Michele Dominici
email: michele.dominici@irisa.fr

Michel Banâtre
email: michel.banatre@inria.fr

A case study of metamorphic housing: the on-demand room

Keywords: Metamorphic housing, sustainable city, resource efficiency, smart home Habitat métamorphique, ville durable, économies de ressources, habitat intelligent

Metamorphic housing is a vision of homes that perform dynamic adaptations of their physical structures, so as to assist their occupants' activities and life cycle.

This paper presents the on-demand room. It is a case study of metamorphic housing, where apartments of a building can be dynamically enlarged when the need arises. The additional space is to all intents and purposes an extension of the apartment, as devices and networks are interconnected and customized.

We present the on-demand room and a model of computation formalizing its behavior and then illustrate the research problems that have to be solved to make it a reality. Finally, we describe our plans to demonstrate the on-demand room in an immersive virtual reality platform.

Introduction

As many other countries in the world, France is going through socio-demographic evolutions, like growth of life expectancy and consequent increase in the number of elderly people, urbanization and resource scarcity. Households experience financial restrictions, while housing costs increase with the raise of real estate and energy prices.

Important questions arise concerning the future of housing policies and ways of living. We observe novel initiatives like participative housing and developing behaviors, including house-sharing, teleworking and longer stay of children in parents' homes.

To tackle the challenges raised by these emerging phenomena, future homes will have to be modular, upgradeable, comfortable, sparing of resources. They should be integrated in the urban context and exchange information with other homes, contribute to reducing the distances to be covered daily and respect the characteristics of the territory where they are located.

We proposed the vision of metamorphic housing as contribution to reaching the aforementioned objectives [START_REF] Baldoni | Comfort-oriented metamorphic house (COMETAE)[END_REF]. Enabled by Information and Communication Technologies (ICT) and adaptive building elements, domestic environments will modify their shape and behavior to support activities and changes during the life cycle of occupants, increase comfort and optimize the use of resources. The same physical spaces will be transformed for different uses, giving inhabitants the illusion of living in bigger, more adapted and more comfortable places.

This paper presents a case study of metamorphic housing, addressing the goals of saving space and energy in an apartment building, while preserving residents' comfort: the on-demand room, illustrated below.

Dynamically extending homes: the on-demand room

The principle of the on-demand room is that apartments of a building can be dynamically enlarged when the need arises. To this end, a floating space is dynamically conveyed from an apartment to another.

Differently from the process of acquiring an additional all-purpose space, like a meeting room in an office building, the on-demand room becomes to all intents and purposes an extension of the apartment. This is obtained by connecting the devices and networks of the on-demand room with those in the apartment, and by customizing the room so that it looks and behaves as if it belonged to the apartment.

The on-demand room can be used as bedroom, teleworking space or any other function. It can also be shared among apartments, to become a place for socialization or co-working.

Depending on the specific deployment, the on-demand room can allow to:

• mutualize built space between residents;

• add flexibility to buildings by enlarging or contracting apartments depending on occupants' characteristics, needs and phases of their life cycle;

• temporarily create private spaces in shared houses or apartments;

• strengthen relations between generations and facilitate in-home care of elderly people, by enabling households to combine the benefits of having a private extra room and a shared space for socializing with neighbors.

Section 2 illustrates in detail the on-demand room, while Section 3 presents the scientific goals to achieve in order to make it a reality. Section 4 describes the rules and means for handling multiple requests of extension, while Section 5 describes the implementation and evaluation strategies. Finally, Section 6 presents related work and Section 7 concludes the paper by illustrating a work plan and some perspectives of generalization of the approach.

Description of the case study

This section presents our case study, by illustrating the dynamic transformations and customizations happening when the space is conveyed from an apartment to another (cf. § 2.1) and the proactive actions that are performed to anticipate future occupation (cf. § 2.2).

In order to simplify the presentation, let us visualize the on-demand room concept, by considering a building floor hosting two apartments and a floating space in between them, as illustrated in Fig. 1.

Dynamic customization of the on-demand room

As we said, the difference between our case study and simply renting an additional room is that the on-demand room becomes an extension of the apartment that requested it. This requires interconnecting the networks and devices of the two spaces and dynamically customizing the room, so that occupants feel like they were in their own dwelling.

Below is a description of the adaptation strategies that dynamically change the on-demand room into an extension of the apartment requesting it.

Physically connecting the spaces When an apartment requests an extension, the floating space is assigned to it. A door appears, allowing occupants to access the space, which thus becomes a new room of the apartment: the on-demand room. This is illustrated in Fig. 2, showing a door connecting the room with apartment A.

Reproducing the owning apartment's ambience

To make occupants feel they did not leave their apartment at all, the room adapts its atmosphere so as to resemble the apartment that requested it: lighting conditions, temperature, music and other aspects of the apartment's ambience can be reproduced automatically.

Controlling simple devices When the room is assigned to a particular apartment, devices like light bulbs, electric blinds or HVAC systems require to be controlled from the apartment that owns the room at the moment, via wall-mounted switches, the apartment's domotic control unit or applications running on apartment's mobile devices (smartphones, tablets, etc.). Additionally, the room itself has a domotic control unit that can manage these devices, for instance when customizing the room ambience or when proactively acting by anticipating future requests (cf. § 2.2).

Interconnecting internal networks Wireless and wired data infrastructures (Ethernet, Wi-Fi, ZigBee, telephone, etc.) of the room and of the apartment that temporarily owns the room must be interconnected, so that equipment (e.g., media streaming and playing devices) can communicate with each other. To preserve occupant's privacy, devices in the room should use the owning apartment's Wi-Fi network, so that data are encrypted with that network's private key, not shared with other apartments.

Redirecting context information Sensor data providing information like presence and temperature must be redirected to the right apartment, for applications like HVAC management and security. For instance, if all the occupants of the apartment move to the on-demand room, the domotic unit must know that someone is still home (and not turn the heating off). Pushing settings to complex devices Devices located in the room, like programmable or learning thermostats, television sets, home theater systems and receivers, digital photo frames, computers, gaming consoles and media players contribute to the comfort and entertainment of occupants and are generally highly customized to user preferences. These devices have to be reprogrammed dynamically with the appropriate settings when the room's owner changes.

Furthermore, these devices sometimes maintain the history of use (e.g., last channel seen on TV, music playlist on a media player). These data have to be stored separately for each apartment, so as to give users the impression of being the only owner of the devices and to avoid disclosing private information.

A simple illustration of some possible aspects of the customization is presented in Fig. 3. The picture shows that the same light bulb is controlled by a switch located in apartment A or B, depending on which one owns the room at that moment. Similar considerations apply to the room's Wi-Fi range extender, presented in the lower part of Fig. 3. The device is dynamically connected to the appropriate router, so as to provide the room's occupants with connection to their apartment's internal network and, through that, to the Internet.

Proactive behavior of the on-demand room

We now illustrate the proactive functionalities of the room, that is, the actions that it performs to anticipate future events. Proactivity is required to solve room usage conflicts before they happen and for actions whose effects are delayed in time. The underlying assumption is that there exist regularities in the time and duration of occupations, due to user habits. This allows a whole new set of rules for the management of the on-demand room, as illustrated below.

Proactively solving conflicts When allocating the room, regular occupations should have higher priority than impromptu requests, so as to foster appropriation and comfort. If an apartment usually requests the on-demand room for teleworking from 2 to 6 pm every working day, priority will be given to this habit over other requests.

If other occupants ask the room at 1:55 pm, the room will be allocated to them anyway, as there is no certainty that the regular request will actually be issued today and at the usual time. However, users will be informed about the higher priority of the possible future request and, when this is issued, they will be asked to leave the on-demand room. The rule is valid unless the apartment that issued the usual request has reached its usage quota for the day/week/month/etc.

Proactive customization As we said, the room adapts its atmosphere so as to resemble the apartment that uses it, giving occupants the impression that they did not leave their dwelling at all. Figure 3: The room's equipment must be dynamically connected to the apartment that temporarily owns it Some of the customizations require sizeable anticipation to produce remarkable effects by when the request is actually issued, like when managing heating, ventilation and air conditioning (HVAC). Giving the order when the room is requested would cause the occupant to wait a certain time before the preferred conditions are reached.

For this reason, some actions are realized proactively, by anticipating regular requests. For instance, heating is turned on early enough for the room to reach the preferred temperature of the user, before the request is actually issued.

Scientific goals and challenges

This section illustrates the scientific goals to pursue in order to make the on-demand room a reality. As we explained previously, when an apartment needs to be extended, a number of transformations are required to make the floating space belong to that apartment. The first goal is thus to dynamically reconfigure the devices and networks of the room, in order to connect them with the apartment that now owns them and to adapt their behavior to the respective preferences. This is described in § 3.1.

We also said that it is of prime importance that room usage conflicts are solved before they occur and that some actions are performed proactively, in order to make up for their delayed effect. The second goal to achieve is thus the prediction of future requests, obtained by discovering and learning user habits, as illustrated in § 3.2.

Goal 1 -Making the on-demand room belong to an apartment

Whenever an apartment is extended, a complex set of reconfigurations are required to make the on-demand room belong to the apartment and to give occupants the impression of owning the room. These were described in § 2.1 and raise the following research questions:

• How to dynamically provide an apartment with exclusive control over the room's equipment and designate it as the receiver of sensor and metering data? This reconfiguration should be entirely transparent for the equipment and sensors, so that real-world deployments can be simplified by using off-the-shelf devices.

• Conversely, how to dynamically reprogram the devices that have to substantially change their behavior after a reconfiguration? For instance, the network the Wi-Fi range extender is connected to has to change, as well as the devices allowed to control the room's blinds.

