Stopbank mapping and control

Satellite(s) LiDAR regional capture, Sentinel-2, Planet Dove, Skysat, WorldView.	Monitoring element LiDAR backscatter, land surface reflectance.

Satellite(s)

LiDAR regional capture, Sentinel-2, Planet Dove, Skysat, WorldView.

Monitoring element

LiDAR backscatter, land surface reflectance.

Description technique

A LiDAR derived digital terrain model can be utilised to map estimations of relative stop bank crest height and highlight any locations where settlement/damage/construction issues have resulted in an elevation below target specification. Choung (2014) describes a mapping approach combining 1 m LiDAR with high resolution (~0.25cm) aerial multispectral imagery (equivalent to the regional LiDAR and aerial photography programs currently run in NZ). Transects can be generated using stream network data, allowing cross sections to be generated at regular intervals along channels. By modelling channel hydrology, design hydrographs can be used to derive estimates of peak flow. Tonkin and Taylor (2018) used a hydrologic model to relate these values to LiDAR derived cross sections at a horizontal resolution of 5 m.

LiDAR derived stream network data can be used to generate regular cross sections for which peak stop band crest height can be calculated.

Accuracy / Resolution

Variable spatial and temporal resolution according to sensors.

Case study

Tonkin Taylor (2018) provided a review of Tauranga & Taupo flood protection scheme (Waikato).
Example of stop bank transect using 2019 LiDAR Regional Capture (Indufor):
https://induforauckland.users.earthengine.app/view/northland-lidar-stopbank-demo

Also fits domain

UAV

Benefits

Allows for synoptic, empirical review of stop bank status with greater granularity than what may be reasonably achieved with field measurement.
Allows optimization of monitoring resource.

Limitations

LiDAR capture represents a snapshot, forming a baseline for which other data sources will be required for ongoing monitoring.
Open access or commercial optical data may be used for this purpose but will likely be insufficient for monitoring stop bank condition.
Recent developments in UAV derived photogrammetry show dense point clouds can be derived of comparable accuracy to traditional aerial LiDAR capture, though this does degrade with vegetation cover (Salach et al., 2018).
Cross section methodology 1-dimensional, accuracy depends on granularity of cross sections.
Stop banks are generally vegetated to some degree, which can introduce uncertainty into the LiDAR terrain model by reducing the number of true ground returns.

Applicability for Northland

Yes.

Would make an ideal use case for recent Northland LiDAR capture and extension to current flood mapping programme.

Techniques applying optical data will be limited in coverage and temporal granularity by the persistent cloud cover in the region, particularly during the winter months. Mature cloud-masking techniques are directly available for open access multispectral data (e.g. Landsat and Sentinel-2). When using commercial data, care must be taken to ensure that there is sufficiently cloud free imagery available, as cloud masking is not as mature and ordering a large volume of imagery to ensure complete cloud free coverage between multiple observations can become cost prohibitive.

Publication references

Özer, I.E., van Leijen, F.J., Jonkman, S.N. and Hanssen, R.F., 2019. Applicability of satellite radar imaging to monitor the conditions of levees. Journal of Flood Risk Management, 12(S2), p.e12509.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/jfr3.12509

Choung, Y., 2014. Mapping levees using LiDAR data and multispectral orthoimages in the Nakdong river basins, South Korea. Remote Sensing, 6(9), pp.8696-8717.

https://www.mdpi.com/2072-4292/6/9/8696

Salach, A., Bakuła, K., Pilarska, M., Ostrowski, W., Górski, K. and Kurczyński, Z., 2018. Accuracy assessment of point clouds from LiDAR and dense image matching acquired using the UAV platform for DTM creation. ISPRS International Journal of Geo-Information, 7(9), p.342.

https://www.mdpi.com/2220-9964/7/9/342

Tonkin and Taylor. 2018. Tauranga Taupo - service level review report. Prepared for
Waikato Regional Council. 57 p.

https://www.waikatoregion.govt.nz/assets/WRC/WRC-2019/TR201822.pdf

Other comments or information

Potential use of spaceborne SAR monitoring for stop bank elevation monitoring, reducing cost and increasing the update frequency possible when compared to airborne LiDAR (Özer et al, 2019).