In this post, we'll utilize various resources to analyze the floodplains and alluvial soils within the Ridley Creek watershed. Because land use in the watershed ranges from undeveloped protected state park lands to urban industrialization, we will compare areas representing each scenario and the risks that a potential flood may pose.
Ridley Creek State Park
This floodplain map from FEMA shows an area within Ridley Creek State Park. The blue zone, Zone A, shows the area around the creek that has a 1% chance of flooding annually. Ridley Creek State Park is preserved land, thus it is mostly undeveloped and therefore there are not any structures in danger of being flooded. However, there are some roads within Zone A which could take on water during a flood event.
This soil map from USDA shows the soil units found in the same area. The top three soil units in the area of interest, by acreage and percentage of area, are as follows:
| GdB | Gladstone gravelly loam, 3 to 8 percent slopes | 130.1 | 12.7% |
| GdC | Gladstone gravelly loam, 8 to 15 percent slopes | 318.3 | 31.2% |
| GdD | Gladstone gravelly loam, 15 to 25 percent slopes | 126.1 | 12.3% |
The Gladstone soils are described as well drained with high runoff. The elevation of the unit ranges from 20 to 160 feet. Typically, the Gladstone series consists of gravelly loam from 0-10 inches; sandy clay loam from 10-22 inches; loam from 22-37 inches; sandy loam from 37-66 inches; and bedrock from 66-80 inches.
Based on the floodplain map and available soil data, this area can be considered an appropriate use of the land, as there are no structures that would be endangered in a flood event. The permeable surfaces will allow for better infiltration of rain and flood waters when compared to an urban area. However, if the 1% annual chance flood does occur, one would need to be vigilant about the roads that are within the floodplain.
Chester/Eddystone
This floodplain map from FEMA shows the area at the mouth of Ridley Creek, where it enters the Delaware River. Here the creek runs between the city of Chester and the borough of Eddystone. Again, the blue zone shows the area that has a 1% chance of flooding annually. In this area, the blue zone is designated Zone AE, which indicates that it has an identified base flood elevation. In this map we also have an orange zone, or Zone X. This area has a 0.2% annual chance flood hazard; it is portions of Zone AE where average flood depth is less than one foot or the drainage area is less than one square mile.
There is also a blue and red banded area, which denotes a regulatory floodway. According to FEMA, a regulatory floodway is defined as "the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated height. Communities must regulate development in these floodways to ensure that there are no increases in upstream flood elevations."
Based on the floodplain map, we can see that this area is heavily developed with both industry and housing within the floodplain of Ridley Creek as well as the Delaware River. Development in this area must be managed carefully to reduce impacts from flood events.
This soil map from USDA shows the soil units found in the same area. This map differs drastically from the Ridley Creek State Park soil map due to urbanization. The top three soil units in the area of interest, by acreage and percentage of area, are as follows:
| UbB | Urban land, gravelly sand substratum, 0 to 8 percent slopes | 572.2 | 52.0% |
| PoB | Pineyneck-Urban land-Greenbelt complex, 0 to 8 percent slopes | 129.2 | 11.7% |
| UhB | Urban land-Greenbelt complex, 0 to 8 percent slopes | 66.1 | 6.0% |
The Urban land, gravelly sand substratum unit is described as buildings and pavement over human-transported material. The elevation of the unit ranges from 0 to 60 feet.
The Urban land-Greenbelt complex soils are described as well drained with high runoff. The elevation of the unit ranges from 0 to 60 feet. Typically, the Urban land-Greenbelt series consists of loam from 0-9 inches; sandy loam from 9-33 inches; sandy clay loam from 33-43 inches; and loamy sand from 43-80 inches.
The Pineyneck-Urban land-Greenbelt complex soils are described as moderately well drained with high runoff. The elevation of the unit ranges from 0 to 60 feet. Typically, the Pineyneck-Urban land-Greenbelt series consists of loam from 0-10 inches; silt loam from 10-29 inches; fine sandy loam from 29-47 inches; loamy fine sand from 47-61 inches; fine sandy loam from 61-70 inches; and loam from 70-80 inches.
Based on the floodplain map and available soil data, this area may be considered a less appropriate use of the land when compared to undeveloped areas of the watershed, as there are numerous structures and roads that would be endangered in a flood event. The large area of paved surfaces and buildings will only increase water accumulation and runoff. However, there is a fair amount of open space within the floodplain to allow safe discharge of flood waters. Development in this area must be regulated and carefully balanced.
References
FEMA. “Flood Maps | FEMA.gov.” www.fema.gov, U.S. Department of Homeland Security, www.fema.gov/flood-maps.
USDA. “Web Soil Survey.” usda.gov, United States Department of Agriculture, websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx.