12 nd International Conference on Urban Drainage, Porto Alegre/Brazil, 11-16 September Page 1 of 8 Accounting for climate change in urban drainage and flooding: contrasting alternative approaches to devising adaptive strategies B. Gersonius 1 *, R. Ashley 1 , A. Jeuken 2 , A. Pathirana 1 and C. Zevenbergen 1 1 UNESCO-IHE Institute for Water Education, Westvest 7, Delft, 2611 AX, The Netherlands. 2 Deltares, Daltonlaan 400, Utrecht, 3584 BK, The Netherlands. *Corresponding author, e-mail b.gersonius@unesco-ihe.org ABSTRACT Two frameworks are presented that can be used to account for climate change uncertainty in investment decision making related to urban drainage and flooding systems, either cause- based or effect-based. In the former, Real-In-Options (RIO) is proposed as an approach to identify the optimal set of adaptive strategies in response to advances in knowledge about future climate change. Other approaches align with the effect-based framework. A relatively simple approach to implementing this framework is to use Adaptation Tipping Points (ATPs), which have been defined as the points where the magnitude of climate change is such that the current strategy can no longer meet the pre-set objectives. Responses for this approach aim to extend the location and timing of these ATPs to some acceptable future point. This paper compares the RIO approach with the ATP approach. The emphasis is on the procedural steps, benefits and limitations. The paper concludes with a summary of the key characteristics to assist in choosing the most appropriate approach. KEYWORDS Adaptation Tipping Points; climate change; flood risk management; Real-In-Options; urban drainage 1. INTRODUCTION Decision making for investments in urban drainage and flooding systems needs to take account of climate change uncertainty. This is because of two features associated with such systems. First, the consequences of investment decisions for these systems have to be lived with for a long time, which means that the associated uncertainties can grow larger. Second, potential irreversibilities in choices can lead to a need for larger construction initially, particularly in 'hard' structural measures; which allows for headroom for later adjustment. New approaches to devising adaptive strategies are needed to address these uncertainties (Kundzewicz et al., 2008). Otherwise, such strategies can be maladaptive, resulting in unnecessary costs of potentially irreversible measures (Barnett and O'Neill, 2010). Two frameworks are presented that can be used to account for climate change uncertainty in investment decision making related to urban drainage and flooding systems (Jones and Preston, 2010), framed around a Driver-Pressure-State-Impact-Response (DPSIR) continuum. The cause-based framework begins by considering the climate system (drivers) and moves through the pressures, state process to predict the impacts. Responses are then formulated to cope with these in a way that maintains expected performance levels. The alternative, effect- based framework starts with quantifying the outcome in the form of impact thresholds that define possible system states, and then the likelihood of attaining or exceeding this outcome is assessed.