Manhasset Bay Protection Committee, Watershed Physical Characteristics

The Watershed’s Size and Boundaries

Approximately 9,947 acres of the watershed’s land surface drains to Manhasset Bay. In general, the watershed’s approximate drainage area boundaries include the western half of the Port Washington peninsula, the eastern third to one-half of the Great Neck peninsula and the area south of Northern Boulevard extending just south of the Long Island Expressway in the Village of Lake Success.

Overall, the 9,947-acre land area of the watershed covers all, or portions of, 19 different communities that are all in the Town of North Hempstead. The communities include 16 villages and 3 unincorporated areas of the Town of North Hempstead. The entire watershed has approximately 200 miles of roads and is estimated to be 20 to 25% covered with impervious surfaces (roads, roofs, parking lots, driveways, etc.) and 75 to 80% pervious surfaces (lawns, woods, shrubs, grass strips, etc.). (back to top)

Subwatersheds

For the purpose of this planning effort, and using existing topography as a basis, the Manhasset Bay watershed has been divided into 12 land-based subwatersheds. The delineation of the 12 subwatershed boundaries is based on topography and not manmade storm water drainage. For instance, in a few locations runoff is discharged to storm water recharge basins that recharge the groundwater. However, if these basins did not exist or if they were taken out of service, the drainage in their services area would discharge to the Bay.

The approximate boundaries of the entire watershed, the 19 communities within the watershed, and the 12 subwatersheds are shown graphically on Figures 1.2-1, 1.2-2 and 1.2-3 while the individual communities located within each of the 12 subwatersheds are presented in Table 1.2‑1, and the estimated land area within each subwatershed is presented on Table 1.2‑2. Furthermore, for the purpose of this planning effort, the 12 subwatersheds have generally been named according to the principal water body or geographic feature within the drainage area, and have been given identification numbers in increasing order moving in a clockwise direction beginning in Sands Point and ending in Kings Point.

If the watershed were divided in a east/west fashion it would have approximately 80 percent of the land area on the east side and 20 percent on the west side and 16 communities on the east side and 6 on the west side. As shown in Table 1.2‑2, the three largest subwatersheds (Whitney Pond, Leeds Pond and Baxter and Mill Pond) account for approximately 59 percent of the watershed acreage while the five largest (Whitney Pond, Leeds Pond, Baxter-Mill Pond, Mitchells Creek, and Kings Point Creek) comprise 76 percent of the total watershed acreage. (back to top)

Watershed and Subwatershed Delineation Map

COMMUNITIES IN THE WATERSHED

(Categorized by Subwatershed)

Subwatershed

Communities

Rank by Size

ID #

Name

Baxter Estates (V)

Flower Hill
(V)

Great Neck (V)

Great Neck Gardens (U)

Kensi-ngton (V)

Kings Point (V)

Lake Success (V)

Manhasset (U)

Manor-haven (V)

Munsey Park
(V)

North Hills
(V)

Plan-dome (V)

Plan-dome Hts (V)

Plan-
dome Manor
(V)

Port Washing-ton (U)

Port Washing-ton
North (V)

Roslyn Estates (V)

Sands Point
(V)

Thom-
aston
(V)

Whitney
Pond

Leeds
Pond

Baxter &
Mill Pond

Mitchell
Creek

Kings
Point
Creek

Barkers
Point

Sheets
Creek

Toms &
Plum
Points

Eastern
Shore

Kings
Point
Pond

South
eastern
Shore

South
western
Shore

ü indicates that the Community has acreage within the Subwatershed.

(V) - Incorporated Village

(U) - Unincorporated area of the Town of North Hempstead

(back to top)

As with many other areas in the Town of North Hempstead and the rest of Nassau County, the Manhasset Bay watershed contains both a variety of land uses and the majority of the land uses being residential of different densities. Based on information contained in Nassau County’s existing computer databases (GIS), there are 11 different land use categories in the Bay’s watershed. These different land use categories are shown graphically on Figure 1.2‑4. As seen from the figure, the predominate land use in the watershed is residential accounting for approximately 65 percent of the land use in the entire watershed. Although this map is based on the latest County GIS data, there may be some discrepancy with actual current uses for some locations (e.g., the industrial area shown on Manhasset Isle is currently vacant). However on a watershed-wide basis, it is sufficiently accurate for planning purposes. The percent breakdown for the watershed of each of the 11 categories is presented in Table 1.2‑3. A breakdown of acreage by community for each subwatershed is presented in Appendix A. (back to top)

As shown in Table 1.2-3, approximately 90 percent of the watershed’s area is currently developed or utilized for one purpose or another, while approximately 10 percent is categorized by the County as “vacant/unknown.” Table 1.2‑4 presents the acreage of vacant/unknown land use in the 16 communities in the watershed. Based on this table, there is potential for new development in certain subwatersheds. Some examples of these areas include the Whitney Estate, land along Leeds Pond, commercial sites along Northern Boulevard, the Shore Road Main Street area of Port Washington, the Commander Oil and Lewis Oil sites, and the waterfront sites in Manorhaven. (back to top)

Using 1997 LILCO figures, 1997 population residing in the watershed (shown in Table 1.2-5) has been estimated to be 43,497. Using LILCO’s 1997 average household size of 2.9 persons per household for the Town of North Hempstead, this would indicate that approximately 15,000 households are located in the Bay’s entire watershed. While Manhasset Bay is important to most of the residents/households in the 19 communities in the watershed, not all 19 communities are located directly on the Bay. Of these, 13 abut the Bay. The 13 communities and their estimated portion of the approximate 12.57‑mile long shoreline are presented in Table 1.2‑6 in order of size of shoreline on the Bay.

The topography of the Manhasset Bay watershed varies greatly from flat in certain areas of Kings Point, Manorhaven and Sands Point to fairly steep slopes in other areas such as the unincorporated areas of Port Washington and Plandome/Manhasset, and portions of the Villages of Kensington, Thomaston and Great Neck. Ground surface elevations in the watershed range from sea level along the 12 to 13-mile shoreline to over 280 feet above sea level in parts of Port Washington (near Port Washington Boulevard just south of Main Street), 250 feet in parts of Manhasset, and 225 feet in parts of Great Neck area.

Much of the watershed is either flat, or with gentle slopes of less than 5 percent, which tends to be fairly stable with respect to erosion potential. However, there are some areas in the watershed with moderate slopes between 5 and 10 percent, which are more prone to erosion under certain conditions and there are a few areas with moderate to steep slopes greater than 10 percent, which are definitely prone to severe erosion under a number of conditions. A map indicating areas having these various ranges of slopes in the watershed is presented on Figure 1.2-5. (back to top)

The most common soil type in the watershed is Urban, Land Montauk - Riverhead. This is a well drained medium to coarse textured soil on low hills. This characteristic lends itself to allowing for good recharge to the groundwater from septic systems, leaching pools, dry wells, etc. Figure 1.2-6 presents the general soil types in the watershed. As shown on the map, there are five different soil types with three of them in over 99 percent of the watershed area. A description of the characteristics of these five soil types (Nassau County Soil Survey, 1987) is presented below.


	Manhasset Bay Protection Committee