Central and West Coast Basin Study
For the Central and West Coast Basin, preliminary studies show a possibility of 45,000 AF / year of recharge, which over a period of time could add 400,000 AF to this aquifer, and reduce water import needs in this area by 12% long term based on the overlying demand. Recharge in this location would require the use of injection wells due to the confined condition of the aquifer.
In addition to the current and future expanded infiltration basins in the San Fernando Valley that capture and infiltrate water from the headwaters of the Los Angeles River into the San Fernando Basin, the River itself provides additional groundwater recharge opportunities further downstream in the system. This flow occurs as a result of dry weather runoff (treatment plant discharges, groundwater upwelling in soft-bottom reaches, and urban dry flow such as irrigation overflow) and wet weather runoff during and after storm events. Existing storage areas, such as the Sepulveda Flood Control Basin, and newly created storage ponds can be used to capture water during rain events (through diversions from the River channel) and then slowly release water back into the River. This excess water, that otherwise would have been lost to the ocean, can then be extracted further downstream and used to recharge the Central Basin.
Two strategic locations along the River have been identified for extraction;
- Los Angeles Forebay, located south of Downtown Los Angeles (approximately between River Miles 16 and 19). Here the aquifer is confined (i.e., overlaid by impermeable soil) and thus will require water to be mechanically forced (or injected). However, the groundwater has a free-surface (i.e., is not pressurized) and as such the effort and cost to recharge here will be less than at other locations further downstream.
- Los Angeles Outfall, located near the mouth of the River, but upstream of the tidally influenced region (approximately between River Miles 3 and 4). This location represents the “last chance” to capture water before it is lost to the ocean. The aquifer here is pressurized (i.e., has a higher hydraulic head) and as such this location will require a larger capital investment and operational costs than recharge in the LA Forebay.
Hydrological modeling analyses have been performed to quantify the volumes of water that could be captured for different infrastructure investment levels (i.e., different pumping/injection capacities and different storage capacities).
The flow rate exceedance curve for the River above the LA Forebay (based on 25 years of calibrated hydrological watershed model1 data) indicates that an appropriate design ‘draw-off’ rate2 is likely to be in the “knee” of the curve in the 100 to 500 cubic feet per second (cfs) range. This is confirmed by preliminary analyses at three draw-off flow rates of 100, 500, and 1000 cfs. Increasing the draw-off rate results in capture increasing logarithmically, rather than arithmetically, at both the LA River Outfall and the LA Forebay location.
The modeling analyses was refined to assess a finer range of draw-off rates. To date, this additional analyses has only been performed for the LA Forebay location.
The Loading Simulation Program in C++ (LSPC) model adapted for the Los Angeles Department of Water and Power’s Stormwater Capture Master Plan (LADWP, 2015) was used to model the flows from the watershed to the LA Forebay, from 1987 through 2011. Flow from three wastewater treatment plants (DC Tillman, LA Glendale, and Burbank) were added to the model as constant point sources. Constant flows from these treatment plants were estimated from recent flow records. Flows from DC Tillman included both flows discharged directly to the Los Angeles River and flows used for environmental uses which eventually flow to the Los Angeles River. The flow rates used were 45 cfs from DC Tillman, 15 cfs from LA Glendale, and 7 cfs from Burbank, for a total of 67 cfs. Examination of the dry weather flows at this location in the Los Angeles River show that approximately 80 cfs was the typical dry weather flow, including the flow from the wastewater treatment plants.
A range of draw-off rates was analyzed under three scenarios. In the first scenario, flow was drawn off at the constant draw-off rate at all times, so both dry and wet weather flows were captured, and flows exceeding the draw-off rate were released downstream. In the second scenario, it was assumed that 50%, or 40 cfs, of the dry weather flow would be captured so the draw-off was initiated at 40 cfs. The third scenario assumed the 80 cfs of dry weather flow was constantly released downstream (or assumed to not be present), and only when the flow in the river exceeded 80 cfs (wet weather) was the draw-off rate initiated. Each scenario also incorporated four different storage strategies, where upstream storage (yet to be developed) could increase the capture efficiency. The storage values applied were zero, 5,000, 10,000, and 15,000 acre-feet that could be used repeatedly as wet weather events seasonally occurred.
Range of Annual Capture Volumes
The results of the stormwater capture analysis are summarized [3 figures below] for these scenarios for different constant draw-off rates.
- When the draw-off rate is applied during both dry weather and wet weather, more flow is captured than when only wet weather flows are captured. The effect of storage cannot be overlooked, and is most pronounced when any storage is developed (between zero and 5,000 acre-feet), however it is clear that as more storage is developed additional water supply is accrued.When the draw-off is applied to both dry weather and wet weather, the average annual capture volume ranges from approximately 8% to 55% of the 262,000 acre feet of total average annual volume of flow to this point in the river. This represents approximately 22,000 acre feet to 145,000 acre feet per year. The capture rate increase more rapidly below 80 cfs because the draw-off rate is always being applied, whereas, at higher draw-off rates, the higher rate can only be applied when that much flow is available in the river, which occurs only during wet weather events.
