Loir Valley Development Project

Copnigh t © I L \ C S"stems ,-\nal\sis .-\pplied to \lanageme nt of \\'ater Resources. Rabat. ~loroc(O. 19HH

LOIR VALLEY DEVELOPMENT PROJECT B. Corbel*,

J.

L. Roy** and

J.

Lefevre***

*H.wiratfc **Loire-Bretaglle Basin Delegation, Orleans, France ***Loire-Bretaglle Basin Agenc)" OrleallS, France

Absl'act. A survey led in 1986 and 1987 on the L oir river basin (8 000 km2, west of France) makes it a good example of an ove ra ll analysis of a catchment area's hydrology and also of the problems set by irregular flows. The survey was launched considering the insisting demand of the riverside residents much concer ned by floods. It was led continously in co ncert with the municipalities, the authorities and t:-'e research department. It made it easier to explain, to make understand the mechanislOls of the flood formation, their rout ing and also to put the emphasis upon th(' importance of the diffe re nt phenomena taken ino account. Then, the interest of development program shown by the survey has been co mpletely acknowledged by all the responsible bodie s. Thi s ambitions program not only deals with all the tec hni ca l means to reduce damage caused by floods (city development control, flood forecast, local works, reservoirs) but also with the water co nsumption development and the necessity to increase low water flows .

Keywocds. France; flooding val le y development project.

low water levels

water quality

IN TRODUC TIO N The Loi r Ri ver is situate d in the western section of the Loire Basin and drains 8 000 km2 over a length of 300 km (F ig ure I). The present hydraulic de ve lopment project is the resu lt of a survey c arried out in the years 1986-1987. I t f or ms a pe rfect example of hydrological analysis in a given basin, treating the problems caused by water supp ly varia tions (flooding, low water levels, water quality, all responsible, in different ways, fo r disrupting development proj ect involving all the organisation co ncer ned with the issue: Management, Loc a l Communities and research units. This analysis was undertaken following the sudden r ise in water levels in April 1983 and later in November 1984 (ref(' r to paragraph 1.3.1). Local inhabitants, repres e nted by their councillors, appealed to the authorities that measures must be adapted to reduc e damage caused by the sudden rising of the river. It was th(,1l decided to car r y out the survey as regards the whole va ll ey, in the view of producing a proje c t c oncerned with development, flood fo r ecasting and supervision of urbanization. Thi s survey was carried out by the company HYDR A TEC, and backed by the State Basin Agency and the two Reg ions through which the Loi r flows. It was directed by a Committee, uniting A dminist ration, Basin Or ganisations and Comrnunities

157

dams

local development

flood forecasting

158

B. Corhel.

l1..

Rm and

l

Lefe\TC

PRESENT A TION OF THE LOIR BASIN

1. 3 Problems

1.1 Geographical - Geo l ogi c al Situation There are t hree (fIgure 2) :

zones

to be examined

up stream section to the left (permeab le limes t one soi l s)

in

this

basin

the Beau ce Pl ain

up stream section to the right: the hilly Per c he regio n, impermeaole so ils, (silex c l ays , marly c ha lk) the western region , downslream seClion , is a diversified zone .

There a'e thre e main problems with whi c h the L oir river is co nfronted. - f l oodi ng - low water-levels - wate r quality

1. 3.1 Flooding Alo ng the banks of the Loi r Ri\·er there "re a number of smal l towns whic h developped around the POft S and mills.

B onneva l

,

<, \

'-,

I

,, I

--

I

, ....

Fig . 2

... - ...... '

....!.

Survey zones

1.2 HYDROLOGY It is mainly water (oc ean-originating rainfal l) co ming from the Pe rc he hi l ls that causes excess r i ver le ve ls, more especia ll y i n the months of Novembe r thro ugh to Apr il. For information only, high-level water flow rates f or return periods of 2, 10 and 100 years are as follow (in m3 /s ) : Ut!lurll peri od

lip lo llC "nl

I oil

f)(lWJl ~ II" " '"

MIII - I.oi r

( year s)

( I\oIHII· ... al )

"I ll

')f1

'11

1S11

l.,n

'f.1I

:11 tJ

' 111

',(,11

,nu

(M o nlllil c/ t )

I oi l ( DII I ';d)

f low

tJp Slrt!~lm 1011 (Bunlll : llill )

O. -/4 111

11 .'>'.1 n .'!)

111

[1.11

MIII- l.ol1

( M o II\oirc / l l

.. ,

Moreover, between 1966 and 1983, the water level d i d no t reach danger limits and no d iff icult ies were encountered for town de\·eloprnent in areas traditiona ll y liable to f lOOding .

1 ')0

Dur ing low water-level periods, wate' comes malnl, from large ground water sources retained in th e l imestone strata of the Beauce reg ion .

It e lurfl petiOli (yea r s)

,\ tone ti me, the incom·e nlences caused by t he high water-IcH'I!, were compe n sa t ed by the eco nomi c Interest based on the river. H owe\·er , today, the economic interest has somewhat diminished; new means of lransport ha\e been inlroduced and water power has been replace d by other types of energy.

,all ~S

m ~/':.

