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Home My WebLink AboutSWM Report-Trails ESE Consultants, Inc. 3103 W. Sheryl Drive Meridian, Idaho 83642 TEL: 208-424-0020 PRELIMINARY STORMWATER MANAGEMENT PLAN FOR THE RESERVE AT VERBENA RANCH SUBDIVISION LOCATED IN: CITY OF CALDWELL, CANYON COUNTY, IDAHO ____________________________ Justin E. Bozovich Professional Engineer Idaho License No. 12449 May 2024 TABLE OF CONTENTS SUBJECT SECTION Narrative and Project Information 1 • Site Information • Stormwater Management Design Stormwater Facility Design 2 • Seepage Bed Design • Drainage Plan SECTION 1 Narrative and Project Information (Site Information, Stormwater Management Design) Site Information Project Location The project site is situated in the City of Caldwell, Canyon County, Idaho. The site is bound by the Deer Flat Caldwell Canal to the northeast, S. 10th Ave to the west, the Forest Canal to the southwest, and agricultural land to the south. The site is primarily comprised of open agricultural fields. Project Scope The applicant proposes to construct two hundred and sixteen (216) single family residential homes on subdivided lots within the approximately 61.61-acre property. The development includes the construction of homes, support roads, sidewalks, stormwater management facilities, and other infrastructure improvements typical of residential construction. Report Scope This report presents the preliminary stormwater management plan for the Trails at Verbena Ranch Subdivision. Included within this report are all the calculations to verify that the stormwater management plan meets the regulations listed in the Caldwell Municipal Stormwater Infrastructure Design Manual from March 2024. Stormwater Management Design Design Criteria The Stormwater Management Plan described herein has been designed according to the following publications and criterion: Caldwell Municipal Stormwater Infrastructure Design Manual, March 2024. The proposed Stormwater Management Design has been prepared to meet the following requirements and guidelines: ♦ The Rational Method (Q=ciA) has been utilized to calculate peak flows. The ‘c’ coefficients for the calculations were based on “Table 1” from Section 2.3.2 in the stormwater design manual. Areas with mainly residential lots were assumed at this time to have a c value of 0.50 and weighted c values were calculated for areas with predominantly roadway area (collector roads). Weighted c summary tables can be found in Section 2 of this report. Rainfall intensities were determined from a chart labeled as “Table 2” from Section 2.3.2 in the stormwater design manual. ♦ Seepage beds have been designed to store and infiltrate the 100-year storm event. Design calculations can be found in Section 2 of this report. ♦ Infiltration rates for the design of the seepage beds were taken from a report titled, “Geotechnical Evaluation for ‘Caldwell Land Auction’” prepared by Geotek Inc., dated