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Home My WebLink About00 - Design Report - Comfort Inn - Drainage Report STORM WA TER MAINTENANCE PLAN Comfort Inn 1370 North 71"Avenue Bozeman, MT 59715 The storm drainage control facilities for the Comfort Inn, consist of overland flow of storm runoff into a single detention basin located on site adjacent to the eastern property boundary. The final grades for the site have been designed so that storm runoff will flow unrestricted to the detention basins from the buildings,parking lots, and the landscaped areas. The purpose of the detention basin is to reduce the peak runoff from the development, and to remove settlable solids, silt, oils, grease, and other pollutants. The outlet structure of the detention basin will discharge settled storm runoff through an eight inch diameter pipe to the drainage channel adjacent to the northeast corner of the property. The Comfort Inn will be responsible for maintenance of the storm drainage facilities within the development. The Comfort Inn shall maintain and mow the landscaped areas forming the detention basins. The outlet structures in the detention basins should be checked periodically, and cleaned if any accumulation of sediment found. The outlet pipes should also be checked for sediment and cleaned on a routine basis. Typically, inspecting the basins twice a year should be sufficient. Any significant accumulation of sediment in the basins themselves should also be removed to retain the capacity of the basins. No fill or other materials shall be placed or stored in the detention or retention basins, as this will reduce the capacity of the basins. DRAINAGE REPORT Comfort Inn 1370 North 7th Avenue BOZEMAN, MONTANA Prepared By: Rocky Mountain Engineers 1700 W. Koch Street, Suite 7 Bozeman,Montana,59715 (406) 586-4859 Table of Contents INTRODUCTION ON-SITE RUNOFF AND DETENTION BASIN CALCULATIONS SUPPLEMENTAL INFORMATION INTRODUCTION The Comfort Inn is a commercial business proposing to expand it's building and it's parking area . The business is on approximately 2.7 acres located at 1370 North 7t Avenue. The site slopes downward to the east and north at grades from 3 to 5 percent. The property drains to a storm drainage channel adjacent to the northeast corner of the property. The site is currently occupied by the Comfort Inn, associated parking lots, and landscaping. This report describes the storm drainage detention facilities planned for the development. The Rational Method was used in this report to determine the storm runoff quantities. This is the most widely used method of determining runoff from small drainage areas such as these. The Rational Formula is: Q=C I A, where Q is the calculated storm runoff, C is the coefficient of runoff for the basin, I is the rainfall intensity, and A is the basin area. Antecedent precipitation for the less frequent,higher intensity storms is accounted for by multiplying by a"frequency factor" (Cf). This factor is equal to 1.10 for the 25 year storm and 1.25 for the 100 year storm. The modified formula for the higher intensity storms is: Q=CfCIA ON-SITE RUNOFF AND DETENTION BASIN CALCULATIONS The Comfort Inn site has been treated as a single area for control of storm runoff. Runoff from the site will flow to a detention basin along the eastern property boundary where the excess runoff from the 10 year storm will be detained. The detention basin will also reduce the amount of settlable solids, silt, oils, grease, and other pollutants in the storm water runoff. The detention basin has an outlet structure consisting of a 15 inch diameter concrete pipe set vertically in the ground at the low point of the basin. The pipe