• Finally, how to make apartments' and room's equipment interoperate when they differ in technologies and/or vendors? This happens frequently in real deployments, where apartments and, possibly, the room have different owners, who make different choices in terms of brands and technologies. For instance, what if apartment A uses EnOcean wireless light switches [START_REF]EnOcean alliance -building automation experts[END_REF], apartment B uses wired light switches and the room's light to control is a Wi-Fi light bulb?

These research problems will be addressed by using a modular architecture for the on-demand room and by dynamically reprogramming appliances, as explained in the two following subsections.

Abstracting physical devices

A partial solution to the aforementioned research problems is given by adopting a modular architecture: the on-demand room's equipment is not directly connected to any apartment. An abstraction module provides the decoupling between the room and the apartments. When the room gets a new owner, the devices and sensors are automatically connected to the new apartment.

Transparency With this approach, reconfigurations are completely transparent for the equipment of the on-demand room and of the apartments. For instance, the room's light continues to answer commands from the same virtual switch and the apartment's switch is still connected to the same virtual light: both the virtual elements are provided by the abstraction module. When and only when the apartment owns the room, switching actions will produce an actual effect on the room's light, as illustrated in Fig. 4.

Interoperability The proposed approach also allows devices to communicate with controllers of different technologies and brands, as the abstraction module can act as a bridge between heterogeneous communication media and protocols.

The general on-demand room modular architecture is illustrated in Fig. 5. As it can be seen, some devices are directly connected to the internal network of the apartment, without going through the abstraction module. It is the case of the network music player in the figure, which communicates with the music streaming server located in the owning apartment (and only in that apartment) thanks to the Wi-Fi range extender. In this case, interoperability between the music player and the streaming server relies on industrial standards like DLNA [START_REF]DLNA for industry[END_REF].

Dynamically reprogramming complex devices

We just described how to seamlessly plug the room in any apartment, without modifying the equipment's behavior. However, changing the on-demand room's owner can sometimes require reconfigurations that are not transparent for the devices. For instance, the ambient temperature set point of a thermostat should be dynamically changed to reflect the preferences of the new room's owner.

Reprogramming through commands In simple cases, devices can be reprogrammed by sending commands to them. For instance, this is true for most recent off-the-shelf thermostats, provided that the required wired or wireless communication infrastructure is available and that a common communication protocol is used by the thermostat and the controlling unit. When these assumptions hold, the change of room's owner will simply be dealt with by sending appropriate reconfiguration instructions to the devices.

Generally speaking, however, there are a number of settings that each user might customize the devices with. For instance, two different users might prefer different brightness and contrast levels for a TV screen, different songs to be reproduced by a music player, different pictures to be shown by a digital photo frame, different channel numbering for a TV set (since different households might have different subscriptions). These changes cannot be realized via simple commands to be sent to devices, so we need to consider a more general way to reprogram them.

Reprogramming in the general case Ideally, each household should feel like they have their own devices, that they can configure and customize. Settings should be stored and retrieved whenever that apartment owns the room, while being replaced when another household uses the room and its devices. This functionality is called multi-user support and some devices provide it natively, like personal computers and few mobile device types and vendors. The challenge is thus to extend the multi-user support to other devices of the consumer electronics industry. To do this, it must be possible to have several "images" of the same device, to be chosen and executed depending on which apartment owns the room. This is similar to what is done in computer science with virtualization: different virtual machine images (called virtual appliances) can be saved and loaded on the same physical machine, allowing to quickly switch from a configuration to another. In our case, the images to be saved and loaded only include the custom data of the device (settings, files, history, etc.) and not the firmware (the software realizing the core functionalities) of the appliance.

Goal 2 -Automatically building a model of user behavior

The proactive functionalities of the on-demand room are the actions that it performs to anticipate future events, including proactive customization and resolution of conflicts (cf. § 2.2). They rely on the ability of the room to learn habits and preferences about its use. The problem consists in observing the behavior of users and discovering correlations among events; whenever these recurring patterns are learned, the room can predict future requests.

This problem can be solved using machine learning techniques. We now briefly present these techniques (cf. § 3.2.1) and choose which ones we need to realize the on-demand room solution (cf. § 3.2.2). Then, we will describe the challenges to face in order to achieve our goal of automatically building a model of user behavior (cf. § 3.2.3).

Existing machine learning approaches

Existing approaches can be divided in two categories: supervised and unsupervised machine learning techniques.

In unsupervised approaches, regularities and interconnections among events are discovered automatically and learned. To this end, several algorithms can be used; two examples are the topic model and the probabilistic context-free grammars [START_REF] Kim | Human activity recognition and pattern discovery[END_REF]. In the topic model approach, the observed traces of simple activities are treated as a text document, where an activity is a word. Several topics are then discovered as recurring patterns of words. In the probabilistic context-free grammars approach, basic activities are characters of a grammar alphabet; rules, i.e., repeated sequences and complex combinations of activities, are automatically extracted from the execution traces.

Supervised approaches learn to automatically classify situations leveraging annotated training data, that is, traces of observed events coming with labels indicating what activity or habit each trace refers to. Supervised approaches include dynamic Bayesian networks, conditional random fields, hidden Markov models, artificial neural networks, support vector machines, k-nearest neighbors, etc. Many of them have been applied to human activity recognition [START_REF] Kim | Human activity recognition and pattern discovery[END_REF].

Which approach?

In our case, the goal is not finding correlations among any possible event, but rather discover the conditions under which the room is used. In this sense, the fact that an apartment requested the room constitutes the label for the previous events, because it states that the previous actions and context are correlated with the request. For this reason, supervised machine learning techniques are the most suitable approach to solve our problem.

We will use supervised machine learning techniques to automatically build a model of user preferences and habits. When considering the on-demand room, these concern occupation habits and respective duration and privacy level, selected HVAC settings, room layout and other custom settings.

Challenges

To use supervised machine learning techniques in our case, it is very important to select the factors or features that contribute to the identification of user preferences and habits, so that the system considers them when learning.

The simplest solution would be considering the features as observable user actions and interactions with appliances, like returning home or moving from room to room. However, we cannot discover correlations only based on user actions. The reason is that these are not sufficient to reliably describe the behavior of occupants, as this depends on many contextual factors and there is not always a direct correlation between actions themselves.

For instance, the system may observe that the resident of apartment A frequently uses the on-demand room just after returning home. This would lead to associating the action of returning home to the action of requesting the on-demand room. However, it also happens that the person does not use the room after coming home and none of the actions performed previously helps distinguishing among the two situations. In this case, action patterns alone do not allow to reliably predict future events.

To address this problem, features should include other aspects of the situation, apart from user actions and interactions. There could be other conditions suggesting that the room is going to be requested. For instance, among many important factors that should be considered during the learning phase, we can cite:

• Temporal aspects: time of the day or day of the week in which the use of the room occurs, day of the year (e.g., National or religious holiday), period of the year (e.g., summer holidays, start of the new school year), etc.;

• Social aspects: presence of certain or all household members and their age.

So, instead of only learning and recognizing user action patterns, the system should also consider the context associated with the actions. If system designers provide the relevant features to machine learning algorithms, these are able to automatically discover regularities.

The challenge is thus to identify relevant and observable factors by analyzing domestic activity and existing examples of shared spaces use (e.g., shared rooms in apartment buildings). This requires an interdisciplinary effort, involving sociologists, psychologists and ergonomists.

Fostering socialization and solving usage conflicts

This section provides two additional categories of specifications of the on-demand room. The first group of specifications, illustrated in § 4.1, addresses the issues arising when multiple apartments request additional space at the same time. The second category of specifications, provided in § 4.2, describes how the on-demand room can be shared between adjacent apartments or even by all the building occupants, so as to foster socialization among building residents and to provide additional flexibility to the on-demand room. This section concludes with a formal specification of the case study as a state machine (cf. § 4.3).

Handling multiple requests

We describe here the working principles of the on-demand room when it comes to managing requests coming from multiple apartments. The presentation is organized in several paragraphs, each illustrating a particular aspect of the operation.

Assignments are inviolable Once the room has been assigned to an apartment x, it will remain so until x releases it. However, there are two cases in which x is misallocated the room. This happens whenever an apartment y requests it and: (i) the room had been booked by y (explicitly or as a result of automatic learning of habits, as explained in § 2.2) before it was assigned to x or (ii) apartment x has reached its usage quota (see below). In both cases, x is notified about the maximum duration of the occupation at the moment of requesting the room.

Conflict resolution When an apartment is misallocated the room for the reasons presented above, a notification will inform occupants that they should quickly leave the on-demand room. In case they refuse to do so, a conflict-resolution procedure, possibly involving a human supervisor (e.g., the building caretaker), will be started.

Economic model

The additional space is pay-per-use, in order to prevent abuses like permanent occupation of the on-demand room by an apartment. When the floating space is not needed by the adjacent apartments, it may be assigned to any other building resident, so as to maximize its occupation. Usage quota There is a limit on the amount of time during which an apartment can use the room. Different usage quotas can be set: maximum continuous use, maximum cumulative use per week/month/year, etc. Usage quotas apply to adjacent apartments and to other building occupants. For the latter users, the quota is calculated as a common cumulative usage time for all residents.

Minimum vacancy Even if an apartment has reached its maximum use per day/week/month, it will be granted access to the room, so as to best exploit the available built surface and maximize revenues (the room is pay-per-use). However, the room will have to be vacated as soon as a request is issued by an apartment that has not reached its quota.

Hygiene and cleanness Hygiene is favored by increasing the ventilation between successive occupancies. Cleanness can be ensured using robotic vacuum cleaners or scheduling human intervention.

Concurrent use of the on-demand room

We specify here some additional functionalities of the on-demand room, which allow to promote socialization among building residents and to mutualize the space to an even higher degree. The underlying idea is that the on-demand room can be used at the same time by several occupants.

The following paragraphs illustrate the material means for sharing the on-demand room and the rules that guarantee a fair use of it.

