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HomeMy WebLinkAboutDrainage Reports - 11/05/1991Final Approved ReP►r Date 4� DRAINAGE REPORT FOR DELLENPACH CHEVROLL-T BUILDING EXPANSION AND PARKING LOT PAVING RESOURCE CON ULTINNTS INC DRAINAGE REPORT FOR DELLENBACH CHEVROLET BUILDING EXPANSION AND PARKING LOT PALING Prepared for Dellenbach Chevrolet by Paul R. Barker, PE Resource Consultants, Inc. 402 West Mountain Avenue P.O. Box Q Fort Collins, CO 80522 October 1991 Ref: 91-600.17 ' DRAINAGE REPORT FOR DELLENBACH CHEVROLET ' BUILDING EXPANSION AND PARKING LOT PAVING Introduction This report summarizes the drainage investigations for the building expansion and paving of a parking lot at Dellenbach Chevrolet, located in the southwest corner of ' Section 26, Township 7 North, Range 69 West of the 6th PM, Fort Collins, Colorado. The site is along the frontage road of South College Avenue between Foothills Parkway. and Swallow Road (see Figure 1). This report consists of four parts: existing drainage ' conditions, proposed drainage conditions, floodplain encroachment, and summary. IExisting Drainage Conditions The existing drainage pattern of the site is shown on Figure 2. The majority of the site drains to the low area at the southwest corner of the property. The outlet for the low area is a sump inlet with an 8" pipe which drains into the existing 48" storm drain along the southern boundary of the site, as shown in Figure 3. The storage capacity of this low area is 2,600 cubic feet which will detain approximately 0.16 inches of runoff from the site. This amount of storage is insignificant for detention purposes. When the sump inlet capacity is exceeded, drainage water overflows onto the Frontage Road at the south street entrance to the site. The lowest point of the Frontage Road between Swallow Road and Foothills Parkway is at the storm drain inlet to the 48" pipe. When ' the storm drain inlet capacity is exceeded, water ponds in the frontage road and backs onto the Dellenbach site. The newly paved parking lot, located along the west boundary of the property ' (see Figure 2), was historically a gravel surfaced parking lot used to store cars and trucks. The drainage from the parking lot (area = 0.78 acre) flows from north to south and is diverted to the east along the south boundary of the site toward the existing low area at the southeastern portion of the site. Peak runoff was computed for the parking lot at concentration point 1 using the rational method for the prepaved condition. Using ' C = 0.5 (gravel parking lot) and slope = 0.5%, the runoff for a 10-year storm event is 1.17 cfs, and a 100-year storm event is 2.28 cfs. I I I I Since the low area is very small and will overflow in a minor rainstorm event, the local watershed was investigated to determine the ponding elevation from runoff at the site. The local drainage for this area is shown on Figure 4. Drainage from Steele's Market was not included since its drainage flows into the storm drain south and west of the site. Also, the abandoned detention pond was not included since it would store the runoff without overflowing. Peak runoff was computed at concentration point 2 using the rational method. Using C = 0.88 and Area = 13.47 acres, the peak runoff for a 10- year storm event is 32.5 cfs and a 100-year storm event is 68.7 cfs. The volume of water ponding in the street and onto the Dellenbach site was estimated using the RESOURCE CON ULTWTS IMC 1 I Sr. Rd .. w 'Lake -•-..