extends a minimum of 18 inches out of the ground, and the top is covered with a concrete lid. A vertical slot of predetermined width is cast in the side of the pipe to allow runoff to flow into the structure. An eight inch pipe leads from the outlet structure to the drainage channel adjacent to the northeast corner of the site.. The acreage of the site was computed, and the historic runoff determined. A value of 0.20 was used for the Coefficient of Runoff(C) for historic conditions. A mass balance computation was then made to determine the required volume of storage. A Coefficient of Runoff was calculated based on the amount of pervious and impervious area on the site. The following tables provide a summary of the area,historic runoff, detention basin storage volume, outlet structure slot width, etc. The other pages that follow show the storage computations for the area. Copies of Figures 22, 23 and 24, from the Storm Water Master Plan for the City of Bozeman, 1982, and other references, are included at the end of this report. SUMMARY OF SITE CHARACTERISTICS TOTAL IMPERVIOUS LANDSCAPED COMPOSITE BASIN# AREA AREA AREA "C" VALUE 1 116,646 sq.ft. 91,403 sq.ft. 25,243 sq.ft. 0.748 SUMMARY OF DETENTION BASIN VOLUME & OUTLET SIZING TIME OF HISTORIC DETENTION APPROX. OUTLET BASIN# AREA CONCENTRATION RUNOFF POND VOL. POND DEPTH SLOT WIDTH (Acres) -HISTORIC (Min.) (CFS) (Cu.Ft.) (Feet) (Inches) 1 2.678 23 0.640 2,490 1.5 1-7/16" The outlet slot width was computed using the formula for a broad crested weir: Q=CLH3i2 Q = Capacity in cubic feet per second (cfs) C = Constant, 2.92 L= Width of Slot H=Depth of Water in Basin k mfort Inn , Basin # 1 (2-12-2000) /1 2.88 = BASIN AREA AU 0.740 = COEFFICIENT OF RUNOFF(C) 0.64 = HISTORIC RUNOFF /H\ DEVELOPED DEVELOPED HISTORIC TIME INTENSITY RUNOFF RUNOFF RUNOFF STORAGE (MIN) (|N./HR) (CF8) (CU. FT.) (CU. FTj (CU. FTj _ 5 3.22 6.4549 1936.48 193.00 1744.48 10 2.05 4.1095 2465.71 384.00 2081.71 15 1.58 3.1673 2850.60 576.00 2274.60 20 1.31 2.6261 3151.29 708.00 2383.28 ' 25 1.13 2.2652 3397.86 SGO�OO 2437.86 30 1 2.0046 3608.35 1152.00 2456,35 35 0.91 1.8342 3830.87 1344.00 2486.87 40 0.83 1.6638 3993.24 1538.00 2457.24 45 0.77 1.5436 4167.65 1728.00 2438�85 50 0.72 1.4433 4330.02 1920.00 2410.02 55 0.88 1.3832 4498.41 2112.00 2388.41 OO 0.64 1,2830 4618.69 2304.00 2314.68 ' ' ' � Circular Channel Analysis & Design Solved with Manning' s Equation Open Channel - Uniform flow Worksheet Name: 829-Comfort Inn Comment : Outlet Pipe Solve For Full Flow Capacity Given Input Data: Diameter. . . . . . . . . . 0 . 67 ft Slope. . . . . . . . . . . . . 0 . 0050 ft/ft Manning' s n. . . . . . . 0 . 010 Discharge. . . . . . . . . 1 . 13 cfs Computed Results : Full Flow Capacity. . . . . 1 . 13 cfs Full Flow Depth. . . . . . . . 0 .67 ft Velocity. . . . . . . . . . 3 .19 fps Flow Area. . . . . . . . . 0 .35 sf Critical Depth. . . . 0 .50 ft Percent Full . . . . . . 100 . 00 % Full Capacity. . . . . 1.13 cfs QMAX @. 94D. . . . . . . . 1 .21 cfs Froude Number. . . . . FULL Open Channel Flow Module, Version 3 . 08 (c) 1990 Haestad Methods, Inc. * 37 Brookside Rd * Waterbury, Ct 06708 Oro ■■■■■■////■/A■I///%ar�■r/■�■■■■■ INFANNIONIONIN ■■■■■�I/■r1//f/,■%/i�%%■■■■ ■■■ r� ■i■■■IONNEaa►ir�► MMIFA ■■■ � ■ ■ ■a■ ■O■E■ ■FAFJNA■■ri rA■aIANON ■■a■■ %■�■ ■r�►�r�r�■�►�far�■ri■■■■■ ■■■ ■��%■i/% /►//J/i/ice■��■�%■/:% . ado fill ■■al r,��r�rirari■ ■c � ��SEEM ■■ ■/Ai/1/////,/.