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Appoquinimink River Watershed Stormwater Management Plan

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Appoquinimink River Watershed Stormwater Management Plan APPOQUINIMINK RIVER WATERSHED STORMWATER MANAGEMENT PLAN NEW CASTLE COUNTY, DELAWARE FINAL May 14, 2010 BL PROJECT NO. 2006-2013-01 APPOQUINIMINK RIVER WATERSHED STORMWATER MANAGEMENT PLAN NEW CASTLE COUNTY, DELAWARE VOLUME I - EXECUTIVE SUMMARY FINAL May 14, 2010 BL PROJECT NO. 2006-2013-01 PREPARED FOR: PREPARED BY: DEPARTMENT OF NATURAL RESOURCES BORTON-LAWSON AND ENVIRONMENTAL CONTROL 613 Baltimore Drive, Suite 300 Division of Soil and Water Conservation Wilkes-Barre, PA 18702-7903 89 Kings Highway Dover, DE 19901 3893 Adler Place, Suite 100 Bethlehem, PA 18017 VOLUME I - EXECUTIVE SUMMARY TABLE OF CONTENTS PAGE A. Introduction ........................................................................................................................1 B. Watershed Description ......................................................................................................1 C. Drainage Area ....................................................................................................................1 D. Methodology .......................................................................................................................2 E. Standards and Criteria ......................................................................................................3 F. Implementation ..................................................................................................................5 PLAN FORMAT The format of the Appoquinimink River Watershed Stormwater Management Plan consists of Volume I, the Executive Summary, Volume II, the Plan Report, and Volume III, the Background Technical Materials. Volume I provides an overview of the why the Plan was needed and a summary of the standards and criteria developed for the watershed. Volume II, the Plan Report, provides an overview of stormwater management, purpose of the study, data collection, all GIS maps, present conditions, projected land development patterns, calculation methodology, and implementation discussion. Volume III provides supporting data, watershed modeling parameters and modeling runs, peak flows, release rates, the existing municipal ordinance matrix, and obstructions inventory. Due to large volumes of data, one copy of Volume III will be on file at DNREC office and one at the New Castle Conservation District office. Large-scale copies of the figures are at the DNREC office and at the New Castle Conservation District office. P:\2006\2013\01\DOCS\Wordprocessing\Report\BLReport\Final Report-20100514\Volume I-Final\APPO-Final - Vol I - ExSum.doc A. Introduction This Plan was developed for the Appoquinimink River watershed in New Castle County, Delaware to partially comply with the requirements of § 4001, Chapter 40, Title 7, of the Delaware Code. The amendment to Chapter 40, Title 7 provides for the establishment of a statewide comprehensive and coordinated erosion and sediment control and stormwater management program to conserve and protect land, water, and other resources of the state. This program is to take into consideration both the quantity and quality of water resources within the state. The Department of Natural Resources and Environmental Control, (DNREC) is required to develop a State Stormwater Management Program in conjunction with appropriate state and federal agencies, conservation districts, other governmental subdivisions of the state, and the regulated community. The code imposes duties, and confers power to the Department of Natural Resources and Environmental Control, conservation districts, municipalities, and counties, and provides for enforcement and appropriations. The amendment gives the DNREC the authority to develop and publish guidelines and criteria for delegation of stormwater program requirements, and the authority to review the implementation of all components and review and approve designated watersheds. The Act requires DNREC to establish criteria for approval of designated watersheds and develop guidelines for stormwater management standards and criteria for application within those designated watersheds. This Plan was compiled to record the findings of the Appoquinimink Watershed Study and document the rationale used to develop standards and criteria for the watershed that are necessary to implement the Plan. The primary focus of the Plan is on engineering and municipal topics which are directly and indirectly related to stormwater. A thorough understanding of these interrelated stormwater issues is necessary to form the basis of the Plan which will be adopted by each municipality within the watershed. Upon approval of this Designated Watershed Plan, all proposed projects within the Appoquinimink River watershed