Hydromorphological Index for Rivers (HIR): A New Method for Hydromorphological Assessment and Classification for Flowing Waters in Poland

The method based on original metric called Hydromorphological Index for Rivers (HIR) was developed in 2017 for the purpose of the monitoring of the hydromorphological status of flowing waters in Poland. It fulfils the requirements of the EU Water Framework Directive (WFD). It allows the assessment of both lowland rivers as well as mid-altitude and highland streams. The proposed system can be used to assess the natural and heavily modified rivers as well as artificial channels. The basis of the proposed system is a field survey, which is supplemented by the analysis of Geographic Information Systems (GIS) data and remote sensing materials. The analysis of the GIS data and remote sensing materials already enable to estimate preliminary classification of the hydromorphological status of the non-surveyed water bodies. On the basis of the field survey, the principal HIR value can be estimated for the considered river site and comparing with the reference conditions, the hydromorphological quality status in the five-class system can be calculated. The properly selected, representative survey sites (one or more depending on the heterogeneity of the environment), enable the classification and evaluation of entire surface water bodies in the framework of the national environmental monitoring. The GIS component of the HIR proved to be useful in verifying the determination of heavily modified water bodies and in assessing the needs of river restoration. It was also applied in the development of the National river restoration program for predicting the impact of the proposed restoration measure on the state of hydromorphology.


INTRODUCTION
The method presented in this paper is based on the original metric called Hydromorphological Index for Rivers (HIR). It was prepared in 2017 for the purpose of river monitoring in Poland (Szoszkiewicz et al. 2017). It can be used for various river types (lowland, mid-altitude and highland), including natural rivers as well as modified and even artificial channels.
The proposed method constitutes an original system of hydromorphological assessment, which was developed based on various Polish and European methods. Many elements of the HIR method are based on the River Habitat Survey (RHS) system, which was developed in the UK (Raven et al. 1998, Boon et al. 2002, Environmental Agency 2003, Szoszkiewicz et al. 2006, Jusik et al. 2014). Moreover, significant elements of the two Polish methods were also used (Adynkiewicz-Piragas 2009, Radecki-Pawlik 2014).
The major purpose of the HIR method development was national monitoring meeting criteria of the Water Framework Directive (WFD) (European Commission, 2000) and European and Polish standard for assessing the hydromorphological features of rivers (EN 14614, 2004). The proposed method can also be utilized to predict the engineering modifications (environmental impact assessment) as well as the results of river restoration measures. Moreover, the HIR system can be used for various scientific ecological investigations of various groups of aquatic organisms.
The HIR is a field survey based method but it is supported by an analysis of the GIS data which deliver preliminary classification of hydromorphological status. Nevertheless, in the result of the field survey, the basic HIR index can be calculated for the stretch of watercourse. Comparing the HIR value with the reference conditions, the hydromorphological quality status in the five-class system can be calculated for this stretch. Representative survey sites enable for the classification of entire rivers or whole water bodies recognized by the national environmental monitoring. The final evaluation of a water body combines an average HIR value from several stretches surveyed in the field and preliminary evaluation based on the GIS data.
The process of the HIR method development is based on the original database consisting of hydromorphological survey records of 1,107 sites, carried out in the field by the authors of this paper in the period 2003-2016 (Fig. 1). This database represents full hydromorphological gradient, from the reference conditions to extreme degradation. The analysis of these data was the basis for developing a river classification method meeting the criteria of the WFD.
So far, three publications summarizing the results of the HIR method have been published -two from the Greater Poland voivodship (Pietruczuk et al. , 2020 and one from the Lower Silesian voivodship (Przesmycki et al. 2019). The GIS component of the HIR proved to be useful in verifying the determination of heavily modified water bodies and in assessing the needs of river restoration (Grela et al. 2019). It was also applied in the development of the National river restoration program, for predicting the impact of the proposed restoration measures on the state of hydromorphology (Biedroń et al. 2020).

