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Home My WebLink AboutGrading Plans - 09/01/2021 (2)CIVIL STRUCTURAL PLANNING SURVEYING GEOTECIiNICAL LANDMARK ; � � � ENGINEERING LOVELAND 970 • 667 -628� TOLL FREE 866379�6252 www. landmarkepc. com August 30, 2021 Project No: STRATH-20H8C- I I-32 I DRAINAGE MEMO East %Z of Lot 9, Bloci< 145, Fort Collins S� (a.lc.a. 330 East Mrytle Street), Fort Coll To Whom It May Concern, 5803 Lockheed Avenue Loveland, Colorado 80538 Grading/Stormwater Plan Accepted � (Building Permit Released) Accepted by: Dan Mogen �ion 09/01/2021 The purpose of the grading plan for the address referenced above is to protect the proposed two-story wood framed residence structure on the lot and adjoining lots from stormwater runoff. The existing subdivision for Fort Collins was established around approximately 1880. The lot is located in an area that was recently revised to Moderate Flood Risl<. According to Claudia Quezada, City of Fort Collins Floodplain Management, since this lot is now located in a Moderate Flood Rislc zone, the City cannot require new construction at or above the BFE but highly recommends the TOF be held as high as possible. The grading for the area in this subdivision will never be able to meet the current drainage and grading standards. This lot is configured as a"Type A" grading which allows for drainage to flow from the rear of the lot to the front of the lot. The newly adopted BFE for this lot is 5089.42(NAVD88) and the proposed Plot Plan shows the Top of Foundation (T.O.F.) elevation to be either 5089.83 for 5% side-yard slope or 5090.08 for 10% side-yard slope both of which are above the BFE. The existing side lot lines on the east and west of the lot are relatively flat. The previous historic drainage for the house that was razed along the west lot line flowed to the front of the lot with the west lot line at an appr•oximate 0.77% grade and the east lot line at an approximate 0.68% grade. These are existing established drainage patterns along the lot lines. These grades cannot be changed without impacting other residences and the adjoining lots. These grades were held in the attached proposed lot line grades. It is recommended gutters be installed on the proposed structure with down spouts at the corners. The roof ridge line will run north and south which would split the flows evenly to the east and west side of the lot. Downspouts should have extensions and be required to discharge slightly inward to the north at the rear and slightly inward at south the front of the structure, in the direction of historic storm-water flows. This Drainage Memo will address the Stormwater Depar-tment concerns. The current lot is 9,500 sq. ft. with an allowed floor area of 3,375 sq. ft. The proposed house has a footprint of approximately 1,815 sq. ft., with a bacic covered porch of approximately 236.66 sq, ft., a front covered porch of approximately 177.0 sq. ft. for a total of approximately 2,228.66 sq, ft., leaving approximately 7,271.34 sq. ft. as landscaped. The razed house had an approximate impervious area of 1,395 sq. ft. The increased impervious area is approximately 833.66 sq, ft. which is less than the allowed 5,000 sq. ft, in Old Town basin, so no detention is required. Attached are copies of the runoff calculations for the previous (historic) site and the new (proposed) site imperviousness. The calculations determined the historic 100-year runoff to be 0.725 cubic feet per second (cfs) and the proposed 100-year runoff to be 0.775 cfs. These calculations use the City of Fort Collins 1.25 multiplier. Thus, the additional 100-year runoff equates to approximately 0.05 cfs. Based on the relatively flat configuration of the lot, it is anticipated that this flow will be approximately equal down the two side lot lines. Therefore, the approximate additional 100-year runoff of 0.025 cfs would flow on the east and west lot lines. Doug Bennett at Stratton Homes Drainage Memo — 330 E. Myrtle Street Fort Collins, Colorado Project