20241204-002: Dredge of Channel Rd. Lagoon Presentation — original pdf

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Miranda Reinhard Environmental Scientist Senior, Watershed Protection Department DREDGE OF CHANNEL RD. LAGOON 4824 LAGUNA LANE, 78746 SP-2023-0376D 1 PROJECT LOCATION [2 PROJECT LOCATION Project Location Lake Austin Capitol 3 [ PROPERTY DATA • Lake Austin Watershed • Water Supply Rural Classification • Desired Development Zone • Full Purpose Jurisdiction • Council District 8 [4 PROPERTY DATA • Environmental features: • Critical Water Quality Zone • Wetland CEF • 15 Wetland Critical Environmental Features (CEFs) [5 VARIANCE REQUEST LDC 25-8-261(C)(9)(a) – CRITICAL WATER QUALITY ZONE DEVELOPMENT 1. The variance request is to allow more than 25 cubic yards of dredging in Lake Austin. 6 VARIANCE REQUEST • Restore channel navigability • Minimum amount of dredge volume to EXISTING restore navigable depth PROPOSED 7 VARIANCE REQUEST Dredge Requirements • LDC 25-8- 261(C)(9)(a) allows 25 CY dredge per site plan • Proposing dredge 4 ft in depth (down to 488.8’ depth) • Area of dredge: 174,000 sq ft • Volume of dredge: 11,237 CY of dredge PROPOSED DREDGE 8 VARIANCE REQUEST 4301 & 4307 Michael’s Cove Similar Property Examples: • 4301 & 4307 Michael’s Cove (SP-2010-0005D) • Project approved in 2010 • Dredge down to 489’ • Removed ~5,000 CY • Taylor Slough Silt Removal Project (SP-93-0380D) • Project approved in 1993 • Dredge down to 488.3’ • Removed ~2,500 CY Taylor Slough Silt Removal Project 9 VARIANCE REQUEST Dredge Requirements • Project proposes similar dredge depth and larger volume of dredge due to larger lagoon area • Proposing dredge 4 ft in depth (down to 488.8’ depth) • Area of dredge: 174,000 sq ft • Volume of dredge: 11,237 CY of dredge PROPOSED DREDGE 10 VARIANCE REQUEST Enhancements • 3:1 H:V slope at shoreline • Limit Dredging within CRZ of trees outside shoreline 11 VARIANCE REQUEST Additional Wetland Mitigation Plantings on land CWQZ Restored with 609S Lake Austin Additional Wetland Mitigation Plantings on shoreline Enhancements: • Additional Wetland Mitigation Plantings on shoreline • Additional Wetland Mitigation Plantings on land • Restore areas disturbed in CWQZ with 609S Wetland CEFs Required Wetland Mitigation 12 VARIANCE REQUEST Enhancements: • Measures to minimize/avoid sediment discharge • Stabilized construction entrance & access road • Silt fencing & mulch logs • Stabilized dewatering area for dredge bags Floating turbidity curtain • • Hydraulic dredging methods Access Road Stabilized construction Entrance Silt fencing & mulch logs Floating turbidity curtain 13 VARIANCE RECOMMENDATION • • Staff determines that the findings of fact have been met: Similar variances have been granted for projects with similar code requirements. The variance • • • Is necessitated by topographic features, not design choice; Is the minimum deviation from the code; and Is unlikely to result in harmful environmental consequences. • Water quality will be equal to or better than water quality without the variance. 14 VARIANCE CONDITIONS Staff recommends this variance, with the following conditions: 1. Protect the natural shoreline areas and larger wetland Critical Environmental Feature (CEF) W-13 by providing a 3:1 H:V slope from the lake bottom elevation at the shoreline to the final dredge depth. 2. Provide additional wetland mitigation plantings (2,125 plantings comprised of an equal number of 6 different species of FACW and OBL species) within 5 feet of the shoreline to a maximum depth of 1 foot. 3. Provide additional wetland mitigation plantings (1,321 plantings comprised of an equal number of 6 different species of FAC and FACW species) on the other 14 existing wetland CEFs on land. 15 VARIANCE CONDITIONS Staff recommends this variance, with the following conditions: 4. Restore all areas disturbed in the Critical Water Quality Zone (CWQZ) per Standard Specification 609S.) Limit dredging within the 25% and 50% Critical Root Zones (CRZ) of trees along the shoreline to outside of the existing concrete retaining walls. 5. 6. Provide measures to minimize/avoid sediment discharge including: 1) stabilized construction entrance and access road from the Channel Rd entrance to the barge access location and, 2) silt fencing and mulch logs downslope of staging, storage and dewatering areas, and 3) stabilized dewatering area for dredge bags, and 4) floating turbidity curtain downstream of the dewatering operation, and 5) floating turbidity curtain surrounding active dredge area. 7. Conduct dredging via hydraulic methods using floating excavator units and bagging of material. 16 THANK YOU Questions? 1717