A CRITICAL
ANALYSIS OF FLOOD CONTROL
PROPOSALS FOR CLEAR CREEK
(prepared for the Galveston Bay Conservation and Preservation Association)
|
|
|
|
|
|
||
|
|
|
A CRITICAL
ANALYSIS OF FLOOD CONTROL
|
||||
 |
Larry Dunbar, P.E. |
Congress has authorized the construction of a flood control project along Clear Creek, consisting of the widening and deepening of the existing channel of Clear Creek and the construction of a second outlet from Clear Lake to Galveston Bay. This second outlet has already been constructed, including the installation of gates to maintain the existing tidal fluctuations between the lake and the bay.
Local residents have expressed serious concerns about this federal project, especially in regards to its potential for increasing flooding around Clear Lake, as well as its negative ecological impacts. As a result, the Harris County Flood Control District (HCFCD) and Galveston County, two of the local sponsors of this federal project, requested that the Corps of Engineers put a hold on further construction of the project until the local sponsors could conduct a review of the project in order to address the concerns of the local residents. This review was recently completed by the HCFCD and a report was submitted to the Corps of Engineers in December of 19997, outlining the results of this review and recommending some modifications to the federal project that purportedly would minimize the concerns of the local residents while still achieving the flood control benefits of the federal project.
The Galveston Bay Conservation and Preservation Association (GBCPA) and Friends of Clear Creek commissioned this critical analysis of these flood control projects being proposed for Clear Creek. The purpose of this analysis is to identify issues of concern with the proposed projects and to develop alternative proposals that are both economically and environmentally preferable.
The Clear Creek watershed, including Clear Lake, has a drainage area of approximately 260 square miles, of which approximately 170 square miles drains into the western end of Clear Lake. Armand Bayou, a major tributary to Clear Creek with a drainage area of approximately 60 square miles, enters the middle of Clear Lake from the north. Clear Creek forms the boundary between Harris County on the north side and Brazoria/Galveston counties on the south side. The far upstream end of Clear Creek is in Fort Bend County.
The current flood plain of Clear Creek, excluding Armand Bayou, covers approximately 12,800 acres, which is about 10% of the total drainage area of Clear Creek. About 10% of this floodplain has already been developed. The majority of this development in the floodplain is located in the Friendswood area between FM2351 and FM528 and in the Pearland area between Telephone Road (S.H. 35) and Cullen Road. There has been a history of flooding problems in these areas, with the latest being in October of 1994. Almost half of the annual flood damages that have been identified along Clear Creek occur in the Pearland area, and about one-fourth of the annual flood damages identified occur in the Friendswood area.
The purpose of the federal project as authorized by Congress was to significantly reduce the flood damages that occur in these areas. The authorized federal project included the Channelization Of Clear Creek From The Western End Of Clear Lake All The Way Upstream To Mykawa Road in Pearland, a distance of approximately 21 miles. This project was designed to carry the 10-year flood within the banks of the new channel, leaving a residual 100-year floodplain. The flood control benefits versus the costs of this project were considered to be economically more preferable to a channelization project that would contain the entire 100-year flood.
Due to the lack of local funding by Brazoria Drainage District No. 4 (BDD4), one of the local sponsors of the authorized federal project, the portion of the federal project within Brazoria County has been put on hold until such time as the local funding become available. Therefore, the “current” federal project consists of the channel improvements from Clear Lake up to the Galveston/Brazoria county line, a distance of about 14 miles, and the second outlet from Clear Lake into Galveston Bay. The “current” federal project, which only extends the channelization up to the Galveston/Brazoria county line, will only help the flooding situation in the Friendswood area, and will have no effect on improving the flooding problems in the Pearland area, where about half of all the flood damages along occur during a 100-year event.
The “current” federal project, as presented by the Corps of Engineers, consists of the deepening and widening of Clear Creek from the western end of Clear lake (River Mile 4.8) up to the Galveston/Brazoria county line (River Mile 18.5), a distance of about 14 miles. The project also include the construction of a second outlet from Clear Lake into Galveston Bay, along with gates that would be generally closed in order to maintain the current tidal regime in the area and would only be opened when Clear Lake water levels exceeded to level of the bay.