February 17, 2022. Per Section 5.3.1 of the stormwater design manual, the tested infiltration rates were reduced to 70% to achieve the design rates utilized in the volume calculations. Only the bottom of the seepage beds were considered for the available infiltration area. ♦ Preliminary site grading was designed to direct all residential lot area and open space areas in such a way as to be fully captured within the site and accounted for in the stormwater facilities. ♦ Sand and grease traps were not designed at this time. The appropriate sand and grease trap size, either 1,000 gallons or 1,500 gallons, will be determined and specified in future design submittals to provide adequate water quality treatment within the required design parameters. SECTION 2 Stormwater Facility Design (Seepage Beds and Drainage Map) Trails at Verbena Ranch PrePlat City of Caldwell Canyon County, Idaho Drainage Area (Ac.) = 3.38 Runoff Coefficient 'c' = 0.5 Bed Length (ft) =100 Total Excavation Volume (cf) = 17,500 Bed Width (ft) = 25 Pipe Size (in) = 18 Bed Depth (ft) =7 Total Pipe Length (ft) = 99 Pipe Storage (cf) = 175 Infiltration Area (sf) = 2500 Remaining Vol. for Stone (cf) = 17,325 Infiltration Rate (in/hr) = 4 Stone Voids (%) = 40% Available Stone Storage (cf) = 6,930 Storm Duration (mins) Storm Duration (hrs) Intensity (in/hr) (1)Peak Flow (cfs)Runoff Volume (cf) (2) Percolated Volume (cf) (3) Storage Volume Needed (cf) 10 0.17 2.11 3.57 2,140 139 2,001 15 0.25 1.78 3.01 2,707 208 2,499 30 0.50 1.24 2.10 3,772 417 3,355 60 1 0.78 1.32 4,746 833 3,912 120 2 0.64 1.08 7,788 1,667 6,121 180 3 0.52 0.88 9,491 2,500 6,991 360 6 0.27 0.46 9,856 5,000 4,856 720 12 0.16 0.27 11,681 10,000 1,681 1440 24 0.10 0.17 14,602 20,000 ‐5,398 MAXIMUM STORAGE VOLUME = 6,991 TOTAL REQUIRED STORAGE VOLUME = 6,991 PROVIDED STORAGE (CF) =7,105 OK DRAINAGE TIME (HRS) =8.5 Drainage Time >>> within 48 hours of 2‐year storm >>> within 120 hours of 100‐yr storm Basin 1 Contributing Area Data Percolation Bed Data Total Bed Volume [Pipe Storage + Stone Storage] (cf) =7,105 100‐YEAR STORM EVENTWHICH IS LESS THAN THE ALLOWABLE 120‐HOUR DRAIN TIME Trails at Verbena Ranch PrePlat City of Caldwell Canyon County, Idaho Drainage Area (Ac.) = 1.93 Runoff Coefficient 'c' = 0.5 Bed Length (ft) = 70 Total Excavation Volume (cf) = 11,200 Bed Width (ft) = 20 Pipe Size (in) = 18 Bed Depth (ft) = 8 Total Pipe Length (ft) = 69 Pipe Storage (cf) = 122 Infiltration Area (sf) = 1400 Remaining Vol. for Stone (cf) = 11,078 Infiltration Rate (in/hr) = 4 Stone Voids (%) = 40% Available Stone Storage (cf) = 4,431 Storm Duration (mins) Storm Duration (hrs) Intensity (in/hr) (1)Peak Flow (cfs)Runoff Volume (cf) (2) Percolated Volume (cf) (3) Storage Volume Needed (cf) 10 0.17 2.11 2.04 1,222 78 1,144 15 0.25 1.78 1.72 1,546 117 1,429 30 0.50 1.24 1.20 2,154 233 1,921 60 1 0.78 0.75 2,710 467 2,243 120 2 0.64 0.62 4,447 933 3,513 180 3 0.52 0.50 5,419 1,400 4,019 360 6 0.27 0.26 5,628 