Loading...
HomeMy WebLinkAboutDrainage Reports - 08/05/2008u July 28, 2008 Wes Lamarque City of Fort Collins Stormwater Utility 700 Wood Street Fort Collins, CO 80521 RE: Casey's Carwash — Drainage Letter Dear Wes, J•R ENGINEERING A Westrian Company We are pleased to submit for your review, this letter, which outlines the revised drainage design for the proposed addition to the Casey's Carwash site. This letter references the design originally approved in 1983 by James H. Stewart and Associates, entitled "Troutman P.U.D. Phase Two". All computations in this report have been completed in compliance with the City of Fort Collins Storm Drainage Criteria, dated May 1984, revised April 1997. The revised layout of the site includes an additional 1,516 square foot automobile detailing building accompanied by an employee break room. Additionally, the existing gravel inventory parking area along the east side of the proposed addition will be paved. In order to anticipate possible retrofit issues that might have impacted the design, JR Engineering met with a city Stormwater representative earlier this year. Per our April 14, 2008 meeting with Basil Harridan, it is our understanding that the proposed design will be allowed to consider both water quality and detention requirements for the proposed addition only, and that the originally approved design will be grand fathered in as it exists today and has for more than 20 years. Many options were considered to provide small, on -site detention of storinwater for the proposed addition. However, options were very limited because of the compact nature of the site and many existing underground utilities. It was therefore determined, that the best option was to maintain or reduce the existing level of impervious coverage, as opposed to trying to find a location and method for storage. To this end, the design proposes to replace the former gravel parking area with pervious concrete pavement. The design will also remove some existing concrete walks that will be replaced by landscaping. Calculations included at the end of the report (refer to Table 1) indicate that the replacement in these areas with more pervious materials actually serve to decrease the overall impervious coverage of the study area slightly. Because the city's criteria manual does not provide a rational method runoff coefficient for porous concrete pavement, the criteria set forth in the Urban Storm Drainage Criteria Manual, USDCM, was utilized as an alternative. However, the method which each manual uses to determine the composite, or area weighted average runoff coefficient 2620 East Prospect Road, Suite 190, Fort Collins, CO 80525 970-491-9888 • Fax: 970-491-9984 • w jrengineering.com differs when considering the 100-year storm event. The city manual applies a correction factor of 1.25 to the c-values listed in Table 3-3 to account for decreased infiltration capacity of the medium. Conversely, the USDCM provides a table of values for various return intervals for each material. For porous concrete pavements, USDCM uses a set of equations to determine the adjusted runoff coefficient based on an effective impervious coverage given by Figure PP-1. In order to rectify the difference between the two, each area's runoff coefficient was multiplied by the 100-year correction factor except for the pervious concrete, which was already determined for the major event as previously described using USDCM. The results were then averaged using typical area -weighted methods. The results, included at the back of the report, demonstrate that both the impervious coverage and composite 100-year runoff coefficient are slightly reduced when compared with existing conditions. Thus, no detention is proposed. Rather, the pervious concrete and landscaped areas will be utilized for water quality treatment. Water quality treatment was designed in accordance with USDCM Volume 3. Based on the procedure outlined in section 3, a 6-hour drain time, and a 2-yr runoff coefficient for pervious concrete, the amount of water quality necessary for the addition was estimated at 79.9 cubic feet. Because a 6-inch thick layer of gravel is needed to adequately support the pervious concrete, the volume provided in the gravel layer alone will provide 264 cubic feet under the proposed design. Further, additional volume will be provided in the void space of the concrete itself. Due to the fact that the volume provided by the proposed design is many times than that which is required by city criteria, the design is assumed adequate. In summary, the proposed design meets or