This channelization project has been designed by the Corps. of Engineers to contain the 10-year flood along the lower reaches of Clear Creek, assuming full development of the watershed. The cost of this portion of the project has been estimated to be $122 Million. However, there would still be a residual 100-year floodplain in the lower reaches of Clear Creek, and over 50 homes in the Friendswood area would still be in this floodplain. Of course, the 100-year would still remain as it is today in the upper reaches of Clear Creek, including the Pearl and area, where over half of all of the flood dames along Clear Creek occur.
When the corps of Engineers evaluated the effect of channelizing Clear Creek on Clear Lake, assuming full development in the watershed, they concluded that the level of Clear Lake would be increased 2.5 to 3 feet during a 100-year storm event. In order to eliminate this adverse impact, a second outlet from the lake to the bay was incorporated into the federal project. This second outlet was designed by the Corps of engineers to remove the additional runoff that would enter the lake, as a result of the channelization project upstream along Clear Creek, so as not to cause an increase in flooding around the lake. The Corps of Engineers relied on their computer modeling of Clear Creek watershed and Clear Lake to demonstrate that, with full development of the watershed, the additional runoff that would be generated by this development and the channelization of Clear Creek could be handled by the second outlet. This additional runoff, as calculated by the Corps of Engineers, would result in about twice as much flow entering the west end of Clear Lake as under current conditions.
Although this second outlet would allow lake waters another path through which they could enter the bay, it also provided another path through which bay waters could enter the lake, such as during high tides and storm surges. This prompted the Corps of Engineers to include gates with this second outlet so that this outlet could be closed most of the time, thereby maintaining the current regime of tidal fluctuations between the lake and the bay. The gates would only be opened when there was a rainfall/runoff event in the watershed that presumably would be large enough to cause the level of the lake to exceed the level of the4 bay (whatever that level happened to be at the time), thereby allowing the flood waters entering the lake to flow out through both the existing opening and the second outlet. This second outlet has already been constructed, including the gates, at a cost of $22 Million.
Besides some widespread public concerns being raised about the adverse environmental impacts of channelizing Clear Creek, the residents around Clear Lake raised concerns about the potential for increased flooding around the lake that would be caused by the federal project. The two local sponsors, the HCFCD and Galveston County, formed the Clear Creek Federal Project Review Team and directed them to review the “current” federal project and evaluate possible alternatives that would be more acceptable to the concerned citizens, yet sill achieve the flood control objectives of the “current” federal project. This group evaluated a number of alternatives and developed a recommended alternative that was supported by the two local sponsors.
The HCFCD recommended alternatives still involves the channelization of Clear Creek, but with a reduced channel width from that of the “current” federal project. In addition, the HCFCD plan includes a bypass channel in the most environmentally sensitive area of the creek, some detention ponds adjacent to Clear Creek and a buyout program for the homes that would remain in the residual 100-year floodplain in the Friendswood area. The estimated cost of the HCFCD recommended alternative is $115 Million.
It is undisputed that the “current” federal project will result in an increase in runoff entering Clear Lake, almost doubling the flows over those estimated for existing conditions. This was the reason for the second outlet. The major concern is what happens if the second outlet cannot be used to release these additional flows entering the lake because the water level in the lake is lower than the water level in the bay. The Corps of Engineer considered this possibility and concluded that high tide and a rainfall/runoff event were two independent events and therefore the likelihood of their occurring at the same time was remote. Also, it was not an important concern to the Corps of Engineers because the amount of damages that would occur as a result of this unlikely event was considered to be far less than the damages resulting from a major flood event or from a major hurricane.
The Harris County Flood Control District (HCFCD) investigated this issue and recently submitted a January 1998 report to the Corps of Engineers that discussed the potential impacts of the “current” federal project on flood levels in Clear Lake. The HCFCD report first concluded that the “current” federal project, as designed, would cause an increase in flood levels in the Nassau Bay area under certain moderate tidal conditions, even if the second outlet was being used. This was discovered by the HCFCD when they analyzed other bay conditions besides the constant 2 feet above mean sea level condition that was utilized by the Corps of Engineers that high tide and a rainfall/runoff event are necessarily independent events. If they occurred at the same time, the HCFCD found that the second outlet was not large enough to remove the additional runoff as calculated by the Corps of Engineers in designing the “current” federal project.
