INTRODUCTION<\/h2>\n
In\u00a0recent decades, Czech Republic (Czechia) has been affected by flood events of\u00a0varying frequency and spatial extent. Simultaneously, the\u00a0country has experienced periods of\u00a0prolonged drought, which have required significant restrictions from the\u00a0perspective of\u00a0water management and agricultural production. Considering the\u00a0recorded and projected climate developments\u00a0[1], the\u00a0occurrence of\u00a0hydrological extremes in\u00a0the\u00a0form of\u00a0droughts and floods can be expected to continue in\u00a0Czechia in\u00a0the\u00a0future.<\/span><\/p>\n Czechia is home to approximately 25,000 ponds and small reservoirs. In\u00a0connection with the\u00a0occurrence of\u00a0hydrological extremes, questions have arisen regarding how existing ponds and small reservoirs can withstand exposure to these two extremes while continuing to fulfil their primary purpose. According to \u010cSN\u00a075\u00a02405, a pond is defined as \u201can artificially drainable reservoir with a natural bottom, primarily used for fish farming\u201d\u00a0[2]. A small reservoir is defined according to \u010cSN\u00a075\u00a02410 as \u201ca\u00a0reservoir with a volume up to the\u00a0normal water level of\u00a0less than 2\u00a0million m\u00b3 and a\u00a0water depth of\u00a0less than 9\u00a0m\u201d\u00a0[3]. The\u00a0purpose of\u00a0such a reservoir may be, for example, storage, flood protection, fish farming, recreational, landscape, economic, water treatment, or remediation. Although these hydraulic structures (HS) represent a potential tool for water accumulation in\u00a0the\u00a0fight against drought, they are limited by their intended purpose and impose specific requirements on the\u00a0hydrological regime. For example, a small reservoir used for fish farming may find it difficult during droughts to ensure supplementary releases for minimum residual flow (MRF). From the\u00a0perspective of\u00a0flood protection, the\u00a0issue concerns securing these existing HS against the\u00a0effects of\u00a0floods with regard to the\u00a0protection of\u00a0the\u00a0surrounding land, property, and human lives downstream. They should be equipped with a sufficiently sized safety spillway; however, this is not always the\u00a0case. At present, existing ponds and small reservoirs exhibit certain\u00a0deficiencies that prevent them from effectively meeting the\u00a0required demands. The\u00a0aim of\u00a0the\u00a0research project, involving co-investigators from the\u00a0T. G. Masaryk Water Research Institute (TGM WRI) and VODN\u00cd D\u00cdLA\u00a0\u2013 TBD,\u00a0a. s. (VD \u2013 TBD), was to evaluate their current problems and assess the\u00a0possibilities regarding compliance with MRF and flood safety.<\/span><\/p>\n The\u00a0first step in\u00a0the\u00a0study was the\u00a0selection of\u00a0pilot sites, primarily in\u00a0the\u00a0South Bohemian Region. In\u00a0selecting a representative set of\u00a050 pilot ponds and small reservoirs, consideration was given not only to issues of\u00a0MRF and flood safety but also to maintaining their diversity. Pilot sites were chosen with a range of\u00a0retained volumes, inundation areas, catchment sizes, design of\u00a0outlet structures, and design of\u00a0safety spillways. The\u00a0locations of\u00a0the\u00a0pilot sites are shown in\u00a0Fig. 1<\/span><\/em>. The\u00a0project was based on available documentation of\u00a0the\u00a0HS, including operational rules and accessible hydrological data.<\/span><\/p>\n MRF is defined in Section 36 of Act No. 254\/2001 Coll., on Waters and on Amendments to Certain Laws (the Water Act), as \u201cthe flow of surface waters that still allows general use of surface waters and ecological functions of the watercourse, taking into\u00a0<\/span>account the\u00a0possibilities for recreational navigation\u201d\u00a0[4]. According to this section, water authorities are obliged to specify the\u00a0MRF in\u00a0the\u00a0water use permit. In\u00a0doing so, they must consider \u201cthe\u00a0conditions of\u00a0the\u00a0watercourse, the\u00a0possibilities for recreational navigation, the\u00a0nature of\u00a0water use, and measures to achieve the\u00a0objectives of\u00a0water protection adopted in\u00a0the\u00a0river basin\u00a0plan\u201d\u00a0[4]. The\u00a0method and criteria for determining MRF are to be based on government regulations.