Privacy levels

The floating space can be used as a common room between adjacent apartments or even be shared by all the building occupants. The possible privacy levels are, from the least to the most stringent one: public (shared with all authorized residents), intimate (shared between the two adjacent apartments A and B) and private (just one apartment). External residents (occupants of any apartment except A and B) can only ask for public privacy level.

Material means of sharing the room When a request is issued with intimate privacy level, two doors appear and allow to access the room from apartments A and B, as illustrated in Fig. 6. In public mode, an additional door connects the on-demand room with the floor landing and thus with the rest of the building (cf. Fig. 7). To prevent people from being locked in the room or out of their dwelling, all doors can become accessible from inside the room to their respective residents, thanks to an identity check mechanism (e.g., fingerprint check).

Satisfying privacy level requests

The privacy level of any accepted request has to be satisfied. Therefore, if the room is currently occupied with a certain privacy level, say intimate, no additional requests with a different level (i.e., private or public) can be accepted.

Negotiations The occupants of the room can express their will to be notified when additional requests are issued during their stay. In that case, a digitally mediated negotiation with other residents could possibly result in reallocating the room or changing the privacy level during use. Occupants can release the room voluntarily or in exchange of other benefits (e.g., financial).

Formal specification of the on-demand room

We provided a detailed description of the on-demand room solution. This section presents a model of computation that formally specifies these working principles.

We first formalize the on-demand room as a finite state machine. Then, we highlight the limitations of this model of computation and propose a more suitable representation as a UML state machine. The on-demand room as a finite state machine To formally specify the on-demand room working principles, we might use a finite state machine. It is a "model of computation consisting of a set of states, a start state, an input alphabet, and a transition function that maps input symbols and current states to a next state" [START_REF] Black | finite state machine[END_REF].

Figure 8 shows a finite state machine, representing a subset of the functionalities of the on-demand room. Operations like keeping track of the number of external requests or dealing with excess of usage quotas are omitted, because the complexity of the model would dramatically increase.

The on-demand room as a UML state machine A UML state machine [START_REF]UML 2.4.1 specification[END_REF] is an alternative formalization that allows to overcome the drawbacks illustrated above. Figure 9 presents a UML state machine describing the on-demand room behavior. Since the UML specifications leave freedom to choose one's own syntax for operations and events [START_REF]UML 2.4.1 specification[END_REF], we followed the statechart description language coming with YAKINDU Statechart Tools 2 [START_REF]Yakindu statechart tools -home[END_REF] plugin for Eclipse.

A UML state machine representation is much more expressive than a finite-state machine. Using this formulation, we can specify the operations associated with entering and exiting states (e.g., enabling or disabling doors), as well as with transitions (e.g., starting occupation timer, notifying occupants).

This formulation is also more compact, because extended state variables can model information like the number of occupants that issued a public request, which allows to know when the room has been released by all of them (cf. Fig. 9). The UML state machine allows us to model the full on-demand room behavior with just 8 states, while the finite state machine required 13 states to only partially model it.

Representing certain functionalities is very difficult with a finite-state machine but is simply done by a UML state machine, like managing requests when the current owner of the room has reached its maximum usage quota. We can see in the right-hand side of Fig. 9 that this is handled via a choice node: some transitions are possible only if the time is out Collection des Publications Internes de l'Irisa c IRISA for the apartment currenly owning the room. Similar choice nodes are required to connect private with public, private with intimate and intimate with public states, but they are omitted to ease the reading of the figure.

Implementation and evaluation

The on-demand room case study will be realized as an interactive application in an immersive virtual reality platform. Stakeholders and potential users will be able to see and interact with the on-demand room, leveraging the virtual environment.

This section presents the iterative methodology for the design and realization of the immersive application (cf. § 5.1), as well as the challenges for its realization (cf. § 5.2) and evaluation (cf. § 5.3).

Overall methodology

The implementation of the virtual environment will be realized in collaboration with architects and designers, so as to produce a technically sound and realistic prototype of the on-demand room. The process will be iterative and each iteration will be characterized by the following phases:

• requirement collection and analysis;

• design of a solution fulfilling the requirements;

• implementation of the solution as an immersive virtual environment;

• evaluation by stakeholders (including end users, companies and public authorities), psychologists, sociologists, etc. At each step, the evaluation phase will produce the requirements for the next iteration (cf. § 5.3).

Realization challenges

In the realization of the immersive environment, there are several goals to be achieved through transdisciplinary collaboration. They are illustrated below.

Architectural modeling

We will need to model a realistic architectural setting, including the layout and appearance of the on-demand room and its neighboring apartments. We will benefit from the help of architects and designers.

We will use 3D architectural modeling software supporting the BIM (Building Information Modeling) standard data sharing format [START_REF]industry foundation classes (IFC) for data sharing in the construction and facility management industries[END_REF]. This format allows to reuse the same model in different, industry-specific software and to realize various engineering processes in parallel. This allows to save time, to avoid errors and to keep a unique, updated representation of the building.

Realizing the virtual environment and the application logic

For our demonstration, we will rely on Immersia [12], a large virtual-reality platform dedicated to real-time, multimodal (vision, sound, haptic, BCI) and immersive interaction. In Immersia, images are rendered on four glass screens, located on the front, on the sides and on the ground, as illustrated in Fig. 10. The platform is 9.6 m wide, 2.9 m deep and 3.1 m high. A tracking system, composed of 16 infrared cameras, enables real objects and people to be tracked within the scene. Images are recomputed as the user moves to fit his point of view, together with high-resolution rendering, active stereoscopy and homogeneous coloring, thereby delivering a visually realistic experience.

The architectural model of the on-demand room will be imported in Unity [START_REF]Unity -architecture[END_REF], a game development software that, used in combination with MiddleVR [START_REF]MiddleVR for unity | MiddleVR[END_REF], allows to deploy, visualize and interact with the model inside Immersia. The working logic of the room, described the previous sections and formalized in § 4.3, will be implemented using Unity scripting facilities.

Realizing a hybrid real/virtual environment

In order to make the immersive experience more realistic, we will integrate in the application few real devices and sensors, realizing a hybrid (also known as mixed reality) application. The real devices might include light bulbs, Wi-Fi range extenders and various sensors. These devices will be controlled by the application, just like virtual ones, and sensors will provide real data about users. For instance, a virtual switch might turn on a real light bulb (or vice versa) and real wearable sensors and handheld devices will allow users to interact with objects and devices in the virtual environment.

Designing suitable interaction modalities

Suitable interaction modalities and interfaces will have to be designed. First of all, interfaces are required to provide occupants with relevant information concerning their use of the room (e.g., cumulative occupation time and financial cost) and its schedule, to be shared with other apartments' occupants.

Appropriate interaction modalities will also be introduced, so that residents can collaboratively define the schedule of the on-demand room and solve usage conflicts.

Evaluation challenges

The prototype of on-demand room will be evaluated under different points of view, including user experience, technical soundness and resource efficiency, as illustrated below.

User experience evaluation We will assess how people appropriate the on-demand room and whether the system answers inhabitants' needs. The appropriation evaluation will establish if users build up their own knowledge of system capabilities and elaborate their own experience leading (or not) to an invisible use.

The immersive virtual environment will allow situating users in realistic conditions of use. Their behavior will be recorded, analyzed and used in interviews involving self-confrontation. This will allow reconstructing their concerns in situation and harvesting their individual and collective points of view (i.e., cognitive, emotional, subjective, etc.).

Decision-making and learning algorithms evaluation We will evaluate how well the interactive application performs with respect to the on-demand room specifications, provided in § 4.3. We will test the system and verify whether it has the expected behavior. Access to the room will have to be granted to apartments having requested it, with the appropriate privacy level and consistently with the state of the room, resulting from its history of use. The reconfiguration and customization of the room will have to follow that decision. Evaluating the ability of the system to learn user preferences and habits requires performing tests that last over time. This is difficult to realize in a virtual reality platform. To overcome this limitation, we will generate synthetic traces of execution, starting from realistic patterns of activity and room use. We will feed the application with the traces and verify whether the patterns are correctly discovered and learned. Based on that model, the system will have to correctly schedule and grant access to the room in a predictive fashion.

Evaluation of space and energy gain The objective of this evaluation phase is to quantify the overall benefits of the on-demand room in terms of space and energy efficiency, compared to a number of additional private rooms, belonging to apartments. Several evaluation indices will be used, including: occupation time per room, ratio of occupied surface over built surface, energy use, etc. The evaluation will be performed on realistic execution traces, compared using those indices, for an apartment building in different configurations: equipped with an additional room per apartment, with a simply shared room and with an on-demand room.

Overall considerations will be issued to verify whether the room allows to find a good trade-off between optimal use of resources and maximization of user comfort, which are often contradictory objectives.

Related Work

Related research activities have been named kinetic, responsive and interactive architecture [START_REF] Fox | Interactive Architecture[END_REF], as well as architectural robotics [START_REF] Weller | Architectural robotics: A new paradigm for the built environment[END_REF]. Some of the major initiatives are illustrated below:

• The Places of Living and Work research theme at the City Science initiative at MIT Media Lab proposes projects of "personalized, transformable urban housing" and "time-shifted, shared space-on-demand for collaborative work" [START_REF]MIT cities[END_REF].

• The Living Architecture Lab at the GSAPP, Columbia University, "aims both to make visible the invisible forces that shape our world, and to explore the potential for architecture to transform in real time based on these forces" [START_REF]Living architecture lab[END_REF].

• The Genetic Architecture group at Universitat Internacional de Catalunya (UIC) "works on applying genetics to the field of architecture in an interdisciplinary way, from two perspectives: the real, natural, and direct, and also the metaphorical, artificial, and digital" [START_REF]Genetic architectures research group[END_REF].

