The risk of ignorance of DEM Height Reference System on coastal inundation modeling (the case of Jakarta)

As a result of continuing land subsidence which is also followed by sea level rise, some lowland coastal areas will experience coastal inundation. The lowlands will gradually become lower than sea level at high tide and even during normal conditions. Some areas where even it is not high tide would experience coastal inundation permanently in certain time. Coastal inundation modeling is important to create, so that we can map potential hazards and disaster risks. With coastal conditions that are usually inhabited by many people, become economic centers and others, then when coastal inundation occurs, material losses will certainly occur. To create a coastal inundation model, parameters are needed including the DEM. The coastal lowlands are represented from the DEM. From the corrected DEM for land subsidence, sea level rise and tidal hydraulic models, with some calculation we will get a coastal inundation model. The most important thing related to this model is the accuracy of the DEM and the height of the reference system. If we do not use an accurate DEM, and do not use the height reference system correctly then we will not get a reliable coastal inundation model. The impact is that we will not be get a correct map the hazards and disaster risks due to the coastal inundation. This paper will demonstrate the risk when we ignore the height reference system, so that we get a miss leading in the dangers and risks of coastal inundation in Jakarta as a case study area. As a result of investigation we can see with incorrect height reference system we found underestimate and or overestimate model and disputed the risk.


Introduction
The impact of land subsidence is seen clearly in cities or urban areas in the coastal region.These impacts can be in the form of coastal inundation (see figure 1), infrastructure damage, expansion of flooding, and sea water intrusion to a decrease in environmental quality [1, 2, and 3].The significant impact of land subsidence is categorized as an ecological disaster.Million of dollar is expected need to be spent due to that ecological disaster in one urban area.In this regard, mitigation and adaptation are very important in reducing disaster risk.In the future if actions taken are less, there will be more significant disaster.One important step on mitigation and adaptation is understanding the coastal inundation through modelling (see also figure 1).Coastal inundation modelling is important to do so that we get potential hazards and disaster risks from the occurrence of it.With coastal conditions that are usually inhabited by many people, become economic centres and others, then when coastal inundation occurs, material losses will certainly occur as well.1245 (2023) 012020 IOP Publishing doi:10.1088/1755-1315/1245/1/012020 2 By definition, Land Subsidence is defined as a decrease in the surface of the ground with respect to a reference system of height such as the sea surface, geoid or ellipsoid surface.Geodetic measurements (e.g., Leveling, GNSS and InSAR) can identify rates per year and magnitude even is small value [4, 5, 6, 7, 8, and 9] in certain periods of times in some part on the surface of the ground.In the term of rate, generally 1-20 centimeter per year and a total 1-4 meter are being observed.

Figure 1. Documentation of coastal inundation in the field (case of Jakarta) along with example of coastal inundation modeling
To create a coastal inundation model, parameters are needed including the Digital Elevation System (DEM), land subsidence information, sea level rise information and tidal wave to define height reference system.The coastal lowlands are represented from the DEM.From the corrected DEM for land subsidence, sea level rise and tidal hydraulic models including definition of height reference system, we will get a coastal inundation model (see figure 2 for illustration).To get accurate data from DEM, leveling surveys are usually used.However, this survey will take a very long time for a large area.Currently the use of LiDAR is widely used to obtain an accurate DEM, although it cannot replace survey leveling.To obtain good land subsidence information, GNSS and InSAR surveys are usually carried out.Meanwhile to get good sea level rise data obtained from satellite altimetry.The most important thing related to coastal inundation modeling are the accuracy of the DEM and the correct choose of height of the reference system.If we do not use an accurate DEM, and do not use the height reference system correctly (in this case it should be High Tide or Mean Sea Level), then we will not get a good or reliable coastal inundation model.The impact is that we will not be correct map the hazards and disaster risk due to the coastal inundation.This paper will demonstrate the risk when we ignore the height reference system, so that we get a miss leading in the dangers and risks of coastal inundation in Jakarta as a case study area.

Data and methods
As mentioned above, when we ignore the height reference system, so that we get a miss leading in the dangers and risks of coastal inundation in Jakarta as a case study area.The ignorance can come from ignorance of the various high reference systems or mistakes in choosing the high reference system.Conceptually there are in fact many high reference systems.Some of them are the High Tide System, Mean Sea Level (MSL), Geoid, Low Tide, Chart Datum, Ellipsoid Surface, and many other systems.For illustrations of various height reference systems, see Figure 3 below.As mentioned above, for modeling coastal inundation the high system should have reference to High Tide or at least Mean Sea Level.In this way we will not find cases of overestimate or under estimate.

Figure 3. Illustration of difference type of height reference system
Mean Sea Level is obtained from the average tide over some periods of observation (e.g.Ideally for about 18 years), while for high tide and low tide is obtained from the tidal range and the average maximum and minimum tide.The geoid is obtained from gravity measurements, while the Ellipsoid Surface is determined from the Earth Ellipsoid and Geodetic Datum modelers.This paper will show the use of several different reference systems for the formation of coastal inundation modeling in Jakarta.The high systems used include Ellipsoid Surface, Geoid EGM2008, Mean Sea Level and High Tide.From the use of some of these systems, we will both understand that when we ignore the height reference system, so that we get a miss leading in the dangers and risks of coastal inundation in Jakarta as a case study area.
The DEM data used comes from modified LiDAR data.This DEM data is quite accurate in modeling coastal inundation.DEM has an accuracy of about 20-40 centimeters.The selection of an accurate DEM is important so that a coastal inundation simulation using several difference height reference systems will give clear results, including when it is correlated with the occurrence of coastal inundation in the field, the results will also be appropriate.

