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1 | P a g e HYDROLOGY STUDY REPORT HIBISCUS HOTEL PHASE III PROJECT SITE: 109C, 109D, 109E, 109R, 109S, 109T LA GRANDE PRINCESSE CHRISTIANSTED, ST. CROIX, USVI 00820 PREPARED BY: 7-1 BONNE ESPERANCE P.O. BOX 8269 CHRISTIANSTED, ST. CROIX USVI 00823 SITE LOCATION THE PALMS HOTEL NORTHSIDE ROAD ST. CROIX AVIS 2 | P a g e TABLE OF CONTENTS INTRODUCTION ............................................................................................................................................. 3 SOILS CLASSIFICATION .................................................................................................................................. 3 CURVE NUMBER (CN) CLASSIFICATION ......................................................................................................... 4 BASIN CHARACTERISTICS .............................................................................................................................. 6 TIME OF CONCENTRATION (Tc) .................................................................................................................... 6 RUNOFF (Q) AND STORAGE VOLUME (V) ................................................................................................... 10 CONCLUSION ............................................................................................................................................... 12 APPENDICES ................................................................................................................................................ 16 3 | P a g e INTRODUCTION This site was previously developed as a hotel property. The proposed improvements encompass renovation of the existing buildings and the addition of three new hotel buildings, a generator building and a swimming pool. The new hotel facility will have a total of 104 units. This hydrology study will focus on a pre-development vs. a post-development analysis to ensure that the proposed improvements do not adversely impact stormwater quality and quantity beyond the pre- existing condition. Any additional impacts will be quantified and properly mitigated in accordance with the Department of Planning and Natural Resources (DPNR) requirements. SOILS CLASSIFICATION For Curve Number (CN) determination, there are four (4) main types of Hydrologic Soils Classification which are as follows: Group A – Soils that have low runoff potential when thoroughly wet. These soils typically have less than 10% clays and more than 90% sand or gravel. Group B – Soils that have moderately low runoff potential when thoroughly wet. These soils typically have between 10% - 20% clays and 50% - 90% sand. Group C – Soils that have moderately high runoff potential when thoroughly wet. These soils typically have between 20% - 40% clays and less than 50% sand. Group D – Soils that have high runoff potential when thoroughly wet. These soils typically have greater than 40% clays and less than 50% sand. 4 | P a g e St. Croix Hydrologic Soils Classification (Source: https://usvi.mapgeo.io) Based on the above, the Hydrologic Soils Classification for the Site is Group A. This soil classification type is synonymous with Group