Preliminary analysis based on the GIS data
The preliminary assessment of the hydromorphological quality of rivers is based on the commonly accessible, generally public spatial data as satellite images (e.g. Geoportal WMS service or Google Earth platform). In this way, the information supportive to field the survey was provided. Moreover, this preliminary assessment can be already utilized by the national monitoring program for the water bodies that are not considered under the field survey scheme.
Preliminary protocol considers the abundance of hydromorphological elements along the river reach or by estimation the frequency of various attributes. The analysis includes the information on hydrological regime, channel sinuosity calculated in 5 class-scale, engineering structures as water management facilities, spillways and flow regulation structures, dams, as well as bridges. Among non-engineering hydromorphological attributes the tree presence in the valley along the surveyed river reach can be recorded based on remote sensing materials.
The preliminary protocol includes several attributes of the river valley, which are analyzed in the 100 m wide buffer for rivers with channel width ≤30 m and whereas in the case of rivers wider than 30 m, the buffer is 1000 m wide. In this way, the information on the abundance of various forms of land use is delivered, distinguishing the urban, agricultural and semi-natural form of management. Moreover, the information on the lateral connectivity of river with its valley is recorded, including the presence of embankments, interembankment zones, ox-bow lakes and wetlands. In the case of large rivers (channels >30 m), the information on the channel dimensions variability, number of bars (side bars and mid channel bars) and islands is recorded.

Preliminary evaluation based on the GIS data
The preliminary assessment is based on the remote sensing materials and spatial databases. In order to assess the presence and abundance of natural elements the GIS Hydromorphological Diversity Score (GIS-HDS) was developed, whereas the GIS Hydromorphological Modification Score (GIS-HMS) informs on the degree of morphological degradation. The range of attributes considered for the GIS-HDS and GIS-HMS estimation is presented in Tables 1 and 2. The number of attributes included depends on the size of the rivers (number of attributes is greater in large rivers).
The preliminary evaluation of the whole river body considers GIS-HDS as well as GIS-WPH which are calculated on the basis of various hydromorphological attributes HDSA and HMSA: where: GIS-HDS -GIS-Hydromorphological Diversity Score; HDSA i -Hydromorphological Diversity Score Attributes; i -attributes 1-7; and GIS-HMS = ∑ where: GIS-HMS -GIS Hydromorphological Modification Score; HMSA i -Hydromorphological Modification Score Attributes; i -attributes 1-6. The result of evaluation based on the analysis of the GIS data enable to estimate GIS-Factor which come out from the GIS-HDS and the GIS-HMS: The GIS-Factor based on the remote sensing data and available spatial databases provides preliminary hydromorphological assessment of the water body, which might be a large section of a river. Moreover GIS-Factor is applied for final hydromorphological classification as a ratio influencing HIR estimated in the result of the field survey.

Field survey approach
Field survey is carried out on a standard stretch of the river with a length of 500 m (streams and brooks with channel width ≤30 m) or 1000 m (watercourses with a channel wider than 30 m).
The survey according to the HIR method includes an evaluation of three zones (Fig. 2): • river channel -includes riverbed (permanently or partially located under water) and bank face (situated between water line and bank top); • coastal zone -river valley adjacent to the bank top (5 m wide in river width ≤30 m and 10 m wide in river width >30 m); • river valley -includes buffer 50 m (river width ≤30 m) or 100 m (river width >30 m) wide adjacent to the bank top.   (Fig. 2, 3). Moreover, the structure of aquatic and bank vegetation as well as land use of a bank zone is recorded in 10 m wide transects. • The second stage is synthetic assessment of the entire river section and it delivers an overview of the entire survey site, where the natural features and modifications, which were not recorded in the previous stage (e.g. occurred between spot checks) are recorded. Moreover, a predominant valley form is described, as well as the channel dimensions and presence of hydroengineering facilities are included.
The field approach of the assessment was taken from a widely used and tested British method of hydromorphological status assessment, the River Habitat Survey, which has been tested under the Polish rivers conditions for over a decade. (Environmental Agency 2003, Szoszkiewicz et al. 2007). The hydromorphological classification of watercourses is based on Hydromorphological Index for Rivers (HIR). Multimetric HIR index combines two indices: Hydromorphological Diversity Score (HDS) and for the Habitat Modification Score (HMS). HDS informs on the presence of natural attributes of the channel, coastal zone and the river valley. Each of the HDS attributes delivers a range of points enabling to calculate HDS of the river stretch ( Table 3). The detailed procedure of scoring is presented in the HIR method manual (Szoszkiewicz et al. 2017). HMS provides the information on the hydromorphological modifications. It includes various forms of fluvial ecosystem transformations, such as profile modifications and reinforcements and presence and abundance of engineering facilities. The HMS value is calculated by summing up the categories presented below (Table 4)

