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Home My WebLink AboutGrading Plans - 07/01/2021July 01, 2021 Dan Mogen, EI, CFM Development Review Engineer Stormwater Engineering & Development Review Division City of Fort Collins Utilities Re: Drainage for 232 Lyons Street Proposed Garage Dear Dan, Grading/Stormwater Plan Accepted (Building Permit Released) Accepted by: Dan Mogen Date: 07/01/2021 Reclamation Village Lot 25 B2102780 As discussed, the plan is to convey all the roof runoff from the garage to the proposed rain garden on the south side of the garage. The additional sand and gravel included with the rain garden installation per the attached detail will alleviate any concerns about the roof area not increasing the runoff to the alley or the adjoining properties. The increased roof runoff will be conveyed into the rain garden and percolate through the sand and gravel over time. I have corrected the imperviousness and rational calculations to account for the gravel apron at the garage door and the gravel at the west side between the garage and the patio. You have in your file two rational calculation files showing that the increase in runoff from entire lot (6000 sf) for the 2-year and 100-year events would only be 0.01 and 0.03 cfs respectively. In fact, the lot runoff is split east and west about the middle of the existing house. The second spreadsheet only deals with the eastern portion of the property (East basin), (4000 sf), which includes the new garage. This calculation shows that the increase will only be 0.01 and 0.04 cfs for the two design storms. In conclusion, it is my professional opinion that the rain garden constructed per the detail provided with the building permit application sans the underdrain, which is not topographically impossible, will contain the negligible increase runoff from the new garage roof thereby not causing any damage to the adjoining properties (234 & 232 Lyons and 231-235 McKinley) and alley between these lots. Sincerely � Y `^"` �� .y� � Mark Oberschmidt, P.E. P.E. 35721 ��PDo Llc� c,��.�•�BERS�; �`�'�` .a :�� y . :� 35721 �; � : o�-ol-zozi ; � .� :� ��`�•S,S/........�� ONAL � 235 Lyons Street moberschmidt@�mail.com (970) 980 3557 June 26, 2021 Dan Mogen, EI, CFM Development Review Engineer Stormwater Engineering & Development Review Division City of Fort Collins Utilities dmogen@fcgov.com Re: Drainage for 232 Lyons Street Proposed Garage Dear Dan, Thanks for taking the time to talk with me on Wednesday about this project. As discussed, the plan is to convey all the roof runoff from the garage to the proposed rain garden on the south side of the garage. The additional sand and gravel included with the rain garden installation per the attached detail should alleviate any concerns about the roof area not increasing the runoff to the alley. The roof runoff will be directed into the rain garden and percolate through the sand and gravel over time. I have corrected the imperviousness and rational calculations to account for the gravel apron at the garage door and the gravel at the west side between the garage and the patio. You also asked that I include detention calculations using the City of Fort Collins 0.35 cfs per acre release rate for this basin even though I contend that they are irrelevant since the roof runoff is not being detained and released off site as in a traditional detention pond design but conveyed directly into the soil below the rain garden. I have provided the requested detention calculations using a historic 2-year, historic 100-year and the 0.35cfs /acre release rate for your records. The rain garden sizing recommendations remain the same as before. Attached please find the following documents requested by the City for the proposed garage at 232 Lyons Street. • Revised Site Plan with rain garden notes • Detention calculations • Imperviousness and rational calculations updated to reflect the gravel on the east and west sides of the garage. • Rain garden cross section Please feel free to contact me if you have any questions. Sincerely c�,� �� Mark Oberschmidt, P.E. P.E. 35721 ��PD o L,c� � �s �o.