Navigating the USGS floodplain study reports to confirm WSELs and discharges:
Navigate to the location in ArcMap/ArcGIS Pro where you are interested in discharges or Waster Surface Elevations (WSELs)
At that location, note the name of the watershed and stream.
Go to the USGS floodplain report for the watershed of interest (see below for summary of reports).
Go to the list of tables in the USGS report and note the page number for the Water-surface Profile Data table corresponding to the stream or watershed of interest.
Go to the relevant Water-surface Profile Data table and locate the section of interest. The station can be easily located by reviewing upstream or downstream roadways or other features noted in the remarks, estimating the distance from the feature on GIS and using stationing information in the second column to locate the section number of interest. The table includes 25-, 50- and 100-year discharges and the corresponding WSELs.
| Watersheds | Open-File Report# | Year completed |
| Accotink Creek | 76-442 | 1977 |
| Cameron Run* | 76-443 | 1976 |
| Pohick Creek | 76-444 | 1977 |
| Difficult Run | 76-459 | 1976 |
| Cub Run | 78-17 | 1978 |
| Bull Run, Little Rocky Run, Johnny Moore Creek, and Popes Head Creek | 77-329 | 1977 |
| Bullneck Run, Scott Run, Dead Run, and Pimmit Run | 78-260 | 1978 |
| Horsepen Run, Sugarland Run, Nichlos Run, and Pond Branch | 78-1028 | 1978 |
| Occoquan River, Wolf Run, Sandy Run, Elk Horn Run, Giles Run, Kanes Creek, Racoon Creek, and Thompson Creek | 79-215 | 1978 |
*Excludes Upper Holmes Run and Tripps Run upstream of Lake Barcroft
All open file reports are available from USGS. Please use the search function at the USGS publications warehouse: https://pubs.er.usgs.gov/. DPWES ( Stormwater Planning Division) also maintains copies of the reports.
Vertical datum:
In 1973, the Sea Level Datum of 1929 was renamed the National Geodetic Vertical Datum of 1929 (NGVD 1929). It appears the USGS reports retained the older name as elevations are referenced to \\u2018Mean Sea Level\\u2019. However, it has always been assumed that this refers to the old Sea Level Datum of 1929 that is now called NGVD 1929.
Evaluation of physical changes
Field data collection for the USGS floodplain studies was completed from about 1960 through 1970. Since physical changes might have occurred in the floodplain after that time, it is important that the area of interest be carefully evaluated to determine if any significant physical changes have occurred. Physical changes typically occur as a result of roadway improvements that significantly alters the stream alignment. Historic (from the 1960\\u2019s and 1970s) for the area of interest and current aerial imagery should be used to evaluate this. If physical changes have occurred, the USGS 100-year WSELs are no longer reliable.
Use of flow values
Discharges in the USGS report were computed using the Anderson method which is one of the approved hydrologic methods in the PFM for computing peak flows for drainage areas over 200 acres. The PFM guidance recommends using the USGS flows when available. However, it should be noted that the flow computation points were relatively sparse and flows at a computation point were typically utilized about 3,000-5,000 feet upstream. Therefore the flow at or near the upstream point before a flow change may be relatively conservative.
Contact: DPWES GIS Services (dpwesgisservices@fairfaxcounty.gov)
Data Accessibility: Publicly Available
Update Frequency: Never
Last Revision Date: 1/29/2024
Creation Date: 1/29/2024
Feature Dataset Name: N/A
Layer Name: STWMGR.USGS_FPL_XSECTIONS
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