Working in Arid and Semi-Arid Climates with Hydromodification Concerns

Transcription

1 Design Techniques for Stream Restoration: Working in Arid and Semi-Arid Climates with Hydromodification Concerns Presented By: Tory R. Walker, PE, CFM, LEED GA September 30, 2014

2 Brief Overview Short History Lesson MS4 (Stormwater) Permit as a Catalyst Regional Factors & Influencers Case Studies

3 Short History Lesson 5 water quality 1 natural 4 Detention & MPDs 2 ditches 3 channels

4 What Are You Looking At? from Stormwater Magazine

5 MS4 Permits as a Catalyst Recent focus on problem of hydromodification Intention is commendable, science is lacking Science has been imported Approach is over simplified Conservative assumptions Result is a convoluted set of rules with little resemblance to reality You can t legislate how nature should behave!

6 Factors & Influencers Precipitation data and geologic data have influenced urban development Consider how MS4 permits should be written to properly address the impacts of hydromodification Present a more effective approach that is scientifically robust & practical MUST UNDERSTAND THESE FACTORS OR ELSE!

7 The Fatal Conceit (coined by Friedrich Hayek) To summarize - the belief that people can manipulate and control complex systems that they don t understand and have good outcomes. Invariably, when they do that, they not only do not have good outcomes, they get all kinds of unintended consequences. Complex systems, by definition, are systems that no one of us can fully understand. Each of us sees only a part of it, and yet when we manipulate it, we continue to be surprised by the results.

8 Climatic Influence The Mediterranean climate in Southern California entices people there to live, work, and play

9 BUT Devastation from occasional winter storms Flooding and mudslides Erosion of channels Sediment deposition downstream Damage to utilities, roads and bridges Loss of life and property Infrequency causes short term memory!

10 Climatic Influence- Seasonal 7.00 Seasonal Precipitation in Oceanside (cm) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 60% 2%

11 Climatic Influence- Long Term Variation

12 Climatic Influence SoCal climatic patterns do not represent most of the U.S.

13 yet the majority of LID and hydromodification management tools have been developed & applied elsewhere, including: Computer Simulation Models & Studies Stream restoration tools & techniques

14 # of days of Rainfall per Year Rainfall Distribution Typical Distributions of Annual Precipitation in Days per Year S. Coastal Vancouver, BC 60 San Diego LIGHT HEAVY EXTREME

15 % of Runoff Rainfall Distribution Typical Distributions of Annual Precipitation as Percentages S. Coastal Vancouver, BC San Diego LIGHT HEAVY EXTREME

16 Climate Vast geographic differences demonstrates need for calibration of continuous simulation hydrologic models LID tools and onsite hydromodification management devices may function differently depending on climate Understand how precipitation infiltrated will behave in soils and underlying geology

17 Geological Influences

18 Urban Environment Established communities have grown around and adapted to environmental conditions and constraints Example: San Diego County currently imports over 80% of its water from outside the County Building and development standards established in recognition of environmental conditions and societal values in the area

19 Building Out an Urban Environment Southern California is largely built out Most desirable land has been developed More challenging (and costly) lands have yet to be developed Some developed land gets redeveloped New development is often adjacent to existing built environment Must carefully consider all potential impacts on both built environment and natural environment

20 Urban Environment & Water Water is still relatively inexpensive and reliable Typical residential water budget is 50% to 60% outside uses Typical landscaping requires 50 inches of irrigation Over-irrigation frequently leads to Slope instability Settlement Mold growth Seepage through foundations Undermining of roads & utilities Now EPA is pushing LID (i.e., infiltration) to reduce hydromodification impacts!

