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HomeMy WebLinkAboutMisc - ALAN PLUMMER A550C:IATES, INC. ErmaorvnENTAL ENGWENS.DESIGNERS SOENTtSTS I I ON-SITE MITIGATION PLAN I I I THE GATEWAY City of College Station, Brazos County, Texas USACE Project No.: 200000749 I I April 15, 2002 Alan Plummer Associates, Inc. Mitigation Plan for"The Gateway" City of College Station,Brazos County,Texas Background "The Gateway" is a retail development located on an approximately 38-acre tract of land within the City of College Station, Brazos County, Texas. The proposed development plan for the site, as shown on Figure 1 included in Appendix A, consists of multiple retail structures and their accoutrements fronting University Drive along the perimeter of the property as well as a proposed Home Depot and additional retail structures and their associated accoutrements located within the interior of the property. Access to the proposed development will be from University Drive and from the State Highway 6 Access Road. A preliminary jurisdictional determination of waters of the United States (U.S.) was conducted by Alan Plummer Associates, Inc. and documented in a report dated February 11, 2002. This report is being submitted to the U.S. Army Corps of Engineers (USACE) along with the project information, including this mitigation plan. Based on the preliminary jurisdictional determination, the jurisdictional waters of the U.S. and adjacent wetlands include two wetland areas, an on-channel pond, a small drainage channel, and an unnamed tributary to Burton Creek. The referenced determination report details information regarding the site hydrology, vegetation, and soil characteristics. The existing storm drainage from off-site and immediately north of University Drive that currently flows through culverts to a linear drainage channel located on the southeastern portion of the project site will be routed through extension of the culverts around the east side of the proposed Home Depot pad site to the north slope of the proposed fill area. Discharge from the culverts will flow through concrete dissipaters constructed within the headwall at the discharge point and flow overland northward toward an existing wetland swale that appears to have been constructed northward across the adjacent property. A portion of this flow may find its way to the small drainage channel that converges with the unnamed tributary to Burton Creek. • Mitigation Plan The Gateway I An approximately 2.4 acre detention/retention basin is being proposed to retain the stormwater runoff and resulting high velocity flows from the introduction of large impervious surfaces associated with the buildings and accompanying parking facilities. Construction of a controlled outfall structure to discharge into the existing unnamed tributary to Burton Creek will be done in conjunction with the detention/retention basin. A vegetated swale to divert runoff from the adjacent property to the west and developments to the southwest will also be constructed around the western and northern perimeter berm of the detention/retention basin to direct flows into the existing on- channel pond located in the northeastern portion of the subject property. The detention/retention basin, controlled outfall structure, and runoff diversion swale, as shown on Figure 2 in Appendix A, will all be located along the northernmost portion of the subject property just west of the unnamed tributary to Burton Creek. Typical cross- sections of the detention/retention basin and controlled outfall structure are included as Figures 3 and 4 in Appendix A, respectively. Avoidance and Minimization After jurisdictional areas on the project site were identified, impacts resulting from the proposed developmental layout, including building footprints, parking facilities, access roadways, and storm runoff control facilities were considered. The goal of the developer was to arrange the proposed facility layout to avoid impacts to jurisdictional areas where possible and then to minimize unavoidable impacts to jurisdictional areas. The location and shape of the building footprints, parking facilities, and access roads were oriented throughout the tract in order to avoid impacts to jurisdictional waters of the U.S. In order to make the project financially feasible for development and to minimize impacts off-site from the increased storm runoff volume and velocities resulting from the introduction of substantial impervious surface area, some impacts to jurisdictional waters and adjacent wetlands were unavoidable. Measures taken to minimize these impacts to the extent possible