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Watersheds tributary to the Ridracoli reservoirFEATURE OF TECHNICAL SERVICE

Subject

Management project of the reservoir «Diga di Ridracoli» according to the

instructions of Ministerial Decree 30 june 2004, under Article 40 Comma

2 of D.L. 11 may 1999 N.152

Carried out by SGAI Srl di E. Forlani & C.

Client Romagna Acque-Società delle Fonti S.p.A

Service length 2005

View of the reservoir «Diga Ridracoli»

Geologic map of the Ridracoli reservoir

RESERVOIR «DIGA DI RIDRACOLI» – Investigation stage, characterisation of the

reservoir and sedimentation process

The reservoir of the Ridracoli dam is located in the province of Forlì-Cesena, within the Foreste Casentinesi

National Park, Campigna Park and Monte Falterona, along of the Bidente river at the confluence with the Rio

Celluzze, at about 10 km from the town of Santa Sofia. The dam is of the arch-gravity type with a height of

103.5m and with a crowning development of 432m. The basin covers an area of 1,035 km2, allowing the

storage of 35.5 million m3 of water. The primary use of the basin is for water supply purposes with annual

regulation of the flow rates of the Bidente River, while the secondary one concerns the hydroelectric production

in the nearby Isola plant. According to the disposal of the Ministerial Decree 30 june 2004, an accurate study of

the basin, dam and hydraulic equipments was carried out, thanks to define the forecast framework of the

management and maintenance activities to carry out for ensure the maintenance and gradual restoration

of the useful capacity of the reservoir, ensuring the operation of the discharge and catchment systems in all

operation conditions. The design approach has seen the realization of accurate geological-geotechnical,

hydrological, hydraulic and environmental studies, thanks to which it is possible to carry out the qualitative

characterization of sediments and suspended solid material; calculate the annual solid transport volume

and the volume of sedimented material; as well as investigating the water quality of the basin. Analyzing

the geological and geomorphological framework of the reservoirs of the Ridracoli reservoir, the presence of

medium tenacity and little erodible lithologies was found, consisting mainly of the marly-arenaceous

formation of Romagna in its various components. The vegetal coverings, made up of thick coppice woods, help

to limit the surface erosion of the soil, reducing even more the supply of debris. Therefore, the volume of

sediment produced annually in each sub-basin was estimated, obtaining an accumulation of debris in the

reservoir for about 41,000 m3/year. Conducting geotechnical, physical-chemical and mineralogical analyzes

on the sediments, the presence mainly of angular-shaped gravel emerged with strong heterogeneity in the

various parameters, indicating a very short moving of the material before deposition. The values of heavy

metals, hydrocarbons, pesticides, Phosphorus, Nitrogen and BOD5 are very low and all parameters verify the

standard value limits. The suspended solid material is present with an amount less than 1% of the solid

transport intercepted by the tidal barrage. In relation to the parameters described and the trends recorded, it is

noted that the temperature, dissolved oxygen and pH are characteristic of reservoirs with good water quality.

The investigation survey showed that the sedimented material for the entire duration of the concession is less

than 5% of the reservoir capacity and, if it was filled from the bottom, it would not reach the elevation of the

bottom discharge tunnel without affecting the useful basin capacity.

Parametri Acque vs. Solidi Sospesi

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

0 50 100 150 200 250 300 350

Solidi Sospesi (mg/l)

BO

D5

(m

g/l)

0

5

10

15

20

25

30

P e

d N

(m

g/l)

Fosforo totale (mg/l P)

Azoto totale (mg/l N)

Azoto ammoniacale (mg/l N)

Azoto nitrico (mg/l N)

BOD5 (mg/l O2)

Water quality analysis: suspended solids, BOD5, Nitrogen and Phosphorus

Spillway crest and stilling basin Bottom discharge tunnel Outlet of the bottom discharge tunnel

Outlet of the middle discharge tunnel

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Checks of the turbidity levels during the draw-off and maintenance operation

OUTLET OF WATER

SUPPLY TUNNEL

OUTLET OF THE MIDDLE

DISCHARGE TUNNEL

ARCH-GRAVITY DAM

WITH SPILLWAY CREST

Aerial view of the reservoir «Diga di Ridracoli»

STILLING BASIN AND

BOTTOM DISCHARGE

TUNNEL

Torbidità Totale e Torbidità Caratteristica Scarico di Fondo

0

50

100

150

200

250

300

350

400

-5 0 5 10 15 20 25 30 35 40 45 50 55 60

Tempo (min.)

To

rbid

ità (

mg

/l) Torbidità Corretta Allo Scarico di

FondoTorbidità Misurata nei punti A,B,C

Volume Rilasciato Totale e dallo Scarico di Fondo

0

20000

40000

60000

80000

100000

120000

140000

-5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

tempo (min.)

Vo

lum

e (

mc)

Volume Totale in Alveo

Volume Totale Rilasciato dallo

Scarico di Fondo

Curva di Torbidità/Volumi Scarico di Fondo

apertura dopo 1 anni

0

50

100

150

200

250

300

350

400

0 5000 10000 15000 20000 25000 30000 35000

Volumi Rilasciati dallo Scarico di Fondo (mc/s)

To

rbid

ità

(m

g/l

)

torbidità

Rilasci Totali a Valle

0

5000

10000

15000

20000

25000

30000

-2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

tempo (min).

Vo

lum

i R

ilascia

ti (

mc)

Scarico di Fondo - mc rilasciati

Totale Volumi in Alveo

Torbidità - Attesa e Limiti

0

2000

4000

6000

8000

10000

12000

14000

16000

-2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Tempo (min.)

