NC Streambed Mapping Project Issue Paper

Size: px

Start display at page:

Download "NC Streambed Mapping Project Issue Paper"

Duane Spencer
5 years ago
Views:

1 ISSUE 1: The Western North Carolina Drainage Area Requirement Issue Several inquiries and discussions in regard to the determination of the 6-acre drainage area requirement as the upstream limit have been initiated for the Streambed Mapping Project. The 6-acre requirement was established as part of Delivery Order 30 for promoting data consistency, completeness and cost effectiveness. Background Implementation Plan Creation The Streambed Mapping Project for Western North Carolina Counties is the result of a technical report entitled, Implementation Plan to Improve the Digital Representation and Mapping of Surface Waters in North Carolina. The report was created in the fall of 2004 in response to Senate Bill 1152, which mandated that a plan be created to improve the mapping of streams and surface waters in the State of North Carolina. Previously it was established that the current stream maps used by government agencies and the private sector were often outdated, inaccurate, and incomplete, which in turn added large expenses to current projects. In order to develop an effective implementation plan, the Geographic Information Coordinating Council (GICC) tasked the Statewide Mapping Advisory Committee (SMAC) with the creation of the Stream Mapping Working Group (SMWG). The SMWG was comprised of federal, state and local agencies. Through a series of meetings with the SMWG, user requirements, mapping specifications and implementation options for the plan were established. This collaborative effort led to the identification of needs for a new statewide digital stream file that could be effectively used and maintained by federal, state and local government agencies as well as the public. The full implementation plan can be found on the North Carolina Stream Mapping Project website ( Technical Documents). The plan was adopted by the Geographic Information Coordinating Council on January 5, 2005 and was presented to the Environmental Review Commission on January 13, The plan was received favorably by both groups. The GICC approved the implementation plan on January 5, Streambed Mapping Project Origination and Basis of Drainage Area Requirement The General Assembly appropriated funds to begin streambed mapping within Senate Bill 7, Hurricane Recovery Act of Senate Bill 7 allocates mapping funds for the 19 counties in Western North Carolina that were declared disaster areas in the Hurricane Recovery Act. The consensus of the SMWG regarding the upstream limits of the stream reaches for the new dataset has been used as a basis for mapping the streambeds for the Streambed Mapping Project. In order to clarify the term streambed, the term streamlines and water bodies shall be used in its place in this document. The SMWG recommended using a 6-acre drainage area as the upstream limit for mapping the streamlines and water bodies. A drainage area standard is necessary to provide data consistency and completeness for the streambed mapping product. First Revision Date: 7/14/2006 Page 1

Methods Method 1 6-Acre Drainage Area Standard The goal for streambed mapping is to maximize data consistency, stream representation and map accuracy in the most cost effective manner.

2 Methods Method 1 6-Acre Drainage Area Standard The goal for streambed mapping is to maximize data consistency, stream representation and map accuracy in the most cost effective manner. It has previously been determined by the SMWG that the 6-acre drainage area will meet the majority of the project s requirements. Figure 1 displays drainage areas in acres and the miles of stream that would be mapped at those specific drainage area requirements. The information is based on calculations for the entire State of North Carolina. Figure 1 indicates that as the drainage area threshold decreases, the linear miles of stream mapped increases. The project cost also increases as the number of mapped streams increases. Figure 1: Drainage Area versus Stream Miles Mapped for the State of North Carolina The 6-acre drainage area requirement also helps to alleviate another need identified by the Stream Mapping Working Group. This is the need for data completeness and consistency. It is important that data collection and attribution remain consistent throughout the 19 identified counties. The current 1:24,000 scale USGS quadrangles contain inconsistencies in data density between them. Figure 2 below shows an area of Watauga County where the Sherwood quadrangle contains a denser stream network than the surrounding quadrangles. First Revision Date: 7/14/2006 Page 2

Figure 2: Consistency Differences in Current USGS 1:24,000-Scale Quadrangles Method 2 Stream Origin Standard The North Carolina Department of Transportation (NCDOT), the North Carolina Division of