exceeds the applicable criteria established by the manual as well as the requirements established during our meeting held on April 14, 2008. Since the design serves to decrease the volume and peak runoff rate leaving the site by slightly reducing impervious coverage, no negative impacts are anticipated downstream. Therefore, the design proposed in this letter and corresponding construction documents is assumed adequate to serve the addition to the building proposed. We greatly appreciate your time and consideration in reviewing this project. If you have any questions or require additional information, please contact us at (970) 491-9888. Sincerely, JR Engineering Prepared by: Jo Deyers Jr. roject Lead Reviewed by: Timothy J. Halopoff, P.E. Land Develo«pment Manager 'INV(W)=5035.98 BOTTOM=503'1.93 ArlEq = 3�.S�.Uy i o� Imo' ICI � I II� I� I FIN. FLOOR III 5038.47 �f I� Ii T �ESGQL��o/t/ ,c3G®G Gon/c G2Av�'G O71-7L 5%93e 20 N V(W) = 5036.08 BOTTOM=5031.9E I VACUUM ISLANDS (TrYP.) I — I STALLS (TYP. ) ---------------------- i I I 50-. I� II I SANITARY MANI RIM=50, �I WATER=50, BOTTOM=501 LL/%/L/ or 2Ei�'LSioN "./-/�RoXri✓Iili�, 01-1-�1116 ITP-Vrs�o. z,y'r - a v-c . 0,95 qo 0,60 `GD ExISh/►G SLi_ �� =' ��oPourA GrzUG. '2yv'E �I II (II � O, OJ I o S � � /�bP05c/J ,cm/�Go�/EE .Q V u ^i.'• I I I � I I a �O ° a oIa 1 v- ° o I V \�I QC ✓ - PtFi2 u�7065 if IQ� ° a 0 i u I WoI al -----------------------... a \� G orlG • c�R�ur v I a �\ I � /�RO�Dos'�J G' (/GrfUv �-� a2� at;��6 ,� • _ 2 � ..- C '�" — 50- i I �ESG2i fioill is�0. �� V..C�" 4�OF'D��.p �ii r3GoG 2�v�5./S O.�ijr 9D ; Co�vc. (ae6) 3 77, 2y (AND a/fln� & D 3 . 32 RI/iOUs EDP/. 2 0o5, 1l t1AUBG 232. Iwo p,so Gf p RUNOFF DRAINAGE CRITERIA MANUAL (V. 1) and the rainfall -runoff relationships change significantly. The'total amount of runoff volume normally increases, the time to the runoff peak rate decreases, and the peak runoff rates increase. Photograph RO-2—Urbanization (impervious area) increases runoff volumes, peak discharges, frequency of runoff, and receiving stream degradation. When analyzing a watershed for design purposes, the probable future percent of impervious area must be estimated. A complete tabulation of recommended values of the total percent of imperviousness is provided in Table RO-3 and Figures RO-3 through RO-5, the latter developed by the District after the evolution of residential growth patterns since 1990. 2.7 Runoff Coefficient The runoff coefficient, C, represents the integrated effects of infiltration, evaporation, retention, and interception, all of which affect the volume of runoff. The determination of C requires judgment and understanding on the part of the engineer. Based in part on the data collected by the District since 1969, an empirical set of relationships between C and the percentage imperviousness for the 2-year and smaller storms was developed and are expressed in Equations RO-6 and RO-7 for Type A and C/D Soil groups (Urbonas, Guo and Tucker 1990). For Type B soil group the impervious value is found by taking the arithmetic average of the values found using _ these two equations for Type A and Type C/D soil groups. For larger storms (i.e., 5-, 10, 25-, 50- and 100-year) correction factors listed in Table RO-4 are applied to the values calculated using these two equations. RO-8 2007-01 Urban Drainage and Flood Control District STRUCTURAL BEST MANAGEMENT PRACTICES DRAINAGE CRITERIA MANUAL (V. 3) 70 co 60 Cn N c rn 0 50 a E 40 I c (D ®' a) 30 a 20 Lu 10A oil All With PP's Except Underdrayso RGP / / RGP w/-Underdrains P.CP.w/ Infiltration 0000 All Other Infiltration is PPs When: to Possible and Allowed 0 0.5 1 1.5 2 (Impervious Trib. Area)/(Porous Pavement Area) Notes: 1. Chart applies only to porous pavements described in Volume 3 of the USDCM, Structural MBPs chapter. Not to be used for other types of porous pavements. 2. Apply the "Effective Percent Imperviousness" to the total area that included the area of porous pavement and the tributary impervious area that can be made to flows uniformly onto PP. 3. Use no more than two units of impervious area for each unit of PP. All impervious areas exceeding this ratio shall be treated as 100% impervious in hydrologic calculations, including runoff volumes. 