MANILA: Deadly floods that have swamped nearly all of the Philippine capital are less a natural disaster and more the result of poor planning, lax enforcement and political self-interest, experts say.

Damaged watersheds, massive squatter colonies living in danger zones and the neglect of drainage systems are some of the factors that have made the chaotic city of 15 million people much more vulnerable to enormous floods.

Urban planner Nathaniel Einseidel said the Philippines had enough technical know-how and could find the necessary financing to solve the problem, but there was no vision or political will.

"It's a lack of appreciation for the benefits of long-term plans. It's a vicious cycle when the planning, the policies and enforcement are not very well synchronised," said Einseidel, who was Manila's planning chief from 1979-89.

"I haven't heard of a local government, a town or city that has a comprehensive drainage masterplan."

Eighty percent of the Manila was this week covered in waters that in some parts were nearly two metres (six and a half feet) deep, after more than a normal August's worth of rain was dumped on the city in 48 hours.

Twenty people have died and two million others have been affected, according to the government, which has said the floods will last for days and longer if more monsoon rains fall.

The deluge was similar to one in 2009, a disaster which claimed more than 460 lives and prompted solemn pledges from government leaders to make the city more resistant to floods.

A government report released then called for 2.7 million people in shantytowns to be moved from "danger zones" alongside riverbanks, lakes and sewers.

The plan would affect one in five Manila residents and take 10 years and 130 billion pesos (2.77 billion dollars) to implement.

But Einseidel said that while there had been some efforts to relocate squatters, they never succeeded.

"With the increasing number of people occupying danger zones, it is inevitable there are a lot people who are endangered when these things happen," he said.

Squatters, attracted by economic opportunities in the city, often build shanties on river banks, storm drains and canals, dumping garbage and impeding the flow of waterways.

"The same people who were already told not to return to the rivers and creeks and floodways are back. They are there again and they are the ones who don't want to leave now."

He blamed the phenomenon on poor enforcement of regulations banning building along creeks and floodways, with local politicians often wanting to keep squatters in their communities to secure their votes at election time.

Meanwhile, on the outskirts of Manila, vital forested areas have been destroyed to make way for housing developments catering to a growing middle and upper class, according to architect Paulo Alcazaren.

Alcazeren, who is also an urban planner, said the patchwork political structure of Manila had made things even harder.

The capital is actually made up of 16 cities and towns, each with its own government, and they often carry out infrastructure programmes -- such as drainage and watershed protection -- without coordination.

"The controls of physical development must not be dependent on political boundaries of towns and provinces," he said.

"Individual cities can never solve the problem. They can only mitigate. If you want to govern properly, you must re-draw or overlay existing political boundaries."

Solutions to the flooding will all require massive efforts such as re-planting the watersheds, building low-cost housing for the squatters and clearing drainage systems, the experts said.

"It will cost billions of pesos but we lose billions anyway every time it floods," Alcazeren said.