• The Hyperbody group is the knowledge center for Nonstandard and Interactive Architecture at the TU Delft [START_REF]Hyperbody[END_REF].

The laboratory works on the change in conception and realization of design due to digital design and production methods and engages the new modes of architectural practice that embrace the shift from standard to nonstandard structures and from static to dynamic buildings.

• The Laboratory for Integrative Design (LID) is a research group from the University of Calgary that brings together researchers from multiple disciplines [START_REF]Laboratory for integrative design[END_REF]. Researchers at LID operate at the intersection between design, its allied disciplines of engineering and production, and other fields, such as computer science, material science, mathematics and biology.

• The Office for Robotic Architectural Media & The Bureau for Responsive Architecture (ORAMBRA) "develops new construction systems and components for buildings. Focusing upon rethinking construction alongside the emergence of responsive technologies, the office is recognized for developing structural shape change technologies that alter the way that buildings use energy" [START_REF]ORAMBRA -The Office for Robotic Architectural Media & Bureau for Responsive Architecture[END_REF].

• The Bartlett Space Syntax Laboratory is the international center of the theory and methodology known as "space syntax" [START_REF]Space syntax laboratory[END_REF]. Its team of researchers studies the effects of spatial design on aspects of social, organizational and economic performance of buildings and urban areas, uniquely combining the creation of software for spatial analysis and design with observation of function. Its theoretical and methodological innovations intersect with a variety of disciplines, including anthropology, archaeology, geography, computer science, psychology, sociology, mathematical modelling, public health and physics.

Conclusions

This paper presented a case study of metamorphic housing called on-demand room. We described the goal of dynamically extending a dwelling, as well as the transformations in the material and digital world that make the additional space belong to it. The paper also presented the strategies for proactively solving conflicts in the use of the limited floating space, which are based on learning user habits. Finally, we illustrated the important efforts required to define, implement and evaluate the case study in an immersive virtual reality platform.

Work plan

Our first steps to reach the goals presented in this paper will consist in introducing suitable strategies to dynamically reconfigure the behavior and connections between devices. A particularly promising approach to realize such transformations is represented by models@run.time: "runtime adaptation mechanisms that leverage software models" [START_REF] Blair | Models@ run.time[END_REF].

We will also immediately start the realization of the immersive virtual reality environment, using the software Unity3D and its plug-in MiddleVR for the immersive capabilities [START_REF]MiddleVR for unity | MiddleVR[END_REF]. This will allow to quickly obtain a demonstrator of the case study and to iteratively improve its specifications.

Considerations concerning the real-world deployment of metamorphic housing will be taken into account. Namely, all the algorithms will be designed so as to work without relying on an external infrastructure, in order to preserve residents' privacy. Furthermore, we will assume that a part of the physical reconfigurations will be performed by humans instead of mechanical effectors. For instance, the on demand room might be cleaned by a person in between uses and people might manually reconfigure a part of the room's furniture to fit their preferences.

Perspectives

The models and algorithms to be introduced through this research are generic and will be applicable to other examples of metamorphic housing. We provide below a non-exhaustive list of potential future case studies, which would benefit from the outcomes of this research.

• Splitting the floating space into separate smaller rooms, thanks to mobile partitions and control algorithms.

This would allow to extend apartments in spite of possibly time-overlapping requests. The models and algorithms used in the on-demand room for allocation of the space and for the dynamic transformations could be reused.

• Dynamically changing the room's internal layout and aspect so as to reflect user preferences, also with respect to declared or expected activity to be performed in the room (e.g., arrangement and type of furniture). The dynamic change of ownership of devices proposed by the on-demand room use case is applicable in this example. Indeed, when a room is entirely reconfigured to change from a kitchen to a bedroom, a number of changes in the operation and interconnection of devices must be realized.

• Defining an extension of the Building Information Modelling (BIM) data sharing format [START_REF]industry foundation classes (IFC) for data sharing in the construction and facility management industries[END_REF], leveraging the experience of this case study. The extension would consist in a formal language that allows to specify the dynamic behavior of the building and of the architectural structures.

Figure 1 :

 1 Figure 1: Plan of the building floor

Figure 2 :

 2 Figure 2: On-demand room assigned to apartment A

Figure 4 :

 4 Figure 4: Abstraction of physical devices: only the light switch of the current room's owner will work

Figure 6 :Figure 7 :

 67 Figure 6: On-demand room shared between apartments A and B (intimate mode)

Figure 8 :

 8 Figure 8: A subset of the functionalities of the on-demand room as a finite-state machine

Figure 9 :

 9 Figure 9: A UML state machine representing the on-demand room operation

Figure 10 :

 10 Figure 10: The immersive virtual reality platform Immersia

Collection des Publications Internes de l'Irisa c IRISA

Acknowledgments

We thank Fondation Rennes 1 "Progresser Innover Entreprendre", Cardinal Edifice, the Chamber of Commerce and Industry of Rennes, Rennes Métropole and Veolia for funding and supporting this research. We also thank Inria and IRISA for providing access to Immersia, as well as the platform's management and support team.

This research was conducted as part of the chair "Smart Home and Innovation", financed by Fondation Rennes 1


Slawomir Jack Giletycz

N Loget

C-P Chang

F Mouthereau

Transient fluvial landscape and preservation of low--relief 1 terrains in an emerging orogen: example from Hengchun 2 Peninsula, Taiwan 3 4

Keywords: transient landscape, orogen, Taiwan, fluvial processes, knickpoint 53 retreat 54 55

The oblique collision of Taiwan orogen led to a progressive southward uplift and 31 emergence of a submarine accretionary wedge. The Hengchun Peninsula located at 32 the southernmost tip of Taiwan island represents the most recently emerged 33 landform of an antecedent submarine surface. In this study we examine the 34 geomorphic evolution of this newly emerged and uplifted landscape. The extraction 35 of geomorphic parameters (knickpoints distribution, steepness index of rivers) 36 indicates a transient nature of the landscape but shows that a simple model of 37 regressive erosion consecutive to sudden uplift is unsatisfactory. Knickpoints 38 migration modeling shows that only the upstream Sizhong basin can be explained 39 by this single process. On the other hand, the occurrence of a relict landscape 40 (Mutan Ponds low--relief) developed at low elevations, and today uplifted in the core 41 of the peninsula, has strongly influenced the drainage evolution. Numerous 42 knickpoints are linked to capture processes around the Mutan Ponds area, 43 indicating a dynamic reorganization of the drainage system. We propose that the 44 landscape of the Hengchun Peninsula is in a transient stage and results from a 45 combination between rejuvenation and drainage rearrangement caused by the late 46 stage of emergence of the south of Taiwan. 47 48 49

Introduction 56 57

The early stage of fluvial landscape development associated with emerging 58 mountain ranges is often difficult to capture in natural systems. First, this is because 59 the juvenile geomorphic surfaces need to be unambiguously distinguished from 60 former submarine processes that shaped the landscape. Second, they are basically 61 transient and therefore the rate at which this landscape is preserved directly 62 depends on tectonic forcing and prevailing erosional processes [START_REF] Burbank | Rates of erosion and their implications for exhumation[END_REF]. 63

Better understanding of these processes as well as the time response of landscape to 64 tectonics or climate in active areas requires an approach that aimed at quantifying 65 the early stages of geomorphic development [START_REF] Willett | On steady states in mountain belts[END_REF]Whipple, 66 2004;[START_REF] Crosby | Knickpoint initiation and distribution within 574 fluvial networks: 236 waterfalls in the Waipaoa River, North Island, New 575 Zealand[END_REF][START_REF] Garcia--Castellanos | Long--term evolution of tectonic lakes: Climatic controls 585 on the development of internally drained basins[END_REF][START_REF] Ramsey | Topographic evolution of emerging mountain belts[END_REF]67 Whittaker et al., 2007;[START_REF] Craddock | Rapid fluvial 571 incision along the Yellow River during headward basin integration[END_REF][START_REF] Walker | Geomorphic evidence for ancestral 669 drainage patterns in the Zagros Simple Folded Zone and growth of the Iranian 670 plateau[END_REF]. 68

Taiwan is classically described as an active orogenic system that reached a steady--69 state [START_REF] Whipple | Fluvial landscape response time: how plausible is steady--state 678 denudation?[END_REF] and where transient stages of landscape development are 70 expected to be hardly preserved. However, due to convergence obliquity that 71 induces uplift to propagate southwards [START_REF] Suppe | Mechanics of mountain building and metamorphism in Taiwan. 667[END_REF][START_REF] Willett | Erosion rates and 693 orogenic--wedge kinematics in Taiwan inferred from fission--track 694 thermochronometry[END_REF], the 72 southern tip of Taiwan appears to be ideally located to assess the role of outboard 73 marine inherited landscape and the geomorphic processes at play during the initial 74 stage of mountain building (Fig. 1). Previous studies in central Taiwan have shown 75 that topographic steady--state can be reached where high erosion rates 2--5 mm/yr 76 are recorded [START_REF] Willett | Erosion rates and 693 orogenic--wedge kinematics in Taiwan inferred from fission--track 694 thermochronometry[END_REF][START_REF] Siame | Erosion rates in an active orogen (NE--Taiwan): A confrontation of 652 cosmogenic measurements with river suspended loads[END_REF]. However, in contrast to the 77 northern and more mature part of the mountain belt, the Hengchun Peninsula in the 6 south is characterized by low erosion rates as indicated by old, pre--orogenic fission--79 track ages [START_REF] Fuller | A thermomechanical wedge 580 model of Taiwan constrained by fission--track thermochronometry[END_REF], indicating this region did not reach an exhumational 80 steady--state. Transient stages of landscapes are mostly characterized by knickpoints 81 in their river profiles linked to a base--level drop or an increase of the tectonic uplift 82 [START_REF] Whipple | Dynamics of the stream--power river incision 674 model: implications for height limits of mountain ranges, landscape response 675 timescales, and research needs[END_REF][START_REF] Bishop | Knickpoint recession rate and 559 catchment area: the case of uplifted rivers in eastern Scotland[END_REF][START_REF] Loget | Mesoscale fluvial erosion 618 parameters deduced from the modeling of the Mediterranean sea--level drop 619 during the Messinian (Late Miocene)[END_REF][START_REF] Whittaker | Bedrock 688 channel adjustment to tectonic forcing: Implications for predicting river 689 incision rates[END_REF]83 Attal et al., 2008;[START_REF] Gallen | Miocene rejuvenation of 583 topographic relief in the southern Appalachians[END_REF]. Numerous works have attempted either to 84 calibrate erosional models or extract tectonic information from river profiles but in 85 most of cases they start from an inherited continental domain or antecedent 86 drainage systems (e.g., [START_REF] Roberts | Estimating Uplift Rate Histories from River Profiles 645 using African Examples[END_REF]. Few works concern the 87 adaptation and organization of a drainage system following a large and recent 88 emerging domain (e. g., [START_REF] Rosenbloom | Hillslope and channel evolution in a marine 648 terraced landscape, Santa Cruz, California[END_REF]. A coupled approach of 89 river incision and organization/reorganization could be used as a marker of the 90 dynamic of emergence of continental domains. 91

