Mad River Valley Erosion Study Final Report May 7, 2012

Transcription

1 Mad River Valley Erosion Study Final Report May 7, 2012 Prepared by: Prepared for: P.O. Box 1085 Waitsfield, VT P.O. Box 255 Waitsfield, VT

2 CONTENTS Introduction... 1 Methodology... 1 Geographic Information System (GIS) Desktop Analysis... 1 Field Priority Indicators... 2 Data Collection & Reduction... 4 Results... 5 Data Gaps & Recommendations... 7 Attachment A GIS contraints analysis maps... 9 Attachment B- scoring matrix Attachment C site photographs Attachment D priority erosion sites Attachment E final design plans Attachment F final design construction details Attachment G GIS shapefiles... 15

3 Mad River Valley Erosion Study Final Report May 7, 2012 Watershed Consulting Associates, LLC page 1 INTRODUCTION Accelerated erosion from roadside swales and culverts is major contributor to water quality pollution in the Mad River Valley towns. The abundance of development on steep terrain with erodible soil conditions makes this road network especially vulnerable to erosion and sedimentation. The severity of the erosion and sedimentation problem becomes clear by touring the Valley roadways during a rainstorm and viewing the chocolate brown runoff flowing directly to the Mad River tributaries and wetlands. In 2011 and 2012 Friends of the Mad River (FMR) along with Watershed Consulting Associates, LLC (WCA), acting as contractor to FMR, conducted an analysis to map, inventory, prioritize, and prepare design plans for back road erosion areas within the five Mad River Valley Towns of Warren, Waitsfield, Fayston, Moretown, and Duxbury. The purpose of the project was to 1) better understand the scope of the erosion problems along the Class 3 and 4 road drainage networks in these Towns, 2) to provide an inventory of erosion areas for the Towns to track the problem areas, 3) to prioritize the erosion areas for impact to water quality by sedimentation, 4) to provide simple, low cost design solutions for a selection of the mapped erosion areas, and 5) to better inform local road crews on preventing road erosion by hosting a workshop in the Towns scheduled for May The project was funded through a Grant provided by the Vermont Department of Environmental Conservation Watershed Grants Program. Typical Mad River Valley Road Erosion Area METHODOLOGY GEOGRAPHIC INFORMATION SYSTEM (GIS) DESKTOP ANALYSIS A constraints analysis was conducted by the Central Vermont Regional Planning Commission (CVRPC) using ArcView GIS. The purpose of the constraints analysis was to use desktop GIS to identify areas of the road network that were likely susceptible to erosion and sedimentation resulting in water quality impact, in order to focus the scope of the field investigation. The constraints analysis considered a total of five parameters, which included stream crossings, width of buffers to streams, width of buffers to mapped State-regulated Class II wetlands, soil erodibility, and road slope. The road network within the five Towns was divided into 100-foot segments, and for each segment the presence or absence of the five constraints was analyzed. Then, for each road, the number of constraints was totaled. The segments of each road with the relative constraints scoring were displayed on a GIS map. In addition a second analysis was performed, which summarized

