Description
Urban sprawl and city growth has led to the rise of impermeable surfaces such as roads, parking lots, and buildings which are resulting in increased stormwater runoff and associated environmental challenges. Traditional engineering has relied primarily on gray infrastructure solutions such as pipes, channels, and detention basins enlargements. However, this approach is costly and does not alleviate the water quality of urban stormwater.
In response to this limitation of the traditional approach, the use of Low Impact Development (LID) or Sustainable Urban Design (SUD) become a natural candidate when aiming to implement a greener engineering solution. The idea of LIDs is to mimic natural hydrological processes. In MIKE+ units such as green roofs, bioretention cells, rain gardens, porous pavements and vegetative swales are supported.
The modelling of LID practices in MIKE+ is divided into 2 main approaches:
- Modelling of LID at screening level - catchment-based approach.
- Detailed hydraulic modelling of individual LID structures - drainage network-based approach.
The second approach offers a method whereby the user has the option of detailed modelling of individual LID structures hydraulically connected to the stormwater pipe network. This approach is based on the concept of soakaway nodes. You may read more about this feature in section 4.4 of our MIKE+ Collection System. User Guide.
In this Knowledge Base Article, the MIKE+ implementation of LID at the screening level - the catchment-based approach – is described.
#01- Numerical Structure and Concept of a Low Impact Development Unit (LID) - Fig. 1-2
Green infrastructure modelling in MIKE+ at a screening level simulates the hydrological processes, water flow, pollutant transport, and other relevant aspects within a stormwater collection system in a” buckets” fashion - see Fig. 1.
This means that the LID unit is divided in layers, each layer representing a bucket which needs to be full before the flow can continue into the next layer (bucket). These layers represent the different components of the units (surface storage, engineered soil, storage capacity, urban drain) all these components are governed by its infiltration capacity and the hydraulic conductivity of the surrounding soil.
Fig. 1 - Bucket schematic structure of the LID implementation in MIKE+
Each LID Unit has a different combination of buckets (layers) depending on its engineered specification, more specific details of this specification can be read in section 4.4 of our MIKE+ Collection System. User Guide
LID units are deployed per catchment, adopting the hydrological parametrization of those catchments. Only the Kinematic Wave Hydrological Model supports the implementation of LID units. Therefore, it is imperative to characterize the catchment properly, especially if multiple LIDs are deployed and if the catchment is large. Moreover, if the location of those LID units is important, it is worth considering splitting the catchment into multiple sub-catchments and characterize them individually - see Fig. 2.
Fig. 2 - Large catchment divided into smaller catchments for a better hydrological characterization
#02 - LID Parameter Description - Figs. 3-4
All LID Units in MIKE+ can be modelled using the Bio Retention Cell concept - see Fig. 3. Depending on the unit engineered structure, a different variation of the layers will be applied.
Fig. 3 - The Bio Retention Cell Concept
The LID Parameter Editor provides the necessary information to define the Bio Retention Cell Unit. These entries are presented in a layer-by-layer fashion, with each layer enabled depending on the type of LID unit described - see Fig. 4.
Fig. 4 - LID Layers Structure in MIKE+
#03 - LID Deployment - see Figs. 5-12
In order to implement LID units in the hydrological model, the units need to be associated with a mother catchment. The hydrological parameters of this catchment will be inherited to the LID tributary area. Bear in mind that hydrological model B, Kinematic Wave Approximation, is the only hydrological model supporting the use of LIDs.
Fig. 5 - LID Deployment and unit geometry definition
The number of units is the quantity of LID units expected to be built on the catchment.
The width parameter is only applied to LID units which account for surface flow, being this the hydrological width of the runoff's outflow face on the LID capture area. The LID units using the hydrological width for the surface flow are roofs, porous pavement, trenches and vegetative swales.
It is important to distinguish in the deployment tab that there are two entries related to areas, nonetheless they represent different components. The LID Unit area represents the surface area of each replicate LID unit, i.e., if the user replicates 10 LID units with 10 square meters area each, the total LID unit area would be 100 square meters. The Collecting Area on the other hand is the tributary area connected to the LID unit whose runoff is treated (conveyed) by the LID unit, this area includes the LID unit area itself.
As can be seen in Fig. 6 the LID Area sums additional green areas to the entirety of the catchment enhancing the Impervious area with permeable surfaces. As previously mentioned, the runoff from the LID solution is routed using the same routing method and parametrization developed for the entire catchment.