3 Figure 2: Consistency Differences in Current USGS 1:24,000-Scale Quadrangles Method 2 Stream Origin Standard The North Carolina Department of Transportation (NCDOT), the North Carolina Division of Water Quality (NCDWQ) and other agency programs related to permitting activities for the State have additional business requirements that are outside the scope of the Streambed Mapping Project. While data consistency and completeness are required, there is an additional need for accurate upstream limits that represent the total length of the first order stream. The starting point, or origin, of the intermittent and perennial stream is essential to planning, permitting and implementation of mitigation and buffer rules within the Division of Water Quality. NCDOT and NCDWQ initiated a project in early 2004 to map and model intermittent and perennial stream origins. The data collected and derived from the NCDOT/NCDWQ effort can be indexed with the final Streambed Mapping Dataset ( # 005). Data Analysis Stream Origins in 3 Areas of NC Three field-derived stream mapping datasets were used to analyze the effectiveness of the 6- acre drainage area standard. These datasets included Greensboro, NC which is located in the Piedmont; Hendersonville, NC, an urban area in the mountains; and White Mountain (Lenoir, NC), a rural mountain site. Stream identification and origins were determined in the field using First Revision Date: 7/14/2006 Page 3

4 the DWQ methodology described in the Stream Identification Manual version 3.1. This methodology was originally developed by DWQ and NCSU along with other agency and private industry personnel, and was implemented as a state policy in Each dataset was spatially analyzed using Geographic Information Systems (GIS) to determine the contributing drainage area to each stream origin. Drainage area data was then compared to a 6-acre threshold to assess the number of origins captured in a 6-acre drainage area. The comparisons of the stream origins to the 6-acre drainage area standard are included in Table 1 below. Table 1: Field Mapped Stream Origin Data Watershed Approximate Land Area (Acres) Number of Intermittent Origins Number of Perennial Origins Total Number of Origins Number of Origins Covered by 6 Acres Greensboro 25, White Mountain Hendersonville* 13, *Flow duration of origins was not determined The Greensboro data showed that 368 origin points (82%) were collected using 6-acres as the upstream drainage limit. Data obtained from Hendersonville indicated that only 30 of 119 (25%) stream origins were missed, or the 6-acre threshold captured 75% of all stream origins. The analysis from White Mountain revealed that 10 of 41 origins (25%) were not captured in the 6 acre stream limit. Data Analysis Stream Origins in White Mountain Additional analysis was performed on the White Mountain data to assess the miles of stream collected by digitizing to the stream origin point. This analysis was done by extending the 6- acre stream shapefile to the stream origin points or in some cases deleting the extra linework so that it stops at the stream origin points. The miles of stream were then calculated for both instances and compared to the original 6-acre streamlines. The results of this analysis are shown in Figure 3 and Table 2 below. In the White Mountain area, the longest distance from a 6 acre stream to a stream origin was 485 ft. Using 485 ft. as the maximum distance needed to intersect a stream origin, it can then be determined that a maximum of 17,453 feet of streams would need to be added to the White Mountain line work to match the stream origins. When this is calculated out across the entire project area, it would add approximately 24,508 miles on top of the existing 55,697 miles of stream in the current project area. First Revision Date: 7/14/2006 Page 4

5 Figure 3: White Mountain Sample of Color Coded Streams Analysis Table 2: White Mountain Stream Origin Analysis Stream Description Color Correlation with Figure 3 Length in Feet Streams Added to Origin Point RED Lines 4,286 Streams not Associated with Origin Point YELLOW Line Length 3,079 Streams Removed to Origin Point GREEN Line Length 3,168 Additional Linework Subtracted from Clipped Line Work Total 6 Acre Stream Coverage In Addition to Origin Points RED Length - GREEN Length 1,118 YELLOW + GREEN - RED 1,961 The results from Table 2 indicate that an additional 1,961 feet of streams were delineated at the 6 acre level than what would have been delineated using the origin points. An important First Revision Date: 7/14/2006 Page 5

