Urban Flood Risk Mitigation model¶

Introduction¶

Flood hazard comes from different sources, including: riverine (or fluvial) flooding, coastal flooding, and stormwater (or urban) flooding - the focus of this InVEST model. Natural infrastructure can play a role for each of these flood hazards. Related to stormwater flooding, natural infrastructure operates mainly by reducing runoff production, slowing surface flows, and creating space for water (in floodplains or basins).

The InVEST model calculates the runoff reduction, i.e. the amount of runoff retained per pixel compared to the storm volume. For each watershed, it also calculates the potential economic damage by overlaying information on flood extent potential and built infrastructure.

The model¶

How it works¶

Runoff production and runoff attenuation index¶

For each pixel $i$, defined by a land use type and soil characteristics, we estimate runoff $Q$ (mm) with the Curve Number method:

(106)¶\[\begin{split}Q_{p,i} = \begin{Bmatrix} \frac{(P - \lambda S_{max_i})^2}{P + (1-\lambda) S_{max,i}} & if & P > \lambda \cdot S_{max,i} \\ 0 & & otherwise \end{Bmatrix}\end{split}\]

Where $P$ is the design storm depth in mm, $S_{max,i}$ is the potential retention in mm, and $\lambda \cdot S_{max}$ is the rainfall depth needed to initiate runoff, also called the initial abstraction ($\lambda=0.2$ for simplification).

$S_{max}$ (calculated in mm) is a function of the curve number, $CN$, an empirical parameter that depends on land use and soil characteristics (NRCS 2004):

(107)¶\[S_{max,i}=\frac{25400}{CN_i}-254\]

The model then calculates runoff retention per pixel $R_i$ as:

(108)¶\[R_i=1-\frac{Q_{p,i}}{P}\]

And runoff retention volume per pixel $R\_m3_i$ as:

(109)¶\[R\_m3_i=R_i\cdot P\cdot pixel.area\cdot 10^{-3}\]

With $pixel.area$ in $m^2$.

Runoff volume (also referred to as “flood volume”) per pixel $Q\_m3_i$ is also calculated as:

(110)¶\[Q\_m3_i=Q_{p,i}\cdot pixel.area\cdot 10^{-3}\]

Calculate potential service (optional)¶

First, $\text{Affected.build}$, the sum of potential damage in $ to built infrastructure, is calculated for each watershed or sewershed $W$:

(111)¶\[\text{Affected.build}_W = \sum_{b ∈ B}a(b,W)·d(b)\]

where

$b$ is a building footprint in the set of all built infrastructure $B$
$a(b,W)$ is the area in $m^2$ of the building footprint $b$ that intersects watershed $W$
$d(b)$ is the damage value in $$/m^2$ (from the Damage Loss Table) for building $b$’s type

We then calculate $\text{Service.built}$, an indicator of avoided damage to built infrastructure, for each watershed $W$:

(112)¶\[\text{Service.built}_W=\text{Affected.build}_W·\sum_{i ∈ W}R\_m3_i\]

where

$i$ is a pixel in watershed $W$
$R\_m3_i$ is the runoff retention volume on pixel $i$

$\text{Service.built}$ is expressed in $$·m^3$. It should be considered only an indicator, not an actual measure of savings.

Limitations and simplifications¶

Runoff production: the model uses a simple approach (SCS-Curve Number), which introduces high uncertainties. However, the ranking between different land uses is generally well captured by such an approach, i.e. that the effect of natural infrastructure will be qualitatively represented in the model outputs. Future work will aim to include a routing over the landscape: ideas include TOPMODEL (there is an R package), UFORE (used in iTree), CADDIES, etc

Valuation approaches: Currently, a simple approach to value flood risk retention is implemented, valuing flood risk as the avoided damage for built infrastructure. Alternative approaches (e.g. related to mortality, morbidity, or economic disruption) could be implemented. Another service metric is the affected population, i.e. the number of people at risk from flooding. This could focus on vulnerable groups only, e.g. related to age, language, etc. See Arkema et al., 2017, for a review of social vulnerability metrics. This metric can be calculated by summing the population in the intersection of the watershed and the flood-prone area.