-..It ........ St' ." E.Lok*''' .. ::-'. St et i - - -CI 1 Ded ... . N rCt Prot ect Rd ° ' JBol eoT n. E w • D yq� O O ro'nti� Juni er Ln. J ^ Birk PL. _ Parker S t Alps 4 vdt Ct. ; — r\}r'1 .F Sheet Dr, ui' `` 4vea x W. O e•, i ¢I E F Stuo Reri i 4 y�r� N l� 'S' Wine Johnson T , /\ non bi e cr N. IYrry r.ne SeMe lr n9 Cie. nl� ... pr. 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I �� S > uoF Co Figure 1: VICINITY MAP 2 z O co o Q n w z 0 �o O Q F- W ~ c (U zziu z z_ ; _ W a cr0E 00 O u o u o U o.. pp »- m 0+� - `_ o °° lXJ 3 OD —4 �j rrrr r -- _ IICP E [ti I I I I I I I N I I� X a�lllllll gooCL II�I�II- `II O4- - :... 0 v n `v v V�-- z0000'— I M?miiiiiiiiii 0 0 t J a o, a •- 3 a, o za f w a.- Po`Wn LLJ U z r N 3 w H 2 U Q GO z w J J w G LL O x w z m O U w F- a a w a z c z O cl a) - LM o� LL 4 0 0 N L - O i- 5 z m w Q a w O Q z a �c 0 J Q U O J d' 4) L IL cumulative runoff method. It was assumed that the 48" storm drain would not restrict flow of water into the inlets. Using an outflow rate of 10.2 cfs for the 10-year event and ' 12.8 cfs for the 100-year event, a maximum volume of 36,210 and 81,989 cubic feet for the 10-year and 100-year storm events was calculated. These volumes correspond to a ponding elevation of 5,023.54 and 5,024.50 respectively, or 1.94 and 2.90 feet ponding ' depth at the sump inlet. ' Proposed Drainage ' The proposed building expansion is shown in Figure 2. The expansion to the building will include an atrium to the main building, additional office space and a showroom. Also, they will be constructing an outdoor display patio. All of these areas ' were historically part of the asphalt parking lot and display areas. There will be no change in runoff from these areas since they were impervious before the building expansion and will continue to be impervious in the proposed building expansion plan. ' The newly paved parking lot's slope and drainage patterns remained the same as the prepaved parking lot. Peak runoff was computed at concentration point 1 using the ' same parameters as before, except for C = 0.95. The runoff for the 10-year storm event is 4.00 cfs and the 100-year storm event is 6.42 cfs. The increase in peak flowrate for the 10-year storm event is 2.83 cfs, and for the 100-year storm event it is 4.14 cfs. Peak runoff was also computed at concentration point 2 using the same parameters as before, except C = 0.90. The runoff for the 10-year storm event is 54.8 cfs and the 100-year storm is 106.4 cfs. The increase in peak flowrate for the 10-year storm is 22.3 cfs and for the 100-year storm the increase is 37.7 cfs. The volume of water ponding was estimated using the same method and parameters as before except composite C = 0.90. The maximum volume of water ' ponded was 36,947 and 81,989 cubic feet for the 10-year and 100-year storm events. These pond volumes correspond to ponding elevations 5,023.56 and 5,024.50, respectively or 1.96 and 2.90 feet ponding depth at the sump inlet. From prepaved to postpaved conditions the ponded water increased in volume by 737 cubic feet and increased in elevation by 