21 Hydromodification Simple Definition: Erosion of waterways and subsequent sediment deposition downstream, often due to modifications in the watershed Examples: Covering the land with impervious surfaces Deprives waterways of sediment Increased runoff volumes, durations, & peaks in those same waterways Flood Control Dams Traps sediment Reduces damaging peak flows & volumes

22 Effects of Urbanization on Hydrology (Peak, Volume, Duration) Disruption of natural water balance Increased flood peaks Increased stormwater runoff More frequent flooding Increased bankfull flows

23 Traditional Response to Hydromodification Impacts addressed by improving (channelizing and hardening) waterways Protecting adjacent lands from periodic flooding Protecting against erosion Often resulting in concrete lining or rock riprap lining Allowing for more adjacent lands to be developed Urban environment encroached upon previously flood-prone lands Habitat was assigned less value by society than potential uses of adjacent lands

24 Hydromodification Degradation Many waterways in Southern California were not improved through the traditional approach Various degrees of degradation resulted Common factors affecting stream degradation include: Percentage and locations of impervious areas within watershed Soils and geology of watershed Sediment sizes within watershed Slope of waterway Time passed since development

25 Oso Creek in San Juan Capistrano

26 Escondido Creek in Escondido

27 Agua Hedionda Creek in Vista

28 Geology Consensus Based Science The Permit FOG Vegetation

29 Hydromodification Myths Common Myths (over-simplification): 1. Precise definition of critical low-flow threshold is fundamental to determine hydromodification impacts 2. A small change in imperviousness of an already heavily altered system will change the erosion response 3. Many heavily altered watersheds and creeks can be restored to natural (pre-columbian) conditions 4. Designing over-sized onsite hydromodification management facilities for new developments may significantly improve current problems

30 Hydromodification and the SoCal MS4 Permit Fails to consider mitigating factors that exist in nature Wetland and riparian vegetation Channel irregularities Ability of many creeks to self-armor Climatic Variation Fails to consider fluvial systems that are in a state of dynamic equilibrium Assumes that critical low-flow threshold is the point at which a natural channel becomes unstable FACT: in Southern California, infrequent episodic events are far more influential!

31 Sediment Transport - Southern California Most sediment is transported during medium to extreme events

32 Sediment Transport - Tijuana River 53% of sediment transported occurs for peak flows larger than Q10 * Typical range valid for non-mediterranean climates is not representative of the optimum range of analysis in Southern California Misguided efforts to tightly control extremely small flows (i.e., 10% of Q 2 ) accomplishes very little of significance Greatly increases size of onsite hydromodification management facilities * 73 years of daily data

33 Sensitivity of an Impacted System Previous studies of natural fluvial systems As little as 3% of impervious area can be sufficient to cause a natural system to begin to degrade or unravel Long term response of fluvial systems to impervious area is not well understood Predictions can be based on similar systems Some systems remain vulnerable (Oso Creek) Some in a state of new dynamic eq. (Escondido Creek) Some in between (needing some level of restoration)

34 Systematic Approach to Hydromodification Management Observe response of fluvial system to previous increase in impervious area over time Study aerial photography and topography GIS analysis HEC-RAS Other fluvial geomorphic tools Fluvial Geomorphic Studies Reasonable Assessments of Vulnerability

35 One Approach to Watershed Rehabilitation Deconstructing urban environment over time Restoration to pre-columbian natural systems All new development needs a zero net impact LID and onsite hydromodification management Existing development would follow suit Required retention of 85 th Percentile runoff Natural condition as a point for comparison If underlying soils unsuitable for infiltration, other LID practices implemented Green roofs; Cisterns and rain barrels REMEMBER FACTORS & INFLUENCERS!

36 A Flawed Approach Society would bear cost of condemning all currently protected properties adjacent to waterways Alternatives will cause pressure to infiltrate against better judgment Displaced homes and businesses would need to relocate (new impacts) Not smart growth

37 A Flawed Approach (cont.) California Environmental Quality Act (CEQA) requires consideration of potential impacts to surrounding land uses, both natural and built Potential impacts Flooding Environmental degradation Impacts to groundwater quality and quantity CEQA checklist addresses groundwater depletion Does not address Increases of infiltration Increases to groundwater levels

38 A Balanced Approach Start with multidiscipline team of experts! Use the best available data, models and tools Rigorous adherence to the scientific method Further develop modeling tools based on local data Application to realities that characterize the Region Climatic Geologic Urban Economic Calibrate parameters

39 MS4 Permit Alternative Compliance Focus on impaired receiving waters rather than onsite LID and hydromodification management Allowing water quality goals to be achieved sooner Establish Water Quality Equivalency (currency) In-lieu fee mechanism (like wetlands program) Numerous stream restoration and rehabilitation projects have been already identified Improves physical, chemical and biological characteristics of waterway