- When the draw-off rate is only applied to 50% of the dry weather flow and during wet weather, the average annual capture volume ranges between approximately 8% and 46% of the 262,000 acre feet total average annual volume of flow to this point in the river. This represents approximately 22,000 acre feet to 120,000 acre feet per year. In this case, the capture rate increases more steadily because the draw-off rate is applied only during wet weather, so the amount of time that the draw-off rate is being applied is approximately the same for the entire range of draw-off rates.
- When the draw-off rate is only applied during wet weather, the average annual capture volume ranges between approximately 5% and 36% of the 262,000 acre feet total average annual volume of flow to this point in the river. This represents approximately 14,000 acre feet to 95,000 acre feet per year. In this case, the capture rate increases more steadily because the draw-off rate is applied only during wet weather, so the amount of time that the draw-off rate is being applied is approximately the same for the entire range of draw-off rates.
The amount of water potentially available in the Los Angeles River for the proposed reinjection project is significant and varies between 14,000 and 145,000 acre feet per year depending on the capture strategy and available upstream storage developed. Additional considerations such as impact of variable inflow rates on the treatment system and re-injection well field will need to be considered prior to selecting a capture strategy.
Effect of Storage and Dry Weather Capture
In order to more clearly assess the effects of storage volume and dry weather capture strategy, results were summarized for an assumed constant draw-off rate of 250 cfs. At this constant flow rate the annual average capture volumes ranged from 45,000 AF (no dry weather capture and zero storage volume) to 121,000 AF (full 80 cfs dry weather capture and 10,000 AF of developed storage). It is noted that these capture volumes are averages based on 25 years of modeling. The capture volume varies substantially from year-to-year as detailed by the Annual and Monthly Summaries that summarize annual and monthly flow volumes, capture volumes, and capture fractions.
Water Supply, Aquifer Recharge, and Resiliency
The primary benefit of the capture is the increase in available supply from local water resources that can be used to recharge local groundwater storage. This is illustrated through consideration of three potential average capture rates of 45, 75, and 100 thousand acre-feet/year (K AF/YR). It is estimated that there is at least 400,000 AF of additional storage capacity in the Central Basin. If all the captured water was diverted to this storage, with no other changes in regional operation strategy, then the storage would be full within 4 to 9 years, depending upon the capture rate. At that point the additional capture would be available for export or other uses.
However, the future regional operational strategy would likely be adjusted to increase the local groundwater draw in order to meet growth in demand and/or reduce water imports. For example, if the local groundwater draw was increased by 45,000 AF/year then water imports could be reduced by 12%.
With increased groundwater draw the addition of the 400,000 AF of storage would take place over a longer time period, as a new equilibrium was established between the injection of captured water, the increase in groundwater draw, and the new volume of stored water in the aquifer.
The increase in local water supplies represents a substantial tangible benefit that can be quantified and used to justify costs. As importantly, the increase in volume of local groundwater storage that can be realized through the capture scheme will increase local resiliency by enabling water to be stored for use during drought years.
Potential Ancillary Benefits
In addition to the water supply benefits, there may be other aesthetic and recreational benefits for local communities realized from the created storage ponds. These benefits will depend upon how often the storage ponds are full or partially full, which can be evaluated by extracting more detailed output from the hydrological model. Sample analysis for a draw-off rate of 250 cfs, no dry weather draw-off, and 10,000 AF of storage indicates that storage is typically maximized in March, with it being completely full in about 40% of years and half full in about 65% of years based upon the 25-year record considered. By contrast, storage is practically zero throughout the summer months.
There is substantial variation in storage and pumping operation from wet years to dry years, as illustrated in the Detailed Time Series output from the model provided for each of the 25 years simulated.
The value of water can vary greatly, provided it makes sense to the beneficiaries. An argument can be made that it could range from $1,000-5,000/ AF, or even higher. Recovery of water from the LA River for Injection may best be compared to the cost of new-water supply alternatives for Southern California.
To evaluate the potential infrastructure costs, a similar project considered by the WRD and LADWP for recovery of dry-weather flow was used as reference. In this reference, the efficiency from operation on was about 75%, so 45,000AF/YR of recharge might be available from 60,000AF/YR of river water, with 7,000 AFY of water returned to the river and 8,000 AFY of water lost to the ocean.
The cost of the infrastructure is potentially $1,500-1,800/ AF, which is a reasonable cost for the water and perhaps $500/ AF lower than the cost of water from regional water recycling projects. Of this $180-210/ AF is associated with waste or $210 million over 30 years.
This money used for sewer infrastructure/fees could alternatively be used to develop treatment wetlands, which provide an environmental improvement.
The wastewater flows may also be used to sustain beneficial uses for wildlife and recreation along a redeveloped river system, which could provide additional value.
Project could qualify for Prop 1 funding – project providing water storage benefits with environmental improvements. If capital was used for river improvements to manage waste up to 25% of capital costs including this costs could be available from grants. Could be $250-300 million.
Potential matching Federal funds would double the amount. Could be $500-600 million.
1 The Loading Simulation Program in C++ (LSPC) model adapted for the Los Angeles Department of Water and Power’s Stormwater Capture Master Plan (LADWP, 2015) and calibrated from 1987 through 2011.
2 The draw-off rate refers to the flow rate that will be directly extracted from the River for delivery to the groundwater basin.