4 Mad River Valley Erosion Study Final Report May 7, 2012 Watershed Consulting Associates, LLC page 2 specific point locations where three, four, or five of the constraints existed at the same location. Highlighting the overall road scores was useful for identifying general road locations where erosion and water quality impact may be present. Identifying specific points was useful for pinpointing the potential trouble spots. The GIS maps are included in Attachment A. Field crews reviewed each GIS map, and prioritized field work based on the constraints scoring. All road sections identified in the GIS analysis were visited, with very limited exception, due to private property and the absence of a road in the case of one Class 4 location. Table 1 below provides a summary of the constraint values considered in the GIS analysis. Constraint Criterion Data Source Stream Crossings Road / Stream intersection Vermont Hydrography Dataset (VHDCARTO, 2010) Stream Buffer Width Class II Wetland Buffer 50 feet Vermont Hydrography Dataset (VHDCARTO, 2010) 50 feet Vermont Significant Wetlands Inventory (VSWI, 2010) Soil Erodibility Kw > 0.36 Slope rise/run > 15% Natural Resource Conservation Service (NRCS) Soil Survey (Geologic_SO, 2011) Vermont Hydrography Dataset DEM (ElevationDEM_VTHYDRODEM, 2005) Table 1. GIS Analysis parameters. FIELD PRIORITY INDICATORS After the completion of the GIS desktop analysis, a series of field priority indicators were developed along with a scoring matrix, which were used by field crews to rank each erosion area impact to water quality during the field mapping effort. These priority indicators included volume of runoff expected through the erosion area, the steepness of the buffer area downstream of the erosion area and upstream of the receiving water, the condition of the ground cover of the buffer area, and finally the opportunity for sediment deposition in the buffer area. Individual erosion areas were scored in the field as high, medium, and low, for each of these indicators. Following the field data collection, scoring for each of the field indicators was imported into an excel spreadsheet to calculate total scores for each area. Each high, medium, and low score was assigned a value of 3, 2, or 1, respectively. In the case of the opportunity for deposition indicator, percentages of 30%, 20%, or 10% were assigned to a high medium or low score, given that depositional areas were assumed to have a weighted importance. A summary table of the field priority indicators is provided in Table 2 below. The complete scoring matrix is provided in Attachment B.

5 Mad River Valley Erosion Study Final Report May 7, 2012 Watershed Consulting Associates, LLC page 3 Priority Indicators Description Score Notes small channel, headwater area Low Volume medium channel, middle of watershed large channel, close to receiving water Medium High Volume indicator refers to the condition at and upstream of the erosion area Velocity/Steepness low slope, <5% moderate slope, >5% and < 15% Low Medium Velocity/Steepness indicator refers to the condition at and steep slope, > 15% High downstream of the erosion area Soil Cover stone vegetation bare Low Medium High Soil Cover indicator refers to the condition downstream of the erosion area Deposition to Stream sheet flow over well vegetated terrain/ channel with turnouts to vegetated terrain channel flow with defined break in slope with some deposition before stream Low Medium Deposition indicator refers to the condition downstream of the erosion area channel flow with no slope breaks, obvious in stream deposition High Table 2. Priority indicators for water quality scoring.

6 Mad River Valley Erosion Study Final Report May 7, 2012 Watershed Consulting Associates, LLC page 4 DATA COLLECTION & REDUCTION Field crews walked/drove/bicycled each priority road segment during the 2011 field season. Field crews carried a Trimble GeoXH GPS unit capable of sub foot accuracy with post processing correction. The drainage system along the road right of way was scanned for areas of erosion, including roadside swales and culvert inlets and outlets. Field priority indicators were imported into the GPS unit as a data dictionary. This allowed customized drop down menus to be used to log the data, which significantly increased data collection efficiency. In addition to the collection of the priority indictors data, descriptive data for the erosion areas was also recorded. This included the general category of the erosion, where the erosion areas were located, an in the case of erosion at culvert locations, specific data about the culvert. Each erosion area was assigned a unique ID that correlated with the road name. The descriptor data is summarized in Table 3. Field technician with GPS Descriptors Type Notes General Erosion Category Erosion Location Rill Gully/Incision Slump In stream / in ditch scour Roadway Roadway Shoulder Ditch Ditched Stream Culvert Headwall Culvert Endwall small channel that could be graded larger channel that could not be repaired by grading failure of road surface carving of stream bank / ditch side slopes