<\/span><\/p>\n The\u00a0legislative procedure to approve the\u00a0government regulation has been ongoing since the\u00a0amendment of\u00a0the\u00a0Water Act in\u00a02010 and has not yet been completed. Currently, the\u00a0valid Methodological Guideline of\u00a0the\u00a0Water Protection Department of\u00a0the\u00a0Ministry of\u00a0the\u00a0Environment for determining MRF values in\u00a0watercourses<\/span> from 1998 (hereinafter referred to as the\u00a0Methodical Guideline)\u00a0[5], along with other related legal regulations, is available. There were several reasons for the\u00a0Ministry of\u00a0the\u00a0Environment of\u00a0the\u00a0Czech Republic (MoE CR) to revise the\u00a0methodological approach regarding MRF\u00a0[6]. Firstly, it can be noted that the\u00a0so-called B\u00edlek Table, according to which indicative MRF values are calculated under the\u00a0Methodological Guideline, was originally intended for the\u00a0dilution of\u00a0wastewater below wastewater treatment plants. Furthermore, based on EU Document no. 31: Environmental Flows<\/span>, it was decided to reconsider the\u00a0existing approach and bring it more in\u00a0line with current standards, such as taking into account the\u00a0needs of\u00a0aquatic ecosystems and dividing the\u00a0MRF into at least two values during the\u00a0year.<\/span><\/p>\n Following the\u00a0amendment of\u00a0the\u00a0Water Act in\u00a02010, TGM WRI was tasked with reassessing the\u00a0approach to determining MRF according to the\u00a0Methodological Guideline and implementing additional requirements into the\u00a0newly proposed approach. This proposed approach became the\u00a0basis for the\u00a0draft Government Regulation of\u00a0the\u00a0Czech Republic on the\u00a0Method and Criteria for Determining MRF<\/span> (hereinafter referred to as the\u00a0Draft Regulation). The\u00a0approach considers regional hydrogeological characteristics, the\u00a0seasonal division of\u00a0MRF values throughout the\u00a0year, and the\u00a0inclusion of\u00a0multiple hydrological parameters in\u00a0the\u00a0MRF calculation. At the\u00a0same time, emphasis was placed on considering the\u00a0needs of\u00a0the\u00a0biological components of\u00a0the\u00a0aquatic environment.<\/span><\/p>\n According to the\u00a0proposed approach, MRF for reservoirs and reservoir systems is determined in\u00a0the\u00a0same way as for watercourses. In\u00a0doing so, consideration is given to the\u00a0reservoir operation and current hydrological conditions in\u00a0the\u00a0watercourse. However, if the\u00a0water management design of\u00a0these HS complies with the\u00a0requirements of\u00a0\u010cSN 75 2405 \u2013 Water Management Design of\u00a0Reservoirs, and if this is necessary for their intended purpose, the\u00a0MRF is determined differently. The\u00a0proposed approach to determining MRF in\u00a0watercourses divides Czechia into four regional areas. This regionalisation takes into account the\u00a0key processes involved in\u00a0the\u00a0formation of\u00a0total catchment runoff, with particular regard to hydrological and hydrogeological conditions. It also considers the\u00a0boundaries of\u00a0fourth-order catchments (according to Strahler). Each area is assigned a specific compensation coefficient. The\u00a0compensation coefficient for a given reservoir is determined on the\u00a0basis of\u00a0its classification within\u00a0the\u00a0appropriate area according to its hydrological catchment order number. Due to the\u00a0introduction of\u00a0seasonal differentiation of\u00a0MRF during the\u00a0year, the\u00a0MRF value is determined for two periods \u2013 the\u00a0main\u00a0season (May to January) and the\u00a0spring season (February to April).<\/span><\/p>\n For the\u00a0main\u00a0season, the\u00a0MRF is determined according to the\u00a0equation<\/span><\/p>\n and for the\u00a0spring season, the\u00a0MRF is determined according to the\u00a0equation<\/span><\/p>\n where:<\/p>\n MRF\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 is \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 minimum residual flow (m3<\/sup>\u00a0\u00b7\u00a0s-1<\/sup>)<\/p>\n Q355d<\/sub> \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 discharge achieved or exceeded on average 355 days per year (m3<\/sup>\u00a0\u00b7\u00a0s-1<\/sup>)<\/p>\n Qa<\/sub>\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/em>long-term mean annual discharge (m3<\/sup>\u00a0\u00b7\u00a0s-1<\/sup>)<\/p>\n Q330d<\/sub> \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 <\/em>discharge achieved or exceeded on average 330 days per year (m3<\/sup>\u00a0\u00b7\u00a0s-1<\/sup>)<\/p>\n K\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 compensation coefficient for the\u00a0given area, the\u00a0value of\u00a0which was derived with regard to the\u00a0requirement to keep the\u00a0MRF as close as possible to 25\u00a0% of\u00a0Qa <\/sub>(area 1: K = 1.2; area 2: K = 1.1; area 3: K = 1.05; area 4: K = 1.07).