2,800 2,828 720 12 0.16 0.15 6,670 5,600 1,070 1440 24 0.10 0.10 8,338 11,200 ‐2,862 MAXIMUM STORAGE VOLUME = 4,019 TOTAL REQUIRED STORAGE VOLUME = 4,019 PROVIDED STORAGE (CF) =4,553 OK DRAINAGE TIME (HRS) =9.8 Drainage Time >>> within 48 hours of 2‐year storm >>> within 120 hours of 100‐yr storm WHICH IS LESS THAN THE ALLOWABLE 120‐HOUR DRAIN TIME Basin 2 Contributing Area Data Percolation Bed Data Total Bed Volume [Pipe Storage + Stone Storage] (cf) =4,553 100‐YEAR STORM EVENT Trails at Verbena Ranch PrePlat City of Caldwell Canyon County, Idaho Drainage Area (Ac.) = 10.75 Runoff Coefficient 'c' = 0.5 Bed Length (ft) = 190 Total Excavation Volume (cf) = 60,800 Bed Width (ft) = 40 Pipe Size (in) = 18 Bed Depth (ft) = 8 Total Pipe Length (ft) = 189 Pipe Storage (cf) = 334 Infiltration Area (sf) = 7600 Remaining Vol. for Stone (cf) = 60,466 Infiltration Rate (in/hr) = 3.5 Stone Voids (%) = 40% Available Stone Storage (cf) = 24,186 Storm Duration (mins) Storm Duration (hrs) Intensity (in/hr) (1)Peak Flow (cfs)Runoff Volume (cf) (2) Percolated Volume (cf) (3) Storage Volume Needed (cf) 10 0.17 2.11 11.34 6,805 369 6,435 15 0.25 1.78 9.57 8,611 554 8,057 30 0.50 1.24 6.67 11,997 1,108 10,889 60 1 0.78 4.19 15,093 2,217 12,876 120 2 0.64 3.44 24,768 4,433 20,335 180 3 0.52 2.80 30,186 6,650 23,536 360 6 0.27 1.45 31,347 13,300 18,047 720 12 0.16 0.86 37,152 26,600 10,552 1440 24 0.10 0.54 46,440 53,200 ‐6,760 MAXIMUM STORAGE VOLUME = 23,536 TOTAL REQUIRED STORAGE VOLUME = 23,536 PROVIDED STORAGE (CF) =24,520 OK DRAINAGE TIME (HRS) =11.1 Drainage Time >>> within 48 hours of 2‐year storm >>> within 120 hours of 100‐yr storm WHICH IS LESS THAN THE ALLOWABLE 120‐HOUR DRAIN TIME Basin 3 Contributing Area Data Percolation Bed Data Total Bed Volume [Pipe Storage + Stone Storage] (cf) =24,520 100‐YEAR STORM EVENT Trails at Verbena Ranch PrePlat City of Caldwell Canyon County, Idaho Drainage Area (Ac.) = 10.48 Runoff Coefficient 'c' = 0.50 Bed Length (ft) = 170 Total Excavation Volume (cf) = 61,200 Bed Width (ft) = 45 Pipe Size (in) = 18 Bed Depth (ft) = 8 Total Pipe Length (ft) = 169 Pipe Storage (cf) = 299 Infiltration Area (sf) = 7650 Remaining Vol. for Stone (cf) = 60,901 Infiltration Rate (in/hr) = 2.9 Stone Voids (%) = 40% Available Stone Storage (cf) = 24,361 Storm Duration (mins) Storm Duration (hrs) Intensity (in/hr) (1)Peak Flow (cfs)Runoff Volume (cf) (2) Percolated Volume (cf) (3) Storage Volume Needed (cf) 10 0.17 2.11 11.06 6,634 308 6,326 15 0.25 1.78 9.33 8,394 462 7,932 30 0.50 1.24 6.50 11,696 924 10,771 60 1 0.78 4.09 14,714 1,849 12,865 120 2 0.64 3.35 24,146 3,698 20,448 180 3 0.52 2.72 29,428 5,546 23,882 360 6 0.27 1.41 30,560 11,093 19,467 720 12 0.16 0.84 36,219 22,185 14,034 1440 24 0.10 0.52 45,274 44,370 904 MAXIMUM STORAGE VOLUME = 23,882 TOTAL REQUIRED STORAGE VOLUME = 23,882 PROVIDED STORAGE (CF) =24,659 OK DRAINAGE TIME (HRS) =13.3 Drainage Time >>> within 48 hours of 2‐year storm >>> within 