Result and discussion
The results of the study show that when we use the EGM2008, Ellipsoid Surface and Geoid, we will get an underestimated model for the case in Jakarta.Seems with this parameter, the coastal inundation seems doesn't exist.Conversely, when we use high tide and Mean Sea Level, the coastal inundation model will be reasonable.When validated with inundation events in the field, the most suitable is when we use the High Tide model.The complete research results can be seen in the following paragraphs, complete with illustrations in Figures 4, 5 and 6.It should be noted that apart from using the right reference system, we still must pay attention to other accuracy, such as the optimum accuracy of DEM and the right value of land subsidence information.
Figure 4a shows when we use the ellipsoid surface height reference system in coastal inundation modeling in Jakarta.In the model results there is no potential for coastal inundation in Jakarta even though there is indeed in the field.The use of an ellipsoid surface shows an underestimate result.Figure 4b shows when we use the EGM2008 Geoid height reference system in coastal inundation modeling in Jakarta.In the same way as the model using the ellipsoid surface, it can be seen that in the model results there is no potential for coastal inundation in Jakarta even though there is facts in the field.In this case the use of the 2008 EGM Geoid shows underestimated results.Figures 4c and 4d use the Mean Sea Level and High Tide as Height reference systems, respectively.From the modeling results, there is a potential for coastal inundation in Jakarta.If it is correlated with events in the field, this model is generally suitable.The most appropriate height reference system for modeling coastal inundation is the High Tide System Model d on figure 4. By using logic, when high tides occur in the field, there will be potential for flooding in several places whose positions are already lower than sea level.It should be noted here that the High Tide system is very local in nature, it will be different in each region.For this reason, good tide monitoring will be the key.
Figures 5a and 5b respectively show when we use the ellipsoid surface height reference system and the 2008 Geoid EGM in coastal inundation modeling in Jakarta overlaid with plotting of inundation events in the field.It can be seen that in the model results there is no potential for coastal inundation in Jakarta even though there is clear fact in the field.The use of the ellipsoid surface and Geoid EGM 2008 once again shows underestimated results.Figures 5c and 5d respectively use the High Tide and Mean Sea Level high reference system in the modeling of coastal inundation in Jakarta overlaid with plotting of inundation events in the field.From the modeling results, there is a potential for coastal inundation in Jakarta.If it is correlated with events in the field, this model is suitable especially when we used High Tide.To get a clearer picture that the most reliable way of modeling is to use the High Tide system, you can see the image below showing the modeling results along with the documentation of coastal inundation in the field.By using logic, when high tides occur in the field, there will be potential for flooding in several places whose positions are already lower than sea level.It should be noted here that the High Tide system is very local in nature, it will be different in each region.For this reason, good tide monitoring will be the key.So from our investigation, it is very clear that the model generated from the High Tide system provides results that are suitable for conditions in the field, so that we can use the results of the model for disaster risk mapping, including making decisions on how to reduce disaster risk.Once again, we can imagine that if the model is underestimated or overestimated, disaster risk reduction efforts can be misguided.

Conclusions
One important step on mitigation and adaptation against land subsidence and coastal inundation is understanding the coastal inundation through modelling.Coastal inundation modelling is important to do so that we get potential hazards and disaster risks from the occurrence of it.With coastal conditions that are usually inhabited by many people, become economic centres and others, then when coastal inundation occurs, material losses will certainly occur.
The most important thing related to coastal inundation modeling are the accuracy of the DEM and the correct choose of height of the reference system.If we do not use an accurate DEM, and do not use the height reference system correctly, then we will not get a good coastal inundation model.The impact is that we will not be correct map the hazards and disaster risk due to the coastal inundation.The prove we can see from research above in the case of Jakarta.The most appropriate height reference system for modeling coastal inundation is the High Tide System.IOP Publishing doi:10.1088/1755-1315/1245/1/0120207 So, do not ignore in using the heigh reference system when we modelled coastal inundation.For some cases we still see publication with ignorance in the use heigh reference system, in this case model produced seem overestimate or under estimate.Latter on there will be miss calculation of real risk of disaster.

Figure 2 .
Figure 2. The figure shows how to create coastal inundation model.From the corrected DEM for land subsidence, sea level rise and tidal hydraulic models including definition of height reference system, we will get a coastal inundation model

Figure 4 .
Figure 4. Visualization of coastal inundation each by using height reference system of ellipsoid surface (a), Geoid EGM 2008 (b), Mean Sea Level (c) and High Tide (d)

Figure 5 .
Figure 5. Visualization of coastal inundation along with plotting coastal inundation in the field each by using heigh reference system ellipsoid surface (a), Geoid EGM 2008 (b), Mean Sea Level (c) and High Tide (d)

Figure 6 .
Figure 6.Visualization of coastal inundation along with plotting coastal inundation in the field and photo field documentations