<\/p>\n In\u00a0the\u00a0proposed approach to determining MRF for reservoirs and reservoir systems, operational conditions and current hydrological conditions are also considered. During reservoir filling and operation, the\u00a0prescribed MRF should be maintained at the\u00a0reservoir outflow. If the\u00a0inflow into the\u00a0reservoir decreases below the\u00a0prescribed MRF value, the\u00a0reservoir outflow should equal the\u00a0inflow, as shown in\u00a0the\u00a0following equations:<\/p>\n <\/p>\n <\/p>\n where:<\/p>\n Qoutflow<\/sub>\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 is\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 outflow from the\u00a0reservoir (m3<\/sup>\u00a0\u00b7\u00a0s-1<\/sup>)<\/p>\n Qinflow<\/sub>\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 inflow into the reservoir (m3<\/sup>\u00a0\u00b7\u00a0s-1<\/sup>)<\/p>\n MRFprescribed\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/sub>prescribed minimum residual flow according to the equations above (m3<\/sup> \u00b7 s-1<\/sup>)<\/p>\n The\u00a0research project Design of\u00a0Ponds and Small Reservoirs in\u00a0Terms of\u00a0the\u00a0Possibility to Comply with MRF and Flood Safety<\/em> (TA CR, no. SS03010230) builds on previous activities of\u00a0TGM WRI related to the\u00a0proposed approach to determining MRF. The\u00a0project adopted and applied this proposed approach. It focused on the\u00a0method of\u00a0determining the\u00a0MRF value (according to both the\u00a0proposed approach and the\u00a0Methodological Guideline) and on the\u00a0possibilities of\u00a0maintaining MRF in\u00a0reservoirs. Consideration was given to the\u00a0HS\u00a0water balance and to the\u00a0real and technically feasible options of\u00a0the\u00a0outlet structure. During the\u00a0legislative process, the\u00a0question arose whether, in\u00a0determining MRF for small reservoirs and ponds, reservoir losses (e.g. through evaporation or seepage) should be taken into account by reducing the\u00a0MRF value accordingly. The\u00a0research project addressed this issue.<\/p>\n In\u00a0Czechia, the\u00a0assessment of\u00a0flood safety for HS is conducted according to \u010cSN\u00a075\u00a02935 \u2013 Assessment of\u00a0the\u00a0Flood Safety of\u00a0Hydraulic Structures. The\u00a0Czech technical standard (\u010cSN) is not generally binding in\u00a0itself (according to Act\u00a0no.\u00a022\/1997 Coll.). Standards become mandatory when they are referenced in\u00a0legal regulations. An example is the\u00a0reference to \u010cSN 75 2935 in\u00a0Section\u00a061 of\u00a0the\u00a0Water Act. The\u00a0application of\u00a0this standard results in\u00a0the\u00a0Flood Safety Assessment of\u00a0the\u00a0Hydraulic Structures<\/span>. The\u00a0assessment is prepared for all types of\u00a0dam construction (local material, concrete, brick, and combined) and applies to all HS categories in\u00a0accordance with Decree no.\u00a0471\/2001 Coll., on Technical Safety Supervision of\u00a0Hydraulic Structures. This issue was addressed in\u00a0the\u00a0project by the\u00a0co-investigator VD \u2013 TBD. As part of\u00a0the\u00a0flood safety assessment, the\u00a0project\u2019s task was to prepare assessments for the\u00a0individual pilot sites and, based on the\u00a0experience gained, to develop guidelines for applying \u010cSN\u00a075\u00a02935 to a characteristic type of\u00a0historical HS (ponds) falling within categories\u00a0III and\u00a0IV under the\u00a0technical safety supervision framework.