Lake County Stormwater Management Commission s Study of Post-Development Isolated Waters of Lake County, Illinois

Transcription

1 Lake County Stormwater Management Commission s Study of Post-Development Isolated Waters of Lake County, Illinois Application of the 80% - 150% Wetland Hydrology Design Criteria IAFSM March 2013

2 Background ~21% of County = waters and wetlands (61,500 acres) ~ 11% (7,000 acres) = Isolated Waters of Lake County (IWLC) Most of County has slowly permeable soils (Hydro Group C) Illinois Association for Floodplain and Stormwater Management 2

3 Background Developed by Independent Technical Advisory Committee (TAC) Amended August 2001 to include isolated wetland regulations after Supreme Court SWANCC Ruling Sets minimum county-wide standards for development: Floodplain Floodway Detention Wetlands Buffers Erosion Control Illinois Association for Floodplain and Stormwater Management 3

4 WDO Wetland Hydrology Requirement ( Rule ) The development design shall maintain between 80% and 150% of the existing condition, 2-year, 24- hour storm event runoff volume from the onsite tributary drainage area to the preserved Isolated Waters of Lake County. (WDO Article IV.E.6) Illinois Association for Floodplain and Stormwater Management 4

5 WDO Wetland Hydrology Requirement ( Rule ) The development design shall maintain between 80% and 150% of the existing condition, 2-year, 24- hour storm event runoff volume from the onsite tributary drainage area to the preserved Isolated Waters of Lake County. (WDO Article IV.E.6) Illinois Association for Floodplain and Stormwater Management 5

6 WDO Wetland Hydrology Requirement ( Rule ) The development design shall maintain between 80% and 150% of the existing condition, 2-year, 24- hour storm event runoff volume from the onsite tributary drainage area to the preserved Isolated Waters of Lake County. (WDO Article IV.E.6) Illinois Association for Floodplain and Stormwater Management 6

7 WDO Wetland Hydrology Requirement ( Rule ) The development design shall maintain between 80% and 150% of the existing condition, 2-year, 24- hour storm event runoff volume from the onsite tributary drainage area to the preserved Isolated Waters of Lake County. (WDO Article IV.E.6) Illinois Association for Floodplain and Stormwater Management 7

8 WDO Wetland Hydrology Requirement ( Rule ) The development design shall maintain between 80% and 150% of the existing condition, 2-year, 24- hour storm event runoff volume from the onsite tributary drainage area to the preserved Isolated Waters of Lake County. (WDO Article IV.E.6) Illinois Association for Floodplain and Stormwater Management 8

9 Design Example Development Site (15-ac) Existing On-Site Tributary Area to IWLC (8-ac) Existing CCN: 74 4-ac 1-ac 1-ac Existing Total IWLC Area (5-ac) Existing Total Tributary Drainage Area to IWLC (32-ac) Illinois Association for Floodplain and Stormwater Management 9

10 Design Example Development Site (15-ac) Proposed On-Site Tributary Area to IWLC (4-ac) 1-ac Proposed CCN: Scenario #1-86 Scenario # ac 4-ac Existing Total IWLC Area (5-ac) Existing Total Tributary Drainage Area to IWLC (32-ac) Illinois Association for Floodplain and Stormwater Management 10

11 Design Example Illinois Association for Floodplain and Stormwater Management 11

12 SMC Study Purpose: Determine Effectiveness of WDO Wetland Hydrology Requirement Method: Compared pre-and postdevelopment data from 86 sites Modeled hydrology: Avg , Range: % Study Years: 2003, 2007, 2008, and 2012 Illinois Association for Floodplain and Stormwater Management 12

13 Mean C Mean C Mean C Mean C Results Floristic Quality All Sites (n=86) Mean Range Cohort 1 (80-95%) (n=19) Mean 1.70 Range Baseline Cohort 2 (95-105%) (n=25) Baseline Illinois Association for Floodplain and Stormwater Management Mean Range Baseline Baseline Cohort 3 ( %) (n=42) Mean Range 13

14 Native FQI Native FQI Native FQI Native FQI Results Floristic Diversity All Sites (n=86) Mean Range Cohort 1 (80-95%) (n=19) Mean Range Baseline Cohort 2 (95-105%) (n=25) Mean Range Baseline Cohort 3 ( %) (n=42) Mean Range Baseline Baseline Illinois Association for Floodplain and Stormwater Management 14

15 Mean Wetness Coefficient Mean Wetness Coefficient Mean Wetness Coefficient Mean Wetness Coefficient Results Wetness (Mean W) All Sites Cohort 1 (n=86) y = x (80-95%) (n=19) y = 0.097x Baseline Baseline Cohort 2 (95-105%) (n=25) y = 0.113x Cohort 3 ( %) (n=42) y = 0.027x Baseline Baseline Illinois Association for Floodplain and Stormwater Management 15

16 Average Max June-July Temperatures ºF Average Growing Season (Apr-Aug) Precipitation (in) Average Max Growing Season (April- Aug) Temperatures F Results Climate Precipitation Temperature (avg. max) June July 65 May Temperature (avg. max) Illinois Association for Floodplain and Stormwater Management 16

17 Conclusions Study confirms the efficacy of the WDO wetland hydrology requirement Floristic quality and diversity have improved, indicating the strength of these ecosystems Floristic quality and diversity are stabilizing after 6-10 years adjusting to post-development hydrology IWLC are maintaining wetland hydrology (W= -1.09) The % range appears to be most effective in maintaining overall wetland integrity Wetland wetness appears to be consistent with design hydrology per cohort analysis Illinois Association for Floodplain and Stormwater Management 17

18 Questions? Full Study Report Available: Juli E. Crane, PWS, CWS Principal Wetland Specialist Lake County SMC 500 W. Winchester Rd. Libertyville, IL Illinois Association for Floodplain and Stormwater Management 18