|Title||Decision Support System for Floodwater Spreading Site Selection in Iran|
|Author(s)||Kheirkhah Zarkesh, M.|
|Source||Wageningen University. Promotor(en): Leo Stroosnijder, co-promotor(en): A.M.J. Meijerink; M.A. Sharifi. - Enschede : ITC - ISBN 9789085042563 - 259|
Land Degradation and Development
|Publication type||Dissertation, externally prepared|
|Keyword(s)||hoogwaterbeheersing - inundatie - waterbeheer - besluitvorming - simulatiemodellen - iran - stroomvlakten - overstroomde gronden - beslissingsondersteunende systemen - flood control - flooding - water management - decision making - simulation models - iran - floodplains - flooded land - decision support systems|
Keywords: DSS, remote sensing, GIS. spatial multi-criteria evaluation, analytical hierarchy process, floodwater spreading, groundwater recharge, Iran.
Most aquifers of semi-arid Iran suffer from over-exploitation of groundwater for irrigation. It is therefore important to augment the groundwater resource by artificial recharge, using flood waters that flow into salt lakes or in the sea. The recharge schemes consist generally of diversion of part of the flood discharges of ephemeral rivers in small to medium sized catchments onto infiltration basins.
Apart from recharge of groundwater, supporting food production and drinking water supplies, the schemes have other benefits, such as mitigation of flood damages and 'greening of the desert'. Many governments, including the one of Iran, place now much emphasis on increasing the number of floodwater spreading schemes.
A large number of factors play a role in the selection of the most suitable sites for deciding on investment in a scheme. These factors pertain to earth science (geology, geomorphology, soils), to hydrology (runoff and sediment yield, infi!tration and groundwater conditions) and to socio-economic aspects (irrigated agriculture, flood damage mitigation, environment, job creation and so on). Hence, the decision depends on criteria ofdiverse nature.
This thesis deals with developing a Decision Support System (DSS) to assist decisions as to where the most suitable catchments and associated infiltration areas are and to work out options of types of schemes, which are adjusted to the characteristics ofthe selected infiltration area (the site available).
After discussion of the bio-physical setting for flood spreading schemes in the Introduction (Chapter 1), attention is given to the selection ofthe desired approach for multi-criteria evaluation in Chapter 2. The Analytical Hierarchical Processes (AHP) approach was considered to be appropriate for the problem at hand and use was made of the spatial extension of this approach in a GIS environment, after structuring all the major criteria for a flood spreading scheme.
Of key importance is of course the expected infiltration of flood water diverted. For such an a-priori estimate the effect of soil textures in a soil column on infiltration and percolation have to be made, as well as an estimate of the effects of sedimentation of clay and sand in a scheme, as well as effect of inundation depth and flooding frequency. One-dimensional soil modelling was done with the SWAP model. Chapter 3, using two pedo-transfer functions for the hydraulic parameters based on textures. It was found that for coarse textured soils there was reasonable agreement between functions used, but quite some differences were found for the soils containing clay and silt. As expected, recharge efficiency was positively affected by inundation depth and by rapid succession of inundations.
Because simulation results differed, the recharge of the complex and large Sorkhehesar scheme was analysed, by developing a spreadsheet programme to work out depths, areas and duration of inundation. Chapter 4. It was found that the Mnalem-van Genuchten transfer function was the most appropriate one.
Given the large number of ephemeral rivers draining hilly catchments and passing through alluvial areas, is necessary to first use a rapid screening method to obtain zones which contain promising areas, for which the main criteria are applicable. The screening depends heavily on interpretation of remotely sensed images, which have the advantage that various aspects are presented in a synoptic view. The interpretation has to have a firm footing in earth science because runoff and sediment delivery processes have to be estimated in qualitative terms, as well as aquifer properties. A number of examples from Iran have been described in Chapter 5, highlighting the importance of transmission losses.
In order to select the most suitable area among the promising ones, the spatial AHP was applied in the Varamin zone under consideration of a multitude of criteria, in Chapter 6. The difficulty in the application was to develop and specify the preferences that are the base of the relative importance values for all the decision criteria involved at different levels, once the various factors had been estimated using standard hydrological and other methods. The use of the linguistic measures of preferences in the pair-wise comparisons made it possible to implement the full procedure, even though data differed much in nature, consistency and quality. In the Varamin zone, the Chandab catchment and infiltration area came out as the ''most satisfying" area; it had the highest sum of utilities for three of the four objectives and the highest score in a combination scenario.
For the Chandab infiltration site, a spreadsheet model was developed to work out various options with regard to type of scheme (shallow basin or deep basin type) and size of scheme, resulting in the expected volumes of recharge during the lifetime of the scheme by assuming desilting operations. Of the additional benefits, the flood damage mitigation could be expressed in monetary terms, and that benefit had a profound effect on the cost per unit volume of recharge water if one opts for the two large sized scheme designs.
To our knowledge, this study presents for the first time information on costs and benefits of a flood water spreading scheme in Iran in a structured manner of various designs adjusted to a particular site.
The final conclusion (Chapter 8) states that the DSS described here, with its three stages of increasing focus and associated data requirements and with the approach for evaluation using a multitude of criteria of diverse nature, has proven to be applicable and an efficient way to select the most appropriate alternatives for making a choice for investment in a flood spreading scheme. Although the emphasis was on Iranian conditions, the DSS is essentially of generic nature and may be applied ~ mutatis mutandis- to other semi arid regions.