A in Table 3.6 below. CURVE NUMBER (CN) CLASSIFICATION The curve number classification is determined based on land use type and hydrologic soil type. The table below indicates CN values for various land uses and hydrologic soil types. SITE LOCATION 5 | P a g e Based on the above, the site falls into three (3) distinct classifications listed as follows: Poor Condition (grass cover < 50%) – this accounts for the grass/vegetative cover within the property. Paved; open ditches (including right-of-way) – this accounts for the paved surfaces and buildings within the property. Gravel – this accounts for the gravel stone ground cover within the property. 6 | P a g e BASIN CHARACTERISTICS The disturbed area of the site is approximately 3.76 acres broken down into three (3) major ground cover types. The table below shows the breakdown along with a Weighted Curve Number for each category and ultimately for the entire site.  PRE-DEVELOPMENT  POST-DEVELOPMENT TIME OF CONCENTRATION (Tc) The time of concentration is necessary to estimate peak discharge and is dependent upon the watershed characteristics. To accurately determine Tc, the hydraulics of each part of the flow path must be considered separately. 7 | P a g e  PRE-DEVELOPMENT In the pre-development, the site has one distinct flow path, which is overland flow as follows:  A-B (Overland Flow) – 717 ft. travel distance @ average slope of 0.6% The flow pattern will be evaluated to determine the Time of Concentration (Tc) which will be used to determine the stormwater water runoff volume. 8 | P a g e  POST-DEVELOPMENT In the post-development, the site has one critical distinct flow path, which include a combination of overland and pipe flow as follows:  A-B (Overland) – 279 ft. travel distance @ average slope of 1.5%  B-C (Pipe Flow) – 302 ft. travel distance @ average slope of 0.03%  C-D (Pipe Flow) – 61 ft. travel distance @ average slope of 0.15% 9 | P a g e The flow pattern will be evaluated to determine the Time of Concentration (Tc) which will be used to determine the stormwater water runoff volume. 10 | P a g e RUNOFF (Q) AND STORAGE VOLUME (V) DPNR Runoff Onsite Storage Requirements as per the Territorial Pollutant Discharge Elimination System - General Permit Number VIGSA0000 is as follows: 1) 3600 cubic feet per acre (1” over 1 acre) or, 2) 2yr – 24 hours storm; whichever is greater 11 | P a g e  PRE-DEVELOPMENT 12 | P a g e  POST-DEVELOPMENT CONCLUSION The proposed improvements result in a net increase in building and impervious areas over the existing condition which is evident in the pre vs. post runoff coefficient increasing from 76 to 81. The storage required for 1” over the entire site area = 0.31 ac-ft. The storage required from a pre vs. post analysis of the 2yr – 24hour storm (0.66 ac-ft – 0.54 ac-ft) = 0.12 ac-ft. Based on the above results, the 1” over the entire site controls. 13 | P a g e 14 | P a g e STORAGE POND LAYOUT FOR A TOTAL VOLUME OF 0.23 AC-FT The balance of 0.08 ac-ft of storage (0.31 ac-ft – 0.23 ac-ft) will be accounted for by utilizing Stormtech Underground Storage Chambers. These chambers will be placed beneath the new asphalt parking lot as illustrated on Plan Sheet 02-C11. 