Japan Agency for Marine-Earth
Science and Technology

The CaMa-Flood model description

Dai Yamazaki
JAMSTEC – Japan Agency for Marine-Earth Science and Technology
4th Sep, 2015

Concepts of the CaMa-Flood development
CaMa-Flood is designed to achieve the following 3 requirement:[1] A global-scale model of all rivers, wetlands and lakes on the Earth.
- Simulating floods in all river basins, including ungauged/data-sparse regions.
- Targeting coupling with global climate models or earth system models.
⇒ CaMa-Flood includes global topography data for global simulations.
⇒ Assumed input to CaMa-Flood is runoff from global land models.

[2] Very high computational efficiency.
- Simulation must be fast enough for use in real-time flood forecast.
- Computational cost must be light enough as a sub-model of a complex GCM/ESM.
⇒ 1-D river routine with sub-grid flood scheme. (2-D hydrodynamics is very heavy)[3] Realistic surface water dynamics.
- Water moves from high place to low place. This is the basic of hydrodynamics.
- Simulation of water level & inundated area, in addition to river discharge.
⇒ Hydrodynamic flow equation. (Kinematic wave is not applicable in flat regions)
CaMa-Flood is the only hydrodynamic model which satisfies the 3 requirement.
However, the model became complex to achieve fast and realistic global simulations.
If any of above requirements is not needed, a simpler model may be suitable for your research.

CaMa-Flood (v3.6) development: Model Assumptions[1] Global topography & hydrography data
- Ground elevation is given by SRTM3 DEM (+SRTM30 above 60N).
- River networks are given by HydroSHEDS flow direction map (+GDBD above 60N).

↑90 m elevation[SRTM3]Elevation is adjusted to satisfy the condition that
downstream is always not higher than upstream
along the HydroSHEDS river networks. [Yamazaki et al., 2012]
Each pixel is assumed to have one flow direction
toward a neighboring pixel to represent river networks.

90 m Flow Direction[HydroSHEDS] →

8:NW

1:N

2:NE

7:W

-

3:E

6:SW

5:S

4:SE

CaMa-Flood (v3.6) development: Model Assumptions[2] Unit-catchment & sub-grid topography- River basins are divided to unit-catchments in order to reduce computational cost.
- *One unit-catchment is assigned to each lon-lat grid box for easy data handling.
- Water stage (level, area) is diagnosed from water storage using sub-grid topography.
- Uniform water level in a unit-catchment. River and floodplain water levels are same.
When 60% of catchment
area is inundated
Water level is 12 m

L
SfDr

Sf

Sr

DfW

ZAc
AfB
Ac
Af

Sub-grid topography

Floodplain Elevation Profile

Sub-grid topography is represented by 6 parameters.
[1] Catchment area, [2] channel length [3-4] channel width + depth,[5] Floodplain elevation profile, [6] Bank-top elevation

These topography parameters (except channel width + depth) are
derived from the high-resolution DEM.

Unit-catchments (the Amazon River)

Floodplain profile is given by a non-parametric increasing functionof resampled floodplain heights above river channels.
- No levee, no depressions in floodplains

CaMa-Flood (v3.6) development: Model Assumptions[3] River network map & discharge calculation.
- Water exchange between unit-catchments only occurs along the river network map.
- Only one downstream is assigned to each unit-catchment by the river network map.
- Discharge to downstream is calculated separately for channel and floodplain.
q: discharge h: depth z: bed elevation
N: roughness R: hydraulic radius isfc: water slope
Neglect advection

gn 2 Q Q  Q 2  Qhz


 gA
 gA

04/3

x  A t

x

x
RA


qn 1 2i 1 2FTCS Differentiationqin1122  gh flowis t(1  gh flown 2 t qin1122 h flow

10 / 3)The shallow water momentum equation is approximated to a local inertial form.
Downstream of each unit-catchment [Bates et al., 2010; Yamazaki, Tanaka & Bates, 2015]
is indicated by the river network mapFloodplain
River ChannelLSfDfDrZWAcAfSSfSrBAc
AfRiver and floodplain discharget  ti S  kti

Upstream

Qkt t  Qit t  Aci Rit t

S: storage Q: discharge (channel + floodplain)
Ac: catchment area R: Input runoff
Water storage in next time step is updated using mass-balance equation.

CaMa-Flood (v3.6) development: Model Assumptions[4] Multi-downstream connectivity scheme- Additional downstream directions can be added to the river network map
to represent bifurcation channels and 2-D floodplain flow. [Yamazaki et al., GRL 2014]

River Network with Multi-downstream

Delineation of bifurcation pathways

High-resolution DEM is analyzed to find potential flow pathways which connect two unit-catchments
without upstream-downstream relationship on the original river network map.
Flow pathways are divided to channel bifurcation and overland connections using the SWBD water mask.
Discharge calculation for these sub-channels is same as main channels (i.e. local inertial equation).

CaMa-Flood (v3.6) development: Model Assumptions[5] Channel cross-section parameters- Channel depth + width are given by a power-low function of mean flow.
- Channel width can be given from a satellite water body map (optional).

Channel depth estimated by power-low

Global Width Database for Large Rivers

There is no global measurement of river channel depth, thus is estimated by a power-low equation.
Channel width is calculated based on the SWBD water map [Yamazaki et al., 2014].

CaMa-Flood (v3.6) development: Model Assumptions[6] Diagnostic downscaling of flood depth- Flood depth can be diagnostically downscaled to the high-resolution DEM
by post processing after the hydrodynamic simulation.
Floodplain
River ChannelLSfDfDrZ

SfSrW
Ac
AfBAc
Af

Water depth above river channel (FLDDPH)
DEM’s height above river channel (FLDDIF)“High-resolution DEM’s height above river channel” and “simulated water depth above river channel”
are compared to calculate water depth of each high-resolution pixel.

CaMa-Flood (v3.6) development: Model Assumptions[7] Other assumptions (or limitations)- Evaporation from water surface and infiltration to soil are not considered.
- Lake/water fall schemes are not yet developed.
These are solved by same equations as rivers.

- No river bank/levee is represented.
This is partly due to depression is not assumed in floodplains.

- No human activities (e.g. dam regulation, irrigation, canals, weirs).
- Water “disappears” at river mouth. No water balance calculation at inland seas.
- Topography does not change in time. No sedimentation or geomorphology process.
- Runoff is given from external land models. No surface-ground waters interaction.

What can(‘t) CaMa-Flood simulate?
Flood wave propagation
No reservoir
operation

Floodplaininundation

DEM error
due to building
No bank defense
No lakebathymetry
Backward flow bywater level reversal
Striping errors
in SRTM3 DEM

Bifurcating
channel flow

Summary: CaMa-Flood model development[1] Concepts: Fast but realistic global hydrodynamic simulation- Global 2D hydrodynamic models (e.g. LISFLOOD-FP) is best for accurate simulations
because of explicit flow calculation on high-res pixels without sub-grid assumptions.
- But 2D hydrodynamic models are computationally heavy. We need faster simulations.
- CaMa-Flood overcomes this difficulty by adopting following assumptions:<1> Sub-grid flood inundation scheme<2> 1D river network + multi-downstream connectivity<3> Diagnostic downscaling by post-processing simulated water level- The dynamics of CaMa-Flood is now mostly similar to 2D hydrodynamic models.

[2] Limitations (both in CaMa-Flood and 2D hydrodynamic models)- Global topography dataset (DEM, Hydrography) still has large errors.
- Channel bathymetry (width + depth) are not well represented.
- Large uncertainty in input runoff forcing.
- Only river & floodplain flow are solved. No lake, human activity, sediment, etc.