Krista Reininga, PE Hydromodification and What it Means for the Design of Stormwater Facilities

Transcription

1 Krista Reininga, PE Hydromodification and What it Means for the Design of Stormwater Facilities

2 Agenda 1. Evolution of Water Quality Facilities 2. Regulatory Response/MS4 Permit Requirements 3. Change in Sizing Methods 4. Simplified Facility Sizing Tools 5. Results and Impacts 6. Questions

3 1. Evolution of Water Quality Facilities

4 1. Evolution of Water Quality Facilities Flood/flow control Prior to 1990 detention used with a focus on flood control Limit post-development peak flows to pre-development peak flows for specified design storms

5 1. Evolution of Water Quality Facilities Shift of Focus to Water Quality Beginning late 1980s early '90s, due to Clean Water Act requirements, new emphasis on: detention, wetlands, grass swales, and structural filters (compost filters) for pollution reduction. Effectiveness monitoring in mid-'90s showed some of the best results in reducing pollutant concentrations from filter type facilities especially vegetated filters.

6 1. Evolution of Water Quality Facilities Emphasis on LID Practices and Facilities Mid to late 1990s through present Smaller facilities distributed upstream (source control) Reduced impervious areas Smaller facilities distributed upstream (source control): rain gardens, planters, green roofs, pervious pavement etc. Benefits Eliminate temperature, aesthetic and mosquito issues with detention LID gets at the source Less and less space available for large regional detention facilities LID preserves natural landscape features Better mimics pre-development hydrology Effective for stormwater quality treatment Reduced pollutant loads

7 1. Evolution of Water Quality Facilities What is Next? Address Erosive Flows/Hydromodification Downcutting/erosion is occurring or anticipated in many of our stream channels due to increased runoff volumes and rates. Increased duration of small storms is having a major impact.

8 1. Evolution of Water Quality Facilities The importance of small storms Hollis, 1975

9 Flow (cfs) 1. Evolution of Water Quality Facilities Example hydrograph from large infrequent event Feb. 96 Storm Simulated Hawkins View Subbasin Time (hours)

10 Flow (cfs) 1. Evolution of Water Quality Facilities Example hydrograph from a small frequent event Hawkins View Sub-basin Tim e (hours)

11 1. Evolution of Water Quality Facilities Early hydromodification studies: (Dunne and Leopold) Geomorphically significant flows range from a lower threshold of flow where bed material begins to move to an upper limit where flood flows are no longer contained in the channel. The frequency and duration of geomorphically significant flows are the primary factors that control channel stability or instability. Frequent flow events move the most sediment over time and maintain the channel dimensions.

12 1. Evolution of Water Quality Facilities What are the geomorphically significant flows? Studies conducted to develop thresholds for discharges when bedload transport begins ODOT (42% of the Q2) Santa Clara Valley (10% of the Q2) USDA Study (ranged from less than Q1 to the Q500) Clark County Mill Creek Basin 50% of the Q2 (four sites ranging from 19% of the Q2 to the Q7) OTAK Western Washington (8% of the Q2) Upper discharge endpoint was also evaluated Channel overtopping event or 10-year event

13 1. Evolution of Water Quality Facilities Channel Forming Flow Thresholds- ESA (Andy Collison) information from recent work done in San Diego County Flow range Bed load (kg) % of total bedload for event <0.1Q Q Q2 2, Q2 - Q >Q

14 2. Regulatory Response/MS4 Permit Requirements

15 2. Regulatory Response/MS4 Permit Requirements Stormwater Regulations come from the Clean Water Act Federal Government (Clean Water Act) State Government (DEQ MS4 Phase 1 NPDES Permit) Local Jurisdictions (MS4 Phase 1 Permit Implementation) Hydromodification assessment Stormwater standards for new and re-development

16 2. Regulatory Response/MS4 NPDES Permit Requirements Hydromodification Assessment Permit Language

17 2. Regulatory Response/MS4 NPDES Permit Requirements Development Standards Previous Permit Language Planning procedures including a comprehensive master plan to develop, implement and enforce controls to reduce the discharge of pollutants from municipal separate storm sewers that receive discharges from areas of new development and significant redevelopment. Such a plan must address controls to reduce pollutants in discharges from municipal separate storm sewers after construction is completed.

18 2. Regulatory Response/MS4 NPDES Permit Requirements Development Standards Current Permit Language target predevelopment site hydrologic functions optimize on-site retention reduce runoff volume, duration, and rates of discharge evaluate, prioritize and include implementation of Low-Impact Development (LID), and treat 80 percent of the annual average runoff volume

19 3. Change in Sizing Methods

20 3. Change in Sizing Methods Key to addressing changes (i.e., hydromodification) Not really changing the types of facilities, but changing the methods for how facilities are sized and designed Focus on volume reduction/infiltration and duration of peak flows

