Flood risk and Soil Study in Sally Ormsby Park

To assess the flooding risk for Sally Ormbsy Park, I used the FEMA (Federal Emergency Management Agency) online interactive tool.  I first searched for Fairfax County, Virginia and then zoomed in on the park and its surrounding residential neighborhood.  Sally Ormsby Park is contained in two FEMA flood plain maps:  51059C0260E and 51059C0280E.  Using a snipping toil, I created one image as seen below taking a portion of each of these flood maps.  The red inked lines I inserted show Barkley Drive as the northern terminus of the park and Prosperity Avenue as the southern terminus of the park.  As can be seen on the map below the entire park is in Zone AE, which is a regulatory floodway.  The residential areas to the west and east of the park are Zone X, which has a .2% Annual Chance of Flood Hazard, which is considered to be an area of minimal flood hazard.

The second image shows the same area, but instead of showing the streets prominently, shows more of an aerial view, but with the vegetation in the floodplain shaded blue.  Again, both Barkley Road and Prosperity Avenue are outlined in red ink to show the north and south boundaries of the park and flooding zones AE and X are shown.

To learn more about the soils of Sally Ormsby Park and this section of the Accotink Creek watershed, I went to the USDA Soil Survey site.  By inputting the state of Virginia and then Fairfax County, I was able to zoom in to Sally Ormsby Park and its environs.  The image below is a soil map of the site.  Once again, I inked in red Barkley Drive to the north and Prosperity Avenue to the south, to delineate the north and south boundaries of the park. 

As can be seen in the map above, the most prominent soil in the park is 29A, followed by 49A in the surrounding area.  Below is the legend for the soil map, which includes these two soils.

In the interactive site, you can click on the soil name and short description and get a readout with more details.  I have provided them below for 29A, the Codorus Silt Loam and 49A, the Hatboro Silt Loam.

29A—Codorus silt loam, 0 to 2 percent slopes, occasionally flooded Map Unit Setting National map unit symbol: 2fjmt Mean annual precipitation: 37 to 49 inches Mean annual air temperature: 45 to 67 degrees F Frost-free period: 185 to 212 days Farmland classification: Not prime farmland
Map Unit Composition Codorus and similar soils: 85 percent Minor components: 5 percent Estimates are based on observations, descriptions, and transects of the mapunit.
Description of Codorus Setting Landform: Flood plains Down-slope shape: Linear Across-slope shape: Linear Parent material: Alluvium derived from igneous, metamorphic and sedimentary rock Typical profile H1 - 0 to 8 inches: silt loam H2 - 8 to 50 inches: loam H3 - 50 to 62 inches: stratified very gravelly sand to loam Properties and qualities Slope: 0 to 2 percent Depth to restrictive feature: More than 80 inches Natural drainage class: Somewhat poorly drained Runoff class: Low Capacity of the most limiting layer to transmit water (Ksat): Moderately high to high (0.57 to 1.98 in/hr) Depth to water table: About 10 to 24 inches Frequency of flooding: Occasional Frequency of ponding: None Available water storage in profile: Moderate (about 8.7 inches)

49A—Hatboro silt loam, 0 to 2 percent slopes, frequently flooded Map Unit Setting National map unit symbol: 2fjpz Mean annual precipitation: 37 to 49 inches Mean annual air temperature: 45 to 67 degrees F Frost-free period: 185 to 212 days Farmland classification: Not prime farmland
Map Unit Composition Hatboro and similar soils: 85 percent Estimates are based on observations, descriptions, and transects of the mapunit.
Description of Hatboro Setting Landform: Flood plains Down-slope shape: Linear Across-slope shape: Linear Parent material: Alluvium derived from igneous and metamorphic rock Typical profile H1 - 0 to 6 inches: silt loam H2 - 6 to 23 inches: loam H3 - 23 to 60 inches: clay loam Properties and qualities Slope: 0 to 2 percent Depth to restrictive feature: More than 80 inches Natural drainage class: Poorly drained Runoff class: Negligible Capacity of the most limiting layer to transmit water (Ksat): Moderately high to high (0.57 to 1.98 in/hr) Depth to water table: About 0 to 18 inches Frequency of flooding: Frequent Frequency of ponding: None Available water storage in profile: Moderate (about 8.6 inches)

Both of these soils are silty loams and their landform is flood plains.  Codorus is somewhat poorly drained and Hatboro is poorly drained.  Neither is prime farmland. 

This information in addition to the FEMA flood risk maps shown above show that a park is a good land use for this section of watershed of Accotink Creek.  In particular, the area on either side of Accotink Creek and Long Branch Creek are regulatory floodways, while the surrounding residential areas have minimal flood hazard.

Resources

FEMA Flood Map Service Center.  Retrieved from https://msc.fema.gov/portal/home .

USDA Natural Resources Conservation Service.  Web Soil Survey.  Retrieved from
https://www.nrcs.usda.gov/wps/portal/nrcs/main/soils/survey/