To

rbid

ità (

mg

/l)

Torbidità Scarico di Fondo in Assenza Diluizione

Torbidità Attesa nel Corpo Recettore

valore massimo

Soglia di Attenzione

Operazione Azione Effetti

-L’invaso è colmo, al termine delle operazioni se di manutenzione si può essere in condizioni di sfioro

-I rilasci, sempre se in condizione di sfioro, seguono già portate nel corpo recettore

INIZIO FASE A -Le fasi iniziali hanno una miscelazione con acque di ottima qualità

-Le condizioni stagionali di portata e torbidità sono quelle naturali

Apertura dello Scarico di

Mezzo Fondo . (se necessario)

Sarà in modo da consentire di diluire opportunamente la successiva onda di maggiore torbidità

all’apertura dello scarico di fondo

INIZIO FASE B -La necessità o meno di aprire lo scarico di mezzo fondo dipende dall’eventuale flusso dallo sfioro

Apertura Parzializzata dello

Scarico di Fondo .

INIZIO FASE C

Modulazione Portata Scarico di

Fondo

INIZIO FASE D

Chiusura Scarico di Mezzo

Fondo

-Dopo il raggiungimento di valori di torbidità bassi dallo scarico di fondo sarà possibile chiudere lo

scarico di mezzo fondo, pur mantenendo accettabili valori di torbidità

INIZIO FASE E -La chiusura di questo avverrà in tempi differenti in caso di sfioro in atto o meno

Fine Operazione – Chiusura

Scarico di Fondo

INIZIO FASE F

4

5 -Dopo le operazioni di manutenzione/spurgo dello scarico

Periodo: da Ottobre ad Aprile.

Preferibile condizione di Sfioro.0

1

2

-I primi volumi d’acqua rilasciati dallo scarico avranno la maggiore torbidità, la parzializzazione ha la

finalità di consentire il massimo rapporto di diluizione con le eventuali acque di sfioro e/o di scarico di

mezzo fondo

3 -Dopo la fase iniziale lo scarico di fondo verrà portato alla portata maggiore definita

Spurgo Scarico di Fondo

0

5

10

15

20

25

30

35

40

-4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

tempo (min.)

Q (

mc

/s)

Sfioro+Mezzo Fondo

Scarico di Fondo

Totale

1 2 3 4 5Operazione n.

Characteristic turbidity curves of the bottom discharge tunnel

MANAGEMENT PROJECT - Opening procedures of the hydraulic discharge system

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FEATURE OF TECHNICAL SERVICE

Subject

Management project of the reservoir «Diga di Ridracoli» according to the

instructions of Ministerial Decree 30 june 2004, under Article 40 Comma

2 of D.L. 11 may 1999 N.152

Carried out by SGAI Srl di E. Forlani & C.

Client Romagna Acque-Società delle Fonti S.p.A

Service length 2005

RESERVOIR «DIGA DI RIDRACOLI» – Management project of the basin, design issues and their resolution

Management project of a reservoir, according to the instructions of the Ministerial Decree 30 june 2004, consists of the preventive definition of the

management and maintenance procedures to carry out for ensure the maintenance and gradual restoration of the useful capacity of the basin,

while limiting the effects, related to the outflow of muddy waters, in the river bed downstream of the dam. In order to define the procedures for opening

the discharge system, for each of these, it is necessary to establish the permissible turbidity curves by estimating the movement of sedimented

material at the site of the floodgates, based on the typical speeds of runoff water. In detail, the Ridracoli dam is equipped with the following hydraulic

equipment: spillway crest, middle discharge tunnel, bottom discharge tunnel and emptying drainage. The spillway crest consists of an overflow

threshold composed by 8 spans of 14m each. To protect the foundation of the structure there is a stilling basin with a tidal barrage of 14m height. The

middle discharge drain, consisting of a 4m diameter tunnel under the left abutment, allows to drain flow rates up to 130 m3/s. The bottom discharge

drain consists of a tunnel on the right abutment, of a length of about 400 m, with a disposal capacity of up to 170 m3/s. To best calibrate the opening

procedures of the bottom discharge tunnel, an opening test was performed, in order to obtain the "characteristic" curve of turbidity according to the

volume released. During the test, not being aware of the sediment intake downstream, it was expected that a good mixing of the water from the bottom

discharge drain with high quality reservoir water would be ensured. It is important to define the frequency between one maneuver and another, since

the turbidity values could increase a lot. In fact, by opening the bottom discharge tunnel after 3 years, a turbidity of 2050 NTU was obtained, equal

to about 6,000mg/l. The turbidity curve obtained made it possible to define the correct opening procedures for the discharge system. Since there are no

sedimentation problems in the reservoir and on the quality of the water, the study focused on the activity of the bottom discharge, so that the turbidity

levels remain acceptable during the flaring and maintenance operations. For this purpose, it was considered to perform turbidity sampling at the height

of the valley bridge every 5 minutes from the beginning of the operations with verification of the limit values and reconstruction of the real curve. In

detail, an operating sequence was considered with activation of the middle discharge drain, in preferable overflow condition, and following the opening

of the bottom discharge drain at a flow rate of 10m3/s for 3 minutes. In this phase the maximum turbidity will be diluted in a 1.5:1 ratio. Once the

peak of turbidity has passed and entered areas compatible with the thresholds, the background drain will be divided into 10m3/s and the background

drain will be raised to 25m3/s. The management plan, in order to avoid local phenomena of abnormal sedimentation nearest the discharge organs,

provides for opening operations of the hydraulic organs regularly, ensuring the opening of the bottom and middle discharge drain at least once a year.