Rivers evaluation based on field survey
The evaluation of a river section takes into consideration both the HDS and HMS indices. The HIR score ranges from 1 (reference conditions) to 0 (extreme degradation): The process of reference conditions determination was carried out on the rivers that have not been subjected to significant anthropogenic pressure with apparent natural fluvial processes taking place. The principal reference criteria were as follows: Both criteria were detected on 217 river sites (19.6% of all analyzed), which were considered as reference ones. This group was used to analyze the HIR diversification among various groups of rivers in Poland and to identify the hydromorphological types of watercourses. In this way, the hydromorphological classification of the river was proposed in the five river type system ( Table 5).
The HIR components form two negatively correlated gradients: HDS forms a gradient of diversity of natural morphological elements of the watercourse and river valley, while HMS is related to the gradient of anthropogenic changes in hydromorphology. The hydromorphological Table 3. Attributes included in the Hydromorphological Diversity Score  transformations of rivers usually lead to a decrease in the heterogeneity of the river habitat (Fig. 4).
The method of calculating the HIR components is similar to the synthetic hydromorphological indicators calculated on the basis of the RHS method (Szoszkiewicz et al. 2012). The RHS equivalent of HDS is Habitat Quality Assessment -HQA, while the RHS equivalent of HMS is also the Habitat Modification Score (Fig. 5).

Water body assessment
River assessment based on a survey of a single site delivers the information limited to the local conditions. Such information is not efficient to characterize the entire river over a distance of tens of kilometers but surveying a single site its hydromorphological quality can be estimated classification for the purpose of local monitoring. Moreover, local assessment is principal for Environmental Impact Assessment (EIA) of the planned channel modifications and various water engineering projects. The local hydromorphological data can be also used for the ecological studies of various aquatic organisms (fish, macroinvertebrates, macrophytes et al.).
The hydromorphological assessment of rivers with a length of several kilometers or tens of kilometers (e.g. entire surface water bodies) requires the field data gathered from 1-3 stretches and analysis of the GIS data. The number of recommends survey sites depends on river heterogeneity of the land use in the river valley zone and varies between one (homogenous land use) to three (large diversity of land use). Three land use categories are distinguished in this process: (1) natural/semi-natural areas, (2) agricultural land, and (3) urban localities. The considered buffer width is 100 m from river both banks (river width ≤30 m) or 1000 m (river width >30 m). The land use category is considered when it covers at least 25% of the buffer zone area (Fig. 6).
The assessment of the hydromorphological state of the entire Water Body consists in calculating the weighted average of the HIR values, taking into account the weighting factor for three forms of land development (calculated on the  where: HIR mean -average HIR value for the whole water body; HIR U -HIR calculated for the survey sites located in an urbanized area; wU -weighting factor for urbanized areas calculated on the basis of their percentage share in the buffer; HIR A -HIR calculated for the survey sites located in an agricultural area; wA -weighting factor for agricultural areas calculated on the basis of their percentage share in the buffer; HIR S -HIR calculated for the survey sites located in a semi-natural area; wS -weighting factor for semi-natural areas calculated on the basis of their percentage share in the buffer. The hydromorphological status of entire water body base od average HIR value (HIR mean ) out of each surveyed stretch (HIR of each field surveyed site) modified by GIS-Factor (based on the preliminary assessment of remote sensing data). Average HIR is a weighted mean value based on HIR of 1-3 stretches representing proportionally various types of land use in the bank buffer along the entire surface water body. Moreover, the final classification of the water body is modified by GIS-Factor derived from on the remote sensing and spatial databases ( Table 6).