��;oBERs�y �"o �� 35721 v: � �. 06/26/2021 � � �°.�.ss�........��' ONA1- � 235 Lyons Street moberschmidt@�mail.com (970) 980 3557 Drainage Calculations Site 232 Lyons Street City Fort Collins West Vine Basin Sub basin 71 State CO Lot Dimensions Length Width Area Area ft ft sf ac 120 50 � Percent Imperviousness Existing Lot Areas sf Roof Concrete Lawn Gravel Total Percent Existing Concrete Area sf Impervious 90% 100% 2% 50% Patio 34.3 19.7 1038 Front Walk 3.0 21.0 Total Proposed New Surface Gravel Roof Concrete Lawn Gravel Total apron 5.0 20.0 0 0 180 west 3.0 20.0 Total Resultant New Lot Areas Roof sf Concrete Lawn Gravel Total Notes 1 The existing building is less than 1022 sf but used the square dimensions from the site plan for the purposes of the imperviousness calculations 2 No change to the western end of the site 3 Garage Dimensions 221�x � 20 �sf 4 existing 4x4 sheet adjacent to SE corner of house increases existing roof area by 16 sf 5 patio area is based on field measures and calculated as a rectangle Conclude: The assumed imperviousness for the West Vine Basin sub basin 71 is 45%. Therefore, the addition of the garage roof and other impervious areas will not significantly increase the drainage runoff from the property to the alley or adversely the basin, the adjacent properties, or alley way from a drainage standpoint. Roof Overall Lot area 0.138 acres Area draining to Alley 0.092 acres see assumption below Existing Lot Areas sf Concrete Lawn Gravel Total C C C C Comp. Q2 Q100 roof concrete gravel lawn C cfs cfs ,,, 0.95 0.95 0.5 0.2 <� V Q = CCfIA Comp C = C2 Roof Concrete Lawn 1478 738 3604 Resultant New Lot Areas Gravel Total C C C C Comp. Q2 Q100 roof concrete gravel lawn C cfs cfs 180 6000 0.95 0.95 0.50 0.20 0.66 0.172 0.751 Assume 12 11100 Cf Cf Comp Comp Tc iph iph 2 yr 100 yr C2 C100 min � 5 2.8511 9.95 V 1 1.25 ��I u Assume 1 2/3 of lot slopes to the east and the alley 2 1/3 of the lot slopes to the west and Lyons Street 3 rainfall intensities from Fort Collins stormwater manual 4 Cf from Fort Collins stormwater manual Lot Setback Front of Delta Delta/ depth (SB) ft House to (LD-SB- Lot (LD) Ridge FH) Depth ft beam ft (FH) � 120 25 15 � 67% of lot slopes to east Delta Delta Q2 Q100 cfs cfs �� Conclude the increase in flow off the roof for both storms is barely measurable therefore the rain garden will adequately capture the runoff from the roof. Direct roof runoff into rain garden via pipes Table 3.2-2. Surface Type - Runoff Coefficients Surface T Runoff Coefficients Hardscape or Hard Surface Asphalt, Concrete 0.95 RooRop 0.95 Recycled Asphalt 0.80 Gravel 0.50 Pavers 0.50 Landscape or Pervious Surface Lawns, Sandy Soil, Flat Slope < 29'0 0.10 Lawns, Sandy Soil, Avg Slope 2-7% 0.15 Lawns, Sandy Soil, Steep Slope >7% 0.20 Lawns, Clayey Soil, Flat Slope < 296 0.20 Lawns, Cla e Soil, Av Slo e 2-7% 0.25 Lawns, Clayey Soil, Steep Slope >7% 0.35 7able 3.2-3. Frequency Adjustment Factors Storm Return Period Frequency Adjustment (years) Factor (C,) 2, 5, 10 1_00 25 1.10 SO 1.20 100 1.25 — v - � � � U Q � i Lfl � L � v O O � +� '� U Q1 � � � U � d` 4J m 7 � � L � a--� Q Q `� � � � `� � � � v � fB i i V OC lD Q � I � U t � C �n � � 4J C � O (7 � � � c � a� t .