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25-613: The Gorges de l'Ardèche is made up of a series of gorges in the river and locally known as the "European Grand Canyon", Located in the Ardèche , in the French department Ardèche , forming a thirty-kilometre long canyon running from Vallon-Pont-d'Arc to Saint-Martin-d'Ardèche . The lower part of the gorge forms the boundary between the Ardèche department and the Gard department. The canyon

50-613: A civil security crisis or standby alert. Precise information on the evolution of any flood is provided. The Ardèche receives water from the Loire river via the "La Palisse" flood barrier and the Lake d'Issarlès. Effectively, the water is collected to feed the EDF hydroelectric plant at Montpezat-sous-Bauzon and is subsequently piped into the Fontaulière river, a tributary of the Ardèche, near

75-645: A dry period in July–August resulting in a decrease of the average monthly rate to the level of 12 cubic metres per second (420 cu ft/s) in July. (See bar chart below.) The VCN3 (minimum flow) can drop to 2.5 cubic metres per second (88 cu ft/s) in a dry year. Floods can be extremely important (usually following storms in the Cevennes). The Qix 2 and Qix 5 are respectively 1,800 and 2,600 cubic metres per second (64,000 and 92,000 cu ft/s) which

100-484: A hydrologic soil group should be done based on measured infiltration rates, soil survey (such as the NRCS Web Soil Survey ), or judgement from a qualified soil science or geotechnical professional. The table below presents curve numbers for antecedent soil moisture condition II (average moisture condition). To alter the curve number based on moisture condition or other parameters, see Adjustments . Runoff

125-463: A particular area. The runoff curve number is based on the area's hydrologic soil group, land use , treatment and hydrologic condition. References, such as from USDA indicate the runoff curve numbers for characteristic land cover descriptions and a hydrologic soil group. The runoff equation is: where The runoff curve number, C N {\displaystyle CN} , is then related C N {\displaystyle CN} has

150-410: A range from 30 to 100; lower numbers indicate low runoff potential while larger numbers are for increasing runoff potential. The lower the curve number, the more permeable the soil is. As can be seen in the curve number equation, runoff cannot begin until the initial abstraction has been met. It is important to note that the curve number methodology is an event-based calculation, and should not be used for

175-452: A single annual rainfall value, as this will incorrectly miss the effects of antecedent moisture and the necessity of an initial abstraction threshold. The NRCS curve number is related to soil type, soil infiltration capability, land use, and the depth of the seasonal high water table. To account for different soils' ability to infiltrate, NRCS has divided soils into four hydrologic soil groups (HSGs). They are defined as follows. Selection of

200-705: Is a natural 60-metre (200 ft) stone arch spanning the river known as the Pont d'Arc (arch bridge). The source of the river lies at 1,467 metres (4,813 ft) above sea level in the Vivarais , near the Col de la Chavade, in the forest of Mazan in the commune of Astet . After the towns of Aubenas and Ruoms , it collects the Chassezac and the Beaume and plunges into its famous gorge below Vallon-Pont-d'Arc . It flows into

225-527: Is a tourist attraction, drawing over a million visitors per year, in addition to a rich historical and archeological site. Most of the canyon is protected; it is governed by the Réserve Naturelle Gorges de l'Ardèche. Notable sights along the canyon include the Pont d'Arc at the beginning of the canyon, a natural arch 60 m wide and 54 m high. Much of the canyon is inaccessible except by water, and canoeing and kayaking are popular sports on

250-1038: Is affected by the soil moisture before a precipitation event, the antecedent moisture condition (AMC). A curve number, as calculated above, may also be termed AMC II or C N I I {\displaystyle CN_{II}} , or average soil moisture. The other moisture conditions are dry, AMC I or C N I {\displaystyle CN_{I}} , and moist, AMC III or C N I I I {\displaystyle CN_{III}} . The curve number can be adjusted by factors to C N I I {\displaystyle CN_{II}} , where C N I {\displaystyle CN_{I}} factors are less than 1 (reduce C N {\displaystyle CN} and potential runoff), while C N I I I {\displaystyle CN_{III}} factor are greater than 1 (increase C N {\displaystyle CN} and potential runoff). The AMC factors can be looked up in

275-537: Is high. QIX 10 is 3,100 cubic metres per second (110,000 cu ft/s) while QIX 20 and QIX 50 respectively rise to 3,600 and 4,300 cubic metres per second (130,000 and 150,000 cu ft/s). The maximum instantaneous flow recorded in Saint-Martin d'Ardèche has been 4,500 cubic metres per second (160,000 cu ft/s) (two-thirds of the average flow of the Danube), while the maximum recorded daily rate

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300-518: Is protected along almost all of its course. These protected areas are: Runoff curve number The runoff curve number (also called a curve number or simply CN ) is an empirical parameter used in hydrology for predicting direct runoff or infiltration from rainfall excess. The curve number method was developed by the USDA Natural Resources Conservation Service , which was formerly called

