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Home My WebLink AboutSWM Report-Place ESE Consultants, Inc. 3103 W. Sheryl Drive Meridian, Idaho 83642 TEL: 208-424-0020 PRELIMINARY STORMWATER MANAGEMENT PLAN FOR VERBENA RANCH PLACE 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 Forest Canal to the northeast, S. 10th Ave to the west, and Vallivue Middle School to the south. The site is primarily comprised of open agricultural fields. Project Scope The applicant proposes to construct fifty-six (56) multifamily units on subdivided lots within the approximately 20.94-acre property. The development includes the construction of 4-plexes, support roads, sidewalks, stormwater management facilities, and other infrastructure improvements typical of residential multi-family construction. Report Scope This report presents the preliminary stormwater management plan for the Verbena Ranch Place 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 0.75 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) Verbena Ranch Place PrePlat City of Caldwell Canyon County, Idaho Drainage Area (Ac.) = 5.57 Runoff Coefficient 'c' = 0.75 Bed Length (ft) =160 Total Excavation Volume (cf) = 44,800 Bed Width (ft) = 35 Pipe Size (in) = 12 Bed Depth (ft) =8 Total Pipe Length (ft) = 159 Pipe Storage (cf) = 125 Infiltration Area (sf) = 5600 Remaining Vol. for Stone (cf) = 44,675 Infiltration Rate (in/hr) = 4.0 Stone Voids (%) = 40% Available Stone Storage (cf) = 17,870 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 8.81 5,289 311 4,978 15 0.25 1.78 7.44 6,692 467 6,226 30 0.50 1.24 5.18 9,324 933 8,391 60 1 0.78 3.26 11,730 1,867 9,864 120 2 0.64 2.67 19,250 3,733 15,517 180 3 0.52 2.17 23,461 5,600 17,861 360 6 0.27 1.13 24,363 11,200 13,163 720 12 0.16 0.67 28,875 22,400 6,475 1440 24 0.10 0.42 36,094 44,800 ‐8,706 MAXIMUM STORAGE VOLUME = 17,861 TOTAL REQUIRED STORAGE VOLUME = 17,861 PROVIDED STORAGE (CF) =17,995 OK DRAINAGE TIME (HRS) =9.6 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) =17,995 100‐YEAR STORM EVENTWHICH IS LESS THAN THE ALLOWABLE 120‐HOUR DRAIN TIME Verbena Ranch Place PrePlat City of Caldwell Canyon County, Idaho Drainage Area (Ac.) = 5.24 Runoff Coefficient 'c' = 0.75 Bed Length (ft) = 260 Total Excavation Volume (cf) = 42,120 Bed Width (ft) = 18 Pipe Size (in) = 12 Bed Depth (ft) = 9 Total Pipe Length (ft) = 259 Pipe Storage (cf) = 203 Infiltration Area (sf) = 4680 Remaining Vol. for Stone (cf) = 41,917 Infiltration Rate (in/hr) = 4.4 Stone Voids (%) = 40% Available Stone Storage (cf) = 16,767 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 8.29 4,975 286 4,689 15 0.25 1.78 7.00 6,296 429 5,867 30 0.50 1.24 4.87 8,772 858 7,914 60 1 0.78 3.07 11,035 1,716 9,319 120 2 0.64 2.52 18,109 3,432 14,677 180 3 0.52 2.04 22,071 5,148 16,923 360 6 0.27 1.06 22,920 10,296 12,624 720 12 0.16 0.63 27,164 20,592 6,572 1440 24 0.10 0.39 33,955 41,184 ‐7,229 MAXIMUM STORAGE VOLUME = 16,923 TOTAL REQUIRED STORAGE VOLUME = 16,923 PROVIDED STORAGE (CF) =16,970 OK DRAINAGE TIME (HRS) =9.9 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) =16,970 100‐YEAR STORM EVENT Verbena Ranch Place PrePlat City of Caldwell Canyon County, Idaho Drainage