D u ..... " :; III : au' L n il (U ulla!)

"

J.'l

7,', 4.',

7. 4

1. 1

The two preceding charts give a c lea' Illu stration of how irregu l ar the water flow rate is .

Only when eXl'eme water levels were reached In r\pril 1983 and No\·ember 1984, ca usi ng damage. in around fifteen towns (inc ludi ng Vendome and L a Fle che : the l argest towns of the Loi r Valley), dit it be came obvious th at something had to be done as regards protecting the region .

1.3.2 L ow water -I e\·e ls Water ca uses stream Beauce regions

as a res ouc ce for agri c ulture. usually most problems In the upstream regi on (up from the ground water sources of the Region ). Both mid -L oir and downslream ha'·e fewe' difficul li es .

1.3.3 Water quality I\n excess of nUlr i ti\·e elements (n llra t es and phosphates ), originating in the leaching of farm so ils and town waste water, encoU'ages eutrophicalion.

1 5~

Loire Valley Deve lopment Project

11. SURVEY DESCRIPTION The surve y was carried out in two steps : I - Analysis of the phenomena and a diagnosi s of the present situation 2 - Possible solutions (va r ious developments, forecasting, .. . ) and project definition

Du e to t he varying influence of the tributaries, the same high water-level does not have the same return period at all points along the L oir, as is illustrat ed in the following tabl e

level

11.1 Phenomena ana l ysis and diagnosi s

I h~ llIr

11

pt~ r jotl

r InodinlJ

11.1.1 H igh water - levels

u"

s tr eam loi r (Bo flfwv a l)

a)

Mill - Luir

Oowns! ream

( Mo n l oi r c/ l)

La i r ( Dlllt al)

Analysis of th e origin and propagation of high water - levels based on : Nuv emhe r 84

a detailed (between sufficient meter and

st ud y of 9 exam ples of high water- levels 1961 a nd 1983) for whic h th ere is information by rainfa ll gauge, liq uid level recorder.

the elaboration and use of a mathematical model of flood propagation in the valley (over a distance of 250 ~m) : using the H YDRA modular programe for Barre de 5t Venant equations, working on a microcomputer. Th is allows both easy and flexible usage, as well as detailed analysis at a reasonable cos t. Su c h ana l ysis has made it possi b le t o examine the respective roles of tr ibutar ie s of the Per c he ( Refer to Figure 3, where the case exami ned is that of the Yerre River during the flooding in 1983)

flOODING APRil 198)

Apr i l In Innllsry t) 1

., tu 7

111 lu 12

111

2 11

')11

'u "/')

I ',

711

1111

So as to classify the town s in relation to one another and to enab l e further survey - that of the value of the chosen dev e lopment, treating the valley as a whole, it was necessary to calcu late the damage on an average yearly basis. Agricu l tura l damage has also been estimated. 11.1.2 L ow water-level In this case, the survey was concerned, on the one hand, with the hydrau li c system of the va lley at low water-level, and the' frequency of dr y per iods, and on the other hand, wi th the needs and perspectives of deve l oping pumping systems.

Diagram illustrating the phenomena of Yerre River

400 - ;. .! ~

at Villavard (1)

The return periods esti mated at :

300 Vene River input

100 -

Natural flood level

flood level without the Yerre input

Tr i buta ry

100 -

o --'--'----24

Fig . 3

(I)

48

._---:___--~.,._:_'Af'I(IL

71

110 144 188

191

1981

118 Hou"

F'looding April 1983 Tributary i mpa c t

Vil l avard is located between Vendome and the confluence with the Braye River.

of

low

water-levels

were

3 - 5 years for the upstream section (further than the Beauce R egion ground water sources) 8 - 10 years for the mid-Loir and downstream sections The damage correspo nding to the losses due to insufficient irrigation during a dry period, were also es t imated on an average yearly basis. Thi s survey indi ca ted the interest in maintaintng a low water level of 2 m3/s using reservoirs up stream from the basin. 11 .1.3 Water quality

Thu s, this analysis has made it possible t o estimate, for the fifteen towns: flow rates (and leve ls) for varying return pe ri ods between 2 and 10 years flow rates of flood ing in 1983 and 1984

The installation of water treatment stations has helped to improve the quality of the water as regards certains parameters (ammonia, organi c matter). Nevertheless, c ontinuing use of phosphates and increased use of ni trates, is st ill encouraging water eutrophication.

With reference to these two recent damage-caustng f l oods , It was therefore pOS Sible to est im ate the vu ln erability of various towns in t erms of return periods, with cr iti cal va lues between 5 and 20 years (betwee n 5 and 10 years for two-thirds of the likely zones ).

The survey does not put forth a solution to thi s problem; this was not its principal aim. However , it is obvious that an increase of the low water - level flow will improve the situatIOn generally.

b)

11. 2 Possible solutions and project proposal

The cos t of the damage caused by various floods has been estimated, following a series of invest igations, in rela tion with the towns, loca l government and econom i c analysis.

Four aspects were examined local

pro tection

of

the

15

high- r i sk

areas

using peak-limit dams - remote prote ction placed upstr eam, to complement local protection, and to he lp in the case of low water-levels

B. Corbel.

160

J.L. Roy

and

J.

