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HomeMy WebLinkAbout1.06 DrainageMRI May 2012 Appendix E Drainage Drainage Report Drainage Exhibit WinTR-55 Current Data Description CDOT Type C Inlet 18" CMP Worksheet 18" CMP Rating Table Hydrologic Soil Group 6 Limited Impact Review Appendix E [This page was left blank intentionally.] MRI Waste Transfer/Recycling Center Limited Impact Review Drainage Report April 2012 Prepared by David M. Kotz, P.E. Schmueser Gordon Meyer, Inc. 118 West 6th Street, Suite 200 Glenwood Springs CO 81601 MRI Waste Transfer/Recycling Center Limited Impact Review Drainage Narrative The proposed MRI Waste Transfer/Recycling Center is located on the 35.3 acre IRMW 11 parcel approximately one mile east of Carbondale in Garfield County. The parcel is zoned Industrial and has seen many different uses since serving as the Mid -Continent Coal Load out several decades ago. On-site drainage infrastructure has evolved over the years on a piecemeal basis as needed to address the particular nuisances that affected the, then current, occupants. IRMW created the IRMW 1 and IRMW 11 parcels with the concept of each parcel handling its own drainage in mind. This generally holds true with only a small portion of the developed IRMW 11 parcel located south of the main building draining westward thru IRMW 1 via the roadside ditch, as it has done historically. MRI's operations will not change historic drainage patterns nor will they produce increased runoff, as no new additional impervious areas will be created. The only planned drainage activities are normal maintenance (ditch and pipe cleaning) and correcting a few deficiencies by installing inlets and storm pipe. Drainage patterns on the site consist generally of surface flow from the steep hillside to the south. This hillside is fairly well vegetated. The ridgeline that delineates the southern boundary of the tributary drainage basins is viewable from the site. There are no major offsite basins affecting the site. Limits of the drainages are shown on the 11" x 17" Drainage Exhibit. A diversion ditch was previously constructed (several years ago but date unknown), to intercept and divert drainage from the steep hillside prior to it reaching the developed areas of the site. The ditch marks the northerly limits of Subarea B above the main building. In cross section, it typically consists of an 8'+/- wide bench that slopes back into the hill with a containment depth of about 18". This ditch currently discharges over a near vertical cut slope to the existing gravel roadway near the southeast corner of the main building. Though there have not been significant problems to date, flow over the steep slope could hasten erosion and saturation could result in slope failure. I:\2011\2011-414 Carbondale Transfer Station\F_LIR App\Application\Appendix E - Drainage\1-MRI-LIR-Drainage.doc 2 To limit the likelihood of those occurrences under proposed conditions, flow from this ditch will be intercepted in an inlet and piped down the slope in an 18" cmp to a riprap splash pad. From that point, flow will continue westward in the existing ditch as it has done historically. Runoff from IRMW II Subareas C — G, ultimately flows to the highly vegetated depressions on the south side of the Rio Grand Trail. These depressions are result of the railroad construction and provide extensive storage and de facto water quality