0.02 feet for the 10-year storm event. For the 100-year storm event the volume did not increase. F000dplain Encroachment The floodplain for this area was established in the Foothills Basin (Basin G) Drainage Master Plan, prepared by Resource Consultants, Inc. in February 1981. The ' 100-year flood elevation for the project site is 5025.0. This floodplain elevation is for fully developed conditions with the proposed drainage improvements. This is a higher flood elevation than -computed for the local runoff of the site described above. The ' RESOURCE CONSULTANTS INC 6 I ' floodplain limits for the project site from the above report are shown on Figure 4. ' The proposed expansion to the building will be at the same floor elevation of the existing building, elevation 5028.7. This will be 3.7 feet above the floodplain elevation. The building expansion will encroach into the floodplain limits shown. This will not have an adverse impact on the floodplain since this area is ponded water behind College Avenue. ' Summary ' The building expansion will not increase the runoff of this site; however, the paving of the parking lot will increase the runoff. The runoff will only increase the ' depth of water over the inlet by 0.02 feet (from 1.94 to 1.96 feet deep) during a 10-year storm event, which is not a significant impact. For the 100-year storm event there will be no additional runoff. The finished floor elevation of the building expansion will be 3.7 ' feet above the floodplain elevation. This expansion will encroach slightly into the floodplain, but will not significantly impact the floodplain. I 1 1.. ' 7 RESOURCE CONSULTANTS INC 1 I 1 i I t 1 1 t APPENDIX Drainage Calculations RESOURCE by La CONUTNNTs, chkd. br INC. subject Fort Collins, Colorado r i ' ' . �rra��.a� C ✓-�t.vEs U.sEEo :__:__ f�svyA�7 ' -►- � 9s . � �7 xq Co�) (ivsw.�rZ�a cw�y) 5 As./ooS. Tv= ' Quo _ O,Si3ra XU• 79) _ /. /7 eFs ' Oro:,_ c•8s S.✓�/!�t CTc= 78� = 2;28 CF5 ' > a �s r i ©-o = (0.gSt S74/oror78) = S!oo cFS i 1 l RESOURCE by .. 4 ! date sheet 0, of S CONSULTANTS, chkd. by date job no. - (oOO- ' INC. subject Q:)q en�oac\N SAct bdu(6,-152 Fort Collins, Colorado -- — t } lQMb�u(� _ G.o tnn. C�.- C • C n�D_ • ' p0�83 e_zn --- . a 2d (0 2 C = I,o CCU = b.b?) pp 3�s 0 0 C\j� - -- o c o Cl) 0 0 c u u) o 0 �Q a�U 4) 'o w 0.2 a 02 c c �.�= �mcc O MJmM k, O.` CfUfa� LL ` d *a C O 00 c`3c°�u LLc0 2Qf V �I Z X W Q a w 0 a z a cc 0 a U O J m rn i% E RESOURCE by� date sheet of COOUTMTS, chkd. by date job no. -GCO° t7 INC. b v a u� Fort Collins, Colorado 1 RESOURCE by TO 'Jve�� date sheet S of ZS CONUTanT5, chkd. by date job no. 91-600,1I 1 INC. subject Fort Collins, Colorado 1 f vy� VVJ _ Tcio=��,SC��3.15=`t�IyHyy �; u DELLENBACH - DRAINAGE CALCULATIONS Rainfall Intensity Duration Frequency 10 year, 1-hour rainfall from Figure 3-1 100 year, 1-hour rainfall from Figure 3-1 - 10 - year - - 100 - year Time Rainfall Intensity Rainfall Intensity 5.00 0.47 5.64 0.75 9.00 10.00 0.73 4.38 1.17 7.02 15.00 0.92 3.68 1.48 5.92 20.00 1.10 3.30 1.73 5.19 25.00 1.20 2.88 1.91 4.58 30.00 1.28 2.56 2.05 4.10 35.00 1.35 2.31 2.18 3.74 40.00 1.41 2.11 2.30 3.45 45.00 1.47 1.96 2.40 3.20 50.00 1.52 1.82 2.48 2.98 55.00 1.57 1.71 2.55 2.78 60.00 1.62 1.62 2.60 2.60 1.62 2.60 I u n Ponding Volume Calculations - Dellenbach Site Existing Conditions Area = 13.47 10-year CCf` = 0.88 Time Intensity Inflow Outflow Difference (Min) (in/hr) (cu.ft.) (cu.ft.) Volume 5.00 5.64 20056 3068 16988 10.00 4.38 31151 6137 25014 15.00 3.68 39259 9205 30054 20.00 3.30 46940 12274 34666 25.00 2.88 51208 15342 35865 30.00 2.56 54621 19411 36210 35.00 2.31 57608 21479 36129 40.00 2.11 60169 24548 35621 45.00 1.96 62729 27616 35113 50.00 1.82 64863 30685 34178 55.00 1.71 66997 33753 33243 60.00 1.62 69130 36822 32308 Max. Storage 36210 Estimated outflow ORFICE FLOW Q = 10.23 cfs Q=C*A*SQRT(2*G*H) C=.5 A=1.9 SF H=1.8' Stage Volume information 100-year CCf = 1.00 Intensity Inflow Outflow Difference (in/hr) (cu.ft.) (cu.ft.) Volume 9.00 36369 3827 32542 7.02 56736 7654 49081 5.92 71768 11481 60287 5.19 83891 15308 68583 4.58 92620 19135 73484 4.10 99409 22962 76446 3.74 105713 26790 78923 3.45 111532 30617 80915 3.20 116381 34444 81937 2.98 120260 38271 81989 2.78 123655 42098 81557 2.60 126079 45925 80154 Max. Storage 81989 Estimated outflow ORFICE FLOW Q = 12.76 cfs Q=C*A*SQRT(2*G*H) C=.5 A=1.9 SF H=2.8' 10 YEAR 00 YEAR Elev. Area Volume Storage ELEV ELEV 5021.6 0.0 0.0 0.0 ' 5022 962.5 128.3 128.3 5023 24750.0 10197.8 10326.1 5024 75050.0 47632.9 57958.9 5023.54 - ponding elev 10 year event 5025 ??? ??? ??? __> 5024.50 -Approx. ponding elev.(extrapolated) 100 year event some water may flow out of the basin along Frontage Road and onto College Avenue Sump Inlet Elevation 5021.60 10 year pond elev. at inlet = 5023.54 10 year pond depth at inlet = 1.94 ' 100 year pond elev. at inlet = 5024.50 100 year pond depth at inlet = 2.90 1 I ' Ponding Volume Calculations - Dellenbach Site Post -paved Conditions t Area = 13.47 10-year c = 0.90 Time Intensity Inflow Outflow Difference ' (Min) (in/hr) (cu.ft.) (cu.ft.) Volume 5.00 5.64 20512 3153 17360 10.00 4.38 31859 6305 25554 15.00 3.68 40151 %58 30694 20.00 3.30 48007 12610 35397 25.00 2.88 52371 15763 36609 ' 30.00 2.56 55863 18915 36947 35.00 2.31 58918 22068 36850 ' 40.00 45.00 2.11 1.96 61536 64155 25221 28373 36316 35782 50.00 1.82 66337 31526 34811 55.00 1.71 68519 34678 33941 ' 60.00 1.62 70701 37831 32871 Max. Storage 36947 Estimated outflow ' ORFICE FLOW Q = 10.51 cis Q=C*A*SQRT(2*G*H) C=.S A=1.9 SF H=1.9' ' Stage Volume information 100-year C = 1.00 Intensity Inflow Outflow Difference (in/hr) (cu.ft.) (cu.ft.) Volume 9.00 36369 3827 32542 7.02 56736 7654 49081 5.92 71768 11481 60287 5.19 83891 15308 68583 4.58 92620 19135 73494 4.10 99409 22962 76446 3.74 105713 26790 78923 3.45 111532 30617 80915 3.20 116381 34444 81937 2.98 120260 38271 81989 2.78 123655 42098 81557 2.60 126079 45925 80154 Max. Storage 81989 Estimated outflow ORFICE FLOW Q = 12.76 cfs Q=C*A*SQRT(2*G*H) C=.S A=1.9 SF H=2.8' ' Elev. Area Volume Storage 10 YEAR 00 YEAR ELEV ELEV 5021.6 0.0 0.0 0.0 5022 962.5 128.3 128.3 5023 24750.0 10197.8 10326.1 5024 75050.0 47632.9 57958.9 5023.56 - ponding elev 10 year event 5025 ??? ??? ??? __> 5024.50 - Approx. ponding elev.(extrapolated) 100 year event some water may flow out of the basin along Frontage Road and onto College Avenue Sump Inlet Elevation 5021.60 10 year pond elev. at inlet = 5023.56 10 year pond depth at inlet = 1.96 ' 100 year pond elev. at inlet = 5024.50 100 year pond depth at inlet = 2.90