Best Management Practices - Land Development

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6. Stormwater Management

6.1 Stormwater Management Design

Hydrological studies have demonstrated that traditional dependence on stormwater collection and conveyance methods has resulted in serious declines of fish habitat through altered flow regimes and channel degradation. Upland developments can create adverse downstream impacts through changes to the stormwater runoff patterns. Urbanization increases impervious surfaces such as roofs, driveways and roads and consequently alters runoff rates.

The historical approach to urban drainage design was to collect stormwater runoff in a system of buried pipes and remove it from the subject development site as quickly as possible. As the carrying capacity of stormwater collection systems and/or receiving streams was reached or exceeded, there was a need to capture and temporarily detain runoff during and immediately after rainfall so that the stored water could then be released later at a controlled safe rate. These approaches unfortunately have not provided adequate protection to riparian and aquatic habitat.

It is critical for the continued survival of salmonids, a number of red and blue listed ecosystems and several wildlife species that we first identify the sensitive resources and then design stormwater management systems around them. Merely applying specific streamside setbacks, leave strips or "buffers" to protect stream corridors will not effectively ensure that these features will be preserved, as the impacts of development extend beyond the riparian areas. Stormwater hydrograph and water quality changes also need to be mitigated.

Degradation of aquatic habitats caused by changes to hydrology through land clearing and development are now becoming better understood. The best method of managing stormwater is to develop and implement an integrated stormwater management plan (ISMP). These planning processes encompass a comprehensive assessment of watershed hydrology and provide a set of recommendations to address impacts to the environment.

Best Management Practices

1. Maintain urban forests by maximizing the percentage of undisturbed tree and shrub habitat during urban growth.
2. Minimize the addition of impervious surface to all urban watersheds.
3. Groundwater infiltration strategies should mimic, restore or enhance pre-development infiltration rates.
4. Incorporate landscape design and other treatment technologies in the engineering work, to maintain pre-development flow regimes, surface water quality, and local temperature ranges.
5. Promote educational tools and stewardship activities to foster an awareness of stormwater concerns.

6.2 Integrated Stormwater Management Planning

Best Management Practice

1. MELP recommends that all OCPs provide the framework to assist with protecting people, private property and environmental resources from damage. Local governments should address the issue of integrated stormwater management through the coordination of watershed-based pre-planning, to ensure a unified and coordinated approach regarding management of storm drainage and land disturbance that can lead to erosion and sediment movement.
2. Stormwater management plans should not only control runoff to protect people and private property but also ensure protection and/or enhancement of the riparian and aquatic habitat.
3. MELP recommends that regional districts and municipalities develop drainage bylaws and specify in those bylaws a maximum percentage of impermeable area of land.
4. To best avoid negative impacts on small streams and downstream properties, all areas zoned for lot sizes less than 2 hectares should have an integrated stormwater management plan designed, implemented and maintained by local government.
5. The plan should acknowledge and provide for maintenance of low flows during summer and fall where low flows are vital for fish and aquatic life and water licensees.
6. The plan should ensure that the water quality of the storm runoff has no negative impact on aquatic life.

6.3 Stormwater Quality and Infiltration

The grading, paving and draining of a new development site often decreases infiltration of soil-water into the ground, decreases the groundwater recharge and increases the potential of water contamination from surface runoff.

Best Management Practices

1. The construction of stormwater seepage trenches and recharge basins may provide some restoration of the groundwater regime.
2. The disposal of some or all of the minor stormwater system flows to the ground may be viable in areas where the subsurface soils are of suitable type and capacity, and the stormwater is not contaminated with harmful substances.
3. The sum of the groundwater and surface water minor stormwater design must accommodate at least the one in ten year storm event.
4. Runoff resulting from larger storms may require surface systems.
5. Water quality treatment facilities may be required to maintain high quality water. The report "Urban Runoff Quality Control Guidelines for British Columbia" by MELP - BC Environment provides information on the effectiveness of the various techniques, and guidance for the construction of stormwater quality treatment facilities.

6.4 Initiatives for Protection of Aquatic Habitat

Since 1992, Fisheries and Oceans Canada and Ministry of Environment, Lands & Parks have been promoting the protection of fish and fish habitat through detention of stormwater flows from urban development areas. Broadly stated, these goals were to limit the 1-in-2 year post-development runoff to rates equal to the pre-development levels and to maintain, as closely as possible, the pre-development hydrology and water quality.

Application, monitoring and research of best management practices, stormwater modeling and planning processes in the past years have increased our knowledge about impacts to aquatic systems from stormwater and urban development. More focus has been placed on minimizing the effects of impervious area, management of more frequent runoff events and emphasizing runoff reduction through infiltration. Systems that incorporate physical and biological systems, including soil-water contact, have been shown to have significant capacity to remove pollutants from runoff.