Area (Ac.) = 4.54 Runoff Coefficient 'c' = 0.75 Bed Length (ft) = 100 Total Excavation Volume (cf) = 36,000 Bed Width (ft) = 45 Pipe Size (in) = 12 Bed Depth (ft) = 8 Total Pipe Length (ft) = 99 Pipe Storage (cf) = 78 Infiltration Area (sf) = 4500 Remaining Vol. for Stone (cf) = 35,922 Infiltration Rate (in/hr) = 4.4 Stone Voids (%) = 40% Available Stone Storage (cf) = 14,369 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 7.18 4,311 275 4,036 15 0.25 1.78 6.06 5,455 413 5,042 30 0.50 1.24 4.22 7,600 825 6,775 60 1 0.78 2.66 9,561 1,650 7,911 120 2 0.64 2.18 15,690 3,300 12,390 180 3 0.52 1.77 19,122 4,950 14,172 360 6 0.27 0.92 19,858 9,900 9,958 720 12 0.16 0.54 23,535 19,800 3,735 1440 24 0.10 0.34 29,419 39,600 ‐10,181 MAXIMUM STORAGE VOLUME = 14,172 TOTAL REQUIRED STORAGE VOLUME = 14,172 PROVIDED STORAGE (CF) =14,447 OK DRAINAGE TIME (HRS) =8.8 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 3 Contributing Area Data Percolation Bed Data Total Bed Volume [Pipe Storage + Stone Storage] (cf) =14,447 100‐YEAR STORM EVENT Verbena Ranch Place PrePlat City of Caldwell Canyon County, Idaho Drainage Area (Ac.) = 1.50 Runoff Coefficient 'c' = 0.75 Bed Length (ft) = 70 Total Excavation Volume (cf) = 12,880 Bed Width (ft) = 23 Pipe Size (in) = 12 Bed Depth (ft) = 8 Total Pipe Length (ft) = 69 Pipe Storage (cf) = 54 Infiltration Area (sf) = 1610 Remaining Vol. for Stone (cf) = 12,826 Infiltration Rate (in/hr) = 4.4 Stone Voids (%) = 40% Available Stone Storage (cf) = 5,130 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 2.37 1,424 98 1,326 15 0.25 1.78 2.00 1,802 148 1,655 30 0.50 1.24 1.40 2,511 295 2,216 60 1 0.78 0.88 3,159 590 2,569 120 2 0.64 0.72 5,184 1,181 4,003 180 3 0.52 0.59 6,318 1,771 4,547 360 6 0.27 0.30 6,561 3,542 3,019 720 12 0.16 0.18 7,776 7,084 692 1440 24 0.10 0.11 9,720 14,168 ‐4,448 MAXIMUM STORAGE VOLUME = 4,547 TOTAL REQUIRED STORAGE VOLUME = 4,547 PROVIDED STORAGE (CF) =5,185 OK DRAINAGE TIME (HRS) =8.8 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 4 Contributing Area Data Percolation Bed Data Total Bed Volume [Pipe Storage + Stone Storage] (cf) =5,185 100‐YEAR STORM EVENT Verbena Ranch Place PrePlat City of Caldwell Canyon County, Idaho WEIGHTED RUNOFF COEFFICIENT, C Surface Type Runoff Coefficient, C Area (SF) Weighted C Drainage Area (Ac.) = 0.47 Asphalt 0.95 11934 11337 Runoff Coefficient 'c' = 0.64 Landscaping 0.20 8510 1702 Total 20444 0.64 Bed Length (ft) = 50 Total Excavation Volume (cf) = 3,000 Bed Width (ft) = 10 Pipe Size (in) = 12 Bed Depth (ft) = 6 Total Pipe Length (ft) = 49 Pipe Storage (cf) = 38 Infiltration Area (sf) = 500 Remaining Vol. for Stone (cf) = 2,962 Infiltration Rate (in/hr) = 4.4 Stone Voids (%) = 40% Available Stone Storage (cf) = 1,185 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 0.63 380 31 349 15 0.25 1.78 0.53 480 46 434 30 0.50 1.24 0.37 669 92 577 60 1 0.78 0.23 842 183 658 120 2 0.64 0.19 1,381 367 1,015 180 3 0.52 0.16 1,684 550 1,134 360 6 0.27 0.08 1,748 1,100 648 720 12 0.16 0.05 2,072 2,200 ‐128 1440 24 0.10 0.03 2,590 4,400 ‐1,810 MAXIMUM STORAGE VOLUME = 1,134 TOTAL REQUIRED STORAGE VOLUME = 1,134 PROVIDED STORAGE (CF) =1,223 OK DRAINAGE TIME (HRS) =6.7 Drainage Time >>> within 48 hours of 2‐year storm >>> within 120 hours of 100‐yr storm Contributing Area Data Basin 5 Percolation Bed Data WHICH IS LESS THAN THE ALLOWABLE 120‐HOUR DRAIN TIME Total Bed Volume [Pipe Storage + Stone Storage] (cf) =1,223 100‐YEAR STORM EVENT Verbena Ranch Place PrePlat City of Caldwell Canyon County, Idaho WEIGHTED RUNOFF COEFFICIENT, C Surface Type Runoff Coefficient, C Area (SF) Weighted C Drainage Area (Ac.) = 1.24 Asphalt 0.95 26368 25050 Runoff Coefficient 'c' = 0.57 Landscaping 0.20 27694 5539 Total 54062 0.57 Bed Length (ft) = 65 Total Excavation Volume (cf) = 7,800 Bed Width (ft) = 15 Pipe Size (in) = 12 Bed Depth (ft) = 8 Total Pipe Length (ft) = 64 Pipe Storage (cf) = 50 Infiltration Area (sf) = 975 Remaining Vol. for Stone (cf) = 7,750 Infiltration Rate (in/hr) = 4.4 Stone Voids (%) = 40% Available Stone Storage (cf) = 3,100 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 1.48 888 60 829 15 0.25 1.78 1.25 1,124 89 1,035 30 0.50 1.24 0.87 1,566 179 1,387 60 1 0.78 0.55 1,970 358 1,613 120 2 0.64 0.45 3,233 715 2,518 180 3 0.52 0.36 3,940 1,073 2,868 360 6 0.27 0.19 4,092 2,145 1,947 720 12 0.16 0.11 4,849 4,290 559 1440 24 0.10 0.07 6,062 8,580 ‐2,518 MAXIMUM STORAGE VOLUME = 2,868 TOTAL REQUIRED STORAGE VOLUME = 2,868 PROVIDED STORAGE (CF) =3,150 OK DRAINAGE TIME (HRS) =8.8 Drainage Time >>> within 48 hours of 2‐year storm >>> within 120 hours of 100‐yr storm Percolation Bed Data Contributing Area Data Basin 6 WHICH IS LESS THAN THE ALLOWABLE 120‐HOUR DRAIN TIME Total Bed Volume [Pipe Storage + Stone Storage] (cf) =3,150 100‐YEAR STORM EVENT Verbena Ranch Place PrePlat City of Caldwell Canyon County, Idaho WEIGHTED RUNOFF COEFFICIENT, C Surface Type Runoff Coefficient, C Area (SF) Weighted C Drainage Area (Ac.) = 0.67 Asphalt 0.95 14484 13760 Runoff Coefficient 'c' = 0.57 Landscaping 0.20 14699 2940 Total 29183 0.57 Bed Length (ft) = 100 Total Excavation Volume (cf) = 4,000 Bed Width (ft) = 5 Pipe Size (in) = 18 Bed Depth (ft) = 8 Total Pipe Length (ft) = 99 Pipe Storage (cf) = 175 Infiltration Area (sf) = 500 Remaining Vol. for Stone (cf) = 3,825 Infiltration Rate (in/hr) = 4.4 Stone Voids (%) = 40% Available Stone Storage (cf) = 1,530 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 0.81 485 31 455 15 0.25 1.78 0.68 614 46 568 30 0.50 1.24 0.48 856 92 764 60 1 0.78 0.30 1,077 183 893 120 2 0.64 0.25 1,767 367 1,400 180 3 0.52 0.20 2,153 550 1,603 360 6 0.27 0.10 2,236 1,100 1,136 720 12 0.16 0.06 2,650 2,200 450 1440 24 0.10 0.04 3,313 4,400 ‐1,087 MAXIMUM STORAGE VOLUME = 1,603 TOTAL REQUIRED STORAGE VOLUME = 1,603 PROVIDED STORAGE (CF) =1,705 OK DRAINAGE TIME (HRS) =9.3 Drainage Time >>> within 48 hours of 2‐year storm >>> within 120 hours of 100‐yr storm Basin 7 Contributing Area Data Percolation Bed Data WHICH IS LESS THAN THE ALLOWABLE 120‐HOUR DRAIN TIME Total Bed Volume [Pipe Storage + Stone Storage] (cf) =1,705 100‐YEAR STORM EVENT April 15, 2024 - Caldwell, IdahoVERBENA RANCH PLACEDRAINAGE CATCHMENT PLANGRAPHIC SCALE0FEET1 inch = ft. Horizontal15075150150Basin 15.57 ac.Basin 2 5.24 ac.Basin 34.54 ac.Basin 14.38 ac.Basin 41.50 ac.Basin 5 0.47 ac.Basin 70.67 ac.Basin 61.24 ac.