20250205-004: Taylor Slough Dredge Maintenance Presentation — original pdf

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Miranda Reinhard Environmental Scientist Senior, Watershed Protection Department TAYLOR SLOUGH DREDGE MAINTENANCE 3704 ½ MEADOWBANK DR, 78703 SP-2024-0019D 1 PROJECT LOCATION • 3704 ½ Meadowbank Dr, 78703 2 [ PROPERTY DATA • Lake Austin Watershed • Water Supply Suburban Classification • Drinking Water Protection Zone • Full Purpose Jurisdiction • Council District 10 3 [ PROJECT DATA The Contemporary Austin – Laguna Gloria Mayfield Park Project Location Lake Austin The Contemporary Austin – Laguna Gloria Mayfield Park Lake Austin 4 [ PROPERTY DATA • Environmental features: • Critical Water Quality Zone • Wetland CEF • 4 Wetland Critical Environmental Features (CEFs) • 2 Rimrock CEFs • 3 Spring 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 EXISTING • Restore channel navigability • Minimum amount of dredge volume to restore navigable depth PROPOSED 7 VARIANCE REQUEST • Sediment Accumulation 8 VARIANCE REQUEST Dredge Requirements • LDC 25-8- 261(C)(9)(a) allows 25 CY dredge per site plan • Proposing dredge ~3 ft in depth (down to 489.2’ depth) • Volume of dredge: 1,771.3 CY of dredge PROPOSED DREDGE 9 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 10 VARIANCE REQUEST Dredge Requirements • Project proposes similar dredge depth and smaller volume of dredge due to smaller footprint • 32 years ago: Dredge down to 488.3’ • Removed ~2,500 CY • Proposing dredge ~3 ft in depth (down to 489.2’ depth) • Volume of dredge: 1,771.3 CY of dredge Taylor Slough Silt Removal Project 1993 PROPOSED DREDGE Taylor Slough Dredge Maintenance 2025 11 VARIANCE REQUEST Required Wetland Mitigation • Wetland CEF Disturbance ~506 SF = 64 plants • Wetland CEF Setback Disturbance ~9,139 SF = 1,172 plants • Total Wetland Mitigation: =1,236 plants 12 Staging Area Bulkhead Modification Area VARIANCE REQUEST Required Bulkhead Mitigation • Bulkhead Modification • Downstream Proposed Staging Area • ~43 LF • 2 plants/3 LF= 29 plants • Total Bulkhead Mitigation: =29 plants 13 VARIANCE REQUEST Required Wetland Mitigation & Bulkhead Mitigation Plantings CWQZ Restored with 609S Wetland CEFs • • • Conditions: Required Wetland Mitigation Plantings • • 1,236 9 species of FAC, FACW, & OBL) Required Bulkhead Mitigation Plantings 29 3 species of FACW & OBL) • • Restore areas disturbed in CWQZ with 609S 14 VARIANCE REQUEST Conditions • Limit Dredging to outside of the 50% CRZ of trees along shoreline and install tree fencing 15 VARIANCE REQUEST Conditions: • Measures to minimize/avoid sediment discharge • Stabilized construction entrance • Hydraulic dredging methods • Stabilized dewatering area for dredge bags • Mulch sock • Rock berms • Floating turbidity curtain Floating turbidity curtain Rock berms Mulch sock Stabilized construction Entrance 16 VARIANCE REQUEST Conditions: • Measures to minimize/avoid sediment discharge • Stabilized dewatering area for dredge bags: • Stage 1: Dumpsters • Stage 2: Filter bags Floating turbidity curtain Rock berms Mulch sock Stabilized construction Entrance 17 VARIANCE REQUEST Conditions: • Measures to minimize/avoid sediment discharge • Stabilized construction entrance • Hydraulic dredging methods • Stabilized dewatering area for dredge bags • Mulch sock • Rock berms • Floating turbidity curtain Floating turbidity curtain Rock berms Mulch sock Stabilized construction Entrance 18 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. 19 VARIANCE CONDITIONS Staff recommends this variance, with the following conditions: 1. Provide required wetland mitigation plantings (1,236 plantings comprised of 9 different species of FAC, FACW, and OBL species) along the shoreline. 2. Provide bulkhead mitigation plantings (29 plantings comprised of 3 different species of FACW and OBL species) along the shoreline. 3. Restore all areas disturbed in the Critical Water Quality Zone (CWQZ) per Standard Specification 609S. 20 VARIANCE CONDITIONS Staff recommends this variance, with the following conditions: 4. Limit dredging to outside of the 50% Critical Root Zones (CRZ) of trees along the shoreline and install tree fencing. 5. Provide measures to minimize/avoid sediment discharge including: 1) stabilized construction entrance and access road from the Meadowbank Drive entrance to the barge access location and, 2) mulch sock around the entire limits of construction (LOC) of the staging, storage and dewatering areas, and 3) two rock berms downstream of the staging, storage, and dewatering areas, and 4) stabilized dewatering area for dredge bags, and 5) floating turbidity curtain downstream of the dewatering operation, and 6) floating turbidity curtain surrounding active dredge area. 6. Conduct dredging via hydraulic methods including bagging of material. 21 THANK YOU Questions? 2222