adjusted Composite ftunoff Coefficients Area = O.Q59 Acres AG(Gravel) A�(Pavement) Sq Ft Sq Ft 0 0 C2 to C1Q o.�o C ioo o.se A.� (Tota1) Sq Ft 2�8 (0.059j Time of Concentration TC = TI + TT - Initial Flo�w Time Tj = [(0.395)(1.1-05)(L)1�2]/(S)l�s C5= 0.30, L= 77 ft, Drop =.90 ft, S= 0.0117 ft/ft TI = io.s Minutes Use T �= 11 Minutes Rational Equation (Q=CLA) (Ah) = 0.059 Acres C2 =.30 I2 = 2.2 Q2 =(C2)(I2)(A) _ �.�4 CFS �ioo = •38 Iioo = 7.4 Qioo = �cioo )�Iioo )�A) _ �.17 CFS Storm �fater Runoff Calculations EXISTING & PROPOSED PERCENT IMPERVIOUS AREAS PERCENT IMPERVIOUS VALUES BASED ON THE FOLLOWING Character Percent of Surface Impervious Streets Paved Gravel lfalks Roofs Lawns Historic Run—off Areas A (Landscape) Sq Ft 4110 A�(Conc) Sq Ft 376 Ag(Rools) sq �t 1270 AG(Gravel) sq Ft 0 100 40 90 90 0 AG(Pavement) Sq Ft 0 Percent Impervious Developed Run—off Areas Ay(Landacape) A�(Conc) sq Ft sq Ft 3140 376 25.7� t1g(Roof9) sq Ft 2240 AG(Gravel) sq �t 0 AG(Pavement) sq Ft 0 At (Tota.l) Sq Ft 5756 At (Total) Sq Ft 5756 Pereent Impervious 40.9%