No. STRATH-20H8C-11-321 Page 2 of 2 The existing lot currently has a low spot located in the southeast corner that allows for historic drainage to flow onto and across the adjacent lot. As Agreed to by Mr. Bennett, a landscape berm will be placed along the front east lot line to divert this flow to the street as has historically occurred. This should reduce, if not eliminate, flow onto the lot to the east. Even though the calculated change in 100-year runoff is relatively small it is suggested to install a 4-inch diameter pvc pipe in the east and west lot line swale. Shallow swales should be used on the lot to direct the run-off flows to these pvc drains. In addition, the downstream end of the pipes shall have a bend installed to divert flows toward the center of the lot and discharge into the grass/landscape area in the front of the house. Assuming no head and minimal slope (0.4%) on the 4-inch pvc the approximate capacity would be 0.91 cfs. Assuming a split flow for the entire lot the total 100-year flow per side of the lot would be approximately 0.39 cfs. This is a conservative number since it includes the runoff from the fr•ont yard that is already there as well as and the runoff from the rear yard. Therefore, the capacity of the 4-inch pipes is approximately 3 times the calculated 100-year flow for half of the lot going down each side. The ends of the drainpipes should discharge on the front grass/landscaped area to allow for the flows to pass through the landscaped area before entering the Old Town drainage basin to provide water quality. Based on this relatively insignificant additional flow, it is our Professional Opinion any impact would be negligible. In fact, with the installation of the 4-inch pipe the side yar-d dr-ainage will now discharge in front of and beyond the proposed new structur-e as well as the structure to the east. This will probably improve the drainage for both these lots. Therefore, based on the areas and information provided above, the proposed structure should not substantially change the historic drainage of the surrounding properties. The site drainage should not impact the proposed structure and should reduce the historic impact on the adjacent properties. Stormwater runoff from the yard, will follow the historic drainage paths and flow around the proposed structure on either side or discharge directly to the front of the proposed structure and will reach the historic discharge location in East Myrtle Street. The site stormwater flow will discharge to the street as has historically occurred. The top of foundation, adjacent finish grades, and spot grades and elevations shown on the Site Plan and the information in this memo should be used to perform the final grading for the site. If you have any questions, please feel free to contact our office. Sincerely, LANDMARK ENGINEERING, LTD. � , �, � �� Rodney . Har•r, P.E. Direc r of Engineering attachements .,``�1� t 1 I I � I!!1�'l, ��eRE�J .' a,�, '.l'r f, f��`�! % F .e..o C� ��''� �s�� N A L `�� '';�ii�iiii��` Area-Weighting for Runoff Coefficient Calculation Project Title: 330 East Myrtle Street, Fort Collins, CO Catchment ID: Lot - Historic II lustration s�a'�e°� ,,,.J#� � � Flow Direc[ion S��e9 7` 1 Subazea 3 Cazchm ent EmfnAuy Instructions: For each catchment subarea, enter values tor A and C. Area-Weighted Runoif Coefficient (sum CA/sum A) = 12.08 *See sheet "Design Info" for inperviousness-based runoff coefficient values. Copy of UD-Rational v1.02a Historic.xls, Weighted C 7/23/2021, 2:29 PM CALCULATION OF A PEAK RUNOFF USING RATIONAL METHOD Project Title: 330 East Myrtle Street, Fort Collins, CO Cafchment ID: Lot - Historic I. Catchment Hydrologic Data Catchment ID = Lot-Hisl Area= 022 Acres Percent Imperviousness = 12.08 °/ NRCS Soil Type = C A, B, C, or D II.Rainfallinformation I(inch/hr)=C1*P11(C2+Td�"C3 �esign Storm ReWm Period, Tr = 10 years (input reWm period for design storm) Ci = 28.50 (input the value of C1) C2= 10.00 (input the value of C2) C3= 0786 (input lhe value of C3) P1= 1.40 