120 hours of 100‐yr storm WHICH IS LESS THAN THE ALLOWABLE 120‐HOUR DRAIN TIME Basin 4 Contributing Area Data Percolation Bed Data Total Bed Volume [Pipe Storage + Stone Storage] (cf) =24,659 100‐YEAR STORM EVENT Trails at Verbena Ranch PrePlat City of Caldwell Canyon County, Idaho Drainage Area (Ac.) = 5.75 Runoff Coefficient 'c' = 0.5 Bed Length (ft) = 115 Total Excavation Volume (cf) = 36,800 Bed Width (ft) = 40 Pipe Size (in) = 18 Bed Depth (ft) = 8 Total Pipe Length (ft) = 114 Pipe Storage (cf) = 201 Infiltration Area (sf) = 4600 Remaining Vol. for Stone (cf) = 36,599 Infiltration Rate (in/hr) = 1.9 Stone Voids (%) = 40% Available Stone Storage (cf) = 14,639 Storm Duration (mins) Storm Duration (hrs) Intensity (in/hr) (1)Peak Flow (cfs)Runoff Volume (cf) (2) Percolated Volume (cf) (3) Storage Volume Needed (cf) 10 0.17 2.11 6.07 3,640 121 3,518 15 0.25 1.78 5.12 4,606 182 4,424 30 0.50 1.24 3.57 6,417 364 6,053 60 1 0.78 2.24 8,073 728 7,345 120 2 0.64 1.84 13,248 1,457 11,791 180 3 0.52 1.50 16,146 2,185 13,961 360 6 0.27 0.78 16,767 4,370 12,397 720 12 0.16 0.46 19,872 8,740 11,132 1440 24 0.10 0.29 24,840 17,480 7,360 MAXIMUM STORAGE VOLUME = 13,961 TOTAL REQUIRED STORAGE VOLUME = 13,961 PROVIDED STORAGE (CF) =14,841 OK DRAINAGE TIME (HRS) =20.4 Drainage Time >>> within 48 hours of 2‐year storm >>> within 120 hours of 100‐yr storm WHICH IS LESS THAN THE ALLOWABLE 120‐HOUR DRAIN TIME Basin 5 Contributing Area Data Percolation Bed Data Total Bed Volume [Pipe Storage + Stone Storage] (cf) =14,841 100‐YEAR STORM EVENT Trails at Verbena Ranch PrePlat City of Caldwell Canyon County, Idaho Drainage Area (Ac.) = 12.20 Runoff Coefficient 'c' = 0.5 Bed Length (ft) = 170 Total Excavation Volume (cf) = 68,000 Bed Width (ft) = 50 Pipe Size (in) = 18 Bed Depth (ft) = 8 Total Pipe Length (ft) = 507 Pipe Storage (cf) = 896 Infiltration Area (sf) = 8500 Remaining Vol. for Stone (cf) = 67,104 Infiltration Rate (in/hr) = 3.5 Stone Voids (%) = 40% Available Stone Storage (cf) = 26,842 Storm Duration (mins) Storm Duration (hrs) Intensity (in/hr) (1)Peak Flow (cfs)Runoff Volume (cf) (2) Percolated Volume (cf) (3) Storage Volume Needed (cf) 10 0.17 2.11 12.87 7,723 413 7,309 15 0.25 1.78 10.86 9,772 620 9,152 30 0.50 1.24 7.56 13,615 1,240 12,376 60 1 0.78 4.76 17,129 2,479 14,650 120 2 0.64 3.90 28,109 4,958 23,150 180 3 0.52 3.17 34,258 7,438 26,820 360 6 0.27 1.65 35,575 14,875 20,700 720 12 0.16 0.98 42,163 29,750 12,413 1440 24 0.10 0.61 52,704 59,500 ‐6,796 MAXIMUM STORAGE VOLUME = 26,820 TOTAL REQUIRED STORAGE VOLUME = 26,820 PROVIDED STORAGE (CF) =27,738 OK DRAINAGE TIME (HRS) =11.2 Drainage Time >>> within 48 hours of 2‐year storm >>> within 120 hours of 100‐yr storm WHICH IS LESS THAN THE ALLOWABLE 120‐HOUR DRAIN TIME Basin 6 Contributing Area Data Percolation Bed Data Total Bed Volume [Pipe Storage + Stone Storage] (cf) =27,738 100‐YEAR STORM EVENT Trails at Verbena Ranch PrePlat City of Caldwell Canyon County, Idaho