In this paper, we examine the landscape evolution of the Hengchun Peninsula where 92 transient geomorphic processes, including patterns of drainage reorganization 93 (capture, knickpoints/knickzones) and relicts of low--relief high--elevation areas are 94 present. We first detailed the main geomorphic features of the peninsula 95 characterized by a N--S low--relief area pinned along the western side of the main 96 divide that are incised successively with time by the western and eastern flanks, 97 respectively. We then discuss on the origin of the main knickpoints positioned 98 around the low--relief area using a knickpoint celerity modeling approach and 99 steepness index maps. Geomorphological analyses of the large drainage basins of 100 the Hengchun Peninsula reveal that river channels recorded recent dynamics of the 101 youngest Taiwan orogen. We show that a simple rejuvenation stage cannot explain 102 the main geomorphic features. A mixed model of knickpoint propagation and 103 drainage rearrangement with captures caused by the late stage of emergence of the 104 peninsula is proposed. These erosive and landscape evolution patterns appear to be 105 compatible with an uplift originated from a deep crustal block related. The recent 106 uplift in the Hengchun Peninsula is caused by the continuing progression of the 107 collision towards the south of Taiwan. 108 109

Geological background 110

The Taiwan arc--continent collision is the result of the convergence between the 111 Chinese continental margin of the Eurasian plate and the Luzon arc belonging to the 112 oceanic Philippine Sea plate (Fig. 1). Collision started 6.5 Ma as inferred from initial 113 orogenic loading in western Taiwan [START_REF] Lin | Cenozoic stratigraphy and subsidence 610 history of the South China Sea margin in the Taiwan region[END_REF] and onset of cooling, above the 114 underthrust continental crust, recorded at 7.1 Ma by low--temperature 115 thermochronological data [START_REF] Mesalles | From submarine continental accretion to arc--continent orogenic 629 31 evolution: the thermal record in the Taiwan mountain belt[END_REF]. The emergence of the mountain 116 belts and coupling with surface processes become efficient after 5 Ma in northern 117 Taiwan and not before 3.5 Ma in southern Taiwan [START_REF] Mesalles | From submarine continental accretion to arc--continent orogenic 629 31 evolution: the thermal record in the Taiwan mountain belt[END_REF]. Unreset 118 apatite fission--track ages in southernmost Central Range and Hengchun Peninsula 119 [START_REF] Fuller | A thermomechanical wedge 580 model of Taiwan constrained by fission--track thermochronometry[END_REF][START_REF] Simoes | Late Cenozoic metamorphism and 658 mountain building in Taiwan: A review[END_REF][START_REF] Mesalles | From submarine continental accretion to arc--continent orogenic 629 31 evolution: the thermal record in the Taiwan mountain belt[END_REF] argue for a slower or 120 more recent exhumation in the region with respect to the northern Taiwan. Overall, 121 these constraints are consistent with the obliquity of the convergence, implying that 122 collisional uplift and exhumation in southern Taiwan is the most recent. 123

The southward propagating model places the Hengchun Peninsula as the most lately 124 emerged region in Taiwan. As a result, the evolution of the fluvial landscape is also 125 probably the most recent, but it is yet to be determined. In particular, the patterns of 126 river captures and regressive erosion along river trunk streams have never been 127 described, thus precluding further quantification of geomorphic processes. 128

The peninsula comprises folded, non metamorphic or slightly metamorphosed, 129

Middle to Late Miocene sandstones and siltstones (Fig. 2). They correspond to 130 turbiditic sequences characterized by slope gravitational instabilities sedimentary 131 features. Lenticular conglomeratic bodies with channeling forms are locally 132 abundant and can be easily identify in the landscape due to their rough topography 133 (see Fig. 2, Shimen conglomerates). They are composed of reworked Miocene 134 sandstones, basalt, gabbros and ultramafic rocks [START_REF] Pelletier | Middle Miocene obduction and late Miocene 632 beginning of collision registered in the Hengchun Peninsula: Geodynamics 633 implications for the evolution of Taiwan[END_REF]. Two 135 other sandstone members can be distinguished on the northwest and southeast tips 136 of the peninsula. Adjacent to the Miocene formation, to the west, there is the Kenting 137 ophiolitic Mélange (Kenting Formation of Fig. 2), interpreted as sheared and locally 138 faulted tectonic mélange that developed from the Late Miocene to Plio--Pleistocene. 139

The studied area is the onshore equivalent of southern accretionary prism. Its uplift 140 and exhumation appear to be driven by the accretion of a transitional crust 141 composed of the extremely--thinned continental basement of the South China Sea 142 that is tectonically stacked with Miocene post--rift deposits [START_REF] Mcintosh | Two--dimensional seismic velocity models of southern Taiwan from 626 TAIGER transects[END_REF]. 143

Well--developed Holocene marine terraces containing abundant remnants of corals 144 are found along the southern and southwestern coasts of the peninsula (Lee and 145 Liew and Lin, 1987;[START_REF] Chen | Holocene radiocarbon dates in Hengchun Peninsula and 564 their neotectonic implications[END_REF] (Fig. 2) uplifted from few to 40 meters 146 above the sea level. Their radiocarbon dating gives ages of 10 kyr suggesting mean 147 uplift around 3.5 mm/yr during Holocene (e.g. [START_REF] Wang | Holocene mean uplift rates across an active plate--672 collision boundary in Taiwan[END_REF]. Coral 148 platforms corresponding to Hengchun Limestone are also observed on 200--meter 149 high cliffs of the southwestern part of the Hengchun Peninsula--Hengchun Hill 150 [START_REF] Cheng | Biostratigraphic study in the west Hengchun hill[END_REF][START_REF] Chen | Reconsideration of the stratigraphy of the western 567 Hengchun Hill[END_REF])(see Fig. 2, Hengchun Limestone). 151

Their ages are estimated around Late Pleistocene (~100 ky) suggesting mean uplift 152 rates around 3--4 mm/y during Pleistocene. Few recent ages are published more 153 inland and only lacustrine deposits in Mutan Ponds low reliefs gives an age of 154 deposition between 2 and 20 ky [START_REF] Yang | Variations in monsoonal rainfall over the last 21 kyr inferred from 703 sedimentary organic matter in Tung--Yuan Pond, southern Taiwan[END_REF]. Synchronism of inland 155 lacustrine sediments and the uplifts of the Holocene marine terraces argue that the 156 uplifts of the marine terraces along the coasts could impact the deposition of the 157 Mutan Ponds within the drainage basins of the Hengchun Peninsula. 158 159

Geomorphic characteristics of the Hengchun Peninsula 160

In Taiwan, a combination of very humid and monsoonal climate coupled with active 161 tectonics causes rapid fluvial erosion forming high--relief terrains and unstable 162 slopes [START_REF] Fuller | A thermomechanical wedge 580 model of Taiwan constrained by fission--track thermochronometry[END_REF][START_REF] Ramsey | Geomorphic constraints on the active 640 tectonics of the southern Taiwan[END_REF][START_REF] Giletycz | Geological structure as a crucial factor 587 facilitating the occurrence of typhoon--triggered landslides: Case from Hsiaolin 588 Village, 2009 Typhoon Morakot[END_REF]. Heterogeneous 163 character of the rock strength or uplift results in local preservation of 164 geomorphological patterns. This particular setting gives us the opportunity to 165 examine the response of fluvial processes and landscape adjustment to short--166 term/transient acceleration of subsurface deformations. Previous studies on Taiwan 167 geomorphology show that drainage systems ultimately control the regional 168 denudation and shape the orogen [START_REF] Whipple | Fluvial landscape response time: how plausible is steady--state 678 denudation?[END_REF][START_REF] Wobus | Hanging valleys in fluvial systems: 696 Controls on occurrence and implications for landscape evolution[END_REF]Ramsey, 10 2006;[START_REF] Ramsey | Geomorphic constraints on the active 640 tectonics of the southern Taiwan[END_REF][START_REF] Stolar | Characterization of topographic 665 steady state in Taiwan[END_REF]. For example, Stolar et al. (2007) 170 suggested that tectonic perturbations are balanced by channel response along the 171 eastern Central Range. This contrasts with southern Taiwan where the drainage 172 system reveals ongoing dynamic adjustment. 173