21 3. Change in Sizing Methods Changing Methods for Facility Design Traditional Approach match peak flows 24-hour storms (rainfall depths) based on recurrence intervals of rainfall depths National Weather Service Rainfall Data for February 5-8, Year SCS Type 1A Synthetic Design Storm r ) /h (in 1.0 l fa 0.8 in a R Total Rainfall Volume = 5.28 in Max. Hourly Rainfall Intensity = 0.9 in/hr r ) 1.2 /h (in 1 l fa 0.8 in a R Total Rainfall Volume = 7.3 in Max. Rainfall Intensity = 0.66 in/hr Time (hr) 0 2/5/96-12:00:00 AM 6:00 12:00 18:00 2/6/96-12:00:00 AM 6:00 12:00 18:00 2/7/96-12:00:00 AM 6:00 12:00 18:00 2/8/96-12:00:00 AM Time (hours) 22

22 3. Change in Sizing Methods

23 Rainfall (in/hr) Flow (cfs) 3. Change in Sizing Methods Problems with Traditional Design Approach Based on Rainfall Recurrence Rainfall (in/hr) Post Dev Pre Dev Post Dev (with discharge control) Hours

24 3. Change in Sizing Methods Changing Methods for Facility Design Newer approach based on long term rainfall record match duration of peak flows range of peak flows that cause in-stream erosion thresholds are based on recurrence intervals of peak flows

25 3. Change in Sizing Methods

26 3. Change in Sizing Methods Flow Duration Matching

27 Flow (cfs) 3. Change in Sizing Methods Flow Duration Matching Impervious Mitigated Post-Project Site Pre-Project Site Q10 0.1Q IMP Reduces Impervious Runoff to Less Than Pre-Project Levels % Time Exceeded

28 3. Change in Sizing Methods Requirements Long-term rainfall data Calibrated hydrologic/hydraulic model capable of running continuous simulations Peak flow thresholds for channel forming flows Statistical analysis of peak flows Sophisticated hydrologic/hydraulic engineer

29 4. Simplified Facility Sizing Tools

30 4. Simplified Facility Sizing Tools Three General Types of Sizing Tools Continuous Simulation Models Runs an actual HSPF model to produce a long term continuous record. Pre-run Long -Term Hydrographs Based on model runs previously conducted to produce long term continuous unit hydrographs for different soil conditions. Pre-run BMP Sizing Factors Based on model runs previously conducted to produce long-term continuous unit hydrographs for different soil conditions and then BMPs previously sized on a unit basis based on those runs (BMP sizing calculator).

31 4. Simplified Facility Sizing Tools Continuous Simulation Models WWHM/BAHM/SAHM examples

32 4. Simplified Facility Sizing Tools Continuous Simulation Models WWHM/BAHM/SAHM examples Advantages allows for significant flexibility in facility specifications able to customize to other areas of the country variety of facility options can evaluate drainage areas in series Disadvantages complex operation for user and administrator may allow for too much flexibility takes significant time to understand, use, and run the model is still in the beta testing mode

33 4. Simplified Facility Sizing Tools Pre-run BMP Sizing Factors Kitsap County Example

34 4. Simplified Facility Sizing Tools Pre-run BMP Sizing Factors Kitsap County Example

35 4. Simplified Facility Sizing Tool Pre-run BMP Sizing Factors Kitsap County Example Advantages simple operation for user and administrator sizing factors all pre-set harder for user to game the system Disadvantages facility dimensions/assumptions are pre-set does not include detention sizing no ability to analyze multiple drainage areas in series or discontinuous drainage areas includes pre-set flow thresholds for channel forming flows

36 4. Simplified Facility Sizing Tools Pre-run Long Term Hydrographs and Pre-run BMP Sizing Factors WES Tool Example LID Sizing similar to simple spreadsheet with embedded sizing factors (i.e., lookup table) Detention Sizing pre-run long-term hydrology for different conditions hydrographs are scaled based on site conditions hydrographs are routed through the detention facility using level pool routing flow duration matching is conducted detention pond sizing is optimized

37 4. Simplified Facility Sizing Tools Pre-run Long-Term Hydrographs and Pre-run BMP Sizing Factors WES Tool Example

38 4. Simplified Facility Sizing Tools Pre-run Long-Term Hydrographs and Pre-run BMP Sizing Factors WES Tool Example Advantages: User-friendly for user and administrator Can t alter methods to developer benefit Is not just a tool for LID but includes detention Detention can be sized to address inclusion of upstream LID Ability to analyze discontinuous drainage areas Disadvantages: Facility assumptions/dimensions are pre-set for LID facilities Includes pre-set flow thresholds for channel forming flows Orifices are included for LID facilities in C and D soils

39 4. Simplified Facility Sizing Tools Retention Standard Tool EPA Calculator Example

40 4. Simplified Facility Sizing Tools Retention Standard Tool EPA Calculator Example Advantages online tool that is very easy to use accepted by EPA includes continuous rainfall records allows for flexibility in lid configurations (e.g., planter depths) Disadvantages does not address flow duration/hydromodification does not consider pre-development conditions would need to be used with a retention standard or a water quality design storm would require additional analysis if you wanted to develop an equivalent retention standard to meet hydromodification goals includes lid only and not larger scale regional detention

41 5. Results/Impacts

42 5. Results/Impacts need for continuous simulation modeling simplified sizing tools are desired need for inclusion of outlet control structures to meter out flows more variation in facility sizing based on soils and outlets need for more rigorous infiltration testing significant maintenance requirements (inventory, mapping, inspections, tracking)

43 6. Questions?