By Mahinda Samarasinghe
My Feb. 19 Forum op-ed, headlined "Boardman Project Needs Rethinking," raised several concerns. So far, no adequate answers have been provided, not even in the recently publicized Environmental Assessment Report.
The major concern is the exposure of Traverse City to a devastating future flood. Clearly, Traverse City will become vulnerable to floods if the three upstream dam-pond systems (Brown Bridge, Boardman and Sabin) on the Boardman River are removed because all the flood waters flowing many miles down the Boardman River end up in Boardman Lake, located at the downstream end at Traverse City.
Flooding potential is so high because, at some places in Boardman Lake, the existing dry-weather water level is only about two to three feet below the surrounding ground level — easily observable close to the Traverse City public library and the condominiums nearby.
Basic mechanics of water flow can be explained in simple terms as follows:
During a substantial rainfall period, as we often witness, any water body such as a river, lake or pond tends to swell. The swelling itself has many causes and effects. The swelling, which causes flooding when banks overflow, also helps develop higher velocities for vast volumes of flood water to be quickly carried away downstream.
The swelling itself is also a phenomenon that provides greater temporary storage capacity for a lake or pond during a peak rainfall event, thus reducing the flood potential. Depending on the dimensions of the lake or pond, the temporary storage volume can be even several times more than its existing dead-water volume. It is this temporary storage of water (which is observable as swelling) that helps avert or mitigate flood damage.
It came to light recently that the Boardman dam and pond prevented, as explained above, possible flooding of Traverse City in September of 1961 from a flash flood resulting from Hurricane Carla, as reported in the 1961 Sept. 15 Holland Evening Sentinel newspaper: "Hurricane Carla, Dumping up to 10 inches of rain in the Traverse Bay region"; "The Boardman River, one of the state's internationally Famous trout streams, became a raging flood in the rain and two dams on the river gave way. The Keystone dam, about 10 miles south of Traverse City, failed and sent a wall of water from the 16 foot high backwater plunging toward the TOWN.
"Boardman Dam six miles from the heart of the city, caught the crushing blow of water, and held, PREVENTING GREAT DAMAGE."
"About four hours later Mayfield DAM on Mayfield Creek some 11 miles south of the city collapsed and another slug of water crashed into the Boardman DAM, BUT AGAIN IT HELD."
This shows how dangerous it is to remove the upper three dam-pond systems, leaving only the Boardman Lake located in Traverse City itself to absorb the full force of a peak flood.
It is quite possible that the forefathers of Traverse City built these dam/pond systems for several reasons, including security against flooding that they may have experienced in the past.
About the author: Mahinda Samarasinghe is a retired civil engineer with a doctoral degree in geotechnical engineering from the University of Kentucky; he has worked in Australia and Sri Lanka as a civil engineer and lives in Traverse City.
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