The URL can be used to link to this page

Home My WebLink About19970722 Ver 1_Mitigation Information_20010712DRAFT Review of the Detailed Wetland Mitigation Plans (Randleman Reservoir Water Supply) for Richland Creek, Reddicks Creek and Hickory Creek Background The Piedmont Triad Regional Water Authority has proposed the construction of a 3000 -acre reservoir that will become a primary water supply for the Triad region The construction of this reservoir will unavoidably impact approximately 121 acres of wetlands. Compensatory wetland mitigation of at least 121 acres is required under North Carolina Law. These plans address how compensatory mitigation will be achieved at these three sites. A total of 10 potential mitigation sites are being considered. It is estimated that these three sites will replace 69 acres (57 %) of wetlands lost due to the construction of the reservoir. All three wetland mitigation plans are similar in detail and scope. Two primary methods are proposed for wetland restoration. Instream structures designed to reduce sediment transport capacity are proposed at Richland and Hickory Creeks, and the establishment of a greentree impoundment is proposed for Reddicks Creek. Both methods will trap sediment and may promote the establishment of forested wetlands. Water quality concerns with sediment and nutrients have been addressed in a narrative fashion in these three reports. Instream Structures Instream structures are proposed primarily along dredged or entrenched stream corridors on relatively low -slope valley floors (<0.009 rise /run). Adjacent floodplains have been abandoned by the incised stream and converted to elevated�terraces not regularly exposed to overbank flooding or wetland hydrodynamics. Properly designed in- stream structures are expected to reduce the degree of channel incision, reduce the rate of groundwater discharge from the floodplain into the channel, increase overbank flooding, reduce sediment transport capacity and increase sediment deposition within vegetated wetlands Greentree Impoundments Greentree impoundments are proposed on more steeply sloped floodplams and stream terraces ( >0.008 rise /run) or pastured sites where relatively severe stream channel degradation and steepening has occurred. In general, a greentree impoundment comprises a floodplam levee and controllable outlet structure that is modified periodically to promote the development of forested wetlands. The elevation of the outlet is typically raised during the winter months to promote ponding, sediment deposition, and waterfowl habitat. Subsequently the elevation of the outlet is lowered in early spring to allow for the growth of vegetation and seedling FOMIT establishment To provide an optimum environment for the establishment of forested vegetation the water depth during the winter is generally dependent on the height (and species) of planted seedlings. Foresters have studied vegetation and species survival in greentree impoundments so a body of literature is available to guide the managers of the impoundments. Reference Areas The spatial patterns of sediment deposition and vegetation structure and composition was studied at four reference impoundments: 1) Beaver Creek above Lake Jordan (Wake Co.), 2) Little Creek above Lake Jordan (Durham Co ), 3) Rocky Branch above Falls Lake (Wake Co.) and 4) Country Line Creek (Caswell Co.). Details on wetland vegetation that could serve as a reference standard were obtained at two sites along the Rocky River in Cabarrus Co. (Details on the location on these sites is missing). Hydrologic Modeling Analyses of stream flow and water surface elevations were modeled using the US Army Corps of Engineers HEC -1 and HEC -2 for 1 -, 2 -, 5 -, 10 -, and 100 -year flood events. Data used as inputs for the models included synthetic storm precipitation data, drainage area, NRCS curve numbers, drainage basin lag time, and channel cross sections that were completed for the Federal Emergency Management Agency (FEMA) flood insurance studies. Groundwater modeling was conducted using DRAINMOD. Richland Creek The site contains an approximately 5450 -foot reach of Richland Creek, and encompasses about 70 acres — 62 acres of disturbed bottomland hardwood forest and 4.5 acres of pastoral land in a watershed of ca. 14 5 square miles. About 2800 linear feet of the creek was dredged and straightened Spoil material was added to the natural stream levee, and subsequent channel degradation have lowered the channel bottom up to 12 feet below the top of the constructed levee. The channel is classified as a G5 (sand dominated gully) based on fluvial geomorphic features developed by Rosgen (1996). The creek is classified as WS_IV CA. Four fixed in- stream weirs (Fig. 14) will be placed within the dredged channel