included modifying the designs of the extended culverts carrying off- Alan Plummer Associates,Inc. Page 2 Mitigation Plan The Gateway site storm runoff and the detention/retention basin for on-site storm runoff collection located along the northern portion of the subject property to avoid the unnamed tributary to Burton Creek and the small drainage channel, and to maintain a majority of the emergent wetland at the headwaters of the unnamed tributary to Burton Creek within the floor of the detention/retention basin. The detention/retention basin will be created by constructing a perimeter berm around the depressional wetland area; therefore, existing grades will be maintained within the basin. Therefore, impacts to the wetland area will be limited to a small portion of the emergent wetland on the east side at the location of the perimeter berm and controlled outflow structure. The proposed impacts will be limited to approximately 0.07 acre of the emergent wetland from excavation and the subsequent grading and fill activities for construction of the berm and outflow structure. Restoration of positive flow through the unnamed tributary to Burton Creek will also be needed. This will entail the removal of the earthen plug located just east of the property line within the channel of the unnamed tributary to Burton Creek so that flows from the detention/retention basin will be carried downstream to Burton Creek. Flows should be restored to an on-channel pond located in the northeastern corner of the subject property then continue through the tributary channel northward to Burton Creek. On-Site Mitigation Plan To provide mitigation for unavoidable impacts as a result of the proposed development, a plan, as shown on Figure 5 in Appendix B, has been developed to create approximately 1.97 acres of emergent marsh in addition to the preserved wetland pool within the proposed detention/retention basin, provide approximately 0.10 acre of enhanced riparian habitat between the detention/retention basin's eastern berm and the unnamed tributary to Burton Creek, and create approximately 1,086.5 L.F. of a vegetated swale to divert flows from the adjacent property to the west and developments to the southwest around the detention/retention basin's western and northern perimeter berm and into the existing on- channel pond. Also, approximately 0.68 acres of berm area will be planted with native grasses, legumes, and other herbaceous species. Alan Plummer Associates,Inc. Page 3 Mitigation Plan The Gateway The topsoil from the impacted portion of the existing emergent wetland and areas adjacent to the emergent wetland will be stockpiled prior to fill activities for the development of the proposed facilities on the site. This topsoil will be used for the final grading of the bottom and lower slopes inside the basin. The existing seed bank within the stockpiled topsoil should quickly revegetate the basin with the existing hydrophytic vegetative species on-site. Outside of the detention/retention basin's eastern berm and along the unnamed tributary to Burton Creek, native trees, shrubs, and herbaceous vegetation (grasses, legumes, and wildflowers) will be planted to both stabilize the area and to enhance the riparian corridor along the tributary. The proposed plantings include 10 canopy trees, 30 shrubs, and a herbaceous seed mixture for enhancement of approximately 0.10 acre of riparian area along the unnamed tributary. An herbaceous seed mixture is also proposed for the stabilization of the disturbed soil areas of the perimeter berm and diversion swale for a total of approximately 0.75 acre. Typically (based on availability), planted canopy trees are 5-gallon container grown size and shrubs are 1 to 3-gallon container grown size. Seeding rates for the suggested herbaceous vegetation are based on recommended amounts for each of the specified species. The species proposed for planting are listed in Table B-1, included in Appendix B. Also included in Table B-1 are the suggested seeding rates for the herbaceous vegetation. It is anticipated that survival rate of the planted tree and shrub species after two years of growth will be greater than 80 percent. If the planted trees and shrubs exhibit less than 80 percent survival, the permittee will replant until at least an 80 percent survival rate is achieved for a two-year period. Appropriate erosion and siltation controls will be used and maintained in effective operating condition during construction, and all exposed soils and other fills will be permanently stabilized at the earliest possible date. Appropriated erosion control best management practices