will have stormwater requirements placed upon them that are consistent with the Plan. B. Watershed Description The Appoquinimink River watershed is located in the southern portion of New Castle County, Delaware. It is one of the first major watersheds located to the south of the Chesapeake and Delaware Canal. The watershed encompasses the towns of Middletown, Odessa, and Townsend. The remainder of the watershed is located within New Castle County. Of the three urban centers located within the watershed, Middletown covers the largest geographic area and is located in the western portion of the watershed. Odessa is the second largest urban area and is located in the center of the watershed with Townsend situated on the southwestern border of the watershed. C. Drainage Area The Appoquinimink River watershed has a total drainage area of approximately 46.6 square miles. There are many tributaries that contribute flow to the river. Major tributaries of the Appoquinimink River include: Appoquinimink River Main Stem, Doves Nest Branch, Drawyer Creek, Deep Creek, Noxontown Pond and Hangmans Run. The main stem of the Appoquinimink is approximately 16 miles long and originates about 2 miles west of Townsend I-1 P:\2006\2013\01\DOCS\Wordprocessing\Report\BLReport\Final Report-20100514\Volume I-Final\APPO-Final - Vol I - ExSum.doc in the southwest part of the watershed. The main stem of the river travels in a northeast direction towards the Town of Odessa, then changes direction about 2 miles east of Odessa, where it flows in a southeast direction until it discharges into the Delaware River Estuary. Large portions of the eastern half of the watershed area comprised of tidal marshes and wetlands and there are several sizeable ponds or lakes located throughout the watershed (Shallcross Lake, Silver Lake, Wiggins Mill Pond, Noxontown Pond and an unnamed pond on Hangmans Run). D. Methodology The Plan was developed from data collected on the physical features of the watershed, such as soils, wetlands, topography, floodplains, dams and reservoirs, stream dimensions, and obstructions. Information on existing problem areas was solicited from the Steering Committee which consisted of representatives from DNREC, the municipalities within the watershed, as well as other interested parties including the New Castle County Conservation District, Appoquinimink River Association, Delaware Department of Transportation and others. Although the Plan is not geared solely toward solving existing problems, knowing where and why they exist aided the engineer in developing the sub-watersheds, identifying points of interests, and understanding the hydrologic flow of the watershed as a whole. Information on existing land use and zoning was also collected. This helped the engineer to determine where and to what extent future development would take place. All of this data was compiled into a geographic information system (GIS) database. The computer model used for the project was the US Army Corps of Engineers Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS). This model was chosen for the project because it can be easily adapted to an urban and/or rural area, it has the ability to analyze reservoir or detention basin-routing effects, and it is accepted by most state and federal agencies. To gain a realistic picture of how stormwater is generated in the Appoquinimink River watershed and moves through the watershed, the model, which was created using the Delmarva Unit Hydrograph, was calibrated using target flows developed from regression models and data obtained from the Federal Emergency Management Administration (FEMA). The process of determining how stormwater runoff flows through the watershed is a complex one. The hydrologic model of the Appoquinimink River watershed was developed with sixty- nine (69) subareas, ranging in size from nine (9) to fourteen hundred (1,400) acres. Using the calibrated hydrologic model the hydrographs of each of these subareas was evaluated at four (4) separate points of interest to understand how watershed timing impacts the flow in different parts of the Appoquinimink River. This analysis was then used to establish management districts for the watershed to better manage post construction stormwater runoff. For this study, the 2-, 10-, 25-, 50-, and 100-year storms were modeled. A precipitation and annual runoff analysis was conducted for the watershed in which the model was used to determine the amount of rainfall that is equivalent to 90% of the annual runoff volume from the watershed. This analysis determined that 90% of the annual runoff occurs from events that provide precipitation depths equivalent to approximately the 1-year storm. In addition to the aforementioned hydrologic modeling efforts, the Plan also identified and performed a cursory evaluation of significant obstructions and
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