15 | P a g e 16 | P a g e In addition to runoff storage calculations, pipe flood routing calculations were also performed to ensure that the piping network is adequately sized to convey the runoff to the retention storage areas. These calculations are included in the attached Appendices. APPENDICES PRE-DEVELOPMENT SITE AREA BREAKDOWN 17 | P a g e POST-DEVELOPMENT SITE AREA BREAKDOWN 109C, 109D, 109E, 109R, 109S, 109T La Grande Princesse, C'sted, St. Croix, USVI 00820 PIPE FLOOD ROUTING SIZING ANALYSIS BUILDTEC, LLC 7-1 Bonne Esperance, P.O. Box 8269, Christiansted, USVI 00823 HIBISCUS HOTEL PHASE III CURRENT ISSUE DATE: 11/29/23 PREPARED BY: DAMIAN CARTWRIGHT, P.E. Project Name: HIBISCUS HOTEL PHASE III Date: November 29, 2023 Project Location: 109C, 109D, 109E, 109R, 109S, 109T La Grande Princesse, C'sted, St. Croix, USVI 008Activity: Drainage Design_Pipe Flood Routing Project #: Sheet #: Engineer: DC Drawing Ref.: DESIGN OBJECTIVE: Determine runoff and pipe sizing for sub catchments in the Hibiscus Hotel Development DESIGN REFERENCEStandard Handbook for Civil Enginers , Merritt/Lofton/Ricketts Design & Constr. Of Urban Stormwter Management Systems, ASCE Drainage Handbook-Hydrology,Oct. 2000, Florida Dept. of Transportation Drainage Handbook-Storm Drains, Aug. 2000, Florida Dept. of Transportation Drainage Manual, Oct. 2000, Florida Dept. of Transportation DESIGN PARAMETERS: 1. Divide area into subcatchments 2. Use Rational formula to determine peak discharge for each sub-catchment Storm Design Period yrs 5 Storm Duration hrs 6 FDOT Design Zone 10 Figure F-21, Drainage Handbook-Hydrology Rainfall Intensity in/hr 4 IDF Curve, Figure F-31, Drainage Handbook-Hydrology Average Slope % 0 to 2 Runoff Coeff., Table T-4, Drainage Handbook-Hydrology Minimum Pipe Flow Velocityft/s 2 Sec. 3.6.1 Drainage Manual C1 (Woodlands) 0.15 Runoff Coeff., Table T-4, Drainage Handbook-Hydrology C2 (Pasture, Grass, and Farmland) 0.20 Runoff Coeff., Table T-4, Drainage Handbook-Hydrology C3 Bare Earth 0.50 Runoff Coeff., Table T-4, Drainage Handbook-Hydrology C4 Rooftops & Pavements 0.95 Runoff Coeff., Table T-4, Drainage Handbook-Hydrology C5 Impervious Pavements 0.95 Runoff Coeff., Table T-4, Drainage Handbook-Hydrology C6 SFR 0.45 Runoff Coeff., Table T-4, Drain. Hndbk-Hydrology (lots < 1/2 Acre) C7 MFR 0.75 Runoff Coeff., Table T-4, Drainage Handbook-Hydrology C8 Commercial & Industrial 0.95 Runoff Coeff., Table T-4, Drainage Handbook-Hydrology 7-1 Bonne Esperance, P.O. Box 8269, Christiansted, USVI 00823 Project Name: HIBISCUS HOTEL PHASE III Date: November 29, 2023 Project Location: 109C, 109D, 109E, 109R, 109S, 109T La Grande Princesse, C'sted, St. Croix, USVI 008Activity: Drainage Design_Pipe Flood Routing Project #: Sheet #: Engineer: DC Drawing Ref.: Sub Catchment Number CA-1 Total Catchment Area, A acres 0.38 Land Use Percentages A1 (Woodlands) 0 A2 (Pasture, Grass, and Farmland) 21.1 A3 Bare Earth 0 A4 Rooftops 34.2 A5 Impervious Pavements 44.7 A6 SFR 0 A7 MFR 0 A8 Commercial & Industrial 0 Total 100 OK Weighted Average Runoff Coeff. 