Preliminary design of a flood abatement control zone (ZAC) in the region of Murcia, Spain

Abstract

The coastal area of the Murcia region in Spain experiences more frequent and more intense flash floods caused by inland heavy rainfall due to climate change. This results in high water levels in towns and cities, leading to risk of loss of life and many financial damages. Currently the region is vulnerable, because there are no hydraulic structures that regulate the flow safely downstream towards the coastline. Instead, at this moment the current channels are inadequate dry rivers. To reduce the risk of flooding, the objective of this design was to design a flood abatement control zone (ZAC). This design followed the design approach for Hydraulic Engineering.

A ZAC consists of a series of dikes that enclose separate reservoirs that are able to temporarily store water. This dampens the peak discharge of the flash flood, which reduces the flood risk of the downstream area. The peak reduction is the main function. The water that is stored is being discharged with a delay, spreading an acceptable discharge over a longer time to discharge the same rainfall event volume.

The first step of the design was to define the system of the ZAC. The ZAC was combined with the creation of an up- and downstream channel with short lengths to fit the structure into the environment. Its design life was set at 50 years, corresponding to a design rainfall event of once per 474 years. To design this structure, it was important to determine the maximum rainfall event discharge and the existing maximum discharge capacity without floods occurring downstream. These 2 factors, together with soil properties and land boundaries, acted as boundary conditions to the system.


In step 2, two locations were considered as potential construction location, both indicated by the client Universidad Polytechnica de Cartagena (UPCT). By applying a multi-criteria analysis (MCA) based on predominantly the water inflow, potential storage area, close company buildings and houses, the more upstream location was chosen.

Step 3: in order to design an adequate ZAC, a model was required to create the before-and-after-construction situation. A hydrological 2D-flow model was created in HEC-RAS. This was closely tied to functional design and design steps were taken iteratively. The 2D-model was able to compute flow in longitudinal and lateral direction, which is strongly needed in a flooded terrain. The most important design parameters to test in the model were the culvert-spillway-structure and the number of reservoirs. The model was validated qualitatively by flood maps from Centro de Descargas del CNIG.

Then, the functional design of the ZAC was done in step 4. The ZAC was placed partially dug into the soil upstream, and partly sticking out of the soil downstream. Upon iteration, it was decided to create 5 reservoirs, because of costs and a smaller marginal peak reduction effect of extra reservoirs. The ZAC is created in combination with a downstream funnel, downstream outflow channel and upstream channel. This means that the reservoirs are enclosed by 2 side dikes and 6 lateral dikes.

Then, detailed design in step 5 followed. The culvert-spillway structure was designed. The aim of this structure is to let water through the reservoirs without overflowing and therefore damaging the dikes. The structure consists of a culvert, spillway and retaining walls. For each element an MCA was set-up to determine the optimal shape, followed by choosing the design alternative. The design is a trapezoidal spillway, an arched culvert and a retaining wall.

With the optimal design, a conceptual design is constructed. In this conceptual design, the reinforced concrete dimensions and governing load combinations are determined. From this, the strength of the ZAC structure is evaluated in the finite element method program DIANA.

Finally, the final design was created. The conclusion is that the combination of structural elements that have been modelled in the system satisfies the aim to reduce the design rainfall event flood wave enough to avoid flood risk to the downstream areas of Murcia. The main uncertainties are the scaling of the data of the design rainfall event and the used soil characteristics. Further research could look into quantitative validation of the 2D-flow model, the implementation of sediment transport in the model and into optimizing bed protection downstream of the ZAC.