Repository logo
 

Search Results

Now showing 1 - 5 of 5
  • The use of detention basins to reduce flash flood hazard in small and steep volcanic watersheds - a simulation from Madeira Island
    Publication . Vieira, I.; Barreto, V.; Figueira, C.; Lousada, S.; Prada, S.
    Madeira, like many mountainous volcanic islands, is susceptible to flash floods. Throughout its history, about 40 large events resulted in more than 1200 victims and countless damages. Recently, urban areas expanded greatly, leading to a higher exposure of the population to flash floods. In order to analyse ways to reduce vulnerability and decrease hazard in the urbanised, flash flood prone watershed of the Machico River, the construction of detention basins (DB) is simulated. A hydrological and hydraulic model was performed to determine if they would be a viable option to protect downstream populations. Modelling suggests that two 12 m high outlet structures, and a storage capacity of 111 298 m3 (DB1) and 121 095 m3 (DB2), would reduce peak discharge by 72%, from a precipitation event with a return period of 100 years. Two identified sen sitive Sections (S) had their fill rates reduced from 130.6% to 79.6% (S1) and from 128.6% to 33.4% (S2), thus preventing channel overflow. A concise eco nomic analysis was made in terms of implementation and maintenance costs, as well as a SWOT analysis highlighting that DB should be regarded as a viable engineering solution to reduce vulnerability to flash floods hazards in volcanic islands with small, steep, and urbanised watersheds.
  • Cloud water interception in the temperate laurel forest of Madeira Island
    Publication . Figueira, Celso; Sequeira, Miguel Menezes de; Vasconcelos, Rita; Prada, Susana
    A cloud belt frequently forms on the windward side of Madeira Island, between 800 and 1600 m a.s.l., as a result of adiabatic cooling of the northeastern trade winds that are forced upward. Temperate laurel forest is the most common vegetation inside that cloud belt altitudinal range. Cloud water interception was estimated by comparing precipitation and throughfall during a hydrological year. It totalled 200 mm (8% of rainfall) during 65 days (3 mm d-1) and seems to constitute a larger fraction of water input during drier months. Multiple linear regression between gauge standard deviation and throughfall throughout rain events shows that cloud interception is common before the onset of rainfall. Its role in the ecohydrology of laurel forest and in the island’s hydrology should be acknowledged. Further studies on this issue should be a priority in order to better understand these dynamics and provide tools for the correct management of this protected forest and the island’s groundwater resources.
  • Cloud water interception in the high altitude tree heath forest (Erica arborea L.) of Paul da Serra Massif (Madeira, Portugal)
    Publication . Prada, Susana; Sequeira, Miguel Menezes de; Figueira, Celso; Vasconcelos, Rita
    Cloud water interception (CWI) occurs when cloud droplets are blown against the forest canopy, where they are retained on the vegetation surface, forming larger water droplets that drip into the forest floor. CWI was measured from 1 October 1997 to 30 September 1999, on a first-line tree heath (Erica arborea), at Bica da Cana, Madeira Island. Rainfall was corrected for wind-loss effect and compared with throughfall and other climatological normals. The CWI depletion rate along a forest stand transect was also analysed during three distinct fog events in 2008. Cloud water was 28 mm day 1, corresponding to 68% of total throughfall and 190% of the gross precipitation. Cloud water correlates directly with monthly normals of fog days and wind speed and correlates inversely with the monthly air temperature normal. CWI has an exponential correlation with monthly relative humidity normal. Cloud water capture depletion along the stand shows a logarithmic decrease. Although a forest stand does not directly relate to a first-line tree heath, this study shows that CWI is a frequent phenomenon in the Paul da Serra massif. Restoration and protection of high altitude ecosystems in Madeira should be a priority, not only for biodiversity, ecological and economical purposes but also for its role in regional water resources.
  • Response to “Comment on fog precipitation and rainfall interception in the natural forests of Madeira Island (Portugal)”
    Publication . Prada, Susana; Sequeira, Miguel Menezes de; Figueira, Celso; Prior, Víctor; Silva, Manuel Oliveira da
    In this paper we discuss the comments given by Regalado and Ritter (2010) on our previous work (Prada et al., 2009). Here, we demonstrate that rainfall correction does not significantly change the results; make clear why Site 1’s fog precipitation values do not seem to be overestimated due to the low number of gauges; present observational evidence that suggest high water yields in Site 1; and why we consider that fog water may be important for groundwater recharge. We also review different methodological, ecological and climatic factors between Madeira and the Canary Islands’ fog precipitation studies that might explain different values between them (Ritter et al., 2008, 2009).