0.79 C = CiAi/A Rainfall Intensity in/hr 4 Peak Flow, Q ft3/s 1.20 Q = CIA Total Rainfall Volume ft3 25994.74 Drainage Pipe Sizing Pipe Material HDPE Mannings No., n 0.012 Sec. 3.6.4, Drainage Manual Pipe Dimension, D in 16.00 Pipe Slope, S ft/ft 0.005 Flow Velocity ft/s 4.21 OK V= 0.590/n x D2/3 x S1/2 , eqn. 21.33b, SHCE Full Flow Pipe Capacity ft3/s 5.88 OK Q= 0.463/n x D8/3 x S1/2 , eqn. 21.33c, SHCE 7-1 Bonne Esperance, P.O. Box 8269, Christiansted, USVI 00823 Project Name: HIBISCUS HOTEL PHASE III Date: November 29, 2023 Project Location: 109C, 109D, 109E, 109R, 109S, 109T La Grande Princesse, C'sted, St. Croix, USVI 008Activity: Drainage Design_Pipe Flood Routing Project #: Sheet #: Engineer: DC Drawing Ref.: Sub Catchment Number CA-2 (Main Trunkline Pipe Check) Total Catchment Area, A acres 0.67 (includes contributing area from CA-1) Land Use Percentages A1 (Woodlands) 0 A2 (Pasture, Grass, and Farmland) 29.9 A3 Bare Earth 0 A4 Rooftops 32.8 A5 Impervious Pavements 37.3 A6 SFR 0 A7 MFR 0 A8 Commercial & Industrial 0 Total 100 OK Weighted Average Runoff Coeff. 0.73 C = CiAi/A Rainfall Intensity in/hr 4 Peak Flow, Q ft3/s 1.95 Q = CIA Total Rainfall Volume ft3 42012.22 Drainage Pipe Sizing Pipe Material HDPE Mannings No., n 0.012 Sec. 3.6.4, Drainage Manual Pipe Dimension, D in 16.00 Pipe Slope, S ft/ft 0.0025 Flow Velocity ft/s 2.98 OK V= 0.590/n x D2/3 x S1/2 , eqn. 21.33b, SHCE Full Flow Pipe Capacity ft3/s 4.15 OK Q= 0.463/n x D8/3 x S1/2 , eqn. 21.33c, SHCE 7-1 Bonne Esperance, P.O. Box 8269, Christiansted, USVI 00823 Project Name: HIBISCUS HOTEL PHASE III Date: November 29, 2023 Project Location: 109C, 109D, 109E, 109R, 109S, 109T La Grande Princesse, C'sted, St. Croix, USVI 008Activity: Drainage Design_Pipe Flood Routing Project #: Sheet #: Engineer: DC Drawing Ref.: Sub Catchment Number CA-2 (Area Drain Pipe Check) Total Catchment Area, A acres 0.29 Land Use Percentages A1 (Woodlands) 0 A2 (Pasture, Grass, and Farmland) 41.4 A3 Bare Earth 0 A4 Rooftops 31 A5 Impervious Pavements 27.6 A6 SFR 0 A7 MFR 0 A8 Commercial & Industrial 0 Total 100 OK Weighted Average Runoff Coeff. 0.64 C = CiAi/A Rainfall Intensity in/hr 4 Peak Flow, Q ft3/s 0.74 Q = CIA Total Rainfall Volume ft3 16023.31 Drainage Pipe Sizing Pipe Material HDPE Mannings No., n 0.012 Sec. 3.6.4, Drainage Manual Pipe Dimension, D in 10.00 Pipe Slope, S ft/ft 0.0025 Flow Velocity ft/s 2.18 OK V= 0.590/n x D2/3 x S1/2 , eqn. 21.33b, SHCE Full Flow Pipe Capacity ft3/s 1.19 OK Q= 0.463/n x D8/3 x S1/2 , eqn. 21.33c, SHCE 7-1 Bonne Esperance, P.O. Box 8269, Christiansted, USVI 00823 Project Name: HIBISCUS HOTEL PHASE III Date: November 29, 2023 Project Location: 109C, 109D, 109E, 109R, 109S, 109T La Grande Princesse, C'sted, St. Croix, USVI 008Activity: Drainage Design_Pipe Flood Routing Project #: Sheet #: Engineer: DC Drawing Ref.: Sub Catchment Number CA-3 (Main Trunkline Pipe Check) Total Catchment Area, A acres 1.07 (includes contributing areas from CA-1 & CA-2) Land Use Percentages A1 (Woodlands) 0 A2 (Pasture, Grass, and Farmland) 36.4 A3 Bare Earth 0 A4 Rooftops 29 A5 Impervious Pavements 34.6 A6 SFR 0 A7 MFR 0 A8 Commercial & Industrial 0 Total 100 OK Weighted Average Runoff Coeff. 