Because the southward propagation of the arc--continent collision is thought to 174 occur at a rate of 55 mm/yr, [START_REF] Stolar | Characterization of topographic 665 steady state in Taiwan[END_REF] estimated that 1.8--2.3 Myr are 175 required to reach topographic steady--state after emergence above the sea level. Due 176 to the space--time equivalence, topographic steady--state becomes apparent at a 177 distance of 100--125 km from the southern tip of Taiwan. At this latitude individual 178 drainage basins yield rapid increase of the uplift of the southern Central Range 179 [START_REF] Ramsey | Geomorphic constraints on the active 640 tectonics of the southern Taiwan[END_REF]. However, [START_REF] Whipple | Fluvial landscape response time: how plausible is steady--state 678 denudation?[END_REF] argues that the timescale for the 180 Quaternary climate fluctuations and fast uplift is not enough to achieve topographic 181 steady--state, thus mountainous landscapes of Taiwan have been capable only to 182 adjust to 'mean climatic conditions'. The irregularity of the drainage system and the 183 high asymmetry of the orogen at the Hengchun Peninsula reflect its landscape to be 184 the youngest of the emerging Taiwan orogen. 185

The only geomorphological study carried out over Hengchun's Peninsula (Ramsey, 186 2006) indicates a dynamic drainage system rearrangement suggesting a short 187 duration of the geomorphic response to tectonic/climatic imprint. [START_REF] Ramsey | Topographic evolution of emerging mountain belts[END_REF] 188 also suggested a lithological control for the drainage basins by the emergence of 189 Holocene coral reef limestones along the coast (Fig. 2). The uplifted limestone acted 190 as a barrier for sediments sourced from perched drainage basins, hence keeping the 191 discharge low enough to trap sediments in the upstream part of the drainage system 192 but this hypothesis hardly explains the initial shaping of low reliefs areas. 193 194 4. Methods 195 4. 1. Topographic and Relief analyses 196 Swath averaged--profiles of topography were built across the studied region to 197 report the main landscape characteristics from the Central Range in the north, down 198 to low--elevated areas of the southern Hengchun Peninsula (Fig. 3). Each profile was 199 taken perpendicular to the divide's strike where the mean, maximum and minimum 200 elevations of the DEM (SRTM 90) along an E--W 10 km window are projected into 201 cross--section planes. 202

In order to track the mean orientations of local relief in the Hengchun Peninsula, we 203 used a 3D analysis approach using a 5x5 m scale DEM (e.g. Ahnert, 1984) collected 204 by the Aerial Survey Office, Forestry Bureau--Taiwan. To carefully map the low--relief 205 areas, we adopted a circular sliding window of 5 km diameter where the mean local 206 relief (difference between the highest point and lowest point) is reported on the 207 center of this relief grid (Fig. 4). 208

Steepness index and concavity index 216

In addition to the two--dimensional topographic analysis of river profiles we 217 estimated the slope--area relationship that is characteristics of erosional processes. 218

The channel slope (S) is linked to the drainage area (A) by an inverse power law 219 such as: 220

𝑆 = 𝑘 ! 𝐴 !! (Eq. 1) 221
where (ks) and (𝜃) correspond to steepness index and concavity index, respectively 222 [START_REF] Hack | Studies on longitudinal stream profiles in Virginia and Maryland[END_REF][START_REF] Flint | Stream gradient as a function of order, magnitude, and discharge[END_REF][START_REF] Snyder | Landscape response to 661 tectonic forcing: Digital elevation model analysis of stream profiles in the 662 Mendocino triple junction region, northern California[END_REF]Lague and Davy, 2003). If we assume 223 that fluvial erosion follows a detachment--limited erosional pattern, the erosion (E) 224 is governed with a stream power law such as: 225

𝐸 = 𝐾𝐴 ! 𝑆 ! (Eq. 2) 226
where K represents the erodibility of rocks, A the upstream drainage area and S the 227 local slope [START_REF] Howard | A detachment--limited model of drainage basin evolution[END_REF][START_REF] Whipple | Dynamics of the stream--power river incision 674 model: implications for height limits of mountain ranges, landscape response 675 timescales, and research needs[END_REF]. 228

In the specific case of steady state where E equal uplift rate (U), equation 2 becomes: 229

𝑆 = (𝑈/𝐾) !/! 𝐴 !!/! (Eq. 3) 230
Where (𝑈/𝐾) !/! and -𝑚/𝑛 represent ks and 𝜃, respectively. In a log--log plot, 𝜃, 231 which is sensitive to the basin hydrology, corresponds to the slope of the regression 232 line whereas ks is the intercept. Both ks and 𝜃, are known to be highly sensitive to 233 tectonic uplift (e.g., [START_REF] Snyder | Landscape response to 661 tectonic forcing: Digital elevation model analysis of stream profiles in the 662 Mendocino triple junction region, northern California[END_REF]. The concavity 𝜃 typically varies in the small 234 range of 0.4 to 0.6. This enables us to adopt a reference concavity index (𝜃 !"# ) 235 where different stream profiles can be directly compared by a normalized steepness 236 index (ksn) [START_REF] Wobus | Hanging valleys in fluvial systems: 696 Controls on occurrence and implications for landscape evolution[END_REF][START_REF] Kirby | Expression of active tectonics in erosional landscapes[END_REF]. We keep in mind that 237 these parameters are often deduced from an equilibrium stage of river profiles. 238 However for transient cases the response time of rivers may be longer than the 239 tectonic forcing as indicated by the presence of knickpoints in river profiles. In that 240 case, one may consider that knickpoint separates an upstream section in a paleo--241 equilibrium stage from a downstream oversteepened part in a final equilibrium 242 stage [START_REF] Whipple | Dynamics of the stream--power river incision 674 model: implications for height limits of mountain ranges, landscape response 675 timescales, and research needs[END_REF]. 243

Here, we have adopted the automated method for extraction of steepness and 244 concavity indexes on stream profiles developed by [START_REF] Whipple | New Tools for 682 Quantitative Geomorphology: Extraction and Interpretation of Stream Profiles 683 from Digital Topographic Data[END_REF]

on a 245

ArcGIS--Matlab environment that can be freely downloaded at geomorphtools.org. 246 247

Modeling knickpoint retreat 248

An important point concerning the morphological evolution of Hengchun Peninsula 249 is to evaluate if this landscape is a result of a uniform or more distributed uplift. A 250 uniform uplift would induce a wave of regressive erosion in a radial pattern in the 251 watersheds [START_REF] Crosby | Knickpoint initiation and distribution within 574 fluvial networks: 236 waterfalls in the Waipaoa River, North Island, New 575 Zealand[END_REF][START_REF] Berlin | Modeling of knickpoint retreat on the Roan 556 Plateau, western Colorado[END_REF]. By contrast, a 252 distributed uplift would induce a mixing of wave of erosion, shifting and capture of 253 rivers (e.g., [START_REF] Jackson | The structural evolution of active fault and 597 fold systems in central Otago, New Zealand: evidence revealed by drainage 598 patterns[END_REF]. 254

To investigate the hypothesis of a wave knickpoint migration, we tested a 255 knickpoint celerity model. The model predicts knickpoints position at each time step 256 [START_REF] Crosby | Knickpoint initiation and distribution within 574 fluvial networks: 236 waterfalls in the Waipaoa River, North Island, New 575 Zealand[END_REF][START_REF] Berlin | Modeling of knickpoint retreat on the Roan 556 Plateau, western Colorado[END_REF]Loget and Van Den 257 Driessche, 2009;[START_REF] Gallen | Miocene rejuvenation of 583 topographic relief in the southern Appalachians[END_REF]) such as: 258 V = dx/dt = CA P (Eq. 4) 259 where V is upstream knickpoint migration rate, C is a constant dimensional 260 coefficient representing erodibility and efficiency of the knickpoint retreat, A is an 261 upstream area and p is a non--dimensional constant for the power law dependence 262 on a drainage area. We have considered a uniform erodibility at the scale of the 263 basin as suggested by the uniform lithology of the Mutan turbidites. 264

We hypothesized an initial upstream area or a starting point for the knickpoint 265 migration as the converging point of main knickpoints in the basin coupled to the 266 presence of knickpoints in all upstream sub--watersheds. In the absence of data for 267 incision or knickpoint migration rates, we tested two end--member classes of 268 parameters for the modeling that fit with response time around 10 4 to 10 5 years for 269 knickpoint migration. We used the model parameters of Crosby and Whipple, 270 (2006) (hereafter abbreviated CW) with V=7.9 10 --7 A 1.1125 (that favors large 271 upstream area with a low efficiency for knickpoint retreat) and the model of Loget 272 and Van Den Driessche (2009) (hereafter abbreviated LVDD) where V = 10 --4 A 0.5 273 (that favors small upstream area with a high efficiency for knickpoint retreat). Both 274 models were stopped when the modeled knickpoint reach the farthest knickpoints 275 (KP5). 276 277

Asymmetry Factor (AF) observations 278

To highlight the areas with potential captures, we have adopted the non--279 dimensional Asymmetry Factor (AF) [START_REF] Hare | Geomorphic indicators of vertical neotectonism 592 along the converging plate margins, Nicoya Peninsula, Costa Rica[END_REF]: 280 AF =100(Ar /At) (Eq 5) 281

where, Ar represents the drainage area to the right of the main trunk stream looking 282 downstream and At is the total drainage area. A symmetric catchment with a mean 283 value of AF = 50 indicates equal development of the drainage basin both on the right 284 and left side of the basin. Decrease or increase of the AF value indicates the 285 development of asymmetry on either side of the drainage area. Such asymmetry can 286 potentially emphasize an area where one drainage basin through a capture adopted 287 new tributaries. In our AF analyses we targeted small drainage basins along the 288 main divide and coasts, where the AF is more sensitive to local variances. 289 290

