Follow the parking instructions for the Hansen Dam Trailhead - The Iron Cross (GC11ZDX). Binoculars will help see the sides of the pit.
Big Tujunga Wash and Little Tujunga Wash merge just upstream from this dam. Their combined watershed (225 square miles) is the largest that drains down into the San Fernando Valley and on into the Upper Los Angeles River. While these lower elevations typically get 4 to 6 inches of rain per year, the higher mountains that drain into the dam can get 24 inches or more. This is due to a rain shadow effect (See San Jacinto EarthCache). Often the rain will come in short intense storms during the winter. This dam is here to temporarily store the storm water and release it down stream over a longer period but lower flow hopefully preventing flooding in the urbanized area below the dam.
The urbanization of the area has created a demand for clean water. One of the important sources of water for the area is ground water. The San Fernando Valley is a very deep, 10,000-foot deep, valley filled with sediments (sand, boulders, silt, etc) from the surrounding mountains. The spaces between all these sediments have filled with water creating a huge underground lake that drains out at the Glendale Narrows. This water is called ground water and can be pulled out of the ground with wells.
In a natural system, a large percentage of precipitation would infiltrate into the soils and recharge the ground water. While infiltration and recharge are often used interchangeably, there is a slight difference. Infiltration is the movement of water into the ground, while recharge is the infiltrated water that makes it all the way down to ground water. The difference between infiltration and recharge would be the water that is pulled back out of the ground by plants (evapotranspiration), and the amount of water that stays stuck to individual grains.
Natural recharge is currently happening upstream of the dam where the channel is unlined. However, urbanization of the San Fernando Valley has significantly reduced the available area for surface water to percolate down to groundwater by covering the land with impermeable concrete and asphalt and has lined the creek and river channels with concrete. At the same time, the growing population creates a growing demand for safe water. The net effect has been increasing the amount of ground water used and decreasing the amount of water to recharge, resulting in a decreasing supply of usable water.
In order to counter this effect, specially designed areas have been set aside to artificially increase the volume of recharge. These areas are called spreading grounds and are not that complicated. They are shallow basins that are filled with water and allowed to infiltrate down into the ground. One set of these spreading grounds, the Hansen Spreading Grounds can be seen to the right of the stream channel that leaves the dam on the other side of the golf course (across the channel from the tall red and white towers). The close up coordinates are provided as a waypoint along with coordinates for the Pacoima Spreading Grounds. After storm events, these basins are filled with some of the runoff, effectively storing the rainwater in the ground for later use. The Hansen Spreading Grounds are estimated to store 43,000 acre-feet per year. Planners estimate that between 1 to 3 families use 1 acre-foot of water each year.
The relationship between the spreading grounds and the ground water can be seen by looking at water in the deep pit to the right of the channel on the far side of the golf course (beyond the ponds in the golf course). This pit, an old open pit mine, was excavated down to the top of the ground water. In the sides of the pit you can see the type of sediment that the water in the spreading grounds would be infiltrating through and how far it has to infiltrate. The sediments seen in the sides of the pit are typical for the surrounding area as this area is an alluvial fan (see Tujunga Wash). Click on the image to the right for a larger version.
Logging requirements:
Send me a note with :
- The text "GC17CFF Hansen Flood Control and Spreading Grounds" on the first line
- The number of people in your group.
- Look at the sides of the pit and describe the types off sediments that would be under the spreading grounds.
- How quickly might you expect water to move through these sediments and how does that relate to the spreading grounds.
Put in your log:
- How much water is in behind the dam and how much water is going down the channel. The water depth markers are just west of the overflow channel. Be general.
- Compare that to how much water is in the spreading grounds and how much rainfall there has been recently.
Advanced Information:
Beyond the pit and a little to the left of the power plant is a growing hill. This hill is the Bradly Landfill and used to be a pit. The recharge at the spreading grounds raises the level of the ground water in the area. As much as the region needs a lot of water, the amount of recharge needs to be carefully regulated to keep the ground water at least 10 feet below the bottom of the landfill to prevent the landfill from contaminating the ground water supply.
The above information was compiled from the following sources:
- Langenheim, V.E. and others, Preliminary Potential-Field Constraints on the Geometry of the San Fernando Basin, Southern California, Open-File Report 00-219, U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY, 2000
- Inventory of Existing Information for the State of the Tujunga Report - Tujunga Watershed Project. The River Project Staff and the Tujunga Watershed Project Team for the California Bay Delta Authority. September 2005
- Chapter IV – Groundwater Basin Reports San Fernando Valley Basins - Upper Los Angeles River Area Basins, Los Angeles Metropolitin Water District, September 2007 http://www.mwdh2o.com/mwdh2o/pages/yourwater/supply/groundwater/PDFs/SanFernandoValleyBasins/UpperLARiverAreaBasins.pdf
- Dubnewych, Steve and others, PROBLEMS ENCOUNTERED ON THE UPPER REACH OF THE NORTHEAST INTERCEPTOR SEWER PROJECT, 2005 RETC PROCEEDINGS, Chapter 22
- Upper Los Angeles River Area Watermaster (Watermaster), 2006a, Watermaster Service in the Upper Los Angeles River Area, 2004-05 Water Year, Los Angeles County, California.
- Bradley Landfill and Recycling Center Transition Master Plan Draft Environmental Impact Report. The City of Los Angeles. December 2005
- Nicholas D. Kilb, SUNY at Stony Brook, Fall 2005, Models of Ponding In Two Flooded Recharge Basins http://www.geo.sunysb.edu/lig/Conferences/abstracts06/kilb.pdf
- Pizzulli, M.E. "Fate of Nitrogen in a Ponded Recharge Basin." Proc. of Geology of Long Island and Metropolitan New York Conf., 24 Apr. 1999, SUNY at Stony Brook, New York. 18 December 2005.