Stormwater Planning: A Guidebook for Brtish Columbia:
Guidance document provides framework for applying rainfall-runoff models to test performance targets
Stomwater Planning: A Guidebook for British Columbia shows how site level solutions fit in to a larger watershed context, and provides a framework for developing landscape-based rainwater management plans at a watershed scale.
“Founded on British Columbia case study experience, the Guidebook formalized a science-based understanding to set performance targets for reducing rainwater runoff volumes and rates. These targets represent the synthesis of biological and hydrological understanding,” reports Kim Stephens, Project Manager and principal author of the Guidebook.
The Guidebook also established a protocol for application of computer models to assess the achievability of performance targets. The following discussion on rainfall-runoff modeling is extracted from Chapter 9 of the Guidebook (i.e. pages 9-14 through 9-17).
The chapter is titled Developing and Implementing an Integrated Stormwater Management Plan (ISMP).
Integrated Stormwater Management Planning
In the late 1990s in British Columbia, the term ISMP gained widespread acceptance by local governments and the environmental agencies to describe a comprehensive, ecosystem-based approach to stormwater planning. The purpose of an ISMP is to provide a clear picture of how to be proactive in applying land use planning tools to:
- protect property from flooding, and
- protect aquatic habitat from erosion and sedimentation
Use of the term is uniqe to British Columbia. The City of Kelowna first used the the term in 1998 to make a clear distinction between ‘suburban watershed management’ and the Province’s ‘integrated watershed management’ process for natural resource management in wilderness watersheds. At the time, this was an important distinction. Local government typcically has control over stormwater in residential, commercial and industrial land uses. It does not necessarily have control over watersheds.
In British Columbia, there has been a change in thinking among drainage practitioners, and the technical language is in transition. This change has seen the single function view of traditional ‘stormwater management’ give way to the integrated and comprehensive perspective that is captured by the term ‘rainwater management’. This sea-change began in 2004.
For more information on this language shift and why, click on Well, What is Rainwater Management Really?
Using Performance Targets to Quantify Watershed Objectives
Performance targets provide a quantifiable way of measuring success in protecting (or restoring) a watershed, and for identifying what needs to be done to achieve a given environmental protection objective.
- Desired protection objectives for significant stream reaches should be translated into performance targets for the catchments draining into those reaches. For example, to maintain or restore the health of a stream reach, an appropriate performance target would be to limit the volume of runoff from land uses in the drainage catchment to 10% or less of total rainfall volume.
- For catchments upstream of drainage ‘hot spots’ (e.g. chronic flooding locations), a more appropriate performance target may be to reduce peak runoff rates from large rainfall events (e.g. 5-year storms).
- Other performance targets relating to the preservation/restoration of significant natural features (e.g. riparian forests, wetlands), measurement of stream health (e.g. B-IBI), protection/improvement of water quality, or instream enhancements (e.g. for habitat or fish passage) should also be established.
A key principle is to establish performance targets that relate directly to the watershed objectives. Download Guidebook Chapter 6 titled Setting Performance Targets and Design Guidelines for further guidance on setting performance targets.
The selected targets should also be monitored over time to ensure that the ISMP is achieving the desired results.
Setting Performance Targets
To establish realistic performance targets for a given watershed, an ISMP must answer questions such as those introduced in Guidebook Chapter 6 and reiterated below:
- What is the existing level of annual runoff volume? What percentage of total annual rainfall volume does it represent? What is the existing Mean Annual Flood (MAF)?
- What are acceptable levels of runoff volume and rate in terms of flood risk and environmental risk? What are the consequences of increased or decreased flows related to land development? Are these consequences acceptable?
- What actions are needed to avoid flooding or environmental consequences?
- How can the necessary actions be staged over time?
- Are the targets to maintain 10% runoff volume and maintain the natural MAF necessary or achievable over time? If not, what levels are?
Modeling Alternative Scenarios
Scenario modeling can be used to assess a range of performance targets, and evaluate options for achieving these targets.
Scenario modeling involves consideration of the complete spectrum of rainfall events that typically occur in a year. Download Guidebook Chapter 6 titled Setting Performance Targets and Design Guidelines for further details regarding the three tiers. An integrated approach to managing these events comprises three components:
- retain the small events or ‘light showers’ at the source (these are denoted as Tier A in the Guidebook)
- detain the large events or ‘heavy rain’ (denoted as Tier B) in detention facilities
- safely convey the extreme events (denoted as Tier C)
The Guidebook introduced the concept of an integrated strategy to address the complete spectrum of rainfall events.
Relationship of Rainfall Spectrum to Watershed Objectives
The balance between the above three components depends on the watershed objectives.
- Stream protection/restoration objectiveswould likely govern scenarios that emphasize source control (e.g. infiltration, rainwater re-use), along with other possible options, such as riparian corridor protection.
- Flood management objectives would likely govern scenarios that place more emphasis on detention and conveyance.
The key is to determine which scenario or blend of scenarios has the best ‘fit’ to address a range of watershed objectives.
