The Real Hahamongna Dilemma

At first glance, you may think this photo is the Arroyo Seco flowing through the Hahamongna.

But this is actually the Arroyo Seco flowing south of Devil's Gate Dam just above Brookside Golf Course and look at that sediment!

One of the saddest legacies of the devastating 2009 Station Fire is that the complete burn of the upper Arroyo Seco Watershed created massive sediment debris flows into the central and lower reaches.

While lots of public outcry is being played out in the media about LA County's plan to remove sediment in the Hahamonga and the impact of that operation on habitat in that basin, no one is talking about what will happen downstream if the dam fails due to sediment buildup overload.

Already, heavy sediment flows into the Central Arroyo Seco have damaged most river restoration enhancements completed in 2008, including complete mortality for the small, yet growing school of native Arroyo Chub fish that was successfully reintroduced. In fact, the backwater pool under the Colorado Street Bridge is so full of sediment, you wouldn't even know that a pool once existed there.

In addition to the habitat damage, the sediment buildup both above and below the Rose Bowl complex is such that, without sediment removal behind the Dam, this popular recreation area could experience a flood or near flood event within the next year or so if rainy seasons replicate last year's deluge. Moreover, LA County's effort to relieve stress on Devil's Gate Dam from sediment buildup has included extra water releases into the Central Arroyo, contributing to the sediment buildup there.

This flooding potentiality was driven home last week during a tour by flood program representatives of the California Department of Water Resources who expressed shock (their actual word) at the level of sediment buildup behind the dam and its growing buildup in the Central Arroyo, especially as it 'backs up' into the channelized area just before the softbed section south of the Parking Lot I picnic area.

What would it mean to Pasadenans and their neighbors if the Brookside Golf Course and the Rose Bowl areas flooded?

First, it would mean no access to highly used recreational amenities, including the Rose Bowl Loop, which draw almost 3,000,0000 people to Brookside Park every year. Secondly, it would mean a major financial and logistics emergency preparedness cleanup effort by the City of Pasadena and LA County which would cost in the tens of thousands of dollars at a time of stressed public agency funding. Finally, it would mean a direct loss of over $4,000,000 in annual revenues that the City of Pasadena relies upon to fund its operating budget.

Here are some sobering facts:

.the 1938 Arroyo Seco flood damaged 85% of Brookside Golf Course and filled the Rose Bowl to its first tier level of seating until the river changed course and spared further damage to the stadium
.the Arroyo Seco Concrete Channel is nearing the end of its useful life
.the lack of federal funding match to complete the US Army Corps of Engineers Arroyo Seco Feasibility Study, first approved in 2003, hampers federal financial support for channel realignment or relocation in the Central Arroyo
.there is a high risk of little federal support in the event of a flood event in Brookside Park because, in part, the Federal Emergency Management Agency (FEMA) never created a flood inundation map for the area after the Arroyo Seco was channelized
.the Arroyo Seco Concrete Channel has never really been challenged with the type of heavy water/sediment flow now possible, so it is unknown whether it will actually 'hold' should a catastrophic spring flood event like the current Ohio and Mississippi River flooding test its strength

Let's hope that the newly created Sediment Removal Advisory Commitee will work with LA County Flood Control Management District leadership, state agencies, and federal officials to find sustainable solutions to Arroyo Seco sediment buildup that takes into account the larger context of impact that goes beyond the mere perimeter of the Hahamongna Basin.