treatment. Little, if any flow ever makes it to the County Road 100 cross culverts. Flows from IRMW II Subareas C & E are tributary to an existing retention pond located northeast of the main building. Google Earth aerial photography shows how this area has changed over the years. Past project plans indicated existence of inflow and outflow culverts. If these pipes exist they are buried and need to be cleaned out. The project plans show appropriate drainage improvements assuming these pipes are not found in a useable condition. The remainder of this report discusses the methodology, procedures and results of the drainage analysis conducted by SGM. Proposed maintenance and design of new facilities is sufficient to accommodate a 25 -year storm. Complete calculations follow the written summary. Refer to the Transfer Bay/ Drainage Plan Sheet 3 of 3 for proposed improvements. Methodology The NRCS TR -55 method was used for estimating the amount of runoff that will occur as a result of a particular precipitation event. This method calculates runoff from individual basins based on the basin area, NRCS curve number, 24-hour precipitation, time of concentration and the travel time to the outlet foe composite watersheds. The WinTR-55 computer program Version 1.00.09 was for this analysis. Complete output follows this text. Drainage Basin Area Seven different drainage basins were delineated to analyze conditions for this project. These are historic basins and will remain the same under proposed conditions. Refer to the 11' x 17" Drainage Exhibit. Subareas A & B are the largest at 27.3 and 14.5 acres respectively. They consist mainly of IRMW I land and drain thru IRMW I to the west. IRMW II Subarea C is 11.8 acres and is tributary to the main gravel access road ditch that flows I:\2011\2011-414 Carbondale Transfer Station\F_LIR App\Application\Appendix E - Drainage\1-MRI-LIR-Drainage.doc 3 westward towards the southeast corner of the main building. Drainage is poor in that area. Flows from 2.0 acre Subarea E will combine with Subarea C flow at the existing retention pond. Subarea F, 1.67 acres of main building and gravel parking, along with Subarea G, 2.76 acre adjacent to the main office, will both drain via sheet flow to the north. Subarea D is located at the very east side of IRMW 11. This 3.6 acre basin drains off to the east. NRCS Curve Number A NRCS curve number is assigned to each basin. The curve number provides the relationship between the amount of precipitation and the amount of subsequent runoff. The curve number is a function of soil type, vegetative -cover, land use, and antecedent moisture conditions. An area -weighted average curve number is assigned to each sub -area based on the variation in soil types, impervious and vegetative cover. The NRCS soil maps for this area of Garfield County show predominantly soil type 116 — Yamo Loam across the lower, developed portions of the site. This is classified as Hydrologic Soil Group B having fairly low runoff potential. The steep hillside above the site is predominantly soil type 55 — Gypsum land- Gypsiorthids complex, which is Hydrologic Soil Group C having slow infiltration and slightly higher runoff potential. A complete NCRS soils report and map by Hydrologic Soil Group appears at the end of this report. Land use and cover also affect the curve number. Vegetation typical of the semi -arid west (Pinyon/Sage/Grass