0.68 C = CiAi/A Rainfall Intensity in/hr 4 Peak Flow, Q ft3/s 2.90 Q = CIA Total Rainfall Volume ft3 62587.30 Drainage Pipe Sizing Pipe Material HDPE Mannings No., n 0.012 Sec. 3.6.4, Drainage Manual Pipe Dimension, D in 18.00 Pipe Slope, S ft/ft 0.0025 Flow Velocity ft/s 3.22 OK V= 0.590/n x D2/3 x S1/2 , eqn. 21.33b, SHCE Full Flow Pipe Capacity ft3/s 5.69 OK Q= 0.463/n x D8/3 x S1/2 , eqn. 21.33c, SHCE 7-1 Bonne Esperance, P.O. Box 8269, Christiansted, USVI 00823 Project Name: HIBISCUS HOTEL PHASE III Date: November 29, 2023 Project Location: 109C, 109D, 109E, 109R, 109S, 109T La Grande Princesse, C'sted, St. Croix, USVI 008Activity: Drainage Design_Pipe Flood Routing Project #: Sheet #: Engineer: DC Drawing Ref.: Sub Catchment Number CA-3 (Area Drain Pipe Check) Total Catchment Area, A acres 0.40 Land Use Percentages A1 (Woodlands) 0 A2 (Pasture, Grass, and Farmland) 47.5 A3 Bare Earth 0 A4 Rooftops 22.5 A5 Impervious Pavements 30 A6 SFR 0 A7 MFR 0 A8 Commercial & Industrial 0 Total 100 OK Weighted Average Runoff Coeff. 0.59 C = CiAi/A Rainfall Intensity in/hr 4 Peak Flow, Q ft3/s 0.95 Q = CIA Total Rainfall Volume ft3 20520.00 Drainage Pipe Sizing Pipe Material HDPE Mannings No., n 0.012 Sec. 3.6.4, Drainage Manual Pipe Dimension, D in 10.00 Pipe Slope, S ft/ft 0.0025 Flow Velocity ft/s 2.18 OK V= 0.590/n x D2/3 x S1/2 , eqn. 21.33b, SHCE Full Flow Pipe Capacity ft3/s 1.19 OK Q= 0.463/n x D8/3 x S1/2 , eqn. 21.33c, SHCE 7-1 Bonne Esperance, P.O. Box 8269, Christiansted, USVI 00823 Project Name: HIBISCUS HOTEL PHASE III Date: November 29, 2023 Project Location: 109C, 109D, 109E, 109R, 109S, 109T La Grande Princesse, C'sted, St. Croix, USVI 008Activity: Drainage Design_Pipe Flood Routing Project #: Sheet #: Engineer: DC Drawing Ref.: Sub Catchment Number CA-4 Total Catchment Area, A acres 1.94 (includes contributing areas from CA-1, CA-2, CA-3 & CA-7) Land Use Percentages A1 (Woodlands) 0 A2 (Pasture, Grass, and Farmland) 37.1 A3 Bare Earth 0 A4 Rooftops 17 A5 Impervious Pavements 45.9 A6 SFR 0 A7 MFR 0 A8 Commercial & Industrial 0 Total 100 OK Weighted Average Runoff Coeff. 0.67 C = CiAi/A Rainfall Intensity in/hr 4 Peak Flow, Q ft3/s 5.21 Q = CIA Total Rainfall Volume ft3 112596.05 Drainage Pipe Sizing Pipe Material HDPE Mannings No., n 0.012 Sec. 3.6.4, Drainage Manual Pipe Dimension, D in 18.00 Pipe Slope, S ft/ft 0.0025 Flow Velocity ft/s 3.22 OK V= 0.590/n x D2/3 x S1/2 , eqn. 21.33b, SHCE Full Flow Pipe Capacity ft3/s 5.69 OK Q= 0.463/n x D8/3 x S1/2 , eqn. 21.33c, SHCE 7-1 Bonne Esperance, P.O. Box 8269, Christiansted, USVI 00823 Project Name: HIBISCUS HOTEL PHASE III Date: November 29, 2023 Project Location: 109C, 109D, 109E, 109R, 109S, 109T La Grande Princesse, C'sted, St. Croix, USVI 008Activity: Drainage Design_Pipe Flood Routing Project #: Sheet #: Engineer: DC Drawing Ref.: Sub Catchment Number CA-5 Total Catchment Area, A acres 0.03 Land Use Percentages A1 (Woodlands) 0 A2 (Pasture, Grass, and Farmland) 33.3 A3 Bare Earth 0 A4 Rooftops 0 A5 Impervious Pavements 66.7 A6 SFR 0 A7 MFR 0 A8 Commercial & Industrial 0 Total 100 OK Weighted Average Runoff Coeff. 