Lefevre

- improving flood forecasting

11.2.4 F lood forecasting

- building regulations in areas liable to flooding.

Until this survey was carrie d out, flood forecasting was the empirical reading of scales, ignoring the influence of both the Yerre and Braye R i vers, w hose role was later found to be far from insignificant (Figure 4) :

11 . 2.1 Local development A series of investigations, surveys, t ogether wit h topographical research and hydraulic analysis, have provided solutions for improving the present situation, in 13 out of the 15 risk areas . Specific critical points will be modified : reducing specific water loss, recalibrating, damming, channeling of main bed ...

P ropaga l io n lim e fr o m U o nn eva l

;.

H ydraulic calculations were carried out using local mathematical models, netted or not, according to the lay - out in question. The model of the valley was used tu esti mate the downstream effect of the planned local deve l opment, and to check that there was a coherence between the development plans proposed for the valley, as a whole . These modifi ca tions allow the critical low water-level return period to be increased from 5 to 10 years. The impro veme nt as regards the yedrly average damage for the whole va lley wou l d amount to 40 % (J 100 000 French Francs as compared to 2 800 000 French Francs) for an Invest ment or approxi mate l y 32 000 000 French Fran cs .

..

X ~.r-__~5~e~c~ u~o~n~J____-..~1r-~5~c~c~t~lo~n~'__+.~ I .~5~c~c~t~io~n~ t __ >-

10 0

ai

.

50

> De la y var ia t ion acc o r d ing to fl ood l e y e l

. . . .

.. .'" u

~

~

0

~

~

. ... .. . . " '"

C

U

~

~

;

E

~ >

0

>-

.=W

.. ~

'"w

Fig.4

11.2.2 Dams In order to com plete valley protec tion, various sites for peak-limit dams have been investigated, four of which have been selected on tributaries of the Loir and the Yerre Rivers. (Figure 2).

It became possible to co mpare water flows which could then be translated into depth, using on l y flow-depth ratios, ca l c ula t ed during deve lopment sur veys.

The im pac t of these dams on flooding in the valley has been tested with the mathematical model of flood propagation.

Th ree levels of equations were put forward, each an improvement on the latter, but requ i ring more equipment each time:

These dam s water-levels.

low

Level I

Equations using pr ese nt day mea suring station equ i pment.

Furthermore, in order to complete the low water-level standby , the building of a fifth dam on the Braye River has been proposed. (Figure 2).

Leve l 2

Equations us i ng modernized measuring stations

Level 3

Equat ions using an automati c network, incorporating pluviographi c figure s or figures from l iquid l evel recorders placed completely up stream.

will

also

be

used

for

avoiding

Economic results have shown, that the dams are not as profitable as local development plans. Therefore, it is co nsidered logical to envisage their building, only on com pletion of local development.

11.2.5 Building regulations i n areas liable to flooding

11.2.3 Development project proposal The project has been broken down into steps of increaSing ambition . Its co ntent s are given in the table hereafter:

In ... eH m

Uc""lullmenl plan

( ,.. il l iun

~ .. I

r t ;,,,~- ,)

L uta! prol ~cl iOIl (1) inc;ludint,j VcndOmc iII"j L;\

A

r leche

,"'it"".. ,,1

...

lu ca l

p . ulf'iI:\',ln

0 .. ""

:

o I~nne

11.7

11.1

ruuu",nJc Z

U.6

7).t.

ILIl

1 LU

! ''IIfonnC riown5t ream

1

17.1

)1.1

[ll.lYr. du .... nstream

1

)U.I)

)U.(,

YI'! I It"

I U T AL (per step) ( M.llioll 1 1<1"1.:5)

IU.4

"'.4

flU.}

ClJMUL"T,V(

HI • •

JZ.8

111.1

TOTAL ( M i lliuns rrancs )

87.7 168 . 8

This section of the proje c t was intentionally l imited to a series of recommenda t ions given to loca l representatives with material support, In the hope that their attention would be drawn to the risks run when building on land liable to flooding, and the interest there is, in developing or applying , in a strict manner, regu lations in this field.

Loire Va lley Development Project

III CONC LUSION a)

The approach used for this survey was based on two essential characteristics : complete analysis of high and low water -l evel problems for the who l e of the L oir Valley, from a physical and economic point of view . co ntinual co nfer rence between the research uni t and the other organisations involved (local communities, and their representatives, and all administrative bodies concerned with water and its management).

The conclusions of this survey, namely the development project , pro\'ide solutions to the problem as a who le, and have been agreed on by all those mentioned above. In this stud y , an important role was played by the mathematical flow model, calculating on the basis of the whole valley for the analysis of the present situation, loca l development plans, cohere nce in the reservoir effect, and for for mulating flood fore casting. b)

It is sti ll too ear l y to be able to eva luate precisely the success of this study.

Planning will be required before steps can be carried out. Ho wever, the methods for forecasting high water-levels have alr eady had a certain success, as the equations were used in January and February 1988 for the high water-levels.

16 1