325-647: The Rhône at Pont-Saint-Esprit . The Ardèche flows through the following departments and communes: The most important tributaries and subtributaries to the Ardèche include: The river has an average discharge of 65 cubic metres per second (2,300 cu ft/s) but experiences severe floods, called "coups de l'Ardèche" (Blows of the Ardèche), in spring and autumn and periods of very low water in summer. During flood events in 1827, 1890, and 1924, it reached 7,800 cubic metres per second (280,000 cu ft/s) and

350-528: The Soil Conservation Service or SCS — the number is still popularly known as a "SCS runoff curve number" in the literature. The runoff curve number was developed from an empirical analysis of runoff from small catchments and hillslope plots monitored by the USDA. It is widely used and is an efficient method for determining the approximate amount of direct runoff from a rainfall event in

375-500: The Ardèche was observed and calculated over a period of 26 years at Saint-Martin d'Ardèche. It amounted to 63.4 cubic metres per second (2,240 cu ft/s) for a surface basin of 2,240 square kilometres (860 sq mi)—i.e. the vast majority of its watershed of 2,430 square kilometres (940 sq mi). The river has seasonal fluctuations: a typical flow around the Cevennes, with high water in autumn and winter being double

400-605: The CN runoff equation becomes: In this equation, note that the values of S 0.05 {\displaystyle S_{0.05}} are not the same as the one used in estimating direct runoff with an I a / S {\displaystyle I_{a}/S} ratio of 0.20, because 5 percent of the storage is assumed to be the initial abstraction, not 20 percent. The relationship between S 0.05 {\displaystyle S_{0.05}} and S 0.20 {\displaystyle S_{0.20}}

425-462: The assumption of I a / S = 0.20 {\displaystyle I_{a}/S=0.20} is usually high. More than 90 percent of I a / S {\displaystyle I_{a}/S} ratios were less than 0.2. Based on this study, use of I a / S {\displaystyle I_{a}/S} ratios of 0.05 rather than the commonly used value of 0.20 would seem more appropriate. Thus,

450-532: The gorge is the Chauvet Cave . Ard%C3%A8che (river) The valley of the Ardèche is very scenic, in particular a 30-kilometre (19 mi) section known as the Ardèche Gorges. The walls of the river here are limestone cliffs up to 300 metres (980 ft) high. A kayak and camping trip down the gorge is not technically difficult and is very popular in the summer. The most famous feature

475-624: The normal, brings the average monthly flow at the first peak of 93 cubic metres per second (3,300 cu ft/s) in October then, after falling to 76 cubic metres per second (2,700 cu ft/s) in December, a new peak occurs from 96 to 102 cubic metres per second (3,400 to 3,600 cu ft/s) in January–March (with a maximum in January). A rapid decline in flow rate follows ending in

500-444: The ratio of I a {\displaystyle I_{a}} to S {\displaystyle S} with hundreds of rainfall-runoff data from numerous U.S. watersheds. In the model fitting done by Hawkins et al. (2002) found that the ratio of I a {\displaystyle I_{a}} to S {\displaystyle S} varies from storm to storm and watershed to watershed and that

525-474: The reference table below. Find the CN value for AMC II and multiply it by the adjustment factor based on the actual AMC to determine the adjusted curve number. The relationship I a = 0.2 S {\displaystyle I_{a}=0.2S} was derived from the study of many small, experimental watersheds . Since the history and documentation of this relationship are relatively obscure, more recent analysis used model fitting methods to determine

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550-464: The river. Overnight camping is not allowed, except for at two bivouac shelters . The cliffs offer habitat to rare birds such as the Bonelli's eagle . (As of 2013 there were only two pairs in the Ardèche, and no more than thirty in all of France.) Humans have lived in caves in the area for over 300,000 years. Over 2,000 caves are found in the gorge, some of them painted; the best-known painted cave in

575-600: The town of Aubenas. Several ancient inscriptions about a college of nautes (Boatmen) in associated rivers have been discovered in the Gard. It is possible that the two rivers concerned are the Ardèche and the Ouvèze. The identification of these two rivers is still pending. However, if it does involve the Ardèche river, the spellings Hentica (from 950), then Ardesca (in the Charta Vetus) have been attested. The Ardèche

600-725: The water level rose to a record 21.4 metres (70 ft) in the gorge. Despite the Ardèche's short length, the flow of the river at 65 cubic metres per second (2,300 cu ft/s) is relatively high—higher than the Gardon at 32 cubic metres per second (1,100 cu ft/s), the Cèze (22 m3/s), the Hérault (44 m3/s), or the Agout (55 m3/s)—major rivers south of the Massif Central but much longer. The inter-annual average flow of

625-487: Was 2,506 cubic metres per second (88,500 cu ft/s). The runoff curve number flowing into the catchment of the river is 897 millimetres (35.3 in) annually, which is very high. The specific flow (Qs) reaches 28.3 litres per second per square kilometre of the catchment. The prefecture of Ardèche has provided a voice server since June 2005 whose objective is to regularly disseminate information messages to allow monitoring of any significant event that might trigger

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