Understory) was referenced in the WinTR-55 curves number tables in conjunction with calculated gravel, impervious areas (roof, asphalt and concrete) to develop composite curve numbers for each subarea. Refer the attached calcs for a breakdown of each basin. Precipitation Major runoff events in this region of Western Colorado are caused by cloudburst type storms that are characterized by short durations of high intensity rainfall. The SCS Type 11 24-hour distribution best represents these types of storms and was used for this analysis. Rainfall depths were taken from the NOAA Atlas 2, Precipitation -Frequency Atlas of the Western United States, Volume 111- Colorado (1973). Values for the 5-, 10-, 25-, 50- and 100 -year are 1.40, I:\2011\2011-414 Carbondale Transfer Station\F_LIR App\Application\Appendix E - Drainage\1-MRI-LIR-Drainage.doc 4 1.60, 2.00, 2.20 and 2.50 inches, respectively. Time of Concentration/Travel Time Time of concentration (TC) is an important parameter in runoff modeling. It is defined as the time it would take for a drop of water falling on the most hydraulically remote point in the watershed to reach the outfall. TR -55 uses a segmental approach consisting of three components: sheet flow, overland flow, and channel flow, to estimate (TC). The length of flow, surface roughness, slope and channel geometry all factor in. Flow velocities are estimated using Manning's kinematic flow equation. Shorter TC's produce more rapid runoff and comparatively higher peaks. This is the case on this project as the longest TC used was aout 11 minutes for Subarea C. A minimum TC of 0.1 hour or 6 minutesis applied in the calculations when areas are small. . Analysis & Results The following table summarizes the drainage characteristics for the drainage basin and presents results of the 5-, 10-, 25- and 100 -year analysis. Complete computer output follows this text. TR -55 Parameters & Results BASIN AREA CN T,, 05 010 025 0100 [ac] [hr] [cfs] [cfs] [cfs] [cfs] Subarea A 27.32 73 0.17 1.62 3.85 9.99 19.70 Subarea B 14.52 73 0.16 0.91 2.13 5.44 10.68 Subarea C 11.81 73 0.19 0.66 1.58 4.15 8.26 Subarea D Subarea E 3.56 2.00 73 85 0.25 0.16 0.57 1.04 1.41 1.41 1.90 2.20 2.71 3.29 Subarea F 1.67 87 0.12 1.12 1.48 2.24 3.26 Subarea G 2.76 81 0.12 0.95 1.37 2.39 3.79 The results show that frequent storm events (5 -yr and less) generate minimal runoff. The 25 -yr flows represent appropriate design values for new facilities and were used in sizing the improvements shown on plan sheet 3 of 3. The 100 -yr flows are presented for a reference check. With any design, the effects of larger storms should be considered. Upsizing of facilities may be warranted if appreciable property damage or public safety concerns will occur with the large event. I:\2011\2011-414 Carbondale Transfer Station\F_LIR App\Application\Appendix E - Drainage\1-MRI-LIR-Drainage.doc 5 Proposed improvements to intercept flows from the Subarea B diversion ditch prior to the vertical drop, consist of a CDOT Type C inlet and 18" cmp running down the steep slope to the west as shown on plan sheet 3 of 3. While sized for the 25 -yr event, these facilities will also handle the 100 -yr event of 10.7 cfs. Ponding depth over the inlet would be about 0.6' based on 20% clogging. The 18" cmp would operate at a headwater depth ratio of about 1.6. Refer to the attached CDOT Type C Inlet capacity chart and