First Impressions
from
Field Visit 100723 to Tijuana River Project
Victor M. Ponce

The sources of water, sediment, and trash in the Tijuana river are on the Mexico side of the border. Solutions will have to consider improved management practices on the other side of the border. Institutional and cultural barriers will need to be tackled to properly contribute to solve the problem. The Municipality of Tijuana would have to engaged.
The channel capacity of the Tijuana River at its entrance to the United States is 3,100 m³/sec, which translates to 110,000 cfs. By comparison, the low-flow channel capacity of the Tijuana river at its estuary is estimated at about 1,000 cfs. Thus, flooding will remain a recurrent problem in the Tijuana delta.
The Tijuana estuary is a delta, where the river spreads its water, sediment, and, in this case, trash. The prevailing slopes are such that spreading will always have the tendency to occur. This is a fact of nature, which cannot be contested.
The Tijuana river delta appears to have been much greater in size. There are unconfirmed reports that the true historic mouth of the Tijuana river is at San Diego Bay. Is this is correct, encroachment by development over the past 100 years would have had the effect of decreasing the delta's size; therefore, increasing the sediment's vertical accumulation, in lieu of horizontal spreading. Thus, progression toward a fresh water marsh may have been accelerated by development.
The headwaters streams near the delta are Los Laureles [Goat Canyon], El Matadero [Smuggler's Gulch] and Los Sauces [Yoghurt Canyon], with headwaters in Tijuana proper. These watersheds have undergone accelerated urbanization in the past 15 years. This has accelerated the delivery of sediments to the U.S. side of the border. The disposal of trash in the canyons on the Mexico side of the border creates a problem of mobilization during floods. Large amounts of trash are transported and deposited on the U.S. side during flood events.
The sediment retention basin at Los Laureles/Goat Canyon appears to be working. There is historic sediment data there, which could be used as a reference for the design of a sediment detention basin in Smuggler's Gulch. Differences in catchment size and degree of urbanization must be taken into account when performing a regionalization of sediment data.
Sediment and trash will continue to remain a problem in the estuary. Even if the trash problem is tackled succesfully, the sediment problem, which is more difficult to solve, will remain. Solutions to reduce sediment sources on the Mexico side of the border are difficult to solve.
One-dimensional flood analyses such as HEC-RAS will not give enough detail or accuracy on a delta and estuary such as the Tijuana River. A two-dimensional water and sediment routing model is recommended for future work.
One must remember that during very wet years, Mexico will not hesitate to fill the Tijuana channel to capacity. That has not happened in the past 40 years, but there is no reason why it could not. If this happens, there is simply no way to avert or mitigate the flood and sediment disaster that would ensue on the U.S. side. Mexico is trying to hold on to as much water as it can, but surplus water in wet years is going to be disposed on the U.S. side.
Accumulations of sediment, accelerated by urbanization and delta constraining, are gradually changing the character of the delta marshes, from saline and transitional to fresh water. Studies by the Tijuana River Estuarine Research Reserve may indicate the extent to which this transformation has taken place in the past 30 years. The only way to mitigate this effect, after the fact, is to collect the sediment in detention basins, and to dispose, sell, or otherwise remove the sediment from the premises, mechanically.
Channel improvements in the main channel Tijuana river will add to its current capacity, but to be effective, would have to run the entire length, from near the border to the sea, which could be expensive. Regardless of its size, it would be out of the question to match the maximum Mexican discharge; so, flooding will remain a threat in the valley (delta).
Directing the flood by way of levees will solve some local problems at the expense of creating other problems in the neighborhood. A 2-yr conveyance channel may be constructed, but it would be subject to overflow for longer return periods.
Stakeholders must bear in mind that the problem of managing water, sediment, and trash in a binational river basin such as the Tijuana is not easy, and there are no clear-cut solutions that everybody will subscribe to. Someone will always be at risk.
An immediate task is to contact Mexican authorities and start the process of engaging them as partners to solve the trash problem from source to mouth. This is a medium-term task which needs to be started as soon as practicable.
Sediment accumulated in the detention basins could be sold for profit to interested parties. Chemical analyses are needed to confirm that it is free from hazardous materials. Sand is a precious resource in Southern California; thus, an immediate problem could be turned into a profit-making activity for the city and county of San Diego. For example, the Los Angeles Department of Public Works sells all the sediment collected in their more than 100 debris basins strung along the San Gabriel Mountains, near Pasadena and neighboring communities.
Additional field visits are recommended to examine design features in more detail. A list of proposed actions by all stakeholders, and the stage of their development, would help focus the design team on the most immediate issues of flood and sediment mitigation.
Who is trying to do what, how, and when? These answers are required to keep the recommended actions of the design team in the proper perspective.
An interdisciplinary, interinstitutional, and international perspective is a must in project planning for the Tjuana river delta. Patchwork solutions will not work.

100723