Download
1 / 41
410 likes | 423 Views
Learn about influent and effluent streams, ephemeral vs. perennial streams, and the importance of stream classification in hydropower development and water management.
E N D
CLASSIFICATION OF STREAMS • Influent and Effluent streams If the GWT is below the bed of the stream, the seepage from the stream feeds the ground-water resulting in the build up of water mound Such streams are called influent streams. Such streams will dry up completely in rainless period and are called ephemeral streams.
The ephemeral streams, generally seen in arid regions, which flow only for a few hours after the rainfall, are of no use for conventional hydropower. However, they can occasionally be used in pure pumped storage schemes, where the actual consumption of water is only marginal.
When the GWT is above water surface elevation in the stream, the ground water feeds the stream, Such streams are called effluent streams. The base flow of surface streams is the effluent seepage from the drainage basin. Most of the perennial streams are mainly effluent streams.
Intermittent and perennial streams. If the GWT lies above the bed of the stream during the wet season but drops below the bed during the dry season, the stream flows during wet season (due to surface runoff and ground water contribution) but becomes dry during dry seasons. Such streams are called intermittent streams.
While in the case of perennial streams, even in the most severe droughts, the GWT never drops below the bed of the stream and therefore they flow throughout the year. For power development a perennial stream is the best; power can also be generated from intermittent streams by providing adequate storage facilities.
A fan-shaped catchment produces greater flood intensity since all the tributaries are nearly of the same length and hence the time of concentration is nearly the same and is less, whereas in the fern-shaped catchments, the time of concentration is more and the discharge is distributed over a long period
Unit Hydrograph Method. The hydrograph of direct surface discharge measured at the outlet of drainage area, which produces a unit depth of direct runoff (i.e., a Pnet of 1 cm over the entire area of the catchment) resulting from a unit storm of specified duration (called unit period) is called a unit hydrograph of that duration.
The area under the hydrograph represents a direct runoff of 1 cm,
The theory of unit hydrograph is based on the following assumptions: (i) The net rainfall is of uniform intensity within its duration (i.e., unit period). (ii) The net rainfall uniformly occurs over the entire area of the drainage basin. (iii) For a given drainage basin, the base period of the hydrographs of direct runoff corresponding to net rains of different intensities but same unit duration, is constant.
(iv) The ordinates of direct runoff hydrographs due to net rains of different intensities (but same unit duration) are proportional. (v) A unit hydrograph reflects all the physical characteristics of the basin.
EX: the following table gives values of measured discharges ata stream-gauging site in a year. Upstream of the gauging site a weir built across the stream diverts 3 and 0.5 Mm3 of water per month for irrigation and for use in an industry respectively. The return flow from the irrigation is estimated as 0.8 Mm3 and from the industry 0.3 Mm3 reaching the stream upstream. Estimate the natural flow and the runoff-rainfall ratio, if the average rainfall is 185 cm and the catcment area is 180 Km2
Application of the Unit Hydrograph. First a unit hydrograph of suitable unit duration is derived from an observed flood hydrograph for the drainage basin due to a known storm (storm loss and net rain). The unit hydrograph so derived can be applied for any other storm (of the same duration but producing different net rain) occurring on the basin and the resulting flood hydrographs can be obtained.
A river catchment has a 2 hour unit hydrograph with the ordinates 0, 3, 11, 35, 55, 66, 63, 40, 22, 9 and 2 m3/s. Assume that the base flow at time t=0 hour is 20 m3/s and linearly increases to 44 m3/s at t=24 hours. • Compute the hydrograph resulting from two successive 2 hour periods of effective rain of 2.0 cm and 1.5 cm respectively. • To prevent downstream flooding, the maximum flow to be released from the catchment is set at 180 m3/s. Calculate the space needed to store the excess water from this event (in m3).
A river catchment has a 2 hour unit hydrograph with the ordinates 0, 3, 11, 35, 55, 66, 63, 40, 22, 9 and 2 m3/s. Assume that the base flow at time t=0 hour is 50 m3/s and linearly increases to 74 m3/s at t=24 hours. • Compute the hydrograph resulting from two successive 2 hour periods of effective rain of 2.0 cm and 3.0 cm respectively. • To prevent downstream flooding, the maximum flow to be released from the catchment is set at 273 m3/s. Calculate the space needed to store the excess water from this event (in m3).
Derive 2 hour 1cm Unit hydrograph from the following S-curve.
Ex: small watershed consists of 1.5 km2 of cultivated area (c = 0.2), 2.5 km2 under forest (c = 0.1) and 1 km2 under grass cover (c = 0.35). There is a fall of 20 m in a watercourse of length 2 km. The I–D–F relation for the area is given by:
Ex:2 the following table gives values of measured discharge at a stream gauging site in a year. Upstream of the gauging site a weir built across the stream diverts 3 Mm3 and 0.5 Mm3 of water per month for irrigation and for use in an industry respectively. The return flow from the irrigation is estimated as 0.8 Mm3 and from the industry at 0.30 Mm3reaching the stream upstream of the gauging site. Estimate the natural flow and the runoff-rainfall ratio if the catchment area is 180 Km2 and the average annual rainfall is 185 cm.
The total quantity of surface water that can be expected in given period from a stream at the outlet of its catchment is known as yield of the catchment in that period.
Annual measurements of rainfall and runoff for a certain watershed were summarized in the table below. Under this scenario, answer the following: 1- Develop a linear relationship between rainfall and runoff. 2-Find out the minimum annual rainfall necessary to produce runoff for this watershed. 3- What would be the runoff for an annual rainfall of 950 mm?