Data Needs¶

Spatial layers for Urban Flood Mitigation may have different coordinate systems, but they must all be projected coordinate systems, not geographic. Raster inputs may have different cell sizes, and they will be resampled to match the cell size of the land use/land cover raster. Therefore, raster model results will have the same cell size as the land use/land cover raster.

Workspace (required): Folder where model outputs will be written. Make sure that there is ample disk space, and write permissions are correct.

Suffix (optional). Text string that will be appended to the end of output file names, as “_Suffix”. Use a Suffix to differentiate model runs, for example by providing a short name for each scenario. If a Suffix is not provided, or changed between model runs, the tool will overwrite previous results.

Watershed Vector (required). shapefile delineating areas of interest, which should be hydrologic units: watersheds or sewersheds.

Depth of rainfall in mm (required). This is $P$ in equation (106).

Land Cover Map (required). Raster of land use/land cover (LULC) for each pixel, where each unique integer represents a different land use/land cover class. All values in this raster MUST have corresponding entries in the Land Cover Biophysical Table. The model will use the resolution of this layer to resample all outputs. The resolution should be small enough to capture the effect of green areas in the landscape, although LULC categories can comprise a mix of vegetated and non-vegetated covers (e.g. “residential”, which may have 30% canopy cover, and have biophysical table parameters that change accordingly)

Soils Hydrological Group Raster (required). Raster of categorical hydrological groups. Pixel values must be limited to 1, 2, 3, or 4, which correspond to soil hydrologic group A, B, C, or D, respectively (used to derive the CN number)

Biophysical Table (required). A .csv (Comma Separated Value) table containing model information corresponding to each of the land use classes in the Land Cover Map. All LULC classes in the Land Cover raster MUST have corresponding values in this table. Each row is a land use/land cover class and columns must be named and defined as follows:

lucode: Land use/land cover class code. LULC codes must match the value column in the Land Cover Map raster and must be integers and unique.

Curve number (CN) values for each LULC type and each hydrologic soil group. Column names should be: CN_A, CN_B, CN_C, CN_D, which the letter suffix corresponding to the hydrologic soil group

Built Infrastructure Vector (optional): shapefile with built infrastructure footprints. The attribute table must contain a column ‘Type’, with integers referencing the building type (e.g. 1=residential, 2=office, etc.)

Damage Loss Table (optional): Table with columns “Type” and “Damage” with values of built infrastructure type (see above) and potential damage loss (in $/$m^2$)

Interpreting Outputs¶

Parameter log: Each time the model is run, a text (.txt) file will be created in the Workspace. The file will list the parameter values and output messages for that run and will be named according to the service, the date and time. When contacting NatCap about errors in a model run, please include the parameter log.

Runoff_retention.tif: raster with runoff retention values (no unit, relative to precipitation volume). Calculated from equation (108).

Runoff_retention_m3.tif: raster with runoff retention values (in $m^3$). Calculated from equation (109).

Q_mm.tif: raster with runoff values (mm). Calculated from equation (106).

flood_risk_service.shp: Shapefile with results in the attribute table:

rnf_rt_idx: average of runoff retention values ($R_i$) per watershed

rnf_rt_m3: sum of runoff retention volumes ($R\_m3_i$), in $m^3$, per watershed.

flood_vol: The flood volume (Q_m3, equation (110)) per watershed.

aff_bld: potential damage to built infrastructure in $, per watershed. Only calculated when the Built Infrastructure Vector input is provided.

serv_blt: $Service.built$ values for this watershed (see equation (112)). An indicator of the runoff retention service for the watershed. Only calculated when the Built Infrastructure Vector input is provided.