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Home My WebLink AboutSWM Report_Heights_Verbena Ranch_v2 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 Revised: September 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.) = 6.30 Runoff Coefficient 'c' =0.5 Bed Length (ft) =110 Total Excavation Volume (cf) =37,400 Bed Width (ft) =40 Pipe Size (in) =12 Bed Depth (ft) =8.5 Total Pipe Length (ft) =109 Pipe Storage (cf) =86 Infiltration Area (sf) =4400 Remaining Vol. for Stone (cf) =37,314 Infiltration Rate (in/hr) =2.6 Stone Voids (%) =40% Available Stone Storage (cf) =14,926 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 6.65 3,988 159 3,829 15 0.25 1.78 5.61 5,046 238 4,808 30 0.50 1.24 3.91 7,031 477 6,554 60 1 0.78 2.46 8,845 953 7,892 120 2 0.64 2.02 14,515 1,907 12,609 180 3 0.52 1.64 17,690 2,860 14,830 360 6 0.27 0.85 18,371 5,720 12,651 720 12 0.16 0.50 21,773 11,440 10,333 1440 24 0.10 0.32 27,216 22,880 4,336 MAXIMUM STORAGE VOLUME = 14,830 TOTAL REQUIRED STORAGE VOLUME = 14,830 PROVIDED STORAGE (CF) =15,011 OK DRAINAGE TIME (HRS) =15.7 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) =15,011 100-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) =115 Total Excavation Volume (cf) =13,800 Bed Width (ft) =15 Pipe Size (in) =12 Bed Depth (ft) =8 Total Pipe Length (ft) =114 Pipe Storage (cf) =90 Infiltration Area (sf) =1725 Remaining Vol. for Stone (cf) =13,710 Infiltration Rate (in/hr) =2.6 Stone Voids (%) =40% Available Stone Storage (cf) =5,484 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 62 1,374 15 0.25 1.78 2.02 1,818 93 1,725 30 0.50 1.24 1.41 2,533 187 2,346 60 1 0.78 0.89 3,187 374 2,813 120 2 0.64 0.73 5,229 748 4,482 180 3 0.52 0.59 6,373 1,121 5,252 360 6 0.27 0.31 6,618 2,243 4,376 720 12 0.16 0.18 7,844 4,485 3,359 1440 24 0.10 0.11 9,805 8,970 835 MAXIMUM STORAGE VOLUME = 5,252 TOTAL REQUIRED STORAGE VOLUME = 5,252 PROVIDED STORAGE (CF) =5,574 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) =5,574 100-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.) = 0.41 Asphalt 0.95 10299 9784 Runoff Coefficient 'c' =0.63 Landscaping 0.20 7728 1546 Total 18027 0.63 Bed Length (ft) =100 Total Excavation Volume (cf) =3,000 Bed Width (ft) =5 Pipe Size (in) =12 Bed Depth (ft) =6 Total Pipe Length (ft) =99 Pipe Storage (cf) =78 Infiltration Area (sf) =500 Remaining Vol. for Stone (cf) =2,922 Infiltration Rate (in/hr) =2.6 Stone Voids (%) =40% Available Stone Storage (cf) =1,169 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 0.55 329 18 311 15 0.25 1.78 0.46 417 27 390 30 0.50 1.24 0.32 581 54 526 60 1 0.78 0.20 730 108 622 120 2 0.64 0.17 1,198 217 982 180 3 0.52 0.14 1,461 325 1,136 360 6 0.27 0.07 1,517 650 867 720 12 0.16 0.04 1,798 1,300 498 1440 24 0.10 0.03 2,247 2,600 -353 MAXIMUM STORAGE VOLUME = 1,136 TOTAL REQUIRED STORAGE VOLUME = 1,136 PROVIDED STORAGE (CF) =1,247 OK DRAINAGE TIME (HRS) =11.5 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) =1,247 100-YEAR STORM EVENT September 15, 2024 - Caldwell, IdahoHEIGHTS AT VERBENA RANCHDRAINAGE CATCHMENT PLANGRAPHIC SCALE0FEET1 inch = ft. Horizontal15075 150150Basin 16.64 ac.Basin 16.73 ac.Basin 26.30 ac.Basin 31.76 ac.Basin 40.41 ac.