With regards to the issue of what would happen if the bay was higher than the lake during a rainfall/runoff event in the Clear Creek watershed, an unsteady flow computer model was developed by the HCFCD to assist them in analyzing the reaction of the lake to runoff entering during high tides in the bay. The conclusion of the HCFCD was that there was virtually no situation where the second outlet could not be used for removing runoff entering the lake. They were able to reach this conclusion because, according to their computer model, the level in the lake quickly reaches the level in the bay. This is of course totally contrary to the findings of the Corps of Engineers in their 1982 report and the justification for installing the gates at the second outlet. While the HCFCD acknowledge that their computer model produces different results than those produced by the Corps of Engineers model, the HCFCD cannot explain the reason for these differences. And they do not have any observed data that demonstrates that no differential exists between the lake and the bay, as was noted by the Corps of Engineers in its Pre-Construction Authorization Report. Thus, the HCFCD was able to use their model results to conclude that there would virtually never by a situation when the gates to the second outlet could not be opened, so that the second outlet could always be used for removing some of the additional runoff entering the lake.
There are some serious questions regarding the accuracy and reliability of the unsteady flow computer model results of the HCFCD. The fact that their model shows virtually no differential in lake level and bay level is contrary to both the findings of the Corps of Engineers and the observations of the local residents around the lake. In addition, their model produced about half of the increase in lake level as that produced by the Corps of Engineers in analyzing the impact from the current federal project without the second outlet. This would suggest that the HCFCD model allows much more eater to pass through the existing outlet than does the prior observations of the Corps of engineers and the local residents. This may be caused by an error in the model. In fact, the HCFCD compared the results of their unsteady flow model to the results of the standard HEC-2 model for the existing outlet from the lake and found that the unsteady flow model produced much more flow leaving the lake than would be shown using the HEC-2 model.
In my opinion, the “current” federal project, as designed, will cause an increase in flooding around Clear Lake. This is due in part to the fact that the “current” federal project was designed based upon the assumption that all future development in the watershed could occur without any runoff controls being provided other than the Corps’ project. This assumption was shown to be incorrect by the HCFCD with the use of an updated HEC-1 model of the Clear Creek watershed. This model showed that, even with the HCFCD’s Regional Flood Control Plan for Clear Creek in effect (which includes the “current” federal project), future development of the watershed would result in a significant increase in the runoff entering the lake that could not be handled by the second outlet. The HCFCD concluded that in order to prevent to have on-site detention in order to maintain existing runoff rates. This is because the second outlet is only large enough to handle the additional runoff that will be caused by the channelization of Clear Creek itself, assuming the runoff from the existing land use is maintained. Of course, the ‘current” federal project was shown to be economically justified primarily on the basis of providing significant benefits to future development. This is now no longer the case.
Even though the second outlet is considered to be capable of handling the additional runoff from the channelization of Clear Creed, according to the HCFCD, the unsteady flow modeling performed by the HCFCD is highly suspect and unreliable in being capable of showing whether the second outlet will always be available for removing additional runoff entering the lake. Clearly this unsteady flow model produces results inconsistent with the HCFCD’s own HEC-2 model, with the Corps’ analyses used to design the “current” federal project, and with the observations of residents living around the lake. It is understood that the Corps of Engineers is planning to develop its own unsteady flow model of the lake in an attempt to address this issue. It is recommended that any computer model developed for this project be calibrated to observed data so that any results from the model can have some credibility and usefulness.
In reviewing the recommended alternative, the same concern exists as with the “current federal project, but to a lesser degree, because the recommended alternative involves less channelization of Clear Creek and therefore less of an impact on the lake. While the recommended alternative reflects less runoff entering the lake due to the reduced channelization and the regional detention, there still is an increase in runoff expected to occur over that which is occurring now. Thus this increased runoff will cause an increase in lake level if the second outlet cannot be used, such as when the bay is higher than the lake.
It is well established that channelization of a stream will cause an increase in flooding downstream due to the reduction of floodplain storage and the increase flow conveyance resulting from an improved channel. The HCFCD itself has studies that show that the improvement of the runoff conveyance system can increase the runoff rates of frequent storms by as much as seven times the rate of existing runoff, without accounting for any increase in imperviousness. Therefore, any project that involves channelization without adequate compensating detention will cause increased runoff downstream. One obvious way to avoid sending additional water downstream is to avoid channelization. The Clear Creek Project Review team evaluated a number of alternatives that did not include channelization, such as regional detention and buyout. The HCFCD recommended alternative incorporated both of these options as a way to allow for a reduction in the amount of channelization being proposed in the “current” federal project. However, they did not totally eliminate channelization from their plan since one of their objectives was to provide for a way to drain the upper portions of Clear Creek.