//.a ■ AN/FURa 0we NEAR 221 ./■ , ■i ■aV 0-10 Fd - �AFrI .1109 CONCENTRATIONHIM WPM ON MEE ON 010 TIME OF FORMULARATIONAL RAINFALL POWER CURVE FREQUENCY X = HR [Y = IN/HR 2 YEAR Y = 0.36 X*60 5 YEAR Y = 0.52 X-64 10 YEAR Y = 0.64X_6 25 YEAR Y =0.78 X, 4 _ 50 YEAR Y =0.92 X_66 100 YEAR Y = 1 .0 1 X 6 o: 5 0 0 I = h oc d r � 4 aN z W i z2 z 3 z w z �u U. J Jon J J z = li Q 2 0: = N E: i I 00 5 10 15 20 25 30 40 50 60 TO DURATION IN MINUTES I D RAINFALL INTENSITY- DURATION CURVES FIGURE BOZEMAN, MONTANA 23 j BASED ON NOAA ATLAS 2,VOL.1 W 5 �- M� � N N pJ N J F- Q N Q O N U.) v to t- W � cr. to cp t0 cD cD cO � t i' f 1 i. cn v � x x x x x x m cr °Cx `�° coo ~ rn O U W = Z 0 6 6 O O z 3 u u n n u n u u a tx ri r >- >- >- r r Z X Q D no Dcr. r _ -- Z r z _j Z Q Q Q Q Q Q Z d ►'� r r r tarn r r z W LL1 z o g Z N 1 N N in d m r� J i rn Z �. a o� M N jPMA NI AON3nO3V:i 11V.INIVH O O u'y O .......... unOH aid S3HONI NI AIISN31NI '1'lti3NlVN b Table One - Typical Runoff Coefficients by Watershed Character Description of Area Runoff Coefficients Business Downtown 0.70 to 0. 95 Neighborhood 0.50 to 0.70 Residential Single-family 0. 30 to 0. 50 Multi-units, detached 0.40 to 0. 60 Multi-units, attached 0.60 to 0. 75 Residential (suburban) 0 . 25 to 0.40 Apartment 0.50 to -0.70 Industrial Light 0 .50 to 0. 80 Heavy 0 . 60 to 0 . 90 Parks, cemeteries 0 . 10 to 0 . 25 Playgrounds 0 .20 to 0 . 35 Railroad yard 0 . 20 to 0 . 35 Unimproved 0 . 10 to 0. 30 Table Two Typical Runoff Coefficients by Surface Type Character of Surface Runoff Coefficients Pavement Asphaltic and Concrete 0 . 70 to 0. 95 -Brick 0 . 70 to 0. 85 Roofs 0 .75 to 0 . 95 Lawns, sandy soil Flat, 2 percent 0 . 05 to 0. 10 Average, 2 to 7 percent 0 .10 to 0.15 Steep, 7 percent 0. 15 to 0.20 Lawns, heavy soil Flat, 2 percent 0 .13 to 0.17 Average, 2 to .7 percent- 0.18 to. 0.22 Steep, 7 percent 0.25 to 0. 35 K i F i4 " - - -- -- r -•s- - - - TABU' - - •�o.�,•_- RUNOFF COEFFICIENTS •(C.) FOR USE IN THE RATIONAL FORMULA Land Use Runoff. Coefficients " (C) Open Land 0.20 Low to Medium Density Residential 0.35 Dense Residential 0.50 Commercial Neighborhood 0 . 60 Commercial Downtown 0. 80 Industrial 0. 80 A basic assumption of the rational method is that the peak runoff rate occurs when the duration of the storm equals the time of concentration. The -time of concentration is the flow time from the most remote point in the drainage to the point in question. It generally consists of overland flow time and channel flow time. Overland flow time may be estimated from the nomograph in Figure 22. An example of its use is provided in Figure 22A. Channel flow time in gutters, ditches, or pipes may be determined by estimating velocities with the Manning equation: V = 1 .486 R2/3 Sl/2 n V - Mean velocity (ft/sec. ) n - Manning roughness coefficient (typical values in Table 5) III - 4 Storm Drainage Specifications It should be noted that the units of Ai and At are immaterial as long as they are the same. The coefficients listed in Tables 1-2 and 1-3 are applicable only for storms of 2 to 10 year frequenci.es. Less frequent higher-intensity storms will require modification of the runoff coefficient to account for antecedent precipitation and the corresponding lessening amount of infiltration and other losses which have a direct affect on runoff. The Rational Method Formula for use with major storms shall. be modified by a frequency factor (Cf) so 'that the Rational Formula now becomes Q = (CCf)IA; CCf < 1.0 where Q = storm floe, (cfs) I = rainfall intensity (in/hr. ) A = drainage area (acres) C = runoff coefficient obtained from Tables 1-2 or 1-3 Cf = frequency factor used to account for antecedent precipitation. The values of C to be used shall not be less than those lisped in Table 1-4 . The product of � x Cf should not exceed 1 .0. TABLE 1-4 FREQUENCY FACTORS (Ct) FOR RATIONAL FOR:V_j% Design Storm Freq. Factor Return Period Cf 2 1.0 5' 1.0 10 1 .0 25 1. 1 SO 1.2 100 1. 25 August 1969 I-g