FLOOD CREST POURING DOWN INDUS, CHENAB RIVERS
The causative outbursts of rain for Pakistan's tragic flooding seem to have happened in the north of Pakistan during the last days of July, mostly the 28th to the 30th, based upon data from the PMD(Pakistan Meteorology Dept).
The data table shows that two-day rainfall reached at least 40 cm, or 16 inches, in Khyber Pakhtunkhwa (Risalpur, to name one site), site of some of the most deadly flash-flooding and property loss. At the Peshawar airport, two-day rainfall was 33 cm, even though normal monthly rainfall is only 4.6 cm. One-day rainfall locally reached 25 cm in Islamabad which, admittedly, has a much wetter summer monsoon than nearly all of Pakhtunkhwa.
Earlier in the month, easternmost Punjab, especially Lahore, was hard-hit.
MAJOR RIVERS SWELLED INTO FLOOD
Runoff from most of the far north and northwest of Pakistan is directly to the upper Indus River, which quickly began to burst its banks at the end of July.
The earlier downpours in Punjab and across the international border already had the Chenab, Sutlej and Jhelum rivers high or in flood by the time of the late-month cloudbursts.
As of today, Aug. 5, the PMD have the Indus River cresting near its meeting with the swollen Chenab outside of Mithankot, southern Punjab.
The estimated top flow of the flood crest is 900,000 to 1,000,000 cubic feet per second ("cusecs").
A like flow ("Exceptionally High Flood Level") is forecast for Guddu, at the Sindh border.
The forecast shows that Sukkur will see the flood crest within about day, the flow being that of the crests farther upstream.
The last major forecast site on the River is for Kotri, near Hyderabad, where the crest is forecast to happen on Aug. 10 and 11.
INDUS IS MUCH LIKE THE NILE
The Indus, like the longer and better known Nile, rises in highlands watered by seasonal, or monsoon, rains.
For much of its length, there is minimal inflow to the Indus, as it crosses what is essentially a desert. Or at least it would be were it not watered by the river's water through the industry of the people.
A good true-color satellite image would show clearly the green of lush croplands grounded in fertile alluvial soil. All this rich earth needs is dependable water and the farmer's care, and it can burst forth with green growth.
GEOGRAPHY SHAPES FLOOD THREAT
Fast-rising (in the geological sense of meaning) mountains, like those rimming the Indus Valley to the north and west, shed vast loads of sediment, which find their way downhill to streams and rivers on the way to the sea (in most instances).
When the main stream or tributaries reach flatlands, as does the Indus, the load of sediment can be too high for the streams to carry. When this happens, it chokes the stream beds and, over time, they tend "wander," thereby distributing the sand, silt and mud in a broad sheet. Much of Pakistan, as well as northern India, has such a landscape.
Image (Credit: Google maps) of Guddu Barrage (low dam)
Satellite images of the Indus show a wide, meandering bed riven into untold channels, sandbars and islands. No doubt, any of these can shift markedly, and appear or disappear during a single flood.
Sometimes, the lowest spots along the river's flood plain are not those followed by the river bed. This happenstance renders such spots exceptionally prone to inundation when such a river "reclaims" its rightful place on the flood plain during high water.
Crops and human habitation along rivers, and foremost desert rivers, are inevitably in harm's way during times of exceptional flow.
In the present instance, this accounts for the stated tragic loss of life (well above 1,000), number of displaced/affected people (3,000,000 or more), and land inundated (in the 100,000s of hectares) along the Indus River basin.
As I understand it, the top flow on the Indus (1,000,000 cusecs) is about 10 times the mean yearly flow of the river.
Unlike the Nile, the Indus does not have one great dam and flood control reservoir (as in the Aswan High Dam and Lake Nasser upstream of Egypt) to moderate flow in its lower reaches. The huge Tarbela Dam, north of Islamabad, is on the upper Indus, upstream of much of the runoff from this July's exceptional rainfall, and thus not a factor in controlling it.
LOW DAMS, FLOOD CONTROL AND IRRIGATION
Instead of major flood control dams, the mid- and lower reaches of the Indus are fitted with low dams, or "barrages." These work rather more to shunt water into supply canals than to have any direct moderating effect upon the flood crest.
At least in theory, the water supply canals can act to sluice away some of the flood flow, and this may be having a significant effect upon flow dynamics on the lndus at this time.
"Barrages" are, north to south, at Jinnah, Taunsa, Guddu, Sukkur and Kotri/Hyderabad. Having never been here myself, I must rely on other means to grasp this setting.
Satellite imagery shows that each of these structures has canals that split away right upstream. These canals then flow parallel to the river itself, their water being distributed to the croplands, towns and cities along the way.
While I have no actual statistics one way or another, it is my understanding that Karachi gets most of its water in this way.
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