Flood events and the hydrology of a complex catchment: the ... · PDF filecomputation of the...

L. Zaggia, A. Zuliani, F. Collavini & R. Zonta CNR – Istituto di Scienze Marine, Venice, Italy

Abstract

An investigation on the hydrology of the drainage basin of the Venice Lagoon was conducted as a part of an extensive monitoring of the freshwater and contaminant delivery from the twelve main tributaries (DRAIN project). The importance of flood events at the scale of the individual sub-basins as well as for the whole catchment was studied in detail by comparing the discharge measured in the tributary streams and time series of rainfall inputs obtained from a specifically created data base. The main results and an overview of the relevant methodological aspects faced by the study are presented.

1 Introduction

A basic understanding of the catchment hydrology is an essential prerequisite in the management of water resources, particularly for flood protection, optimisation of conservative and consumptive uses (i.e. power production and land irrigation), lowland reclamation and monitoring of contaminant transport. This knowledge is even more necessary when considering systems whose morphology has been intensely modified, as the drainage basin of the Venice Lagoon. The behaviour of this system is, in fact, quite complex and strongly different from that of natural catchments. The peculiar morphology combines with the distribution of rainfall inputs and the variety of aspects connected to the hydraulic management, determining a spatially diversified and variable response [1]. To provide an estimate of the annual transfer of freshwater and pollutants and to set-up and test methodologies for a continuous monitoring of the main tributary streams, a three-years research project was started in 1998 (DRAIN

Coastal Environment V, incorporating Oil Spill Studies, C. A. Brebbia, J. M. Saval Perez & L. Garcia Andion (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-710-8

Venice Lagoon

Flood events and the hydrology of a complex catchment: the drainage basin of the

project: DeteRmination of pollutAnt INputs from the drainage basin into the Venice Lagoon). Given the complexity of the system and the absence of a previous reference framework, an investigation on the basin hydrology including the role of flood events in the overall transport of materials was considered a priority task for the project. For each of the storms occurred during the monitoring period, the runoff of individual sub-basins was therefore studied in detail to obtain a description of space and time variability of the system response to the rainfall inputs. The methodological aspects and the main results of the research are here described and discussed.

2 Study area

With a total surface of about 1850 km2 and a population of about a million inhabitants, the drainage basin of the Venice Lagoon (Figure 1) includes some important urban centres, remarkable zootechnical and agricultural activities (about 70% of the surface is cultivated soil), as well as medium to small size industrial districts.

Figure 1: Schematic map of the Venice Lagoon and its drainage basin. The

investigated tributary sub-basins are listed: 1 Silone; 2 Vela; 3 Dese; 4 Osellino; 5 Scolmatore; 6 Lusore; 7 Naviglio Brenta; 8 Taglio Nuovissimo; 9 Lova; 10 Montalbano; 11 Morto; 12 Cuori.


164 Coastal Environment V, incorporating Oil Spill Studies

It is mainly constituted by a low-gradient floodplain: the northern area, which belongs to the Venetian piedmont zone, is the more elevated with a maximum altitude of 50 m above the mean sea level. The southern territories, as well as a large part of the areas bordering the lagoon, globally accounting for about 40% of the basin surface, lie instead below the sea level with altitudes as low as 2-4 m. The basin comprises an ensemble of tributary sub-basins having different morphology and extension, from a few to hundreds of square kilometres. Twelve main tributaries account for about 90% of the whole drainage surface and 97% of the mean total freshwater discharge [2]. Depending on the both local elevation and presence of external inputs, by springs or lateral derivations, these streams have different hydraulic regimes, from natural to mechanical, and their flow can be regulated and diverted. The morphology of the drainage network derives from the merging of the natural hydrography, which has a dendritic aspect, with the more regular pattern of the artificial canals, constructed at different times, for irrigation or land reclamation purposes. Several connections also permit water exchanges among contiguous sub-basins, both in normal conditions and during flood events. Therefore, the hydraulic pathways are generally complex and not univocally established.