will be employed during construction activities along the upland portions of the project site to capture sediment in storm runoff from exposed soil areas and to minimize impacts to any receiving stream. A silt fence will be installed to filter Alan Plummer Associates,Inc. Page 4 Mitigation Plan The Gateway overland flow from areas disturbed during construction activities to prevent adverse impacts to receiving waters. Monitoring Plan Monitoring of the mitigation plantings will be conducted by Alan Plummer Associates, Inc. or another qualified biologist, and photographs and data documenting the survival of the plantings will be submitted to the USACE on an annual basis, due October 1 each year, beginning October 1, 2002. If the planted trees and shrubs exhibit less than 80 percent survival two years after planting, the permittee will replant until at least an 80 percent survival rate is achieved. Supplemental planting of herbaceous vegetation will be conducted as necessary to achieve a minimum 80 percent vegetative cover after two years of growth. The monitoring program and submittal of annual reports will continue until the USACE has been satisfied that the specified survival has been achieved. Deed Restriction The permittee shall dedicate in perpetuity by deed restriction the mitigation areas, as described above. This dedicated area shall not be disturbed, except by those activities that would not adversely affect the intended purpose, extent, condition, and function of the area. The permittee shall survey the mitigation area, develop an appropriate deed restriction for the surveyed area, submit the draft deed restriction to the USACE for review and approval, and record the USACE-approved deed restriction with the Brazos County Clerk. The permittee shall provide a copy of the recorded deed restriction to the USACE within six months after issuance of the Section 404 permit. This restriction shall not be removed from the deed or modified without the written approval of the USACE and conveyance of any interest in the property shall be subject to the deed restriction. Alan Plummer Associates, Inc. Page 5 I I _ TABLE 1 MITIGATION PLANTING IMitigation Planting for Enhanced Riparian Area ICanopy Trees Common Name Scientific Name Number Bur Oak Quercus macrocarpa 3 I Shumard Oak Water Oak Quercus shumardii 3 Quercus nigra 2 Pecan Carya illinoensis 2 ITotal Hard-Mast-Producing 10 Small Trees and Shrubs ` Common Name Scientific Name Number r Smooth Leaf Elbowbush Forestiera pubescens var. glabrifolia 5 American Beautyberry Callicarpa americana 4 I Coralberry Eastern Redbud Symphoricarpos orbiculatus 5 Cercis canadensis 4 Rusty Blackhaw Viburnum rufidulum 4 Southern Wax-myrtle Myrica cerifera 4 I Rough-leaf Dogwood Total Cornus drummondii 4 30 IHerbaceous Seed Mixture(for both enhanced riparian area and perimeter berm) Seeding Rate I Common Name Native Grass Mixture Scientific Name Pounds/Acre 8 lb/acre Big Bluestem Andropogon gerardii I Eastern Gamagrass Green Sprangletop Tripsacum dactyloides Leptochloa dubia Switchgrass Panicum virgatum Blue Grama Bouteloua gracilis I Sideoats Grama Bouteloua curtipendula Buffalograss Buchloe dactyloides Sand Lovegrass Eragrostis trichodes I Little Bluestem Schizachyrium scoparium Indiangrass Sorghastrum nutans Seeding Rate Common Name Scientific Name Pounds/Acre Herbaceous Seed Mixture Winter Vetch Vicia dasycarpa 20-30 lb/acre White Clover Trifolium repens 3-4 lb/acre Slender Lespedeza Lespedeza virginica 25-35 lb/acre Illinois Bundleflower Desmanthus illinoensis 15 lb/acre Showy Partiridgepea Chamaecrista fasciculate 15-20 lb/acre Bluebonnet Lupinus texensis 20 lb/acre • Huisache Daisy Amblyolepis setigera 8 lb/acre Prarie Verbena Verbena bipinnatifida 6 lb/acre Lazy Daisy Aphanostephus spp. 1 lb/acre Indian Blanket Gaillardia pulchella 10 lb/acre Greenthread Thelesperma filifolium 6 lb/acre I Mitigation Planting for Diversion Swale Area Native Grasses and Herbaceous Seed Mixture Seeding Rate Common Name Scientific Name Pounds/Acre Native Grass Mixture 30 lb/acre Big Bluestem Andropogon gerardii Eastern Gamagrass Tripsacum dactyloides Green Sprangletop Leptochloa dubia Prairie Wildrye Elymus canadensis Switchgrass Panicum virgatum Herbaceous Seed Mixture 20 lb/acre Clasping Coneflower Rudbeckia amplexicaulis Cutleaf Daisy Engelmannia pinnatifida Scarlet Sage Salvia coccinea Plains Coreopsis Coreopsis tinctoria Illinois Bundleflower Desmanthus illinoensis Black-eyed Susan Rudbeckia hirta Pink Evening Primrose Oneothera speciosa Maximilian Sunflower Helianthus maximiliani Obedient Plant Physostegia intermedia Pitcher Sage Salvia•azurea