� � � co � C � 0 � � J � � (6 a v N � O O i 0 0 0 O n N � � � Q +� � � � � L � � � � � •� � � v O � OJ (7 Q C '6 � � � � � � � � O � Q � � N � o `" � c��o i � N f0 � ci1 U � co N Q 0 � � � v C N t � 0 0 �I .� � N Q C N � � � C .� � ( z S ( 4 < t ( ( C < S N � z i Q 4 Q ( l ( < 4 u < � � 1 .0 S � � > ( t w i +� � O t o c s 4 . � < i s c < S �p S +� ( � � S < N Z � � � C U S C , V � 3 � 3 � ) � � ' m 0 � Z � 3 N N � 3 � Q- iA Q � � i 41 N � � _ •� � ) � C � ' � � a. a-.+ � � U 3 � �� T � � �' J � ,� 1 Q � - � � � � � iO L 3 � G�p = a� � _ � '� _ N L ' � Q1 3 U � ) � � O 7 N � 3 `� O iA >' � � a� � +� � � �C � G � ) 'X � 3 � v � Q > _ (O � 3 � U ) � — T , � L � � � - N � - C � co � � C 7 3 � O O 3 •� � Q � �0 � v � 3 t .� O _ H � � Detention Calculation Summary Release Rate Area Release Volume Description ac rate Required cfs cf Historic 2-yr 0.09 0.17 315 Historic 100-yr 0.09 0.72 12 0.35 cfs/ac 0.09 0.03 783 Proj No: 1552-01-98 Shear Engineering Corporation POND CUHP 6/26/2021 ention Volume Required For 100 -Yr Storm Jason Jones Back Yard Release Rate is the historic CUMULATIVE RLTNOFF METHOD Q2 from the site FOR SIZING DETENTION PONDS Design Ston1� 100 year Runoff Coefficient 0.82 Release Rate 0.17 cfs Area 0.09 acres Initial Time 5.00 Minutes Time Increment 5.00 Minutes Inflow Inflow Release Storage Storage Max Time ntensit Rate Volume Volume Voluine Volume Volume min iph cfs cf cf cf ao-ft *** 5.00 9.95 0.76 228 51 177 0.00 10.00 7.72 0.59 353 102 251 0.01 15.00 6.52 0.50 447 153 294 0.01 20.00 5.60 0.43 512 204 308 0.01 25.00 4.98 0.38 570 255 315 0.01 '�** 30.00 4.52 0.34 620 306 314 0.01 35.00 4.08 0.31 653 357 296 0.01 40.00 3.74 0.29 685 408 277 0.01 45.00 3.46 0.26 712 459 253 0.01 50.00 3.23 0.25 739 510 229 0.01 55.00 3.03 0.23 763 561 202 0.00 60.00 2.86 0.22 785 612 173 0.00 65.00 2.72 0.21 809 663 146 0.00 70.00 2.59 0.20 830 714 116 0.00 75.00 2.48 0.19 851 765 86 0.00 80.00 2.38 0.18 871 816 55 0.00 85.00 2.29 0.17 891 867 24 0.00 90.00 2.21 0.17 910 918 -8 0.00 95.00 2.13 0.16 926 969 -43 0.00 100.00 2.06 0.16 943 1020 -77 0.00 105.00 2.00 0.15 961 1071 -110 0.00 ll0.00 1.94 0.15 976 ll22 -146 0.00 115.00 1.89 0.14 995 1173 -178 0.00 120.00 1.84 0.14 1010 1224 -214 0.00 Detention Volume Required = 315 cf Detention Volume Required = 0.01 ac-ft Page 2 of 4 Proj No: 1552-01-98 Shear Engineering Corporation POND CUHP 6/26/2021 ention Volume Required For 100 -Yr Storm Jason Jones Back Yard Release Rate is the historic CUMULATIVE RLTNOFF METHOD Q 100 from the site FOR SIZING DETENTION PONDS Design Ston1� 100 year Runoff Coefficient 0.82 Release Rate 0.72 cfs Area 0.09 acres Initial Time 5.00 Minutes Time Increment 5.00 Minutes Inflow Inflow Release Storage Storage Max Time ntensit Rate Volume Volume Voluine Volume Volume min iph cfs cf cf cf ao-ft *** 5.00 9.95 0.76 228 216 12 0.00 �`** 10.00 7.72 0.59 353 432 -79 0.00 15.00 6.52 0.50 447 648 -201 0.00 20.00 5.60 0.43 512 864 -352 -0.01 25.00 4.98 0.38 570 1080 -510 -0.01 30.00 4.52 0.34 620 1296 -676 -0.02 35.00 4.08 0.31 653 1512 -859 -0.02 40.00 3.74 0.29 685 1728 -1043 -0.02 45.00 3.46 0.26 712 1944 -1232 -0.03 50.00 3.23 0.25 739 2160 -1421 -0.03 55.00 3.03 0.23 763 2376 -1613 -0.04 60.00 2.86 0.22 785 2592 -1807 -0.04 65.00 2.72 0.21 809 2808 -1999 -0.05 70.00 2.59 0.20 830 3024 -2194 -0.05 75.00 2.48 0.19 851 3240 -2389 -0.05 80.00 2.38 0.18 871 3456 -2585 -0.06 85.00 2.29 0.17 891 3672 -2781 -0.06 90.00 2.21 0.17 910 3888 -2978 -0.07 95.00 2.13 0.16 926 4104 -3178 -0.07 100.00 2.06 0.16 943 4320 -3377 -0.08 105.00 2.00 0.15 961 4536 -3575 -0.08 ll0.00 1.94 0.15 976 4752 -3776 -0.09 115.00 1.89 0.14 995 4968 -3973 -0.09 120.00 1.84 0.14 1010 5184 -4174 -0.10 Detention Volume Required = 12 cf Detention