3 Methods

One typical problem encountered in the monitoring of previously ungauged streams is the relatively limited extent of the observation period compared to the variability of flow, which is determined by natural climatic fluctuations as well as short and medium term trends. The absence of long discharge records for the drainage basin of the Venice Lagoon prevents the construction of flow-duration curves, which are fundamental for the definition of rating curves and long-term evaluations. Therefore, the significance of the estimates based on our monitoring can only be assessed on the basis of a comparison with the trend of climatic variables for which long records are instead available. With this aim, a database of daily rainfall for 39 rain gauge stations distributed on the drainage basin was realised using data from the bulletins printed by the Italian Hydrological Service (Ufficio Idrografico e Mareografico dei Servizi Tecnici Nazionali, Venezia). For the period 1921-2000, the database permitted to compute time series of daily precipitation for individual stations at the sub-basin and basin scales, and the execution of a flood-frequency analysis [3]. The freshwater flow of the twelve main tributaries was continuously monitored for an overall period of about 2 years. The position of the discharge gauge sections was selected carefully, following the U.S. Geological Survey indications. They were located in the close proximity of the lagoon border to intercept the contributions from the largest possible area of each sub-basin. As a main consequence, for seven of the investigated streams the monitoring reach


Coastal Environment V, incorporating Oil Spill Studies 165

was affected by the tide excursion, requiring an accurate measurement and computation of the stream flow. The geometry of the sections was determined by topographic profiles referred to the mean sea level and a stage-wetted area relationship was obtained by means of a CAD software. Time series of current speed (index velocity) were obtained by electromagnetic self-recording flow meters (InterOcean mod. S4, USA) positioned in the central axis of the section. The instruments also acquired water level, temperature and conductivity, with a sampling interval of 15 minutes. A large number of calibration measurements performed with the normal velocity-area method [4] was used to calibrate the index velocity to the average channel velocity [5]. The procedure implies the use of specific calibration coefficients for the full range of flow, and for the different tide conditions in tide affected sections [2]. The so obtained time series were filtered using a 25-h forward and reverse moving average in order to remove the effects of tidal excursion, permitting the reconstruction of the instantaneous discharge hydrograph.

4 Results and discussion

The statistical analysis of time series of rainfall for each of the spatial scales covered by the database, produces a large variety of information, most of which are useful for medium and long-term hydrological balances. The different climatic characteristics are emphasised by the non-homogeneous spatial distribution of the rainfall inputs. As shown by the hysoiet map of the mean annual rainfall for the period 1921-2000 (Figure 2), the area of maximum precipitation, with average values higher than 1000 mm, covers the northern and northwestern sectors of the drainage basin. This trend is also reflected by the distribution of rainfall associated to storms of regional importance with significant implications in the generation of peak discharges, particularly for the sub-basins of this area. The annual rainfall decreases in the southeastward direction, individuating a zone of minimum precipitation over the southern basin. The relatively lower inflow in this sector combines with the characteristics of drainage, which is mainly mechanical, determining both minimum values of the runoff coefficients [2]. Storms tend to have maximum effects on the lagoon border, whereas the rainfall propagation in the area of the lagoon and along the coastlines is inhibited. This feature explains the extension of the area of minimum inflow over the lagoon basin observable in the map of Figure 2. The flood-frequency analysis permitted to estimate the return periods (τ) of storms occurred during and immediately after the monitoring period (Table 1), until December 2000. The more important storm occurred on October 1998. Data of Table 1 show a non-uniform spatial distribution of the associated rainfall: return periods for the southern sector are in fact much lower than those of the remaining basin, although they are still characteristic of extreme events. For the drainage basin, the average rainfall of five-days duration had a τ of 10 years.



Figure 2: Hysoiet map of the mean annual rainfall depth on the drainage basin,

for the period 1921-2000. The 39 rain gauge stations are indicated. Within the monitoring period, other three high-magnitude events occurred. The return period of the 7 November 1999 event varies from 2 to 5 years and is below 2 years only for two sub-basins. Because of its short duration (approximately 12-h), this storm would be more appropriately analysed in the context of a study of intense rainfall, for which a higher τ should be assigned. As no other events with similar characteristics were recorded in the course of the project, it was considered as an event lasting one day. The storm occurred on 7-9 May 2000, which is a recurrent event (τ ranges from <2 to 2-3 years) has the characteristics of a local phenomenon. It affected significantly only one of the sub-basins of the northern sector (Osellino). The storm of October 2000, occurred after the end of the project monitoring, shows a patchy distribution with τ variable from a maximum of 12 to less than 2 yrs. For the whole drainage basin, the return period was equal to 9 yrs, a value mainly determined by the extreme depths recorded in the sub-basins of the southern sector. Besides the spatial and temporal distribution of rainfall, other factors, such as variable regimes, drainage mechanisms, tide influence and different lag-times affect the generation and evolution of runoff in the individual sub-basins.