inches (input one-hr precipitation--see Sheet "Design Info") III. Analysis of Flow Time (Time of Concentration) for a Calchment Runoff CoeKcient, C = 0.31 Overide Runoff Coefficient, C= (enter an overide C value if desired, or leave blank to accept calculated CJ 5-yc Runoff Coeffcient, C-5 = 0.22 Overide 5-yr. Runoff Coefficient, C= (enter an overide G5 value if desired, or leave blank to accept calculated G5 J Illustration ooerland Reach 1 }]a� Reaeh 2 � ReachJ LEGEND � Hegimiing Flmv Directio �— Catclunent Bowda.y Paved Areas & 1 Shallow Paved Swales Type II Meadow II Field II Pasture/ Lawns Calculations: � Slope Length S L ff/ft ft input input 1.0067 189 Sum 789 Bare �� Swales/ 5-yr NRCS Flow Flow Runoff Canvey- Velocity Time Coeff ance V Tf G5 fps minules Regional Tc User-Entered Tc IV. Peak Runoff Prediction Rainfall Intensity at Compuled Tc, I= 2.45 inch/hr Rainfall Intensity at Regional Tq I= 3.64 inch/hr Rainfali Intensity at User-Defined Tc, I= 3.64 inch/hr Peak Flowrate, Qp = 0.17 cfs Peak Flowrate, Qp = 025 cfs Peak Flowrate, Qp = 025 cfs Copy of UD-Rational v7.02a Historic.xis, Tc and PeakQ 7/23/2027, 2:28 PM ICALCULATION OF A PEAK RUNOFF USING RATIONAL METHOD I Project Title: 330 East Myrtle Street, Fort Collins, CO Catchment ID: Lot - Historic I. Catchment Hydrologic Data Catchment ID = Lot-Hist Area = 0.22 Acres Percent Imperviousness = 12.08 % NRCS Soil Type = C A, B, C, or D II. Rainfall Information I(inchlhr) = C1 ' P1 1(C2 + Td)^C3 Design Storm Return Period, Tr = 100 years (input return period for design storm) C1 = 28.50 (input the value of C1) C2= 10.00 (input the value of C2) C3= 0.786 (input the value of C3) P1= 2.83 inches (input one-hr precipitation--see Sheet "Design Info") III. Analysis of Flow Time (Time of Concentration) for a Catchment Runoff Ccefficient, C = 0.54 Overide Runoff Coe�cient, C= (enter an overide C value if desired, or leave blank to accept calculated C.) 5-yr. Runoff Caefficient, C-5 = 0.22 Overide 5-yr. Runoff Coefficient, C= (enter an overide C-5 value if desired, or leave blank to accept calculated C-5.) Illustration Calculations Reach Slope ID 5 wh input Overland 0.0067 1 2 3 4 Length 5-yr L Runoff Coeff ft C-5 NRCS Flow Flow Convey- Velocity Time ance V Tf fps minutes input output output Suml 189 IV. Peak Runoff Prediction Rainfall Intensity at Computed Tc, I= 4.94 inchlhr Rainfall Intensity at Regional Tc, I= 7,35 inch/hr Rainfall Intensity at User-Defined Tc, I= 7.35 inch/hr Copy of UD-Rational v1.02a Historic.xls, Tc and PeakQ Regional Tc = 11.05 User-Entered Tc = 11.05 Peak Flowrate, Qp = D.58 cfs Peak Flowrate, Qp = 0.86 cfs Peak Flowrate, Qp = 0.86 cfs ��g � �' ` a � � � 7/23/2021, 2:27 PM Area-Weighting for Runoff Coefficient Calculation Project Title: 330 East Myrtle Street, Fort Collins, CO Catchment ID: Lot - Proposed Illustration Instructions: For each catchment su6area, enter values for A and C. EGETID: lojs• Direction � �atchment 3ound�•y Area-Weighted Runoff CoeTficient (sum CA/sum A) = 23.21 *See sheet "Design Info" for inperviousness-based runoff coefficient values. Copy of UD-Rational v1.02a Historic.xls, Weighted C 7/23/2021, 11:21 AM CALCULATION OF A PEAK RUNOFF USING RATIONAL METHOD Project Title: 330 East Myrtle Street, Fort Collins, CO Catchment ID: Lot - Historic I. Catchment Hydrologic Data Catchment ID = Lot-Hist Area = 0.22 Acres Percentimperviousness= 2321 % NRCS Soil Type = C A, B, C, or D II. Rainfall Information I(inchlhr) = C7 * P1 /(C2+Td)^C3 Design Storm Retum Period, Tr = 10 years C1 = 28.50 C2= 10.00 C3= 0.786 P1= 1.40 inches (input reWrn period for design siorm) (input ihe value of C1) (input the value of C2) (input the value of C3) (inpui one-hr precipitation--see Sheet "Design Info") III. Analysis of Flow Time (Time of Concentration) for a Catchment Runoff CoeKcient, C = 0.36 Overide Runoff Coeffcient, C= (enter an overide C value if desired, or leave blank to accept calculated C.) 5-yc Runoff Coeffcient, C-5 = 027 Overide 5-yr. Runoff Coeffcient, C= (enter an overide G5 value if