<\/span><\/p>\n The\u00a0main\u00a0principles for preparing a flood safety assessment according to\u00a0\u010cSN\u00a075\u00a02935 are as follows [7, 8]:<\/span><\/p>\n The\u00a0Flood Safety Assessment has the\u00a0following standardized structure and\u00a0chapter designations:<\/span><\/p>\n Comparison of\u00a0MRF values determined according to the\u00a0Methodological Guideline and the\u00a0Draft Regulation (Tab. 1<\/span><\/em>) shows that applying the\u00a0approach in\u00a0the\u00a0Draft Regulation does not cause significant changes in\u00a0MRF values. Specifically, for 22 pilot sites, the\u00a0difference in\u00a0MRF between the\u00a0current Methodological Guideline and the\u00a0Draft Regulation was up to 5\u00a0%; for 9 sites, up to 10\u00a0%; for 7 sites, up to 15\u00a0%; and for 4 sites, the\u00a0difference exceeded 20\u00a0%. For 7 pilot sites, the\u00a0values were similar. It is worth noting that, in\u00a0some cases, the\u00a0MRF value specified in\u00a0the\u00a0water use permit differs from the\u00a0calculated guideline values according to the\u00a0Methodological Guideline. This discrepancy is either due to an older water use permit or was determined differently for a\u00a0specific reason. Comparison of\u00a0MRF values for selected sites, considering their classification into regions, is shown in\u00a0detail in\u00a0Fig. 2<\/em>.<\/span> The\u00a0pilot sites, divided into regions based on the\u00a0regionalization in\u00a0the\u00a0Draft Regulation, fall into Area\u00a03 and Area\u00a04. The\u00a0data analysis also showed that, for the\u00a0pilot sites, the\u00a0Q\u2083\u2085\u2085d<\/span><\/sub> flow was not undershot, with one exception (Fig. 3<\/span><\/em>). The\u00a0Draft Regulation sets Q\u2083\u2085\u2085d<\/span><\/sub> as\u00a0the\u00a0minimum allowable MRF value, because this flow represents the\u00a0threshold of\u00a0hydrological drought.<\/span><\/p>\n <\/p>\n <\/p>\n <\/p>\n The\u00a0project also focused on assessing the\u00a0relevance of\u00a0reducing the\u00a0prescribed MRF by losses due to evaporation and seepage into the\u00a0subsoil. Seepage into the\u00a0subsoil is related to hydrogeological conditions at the\u00a0HS\u00a0location. The\u00a0evaluation of\u00a0evaporation losses, presented in\u00a0Figs. 4<\/span><\/em> and 5<\/span><\/em>, shows an\u00a0increasing trend over successive decades. However, reducing the\u00a0MRF by evaporation losses is not acceptable for small reservoirs and ponds, as it would lead to a reduction of\u00a0the\u00a0MRF itself.<\/span><\/p>\n <\/p>\n The\u00a0research project examined the\u00a0feasibility of\u00a0maintaining MRF in\u00a0reservoirs, taking into account the\u00a0hydrological balance of\u00a0the\u00a0HS and the\u00a0actual and technically feasible capabilities of\u00a0the\u00a0outlet structure. A simplified hydrological balance of\u00a0the\u00a0reservoir, simulated using the\u00a0MAVONA application\u00a0[9], is shown in\u00a0Fig. 6<\/span><\/em> and indicates that strict adherence to the\u00a0MRF at the\u00a0reservoir outlet significantly affects the\u00a0required reservoir volume. Uncompromising compliance with MRF throughout the\u00a0year is not always realistic, especially if inflow to the\u00a0reservoir drops below the\u00a0prescribed MRF. In\u00a0such cases, it would be necessary to supplement the\u00a0outflow from the\u00a0stored reservoir volume, which may compromise some of\u00a0the\u00a0HS functions. On the\u00a0other hand, maintaining the\u00a0MRF is essential to prevent negative impacts on the\u00a0hydrological regime of\u00a0the\u00a0watercourse downstream of\u00a0the\u00a0reservoir caused by reduced inflow. For these reasons, operational conditions and current hydrological conditions are considered in\u00a0the\u00a0method for determining MRF, as described earlier in\u00a0the\u00a0methodology. The\u00a0technical design of\u00a0the\u00a0outlet structures may represent a limiting factor in\u00a0releasing the\u00a0required MRF, since these outlets are technically adapted to the\u00a0reservoir\u2019s functions. For more flexible regulation of\u00a0outflow according to current hydrological conditions, it would be necessary to modify the\u00a0technical design of\u00a0the\u00a0outlet structures and establish an appropriate inspection frequency. Typical outlet facilities used in\u00a0ponds and small reservoirs include a Monk drainage system, sluice gate, slide gate, flap gate, and others. However, none of\u00a0these allow flexible manipulation of\u00a0outflow in\u00a0accordance with changing hydrological conditions. To ensure effective compliance with MRF, it is essential to define requirements that are meaningful, technically feasible, and practical, given the\u00a0large number of\u00a0ponds and small reservoirs in\u00a0Czechia.