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Home My WebLink AboutSWM Report-Heights ESE Consultants, Inc. 3103 W. Sheryl Drive Meridian, Idaho 83642 TEL: 208-424-0020 PRELIMINARY STORMWATER MANAGEMENT PLAN FOR HEIGHTS 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 Forest Canal to the north and west, existing agricultural land to the south, and S. 10th Ave to the east. The site is primarily comprised of open agricultural fields. There are several existing agricultural structures to be removed at the time of construction. Project Scope The applicant proposes to construct fifty-five (55) single family residential homes on subdivided lots within the approximately 15.56-acre property. The development includes the construction of homes, local 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 Heights 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) Heights at Verbena Ranch PrePlat City of Caldwell Canyon County, Idaho Drainage Area (Ac.) = 6.64 Runoff Coefficient 'c' = 0.5 Bed Length (ft) =125 Total Excavation Volume (cf) = 40,000 Bed Width (ft) = 40 Pipe Size (in) = 12 Bed Depth (ft) =8 Total Pipe Length (ft) = 124 Pipe Storage (cf) = 97 Infiltration Area (sf) = 5000 Remaining Vol. for Stone (cf) = 39,903 Infiltration Rate (in/hr) = 2.6 Stone Voids (%) = 40% Available Stone Storage (cf) = 15,961 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 7.01 4,203 181 4,023 15 0.25 1.78 5.91 5,319 271 5,048 30 0.50 1.24 4.12 7,410 542 6,869 60 1 0.78 2.59 9,323 1,083 8,239 120 2 0.64 2.12 15,299 2,167 13,132 180 3 0.52 1.73 18,645 3,250 15,395 360 6 0.27 0.90 19,362 6,500 12,862 720 12 0.16 0.53 22,948 13,000 9,948 1440 24 0.10 0.33 28,685 26,000 2,685 MAXIMUM STORAGE VOLUME = 15,395 TOTAL REQUIRED STORAGE VOLUME = 15,395 PROVIDED STORAGE (CF) =16,058 OK DRAINAGE TIME (HRS) =14.8 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) =16,058 100‐YEAR STORM EVENTWHICH IS LESS THAN THE ALLOWABLE 120‐HOUR DRAIN TIME Heights at Verbena Ranch PrePlat City of Caldwell Canyon County, Idaho Drainage Area (Ac.) = 5.67 Runoff Coefficient 'c' = 0.5 Bed Length (ft) = 110 Total Excavation Volume (cf) = 35,200 Bed Width (ft) = 40 Pipe Size (in) = 12 Bed Depth (ft) = 8 Total Pipe Length (ft) = 109 Pipe Storage (cf) = 86 Infiltration Area (sf) = 4400 Remaining Vol. for Stone (cf) = 35,114 Infiltration Rate (in/hr) = 2.6 Stone Voids (%) = 40% Available Stone Storage (cf) = 14,046 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 5.98 3,589 159 3,430 15 0.25 1.78 5.05 4,542 238 4,303 30 0.50 1.24 3.52 6,328 477 5,851 60 1 0.78 2.21 7,961 953 7,007 120 2 0.64 1.81 13,064 1,907 11,157 180 3 0.52 1.47 15,921 2,860 13,061 360 6 0.27 0.77 16,534 5,720 10,814 720 12 0.16 0.45 19,596 11,440 8,156 1440 24 0.10 0.28 24,494 22,880 1,614 MAXIMUM STORAGE VOLUME = 13,061 TOTAL REQUIRED STORAGE VOLUME = 13,061 PROVIDED STORAGE (CF) =14,131 OK DRAINAGE TIME (HRS) =14.8 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) =14,131 100‐YEAR STORM EVENT Heights 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.) = 1.76 Asphalt 0.95 20139 19132 Runoff Coefficient 'c' = 0.64 Landscaping 0.20 13818 2764 Total 33957 0.64 Bed Length (ft) = 150 Total Excavation Volume (cf) = 14,400 Bed Width (ft) = 12 Pipe Size (in) = 12 Bed Depth (ft) = 8 Total Pipe Length (ft) = 149 Pipe Storage (cf) = 117 Infiltration Area (sf) = 1800 Remaining Vol. for Stone (cf) = 14,283 Infiltration Rate (in/hr) = 2.6 Stone Voids (%) = 40% Available Stone Storage (cf) = 5,713 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 2.39 1,437 65 1,372 15 0.25 1.78 2.02 1,818 98 1,721 30 0.50 1.24 1.41 2,533 195 2,338 60 1 0.78 0.89 3,187 390 2,797 120 2 0.64 0.73 5,229 780 4,449 180 3 0.52 0.59 6,373 1,170 5,203 360 6 0.27 0.31 6,618 2,340 4,278 720 12 0.16 0.18 7,844 4,680 3,164 1440 24 0.10 0.11 9,805 9,360 445 MAXIMUM STORAGE VOLUME = 5,203 TOTAL REQUIRED STORAGE VOLUME = 5,203 PROVIDED STORAGE (CF) =5,830 OK DRAINAGE TIME (HRS) =14.9 Drainage Time >>> within 48 hours of 2‐year storm >>> within 120 hours of 100‐yr storm Basin 3 Contributing Area Data WHICH IS LESS THAN THE ALLOWABLE 120‐HOUR DRAIN TIME Total Bed Volume [Pipe Storage + Stone Storage] (cf) =5,830 100‐YEAR STORM EVENTPercolation Bed Data Heights 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.) = 1.06 Asphalt 0.95 24974 23725 Runoff Coefficient 'c' = 0.60 Landscaping 0.20 21282 4256 Total 46256 0.60 Bed Length (ft) = 190 Total Excavation Volume (cf) = 7,600 Bed Width (ft) = 5 Pipe Size (in) = 12 Bed Depth (ft) = 8 Total Pipe Length (ft) = 189 Pipe Storage (cf) = 148 Infiltration Area (sf) = 950 Remaining Vol. for Stone (cf) = 7,452 Infiltration Rate (in/hr) = 2.6 Stone Voids (%) = 40% Available Stone Storage (cf) = 2,981 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 1.36 813 34 779 15 0.25 1.78 1.14 1,029 51 978 30 0.50 1.24 0.80 1,434 103 1,331 60 1 0.78 0.50 1,804 206 1,598 120 2 0.64 0.41 2,960 412 2,548 180 3 0.52 0.33 3,608 618 2,990 360 6 0.27 0.17 3,746 1,235 2,511 720 12 0.16 0.10 4,440 2,470 1,970 1440 24 0.10 0.06 5,550 4,940 610 MAXIMUM STORAGE VOLUME = 2,990 TOTAL REQUIRED STORAGE VOLUME = 2,990 PROVIDED STORAGE (CF) =3,129 OK DRAINAGE TIME (HRS) =15.2 Drainage Time >>> within 48 hours of 2‐year storm >>> within 120 hours of 100‐yr storm Percolation Bed Data Contributing Area Data Basin 4 WHICH IS LESS THAN THE ALLOWABLE 120‐HOUR DRAIN TIME Total Bed Volume [Pipe Storage + Stone Storage] (cf) =3,129 100‐YEAR STORM EVENT April 29, 2024 - Caldwell, IdahoHEIGHTS AT VERBENA RANCHDRAINAGE CATCHMENT PLANGRAPHIC SCALE0FEET1 inch = ft. Horizontal15075 150150Basin 16.64 ac.Basin 16.73 ac.Basin 25.67 ac.Basin 31.76 ac.Basin 41.08 ac.