7 Mad River Valley Erosion Study Final Report May 7, 2012 Watershed Consulting Associates, LLC page 5 Descriptors Type Notes Culvert Data (Culvert Sites Only) Diameter inches Material Condition assumed ownership Drop from outlet to stream Bankfull width at outlet metal/plastic/concrete poor/fair/good Town/Private inches - live stream crossings only inches - live stream crossing only Table 3. Erosion area descriptors. Photographs were also collected, and are included in Attachment C. Pathfinder office software was used to post process the erosion area GPS data to increase accuracy. GPS data was exported to ESRI shapefile format. A point and line shapefile was produced that included all mapped erosion areas. The overall priority score of each erosion area was included as a field in the shapefiles. RESULTS A total of 368 erosion areas were mapped across the five towns. Sites having the highest priority scores in each Town were organized into a spreadsheet and evaluated. A total of 32 sites were included on this reduced list. Of the 32 sites, it was determined that several of the sites will require full engineering design for repair, given the complexity of the likely design solution. The need for an engineering design on these sites was noted in the GIS database. For several other sites, it was determined that a repair had been completed or was being initiated, since the time the erosion areas were first mapped. The remaining 19 sites were selected for a design repair. The list of the design sites is included in Table 4 below. The list of the 32 priority sites initially evaluated is included on Table 5 in Attachment D. Site plans for all of the final design sites are provided in Attachment E. Construction details are provided in Attachment F. GIS shapefiles for all erosion sites and also for the design sites are provided in Attachment G.

8 Mad River Valley Erosion Study Final Report May 7, 2012 Watershed Consulting Associates, LLC page 6 SITE_ID Longitudinal Slope (%) Bottom Width (ft) Depth (ft) Top Width (ft) Flowing (y/n) Town DR y Duxbury DR n Duxbury WH23 5 3'-gully; 2' upslope ditch 3' gully; 2' upslope ditch 3' gully, 7' upslope ditch y Duxbury CFR y Fayston KV y Fayston PB y Fayston TH n Fayston TH n Fayston TH y Fayston FR y Moretown MC n Moretown SB n Moretown SB n Moretown SB n Moretown PH y Waitsfield AR n Warren AR n Warren BEHN y Warren PMR n Warren Table 4. Final design sites. Design repairs consisted of simple site plans that provide locations of the erosion area and a reference to a corresponding construction detail. The site plans also provide construction material quantity information to allow Town crews to price the specified materials.

9 Mad River Valley Erosion Study Final Report May 7, 2012 Watershed Consulting Associates, LLC page 7 The final GIS shapefiles, site plans, construction details, and photo log will be provided to each of the five Towns. The data will serve to help the Towns track erosion area problems, and to repair the final design sites using the Town road crews. For the sites that require full engineering design to repair, the Towns can use this information to help inform future decisions on hiring a design firm to address these areas as funding becomes available. DATA GAPS & RECOMMENDATIONS Local roadways that were not highlighted in the GIS desktop analysis were not evaluated, and therefore it can be expected that additional unmapped erosion areas exist on local roads in the five Towns. Sites were initially mapped before Tropical Storm Irene in August 2011, and therefore, conditions may have changed at some of the mapped areas. However, all 19 design sites were revisited after Tropical Storm Irene in the Fall of 2011, and it was determined that these areas did not change significantly as compared to when they were mapped initially. It is recommended that the specified designs for repair of the 19 erosion areas be implemented during the field seasons. Towns may decide to apply to the Northern Vermont Resource Conservation and Development (RC&D) Council for funding the repairs through a Category B Better Backroads (BBR) grant. In this case, the design information in this report should be utilized to complete the BBR application, which is due in the fall of The GIS inventory of the erosion sites should be updated periodically to reflect repairs made to the drainage system. Once the initial 19 sites have been addressed, additional mapped sites could be targeted for repair based on the priority scoring. Sites identified for a full engineering design should also be reviewed by the Towns and prioritized for inclusion in Capital Budgets.

10 Mad River Valley Erosion Study Final Report May 7, 2012 Watershed Consulting Associates, LLC page 8

11 ATTACHMENT A GIS CONTRAINTS ANALYSIS MAPS

12 ATTACHMENT B- SCORING MATRIX

13 ATTACHMENT C SITE PHOTOGRAPHS Refer to digital archive Attachment C Site Photographs

14 ATTACHMENT D PRIORITY EROSION SITES

15 ATTACHMENT E FINAL DESIGN PLANS

16 ATTACHMENT F FINAL DESIGN CONSTRUCTION DETAILS

17 ATTACHMENT G GIS SHAPEFILES Refer to digital archive Attachment G GIS Shapefiles