THE LEAKING SHADOWLAKE

Measurements and data collection indicate that the leakage from the lake averages 99 gallon per minute resulting in a yearly loss of 52 million gallons. HOA has a contract with MUD titled The Detention Pond Agreement, which defines the responsibilities for the operation of MUD’s storm water detention facility including the Shadowlake and the dry detention area to the east. MUD’s responsibilities include to ensure that the lake can hold water and HOA’s for the aesthetic aspects including erosion of the shoreline. Hence the responsibility of HOA is to enforce the homeowners interests towards MUD who is responsible for mitigating the leakage.

Leakage takes place under the foot of the berm amounting to 20% of the total and measurable in the outlet channel to the Bayou. The effort to remedy the remaining 80% would include draining the Shadowlake and seal the bottom to an undefined extent possibly costing millions of dollars. As this is cost prohibitive an alternative acceptable to HOA by other means should be considered as all homeowners are also MUD taxpayers.

I and other homeowners have since the beginning of January until the end of September this year monitored the lake level weekly including measuring the leakage flowing through the outlet channel to the bayou. Data on rainfall and evaporation have been provided by David Keller.

HCFCD carried out soil investigations for the Driscoll Park detention facility project using the renowned geotechnical engineering company Fugro. The report includes an indication that an approximately 700 feet wide strata to depth of 30 feet through which water leaks extends to the Shadowlake under the berm as indicated below. MUD recently approved a project to inject foam to a depth of 8 feet at the 700 feet but their Engineer decided to carry it out at a different location without achieving the intended result.

The soil investigation report (click here to view) includes a statement quoted below included under paragraph 1.1 Project Description that the leaking strata can affect the water level in the Shadowlake .

Quote

We understand that a single water well is currently used to supply make-up water to Shadow Lake in order to maintain the lake level at a minimum desired elevation. The water supply from the well offsets water losses from evapotranspiration and from seepage through the subsurface soils. The HCFCD has requested that the impact of the proposed improvements on water loss due to seepage from Shadow Lake also be considered during our study.

Unquote 

An alternative to a million dollar solution has been proposed to MUD is estimated to cost $150,000 including obtaining permission from HCFCD to rearrange the detention capacity. The proposal comprises converting the dry detention area to the east of the lake for capturing rain and flood water from the bayou as well as serving as a holding area for water pumped from the Bayou at high water levels. Water in the capturing area together with pumping from the bayou directly to the lake compensates for the 52,000,000 gallons leakage.

The yearly 17,000,000 gallons average rainfall in the capture area as well as each 13,200,000 gallons fill of the same by flooding from the bayou are free. Lowering of the maximum lake level reduces the erosion impact of waves on the shoreline.

 
   
The 13,200,000 gallons reduced capacity of the detention facility is replaced by lowering the maximum water level in the lake by 11 inches with the result shown in the below picture. A lower maximum water level reduces the impact of erosion caused by waves.

Below are comments to the non-technical opinions offered by HOA’s lake manager Mac McCune in response to questions by the author of the September newsletter article.

Question: A statement made in this homeowner’s emails is that a repair to the lake’s berm may require going down approximately 30' instead of 8' based on a study done by HCFCD for the adjacent Mike Driscoll Park. Our engineer and maintenance contractor state that repairs using injections of Uretek have stabilized and sealed leaks in the earthen berm. Is it cost effective to bore down 30' to inject additional Uretek?

Mac: In my 40 years of dealing with lakes and ponds, I have never seen the need to bore 30’. To inject Uretek at that depth would be assuming that any seepage from the lake is specifically occurring under the lake immediately adjacent to the berm when, in fact, any seepage could be occurring in other areas around the lake and traveling to an area of least resistance. Aside from a needle in a haystack, this is like trying to prevent any connection to subsurface groundwater and would require a “curtain” of Uretek around the entire perimeter of the lake as well as the basin. In other words, you would be better off draining the lake, mucking it out and re-lining the lake with a 30+ PI clay assuming soil borings indicate a problem.

 

Comment: The information included in the soil investigation report commissioned by HCFCD indicates that the leaking strata extends to 30 feet below the lake bottom. Decisions about remedies should be based on technical information and not 40 years of non-technical observations at various locations not necessarily comparable with the Shadowlake location.

Observations by the maintenance contractor of water accumulations on the surface are not valid engineering input and likely are accumulations from rain in the lower area. Measurement of the flow in the outlet channel to the bayou indicates that the 20% leakage has not changed after the foam injection project.

Question: In your experience, have you seen dry basins converted to hold water to pump back into lakes?

Mac: Yes. However, this would entail excavating down below the required detention capacity, so detention volume is not impacted. In addition to being very expensive, it’s possible that the soils below the existing detention basin are not suitable to hold water and soil borings would be required prior to finalizing any plan. Then comes the question of how large and deep the supply reservoir should be. Construction costs would include excavation, any possible clay liner depending on whether the clay is onsite or imported, hauling the spoils offsite and a lift station with discharge to the HOA lake. Once these costs are determined, a feasibility study would be needed to determine how much would be saved in makeup well water costs as well as the capital cost ROI (Return on Investment).

Comment: The solution to compensate for the reduced flood control volume comprises lowering of one spillway shown in the below picture by 11 inches, which can be achieved for $500. The resulting lake level is shown in the picture above.

Question: The homeowner states in his emails that the community could install electricity and pumps on a tributary to Brays Bayou at a cost of $10,000 that "facilitates pumping water to the Shadowlake from the Bayou at high water levels.” Would this plan re-quire HCFCD’s permission to pump from a bayou tributary? Would it cost approximately $10,000 given the constraints mentioned of a shallow detention basin not designed to hold water for long periods of time?

Mac: Not sure about the cost, but I am sure it would require permission and the acquisition of water rights as there is no free water. In addition, the accumulation of off-site water quality issues is certainly a concern.

Comment: The City of Houston being the custodian of the water in the bayou has confirmed to HOA that pumping can be carried out at a cost of $0.50 per one thousand gallons.

Captured rain and flood waters are free and $10,000 yearly saving is achieved by irrigating the areas at the Lake Section using the lake the source in lieu of city water. This was the method used prior to the Subsidence District imposed a restriction of using the lake as a source after the unlawful pumping from the well. The proposed solutions offer the possibility to use the lake as the source for irrigation in areas north of the Bayou saving $50,000 per year.

The water quality at high levels in the Bayou is identical to when the lake is filled by flooding.

Question: The homeowner states that the cost for his plan to store water in the dry basin would be minimal. Can a shallow, normally dry basin originally constructed to quickly drain overflow water into Brays Bayou during heavy rain events such as Hurricane Harvey be inexpensively converted to hold water for longer periods of time?

Mac: See above. I doubt the cost of this project would be even close to “minimal”. The supply reservoir would need to be at least six feet deep to reduce stagnation and nuisance aquatic weed and algae growth. Therefore, it is likely that someone would have to pay to maintain the supply reservoir so that the lift station does not become blocked with debris as well as pay for the operating costs.

Comment: The cost including obtaining a permit to rearrange the flood control volume, 700 feet by 250 feet liner to cover the leaking strata in the dry detention area, construct a spillway in the outlet channel to the Bayou, pump well, bring electricity supply across the bayou, 4000 gpm and 200 gpm pumps is estimated as $150,000.

 

Tor Persson, tpp@twindog.com