Flowmaster normal depth rating chart for the 18" pipe size utilized on this project. The other proposed inlets and pipes proposed carry less flow than contemplated above and are more than adequate for the 25 -yr event. Summary This drainage report in conjunction w/ the accompanying plans is sufficient to meet the requirements of the Garfield County ULUR of 2008 with respect to this LIR application. Runoff volumes from the site will remain essentially unchanged as minimal construction is planned and additional impervious areas are not contemplated. A key provision of any drainage plan is that proper grading away from structures be maintained. Given existing grades and the minor maintenance and improvements proposed, there is little risk of drainage adversely affecting waste transfer or recycling operations. I:\2011\2011-414 Carbondale Transfer Station\F_LIR App\Application\Appendix E - Drainage\1-MRI-LIR-Drainage.doc 6 INIIIN 1 - ,4_11 WillarIV kilo ... ile * Air GRAPHIC SC 1 Jack MO it . IRMW TOTAL ‘4,152,592 sq. ft. 95.333 acres , . Jo • ) .-h .„'S. , # . 4 , 0,•di 4 " 44 Air , . f ' PRELIMINARY' NOT FOR NSTRUCTION seistilMULLOORD/11,1 WWI d, •I4•1 SCHMUESER GORDON MEYER 1 1 11 W. elll elPILLT,eUrTE ZOO GLENWOODSPIMIUM. ea-BIGOI ZITO sma- 1 cc." FAX 1070) cma-aisNiu ca-GRIGG ECI709. OX54117Z7 GIAG41/1414. co 4070i C*1 I -5355 Km FAME TRANSFER/ RECYCZHVG CENITZR DRAINAGE NM is. grip -nu bre 14PF J-de-Pir I* NM Ire as WinTR-55 Current Data Description --- Identification Data --- User: DMK Date: 4/4/2012 Project: MRI Waste Transfer Center Units: English SubTitle: IRMW I & II Drainage Areal Units: Acres State: Colorado County: Garfield Filename: C:\Documents and Settings\dkotz\Application Data\WinTR-55\MRI-Rev4-4-12.w55 - -- Sub -Area Data --- Name Description Reach Area(ac) RCN Tc A IRMW I (WEST) Outlet 27.32 73 .17 B IRMW I (EAST) Outlet 14.52 73 .156 C IRMW II (UPPER WEST) Outlet 11.81 73 .189 D IRMW II (UPPER EAST) Outlet 3.56 73 .126 E IRMW II (LOWER BENCH E) Outlet 2 85 .162 F IRMW II (LOADOUT BLDG) Outlet 1.67 87 .12 G IRMW II (MRI OFFICE) Outlet 2.76 81 .121 Total area: 63.64 (ac) - -- Storm Data -- Rainfall Depth by Rainfall Return Period 2 -Yr 5 -Yr 10 -Yr 25 -Yr 50 -Yr 100 -Yr 1 -Yr (in) (in) (in) (in) (in) (in) (in) 1.0 1.4 1.6 2.0 2.2 Storm Data Source: User -provided custom storm data Rainfall Distribution Type: Type II Dimensionless Unit Hydrograph: <standard> 2.5 1.0 WinTR-55, Version 1.00.09 Page 1 4/4/2012 11:10:05 AM DMK MRI Waste Transfer Center IRMW I & II Drainage Garfield County, Colorado Storm Data Rainfall Depth by Rainfall Return Period 2 -Yr 5 -Yr 10 -Yr 25 -Yr 50 -Yr 100 -Yr 1 -Yr (in) (in) (in) (in) (in) (in) (in) 1.0 1.4 1.6 2.0 2.2 Storm Data Source: User -provided custom storm data Rainfall Distribution Type: Type II Dimensionless Unit Hydrograph: <standard> 2.5 1.0 WinTR-55, Version 1.00.09 Page 1 4/4/2012 11:10:05 AM DMK MRI Waste Transfer Center IRMW I & II Drainage Garfield County, Colorado Watershed Peak Table Sub -Area Peak Flow by Rainfall Return Period or Reach ANALYSIS: 5 -Yr 10 -Yr 25 -Yr 50 -Yr 100 -Yr Identifier (cfs) (cfs) (cfs) (cfs) (cfs) (cfs) SUBAREAS A .00 1.62 3.85 9.99 13.63 19.70 B .00 0.91 2.13 5.44 7.41 10.68 C .00 0.66 1.58 4.15 5.70 8.26 D .00 0.25 0.57 1.41 1.90 2.71 E 0.41 1.04 1.41 2.20 2.63 3.29 F 0.50 1.12 1.48 2.24 2.64 3.26 G 0.26 0.95 1.37 2.36 2.91 3.79 REACHES OUTLET 1.17 6.02 11.82 27.13 36.10 