Juan B. Marco

Efficiency of Storm Detention Tanks for Urban Drainage Systems under Climate Variability

Climate change effects on combined sewer systems efficiency is a great matter of concern. In fact, changes in rainfall regime could significantly affect combined sewer overflows (CSOs) into receiving water bodies. Given that CSOs are a significant source of pollution, a better understanding and modeling of climate change effects on urban drainage systems is a compelling requirement to support design of adaptation strategies. This paper aims at studying the resilience of storm water detention tanks efficiency with respect to changes in rainfall forcing. In detail, an analytical probabilistic model is proposed to assess overflow reduction efficiency and volumetric efficiency of detention tanks depending on behaviors of climate and urban catchment. Sensitivity of tank efficiencies is evaluated under assigned changes in rainfall forcing. Results show that resilience of storm tanks benefits from filtering of climate change effects operated by the urban catchment.

Flood risk mapping

Flood risk maps are the essential tools for land use planning in flood-prone areas. The basic criteria for mapping are usually chosen according to flood return periods. Sometimes, the expected water depth or dynamic considerations are used instead. These criteria are discussed on mapping examples from several countries. To draw a flood risk map, four phases are usually recognized: hydrologic, geomorphic, hydraulic and land use. Each of these phases poses different problems and requires relevant methodologies to accomplish them. As an example, the program for flood risk mapping of the Valencia Region in Spain is presented. A flood risk map is considered as a preliminary yet necessary initial step for all regional development policies.

Partial area coverage distribution for flood frequency analysis in arid regions

Convective rainfalls of high intensity are largely responsible for most large floods occurring in arid regions. Convective storms are limited in size and rarely cover an entire basin. Therefore flood frequency is affected by the distribution of storm coverage c, which is the fraction of basin area upon which rainfall occurs. Analytical probability density function (pdf) estimates of partial storm coverage are presented as a function of basin area sc and the probability distribution function for storm radius rs Conditional pdfs for the covered area fraction c, given the storm radius f(c/rs), are obtained as a compound distribution with finite probability for maximum coverage and continuous density for smaller values. The conditionality assumption is removed by assuming an exponential storm radius marginal distribution to obtain the catchment coverage pdf. This pdf is compound, with discrete probability for complete catchment coverage P(c=1) and a continuous function for partial overlap. The continuous function combines a Weibull pdf and linear functions of c. Both P(c=1) and f(c)) depend on χ, the average catchment-to-storm radius ratio. The distribution explains the partial coverage data for the Walnut Gulch basin in Arizona. Departures were observed, however, for slightly less than complete coverage, namely, for 0.9 < c < 1. A conceptual event classification and a method were designed to include partial coverage effects on flood frequency distribution.

Urban flooding: the flood-planned city concept

Urban flooding can be generated by two different mechanisms. A town or city occupying a part of the flood plain can be flooded by external floods, or else flooding can be generated by internal urban runoff. In this second case, a completely different hydrology occurs, governed by technologies of urban hydrology and hydraulics. The same flooding protection, however, can be applied for both mechanisms. Urban-generated floods are characterized by specific mechanisms resulting from highly impervious urban areas. Urban hydraulics also has particular features. Insufficient storm drainage capacities basically underlie all urban runoff flooding. Nevertheless, the surface drainage system is the key problem in most cases. In this situation, flooding takes place at runoff concentration areas. Urban design for flood protection includes several types of measures, usually termed flood-proofing. This concept of American origin should be generalized for European urban areas which are densely populated. It should encompass the conception of the entire city. A city under flood should be designed as an artificially created differential flood. This urban design concept is illustrated by several examples. Building codes should use appropriate technical standards for foundations, insulation, basements, lifts, etc. Finally, public services and facilities should be treated in such a way that they suffer the least possible damage from flooding. Flood-proofing of a city should be replaced by the concept of a flood-planned city.

The Segura River Basin Model. Disaggregation in a Semiarid Environment

The Segura basin extends over 14,925 km2 with a mean annual flow of 964 Hm3. The river is located in southeastern Spain, the most arid zone of the country. The mean annual rainfall over the basin amounts to some 300 mm. From Roman times onwards an extremely good climate has produced intensive water use for irrigation and urban supply with the result that this is the most exploited river in the country. Only 60 Hm3/year are delivered to the sea, most of this in the form of floods, so consumption amounts to 93% of the water resources.