In light of this information and other research, the requirements for what is best, to protect fish and fish habitat must be updated.

• The first priority is being placed on source control and runoff reduction. This is accomplished by reducing impervious areas and retention of runoff by infiltration or long-term storage.
• Second, greater volumes of total runoff should be treated to remove pollutants that degrade water and habitat quality. Smaller, frequently occurring runoff events must be routed to areas where combined biological and physical processes can improve runoff water quality.
• Third, rates of runoff from events less than 1-in-5 year return periods should be detained to reduce hydraulic and hydrological impacts to stream systems and habitats.

Given the current state of knowledge, these guidelines and BMPs represent what best protects fish and fish habitat from the impacts of stormwater runoff.

In January 2001, an initiative was started that will give clearer direction to land managers and property owners concerning stormwater management and best management practices. The Ministry contact for the project is Peter Law 250 751-3229, who will provide further information including hydrological design criteria upon request. Results of this project will be incorporated in future versions of this reference document.

The following is a list of suggested parameters forming the regional strategy initiative for control of stormwater.

6.4.1 Volume Reduction (VR)

Objective

To reduce and mitigate the total runoff volume caused by increased urban development and subsequent increased impervious areas, as well as to maximize the amount of runoff returned to shallow groundwater via recharge.

Suggested Non-Structural BMP

• Discharge volumes from the post-development 6 month/24 hour events are infiltrated to the ground rather than discharged to storm systems.

Suggested Structural BMP

• Ground infiltration systems, biofiltration swales or burrows, long-term storage in constructed wetlands or ponds or through EIA reduction

6.4.2 Water Quality (WQ)

Objective

To mitigate water quality impacts to fish habitat by collecting and treating "first flush" and more frequent runoff events of smaller storms.

Non-Structural BMP

• Collect and treat the volume of the 24-hour storm event equaling 90% of the total rainfall from impervious areas with suitable BMPs.
• Rate-of-discharge will provide suitable hydraulic retention time so as to maximize water quality.

Structural BMP

• Biofiltration swales or burrows, constructed wetlands, exfiltrating dry detention pond systems

6.4.3 Detention or Rate Control (RC)

Objective

To restrict the post-development peak runoff flow rate to that of the pre-development peak runoff flow rate for selected design return periods.

Non-Structural BMP

• Design BMPs so the post-development flows match the volume, shape and peak instantaneous rates of pre-development flows for the 6 month/24 hour, 1-in-2 year/24 hour or 1-in-5 year/24 hour precipitation events.

Structural BMP

• Suitable BMPs include extended dry detention ponds, constructed wetlands or wet ponds, storage swales.

6.4.4 Watershed Based Implementation

These are general criteria for development and application of best management practices (BMP):

• BMPs should be based on site and watershed planning processes with appropriate application for resource values, site suitability, basin size and type of development
• BMPs should be ecosystem-based, utilizing physical and biological processes providing both water quality and quantity benefits
• BMPs design is based on existing predevelopment conditions, but should address cumulative development impacts or community requirements for enhancement or restoration of aquatic conditions within a watershed
• BMPs should not be sited or designed with footprint impacts to existing fish habitat

Unless otherwise determined on a site-specific basis, BMPs should be designed and implemented in the following priority:

1. Volume Reduction (VR)
2. Water Quality (WQ)
3. Runoff Control (RC)

 

Figure 5. A guide for selection and targeting of BMPs on a watershed or catchment level, acknowledging space and infrastructure limitations in redevelopment scenarios:

Redevelopment Scenario	BMP Implementation	Potential Watershed Coding
Rural Residential to Urban Residential (0 - 10 units/ha and IA* < 30%)	VR, WQ, RC
Residential Densification to Multifamily (10 - 20 units/ha and IA* < 60%)	VR, WQ
Multifamily to Urban (> 20 units/ha and IA* > 60%)	VR or WQ

* - impervious area

Due to the wide range of site-specific conditions encountered prior to implementation of stormwater BMPs, it is important to determine the best BMP investment for the site characteristics, past and future development history. Note that the use of volume reduction or water quality BMPs can be functionally interchangeable. Use of site specific or local BMPs may be part of a catchment or sub-watershed plan resulting in layering of BMP types. For example, VR reduction could be implemented at the lot level (e.g., roof top infiltration via swales) requiring specific WQ controls at effective impervious surface area (EIA) sources (e.g., biofiltration, pocket wetlands at parking lots, road systems) and RC BMPs at sub-catchment scales (e.g., wet detention/wetlands at neighbourhood level).

Reference

Chilibeck, Barry and Megan Sterling, 2000. Urban Stormwater Guidelines and Best Management Practices for Protection of Fish and Fish Habitat. Department of Fisheries and Oceans, Vancouver.

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