desired, or leave blank to accepl caiculaled G5.) Illustmlion Reach 1 Reuh7 NRCSPIand Heavy T e Meadow Conveyance 2.5 CalCula�ions: Reach ID Overland averland LEGIND llmv Q Begimting FRV�V D11ECti0] C9{fi1P1PTt S L Runoff Coeff H/N ft G5 nput inpul outpul 0067 189 0.: Convey- Velocity Time ance V Tf fps minutes Regional Tc User-Entered Tc IV. Peak Runoff Prediction Rainfall Intensity at Computed Tc, I= 2.53 inchlhr Rainfall Intensity at Regional Tc, I= 3.64 inch/hr Rainfall Intensiry ai User-Defined Tc, I= 3.64 inch/hr Peak Flowrete, Qp = 020 cfs Peak Flowrate, Qp = 028 cfs Peak Flowrate, Qp = 0.28 cfs Copy of UD-Rational v1.02a Historicxis, Tc and PeakQ 7/23/2021, 1120 AM CALCULATION OF A PEAK RUNOFF USING RATIONAL METHOD Project Title: 330 East Myrtle Street, Fort Collins, CO Catchment ID: Lot - Proposed I. Catchment Hydrologic Data Catchment ID = Lot-Hist Area = 0.22 Acres Percent Imperviousness = 23.21 °/a NRCS Soil Type = C A, B, C, or D II. Rainfall Information I(inchlhr) = C1 * P1 /�C2 + Td)^C3 Design Storm Return Period, Tr = 100 years (input return period for design storm) C1 = 28.50 (input the value of C1) C2= 10.00 (input the value of C2) C3= 0.786 (input the value of C3) P1= 2.83 inches (input one-hr precipitation--see Sheet "Design Info") III. Analysis of Flow Time (Time of Concentration) for a Catchment Runoff Coefficient, C = 0.56 Overide Runoff Coefficient, C= (enter an overide C value if desired, or leave blank to accept calculated C.) 5-yr. Runoff Coefficient, C-5 = 0.27 Overide 5-yr. Runoff Coefficient, C= (enter an overide C-5 value if desired, or leave blank to accept calculated C-5.) Illustration CaICUlationS: Reach Slope Length ID S L wh n input input Overiand 0.0067 189 1 2 3 4 IV. Peak Runoff Prediction Rainfall Intensity at Computed Tc, I= Rainfail Intensity at Regional Tc, I= Rainfall Intensity at User-Defined Tc, I= 5-yr NRCS Flow Runoff Convey- Velocity Coeff ance V C-5 fps output input output 0.27 N/A 0.13 Computed Tc Regional Tc User-Entered Tc Flow Time Tf minuces output 23.38 11.05 11.05 5.12 inch/hr Peak Flowrate, Qp = 0.62 cfs 7.35 inch/hr Peak Flowrate, Qp = 0.89 cfs 7.35 inch/hr Peak Flowrate, Qp = 0.89 cfs d�� - L �� `S � 1 Copy of UD-Rational v1.02a Historic.xls, Tc and PeakQ 7/23/2021, 11:46 AM PLOT PLAN EAST 1/2 OF LOT 9, BLOCK 145, FORT COLLINS 330 EAST MYRTLE STREET, FORT COLLINS, COLORADO S89' 59' 23"E 50.28� ALLEY I I � � — — � — — — — — — � LOT SIZE-9,500 SQ FT �� ALLOWED FLOOR AREA-3,375 SQ FT �� I j I � o i i � DS = DOWN SPOUT �� I i I N �„-� � N �� Q � Q wwco Q�� Q r!j � N � � � � Q >>� ��z wwQ ��_ � � U �o� W J W Z O Z BFE=89.42 � � ai � 3 N �i- O O O N 5.1 89 29/ � 38' OF 4" �RAIN PIPE Wlhi 0.47 SIOPE DAYLIGHTING WTO VEGETATIVE LANDSCAPE INV IN 89.21 O � 0 � �HX=�VtSy.JtS MIN=5089.33 22.5' BEND IN V. OUT 89.05 � � N � � I —EXISTING � FLOODPLAIN � I �INEFFECTIVE I AREA � � � �0 1�16.1 � 15. 20.00' � iri N r � 1.67' PROPOSED HOUSE �I I, � T.O.F.=5090.08 � 3 a OR 5089.83 -� N F.G.=5089.33 I OR 5089.58 � \ 1.67' g.00' � � � N 13.00' c�v `� COVERED 6.1 '— i � nn' 00 PORCH _ rn � 3 N d- O O O � 6' �88 Ss I�, SIDEYARD SWALE � INV IN 88.51 25' OF 4" PVC � 0.40% SIOPE, DAYLIGHTING INTO VEGETATIVE LANDSCAPE s.o7 SPOT � � � � � � � I I ' 1 OUT S89' 59' 23"E 50.28' aah��y EAST MYRTLE ST ��8838 � ti �x h �� W� h � hN �Z� o �� [ ��' �, hqW � O h �'w ai a�'� �Wp h �'�� `� � 2tih � �� w �� E h �22 � �� p� o'"v� W 0.'�W � �� ti �� q Cti �C F, U p �" �� yN � v�ih v�i w��Y q � � h wo q �ZS � qqN � Oti F' q�ry, a w� a �� p �w h �q O h Q[��, w �w� � � o� � ��2 � o N� a �ww 2 G+ 'J ti W � �i O � � p 'w,y�+ 'J. �sl �U GL 2 �� h ti�r� h O �a' q �N� b W�x � �xwy� U] F' � N ��2 � �� owq �� o �� w Nh�2� U�o~o � � � � � � h � V � ohwwti Zhwti� �LUw�'Wv'�op.�0., iri��Z�o�W�F'�Wv� tiF2�qQ��Q 0 .� � � O � .'l. 2.;ots��i��+i�vv��i STRATTON CUSTOM HOMES Snyder Land Surveying Inc, n�' SITE PLAN Profess�onal LanO Surveyor Erik JaY Snyder: PL$ �H555 0:970-686-5157 pA�: C: 74��992-9754 .µ��s�;:�e�� ds��Y°;$:�"m� 3/31 /2021 JOB "o: STRA-20-06-017 SCALE: 1 "= 20'