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Floodplain Development Permit Package,July 2019 <br /> 1. BACKGROUND AND INTRODUCTION <br /> Westervelt Ecological Services has proposed a mitigation banking project at the confluence of the Big <br /> Thompson (BTR) and South Platte (SPR) rivers near Evans, Colorado. This mitigation involves stream and <br /> floodplain wetland restoration along a heavily incised reach of the lower BTR. The purpose of this report <br /> is to summarize the proposed restoration activities and to discuss the associated hydrologic and hydraulic <br /> conditions to support the issuance of a floodplain development permit. Permit applications and <br /> authorizations are located in Attachment A. Floodplain maps and a project design sheet are located in <br /> Attachment B. Deeds for the project parcels are located in Attachment C. <br /> The project site is located on a 72-acre parcel in Weld County, CO near the town of Evans. Excavated <br /> material will be spoiled on an adjacent 28-acre parcel.The project reach encompasses the lower 5,800 ft <br /> of the BTR,terminating at its confluence with the SPR, and includes the adjacent floodplain. The project <br /> site is located within a Zone A flood hazard area and can be found on FEMA tile 08123C-1707E. The <br /> existing alignment of the BTR within the project reach is greatly incised and disconnected from the <br /> adjacent floodplain. Historically,the project location was likely an active, dynamic floodplain for both the <br /> Big Thompson and South Platte Rivers which experienced frequent episodic flooding. Due largely to water <br /> development activities on both the BTR and SPR throughout the 1900s and early-2000s, floodplain <br /> connectivity and overland flow has been drastically reduced at the project site. The floodplain areas of <br /> the project site have transitioned to a primarily static upland vegetation community, resulting in an overall <br /> loss of ecological function. This reach of the SPR is discussed as a high priority for restoration in the Middle <br /> South Platte River Restoration Master Plan (MSPRMP) (CDM, 2016). <br /> This mitigation bank is currently undergoing the approval process with the U.S. Army Corps of Engineers, <br /> and is expected to be constructed in late 2019. Additionally, an application for a Nationwide Permit 27 <br /> for Section 404 is also being reviewed by USACE. The City of Evans, CO has expressed that they do not <br /> have additional floodplain permitting requirements, but this package will be submitted to the city <br /> engineer for their information and review. <br /> 2. PROJECT NARRATIVE <br /> 2.1. SITE RESTORATION PLAN <br /> The project includes excavation of floodplain benches along the mainstem of the BTR to increase <br /> floodplain connectivity and reduce overall stage and velocity of high flows. Approximately 4,020 linear <br /> feet of backwater channels will also be excavated along existing relic/abandoned channels,with additional <br /> associated wetland floodplains (Figure 1). <br /> Floodplain Benches <br /> Floodplain benches are created by regrading the existing banks of a degraded or incised channel and <br /> establishing a new bankfull bench which creates a lower floodplain within the river corridor (Figure 2). A <br /> healthy riparian zone that is inundated at frequent intervals promotes regeneration of riparian plant <br /> species and acts as a sediment trap and pollutant filter(CDM, 2016). The MSPRMP makes several refences <br /> to floodplain benches as a preferred restoration technique, and further recommends bench widths of 60 <br /> feet or wider(CDM, 2016). <br /> Headwaters Corporation 1 of 7 <br />