Modeling Cumulative LID Features for Floodplain Impacts in an Urban Watershed in Houston, TX

Transcription

1 Modeling Cumulative LID Features for Floodplain Impacts in an Urban Watershed in Houston, TX Christina Hughes, Phil Bedient May 2,

2 Modeling Cumulative LID Features for Floodplain Impacts in an Urban Watershed in Houston, TX Christina Hughes, Phil Bedient May 2,

3 Research Questions How can LID be best represented in a hydrologic model? To what degree can spatial resolution enhance LID modeling capabilities? Is there an appreciable difference in the hydrologic effect of modeling LID as a weighted change in parameters? What cumulative impact on water quantity can small-scale LID features have when implemented across a large area? What combinations of LID features are most effective? 3

4 Background - LID in Houston The Woodlands LID Design Competition Harris County LID Design Criteria Bagby St. receives national recognition 2013 COH Cottage Grove pilot project Limitations: Convention Quantity vs quality Limited space in urban environment Unique topography and climate 4

5 Bagby Street Bioretention/Rain garden Greenroads Certified Development Project Series of rain gardens treatment train Land Use: Commercial/Residential 5

6 Rain Barrels Capacity: varies ( gal) Less storage than cisterns Easily installed at downspouts Individual initiative No current estimate on use Land Use: Residential 6

7 Distributed Modeling Allows for more spatial variation than lumped Higher resolution at large scale Gridded data availability Rainfall Urban Reservoir Channel Riparian Successfully used to model Houston area watersheds: Doubleday et al Teague et al Duncan et al Fang et al

8 Watershed Modeling Lumped Model vs. Distributed Model Uniform parameters 8

9 LID Modeling in The Woodlands, TX Simulated existing LID features by changing roughness, imperviousness, adding detention Compared to undeveloped and theoretical conventional urbanization Significant impact of watershed-scale LID >100% peak and volume reduction for 100-year storm Riparian corridors Large scale detention Roughness associated with land cover 9 Source: Doubleday et al. 2013

10 White Oak Bayou 111 square mile watershed 151 miles open streams Harris County Urban 90% Developed (2011) 18% High 39% Medium 23% Low 10 Source: HCFCD; NLCD 2011

11 White Oak Bayou HEC RAS 10 year Floodplain 11

12 Vflo Model Elevation LiDAR 2001 Land cover NLCD 2011 Soil NRCS 2003 Land Use - HGAC 2011 Channels & gauges Land Use Land Cover Resolution: 50,000 cells Cell: 2.8 acres (350 ft x 350 ft) Soil DEM 12

13 Vflo Model Channel Overland 13

14 Model Performance and Calibration (1) Hurricane Ike in; 48 hrs (2) July in; 168 hrs (3) September in; 24 hrs (1) (2) Parameters adjusted: overland roughness channel roughness hydraulic conductivity soil depth initial saturation (3) 14

15 Modeling LID Rain gardens residential Increased roughness and infiltration Rain barrels residential Pure abstraction Green roofs public buildings Increased roughness and infiltration Decreased impervious surface Bioswales high-traffic roadways Increased roughness and infiltration Decreased impervious surface Design Storms 10 yr, 2 yr 41% Residential land use 15

16 Methodology - Residential LID Assumptions: 1 home = 2000 sq ft building with 2000 sq ft yard 30.6 homes per 2.8 acre cell 100% of homes have one rain garden 100% of homes use one rain barrel Home No change in imperviousness assume features built in existing pervious area 16 Yard/Road

17 Methodology Representing LID Features Weighted by area Rain garden: 225 sq ft area (5 ft x 45 ft) 3 ft depth, sandy loam n = 0.4 (McCuen 2005) No detention Modeled by increased roughness and Green & Ampt parameters Rain barrel: Rain barrel 350 gallons per home 2000 sq ft of roof area captured Modeled as 1.55 in abstraction for entire cell Roof Rain Garden 17

18 Results 7.6 in/24 hr (10 yr) 4% reduction in peak 2% reduction in volume White Oak Outlet - 10 Year Event Current: cfs Rain Garden: cfs Combined: cfs Discharge (cfs) /1/2007 0:00 6/2/ :00 6/4/2007 0:00 Time Current Conditions Rain Gardens Rain Gardens and Rain Barrels

19 Results 7.6 in/24 hr (10 yr) Upper White Oak - 10 Year Event Current:13700 cfs Rain Garden: cfs Combined: cfs 5% reduction in peak 3% reduction in volume Discharge (cfs) /1/2007 0:00 6/2/2007 0:00 6/3/2007 0:00 6/4/2007 0:00 Time Current Conditions Rain Gardens Rain Gardens and Rain Barrels 19

20 Results 4.4 in/24 hr (2 yr) White Oak Outlet - 2 Year Event 10% reduction in peak 5% reduction in volume Current: cfs Rain Garden: cfs Combined: cfs Discharge (cfs) /29/2014 0:00 4/30/2014 0:00 5/1/2014 0:00 Time 20 Current Conditions Rain Gardens Rain Gardens and Rain Barrels

21 Results 4.4 in/24 hr (2 yr) Upper White Oak - 2 Year Event Current: 6700 cfs Rain Garden: 6400 cfs Combined: 5900 cfs 12% reduction in peak 8% reduction in volume 5000 Discharge (cfs) /29/2014 0:00 4/29/ :00 4/30/2014 0:00 4/30/ :00 5/1/2014 0:00 Time Current conditions Rain Gardens Rain Gardens and Rain Barrels 21

22 Observation Largest reduction upstream Higher density of LID features White Oak Bayou Total: 41% Residential Upper White Oak Bayou: 49% Residential Highest LID density = Little White Oak Bayou (59% res.) Lowest LID density = Cole Creek (14% residential) Cole Creek Little White Oak Bayou 22

23 Cole Creek - 2 Year Event Little White Oak - 2 Year Event Current: 1400 cfs Rain Garden: 1400 cfs Combined: 1390 cfs Current:4400 cfs Rain Garden: 4300 cfs Combined: 3800 cfs Discharge (cfs) Discharge (cfs) /29/2014 0:00 4/30/2014 0:00 5/1/2014 0:00 Time 0 4/29/2014 0:00 4/30/2014 0:00 5/1/2014 0:00 Time Current Conditions Rain Gardens Rain Gardens and Rain Barrels 1% reduction in peak 2% reduction in volume 14% reduction in peak 9% reduction in volume 23

24 Conclusions Fully distributed model useful for representation of individual LID features across large area Higher density LID leads to a greater cumulative effect on downstream flows Up to 9% volume reduction and 14% peak reduction for 2 yr storm using only rain gardens and rain barrels LID feature size and scale has appreciable effect on stormwater flows Large-scale features (Woodlands) more effective for large storms 100 year storm wash out small-scale LID features Conservative estimation of an optimistic scenario Future work/ in progress: Combination of LID features and comparison of impact (green roofs, bioswales) Impervious to pervious area routing and interception via higher resolution nested subareas Floodplain impact analysis 350 ft 350 ft 24