Results 291

Topographic and relief analyses 292

The Central Range of Taiwan evolves from a well--organized transverse drainage 293 system that progressively turns southwards into asymmetrical and disordered 294 drainage system stretched over the whole Hengchun Peninsula (Fig. 3). Across the 295 Central Range, the highest elevation on the swath profile (mean, maximum or 296 minimum altitudes) is located near the divide (Fig. 3a). This contrasts with the 297 Hengchun Peninsula where the highest elevation and main divide split in two 298 different eastern and western subdomains and associated divides (Fig. 3b). 299

For instance, the highest domain of the peninsula (~1000m) is observed in the 300

Lilong Range that sets close to the western coast. Conversely, the Mutan Ponds in 301 the eastern part of the peninsula is lower (~400m) but are located along the main 302 divide (Fig. 3b). 303

Relief map (Fig. 4) shows that the maximum local relief (between 400 m and 800 m) 304 is located in the northwest part of the Hengchun Peninsula, in the Lilong Range. The 305 southern and eastern parts of the peninsula are characterized by an alternation of 306 moderate (between 250 m and 400m) and low reliefs (lower than 250m). Figure 4 307 further reveals that the both low and high relief domains are oriented N--S. 308

The main drainage basins are Fengkang, Sizhong and Gutou (Fig. 5). They drain over 309 60% of the peninsula's surface (of the area of 3 x 10 2 km 2 ), while over 20 basins 310 distributed along the coasts have drainage areas in the 2 to 5 km 2 range with only 311 three basins of ~1 km 2 and two of ~20 km 2 . The significant feature is that 312 headwaters of the two main basins (Fengkang and Sizhong) are positioned in the 313 low--relief area of the Mutan Ponds, while their outlets set at the high--relief region in 314 vicinity of the Lilong Range. 315

They drain northern and central Hengchun Peninsula and therefore are the most 316 suitable targets to examine the fluvial network response of the landscape to the base 317 level change. Gutou drainage basin at the southern part of the peninsula flows into 318 the Pacific Ocean to the east. Its whole basin is situated in the low--relief zone and 319 together with Mutan Ponds in the north form the youngest landscape of the 320 emerging wedge (Fig. 4). Headwaters of the Fengkang, Sizhong and Gutou basins 321 begin at comparable elevations of 350--400 meters, but only Sizhong and Fengkang 322 expose apparent shrinkage of the relict landscape. 323

Fengkang basin channels expose knickpoints in the upstream area. This contrasts 324 with the Sizhong basin that is characterized by a more extensive distribution of the 325 knickpoints, probably reflecting a more dynamic response to the relative base level 326 fall. In the Gutou basin, knickpoints are apparent only in the vicinity of the drainage 327 divide (Fig. 5). East of the main divide, a number of knickpoints reveal advancing 328 fluvial processes, however their irregular vertical distribution suggest different 329 origin. Some of knickpoints located close to the main divide are related to capture 330 events of the Sizhong basin at the Mutan Ponds area. 331 332

Long river profiles, steepness and concavity indices 333

River profiles are commonly used to infer the uplift pattern in active tectonic area 334 [START_REF] Snyder | Landscape response to 661 tectonic forcing: Digital elevation model analysis of stream profiles in the 662 Mendocino triple junction region, northern California[END_REF][START_REF] Kirby | Quantifying differential rock--uplift rates via stream 600 profile analysis[END_REF][START_REF] Kirby | Expression of active tectonics in erosional landscapes[END_REF]. We 335 extracted the main profiles of Fengkang, Sizhong and Gutou (trunk and tributaries) 336 (Fig. 6). One particular feature of these basins is that headwaters of main trunks are 337 located at low--elevation close to the Mutan ponds and are thus at lower altitudes 338 than their tributaries. None of three basins presents full graded or concave up 339 profiles arguing for a transient stage. They present numerous knickpoints and 340 knickzones that are located in the upstream part of the trunks and in the tributaries 341 that join them at a maximum distance of 10--15 km from the headwater. Knickpoints 342 are located at an altitude of about 300m in the Fengkang basin, between 200 and 343 500 m for Sizhong basin, and between 200 and 400 m for Gutou basin. 344

The slope--area relationship for Fengkang, Sizhong and Gutou main trunks show the 345 same pattern with some remarkable breaks in regression lines corresponding to the 346 position of knickpoints (Fig. 6). 𝜃 values for Fengkang river are 0.42 and 0.43 347 upstream and downstream of the break, respectively. We choose a value for 𝜃 !"# of 348 0.45 consistent with previous works on Taiwan [START_REF] Wobus | Hanging valleys in fluvial systems: 696 Controls on occurrence and implications for landscape evolution[END_REF]. This 𝜃 !"# value 349 corresponds to ksn values of 7.19 and 33.4 upstream and downstream of the break, 350 respectively. This kind of pattern with a break in the slope--area relationship and a 351 sudden rise of ksn values upstream may be interpreted as a transient profile due to 352 an increase of uplift rate (e.g., [START_REF] Whipple | This is well documented for Fengkang, Sizhong and Gutou 444 rivers (Fig. 6). Both geomorphic markers--low reliefs and ksn distribution maps 445 reflect a transient landscape evolution consecutive to an increase of the uplift rate[END_REF]. We obtained similar values for 353 Sizhong's ksn (9.52 to 30) but the increase is lower toward the south for Gutou basin 354 (11.3 to 16.4). We also calculated ksn values at the scale of the Hengchun Peninsula 355 with the auto--ksn tool developed by [START_REF] Whipple | New Tools for 682 Quantitative Geomorphology: Extraction and Interpretation of Stream Profiles 683 from Digital Topographic Data[END_REF] that allows to do 356 regression over a specified moving window (here 0.5 km) (Fig. 7). Results obtained 357 confirm the pattern observed with relief maps. Highest values of ksn (60--200) are 358 located in northwest part whereas lowest values (0--15) are located in the southern 359 part and in the Mutan Ponds. With this analysis we confirm that ksn values are lower 360 in the upstream part than downstream for Fengkang, Sizhong and Gutou basins. 361 This trend is not observed on rivers that flow toward the east. This reflects the fact 362 that inherited low--relief domains also characterized low ksn values are confined on 363 the western side of the main divide and are incised with rivers flowing toward the 364 west. 365 366

Modeling knickpoint retreat: Is there a wave of regressive erosion in the Hengchun 367

Peninsula? 368

The only basin where ksn in the main trunk is higher or in the same order than 369 tributaries is the Sizhong basin and could correspond to a local wave of regressive 370 erosion. 371

If one examines the map distribution of knickpoints on the Hengchun Peninsula, 372 they do not appear to follow a full radial pattern in the different watershed of the 373 peninsula except for the Sizhong basin (Fig. 5). We further notice that ksn in main 374 trunks are not statistically higher than their tributaries (Fig. 7) as commonly 375 expected for a wave of knickpoint migration [START_REF] Wobus | Hanging valleys in fluvial systems: 696 Controls on occurrence and implications for landscape evolution[END_REF]. Results of 376 knickpoint migration modelling reveal that none of two end--member models can fit 377 both west and east sub--catchments of Shizong (Fig. 8). In the west sub--catchment, 378 the celerity model derived from the parameters used in CW model fits better the 379 position of the five current knickpoints than those derived from LVDD model. The 380 position of three knickpoints is well reproduced and the two others do not exceed 1 381 km of misfit assuming CW model for a time duration of 17 Ky. With the LVDD model 382 only one knickpoint is reproduced and four with misfit higher than 1 km for a time 383 duration of 36 ky. In the east sub--catchment, the LVDD model well reproduced the 384 position of one knickpoint and the other one with error of more than 1 km. In 385 contrast, the CW model reproduces both knickpoints within an error of more than 1 386 km and less than 1 km, respectively. 387 Two conclusions arise from the quantitative modeling of knickpoint retreat. First, if 388 our starting point of knickpoint migration is valid, then the response time according 389 to 10 4 --10 5 celerity parameters can be estimated to about 17 Ky (according to the 390 best fit model) in the west sub--catchment of Sizhong. In the Hengchun Peninsula, a 391 wave of knickpoints migration can be observed but is limited to a local scale (here 392 ~20 km²) that excludes a uniform uplift of the whole peninsula. Second, knickpoints 393 migration both in the eastern and western sub--catchments cannot be reproduced 394 with a simple celerity modeling. By testing two end--member models that are 395 assumed to reproduce the whole range of knickpoint regressive erosion processes, 396 we rule out an incomplete parametric study. We rather suspect changes in the 397 upstream drainage area that can be explained by a process of capture of one sub--398 catchment by another one. This piracy process could explain why the altitude of the 399 graded part of each trunk profile, for west and east sub--catchments, is different. 400 401

River capture 402

A significant part of knickpoints cannot be explained by a wave of regressive erosion 403 moving upstream and incising an uplifting inherited landscape. The observation of 404 capture points instead is remarkable in the Hengchun Peninsula. This argues for 405 drainage reorganization caused by piracy or capture effects, especially in the 406 Sizhong basin where this mechanism is probably the most significant. For example, 407 a number capture points can be observed along the main divide suggesting its 408 westward movement. Fieldwork investigations carried out along the main drainage 409 divide confirm very active regressive erosion upstream of the eastern drainage 410 basins (Fig. 9). 411

Figure 10 shows a good example where part of the Sizhong basin was captured by 412 eastward draining tributary of the Xuhai channel. The knickpoint observed along 413 the western tributary is very likely a capture point as indicated by the abrupt 414 change of flows above the knickpoints and the large inflexion of the divide (Fig. 10b, 415 red arrow). 416

The upstream reaches of Xuhai basin display an asymmetrical pattern (Fig. 10c). 417

Using the shape of the main drainage divide, flow direction of the tributaries 418 together with assumption that knickpoints are actually capture points, we obtained 419 a possible drainage arrangement before the capture with a symmetrical basin model 420 (Fig. 10d). 421

