A little hydrology
Normally, you see water flowing in a creek because the stream is in balance with the goundwater. The stream usually runs in a valley, biting down to reach the level of the ground water. You might say the stream is just the visible part of the groundwater.
If there's an excess of groundwater, or the groundwater level is higher than the stream, groundwater will flow into the stream, augmenting it. If the groundwater level is lower than the stream, water from the stream flows into the ground, trying to raise the level of the groundwater.
In some cases, something might cause the groundwater level to recede from the level of the stream. There might be a large well nearby, or something else drawing away groundwater. If this lowered level persists for some distance, of if the stream is only a trickle, then the dry ground will suck up all the water in the stream, and the stream will disappear. It's not uncommon in the mountains or deserts to see a stream appear and disappear, depending on the level of the groundwater.
Sunset Village Creek originates in a shallow basin bounded roughly by Mineral Pt., Larkin, Hillcrest, and S.Owen. Because it's a shallow basin, the groundwater is close to the surface, and the creek often runs on the surface through the park.
But in the Hillcrest-Upland Greenway, the water table apparently withdraws from the level of the creek, and so the creek dries up in the lower part of the greenway. Usually, however, there are a few pools in the upper portion, even some time after a rain. The water table is evidently on the surface or close to it here. That is why I proposed some pools for the upper greenway in the plan. I simply wanted to capitalize on the existing situation--for the sake of wildlife.
Pools at the upper end of the greenway--a place for wildlife to drink.
It's worthwhile asking: "Why does the water table withdraw from the surface in the lower greenway,causing the stream to dry up?"
- There's a steep slope to the west here, and generally speaking, water tables often withdraw from the surface in hills.
- More likely, the groundwater here is leaking into the sanitary sewer pipes. We heard there were a number of leaks, but we don't smell sewage on the surface. This suggests that groundwater is leaking into the sewer, rather than sewage leaking out.
- In a similar vein, the groundwater might simply be flowing in the trench through gravel alongside the old pipe. In other words, the stream is flowing underground, following the pipe.
Riprap is death for a stream. Dumping riprap will usually raise the surface, separating the stream from the groundwater. In addition, the riprap creates a lot of little pockets between rocks, through which the water flows. So, if it's only a trickle, water will flow through these spaces at the bottom of the riprap layer, out of sight. You will only see water on the top of the riprap if there's enough water to fill all those spaces, or if sediment eventually fills all those spaces after five or ten years. Even if sediment fills the spaces, water won't be visible if the water table is still below the top of the (elevated) riprap.
If you want to see how water becomes invisible after riprap is applied, just look at the riprap along the SW Bike Trail, just below the Glenway Golf Course.
Some residents are concerned about mosquitoes. Of course, mosquitoes and other aquatic insects are the base of the food chain, attracting other wildlife. But riprap can create a mosquito problem because the little pools of water under the riprap will be out of sight. Visible pools can be treated with a larvacide if necessary--invisible pools can't be treated.
In summary, I agree mostly with Lisa Coleman's email comments about the issue. Water will be visible in the channel during times of high flow, such as spring thaw or during a storm. At other times, you and the wildlife can kiss the water goodbye.
Stability is important for nature
Wildlife need to be able to count on water. If the water is visible sometimes, but invisible most of the time, that's bad for wildlife. This alternation of floods with dry channel makes for a very sick stream.
The water in the upper part of the greenway will disappear because the channel will be raised, the water will flow between the cavities in the riprap, and because water will also sink down to the gravel packed around the new sewer pipe. Here the pipe and the gravel will act like a "French drain."
My friend Liz McBride had standing water problems near her back door. She had French drain (a pit filled with gravel) installed there, and has never seen the water again.
There has been talk about placing an impervious "clay cap" over the new sewer trench, under the riprap, to help keep water on the surface. Given the other reasons for water going underground, I don't think a clay cap would make any difference. Only paving the bottom of the channel, as an alternative to riprap, would work--as Tim Kessenich has demonstrated in his yard.
The stream in Tim's yard.
Paving streambed as he did, or improving the watershed, are the only ways to keep the stream on the surface.
Meanwhile, back to the watershed
Once upon a time, our little basin of Sunset Village was perhaps a wet sedge meadow, bordered by some low, oak-covered hills. A little stream trickled out of the basin--serving as home for green herons, phoebes, wood thrushes, and other birds and animals that came to drink.
The little stream trickled down a steeper slope, then joined a larger creek flowing through wetlands on present-day Midvale Blvd, Lucia Crest Park, and University Avenue. Eventually it reached large wetlands bordering lake Mendota, now the site of playing fields and parking lots. Old sand dunes from glacial times, covered in 1850 with oak savanna (the vet hospital), overlooked these expansive wetlands. Fast-growing wetland plants sucked any nutrients or mud from the water that had escaped from the wet meadows of Sunset Village.
In those days, Lake Mendota was almost crystal clear, perhaps with a slight color of tea. The Winnebago would drink from it as they waded along its shore. Harry Whitehorse, who lives near Madison and grew up on her waterways, can almost remember those days.
But in the 1950s, much of Sunset Village was roofed or paved; and two churches paved large parking lots. Resulting floodwaters then rushed down the ravine, eroding it. With unruly runoff racing to University Avenue, there were no waters to recharge the soil of the basin. So the water table dropped, and the little stream mostly dried up.
Having lost the battle to save the last pools of this little stream in the greenwa--this little memory of the past--we must refocus on restoring the watershed of Sunset Village to health. This is our only hope to keep a little water on the surface, where children can learn of its charms.
There will be many benefits to increasing infiltration of rainwater in our neighborhood:
- More gardens and interesting green space. The city would help pay for them.
- More wildlife
- More diverse play spaces for children
- Better flow of the stream in the park
- Reduced flooding on University Avenue
- Healthy lakes (surface flow goes to Mendota; groundwater flow to springs around Wingra)
- Lowered costs for pumping drinking water
- Save on your water bill (the stormwater part)
- And finally, a more verdant greenway (because there's more groundwater for the plants)
Neighbors working togethe.........................................................................
to create rain gardens on Adams Street.
Photo from city website. More
to create rain gardens on Adams Street.
Photo from city website. More
Question: Should we leave out the clay cap, to give the stream a chance to recharge the groundwater?
It depends on your goal--whether you want the stream to recharge groundwater or whether you want it on the surface for people and wildlife. Since the great bulk of the water just rushes through the greenway during floods, it's not going to do much work for recharging groundwater. The clay cap won't make any difference. We should concentrate on recharging groundwater further upstream, with downspout rain gardens, terrace rain gardens, and porous pavement. If the decision to use riprap hadn't already been made, I would vote to have some open pools in the greenway for people and wildlife.
Question: What makes you an "expert" on groundwater and stormwater?
I spent my youth playing around lakes and streams. When I built sand castles, I gave them waterworks--the moat really worked. I explored a stream in my neighborhood. I still spend a lot of time out with my camera, exploring streams and pipes. By burying streams, and destroying our watersheds, I'm afraid today's children will be deprived of these pleasures, and will be denied these ways to find out how the world works.