0.70 C = CiAi/A Rainfall Intensity in/hr 4 Peak Flow, Q ft3/s 0.08 Q = CIA Total Rainfall Volume ft3 1815.05 Drainage Pipe Sizing Pipe Material HDPE Mannings No., n 0.012 Sec. 3.6.4, Drainage Manual Pipe Dimension, D in 10.00 Pipe Slope, S ft/ft 0.008 Flow Velocity ft/s 3.89 OK V= 0.590/n x D2/3 x S1/2 , eqn. 21.33b, SHCE Full Flow Pipe Capacity ft3/s 2.12 OK Q= 0.463/n x D8/3 x S1/2 , eqn. 21.33c, SHCE 7-1 Bonne Esperance, P.O. Box 8269, Christiansted, USVI 00823 Project Name: HIBISCUS HOTEL PHASE III Date: November 29, 2023 Project Location: 109C, 109D, 109E, 109R, 109S, 109T La Grande Princesse, C'sted, St. Croix, USVI 008Activity: Drainage Design_Pipe Flood Routing Project #: Sheet #: Engineer: DC Drawing Ref.: Sub Catchment Number CA-6 Total Catchment Area, A acres 0.11 Land Use Percentages A1 (Woodlands) 0 A2 (Pasture, Grass, and Farmland) 54.5 A3 Bare Earth 0 A4 Rooftops 45.5 A5 Impervious Pavements 0 A6 SFR 0 A7 MFR 0 A8 Commercial & Industrial 0 Total 100 OK Weighted Average Runoff Coeff. 0.54 C = CiAi/A Rainfall Intensity in/hr 4 Peak Flow, Q ft3/s 0.24 Q = CIA Total Rainfall Volume ft3 5144.04 Drainage Pipe Sizing Pipe Material HDPE Mannings No., n 0.012 Sec. 3.6.4, Drainage Manual Pipe Dimension, D in 10.00 Pipe Slope, S ft/ft 0.0033 Flow Velocity ft/s 2.50 OK V= 0.590/n x D2/3 x S1/2 , eqn. 21.33b, SHCE Full Flow Pipe Capacity ft3/s 1.36 OK Q= 0.463/n x D8/3 x S1/2 , eqn. 21.33c, SHCE 7-1 Bonne Esperance, P.O. Box 8269, Christiansted, USVI 00823 Project Name: HIBISCUS HOTEL PHASE III Date: November 29, 2023 Project Location: 109C, 109D, 109E, 109R, 109S, 109T La Grande Princesse, C'sted, St. Croix, USVI 008Activity: Drainage Design_Pipe Flood Routing Project #: Sheet #: Engineer: DC Drawing Ref.: Sub Catchment Number CA-7 Total Catchment Area, A acres 0.51 Land Use Percentages A1 (Woodlands) 0 A2 (Pasture, Grass, and Farmland) 2 A3 Bare Earth 0 A4 Rooftops 3.9 A5 Impervious Pavements 94.1 A6 SFR 0 A7 MFR 0 A8 Commercial & Industrial 0 Total 100 OK Weighted Average Runoff Coeff. 0.94 C = CiAi/A Rainfall Intensity in/hr 4 Peak Flow, Q ft3/s 1.91 Q = CIA Total Rainfall Volume ft3 41199.84 Drainage Pipe Sizing Pipe Material HDPE Mannings No., n 0.012 Sec. 3.6.4, Drainage Manual Pipe Dimension, D in 18.00 Pipe Slope, S ft/ft 0.001 Flow Velocity ft/s 2.04 OK V= 0.590/n x D2/3 x S1/2 , eqn. 21.33b, SHCE Full Flow Pipe Capacity ft3/s 3.60 OK Q= 0.463/n x D8/3 x S1/2 , eqn. 21.33c, SHCE 7-1 Bonne Esperance, P.O. Box 8269, Christiansted, USVI 00823 Project Name: HIBISCUS HOTEL PHASE III Date: November 29, 2023 Project Location: 109C, 109D, 109E, 109R, 109S, 109T La Grande Princesse, C'sted, St. Croix, USVI 008Activity: Drainage Design_Pipe Flood Routing Project #: Sheet #: Engineer: DC Drawing Ref.: Sub Catchment Number CA-8 Total Catchment Area, A acres 1.04 Land Use Percentages A1 (Woodlands) 0 A2 (Pasture, Grass, and Farmland) 0 A3 Bare Earth 72.1 A4 Rooftops 27.9 A5 Impervious Pavements 0 A6 SFR 0 A7 MFR 0 A8 Commercial & Industrial 0 Total 100 OK Weighted Average Runoff Coeff. 0.63 C = CiAi/A Rainfall Intensity in/hr 4 Peak Flow, Q ft3/s 2.60 Q = CIA Total Rainfall Volume ft3 56209.42 Drainage Pipe Sizing Pipe Material HDPE Mannings No., n 0.012 Sec. 3.6.4, Drainage Manual Pipe Dimension, D in 12.00 Pipe Slope, S ft/ft 0.0075 Flow Velocity ft/s 4.26 OK V= 0.590/n x D2/3 x S1/2 , eqn. 21.33b, SHCE Full Flow Pipe Capacity ft3/s 3.34 OK Q= 0.463/n x D8/3 x S1/2 , eqn. 21.33c, SHCE 7-1 Bonne Esperance, P.O. Box 8269, Christiansted, USVI 00823