In their report to the Corps of engineers, the HCFCD evaluated a buyout alternative that they estimated would cost about $70 Million, about half the cost of the current federal project or the HCFCD recommended alternative. This Buyout Plan reportedly involved the acquisition of 400 homes in the Friendswood area as representing all the homes in the 100-year floodplain within the limits of the “current” federal project. The reason why this alternative was eliminated from being given any further consideration was that it did not meet the objectives established by the HCFCD of providing for the draining of the floodplain in the upper reaches of the creek. One must question why this must be an objective of a flood control project that is supposedly addressing existing flooding problems versus providing opportunity for the future development of the existing natural floodplain.
The Buyout Plan is acknowledged to have the least environmental impacts of any alternative analyzed. Normally, buyout is found not to be the preferred plan because of the high cost associated with such an alternative versus other plans. However, in this case, the buyout alternative is actually the least costly alternative. It makes no sense whatsoever to spend about $120 Million for a flood control project to protect 400 homes located in the 100-year floodplain when it would only cost half that amount to buy all of those homes and maintain that land as nature’s detention pond. This proposed plan of buyout would also cause no increase in runoff entering Clear Lake and therefore would have no potential of causing an increase in flooding around the lake, unlike the “current” federal project and the HCFCD recommended alternative.
The only “drawback” that has been raised to this proposed buyout plan is that it does not eliminate the “current” floodplain along Clear Creek and does not provide a way to totally eliminate the floodplain in the upper reaches of the creek. However, this does not mean that the existing flooding problems in the upper reaches of the creek cannot be economically resolved nor that future development in the watershed cannot occur. In fact, the Brazoria Drainage District #4 (BDD4) has developed a flood control plan for their district, which includes most of the Clear Creek watershed upstream of the “current” federal project limits. This plan assumes that Clear Creek will not channelized downstream and proposes some selected channelization of tributaries along with regional detention in order to address the current flooding problems within the district without causing an increase in flood flows downstream. Surprisingly, this plan is estimated to cost about one fifth the cost of the Regional Flood Control Plan developed by the HCFCD to solve virtually the same existing flooding problems. The plan of BDD4 also does not include any channelization of Clear Creek, but rather utilizes regional detention, possibly in conjunction with levees, to address the existing flooding problems along the creek in the Pearland area. And both plans required all future development to have on-site detention to prevent any increase in flows downstream.
The “current” federal project, as well as the HCFCD recommended alternative, is designed to result in an increase in runoff entering Clear Lake, and therefore will have the potential of increasing flooding around the lake. Therefore, it is proposed that the current federal project and the HCFCD recommended alternative be rejected as viable plans for solving the existing flooding problems along Clear Creek in the vicinity of Friendswood. Rather, the flood control project that should be seriously evaluated and pursued is the proposed plan of total buyout (at a fair price) of those homeowners who wish to sell their homes who are located in the 100-year floodplain of Clear Creek between I-45 and the Galveston County line, estimated to be a total of 400 homes.
Most likely, only those people who have experienced significant damages in the past or fear such in the future would opt to be bought out. This, of course, would result in eliminating the majority of the flood damages that the current plans are also attempting to eliminate. Yet those plans are doing so by removing the majority of the existing floodplain which nature has provided as flood protection for Clear Lake, even in areas where the current floodplain is not causing any damage.
In addition, the proposed buyout plan would allow the currently constructed second outlet to be used as a flood control project for Clear Lake, providing reduced flood levels around the lake; thereby providing significant flood control benefits for the proposed plan that are not provided by either of the current channelization projects. The current project uses the second outlet as a means to avoid causing more flooding around the lake; the proposed plan would use the second outlet to help reduce flooding around the lake.
The proposed buyout plan is the least costly alternative, has the least adverse impact on the environment, maximizes the flood control benefits along Clear Creek for those who need it and want it, does not restrict future development upstream, avoids any potential for increasing flooding around Clear Lake (in fact provides additional flood control benefits around the lake), preserves the natural flood storage capacity of Clear Creek, and maintains the natural beauty of Clear Creek.