Fig. 6 - LID Deployment and unit geometry definition
Once the LID units have been characterized and deployed to different catchments the runoff simulation can be executed. The anticipated outcome is a composite hydrograph that combines runoff from the LID solution with catchment runoff that has been reduced by area connected to the LID solution. In the example below it is possible to notice the change in the hydrograph due to an increased infiltration by implementing LID solutions in the catchment. This reduction in the hydrograph is achieved by enhancing the catchment with infiltration losses and increasing the storage capacity by accommodating LID structures. See Fig. 7
Fig. 7 - Runoff hydrograph resulting from LID deployment
Additionally to the runoff MKE 1D result file (*RR.res1d) the user can request a time series file (DFS0) which describes in detail the flow through the unit. Accounting for the different layers conforming the LID structure. The information included in the time series file will be the inflow from the surface, the flow between the layers (buckets), what is the storage level at a layer and finally the output from the unit to the surrounding soil. The time series content is dynamic and change depending on the type of LID unit deployed. There are 13 columns in the DFS0 file, they are described in detail in section 4.4 of our MIKE+ Collection System. User Guide. As they contain information related to flow infiltration, flow, storage depth, moisture content and volume it is recommended to do selection of items while presenting them making sure that the item unit is consistent. - see Fig. 8.
Fig. 8 - Content of the LID Unit DFS0 result time series
When all-time series items are presented, there are two Y axis shown for the graphic visualization. The Y Axis on the left presents the units for rate items, such as rain intensity, infiltration or evaporation loss rate. The Y Axis on the right is used to present items with accumulated format such as depths, water level and volume accumulation. - see Fig. 9.
Fig. 9 - Visual time series of the LID Unit DFS0 file
For example, analyzing a LID bioretention cell deployed to a semi-impervious catchment, one may need to discretize the time series in order to understand the water balance inside the LID unit. In the beginning of the simulation most of the rain precipitating over the LID unit is flown from the soil layer to the storage layer of the unit, as it can be seen in the Fig. 10 (column 2 vs column 8)
Fig. 10 - Tabular view of time series of the LID Unit DFS0 file
As the simulation continues the water flow changes through the different layers (LID unit buckets). As the inflow from the connected area flows in, the rain continues and the flow from the soil to the storage layers is increased. - see Fig. 11
Fig. 11 - Tabular view of time series of the LID Unit DFS0 file
Furthermore, it is feasible to observe that the unit's infiltration gets the majority of the rain and inflow from connected areas, while the drain flow takes some of the flow out of the unit. There is some storage internally in the unit, and it is noticeable that the flow rate between layers accounts for the sum of inflow and rain intensity rates placed between levels. - see Fig. 12.
Fig. 12 - Tabular view of time series of the LID Unit DFS0 file
#04 - Calculating the water balance for a LID unit – see Figs. 13-14
At a particular time step you may account for the inflow and rain as the sources of water in the LID unit and the processes such as evaporation and infiltration as outflows on top of this you may take into account if the unit contains an under-drain feature in this case the flow through the under drain will add up to the outflows. i.e., at the peak of the rain event the sum of inflows (red square) is 228 mm/hr and the source of all outflows (green square) is 228 mm/hr. At this time step the flow between layers (surface-soil-storage) is working at its maximum leak capacity (10 mm/hr) - see Fig.13 and 14.
Fig. 13 - Tabular view of time series of the LID Unit DFS0 file at the peak of rain
Fig. 14 - Bioretention cell soil infiltration settings
Conclusion
The primary goal of green infrastructure modeling using Low Impact Development is to:
- Evaluate how well the units can retain, detain and infiltrate stormwater runoff. This is achieved by quantifying volume reduction and reducing peak flows.
- Identify the most effective design options to maximize stormwater management performance by creating green infrastructure deployment scenarios.
- Determine the best management practices in terms of unit's type, catchment's location and units dimensioning to cope with increasing rainfall intensities to understand future conditions of resilience and adaptability.
When modelling LID units in MIKE+ make sure to understand the internal flow of water volume inside the units and review the dfs0 time series file. A look into the time series file will give a deeper understanding of the unit behavior.
In conclusion understanding the LID models will allow experts to optimize designs, assess performance and evaluate the environmental benefit of integrating green infrastructure into stormwater management plans.
FURTHER INFORMATION & USEFUL LINKS
Manuals and User Guides
MIKE+ Documentation Index
MIKE+ Collection System User Guide
Release Notes
MIKEPlus Release Notes