APPENDIX F SEDIMENT DEPOSITION AND HABITAT CONVERSION ANALYSIS

Transcription

1 APPENDIX F SEDIMENT DEPOSITION AND HABITAT CONVERSION ANALYSIS San Dieguito Lagoon W-19 Restoration Project Draft EIR March 2017 Appendices

2 Introduction: The following is an analysis to provide the volume of sediment expected to be deposited in the W-19 created wetlands and the likely loss/conversion of wetlands area following major flood events. The analysis is based on the predicted depth and distribution of sediment deposition from the Army Corps of Engineer s AdH computer model for the proposed project and Alternatives. The conversion analysis assumes that adaptive management would remove sediment in the interior wetland areas for the proposed project and Alternative C, but no sediment removal in the interior wetlands of Alternative B and riverside wetlands for Alternative C. In areas where sediment is not removed the analysis assumes that over time the habitat will convert to the habitat type supported by the revised inundation frequency, e.g., if the inundation frequency of a mudflat area cover with sediment is reduced to 20%-40%, the mudflat is assumed to covert to low marsh. Table 1 shows the assumed habitat breaks and inundation frequencies assuming no sea level rise. TABLE 1 Target Habitat Elevation Range (ft, NGVD29) Inundation Frequency Transition Zone > % High Marsh +3.0 to %-4% Mid Marsh +1.9 to %-20% Low Marsh +1.4 to %-40% Intertidal Mudflat -0.3 to %-100% Subtidal -2.0 to % Background: The existing conditions over the reach of the river where the proposed wetlands are to be built has already reached a point in the evolution of the geomorphology of the system that the offchannel flood plain was filled to an elevation controlled by the flood levels themselves. Creating wetlands requires lowering the flood plain elevation, which puts the system back into a morphological state when the river is seeking the most hydraulically efficient path-way to the ocean. Modeling shows that without barriers, the most hydraulically efficient pathway for large flood flows becomes splitting off flow through the created wetlands. Alternative B represents the condition of wetlands without barriers to extreme flood levels. The SCE W4/W16 is an example of created wetlands with barriers to protect the wetlands against flooding and sedimentation. The modeling showed that under existing conditions as well as with the W-19 project or any of the alternatives, there was no sediment deposition in W4/16 interior

3 wetlands. Modeling showed that Alternative B created a new pathway for splitting river flows in the created wetlands with 25-year or larger storm events resulting in large volumes of sediment in the interior wetland areas. Of the three alternatives studied, Alternative A, the proposed project provides the wetlands with the most protection from flood flows and accompanying sediment. Alternative C is similar to Alternative A, with the addition of a riverside salt marsh. The riverside salt marsh has no barriers to flood flows and therefore is subject to sediment deposition. Findings: Sediment Deposition Table 2 quantifies the volume of sediment deposition predicted by the model for the proposed project and each alternative by storm by wetland area. TABLE 2. SEDIMENT DEPOSITION VOLUME (CY) BY ALTERNATIVE BY STORM FREQUENCY Alternative/Wetland Area Proposed Project (Alternative A) Alternative B Alternative C 25-yr 50-yr 100-yr 25-yr 50-yr 100-yr 25-yr 50-yr 100-yr Inlets 7,000 24,000 19,000 3,000 20,000 27,000 7,000 23,000 12,000 Interior Salt Marsh 0 5,000 30, , , ,000 Interior Brackish Marsh 0 8,000 28,000 16,000 44,000 62, ,000 Riverside Marsh (Alt C) NA NA NA NA NA NA 53, , ,000 Total Sediment Volume (cy) 7,000 37,000 77,000 19, , ,000 60, , ,000

4 As shown in Table 2, the proposed project/alternative A is expected to have the least amount of sediment deposition consistent with the highest level of wetlands protection. Alternative C has the highest volume of total deposition, almost all of which is in the unprotected riverside marsh. The interior salt marsh and brackish marsh for Alternative C are protected from the river resulting in similar sediment deposition as the proposed project/alternative A and greatly reduced flooding and sediment deposition as compared to Alternative B. Alternative B s interior salt marsh and interior brackish marsh are unprotected and are predicted to receive approximately three to six times the volume of sediment as the proposed project. Habitat Conversion In comparing alternatives a question arose as to the impacts of the large sediment deposits. Sediment removal is planned for the inlets for the project and both Alternatives. The interior area of the salt marsh and brackish marsh for the proposed project and Alternative C will receive adaptive management to insure habitat areas are restored as required. Without maintenance removing the sediment and assuming no sea level rise, habitat areas with greater than 3 to 6 inches of sediment will likely die. In addition, sediment deposition reduces the frequency of tidal inundation for the area covered with sediment and can reduce or totally block tidal flows to interior wetlands. Tidal areas where sediment reduces the frequency of tidal inundation are expected to convert over time to the habitat supported by the reduced frequency of inundation. Table 3 below shows how the unmaintained wetlands habitat for Alternatives B and C will convert assuming only inlet maintenance for Alternative B and no maintenance for the riverside wetlands in Alternative C. Table 3 assumes that salt marsh areas left isolated by deposition blocking tidal flows will convert to high marsh.