Therefore, a discharge hydrograph for the whole drainage basin can only be obtained by integrating the mean daily discharge of the tributaries. Table 1: Return periods (τ) and average rainfall depths (h) of storm events

occurred in the period May 1998–December 2000. The durations (t) of the storms, in days, are also indicated.

Date Silone-Vela Dese Scolmatore Osellino

t h τ t h τ t h τ t h τstart end (days) (mm) (yrs) (days) (mm) (yrs) (days) (mm) (yrs) (days) (mm) (yrs)

05/10/98 09/10/98 5 132.5 7 5 131.7 10 5 119.5 8 5 168.0 5508/04/99 09/04/99 2 64.6 2 2 62.7 2 2 58.8 <2 2 51.7 <221/10/99 25/10/99 5 88.4 <2 5 95.2 3 5 116.9 7 5 117.0 507/11/99 07/11/99 1 46.4 <2 1 59.0 4 1 66.9 5 1 64.8 506/05/00 09/05/00 4 116.3 807/05/00 09/05/00 3 75.1 <2 3 91.0 3 3 79.9 307/10/00 09/10/00 3 88.7 4 3 97.5 6 3 100.5 707/10/00 11/10/00 5 79.9 <2

Date Lusore Taglio-Naviglio Lova

t h τ t h τ t h τstart end (days) (mm) (yrs) (days) (mm) (yrs) (days) (mm) (yrs)

05/10/98 09/10/98 5 145.2 40 5 170.0 100 - - -08/04/99 09/04/99 2 54.9 <2 2 52.0 <2 2 48.6 <221/10/99 25/10/99 5 102.0 4 5 106.9 5 5 90.6 306/11/99 07/11/99 2 67.7 3 2 75.3 307/11/99 07/11/99 1 68.4 506/05/00 09/05/00 4 78.7 2 4 72.2 207/10/00 08/10/00 2 93.6 1007/10/00 09/10/00 3 92.5 407/10/00 11/10/00 5 95.6 3

Date Montalbano Morto Cuori Total basin

t h τ t h τ t h τ t h τstart end (days) (mm) (yrs) (days) (mm) (yrs) (days) (mm) (yrs) (days) (mm) (yrs)

05/10/98 09/10/98 5 101.9 5 5 91.1 4 5 88.2 4 5 116.2 1008/04/99 09/04/99 2 44.7 <2 2 39.8 <2 2 48.6 <209/04/99 10/04/99 2 44 <221/10/99 25/10/99 5 86.3 3 5 91.3 4 5 85.9 4 5 90.8 406/11/99 07/11/99 2 69.0 3 2 52.7 207/11/99 07/11/99 1 46 <2 1 56.5 507/05/00 09/05/00 3 50.3 <230/09/00 05/10/00 5 62.4 <207/10/00 09/10/00 3 109.0 12 3 89.1 7 3 94.5 11 3 91.4 9

If a value of 1.5 times the mean annual discharge is taken as a threshold to identify significant events, flood conditions occurred on a total number of 26 days. Compared to the mean annual value of 34.5 m3/s, the mean discharge in flood conditions is 75.4 m3/s, while the corresponding volume transferred to the lagoon is as high as 16% of the total annual runoff. The hydrograph of Figure 3 shows as almost all the storms induced significant peaks of the daily discharge, with a higher frequency of the events in the spring and autumn months. The maximum value of 237 m3/s was recorded after the storm of 7 November 1999. On a hourly scale, the estimated peak discharge associated to this event was 344 m3/s [2]. For minor floods the results indicate that the response of the basin to the meteorological inputs is strongly dependent on the initial state of the system. This determines different effects in relation to the hydrological constraints such as temperature, soil moisture conditions and the state of vegetation cover. An interesting example comes from the comparison of the events occurred on 26–30



March and 1–4 April 2000. Although the second flood was generated by a minor rainfall, the overall runoff of the basin was considerably higher than in the first case. On the opposite, the events of 1–4 April 2000 and 21–25 October 1999, which yielded a similar runoff, were generated by a completely different inflow, with the first event having only one third the rainfall of the second that occurred after a long dry period.

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Figure 3: Trend of the discharge and average daily precipitation for the drainage basin.

For the analysis of flood events at the scale of the individual sub-basins, time series of instantaneous discharge are required to account for the short response-time characterising the investigated systems. As some of the measurement sections are subjected to the tidal excursion, data were filtered with a 25-h forward and reverse moving average to remove the oscillating trend from the original time series. The filtered data allow for a better description of the phenomena without causing shifts in the position of flood peaks. The resulting hydrograph can be therefore considered as the best approximation of the actual discharge curve for the specific tributary. The effects of a different intensity of rainfall on the trend of discharge and on the shape of the flood hydrograph are emphasised by the comparison of the events of October 1998 and November 1999 for the most important tributaries (Dese, Figures 4 and 5). In both cases an exceptional tide also occurred causing modulations of the streamflow, even during the peak phases. Despite the inflow volumes, as well as the overall runoff, are distinctly higher in the first event, the peak discharges are almost equal. The flood response of the drainage basin and its constituent sub-basins to long-duration events (5 days) was preliminarily investigated by a rainfall-runoff



analysis. The runoff volumes were estimated by integrating the filtered discharge curve within the interval individuated by the rainy period and the lag-time.

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Figure 4: Trend of the discharge and average daily precipitation for the Dese

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Figure 5: Trend of the discharge and average daily precipitation for the Dese

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For some of the tributaries a linear relationship between rainfall and runoff volumes has been found (Figure 6). The correlation improves if the summer events are separated from those occurred in the remaining periods. The slope of the rainfall-runoff regression is related to the characteristics of the sub-basins. For example, a relatively larger extension of impervious urban surfaces determine a relevant influence on the runoff response of the streams to storms, resulting in a considerably higher slope. Among the more rural sub-basins the variability of the slope, particularly in the summer period, can be ascribed to the different irrigation practices. Besides representing a reference framework for the evaluation of the hydraulic risk, the rainfall-runoff relations permit to estimate the freshwater yield for storm events of an assigned magnitude as, for example, those occurred after the end of the monitoring period (Figure 6). Other tributaries show a poor or no correlation. The presence of external inputs and flood diversions tends, in fact, to hide the natural “signal” from the catchment terrain, while the storage of water masses for irrigation purposes limits the runoff in tributaries characterised by total or partial mechanical drainage. The runoff therefore initiates only after the exceeding of predefined stages in the main collectors, reaching the closing sections with a considerable delay. When considered as a single morphological unit, the drainage basin also shows the existence of a functional relation between the inflow and runoff, with a partitioning of summer and autumn–spring events which closely matches that of sub-basins having a more natural behaviour.

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Figure 6: Rainfall and runoff volumes of the main floods on the Dese sub-basin

(grey dots = summer, black dots = autumn-spring). The open circles represents the runoff estimated on the basis of the regression lines for the events occurred after the end of the monitoring.



5 Conclusions

Different aspects of hydrological nature affect the generation of runoff from the drainage basin of the Venice Lagoon, and its constituent sub-basins. Morphological characteristics and the spatial distribution of rainfall, may in fact combine in different ways with the management of water resources. Under particular circumstances, this can minimise the effect of floods or, on the contrary, can enhance the effect of low magnitude events. With regard to many tributaries, one of the main factor affecting the propagation of the flood and the short-time evolution of discharge is the tidal oscillation, which varies over time, adding a further variability in the response to storm events. Finally, the existence of a linear relationship between rainfall and runoff volumes for some sub-basins is a powerful tool for hydrological balances and predictions of the medium and long-term trends.

Acknowledgements

We wish to thank APAT, Venice, for providing hydrological reports which were necessary for the realisation of the database of average rainfall depths for the drainage basin. A special thank to Dr. M. R. Simpson of USGS, Sacramento, USA, for the discussion of the filtering procedure for discharge data. The research was carried out on behalf of Consorzio Venezia Nuova, concessionaire of the Italian Ministry of Public Works - Venice Water Authority, as part of the operations designed to safeguard Venice Lagoon (law 798/84).

References

[1] Zonta, R., Costa, F., Collavini, F. & Zaggia, L., Objectives and structure of the DRAIN project: an extensive study of the delivery from the drainage basin of the Venice Lagoon (Italy), Environment International, in press.

[2] Zuliani, A., Zaggia, L. & Zonta, R., Freshwater discharge from the drainage basin to the Venice Lagoon (Italy), Environment International, in press.

[3] Gilman, C. S., Rainfall (Section 9). Handbook of Applied Hydrology, ed. V. T., Chow, McGraw-Hill: New York, pp. 1-68.

[4] Wahl, K.L., Thomas, Jr., W.O. & Hirsch, R.M, Stream-Gaging Program of the U.S. Geological Survey. U.S. Geological Survey Circular 1123, p. 22, 1995.

[5] Simpson, M. R. & Bland, R., Methods for Accurate Estimation of Net Discharge in a Tidal Channel, IEE Journal of Oceanic Engineering, 25 (4), pp. 437-445, 2000.



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