50.84 WinTR-55, Version 1.00.09 Page 1 4/4/2012 11:10:05 AM DMK MRI Waste Transfer Center IRMW I & II Drainage Garfield County, Colorado Hydrograph Peak/Peak Time Table Sub -Area Peak Flow and Peak Time (hr) by Rainfall Return Period or Reach ANALYSIS: 5 -Yr 10 -Yr 25 -Yr 50 -Yr 100 -Yr Identifier (cfs) (cfs) (cfs) (cfs) (cfs) (cfs) (hr) (hr) (hr) (hr) (hr) (hr) SUBAREAS A .00 1.62 3.85 9.99 13.63 19.70 n/a 12.08 12.06 12.04 12.04 12.03 B .00 0.91 2.13 5.44 7.41 10.68 n/a 12.07 12.05 12.04 12.03 12.03 C .00 0.66 1.58 4.15 5.70 8.26 n/a 12.09 12.07 12.05 12.04 12.04 D .00 0.25 0.57 1.41 1.90 2.71 n/a 12.05 12.04 12.02 12.02 12.01 E 0.41 1.04 1.41 2.20 2.63 3.29 12.03 12.02 12.01 12.00 12.00 11.99 F 0.50 1.12 1.48 2.24 2.64 3.26 12.01 11.96 11.96 11.95 11.95 11.95 G 0.26 0.95 1.37 2.36 2.91 3.79 12.03 12.02 12.01 11.96 11.96 11.95 REACHES OUTLET 1.17 6.02 11.82 27.13 36.10 50.84 WinTR-55, Version 1.00.09 Page 1 4/4/2012 11:10:05 AM DMK MRI Waste Transfer Center IRMW I & II Drainage Garfield County, Colorado Sub -Area Summary Table Sub -Area Drainage Time of Curve Receiving Sub -Area Identifier Area Concentration Number Reach Description (ac) (hr) A 27.32 0.170 73 Outlet IRMW I (WEST) B 14.52 0.156 73 Outlet IRMW I (EAST) C 11.81 0.189 73 Outlet IRMW II (UPPER WEST) D 3.56 0.126 73 Outlet IRMW II (UPPER EAST) E 2.00 0.162 85 Outlet IRMW II (LOWER BENCH E) F 1.67 0.120 87 Outlet IRMW II (LOADOUT BLDG) G 2.76 0.121 81 Outlet IRMW II (MRI OFFICE) Total Area: 63.64 (ac) WinTR-55, Version 1.00.09 Page 1 4/4/2012 11:10:05 AM DMK MRI Waste Transfer Center IRMW I & II Drainage Garfield County, Colorado Sub -Area Time of Concentration Details Sub -Area Flow Mannings's End Wetted Travel Identifier/ Length Slope n Area Perimeter Velocity Time (ft) (ft/ft) (sq ft) (ft) (ft/sec) (hr) A SHEET 100 0.6000 0.240 0.109 SHALLOW 495 0.7300 0.050 0.010 CHANNEL 612 0.0261 0.035 5.00 10.00 4.359 0.039 CHANNEL 528 0.1780 0.040 4.00 5.66 12.222 0.012 Time of Concentration .17 B SHEET 100 0.5200 0.240 0.116 SHALLOW 525 0.6800 0.050 0.011 CHANNEL 541 0.0370 0.035 5.00 10.00 5.182 0.029 Time of Concentration .156 C SHEET 100 0.5000 0.240 0.117 SHALLOW 596 0.6600 0.050 0.013 CHANNEL 420 0.0100 0.030 2.00 5.66 2.482 0.047 CHANNEL 329 0.1000 0.030 2.00 5.66 7.616 0.012 Time of Concentration .189 D SHEET 100 0.6000 0.240 0.109 SHALLOW 735 0.5700 0.050 0.017 E SHEET SHALLOW F SHEET SHALLOW G 100 0.0200 0.050 388 0.0260 0.050 100 0.0300 0.050 142 0.0210 0.050 SHEET 100 0.0200 0.050 Time of Concentration Time of Concentration Time of Concentration Time of Concentration .126 0.121 0.041 .162 0.103 0.017 .12 0.121 .121 WinTR-55, Version 1.00.09 Page 1 4/4/2012 11:10:05 AM DMK Sub -Area Identifier MRI Waste Transfer Center IRMW I & II Drainage Garfield County, Colorado Sub -Area Land Use and Curve Number Details Land Use Hydrologic Sub -Area Curve Soil Area Number Group (ac) A B C Paved parking lots, roofs, driveways B .276 98 Paved parking lots, roofs, driveways C .74 98 Gravel (w/ right-of-way) B 1.29 85 Dirt (w/ right-of-way) B .4 82 Pinyon - juniper (fair) C 20 73 Sagebrush (w/ grass understory) (fair) C 4.61 63 Total Area / Weighted Curve Number 27.32 73 Pinyon - juniper (poor) C 2 85 Pinyon - juniper (fair) C 8.52 73 Sagebrush (w/ grass understory) (poor) C 1 80 Sagebrush (w/ grass understory) (fair) C 3 63 Total Area / Weighted Curve Number 14.52 73 Gravel (w/ right-of-way) B 1.03 85 Dirt (w/ right-of-way) B .23 82 Pinyon - juniper (fair) C 9.55 73 Sagebrush (w/ grass understory) (fair) C 1 63 Total Area / Weighted Curve Number D Gravel (w/ right-of-way) B Pinyon - juniper (fair) C Total Area / Weighted Curve Number E Gravel (w/ right-of-way) F G Total Area / Weighted Curve Number B 11.81 73 .12 85 3.44 73 3.56 73 2 85 2 85 Paved parking lots, roofs, driveways B .31 98 Gravel (w/ right-of-way) B 1.36 85 Total Area / Weighted Curve Number 1.67 87 Open space; grass cover 50% to 75% (fair) B .91 69 Paved parking lots, roofs, driveways B .51 98 Paved parking lots, roofs, driveways C .36 98 Gravel (w/ right-of-way) B .73 85 Sagebrush (w/ grass understory) (fair) B .25 51 Total Area / Weighted Curve Number 2.76 81 WinTR-55, Version 1.00.09 Page 1 4/4/2012 11:10:05 AM r 0 TYPE C MEDIAN INLET CAPACITY - for Bingle grate installations -for two grate installations multiply single grate discharge by 1.8 -for three grate installations multiply single grate discharge by 2.5 -additional capacity reduction le necessary where heavy debris exists USE FOR DESIGN (based on 20Z reduction for debrie) CAPACITY (no debris present) 10 20 30 40 50 60 CAPACITY (c f s) CDDT Drainage Design Manual su!wa u(dois MRI Limited Impact Review Drainage Worksheet for Circular Channel Project Description Worksheet 18" CMP Flow Element Circular Charm( Method Manning's Fora Solve For Full Flow Capac Input Data Mannings Coeffic 0.015 Channel Slope 005000 ft/ft Diameter 18.0 in Results Depth 1.50 ft Discharge 6.44 cfs Flow Area 1.8 ft2 Wetted Perime 4.71 ft Top Width 0.00 ft Critical Depth 0.98 ft Percent Full 100.0 % Critical Slope 008586 ft/ft Velocity 3.64 ft/s Velocity Head 0.21 ft Specific Energ 1.71 ft Froude Numbe 0.00 Maximum Disc 6.92 cfs Discharge Full 6.44 cfs Slope Full 005000 ft/ft Flow Type N/A Title: MRI Limited Impact Review is\...\b_calcs\drainage\mri-18inchcmp.fm2 schmueser gordon meyer inc 04/10/12 11:34:00 AM © Haestad Methods, Inc. 37 Brookside Road Waterbury, CT 06708 USA +1-203-755-1666 Project Engineer: DMK FlowMaster v7.0 [7.0005] Page 1 of 1 Table Rating Table for Circular Channel Project Description Worksheet 18" CMP Flow Element Circular Channc Method Manning's Fora Solve For Full Flow Capac Input Data Mannings Coeffic 1015 Diameter 18.0 in Attribute Minimum Maximum Increment Channel Slope (ft/ft) 0.005000 0.050000 0.005000 Channel Slope (ft/ft) Discharge (cfs) Depth (ft) Velocity (ft/s) Flow Area (ft2) Wetted Perimeter (ft) Top Width (ft) D.005000 6.44 1.50 3.64 1.8 4.71 0.00 0.010000 9.10 1.50 5.15 1.8 4.71 0.00 D.015000 11.15 1.50 6.31 1.8 4.71 0.00 0.020000 12.87 1.50 7.29 1.8 4.71 0.00 D.025000 14.39 1.50 8.15 1.8 4.71 0.00 D.030000 15.77 1.50 8.92 1.8 4.71 0.00 D.035000 17.03 1.50 9.64 1.8 4.71 0.00 0.040000 18.21 1.50 10.30 1.8 4.71 0.00 D.045000 19.31 1.50 10.93 1.8 4.71 0.00 D.050000 20.36 1.50 11.52 1.8 4.71 0.00 Title: MRI Limited Impact Review is\...\b_calcs\drainage\mri-18inchcmp.fm2 schmueser gordon meyer inc 04/10/12 11:36:15 AM © Haestad Methods, Inc. 37 Brookside Road Waterbury, CT 06708 USA +1-203-755-1666 Project Engineer: DMK FlowMaster v7.0 [7.0005] Page 1 of 1 \ 0 \ \ / /\ \\ {3 0_m o Eo o> 0I < E 22 /f / ƒ \ \ / $ > MAP INFORMATION MAP LEGEND Map Scale: 1:8,440 if printed on A size (8.5" x 11") sheet. The soil surveys that comprise your AOI were mapped at 1:24,000. Warning: Soil Map may not be valid at this scale. c\f co \ / \ e @ ° 0 3 a ? / \2 ao ƒ\\ 7 \ - 2// F.., 8, ) $= F a: =r =8/2 ) t ° \ \j0 \%2) �0 < ° ==2) ) ]Kg Cr)0 0 = 22« \\\7 o ca 22 k -5\\ E 0Z < ( 2 %�,� o- = n ]_/, / °5§ \ < ® [ 0a)< a { _ % § = t . 