6 note is that most of the additional streams collected at the 6 acre level are streams that only flow during heavy rains. Data Analysis Stream Origins in Greensboro A separate analysis was also performed using the City of Greensboro s stream study data. This data was used to assess the percent of field-determined stream origins captured within a 6-acre drainage area as compared to other drainage area limits. The results of this analysis are shown in Table 2 below. Table 3: Stream Origin Capture Greensboro, NC The data indicates as the drainage area for mapping the headwaters of streams decreases, the number of breakpoints captured increases. At a 20-acre drainage area requirement, 53% of the intermittent / perennial breakpoints are captured, and at a 6-acre drainage requirement the percentage of breakpoints captured increases to 95%. The results displayed in Table 3 indicate that using a drainage area requirement for mapping the headwaters of streams will capture intermittent as well as perennial streams. Given the relative low percentage of missed streams illustrated by these datasets, the recognition that DWQ stream origin breaks can be indexed to the final map product and the essential need for cost effectiveness and mapping consistency, the 6-acre drainage area standard for generating the Streambed Mapping Dataset is the most feasible. Recommendations Mapping the streamlines and water bodies to the 6-acre requirement is the recommended solution for the 19 counties in Western North Carolina. This solution provides a uniform product throughout the Streambed Mapping Project study areas. As the project moves to other regions of the state, this drainage area requirement may change according to changes in topography and regulations. Concerns regarding the 6-acre drainage area threshold for streams by DWQ and DOT staff centered on the need for accurately represented stream length. All participating agencies in the SMWG recognize the stream length need, but also balance that need with mapping consistency and cost across the state. Mapping all the origins of the state within the time frame allotted for First Revision Date: 7/14/2006 Page 6

7 the Streambed Mapping Project would cost in excess of 0.5 billion dollars. Since a DWQ project is currently underway to map origins across the state, this data can easily be integrated into the Streambed Mapping Project as it becomes available. The business and technical procedure will be determined as part of the Data Maintenance Plan. Issue paper #005 addresses these needs and procedures. For this reason, the 6-acre standard should not be revised at this late date. Discussion Summary Date Discussed: December 7 th, 2005 Discussion Participants: Scott Edelman, Melani Harrell, Terri McLean, Joe Sewash, Silvia Terziotti, Periann Russell Summary of Discussion The 6-acre drainage area requirement was established by the Stream Mapping Working Group (SMWG) in the fall of 2004, and will be used as the upstream mapping limits for the Streambed Mapping Project in Western North Carolina. This drainage area requirement will need to be revisited when the project moves to other regions of the state. It was noted that the SMWG also cited a need for data consistency across the state. Suggestions from the discussion on items to incorporate in the issue paper included: How agencies use the digital streamline and water body data Agencies that were involved in the creation of the Implementation Plan to Improve the Mapping and Digital Representation of Surface Waters in North Carolina How consensus from the SMWG was derived Who approved the Implementation Plan Other agencies that may be performing similar projects During the Advisory Committee Meeting held on July 14, 2006, the additions and revisions to the issue paper were reviewed. A section entitled Data Analysis Stream Origins in 3 Areas of NC was added to the issue paper by NCDWQ. This analysis utilized three field-derived stream mapping datasets to analyze the effectiveness of the 6-acre drainage area standard. Watershed Concepts also added a section to the issue paper called Data Analysis Stream Origins in White Mountain. White Mountain data was assessed to determine the miles of stream collected by digitizing to the stream origin point versus digitizing to the 6-acre upstream limit. Final Guidelines The final guidelines for this issue paper are to use the recommendations provided in Method 1. This method utilizes the 6 acre drainage area standard for the upstream limit of streamlines collected for the Streambed Mapping Project. First Revision Date: 7/14/2006 Page 7

Development of Improved Stream Mapping for NC

Development of Improved Stream Mapping for NC Presentation for Soil Science Society of North Carolina NC Division of Water Quality January 21, 2009 What are stream maps used for? Riparian buffer rules