Helping my landlord put some swales in at his lavender farm in the central coast of California to catch some of this godzilla el niño rain we're supposedly going to be getting this winter. I'm a little stuck on placement.
Check out the picture below for the setup. Basically, everything slopes down from this house in the top right corner to the creek in the bottom right corner. Right now, when it rains, most water runs down the three roads leading from his house (one from top house, two from carport/airstream pad below), part the lavender fields, and straight into the creek.
I want to capture the water running down the roads, plus overflow from the pond, and sink it into the soil above the lavender field via swales. The contours I've laid out with an A-frame are shown as red lines A - E. They are ~1 foot contour lines. I'm definitely planning to construct a swale on contour A. Unfortunately, the other contours don't work out as nicely and quickly head from the road straight into the lavender. My first thought is to construct swales on each of those line segments and have water cascade into each via spillway, as that seems like the only way to get swales running the entire length of the field, and capture rainwater flowing pay the field. Seems kinda wonky though, and may look a bit weird. Hoping someone will either tell me is not as wonky as it seems, or have another suggestion. One additional issue i foresee is that swale E will fill up very quickly due to all the runoff flowing down the road on the far right, and is at the lowest elevation of those contours. m hoping some of you may have a better or didn't suggestion.
The first thing that struck me was that you didn't have any plans for catching water Higher on the property. Looking at the picture, I would be trying to cram as much water catchment in that space between the house and the little foresty area. Right where you've written "slope" in green...Starting at the "E" in slope, I'd see about building a pond and swale system that you could use for gravity watering/feeding. The higher up, the better.
You're right about the road messing with the lower swales. You may have to make some adjustments by reconfiguring the swale lines or by adding some material between the lavender plants to divert water to the right places. This is usually the trouble with earthworks on established crop lands. How to fit curved lines into straight rows. In this case (drought) you might consider the risk of removing some plants to make room for the swales that will save the remaining plants from a slow water less death.
With the area below the road, I would find the Lowest High point along the road edge and start there. Doing that should allow you to make some placements that won't run across the road and mess up traffic. I don't know how much rain you're expecting but make sure that you space the swales apat enough to make the most of the catchment. Too close together and they will fill and overflow. Too far apart and they don't hold enough water to make it worth it. These distances depend on slope, rainfall, infiltration, soil type and patience.
I'd focus on swale a. Connect it to the current existing pond to help keep it full, build that same contour on the other side towards the other road and build a speed bump on that road to direct water into the swale. Set the spillway at close to the top of the lavender patch. Use the pond for irrigation. If you desire a second swale I would cut e all the way through the lavender patch and extend it past so that it could pick up water from swale a.
Thanks both for the input. Craig, in regards to water storage higher up. .. that barren area where the "E" in "slope" is is a very steep slope, ~60%. Not safe for water storage earthworks. I actually put another post up in this forum 2 weeks ago about what to do with that steep slope. there is a second pond, where the "S" is in "slope". The overflow from that pond fills the labeled pond in the photo. Both of these are used to fertigate the lavender. However, trying capture some of this road runoff and create some in-soil water storage happen, as we live in a climate with more evaporation than precipitation.
Since we have the excavator rented all day, do you all think it's not worth it to do those smaller swale segments, B through D, and have armored spillways cascade into each lower one from the lettered swale above? Im trying to create water storage stretching across th entire field, and my landlord doesn't like the idea of ripping out lavender!
If your just renting an excavator and playing around for a day then you might as well try it. Swales b through d could work as you say, sorta fish scale swales. The spill way for each would have to be at the side of the road to catch the next swale and I would line it heavy with rocks. Each tip of the swale opposite the road would have to curve up and be the highest point to allow water to sit. Being your in a drought rain type of cycle the amount of run off could be tremendous. If you implement the b through d swales, I think it would be crucial to cut swale e entirely through the lavender and well into the next field so you can set the spillway for e directly into the creek ...far far away from that house at the bottom of the lavender patch.
What I mean is that I would make my first swale so that It picked up as much of the water off of the road as possible and spread it out along the greatest distance possible. If I'm seeing your map correctly then the roadway is always higher in elevation than the lavender field. That being the case you could probably get by with only a couple swales if the ones you've drawn are spaced one foot in elevation. I would Keep Line A, but shorten it on the left side so that it's not diverting water away from the very top of the lavender field on the left. I would do away with Line B and D. Lines C and E would be shortened on the right hand side to avoid cutting the driveway. These same lines would be extended as far left into the field as they can go to divert as much water back into the center of the field as possible. I would also try to add one more swale lower that the ones you have drawn. Perhaps a larger to catch any extra run off from really big storms. Ideally it would go just below the crop and would catch all the water and nutrients in a catch crop. This extra catch crop could be used for chop and drop mulch or something similar.
I may be just seeing things but it looks like the plants along the right side of the field along both roadways are bigger than those on the opposite side of the field. There also appears to be some thin and bare spots in the center of the field and to the left. It would be nice if one of the swales happened to line up with those bare spots.