2_o Q =\\ { n o Ea k» �\\} ( )/4 Eo / /f 22 G12m c ��� 22 f)/ ,cc\ »_ ] -tee (0 CU 2/§) §u= 77 k}\ 2 ///\ a®E; ) )Ct— �= <° G 9§= E E/ c D§ §7 za 9 wt2] _± a -k /j27 ®w 6-t> eE 2'5»: e= o 6- >) f] /_\ /f e{ G- S ° 2«a ° §e %$3% #/ »5/ o. /a< ) E\)2 E2®£ ID S®t ® ^ -0--o co =S2 = 0 s> - 7L / of co w•E as EE \}3 (/ \02 0 &S2± E 0 \\ k< to < / Soil Map Units te \ \ 0 , E ƒ< 2 = 0 0 \ 0 \ \ 3l \ 1171171171171171EID \ 0 \ a as Transportation ) Interstate Highways Local Roads Hydrologic Soil Group–Aspen-Gypsum Area, Colorado, Parts of Eagle, Garfield, and Pitkin Counties IRMW 1 & 11 - Hydrologic Soil Groups Hydrologic Soil Group Hydrologic Soil Group— Summary by Map Unit — Aspen -Gypsum Area, Colorado, Parts of Eagle, Garfield, and Pitkin Counties (C0655) Map unit symbol Map unit name Rating Acres in AOI Percent of AOI 13 Atencio-Azeltine complex, 3 to 6 percent slopes B 28.1 16.1% 55 Gypsum land-Gypsiorthids complex, 12 to 65 percent slopes C 87.3 50.1% 92 Redrob loam, 1 to 6 percent slopes C 2.8 1.6% 106 Tridell-Brownsto stony sandy loams, 12 to 50 percent slopes, extremely stony B 23.8 13.7% 116 Yamo loam, 12 to 25 percent slopes B 32.3 18.5% Totals for Area of Interest 174.2 100.0% Description Hydrologic soil groups are based on estimates of runoff potential. Soils are assigned to one of four groups according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly wet, and receive precipitation from long -duration storms. The soils in the United States are assigned to four groups (A, B, C, and D) and three dual classes (ND, B/D, and C/D). The groups are defined as follows: Group A. Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission. Group B. Soils having a moderate infiltration rate when thoroughly wet. These consist chiefly of moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately coarse texture. These soils have a moderate rate of water transmission. Group C. Soils having a slow infiltration rate when thoroughly wet. These consist chiefly of soils having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture. These soils have a slow rate of water transmission. Group D. Soils having a very slow infiltration rate (high runoff potential) when thoroughly wet. These consist chiefly of clays that have a high shrink -swell potential, soils that have a high water table, soils that have a claypan or clay layer at or near the surface, and soils that are shallow over nearly impervious material. These soils have a very slow rate of water transmission. If a soil is assigned to a dual hydrologic group (ND, B/D, or C/D), the first letter is for drained areas and the second is for undrained areas. Only the soils that in their natural condition are in group D are assigned to dual classes. US DA Natural Resources raja Conservation Service Web Soil Survey National Cooperative Soil Survey 2/27/2012 Page 3 of 4 Hydrologic Soil Group—Aspen-Gypsum Area, Colorado, Parts of Eagle, IRMW 1 & 11 - Hydrologic Soil Groups Garfield, and Pitkin Counties Rating Options Aggregation Method: Dominant Condition Component Percent Cutoff: None Specified Tie-break Rule: Higher US DA Natural Resources raja Conservation Service Web Soil Survey National Cooperative Soil Survey 2/27/2012 Page 4 of 4