A key aspect of scenario development will be to consider what can be done at the site level to retain the small events, given constraints such as soil conditions, hydrogeology, topography and land use. Further data collection may be required to assess the feasibility of achieving performance targets.
A computer model is a decision support tool. A model can help evaluate alternative scenarios, but it does not make decisions. Sometimes there is a tendency to over-emphasize the value of modeling. The reliability of model output depends on the quality of the input data, and especially on the judgement of the modeler in making critical assumptions.
A fundamental principle is that the level and/or detail of modeling should reflect the information needed by decision makers to make an informed decision. The modeler must always take a step back and ask three key questions:
- Why is the model being built?
- How will the model be applied?
- What problems will the model help us solve?
The graphic below illustrates the four main levels (or applications) of drainage modeling. Moving down the pyramid reflects an increasing level of detail, and hence investment of resources.
At this stage of the ISMP process, modeling should be at a strategic (i.e. conceptual or overview) level to provide basic information to support the decision making process.
Modeling tools take on added importance once the focus shifts to the functional planning and design of proposed stormwater management facilities. More data is required at this level of modeling.
Data Requirements for Strategic Level Scenario Modeling
Continuous rainfall data (in time increments of one hour or less) is the key data requirement for scenario modeling. Ideally, site-specific rainfall data should be used, but even data from a location with similar rainfall characteristics can be used at this stage.
At this strategic level of modeling, the other model inputs (e.g. regarding land use and soil conditions) should be estimated based on the best available information. Where there is high degree of uncertainty regarding certain parameters, a range of assumptions may be tested, and data collection efforts can then be targeted to refine these assumptions.
The appropriate type of modeling will depend on the characteristics of the scenarios being modeled, as discussed next.
Types of Modeling: Single Event versus Continuous Simulation
There are two types of modeling: ‘single event’ and ‘continuous simulation’. Single event typically means a storm duration up to 24 hours. Continuous simulation typically covers a year or a multi-year period, with time-steps up to 1 hour. Their respective applications are summarized as follows:
- Single Event Modeling– acceptable for most applications of Tier C flood risk management
- Continuous Simulation Modeling – required for Tier A rainfall capture, Tier B runoff control, and some applications of Tier C flood risk management
For both types of modeling, measured rainfall data (rather than artificial ‘design storms’) should be used as input data. For further discussion on the three ranfall tiers, download Guidebook Chapter 6 titled Setting Performance Targets and Design Guidelines.
Continuous Simulation for Source Control (Tier A) and Detention (Tier B)
The distinction between Tier A and Tier B modeling is that Tier A requires volume-based thinking, whereas Tier B involves flow-based thinking. Conventional modeling packages are flow-based, and thus most appropriate for modeling detention (Tier B) and conveyance (Tier C) scenarios.
Models may be hydrologic (i.e. simulate runoff response), hydraulic (i.e. perform flow routing functions), or both. A selection of flow-based models is provided below for reference purposes. The appropriate model type depends on the scenario being modeled.
Note that the level of effort and amount of data required to apply these models is highly variable. Some of these models require a high level effort, which may not be suitable for scenario modeling applications at the strategic level.
Because Tier A simulation is volume-based, it is described as Water Balance modeling. Since the focus of stormwater source control is on runoff volume reduction, Water Balance Modeling is most appropriate for source control scenarios. The Water Balance Model (WBM) is an example application. For details, download Guidebook Chapter 7 titled Site Design Solutions for Achieving Performance Targets.
Source Control Scenario Modeling
Whereas the use of conveyance and detention are relatively well understood stormwater management strategies, the use of source control is less well-known. Discussion among ISMP participants is likely to focus on whether source controls are effective or practical in the context of watershed-specific conditions.
Generating source control scenarios through Water Balance modeling can be a critical tool in informing this discussion. For details, download:
Guidebook Chapter 7 titled Site Design Solutions for Achieving Performance Targets; and
Guidebook Chapter 8 titled Watershed Context for Site Design Solutions .
Model scenarios can provide guidance for selecting source control options to achieve catchment-specific performance targets. Further data collection should focus on collecting the information needed to determine whether these options are achievable. For example, if infiltration is identified as an option for achieving performance targets in a particular drainage catchment, a key information need would be to determine soil conditions in that catchment.
Flood Management Scenario Modeling
The primary purpose of modeling for flood management purposes (i.e. Tier C) is to assess the conveyance capacity of drainage facilities installed at stream crossings. The level of preciseness in quantifying design flows is not critical because rated capacity is not the governing consideration.
Physical adequacy normally governs the acceptability of a drainage installation. For more information, download Guidebook Chapter 6 titled Setting Performance Targets and Design Guidelines. Hence, the real purpose in comparing design flows to rated capacities is to provide a relative measure of the degree of risk. This comparison helps elected officials make decisions to invest in drainage facility upgrades and/or replacement.
For certain flood management scenarios, continuous simulation modeling would be more appropriate. For example, continuous simulation would be needed to provide an idea of the extent and duration of flooding over an extended period of time under different detention and/or flow conveyance scenarios.