<\/span><\/p>\n <\/p>\n Current hydrological data for the\u00a0present reference period should be a\u00a0key basis for determining MRF, as they reflect the\u00a0current climatic conditions. With the\u00a0development of\u00a0climatic conditions, hydrological characteristics in\u00a0the\u00a0catchment and at the\u00a0HS also change. The\u00a0project addressed the\u00a0question of\u00a0whether including the\u00a0preceding decade in\u00a0the\u00a0reference period would affect the\u00a0MRF value. During the\u00a0previous decade, both floods and droughts occurred in\u00a0Czechia. The\u00a0project concluded that values of\u00a0M<\/span>-day discharges,\u00a0<\/span>as\u00a0well as the\u00a0MRF, may decrease. There is a possibility that changing the\u00a0reference period for hydrological data will alter the\u00a0MRF requirements for HS.<\/span><\/p>\n The\u00a0assessed pilot sites are represented according to their category under the\u00a0technical safety supervision system, with 34 sites falling into Category III and 16 sites into Category\u00a0IV* (IV* denotes significant HS of\u00a0Category\u00a0IV). The\u00a0required safety level, expressed as the\u00a0return period of\u00a0the\u00a0theoretical DFW, is 1,000 years for the\u00a034 Category III sites, 200 years for 13 of\u00a0the\u00a0IV* sites, and 100\u00a0years for 3 of\u00a0the\u00a0IV* sites.<\/span><\/p>\n Based on knowledge and experience gained during the\u00a0project and from previous practice in\u00a0preparing safety assessments, a draft methodology for the\u00a0\u010cSN 75 2935 application was proposed. The\u00a0draft methodology serves both as a guide for preparing an assessment under \u010cSN 75 2935 for historic\u00a0HS, with the\u00a0aim of\u00a0simplifying and streamlining the\u00a0work, and as a source of\u00a0suggestions for updating the\u00a0standards \u010cSN\u00a075\u00a02935 \u2013 Assessment of\u00a0Flood Safety of\u00a0Hydraulic Structures and \u010cSN\u00a075\u00a00255 \u2013 Calculation of\u00a0Wave Effects on Structures at Reservoirs and Impoundments. The\u00a0formulated principles for preparing the\u00a0assessment, presented in\u00a0the\u00a0form of\u00a0a draft methodology, are available in\u00a0the\u00a0final project report\u00a0[7]. An example of\u00a0selected recommendations is provided below\u00a0[7]:<\/span><\/p>\nMETHODOLOGY<\/h2>\n
Input data<\/h3>\n
![\"\"](\"https:\/\/www.vtei.cz\/wp-content\/uploads\/2025\/10\/Taborikova-fig-1.jpg\")
<\/a>\n<\/h2>\n
Fig. 1. Map of\u00a0pilot locations<\/h6>\n
Minimum residual flow (MRF)<\/h3>\n
<\/a>\n<\/a>\n<\/a>\n<\/a>\nFlood safety<\/h3>\n
\n
\n
RESULTS AND DISCUSSION<\/h2>\n
Minimum residual flow (MRF)<\/h3>\n
Tab. 1. Comparison of the method of determining the minimum residual flow (MRF) according to the draft Regulation of the Government of the Czech Republic on\u00a0the\u00a0method and criteria for determining the MRF in the 2019 version (MZP NV) and according to the Methodical Instruction of the Department of Water Protection of\u00a0the\u00a0Ministry of the Environment to Determine the Values of MRF in Watercourses from 1998 (MZP MP 1998) shown as their percentage difference<\/h5>\n
<\/a>\n<\/a>\nFig. 2. Comparison of the MRF according to the draft Regulation of the Government in the 2019 version (yellow column), the MRF according to the Methodical Instruction from 1998 (green column) and the MRF determined in the water management permit (blue column) for selected locations in Area 3 and Area 4<\/h6>\n
<\/a><\/h6>\nFig. 3. Comparison of the method of determining the MRF according to the draft Regulation of the Government in the 2019 version and according to the Methodical instruction from 1998 with the M-day flows Q355<\/sub> and Q330<\/sub><\/h6>\n
<\/a>Fig. 4.\u00a0 Example of average decadal evaporation for the main season and\u00a0for\u00a0the\u00a0secondary spring season for one pilot site<\/h6>\n<\/h6>\n
<\/a><\/h6>\nFig. 5. Example of average decadal monthly evaporation for one pilot site<\/h6>\n
<\/a><\/h6>\nFig. 6.\u00a0 Illustration of volume in the reservoir when complying with the MRF (blue\u00a0curve) and not complying with the MRF (orange curve) at the reservoir outlet for the\u00a0period from 2010 to 2021<\/h6>\n
Flood safety<\/h3>\n
\n