40 Oso Creek Watershed Watershed area is 20 sq. miles, creek is 13.5 miles long From Upper Oso Reservoir, through Mission Viejo, to North San Juan Capistrano Tributary to Trabuco Creek Watershed is highly urbanized, creek is mostly hardened

41 Impaired Area Reach End of hardened channel to Trabuco Creek Within City of San Juan Capistrano Both public and private property Vital infrastructure (roads, railroad, water, sewer) Land use historically agriculture

42 Area Issues Active erosion due to hydromodification (upstream development) Sedimentation downstream Trabuco Creek is critical habitat for Steelhead Trout Loss of natural creek functions Decreased flood capacity downstream Development of private land infeasible Threat to existing infrastructure

43 May 1994

44 June 2002

45 April 2005

46 November 2009

47 March 2011

48 Issues to Identify in Restoration Plan Stabilization of creek bed and banks Re-establishment of natural creek functions Potential groundwater replenishment Protection of adjacent and downstream properties Downstream flood capacity preserved Protection of existing infrastructure, such as railroad and sewer lines

49 Stabilize Creek Bed and Banks

50

51 Re-Establish Natural Creek Functions

52 Groundwater Replenishment

53 Protection of Property

54 Preserve Downstream Flood Capacity Trabuco Creek at San Juan Creek

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56

57 Conceptual Design Typical Plan View Re-establishment of natural creek profile Creek bed stabilization (grade control) Creek bank stabilization (rock groins)

58 Conceptual Design Typical Profile Re-establishment of Natural Creek Profile

59 Creek Bed Stabilization (Natural Rock Grade Control)

60 Benefits of Grade Control Reduces velocity Reduces bed and bank erosion Promotes sediment buildup Protects existing infrastructure Enhances aquatic biology with riffle pool stream profile Replenishes groundwater Restores bed and bank vegetation Natural attenuation of peak flows

61 Creek Bank Stabilization with Rock Groins Reduces velocity Reduces bank erosion Allows for habitat variation Preserves low flow channel Stabilizes channel bank Plan View Cross Section

62 Agua Hedionda Area Reach About 35 miles north of downtown San Diego Includes portions of Carlsbad, Vista, Oceanside, San Marcos Approximately 31 square miles & 37 linear miles of streams Terminates at the Agua Hedionda Lagoon

63 Agua Hedionda Creek Stable System (circa 1900) High Groundwater Recharge Equilibrium Sediment Transport High Floodplain Function Low Discharge Velocities Dynamic and Broad Riparian Zone Diverse Riparian Habitat Sustainable Vegetation Mosaic Cool Seasonal Aquatic System

64 Agua Hedionda Creek - Present Early Incised System (Present) Depressed Groundwater Table High Groundwater Export Low Groundwater Recharge High Channel Bank Erosion Confined Low Floodplain Function High Discharge Velocities Diminishing Riparian Zone Width Low Diversity Riparian Habitat Poor Riparian Plant Recruitment Cool Seasonal/Perennial Aquatic System

65 Agua Hedionda Creek - Present

66 Agua Hedionda Creek Restoration Project Floodplain Inundation Map Detailed HEC-RAS study Surveyed cross-sections 50 feet apart on average Incorporated hydraulic parameters specific to each cross section Useful for sediment transport analysis Evaluated 2-year, 10-year & 100-year

67 Preliminary Design Proposed Structures 9 grade control structures - 6 larger structures - 3 smaller (riffle) structures

68 Preliminary Design Proposed Structures

69 Preliminary Design Proposed Structures 5 areas of Bank Stabilization Natural stone toe with plantable gabions Natural stone toe to top of bank (2:1 slope) - Requires more room - Many mature trees limit its use

70 Agua Hedionda Creek Mature Incised System (circa 2040) Groundwater Conditions Same High Channel Bank Erosion Confined Low Floodplain Function High Discharge Velocities Diminishing Riparian Zone Width Low Diversity, Narrow Riparian Habitat High Scour Plant Loss Opportunistic Exotic Plant Recruitment Warm Seasonal/Perennial Aquatic System

71 Agua Hedionda Creek Restoration Project Preliminary Design Grade Control Structures

72 QUESTIONS? Tory Walker (760)