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1 | P a g e HYDROLOGY STUDY REPORT PROJECT SITE: V.I. FIRE & EMERGENCY MEDICAL SERVICES (VIFEMS) 1 COTTON VALLEY CHRISTIANSTED, ST. CROIX, USVI 00820 PREPARED BY: 7-1 BONNE ESPERANCE P.O. BOX 8269 CHRISTIANSTED, ST. CROIX USVI 00823 SITE LOCATION COAKLEY BAY CONDOMINIUMS THE REEF CONDOMINIUMS 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 ................................................................................................................................................ 15 3 | P a g e INTRODUCTION This site was previously developed and has a considerable amount of development that currently exist. The proposed improvements primarily encompass the replacement of the existing 3,600 square feet antiquated fire station building with a new expanded state-of-the-art facility measuring approximately 7,400 gross square footage. The new facility will be constructed immediately north of the existing facility which must remain operational throughout the duration of construction. Only after the new facility is placed into service will the existing facility be demolished. 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 B. This soil classification type is synonymous with Group D 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 cover within the property. Paved; open ditches (including right-of-way) – this accounts for the paved surfaces within the property. Commercial and business – this accounts for the buildings within the property. 6 | P a g e BASIN CHARACTERISTICS The disturbed area of the site is approximately 1.22 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) – 247 ft. travel distance @ average slope of 2.3% 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 distinct flow path, which include a combination of overland and pipe flow as follows:  A-B-C (Overland and Pipe Flow) – 296 ft. travel distance @ average slope of 2% 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 As can be noted from the above calculations, the 2yr – 24 hr. storm event generates the most runoff in the pre-development/ current site condition. 12 | P a g e  POST-DEVELOPMENT As can be noted from the above calculations, the 2yr – 24 hr. storm event generates the most runoff in the post-development. CONCLUSION The proposed improvements result in a slight net increase in building and impervious areas over the existing condition which is evident in the pre vs. post runoff coefficient increasing from 84 to 87. The additional storage required to offset this adverse impact is only 0.01 ac-ft (0.16 ac-ft – 0.15 ac-ft). However, rather than provide just 0.01 ac-ft, enough storage will be provided to cover at least 1-inch of runoff over the entire disturbed site which equates to 0.10 ac-ft. Site layout allowed for a larger storage volume resulting in a retention pond totaling 0.20 ac-ft. 13 | P a g e 14 | P a g e STORAGE POND LAYOUT FOR A TOTAL VOLUME OF 0.20 AC-FT 15 | P a g e APPENDICES PRE-DEVELOPMENT SITE AREA BREAKDOWN 16 | P a g e POST-DEVELOPMENT SITE AREA BREAKDOWN