4. Whenever impervious areas cannot be made to run onto the pervious areas in a uniform sheet -flow fashion, identify individual areas and what ratios apply to each and then composite them reating each as a separate area. Figure PP-1—Interim Recommended Effective Percent Imperviousness for Porous Pavements (Based on the Ratio of the Impervious Area Tributary to Porous Pavement) S-34 2008-04 Urban Drainage and Flood Control District 19 RUNOFF DRAINAGE CRITERIA MANUAL (V. 1) in which: i = % imperviousness/100 expressed as a decimal (see Table RO-3) CA = Runoff coefficient for Natural Resources Conservation Service (NRCS) Type A soils CB = Runoff coefficient for NRCS Type B soils CcD = Runoff coefficient for NRCS Type C and D soils KA = Correction factor for Type A soils defined in Table RO-4 KcD = Correction factor for Type C and D soils defined in Table RO-4 Table RO-4—Correction Factors KA and KcD for Use with Equations RO-6 and RO-7 Storm Return Period NRCS Soil Type 2-Year 5-Year 10-Year 25-Year 50-Year 100-Year Cand D 0 -0.101+0.11 -0.18i+0.21 -0.28i+0.33 -0.33i+0.40 `039+046: A 0 -0.08i + 0.09 -0.14i + 0.17 -0.19i + 0.24 -0.22i + 0.28 -0.25i + 0.32 The values for various catchment imperviousnesses and storm return periods are presented graphically in Figures RO-6 through RO-8, and are tabulated in Table RO-5. These coefficients were developed for the Denver region to work in conjunction with the time of concentration recommendations in Section 2.4. Use of these coefficients and this procedure outside of the semi -arid climate found in the Denver region may not be valid. The UD-Rational spreadsheet performs all the needed calculations to find the runoff coefficient given the soil type and imperviousness and the reader may want to take advantage of this macro -enabled Excel workbook that is available for download from the District's web site www.udfcd.org under "Download" —"Technical Downloads." See Examples 7.1 and 7.2 that illustrate the Rational method. The use of the Rational method in storm sewer design is illustrated in Example 6.13 of the STREETS/INLETS/STORM SEWERS chapter. I C D : - ©. 31(o -f o• yG /sD Y° J eD 70.3 3 - 2 y, - &Gp ' 0 RO-10 2007-01 Urban Drainage and Flood Control District DRAINAGE CRITERIA MANUAL (V. 1) RUNOFF Table RO-3—Recommended Percentage Imperviousness Values Land Use or -T Surface Characteristics Percentage Imperviousness Business: Commercial areas 95 Neighborhood areas 85 Residential: Single-family Multi -unit (detached) 60 Multi -unit (attached) 75 Half -acre lot or larger Apartments 80 Industrial: Light areas 80 Heavy areas 90 Parks, cemeteries 5 Playgrounds 10 Schools 50 Railroad yard areas 15 Undeveloped Areas: Historic flow analysis 2 Greenbelts, agricultural 2 Off -site flow analysis (when land use not defined) 45 Streets: Paved 100 Gravel (packed) 40 Drive and walks 90 Roofs 90 Lawns, sandy soil 0 Lawns, clayey soil 0 * See Figures RO-3 through RO-5 for percentage imperviousness. CA = KA + (1.31i3—1.44iz + 1.135i — 0.12) for CA >_ 0, otherwise CA = 0 (RO-6) j o o yor r CCD = KCD + (0.858i3 — 0.786i2 + 0.774i + 0.04) (RO-7) o.33r o.8s$! ssi;-o.7�6(.3s)Z r o,���lo.ss)fo.cY CB = (CA + CCD)12 2007-01 Urban Drainage and Flood Control District C .S$ G0100 D 2 -WwMJMMM His`toiicaUEzistin Con_ditio_ns Object Area I % of Total Area 2- r C value Adjusted 100- r C value Building 1213 0.202 0.950 1 Concrete 1220 0.204 0.950 1 Gravel 3560 0.594 0.500 0.625 su_m = 5993gvera e = 0.777 Proro osed�Conditions Object Area % of Total Area 2- r C value Adjusted 100- r C value Building 2735 0.456 0.950 1 Concrete 377 0.063 0.950 1 Gravel 273 0.046 0.500 0.625 Lan scaping 603 0.101 0.150 0.19 Pervious Concrete 2005 0.335 0.200 0.6 sum =FBI 5993 Average = 0.768 Table 1 - 100-year Runoff Coefficient From USDCM Project: CASF `f S CAe- JPtSt job No: 31 �7 - Z Client: Subject: By: Chk. By: Date: W 3 /O g Sheet No: I of I ft) J•R 'ENGINEERING A Westrian Company T ,, !! t'o((i 5 T-yaEVVIEJ�JT t L�TtIZ�L.TY: i 1V0�u-wtE - I , Vot_ttyt�E?= W-QtGV _�� iT T I I � FfZo ! FSGu ! � _ i� -ct/ = C�rL-Clgi�a�3% I ,_ II I.Q�zTf6- -I , , I 1 1 I I pia t.S�less' z- �_S7. - J 3 r ! 3T:- r - - G-= rM�C► --1- i I I -(- � � I I I! I! i • I t ! , `+L, -t I —I I �— _f T- IIIII ! I �t..�7"�•N I I I I I I 1fOLlt►vt� I I I I I I I Gyt��l I I I I 111 I � �I--I-. 3-_ I 1 _ _ij� i I!1 ! , Li1� 1 1 1 1 1 �-Tr- '-•- - 1 1 I A_I 1 l 1 11 - f ! _ __ �����,�� -U V�ay.vY� _�qL! r j` l 1 �t� �7 I ! �/ot u„nE �Rovs�ep SN_�JC1--LA - -�T S_ '2 �� 32o_F'F -CD: - 1 `i oT -� � 1, 1 1 I I T' i I I -i ! I ��� i i I !i�Q£A' I I t 1 • I i !1 IF L _ t__ I �! I !iT i l II HH I i + i! I I i f ! 1.-4 __ I _ J_—TI I I I ! fi II I ! i!i II +! - - L I- , I�� ii i ' ' � � I i iT i� ! i � �` I �� i !_ I i i �� , i I I •+ , i_, , i ' I J I I! i! J I�!�I —1 1 L I( j l 1+ i r -! rl� F ��(� l 1 LI I i 1 -t- 1 I l I '' t i T-T i I iTl f I i t i I! ! ! i i!! i '--I ! �T i- �--� t T-f-+ T I Tl--T i 1 —T+ -