Drainage Area (Ac.) = 5.49 Runoff Coefficient 'c' = 0.5 Bed Length (ft) = 90 Total Excavation Volume (cf) = 30,960 Bed Width (ft) = 43 Pipe Size (in) = 18 Bed Depth (ft) = 8 Total Pipe Length (ft) = 178 Pipe Storage (cf) = 315 Infiltration Area (sf) = 3870 Remaining Vol. for Stone (cf) = 30,645 Infiltration Rate (in/hr) = 3.5 Stone Voids (%) = 40% Available Stone Storage (cf) = 12,258 Storm Duration (mins) Storm Duration (hrs) Intensity (in/hr) (1)Peak Flow (cfs)Runoff Volume (cf) (2) Percolated Volume (cf) (3) Storage Volume Needed (cf) 10 0.17 2.11 5.79 3,475 188 3,287 15 0.25 1.78 4.89 4,397 282 4,115 30 0.50 1.24 3.40 6,127 564 5,562 60 1 0.78 2.14 7,708 1,129 6,579 120 2 0.64 1.76 12,649 2,258 10,391 180 3 0.52 1.43 15,416 3,386 12,030 360 6 0.27 0.74 16,009 6,773 9,236 720 12 0.16 0.44 18,973 13,545 5,428 1440 24 0.10 0.27 23,717 27,090 ‐3,373 MAXIMUM STORAGE VOLUME = 12,030 TOTAL REQUIRED STORAGE VOLUME = 12,030 PROVIDED STORAGE (CF) =12,573 OK DRAINAGE TIME (HRS) =11.1 Drainage Time >>> within 48 hours of 2‐year storm >>> within 120 hours of 100‐yr storm WHICH IS LESS THAN THE ALLOWABLE 120‐HOUR DRAIN TIME Basin 7 Contributing Area Data Percolation Bed Data Total Bed Volume [Pipe Storage + Stone Storage] (cf) =12,573 100‐YEAR STORM EVENT Trails at Verbena Ranch PrePlat City of Caldwell Canyon County, Idaho WEIGHTED RUNOFF COEFFICIENT, C Surface Type Runoff Coefficient, C Area (SF) Weighted C Drainage Area (Ac.) = 0.75 Asphalt 0.95 18161 17253 Runoff Coefficient 'c' = 0.62 Landscaping 0.20 14451 2890 Total 32612 0.62 Bed Length (ft) = 50 Total Excavation Volume (cf) = 4,500 Bed Width (ft) = 15 Pipe Size (in) = 18 Bed Depth (ft) = 6 Total Pipe Length (ft) = 49 Pipe Storage (cf) = 87 Infiltration Area (sf) = 750 Remaining Vol. for Stone (cf) = 4,413 Infiltration Rate (in/hr) = 4.5 Stone Voids (%) = 40% Available Stone Storage (cf) = 1,765 Storm Duration (mins) Storm Duration (hrs) Intensity (in/hr) (1)Peak Flow (cfs)Runoff Volume (cf) (2) Percolated Volume (cf) (3) Storage Volume Needed (cf) 10 0.17 2.11 0.98 586 47 540 15 0.25 1.78 0.82 742 70 672 30 0.50 1.24 0.57 1,034 141 893 60 1 0.78 0.36 1,301 281 1,020 120 2 0.64 0.30 2,135 563 1,572 180 3 0.52 0.24 2,602 844 1,758 360 6 0.27 0.13 2,702 1,688 1,014 720 12 0.16 0.07 3,202 3,375 ‐173 1440 24 0.10 0.05 4,002 6,750 ‐2,748 MAXIMUM STORAGE VOLUME = 1,758 TOTAL REQUIRED STORAGE VOLUME = 1,758 PROVIDED STORAGE (CF) =1,852 OK DRAINAGE TIME (HRS) =6.6 Drainage Time >>> within 48 hours of 2‐year storm >>> within 120 hours of 100‐yr storm Percolation Bed Data Contributing Area Data Basin 8 WHICH IS LESS THAN THE ALLOWABLE 120‐HOUR DRAIN TIME Total Bed Volume [Pipe Storage + Stone Storage] (cf) =1,852 100‐YEAR STORM EVENT Trails at Verbena Ranch PrePlat City of Caldwell Canyon County, Idaho WEIGHTED RUNOFF COEFFICIENT, C Surface Type Runoff Coefficient, C Area (SF) Weighted C Drainage Area (Ac.) = 1.04 Asphalt 0.95 25951 24653 Runoff Coefficient 'c' = 0.63 