and designed to avoid having the creek short- circuit the weir. The goal is to encourage sediment deposition behind the weir and encourage the stream to migrate over a restored floodplain (p 31) Hickory Creek This site contains a 3400 -foot reach of Hickory Creek and two valley types to a watershed of ca. 8.7 square miles. The lower half of the site consists of a relatively broad (500 -900 feet) and abandoned floodplain along a 1400 reach of the creek In this section the valley slope measures about 0.004 rise /run. The upper reaches of the site the DRAFT floodplam narrows to widths of less than 200 feet and the slope steepens to an average 0.007 rise /run. The main -stem was dredged and straightened in the last several decades. The channel is classified as a G4 (gravel dominated gully) based on fluvial geomorphic features developed by Rosgen (1996). The creek is classified as WS -IV CA. The restoration plan proposed at this site is stated to consist of in- stream structures (Table 1), and one greentree impoundment structure (p.50 and Figure 16) Four rock check dams will be installed within dredged and straightened stream channels. Reddicks Creek This site contains an approximately 3900 -foot reach of Reddicks Creek and lies within a watershed of ca. 9.1 square miles. It flows into Hickory Creek 1 1 miles downstream. The channel is classified as a G4 (gravel dominated gully) based on fluvial geomorphic features developed by Rosgen (1996). The stream is classified as WS -fV CA. The proposed restoration plan includes the construction of four greentree impoundments along Reddicks Creek. Geomorphic Changes and Vegetation Restoration All three plans promote sediment retention upstream of the in- stream structures and greentree impoundments. Although this is admirable goal, it is not clear on how sediment deposition may affect the survival of seedlings of woody plants. Considerable changes in floodplam microtopography can be expected after the in- stream structures and impoundments are constructed. Restoration of Forested Plant Communities All three plans provide details on the proposed plant communities Species lists include mast - producing species. Overall species selection is very good. However, the proposed Randleman Reservoir has received extensive review by regulatory agencies regarding projected nutrient concentrations. One proposed plant species (Tag Alder, Alnus serrulata) fixes nitrogen and may serve as a source of nitrogen in adjacent waters. The ability of alder to contribute a significant amount of nitrogen to Alaskan lakes is discussed in Wetzel (198_, Limnology). Monitoring Plan Water table elevations and survival of transplanted woody vegetation will be monitored The water table will be measured in monitoring wells installed in accordance with US Corps of Engineers WRP Technical Note HY- IA -3.1 (Installing Monitoring Wells /Piezometers in Wetlands, August 1993). Automated recording wells are not proposed; thus the frequency of sampling proposed is weekly during spring and early summer and intermittently through the remainder of the growing season. The success criterion is to achieve saturation within one food of the soils surface for 5% of the growing season. The US Army Corps of Engineers uses saturation for 5% of the growing DRAFT season to establish jurisdiction, but generally 12.5% of the growing season is used to establish successful hydrologic restoration Establishment success of transplanted vegetation will be sampled through randomly placed plots (0.11 acre) representing a 4 -5% sample at the study sites. In general, species survival is the primary focus of sampling However, some consideration is provided for the relative density among species. No one species will be allowed to exceed representing 20% of a goal of 320 stems /acre after three years The plans state "additional stems of a particular species above the 20% threshold will be discarded from the statistical analysis " It is not clear on what is meant by this statement In addition it is not clear on whether vegetation sampling will include seedling established naturally. Herbaceous species will be noted, but no quantitative sampling is proposed for this stratum. Species data should include the wetland indicator status (OBL, FACW, etc.) as part of the data sets and summaries. Submittal of Data The plans propose submittal of data in tabular or electronic (Excel) format. I prefer data submittal in an electronic format The use of MS Excel is acceptable, however spreadsheets must be simple as possible and easily converted to text files so that the data can be read by other statistical software packages. References Larsen et al (1980), which is cited on the figures for "Reference: Lake Shoreline" is not listed in the "References" section of the reports.