Volume Required = 0.00 ac-ft Page 3 of 4 Proj No: 1552-01-98 Shear Engineering Corporation POND CUHP 6/26/2021 ention Volume Required For 100 -Yr Storm Jason Jones Back Yard Q release is based on 0.35 CUMULATIVE RLTNOFF METHOD cfs/acre release rate in this FOR basin per FC SIZING DETENTION PONDS Design Ston1� 100 year Runoff Coefficient 0.82 Release Rate 0.03 cfs Area 0.09 acres Initial Time 5.00 Minutes Time Increment 5.00 Minutes Inflow Inflow Release Storage Storage Max Time ntensit Rate Volume Volume Voluine Volume Volume min iph cfs cf cf cf ao-ft *** 5.00 9.95 0.76 228 9 218 0.01 10.00 7.72 0.59 353 19 334 0.01 15.00 6.52 0.50 447 28 419 0.01 20.00 5.60 0.43 512 38 475 0.01 25.00 4.98 0.38 570 47 522 0.01 30.00 4.52 0.34 620 57 564 0.01 35.00 4.08 0.31 653 66 587 0.01 40.00 3.74 0.29 685 76 609 0.01 45.00 3.46 0.26 712 85 627 0.01 50.00 3.23 0.25 739 95 644 0.01 55.00 3.03 0.23 763 104 659 0.02 60.00 2.86 0.22 785 113 672 0.02 65.00 2.72 0.21 809 123 686 0.02 70.00 2.59 0.20 830 132 697 0.02 75.00 2.48 0.19 851 142 709 0.02 80.00 2.38 0.18 871 151 720 0.02 85.00 2.29 0.17 891 161 730 0.02 90.00 2.21 0.17 910 170 740 0.02 95.00 2.13 0.16 926 180 746 0.02 100.00 2.06 0.16 943 189 754 0.02 105.00 2.00 0.15 961 198 762 0.02 ll0.00 1.94 0.15 976 208 769 0.02 115.00 1.89 0.14 995 217 777 0.02 120.00 1.84 0.14 1010 227 783 0.02 �`** Detention Volume Required = 783 cf Detention Volume Required = 0.02 ac-ft Page 4 of 4 � � L � Jo�� -. �` s.� � � . - -- , �:; R_ ; . ,,� � , �� 5 �3 --� �- GRAVEL RAMP k -- - __zj"-." --- � �. - -- � �;s~t_ � � � � �� '`� �V<•I�� . � � t �c , P< <� `. EK , s �,-,; � � _ r C� C f C�'C. � �"'("' � i='r . -? � ..'. I i '� -��: ��c � 1 1 .�i �� r 1�: ri � �� E �� - -- . - - — - � ,\� ,. 4x � _- <v� � �f . . -- � ' � � �,�;�, L,�; ,,�' 1 r� r a � ' ✓ -' �'� �1 > ��Ol,�� fyr5'C� l;� � � GRAVEL BETWEEN GARAGE AND PAT I O Cr X ��}�c� , l_ _. ' E X' i� ' � ��=�-- , l �,' ; , \ � � �I �� i � __ , Approximate location of Rain Garden , .-�,-�_ � � 'r. �, �� s S Se`} �,e� E�. � � � . :.�-' {.: ,, .:_,'t �` - . Revised 06/26/2021 Alternate 1 1- Install rain garden along south side of garage. Minimum 5-ft from foundation 2 - Minimum required rain garden area is 30.8 sf. 3a - Downspouts from roof gutters have piping connnect that conveys stormwater from roof to rain garden. Alternate 2 See 1 and 2 above 3b - Install rain barrels at NW and SW corner and connected to downspouts. Rain barrels have piping to convey stormwater to rain garden. 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'�o J I r� I Z -X x�.;i' � I�� E _Q eSt - �E ° �' �� N x N rk� :.� x «� c r- z '-'-' s�i` �'-�^ � _ r v W W = LL = m 'i � J W � N Q E � w J d � _ awN� � � � Z � X � E � O�a' a � � � � W Q z 5 � J � z ¢ w 2 a w K F w z O U Drainage Calculations Site 232 Lyons Street City Fort Collins West Vine Basin Sub basin 71 State CO Lot Dimensions Length Width Area Area East Basin = Area ac ft ft sf ac 2/3 of overal site sf 120� 50� � � � �� 4000 0.�� Percent Imperviousness Existing East Basin Area sf Roof Concrete Lawn Gravel Total Percent Existing Concrete Area sf Impervious 90% 100% 2% 50°/o Patio 34.3 19.7 519 Front Walk 3.0 21.0 Total Proposed New Surface Area sf Gravel Roof Concrete Lawn Gravel Total apron 5.0 20A ' 440 0 0 180 west 3.0 20.0 Total Developed East Basin Area sf Roof sf Concrete Lawn Gravel Total Assume Tc min 5 C roof 0.95 C roof 0.95 12 11100 Cf iph iph 2 yr 2.85 9.95 1 Existing East Basin C concrete C C Comp. gravel lawn C 0.95 0.5 0.2 Developed East Basin C concrete C C Comp. gravel lawn C 0.95 0.5 0.2 Cf 100 yr 1.25 Area Q2 ac cfs Area Q2 ac cfs Q100 cfs Q100 cfs Conclusion Delta Delta Q2 cfs Q100 cfs Increase in Q is negligble