Landscaping 0.20 19295 3859 Total 45246 0.63 Bed Length (ft) = 50 Total Excavation Volume (cf) = 7,200 Bed Width (ft) = 18 Pipe Size (in) = 18 Bed Depth (ft) = 8 Total Pipe Length (ft) = 49 Pipe Storage (cf) = 87 Infiltration Area (sf) = 900 Remaining Vol. for Stone (cf) = 7,113 Infiltration Rate (in/hr) = 3.5 Stone Voids (%) = 40% Available Stone Storage (cf) = 2,845 Storm Duration (mins) Storm Duration (hrs) Intensity (in/hr) (1)Peak Flow (cfs)Runoff Volume (cf) (2) Percolated Volume (cf) (3) Storage Volume Needed (cf) 10 0.17 2.11 1.38 830 44 786 15 0.25 1.78 1.17 1,050 66 984 30 0.50 1.24 0.81 1,463 131 1,332 60 1 0.78 0.51 1,840 263 1,578 120 2 0.64 0.42 3,020 525 2,495 180 3 0.52 0.34 3,681 788 2,893 360 6 0.27 0.18 3,822 1,575 2,247 720 12 0.16 0.10 4,530 3,150 1,380 1440 24 0.10 0.07 5,662 6,300 ‐638 MAXIMUM STORAGE VOLUME = 2,893 TOTAL REQUIRED STORAGE VOLUME = 2,893 PROVIDED STORAGE (CF) =2,932 OK DRAINAGE TIME (HRS) =11.2 Drainage Time >>> within 48 hours of 2‐year storm >>> within 120 hours of 100‐yr storm Basin 9 Percolation Bed Data Contributing Area Data WHICH IS LESS THAN THE ALLOWABLE 120‐HOUR DRAIN TIME Total Bed Volume [Pipe Storage + Stone Storage] (cf) =2,932 100‐YEAR STORM EVENT Trails at Verbena Ranch PrePlat City of Caldwell Canyon County, Idaho WEIGHTED RUNOFF COEFFICIENT, C Surface Type Runoff Coefficient, C Area (SF) Weighted C Drainage Area (Ac.) = 0.95 Asphalt 0.95 21127 20071 Runoff Coefficient 'c' =0.58 Landscaping 0.20 20197 4039 Total 41324 0.58 Bed Length (ft) =140 Total Excavation Volume (cf) =5,600 Bed Width (ft) =5 Pipe Size (in) =18 Bed Depth (ft) = 8 Total Pipe Length (ft) =139 Pipe Storage (cf) = 246 Infiltration Area (sf) =700 Remaining Vol. for Stone (cf) =5,354 Infiltration Rate (in/hr) =4.5 Stone Voids (%) = 40% Available Stone Storage (cf) =2,142 Storm Duration (mins) Storm Duration (hrs) Intensity (in/hr) (1)Peak Flow (cfs)Runoff Volume (cf) (2) Percolated Volume (cf) (3) Storage Volume Needed (cf) 10 0.17 2.11 1.17 702 44 658 15 0.25 1.78 0.99 888 66 822 30 0.50 1.24 0.69 1,237 131 1,106 60 1 0.78 0.43 1,556 263 1,294 120 2 0.64 0.35 2,554 525 2,029 180 3 0.52 0.29 3,113 788 2,325 360 6 0.27 0.15 3,232 1,575 1,657 720 12 0.16 0.09 3,831 3,150 681 1440 24 0.10 0.06 4,789 6,300 ‐1,511 MAXIMUM STORAGE VOLUME = 2,325 TOTAL REQUIRED STORAGE VOLUME = 2,325 PROVIDED STORAGE (CF) =2,387 OK DRAINAGE TIME (HRS) =9.1 Drainage Time >>> within 48 hours of 2‐year storm >>> within 120 hours of 100‐yr storm Contributing Area Data Percolation Bed Data Basin 10 WHICH IS LESS THAN THE ALLOWABLE 120‐HOUR DRAIN TIME Total Bed Volume [Pipe Storage + Stone Storage] (cf) =2,387 100‐YEAR STORM EVENT April 26, 2024 - Caldwell, IdahoTRAILS AT VERBENA RANCHDRAINAGE CATCHMENT PLANGRAPHIC SCALEFEET1 inch = ft. Horizontal3000150300300Basin 1 3.38 ac.Basin 21.93 ac.Basin 310.75 ac.Basin 11.04 ac.Basin 410.48 ac.Basin 55.75 ac.Basin 612.20 ac.Basin 75.49 ac.Basin 80.75 ac.Basin 91.04 ac.Basin 100.95 ac.