5 TABLE 3. HABITAT CONVERSION HABITAT WETLAND AREA SOUTH OF INITIAL AREA (AC) WETLAND AREA NORTH OF & BRACKISH MARSH (Alt B) HABITAT AREA CONVERSION TABLE FOR ALTERNATIVES B AND C AFTER 100-YR STORM EVENT AREA (AC) DIFFERENCE 1) AREA (AC) DIFFERENCE 1) AREA (AC) DIFFERENCE 1) AREA (AC) DIFFERENCE 1) ALTERNATIVE B OPEN WATER (-2.0-(-0.3)) MUD FLAT ((-0.3)-1.4) LOW SALT MARSH ( ) MID SALT MARSH ( ) HIGH SALT MARSH ( ) SUBTOTAL WETLAND SUBTOTAL BRACKISH MARSH ( ) ALTERNATIVE C & BRACKISH AFTER 50-YR STORM EVENT & BRACKISH OPEN WATER (-2.0-(-0.3)) MUD FLAT ((-0.3)-1.4) LOW SALT MARSH ( ) MID SALT MARSH ( ) HIGH SALT MARSH ( ) SUBTOTAL WETLAND HABITAT WETLAND AREA SOUTH OF INITIAL AREA (AC) WETLAND AREA NORTH OF & BRACKISH MARSH (Alt B) AFTER 25-YR STORM EVENT AREA (AC) DIFFERENCE 1) AREA (AC) DIFFERENCE 1) AREA (AC) DIFFERENCE 1) AREA (AC) DIFFERENCE 1) ALTERNATIVE B OPEN WATER (-2.0-(-0.3)) MUD FLAT ((-0.3)-1.4) LOW SALT MARSH ( ) MID SALT MARSH ( ) HIGH SALT MARSH ( ) SUBTOTAL WETLAND SUBTOTAL BRACKISH MARSH ( ) ALTERNATIVE C OPEN WATER (-2.0-(-0.3)) MUD FLAT ((-0.3)-1.4) LOW SALT MARSH ( ) MID SALT MARSH ( ) HIGH SALT MARSH ( ) SUBTOTAL WETLAND NOTE: Difference 1) = Habitat wetland area (ac) after storm event - Initial habitat area (ac) before storm event Habitat type defined by inundation frequency. Where wetland area below elevation 4.5 is isolated from tidal inundation by sediment deposition after storm event, assumed wetland area converted to high marsh & BRACKISH AFTER 10-YR STORM EVENT & BRACKISH

6 Habitat Conversion Summary: Assuming the interior wetlands of Alternative B and the riverside wetlands of Alternative C are not maintained and allowed to evolve and change based on river sediment deposition, the following summarizes the expected wetland conversion shown in Table 3 by storm event: Alternative B conversion compared to initial habitat o 25-yr Event Area of mudflat habitat triples, going from 5+ Ac to 17+ Ac. 30% increase (3 Ac) in Mid Marsh Reduction in all other salt marsh habitats Overall reduction of salt marsh by 2+ Ac A 5 Ac or 30% reduction in brackish marsh area o 50-yr Event Area of mudflat habitat increases 250% to 14+ Ac. 20% increase (2 Ac) in Mid Marsh Reduction in all other salt marsh habitats Overall reduction of 10% (6 Ac) in salt marsh A 12+ Ac or 70% reduction in brackish marsh o 100-yr Event Reduction in all salt marsh habitats Loss of 2/3 (39+ Ac) of salt marsh habitat A 12+ Ac or 70% reduction in brackish marsh Alternative C conversation of riverside wetlands o 25-year Event Overall loss of 20% (4 Ac) of initial 22 Ac of salt marsh Increase in Mudflat (1 Ac) and Mid Marsh (1 Ac) o 50-year Event Overall loss of 40% (9+ Ac) of salt marsh Less than ¼ Ac of open water remains, less than 1 Ac of low marsh remains Increase in High Marsh (2+ Ac) o 100-year Event Overall loss of 60% (12+ Ac) of salt marsh No open water remains, less than 1 Ac of low marsh remains Increase in high marsh by 40% (1+ Ac)