HERMOSA - The Battle Creek floodplain is still considered prime real estate for housing development, despite an Aug. 17 flood that shredded much of the existing Ferguson Subdivision.

City officials say they are confident that housing planned for the adjoining Heartland Subdivision - which is platted for up to 58 additional homes in the lowlands of the southeast part of town - can and should move forward.

But they also plan to study engineering options that could divert floodwaters away from homes and prevent the Dakota, Minnesota & Eastern Railroad grade from again becoming a massive dam above vulnerable structures below.

That's what happened the night of the flood, when raging waters of Battle Creek backed up into a small lake behind the railroad grade. The water eventually overtopped the grade, which ruptured and sent a powerful torrent pouring down into the Ferguson Subdivision.

No one died, but three houses were ripped off their foundations and most of the remaining 31 homes in the subdivision were damaged. So, too, perhaps, was the confidence some people have in the idea of floodplain development.

Linda Hasselstrom is among them. A writer and ranch owner from Hermosa who owns land immediately downstream from the damaged subdivisions, Hasselstrom said the flood should inspire serious discussions about the wisdom of building homes in the lowlands of a watershed.

"I think this is an extremely significant point. Anyone who has looked with any care at that land would have to know the area of that subdivision was likely to flood with enough rainfall," Hasselstrom said. "That's why we have hayfields in that area - broad, flat, silt-covered soil from eons of floods along Battle Creek."

City officials who support the floodplain developments acknowledge that the Aug. 17 flood affirmed historic realities. But Hermosa City Board Trustee Dan Holsworth also said the flood was an extraordinary act of nature that simply swamped all of the federally approved floodplain regulations and modifications that were followed in planning and constructing the Ferguson Subdivision.

"When you get that kind of Mother Nature dumping on you, there's no way you could ever plan for it," Holsworth said. "There's people who got water in this (flood) who never had it before."

Although the exact amount of rainfall is unclear, there were official reports of at least five inches in the Battle Creek watershed above Hermosa. And unofficial rain gauges tended by rural residents upstream recorded 8 inches and more.

Most of the rain fell within two hours. The flows that surged down the Battle Creek watershed - including Grace Coolidge Creek, which joins Battle Creek just west of Hermosa - were comparable to those of the Rapid City flood of 1972.

In that calamitous event, a powerful, slow-moving Black Hills weather system overpowered flood-control mechanisms, ravaged communities, especially Rapid city, and killed 238 people.

Hasselstrom said that 35-year-old event, when added to the Aug. 17 calamity, should call into question labels like "100-year-floodplain," which is how most of the subdivision land is categorized.

"If folks start talking about hundred-year floods, I have to differ," she said.

Ronald Bengs of Advanced Engineering and Surveying of Rapid City, who serves as city engineer for Hermosa, said that regardless of the percentage of occurrence, the Aug. 17 downpour clearly constituted an extraordinary storm. With peak rainfall at one inch every 15 minutes, the flows were too much for even well-planned developments, Bengs said.

"I'm guessing this was something above a 100-year event, but I don't know that for certain," he said.

"I just think it makes good engineering sense to take a look at this event and see what options are available. What things could be done to minimize the risk."

Options to be considered are likely to include open-channel ditching and other storm-sewer improvements to divert and distribute water and prevent buildup, Bengs said.

The effect of the railroad grade, which has been temporarily repaired, is also going to be examined, he said.

Bengs said he is trying to clarify the chain of events that led up to the ruptured grade and how that could be avoided, even in the event of another rare and powerful deluge.

"When the railroad was overtopped, I believe, it started eroding and the railroad berm gave way, and we essentially had a wall of water come through the subdivision," he said. "What long-term mitigation or modifications can be done to address that? Absolutely that will be looked at."

It isn't clear after the fact if the trestle area where the railroad crosses Battle Creek was partially plugged by debris or whether the water just came in such volume the opening couldn't handle it, Bengs said. The trestle was relatively clear the day after the flood, he said.

The flood has prompted speculation about whether the Ferguson Subdivision was improperly designed or wrongly approved by the city board. Bengs said the Federal Emergency Management Agency, which regulates floodplain development and administers flood insurance at the national level, was involved during the planning and construction of the Ferguson Subdivision.

The Ferguson Subdivision is essentially completed. There are 36 platted lots, 34 of which have homes that were constructed during the last five to seven years. Most of those homes suffered at least some damage from the flood.

In the newer adjacent Heartland Subdivision farther from the railroad berm there are 62 platted lots, only four of which have been built upon so far. Those four homes suffered relatively minor damage.

But Bengs said a number of homes would have been damaged in the flood if Heartland had been further along in development.

"If that subdivision had been full, I think we would have seen similar damage in that subdivision, because of what transpired when the railroad broke. You have a wall of water," he said. "Damage overall in Heartland probably would have been a bit less, because the water would have tended to fan out as the velocity and depth of water decreased."

The city and subdivision developers will want to do what they can with engineering when construction resumes in order to prevent another lake from forming and another wall of water from smacking into homes, Bengs said.

With those safeguards in place and authorization through FEMA and the city board, the subdivision should be allowed to expand, he said.

"I don't see anything that would stop house construction, as long as it had the adequate permitting in place," he said.

Hasselstrom worries about that and thinks others should as well.

One of the homes washed off its foundation in the latest flood ended up at least a half mile away, on a bit of higher ground out in one of her hay fields.

Considering a major flood hit 35 years ago and again Aug. 17, Hasselstrom said it could happen again sooner than many might imagine.

"Today, people ignore the lay of the land and build homes in vulnerable areas," she said. "Sometimes a natural catastrophe catches up with them."

Holsworth said city officials won't forget that possibility as development in the floodplain progresses.

"We had 10 or 12 inches of rain. That's a phenomenon nobody can plan for," he said.

"But I'm sure it will make everybody a little smarter and a little clearer on how things should be so that it never happens again."

Contact Kevin Woster at 394-8413 or kevin.woster@rapidcityjournal.com