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Streams in Albemarle County, some may be partially in or out.
Utilizing information from several existing individual datasets, a new dataset has been developed to identify the perennial stream segments in Virginia’s portion of the Chesapeake Bay Watershed (watershed).
Reference datasets that were used to develop this new perennial dataset include: The Virginia Information Technology Agency’s (VITA) Virginia LIDAR dataset, which is available for the entire watershed and, in many instances, contains multiple years of data for certain locations; SSURGO dataset that includes information about soils as collected by the National Cooperative Soil Survey over the course of a century; 2002 VGIN-created stream network (derived from digital elevation models (DEMs)); and The United States Geological Survey’s National Hydrography Dataset (NHD).
Process to develop base dataset of stream segments
Due to the spatial inaccuracy of existing spatial databases representing hydrology, it was necessary to create new lines that represent the center of stream channels. A precise representation of the location of the stream channel is necessary to assist with determining compliance with §62.1- 44.123.
VITA’s Virginia LIDAR data was the primary base data layer used to create the dataset. The LIDAR dataset was used to create precise digital elevation models (DEMs). The DEMs were then used to create the general stream features across the watershed. Aerial imagery base maps and the Virginia HUC features map were also included to create this initial stream feature layer.
To further refine the initial stream feature layer, the DEMs were used to model hydrologic processes including the accumulation of water and the direction of the flow of water. As there may be regional variations in the effectiveness of this process, quality control testing was conducted to ensure that the results accurately reflect the actual hydrology in each region. The stream features were then subdivided into smaller segments and these smaller segments were reviewed for perennial identification.
Identifying perennial stream segments from the base dataset
Reference datasets provide additional information for each stream segment and assist with the identification of whether a segment is perennial or intermittent. Each stream segment had information overlaid from the reference datasets to provide additional details that may impact that segment’s identification.
There were three reference datasets utilized to identify a stream segment as perennial.
1. The SSURGO individual soil classifications were used to examine whether wet or flooded soils were present in a certain stream segment. (20% of stream segment needed to intersect with the polygon features classified as OCCASIONAL, COMMON, FREQUENT, or VERY FREQUENT in the soils layers)
2. To account for the impacts of a year with low rainfall on a stream segment, a dataset of water features developed by VGIN from data collected in 2002 was used. There was a severe drought statewide during 2002, therefore, a stream segment included in this dataset has a high likelihood of being perennial. (60% of stream segment needed to intersect a 50ft buffer around the centerline of the VGIN 2002 dataset)
3. The NHD dataset contains information indicating potential water flow through stream segments; this information can be used to assist with the identification of segments in the new base datasets that are likely to be perennial. This dataset’s resolution is not the same scale as the new base stream segment dataset; therefore, a larger search threshold is used for comparison of NHD features with the new base dataset. (40% of stream segment needed to intersect a 60ft buffer around the centerline of perennial streams in the NHD)
As each of the reference datasets contain different characteristics of a perennial stream segment, a segment was only identified as perennial when two of the three reference datasets contained perennial characteristics for that segment.
Quality control for the final dataset
Once stream segments were identified as perennial or intermittent, several quality control processes were undertaken to ensure the integrity of the final dataset including the use of tools to check the processing of all connected features in the dataset. As an example of such processes, stream segments were reviewed to ensure all segments upstream of an intermittent feature were similarly identified as intermittent. For each hydrologic area (12-digit HUC codes), four segments were sampled to ensure the identification of those segments was accurate. Each segment was examined to confirm the correct reference dataset information had been utilized as well.
Additionally, the four segments were evaluated against aerial imagery for certain characteristics of perennial stream segments. This evaluation looked for the presence of a stream bed and bank (which is indicative of perennial water flow); stream sinuosity (streams with low channel slope gradients will typically show high sinuosity if the stream is perennial); and the appearance of riffles or pools (these repeated combinations can be observed readily in perennial streams).
Additional efforts were also made to review the final dataset for any discrepancies that may be related to karst topography as part of the quality control process.