The regressive erosion that led to river capture is not only observed at the main 422 drainage divide. For instance, the 'pre--capture' geometry is seen in the vicinity of 423 the downstream of the Sizhong River (Fig. 11). Zhushe tributary of the Baoli River 424 flows nearly 100 meters above the meander of the Sizhong main channel. An outside 425 band of the Sizhong meander undercuts a hill slope removing western tributaries of 426 the Baoli channel. This reflects that continuous regressive erosion is currently 427 triggering a capture by merging the upper part of the Baoli basin with the Sizhong 428 basin. This will result in the migration of the drainage divide toward the east. A very 429 low Asymmetry Factor (AF = 20) of the central and upper part of the Baoli channel 430 confirms the role of capture dynamics. 431 432

Discussion 433

The geomorphological analysis performed on the Hengchun Peninsula argues for a 434 transient stage of the landscape evolution of the southern Taiwan (Ramsey et al., 435 2007;[START_REF] Stolar | In this case, the drainage rearrangement is no related to a 458 'rejuvenation' of the topography, but rather to a birth of a new landscape. The 459 studied landscape is recent because marine deposits younger than 100 ky[END_REF]. The presence of preserved low reliefs may be classically 436 attributed to a change of uplift not yet counterbalanced by fluvial erosion (e.g., 437 [START_REF] Davis | The geographical cycle[END_REF]. The normalized steepness indexes of rivers (ksn) are also strongly 438 correlated to relief maps (Fig. 4 and7) with an increase of ksn downstream of 439 1990) whereas Hengchun limestone suggests a surface uplift around 100--200 m for 463 the last 100 ky [START_REF] Cheng | Biostratigraphic study in the west Hengchun hill[END_REF][START_REF] Chen | Reconsideration of the stratigraphy of the western 567 Hengchun Hill[END_REF]. Assuming the 464 southward propagating model, the ancient coastline and outlet of drainage systems 465 was positioned between the present low--relief of Mutan Ponds and the current 466 coastline. Consequently, before this emergence in Late Pleistocene times, the low 467 relief of Mutan Ponds should be in a direct continuation with the coastline. The 468 landscape during this initial period was likely composed by reduced EW--directed 469 transverse streams, and by the N--S drainage in the Mutan Ponds area at the 470 southern limit of the island (Fig. 12a). The presence of current low reliefs with a N--S 471 trend in the Gutou drainage basin (Fig. 4) argues for such a physiography during 472 this pre--emergence period. The post 100 ky emergence of the submarine 473 morphology resulted in an asymmetrical arrangement of the drainage pattern. The 474 sudden emersion triggers a wave of regressive erosion upstream in drainage basins 475 that were connected to the newly emerged prism. We have documented this wave of 476 regressive erosion in western drainage basins especially for the Sizhong basin. This 477 kind of pattern where transverse drainages coexist with or capture inherited 478 longitudinal paleo--drainages at the termination of tectonic emerging structures has 479 been described in growing folds (e.g. [START_REF] Ramsey | Fold evolution and drainage 642 development in the Zagros mountains of Fars province, SE Iran[END_REF]. 480 One other possible effect of this emerged asymmetric area to the west is the shift of 481 the drainage divide towards the center of the peninsula, triggering the isolation of 482 the initial low reliefs (Mutan Ponds) (Fig. 12b). Such scenario could explain the 483 current inflection of the drainage divide inside the Hengchun Peninsula. 484

Then, the continuation of regressive erosion of western catchments (Fengkang and 485 Sizhong) will eventually capture these low reliefs. If we consider an endoreic system 486 in Mutan Ponds due to lake deposits in Holocene times up to 2 ky [START_REF] Lee | Pollen records from southern Taiwan: 607 implications for East Asia summer monsoon variation during the Holocene[END_REF], 487 the capture event in the upstream part of Fengkang and Sizhong could be only few 488 thousand years old. This can explain why we observe some misfits between eastern 489 and western sub--catchments in Sizhong basin. 490

As the surface uplift propagates southwards, the main drainage divide remained at 491 the eastern flank of the peninsula. The continuous uplift of the low--relief Mutan 492

Ponds increases incision from the east--draining basins. This wave of regressive 493 erosion leads to the second stage of captures. As a result, the eastern basins enlarge 494 their areas due to the west--draining catchments and consequently move the main 495 divide to the west (Fig. 12c, red arrows). Documented captures of the east--draining 496 basins as well as potential points of future piracies substantially support this 497 concept (Fig. 10 and11). However, the locations of the high and low reliefs as well 498 as the distribution of normalized steepness index ksn suggest the western low--relief 499 terrains to be missing (Fig. 4). It can be related to a submarine topography west to 500 the Hengchun Peninsula where the slopes are too steep to emerge a vast low--relief 501 area or due to fast uplifts of the narrow accretionary wedge generated submarine 502 landslides removing the west flank of the low--relief areas. 503

In summary, the model describing the evolution from the emerging submarine 504 surface to the birth of a new landscape support the transient character of the 505 Hengchun Peninsula. The drainage rearrangement pattern reveals localized waves 506 of knickpoints retreat and shrinkage of the relict landscape as a coherent process 507 (Fig. 13). One of the significant assessments of this study is that the landscapes of 508 the emerging orogens even in the short time span appear to be justified. Our 509 geomorphological analysis and proposed evolutional model does not allow 510 distinguishing between the different modes of crustal accretion proposed for the 511 Central Range of Taiwan further North: underplating of a subducted crust (Simoes 512 et al., 2007) or ductile flow in the deep crust [START_REF] Yamato | Taiwan mountain building: insights from 699 2--D thermomechanical modelling of a rheologically stratified lithosphere[END_REF]. However, our 513 data and results support a lack of distributed shortening/thrusting across the 514 peninsula, in the southern Central Range. This suggests the region is uplifting as a 515 crustal block, with shortening being accommodated by aseismic deformation in the 516 deeper crust. This might support the onset of crustal shortening below the shallow 517 accretionary wedge as proposed from nearby offshore observations (McIntosh et al., 518 2013). 519 520

Conclusions 521

In this study, we focused on the emerging topographic ridge at the southernmost tip 522 of Taiwan orogen, which is the onboard continuity of the offshore accretionary 523 White cross represents a modeling of the migration with a celerity modeling where dx/dt=0.0001A 0.5 [START_REF] Loget | Wave train model for knickpoint migration[END_REF]. Blue triangel represents the modeling with dx/dt=0.0000079A 1.1125 [START_REF] Crosby | Knickpoint initiation and distribution within 574 fluvial networks: 236 waterfalls in the Waipaoa River, North Island, New 575 Zealand[END_REF]. Time stopped for all points at T=12 ky and T=17ky, respectively. SP--represents the starting point.

Field

  campaigns were conducted from 2011 to 2014 to establish a direct control on 209 geomorphological features recognized from the DEM. In particular, areas of 210 potential river capture were investigated to evaluate the possible control by local 211 lithological or artefacts. Overall, field surveys allowed us to improve our 212 understanding of the incision processes in upstream regions in complement to 213 quantitative analyses. 214 12 215

  prism. The extraction of geomorphic parameters shows a transient nature of the 524 topography of the Hengchun Peninsula in particular attested by knickpoints 525 distribution and steepness index of rivers. However, we demonstrate that a simple 526 model of regressive erosion following uplift as a rejuvenation of the topography is 527 unlikely at the scale of the entire Peninsula. Knickpoint celerity modeling shows that 528 it is restrained to the upstream part of the Sizhong basin. On the other hand, the 529 current asymmetrical drainage system, linked to a main divide flanked on the 530 eastern side of the Peninsula and where N--S low reliefs (Mutan ponds area) are 531 pinned, is currently affected by numerous capture processes. This indicates a 532 progressive rearrangement of the drainage system confirming the transient nature 533 of the topography. We have proposed an evolutional model of the Hengchun 534 Peninsula that can explained: (i) how from an antecedent N--S drainage (due to a 535 paleo--northern coastline) it is possible to generate low relief and offset divide (ii) 536 then uplifted and captured by E--W transverse drainages (due to a progressive 537 emergence and shifting coastline). In other words, the Hengchun Peninsula's 538 topography results from a mixing between rejuvenation and drainage 539 reorganization due to the emergence of the southern domain of Taiwan. The scale of 540 this landscape perturbation and the lack of localized tectonic structures argue for an supported by National Science Council, Taiwan (NSC 101--2116--M--545 008--006、NSC 102--2116--M--008 --003、MOST 103--2116--M--008--024)., 1984. Local relief and the height limits of mountain ranges. American J. 551Science, 284, 1035--1055. 552

Fig. 1 .

 1 Fig. 1. Geological setting of Taiwan arc--continent collision. Left: map showing the

Fig. 2 .

 2 Fig. 2. Geological map of the Hengchun Peninsula (modified after CGS 1989; Chang

Fig. 4 .

 4 Fig. 4. Relief map of the Hengchun Peninsula (computed with 5 km sliding window).

Fig. 5 .

 5 Fig. 5. Drainage basins of the Hengchun Peninsula: 1--Fengkang, 2--Sizhong, 3--Gutou

Fig. 7 .

 7 Fig. 7. Normalized steepness index (ksn) of the channels at Hengchun Peninsula 9

Fig. 9 .

 9 Fig. 9. Photograph looking to the west showing active regressive erosion in the

Fig. 10 .

 10 Fig. 10. Capture processes in the western part of the Xuhai basin. a) DEM with

Fig. 11 .

 11 Fig. 11. Capture processes of the Zhushe River (tributary of the Baoli River) by the

Fig. 12 .

 12 Fig. 12. Evolutionary model of the Hengchun Peninsula. (a) Tip of an emerging

Fig. 13

 13 Fig. 13. 3--D view of the Hengchun Peninsula's topography and its main basins. White